Science topic

# Convection - Science topic

Transmission of energy or mass by a medium involving movement of the medium itself. The circulatory movement that occurs in a fluid at a nonuniform temperature owing to the variation of its density and the action of gravity. (McGraw-Hill Dictionary of Scientific and Technical Terms, 4th ed; Webster, 10th ed)
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Hi ,
I am doing a conduction and convection heat transfer analysis in a layered block. The middle layer emits heat flux which is transfered via conduction and convection to the top surface on both sides. I am getting the following error. Would anyone know what this means ?
***ERROR: STRESS - DISPLACEMENT ELEMENTS OR OTHER ELEMENTS WITHOUT TEMPERATURE             DEGREE OF FREEDOM ARE NOT ALLOWED IN A HEAT TRANSFER ANALYSIS
Many thanks for any help.
assign the mesh geometry for heat transfer and not for 3D Stress
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In last 20 years, the single phase convection heat transfer studies of transitional and turbulent flow in noncircular ducts with asymmetrical heating are being attempted using numerical methods and CFD mostly without experimental verification of the results; an important shift from earlier experimental approach. I feel this requires thread bare discussion regarding accuracy of the results and other aspects.
I quite agree, which is why students need experienced faculty to warn and coach them on such details. Even if the professor's experience is in a somewhat different area, there are still similar challenges in any experimental study. Another example of this problem: I know a professor who has published a dozen papers on nuclear reactor design yet has never set foot inside a nuclear plant. This is like giving a seminar on horseback riding yet having never actually touched a horse, let alone ridden one.
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Double-diffusive convection is a mixing process that occurs when two fluid components diffuse at different rates interact. Upwelling of nutrients and vertical heat and salt transport in oceans, for example.
Source:
Animasaun I. L. (2016). Double diffusive unsteady convective micropolar flow past a vertical porous plate moving through binary mixture using modified Boussinesq approximation. Ain Shams Engineering Journal 7(2), 755 – 765. http://dx.doi.org/10.1016/j.asej.2015.06.010
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do i need to apply temperature , convection and radiation loads all simultaneously or can i apply it by using temperature only?
U can use these codes if u work on ANSYS APDL
/SOL
DELTIM,60,0,0
OUTRES,ERASE
OUTRES,ALL,ALL
KBC,0
TIME,7200
/SOLU
SOLVE
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It is shown that a comparison of the convection heat transfer coefficient of a thin water bearing fracture, αw, with the conduction heat transfer coefficient of the surrounding rock, αr, leads to a drastic αw >> αr difference, called α¬discrepance.It does follow that it is the rock, and not the water, which governs the heat transfer.
This looks like a transient heat transfer problem being modelled as steady state one. The main debate issue on such problems is always the boundary conditions assumed to make the mathematics feasible: traditionally either constant temperature or constant heat flux were used, while neither is the real case.
Transient conduction into a semi-infinite solid is covered in Carslaw and Jaegers old, but excellent, book.
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Hi, Where can I find the solution manual of the book - An Introduction to Convective Heat Transfer Analysis? I searched on the internet, but couldn't find it. Thanks in advance.
Shiron Thalagala Hi, did you find the solution manual. I urgently needed it and search the internet throughout.
Thanks
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I am trying to predict the temperature of the hot rolled steel section after cooling down from 1300 °C. My predicted temperature reached room temperature at 30 minutes, but journal prediction only came to room temperature at 150th minutes. I considered all input data as per the journal (DOI: 10.1061/(ASCE)ST.1943-541X.0001739). I am sceptical about modelling input parameters like convection and radiation interaction properties. I want to know where I made a mistake. Kindly help me with this. Please provide your valuable suggestions and, if possible, any input data support my study.
Note: I have attached my Abaqus file and my predicted result
Thank you, Professor Simon Smith , I will try with 2D models. Then I will proceed with further mesh sensitivity study.
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I have brinjal line have trait for wilt resistant that identified by phenotypic data, than I go for cross them with wilt susceptible line, generate F1 and F2, BC1 BC2. based on chai square test and I know this a recessive resistant gene. but now want to know that trait governed by how many gene? how to know pattern of inheritance that gene? or any other method to prove and validate these trait is recessive resistant. finally after know about all genetics which breeding method I should preferred to integrate this trait in promising variety.
Note: I am convectional breeder so want more suggestion on convectional method and also i appreciate advance molecular breeding based suggestion, thank you ...
Dear @Vekariya Rajesh For detailed genetic analysis, you should have 2-3 wilt resistant lines and equivalent number of wilt susceptible lines. Typically such lines should have distinct pedigree. In addition to what you have narrated, also make resistant to resistant and susceptible to susceptible crosses in order to rule out many a possibility. By analysing your phenotypic data, you can detect only such number of genes for which selected parents will differ. For the rest, I endorse what has been mentioned by @Narendra Kumar Singh.
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Hello,
I am trying to understand the method to calculate convective heat transfer and radiative heat transfer in comsol. The geometry is a simple rectangle (2D case). Two opposite sides are insulated and other two opposite sides have heat flux boundary condition with respective temperature and h values. Material selection is air.
I have surface to surface radiation model for radiation. The two sides which have heat flux boundary condition are selected as diffuse surface and emissivity values are defined.
When I look at the results convective and radiative heat transfers are zero. But the conductive heat flux has some value. I do not understand what is going in the calculation.
Can someone explain how to find these values?
Thank you.
You need to add a fluid flow physics and couple the velocity field to the temperature field.
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Hello Everyone,
I am trying to model heat transfer between two coaxial cylindrical rods.
The outer rod is hollow and having the same inner diameter as of inner rod.
The thermal conductivity and coefficient of thermal expansion of outer rod are higher than inner rod, so there is possibility of gap between two rods at higher temperature (around 1000 deg Celsius)
The possible mode of heat transfer are conduction(when in contact), radiation and convection (when gap generated).
The inner rod is solid and is being heated with joule heating. I want to measure the temperature at the outer surface of outer rod.
Any ideas how to model this in star-ccm+ or in any cfd software? In software I can either model it as in contact or with some air gap. Don't know how to model both in same model.
Thank you
It is too important to determine your model boundary conditions and the heat transfer mode that you will simulate. You can start with a 2D model and you can simulate using the COMSOL program.
Regards
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I am trying to model a diffusion process within a laminar flow. I want to plot Sherwood Number across my domain. What i try to do is expressing Sherwood Number as convective flux magnitude/diffusive flux magnitude all over the domain but the results seem to be wrong when compared to correlations. Can anyone suggest me another method to find Sherwood Number in COMSOL?
This is quite difficult, first of all you need to calculate numerically the mass transfer coefficient which by the way depends on what you're looking at. If you do not want to pass through the formal definition and you want tu use fluxes, first of all check that COMSOL calculated fluxes are in agreement with the expected results. Most of time, mesh dependencies are found and this has to be avoided.
Regards
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I am working on mixed convection in an enclosure with inlet and outlet openings with Lattice Boltzmann method "LBM". in some "Re" my results have a good agreement with the other works. But when For example "Re" is 500, my results is different with the reference article. my question is: what should I do for high "Re" ? should I increase my Lattice or reduce inlet velocity? I did these changes but it didn't change my results as good as should be.
Read this research carefully. I assure you that you will get the right answer.
PhD Thesis (English translation) : "Introducing a new and entropic kinetic model for simulating incompressible viscous flows"
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hi,
I am trying to simulate thrombus (clot) formation under blood flow, I have to solve the convective mass transfer to find the concentration of Red blood cells, by growing the clot the velocity profile will change. I am looking for a way to couple these equations to transfer data between them.
Khalid B. Saleem Thanks for your answer. I tried to do it, but I was not successful. I am new on Anysy Fluent. But, as far as I know, species transport is not valid for blood flow. It is only valid for mixed flows which already has been defined on the fluent database. Am I wrong?
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I am doing a welding simulation and using element birth/death in the following way:
Thermal analysis:-
1) Conductivity matrix, Convective matrix and Heat capacity matrix of kill.ed elements are multiplied by a factor of 10^(-6).
2) Heat input vectors (load vectors) are made zero for kill.ed elements
3) After solving a particular load step, fictitious values of temperature are obtained for the nodes within the kill.ed elemental zone. These fictitious temperature values of kill.ed nodes are made equal to ambient temperature at that load step.
Structural analysis (small deformation theory):-
1) Stiffness matrix of kill.ed elements are multiplied by a factor of 10^(-6).
3) After solving a particular load step, fictitious values of displacement are obtained for the nodes within the kill.ed elemental zone. These fictitious displacement values of kill.ed nodes are made equal to zero at that load step.
4) The fictitious elastic and plastic strains for the nodes in the kill.ed zone are made zero, at that load step.
5) The stresses in the kill.ed zone are made zero at that load step.
6) Apart from the elements in the small gap between plates, the other bulk elements that melt are also kill.ed in that load step, and points 1 to 5 (structural analysis) above are applied on them.
7) Elements are made to born structurally once their temperature starts dropping down from the solidus temperature. (i.e from start of solidification). The incremental stresses in the non-linear simulation are added from this start of solidification.
Can anyone provide their insights on how correct is my above approach?
Can there any alternative way to implement this process?
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The concrete contains phase change material and thermal couples inserted in the concrete at different thicknesses. The objective of the study is to quantify how much energy the samples can store.
I am using the equations developed by M. Pomerantz, B. Pon, H. Akbari, and S.C. Chang "The Effect of Pavements' Temperatures on Air Temperatures in Large Cities" April 2000
Which is similar to Energy storage (W/m2) = radiation + convection - conduction.
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Hi
I have been trying to find the Nusselt number to calculate the convective heat transfer coefficient for nano fluids (SiO2) in PVT applications.
This convective heat transfer coefficient is used as an input data in TRNSYS software for PVT system modelling. Many numerical models are proposed by researchers to find the Nusselt number. However, I am not able to distinguish the most suitable one.
Hi, there are a lot of studies related to this kind of work, try
Numerical and experimental investigation of boiling nanofluids in vapour absorption refrigeration
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Dear all,
I'm confused with the mass flow air's role in Indirect Rotary Dryer.
As far as I know, the air is there to carry evaporated moisture whereas the steam-tube is the one who will dried the product through heat transfer. Is that correct?
If that's the case, since the mass flow air's temperature will rise automatically through convection from the steam tube (which I consider as heat loss to air) then we don't have to calculate additional energy to raise the mass flow air from initial air temperature to the exit temperature do we ?
I only consider:
1. heat to raise products(dry+wet) from initial temperature to the desired temperature as sensible heat
2. latent heat for evaporating the water.
3. Heat loss due to radiation and convection to the surrounding (including heat loss to the said airflow)
In total, those are the heat requirement that must be supplied by the steam through heat transfer (conduction+convection) aren't they?
Regards
I agree with Rani, Nice explanation
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Hello,
I am working on the effect of fluid-structure interaction(FSI) on mixed convection. I am using COMSOL Multiphysics. I followed their examples of FSI. And by the way, I learned from them, I try to validate some previous works on the effect of FSI on mixed convection. I managed to validate a result from previous work but others did not match. I tried more after that but still, my results did not match with them. I am attaching the model that I developed for study purposes.
Thank you
Hi,
Please check your BCs. For expl. The initial value is 0 K and the temperature is 0 K.?! Your parameter table is bad, because, do not have units inside Expression. All BCs in the physics interfaces are automatically wrong too.
Regards
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My understanding is that whenever there is a temperature difference in a fluid, then there will be convection current. But this is not always the case, in a nonpolar fluid, with the IR camera, I only saw ring patterns spread out around the heat source, but no convection current was seen. Could anyone help explain this? Does the convection only happens under a specific conditions? I really appreciate your help.
temperature difference is not the only factor that determines the onset of convection in the flow. Factors like gravity and other effects (e.g., electric and magnetic forces in specific types of flow) need to be considered too. For example, convection can be suppressed under stably stratified flow even there is a presence of temperature difference in the fluids. Also IR imaging has limited resolution, it may not have the sensitivity to capture some fast and small convection effects...
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I want to determine the Nusselt number (Nu=hDh/k) in the heat exchanger. In calculate of heat transfer coefficient (h=q''/(Tw-Tb)), for the q'' parameter, I use the total heat flux of the channel walls that is the contact with the fluid and use the average temperature of these walls for the Tw parameter. In this equation, I need the bulk temperature (Tb). How can I calculate it in Comsol multiphysics?
V. The recursive least squares algorithm (RLS) allows for (real-time) dynamical application of least squares (LS) regression to a time series of time-stamped continuously acquired data points. As with LS, there may be several correlation equations with the corresponding set of dependent (observed) variables. RLS is the recursive application of the well-known LS regression algorithm, so that each new data point is taken in account to modify (correct) a previous estimate of the parameters from some linear (or linearized) correlation thought to model the observed system. For RLS with forgetting factor (RLS-FF), acquired data is weighted according to its age, with increased weight given to the most recent data. No prior 'learning phase' is required.
VI. Application example ― While investigating adaptive control and energetic optimization of aerobic fermenters, I have applied the RLS-FF algorithm to estimate the parameters from the KLa correlation, used to predict the O2 gas-liquid mass-transfer, while giving increased weight to most recent data. Estimates were improved by imposing sinusoidal disturbance to air flow and agitation speed (manipulated variables). The proposed (adaptive) control algorithm compared favourably with PID. The power dissipated by agitation was accessed by a torque meter (pilot plant). Simulations assessed the effect of numerically generated white Gaussian noise (2-sigma truncated) and of first order delay. This investigation was reported at (MSc Thesis):
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It is said that when Reynold's number is much lesser than 1 then convective terms can be removed/neglected. Does this happen in FLUENT also?
Hi,
Actually no.
We have to set Reynolds number
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Dear all, I am using OpenFoam 5.0 and working on heat transfer simulation using chtMultiRegionSimpleFoam solver.
1. I have a temperature data file which equals to the face center length of a surface region. This temp file has "0" temp in some faces and rest with variable temperature. (say out of 1000 temp values, 400 are 0s and rest are varying). This is a Dirichet BC.
2. I have used this temperature data and simulated it.
But now, I need this "zero" temperature value to be changed to convection BC. I know the parameters for convection BC. The convection parameters are as given below.
type externalWallHeatFluxTemperature;
mode coefficient;
Ta constant 22;
h uniform 10;
kappaMethod solidThermo;
value uniform 30;
But, how do I change the zero temperature cells to different boundary condition? Or, how can I create a Patch based on this temp=0 faceCentre cells? 3. I checked with SetFields (FieldToCell), but I can only give scalar value there and not BC.
I have attached two images of the images, one from top and isolated view. The blue color shows the 0 temperature where BC need to be changed.
I hope the question is well understood. Any leads will be appreciated.
Regards,
Sunag R A.
Hi Sunag,
If I understand clearly, you want to define two different boundary conditons on one single patch. So, is it possible for you to split the current patch into two different patches, where you can easily define their own boundary condition? We usually put the regions with same boundary conditions into one patch.
Best,
Changchang
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Hello all,
I am a research scholar in the field of atmospheric science. I am willing to discuss with you about convection.
Convection is an important process in reality and in the weather-climate models. There are commonly two ways of treating convection: quasi-equilibrium (QE) convection and non-QE (e.g., triggered) convection. In the QE convection category, the convection may be considered to be in a state of statistical equilibrium with the large-scale convection. While for triggered-like non-QE convection, high-frequency (> 0.5 CPD), small-scale waves will interact with the convection, in which QE does not work anymore.
Can anyone kindly explain more for me comprehensively the difference between QE and non-QE convection?
I appreciate any answers from you!
Thanks,
Yuntao Wei
Correction:
Sorry, it should read: "In the QE convection category, the convection may be considered to be in a state of statistical equilibrium with the large-scale circulation."
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Hello
I am running a simulation on Abaqus based on a python script for a simple thermal model. The model is a shell box with nine surfaces total, based on partitions made. The density and conductivity are assigned. Boundary condition is a surface film condition for convection with sink temperature of 20ºC. Thermal load is a surface heat flux assigned to be from inside to outside as shown of image boxLoads.PNG, and the problem is that i was expecting a heat flux to be more distributed along the faces of the box instead of being concentrated only on the borders of the face shown on boxResultsHFL.PNG, so maybe i am doing something wrong on the python script.
The objective is to define a simulation that accounts for a heat flux from inside the box to the outside of the box, and it is supposed to be along all surfaces to study the effect of thickness in heat flux. Can someone help me finding the error behind this strange results focused on just one face?
The code in python as well as figures that show my pronblem are attatched here.
Nuno
Hi Nuno,
Here are a few notes on the model you have developed.
1. The model is made of shell elements and the elements have 5 integration points through their thickness.
2. Shell elements have inside and outside surfaces (in Abaqus these are often called SNEG and SPOS).
3. The surfaces of a shell element are defined in terms of the element normal. The SPOS side of the shell is on the side of the shell in the positive direction of the element normal.
4. All of the element normals on your box point outwards, so SPOS is on the outside of the box.
5. The heat flux is applied to the same "side" of the element as the film. For example, where the flux is applied to the side SPOS, the film is also applied to SPOS. (There is only one case where the film and flux are applied to the SNEG side.)
6. You are performing a steady state analysis. This means that the solution will only determine the temperature distribution equivalent to a long time.
7. I think that the steady state temperature distribution when the flux and film are on the same side is equal to the ambient because the flux just goes straight in and then leaves immediately without heating the box.
Therefore, I suggest that you should have the film on one face and the flux on the other face. Also, I think all fluxes should be one the same side (e.g. SPOS).
I hope this helps.
Simon
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I am trying to solve the transient heat conduction problem using Newton raphson method and backward Euler time integration. Please help me know how to use them together to solve non-linear transient heat conduction problem.
My problem is non-linear due to temperature dependent properties like thermal conductivity, specific heat and density.
I am using 8-noded brick element.
Applied load is heat flux on one surface.
Boundary condition is surface convection on other surfaces.
Initial temperature is known.
There is no need for the Newton-Raphson method in the problem you describe. There is also no point using backward Euler. You can easily solve this using the forward (or conventional) Euler method. As Akhan says, this is a partial differential equation. Since you mention a "brick" element, I presume you want a 3D solution. This is easily accomplished with finite differences. You could even do it in an Excel spreadsheet. The nodes on one face are controlled by the heat flux condition. The other exterior nodes are controlled by the convection condition. The interior nodes are calculated from Laplace's equation, which reduces to the average of the surrounding nodes. Set all the nodes to some initial temperature and then step through time. If solution becomes unstable, cut the time step in half. I have attached a spreadsheet with a 1D and 2D steady-state example that crunches until it settles out using Excel's internal iterative solution feature. 3D is a little more complicated, but not much. I also included a Crank-Nicholson example, but it's hardly necessary for your problem.
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Hi,
I'm trying to simulate a heat loss problem from a tank using ANSYS FLUENT. I want to study the transient temperature trends and thermal stratification in the tank over a period of 5 days. Heat loss is just meant to be by natural convection, so i'm also using the boussinesq model. What is the quickest way to approach this problem? I currently using a time step of 0.1 seconds and it takes a really long time for a single time step to converge. If i'm to continue at this rate, it would problably take several months just to simulate 5 days. Accuracy is not of so much importance for now. I just need a rough estimate. Any ideas?
Thanks
sir, I have also a problem with heating water in a tube, in the transient thermal module of Ansys. the problem is that when I fine the mesh the results are not converging, and at coarse mesh the results comes quite good
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Dear all,
I am writing an Iron Python script on Ansys Mechanical for a thermal problem.
When a convection load is created (A), in order to define the Film coefficient (B), I need to perform a click on the dropdown list and then, I can select the Tabular option (C). (See attached figure).
However, if I want to automatize this task, I can not do it manually. Therefore, my question is how to define a film coefficient table by command line? (i.e. to perform task (C) ).
Any suggestion will be appreciated.
Best regards,
Ramiro M.
Ps: After reading the Ansys ACT manual, they say:
Field Convection.FilmCoefficient { get; }
Gets the FilmCoefficient.
Remarks
This property cannot be set. When one wants to change its value, one should operate on the returned Field object, either on its inputs or on its output variables .
Following
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Hello together,
first of all, i have a question about breaking the cell walls of bacteria. in my case, i am treating rhodoccus bacteria and i decided to use the sonication method, but after testing with the flowcytometer, it was unsuccessful. does anyone know of a preliminary method to break the cell walls or can recommend another method?
secondly, i wanted to determine biomass and would have used the convection oven method, but i don't know how best to do this. does anyone have a tip or another method to suggest?
Il faut faire dosage
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I am trying to solve the problem with dimensionless numbers. But while I tried, I failed to find all the parameters from the papers. How can I include the nanofluid properties in the simulation? I am using Comsol 5.5.
Thank you, Oras Khudhayer Obayes. This was really helpful.
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I have a cylindrical geometry as shown in image. The gas is entering through inlet at a known mass flow rate and pressure at room temperature. The inside of the cylinder is at a high temperature. The gas is entering the chamber, getting heated and exiting through outlet. How to calculate the pressure inside the cylinder wall? As for convection heat transfer between the cylinder wall surface and heated gas how to calculate the convection heat coefficient?
And what happens to the gas flow behavior (temperature, velocity, pressure) when it enters from small cross section (room temperature) area to a bigger cross section area (higher temperature)?
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hello,
I hope that you are doing well and that the beginning of March is excellent. First of all, I would like to thank the researchers who responded to my last discussion on terrestrial albedo.
Today's discussion is about a problem I found in the numerical modeling of the convection phenomenon (heat transfer)
the goal is to dimension the prototype to be able to dimension it, so I used the finite difference method to discretize the equations of order 1 (I discretized time by a forward scheme and space by a backward scheme).
Despite this and after I wrote a program on python to be able to extract the results, I found values that are not logical for example the water temperature by heating with a radiation value of the order 500 W/m^2 -10 °C.
someone will give me an idea on what I should do or change ... ? the scheme? .........
cordially
Firyal LATRACHE
I wrote a lot of such programs. Usually such a problem comes from incorrect derivation of equations or some small typo in the script.
Filippo Maria Denaro idea is very good, start with something very simple, e.g. 1D time dependent problem with analytical solution - compare your results.
Such problem can be caused also by the stability of the schema. Have you set a stability condition of the solution (range for dx and dt)?
You can look at this article, just assume that eps or u is equal to zero:
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In ansys fluent when we apply the temperature at boundary condition it gets fixed to it and even after convection cooling it remains the same. I need to assign 293K for block placed in a room and get it cooled by convection. I tried to patch the block zone with initial temperature but it didn't worked. I have attached the file below.
Navid Zehtabiyan-Rezaie I used that too. But in that case blocks initial temperature was considered as 2C by the system. How can I initiate it with 20C. I tried patch too but it didnt worked
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I'm currently designing a single pass shell and tube heat exchanger. In the Nusselt number equation I have chosen to use, it requires me to calculate the Prandtl number at the SURFACE of the tube, shell side. I'm trying to find what the temperature at the surface of the tube will be. I've tried calculating this using constant heat flux however I do not know what the convective heat transfer coefficient is yet so I cannot use this method. Any suggestions?
From Newton's laws of Convectional Cooling
QS = h(Ts-Ta)
Where Qs = Heat loss by convection to Ambient
h = heat transfer coefficient
Ts = Temperature of the Surface
Ta = Temperature of the Ambient
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I know many textbooks include empirical relations between Nu, Re_x, and Pr inside a turbulent boundary layer on a flat wall.
However, I want to directly calculate h from known boundary layer information, e.g. velocity, pressure, turbulence intensity, and else.
Is there any textbook or paper I can refer to?
Sincerely, DH.
As mentioned by M.M. Hathal, for fast reasonable ansers you can use an integral method. Basic is discussed in the excellent book of A. Bejan "Heat Transfer". For more advanced models you should use a numerical model...If you do such calculations, do compare your results with simple empirical scaling laws or results of integral methods. At least: DO PRESENT YOUR RESULTS IN DIMENSIONLESS FORM! You will avoid dramatic errors and limit calculations to a minimum if you are interested in the influence of the variations of specific parameters. Good luck. Mico
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Hi All,
I am modeling a heat transfer problem using Modelica tool. I am not able to assign user defined fluid properties. Also not sure how to take care of conduction effect and material properties of a tube. Please suggest some solution.
Do the models in Modelica.Fluid help you? What kind of fluid do you want to model? A lot of medium models can be found in Modelica.Media, some can be simulated using ExternalMedia + CoolProp.
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Does anyone know any article on free convection heat transfer from an oblate spheroid with uniform heat flux? All articles I've found are about isothermal oblates.
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Hello, I have a wall which is affected by a heat flux and it is cooled by convection..How can we do this in FLUENT?
Thanks
Hello dear
Orauno Rejoice Oluchukwu
, you are welcome. Okay, I showed the detailed procedure in my tutorial. We cannot use the heat flux + convection at the same time. We simulate the solar rays using the solar ray tracing.
To understand more about your case, send me the details with some snapshots and documents by e-mail:
Danke!
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Hello Everyone,
I am trying to apply the convective heat flux i.e., -n.q= h(T_ext-T) where "q"=-k ∇ T
So, If my geometry is selcted as "Form Union(fin)", I cannot select the boundary as shown in the attached figure.
But when I select the geomtery as "Form Assembly(fin)", I can select the corresponding boundary but then gaussian heat source couldn't apply on it.
Regards,
Akash Meena
Hi, Akash Meena
have you solved this question? I met the same problem.
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Hello, the software I use is ANSYS Fluent. The subject of my research is the analysis of temperature distribution in the oven. At present, I have a very simple model to test, just how to try the heat of the heater to pass the dance. Or problems such as inability to converge during calculations, floating-point changes, etc. are also repeated.
1.
In the current model, the solid part has a heater, box, and door; and the fluid part has internal air, external air, and a simple cylindrical fan. There is a 0.5mm gap between the door and the box so that the internal air can be transmitted to the external air.
2.
In the cell zone condition, there is an energy parameter of 3570000 (W/m3) for setting the heater.
3.
In the boundary conditions, the six sides of the outside air, the side where the inside air connects to the outside air, and the fan exhaust are set as pressure-out. The fan inlet is set to velocity-intel, and the parameter is 1m/s. The heater, cabinet, and door surface are all set to convection, and the value of heat transfer coefficient is only 500W/m2-K for the heater, and 25W/m2-K for other parameters.
4.
In solution methods, after changing the gradient mode to green-gauss cell-based, numerical calculations are performed.
The above is a rough setting. I wonder if there are any more experienced or professional people who can help me with the problem?
When you are modelling something complex, it is always good to take an incremental approach as opposed to setting up everything in one and then get stuck when it doesn't solve.
So in this case, try building up the model piece-by-piece. First include only the air inside the oven. Does it run? If so, include the walls with heat transfer, then include the heat generation, then the outside air, then the connection between inside and outside, etc. Every time you modify the model, run it, debug as required, evaluate the results and only go forward when you are happy with the state of the model so far.
Otherwise it is very difficult to tell what is the exact issue. It can be something trivial, like a few poor quality elements, non-physical boundary conditions, mistyped material properties, and so on, but you wouldn't know. If you develop the model step-by-step, when it stops solving you will know that it is very likely due to the latest change you made to the model.
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The time step constraint is not sufficient to guarantee the stability of numerical scheme in any case. This constraint is only for the convection terms. Additional constraint is required for the source term. Can you please help me that what are those additional constraints if source term?
If by 'stability constraint' it is meant 'restriction on the Courant number', clearly this only refers to discretized convection (I prefer to say 'advection') equations. Any other term that you have in your (spatially) discretized problem may entail other, possibly more restrictive, stability conditions. A linear source term like
the right hand side of
y'=-k*y
entails a stability restriction of the kind | k* h| < constant where h is the time step
and the constant depends on the time discretization method (constant =2 for explicit Euler). A source term that does not depend on the solution, like the right hand side of
y'=f(t)
where t is time interpreted as the independent variable strictly speaking does not entail a stability constraint, since the discretization of this equation is equivalent to the computation of a definite integral of f. However, if your time step h is such that the behaviour of f is not properly sampled (imagine for example f(t)=cos(omega*t) with omega = O(10) and h=O(1) ) then your discretization will never be giving you accurate results and h must be reduced to sample f properly.
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Temperature gradient in well is sometimes estimated by either a temperature survey or running wireline formation tools. Meanwhile, well is generally full of a fluid whereby fluid movement is governed by either force convection in the flowing condition or free convection in the shut-in condition. Both mechanisms influence on the estimated temperature gradient for some extends. Since estimating reservoir temperature at datum depth needs a temperature gradient (or geothermal gradient), I seek any advice that can shed a light on the most reliable method for estimating the temperature gradient.
In fluid mechanics, temperature gradient is generally related to the category of equilibrium thermodynamics or nonlocal thermodynamics. To the best of my knowledge, if you have a fluid flow with a small Knudsen number, you can consider it as a continuum, and you can refer to Navier-Stokes equations to predict the flow regime. Here, you can introduce an efficient slip boundary condition to describe the temperature, pressure and velocity gradient.
In computational fluid dynamics (CFD), both COMSOL multiphysics and ANSYS can simulate the heat transfer behavior based on finite-element method.
Best Regards,
Houssem
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What is penetrative convection? Also please give some examples if possible.
Penetrative convection is a type of natural convection which is induced in a thermally stratified layer of fluid due to thermo-convective instability.
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In general stove heat moves from the bottom to top in even manner. But in case of micro oven most of the times top layer only we get more heat compare to bottom. What could be the possible reason.
Yes Ali Lateef the basic mechanism of heat transfer is radiation but also there is a small ratio of convection
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I have a wet object to dry by convection. The "object" can be treated as a porous medium or semi-solid. Do you have any idea about solving it in Fluent. This is a conjugate heat and mass transfer problem. Air will hit the object and object will lose moisture by convection. I am wondering how long it takes for an object to dry. Thanks
Thanks Ahmad, Thomas, Christiano and Yagnit. Yes, Fluent can solve conjugate heat transfer problem. The problem is to add mass transfer analysis. I was thinking about using a UDF. But, I did not know how and where. As Christiano said, the problem is finding the mass transfer "h" coefficient around the object over time. Still struggling!
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I am working on a problem in which a material sample is shredded to a fine particle size, and i am working on heat transfer equations for this bulk material. I was thinking that once the material is shredded, it will be able to conduct heat more easily, and thus the shredded material would have a higher thermal conductivity than the unshredded material. I think this because in my experience with starting camp fires, you have to use small tinder and kindling in order to start the fire, rather than starting with a large log. It also may be possible the thermal conductivity stays constant, however the smaller particle size allows for higher rates of convective heat transfer. Any info on this topic or relevant studies would be much appreciated!
In your case, the new bulk material will have different conductivity from the original because there will be air and contact resistance between the shredded pieces. However, more details about your cases are needed to better assess the heat conduction.
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If answer is yes, how? Can we analyse the transition from laminar to turbulent mixed convection in this commercial software? Else, can you suggest any other commercially available software suitable for it.
Following
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Hey,
Here are some resources for modelling of convective heat transfer.Tysm all for this
There are many excellent texts on fluid mechanics, computational and analytical. Please learn the science (theory) before the calculations (software). Don't just run some case with Fluent and think that you understand the subject matter—even if some university will let you get away with this easy path. While it may seem tough now, expectations for graduate students have plummeted over the past half-century. You must climb the mountain in order to see the view. Trust me, it's worth the extra time and effort.
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The Ludwig-Soret model is used to describe thermal diffusion (separation of mixtures in the presence of a temperature gradient). Meanwhile, buoyancy models are used to describe double diffusive free convection. This led to the conclusion that the question needs revision.
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Hello ! how we can prevent convection current in liquid? Are there any ways to prevent the convection current in liquid or not??
You can reduce by increasing the liquid volume and container surface area as this will increase the characteristic length (would recommend container rectangular geometry than circular geometry). if it is possible to put foam in your liquid, this will reduce the convection by half as this will significantly reduce the liquid heat currents.
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Hello everyone, please suggest some good journal for the convection in lid -driven porous cavity.
I am not emphasizing the term! The person who asked the question introduced the term to this discussion. It certainly shouldn't be called a porous lid-driven cavity because that suggests that the lid is porous. No, it is a porous cavity where the flow is created by a moving lid, so the lid drives the flow. So it might be best to say that it is a lid-driven flow in a porous cavity. This now makes sense.
The existence of a large number of papers which deal with flows in lid-driven porous cavities does not necessarily mean that it is a physically realistic to set up in practice. Academic research is not democratic, specifically that something is deemed to be correct because a large number of people say that it is. It is my belief at present that if a porous medium has a low porosity and Darcy's law applies, then the presence of a moving lid does nothing. No doubt such a lid will have an effect if the porous medium is extremely sparse, and therefore there will some "critical" porosity when the effect of the lid is confined to a very thin momentum boundary layer but its presence is not felt in the rest of the cavity.
There are, of course, some very serious mathematical singularities at the corners of such cavities where the mathematical model has discontinuous boundary conditions. An experimental model will possibly have leaks or frictional heating from such moving contacts.
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Hi everyone,
I'm right now trying to simulate a double piped heat exchanger in star ccm+. I've got everything set up and ran a few times. However, it has been giving me weird results. I think I got boundary conditions on interfaces wrong. Can someone help me with this? For this DPHE, there are 4 interfaces which are water/inner tube inside surface, air/ inner tube outside surface, air/turbulator, tubulator/inner tube outside surface and air/outer tube inside surface. For fluid/solid interfaces I set the thermal condition to be convection and used fluid temperature as ambient temp. For solid/solid interface which is the turbulator/inner tube outside surface, I set it to be adiabatic but I know this is wrong because there should be conduction between turbulator and the tube. So How do I set conduction for this interface? Also, the fluid temperature is going to change during the simulation, which means the ambient temperature is changing for all convection interfaces. Is the ambient I set for convection giong to change itself during the simulation? Much appreciated for any help.
Hi do you mean the dynamic and kinetic conditions?
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I want to calculate natural convection in the horizontal rectangular enclosure (heated from below) and in the vertical rectangular enclosure by using the Nusselt correlation. However, I can not find the Nusselt correlation for the high aspect ratio and high Rayleigh number. Does anyone of you know these correlations and if yes, can you please share the references with me. Thank you.
Best Regards.
You can find the following articles useful
International Journal of Heat and Mass Transfer
Volume 139, August 2019, Pages 121-129📷
Experimental investigation on very-high-Rayleigh-number thermal convection in tilted rectangular enclosures
You can change the tilting angle to get vertical or horizontal enclosure
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If you think you have some amazing pictures of clouds, please share here. Don't forget to mention the location, time and date, and picture credits (if picture is taken by someone else). Would be helpful if you can also add the cloud type.
Note: This activity is only for the discussion. Not for any commercial or research activities.
Cheers,
Shipra
Hi Shipra,
Your observation is correct, this is indeed the example of mammatus clouds.
When we talk about interesting cloud shapes, we most often think about the forms visible from the surface. Recently, I realized that after expanding the scale of observations by considering satellite images, we can observe even more astonishing forms such as the Actinoform clouds, of the coast of Australia, January 29, 2020, from NASA which are shown in the attached image.
These cloud forms are too large tom be seen from the surface.
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Dear all:
I need help, I hope there is someone out there who can help me.
I finish drawing the 2D geometry of an Air Solar Collector with an undulated absorber plate (in contrast with a flat plate absorber). This is a ~~~~~ shape plate to capture the sun radiation.
- Single Phase FLuid Flow - Laminar Flow
- Heat Transfer in Solids
- Heat Transfer in Fluids
But when I run the simulation, even though I have the Heat Transfer in Solids and the Heat Transfer in Fluids modules included in my model, the Results doesn't show whatsoever the Graphs for the Temperature Distribution, or contrours. Also the Pressure Results Graph doesn't shows anything
I'am attaching two images: on of my model's geometry, and one with the solution of the Velocities distribution, but this is as much as I can arrive to.
I'm sure I am doing something wrong with the Boundary Conditions I'm entering.
These are all the Boundary Conditions I'm trying to use :
i. Constant Radiation Heat Flux at the top of the glass cover.
ii. Constant Convection Heat Flux as Losses from the top of the glass cover to the ambient.
iii. Tried to establish a Constant Conduction Heat Flux from the bottom of the collector to the surroundings (to represent the Heat Conduction Losses from the bottom of the Collector to the roof and environment).
iv. Isothermal Walls at both of the sides of the air cavity.
v. The No-slip Boundary Condition (u=v=0) at the inside of all the walls of the air cavity.
vi. An entering Air Mass Flow from the left to the right of the bottom air cavity.
vii. I wanted to establish also and exiting Mass Flow at the right side of the bottom cavity, but couldn't. So instead, I established this Condition as a Pressure at the Outlet, as the Boundary Condition.
So I'm pretty sure I am doing something wrong with one or several of the BC.
I hope someon can give me some help me. I'll appreciate it
Thank You !
Useful information can be found in the attached files.
Regards,
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Greetings!
Is it possible to find the amount of Heat Flux leaving from a surface in a 3D steady-state heat transfer analysis in ABAQUS (v6.14 standard Explicit).
I'm doing a 3D steady state heat transfer analysis on sintering process and i want to know the heat loss from each surface due to convection.
Module: "Step"
"Output" tab -> "Integrated Output Sections" -> "Create" (Alt+o+i+c)
Select the surface of interest
"Output" tab -> "History Output Requests" -> "Create" (Alt+o+h+c)
Domain: "Integrated Output Section" : <your_section>
Output variables: check "SOH" or "SOD" for heat transfer or mass diffusion, respectively
*Submit job*
Module: "Visualization"
"Result" tab -> "History Output" (Alt+r+h)
Select <your_section> -> "Save As" -> <xy_data_name>. You can also plot it.
To export to excel:
"Plug-ins" (Alt+g) -> "Tools" -> "Excel Utilities"
Select <xy_data_name> -> OK
It should open up in Excel.
(Using Abaqus/CAE Student Edition 2019)
Note: for the Alt commands to work, my experience is that you have to hold the alt key, then press the subsequent keys sequentially without letting go of alt.
This video (not mine) was also useful for me getting started up (I was doing mass diffusion):
Abaqus Tutorial Videos - How to Perform Steady State Heat transfer analysis of a plate, by TrendingMechVideos
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I have used finite difference method to discretise the equation and have written a code for it on mathematica. Anyone knows how to write a code for the truncation error that is the order of accuracy? I am not very familiar with mathematica.
All essentiall elements required to perform a numerical analysis of the finite difference schemes are discussed In the following sites:
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Dear all :
I'm trying to solve the following problem using COMSOL Multyphysics.
I have an enclosed geometry (please see the attached images) with a top flat surface (the celing) and an undulated shaped bottom surface (the floor). And I have Air as a medium enclosed inside this cavity.
So, I want to solve for fluid Velocities (u and v) distribution of the air inside the cavity, and for its Temperature distribution too.
So, I already included the physics modules: Laminar Flow (for single-Phase Fluid Flow) and Heat Transfer in Fluids.
However, I'm sure I'm doing something wrong witht he Boundary Conditions (it can be observen from the first attached image, that I don't get any velocities distribution for the air mass).
The BC I am trying to stablish are the following :
i. Isothermal Walls in all the interior walls of the cavity.
ii. No-slip Boundary Condition (u=v=0) in all the walls (on the inside) of the cavity.
iii. A constant Temperature at the upper flat surface
iv. A constant Temperature at the bottom (wavy floor) surface of the cavity.
Where the problem could be at ? The Velocity Stream Lines and Temperatures Contours should show a kind of oval spiral pattern at the top of each of the 'valleys' of the wavy bottom surface. But as depicted in the image, I don't get any valocities distribution.
I will really appreciate if someone can give some help.
Thank you all !
Best Regards !
Thank you very much for your help, I already adjusted the "Pressure Point Constraint" and the reference value for the Pressure, and nothing.
I'll send you my model on Inbox, I'm using the Comsol's 5.1. ver
Thank you again , hopefully I was close to the answer, with the model, I'm sure you will be able to tell me what my problem was.
Regards !
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Hello,
I am working on natural convection and radiation heat transfer problem.
Since my domain contains solid objects with finite thickness surrounded by fluid, I have to solve conduction in the solid and convection in fluid. At the interface between the solid-fluid, 'coupled' boundary condition is used.
For radiation, I am using S2S model. From Fluent theory guide, I understood that radiation is modeled as a source term in Energy equation.
I have the following queries in this context:
1. Whether the source term to account radiation is applied in the energy equation for the fluid region only? Or is it applied in the solid region also? Or a single energy equation is solved for solid and fluid domains?
2. At the solid-fluid interface, how the 'coupled wall' boundary condition is treated? Is it the heat flux balance between conduction, convection and radiation?
At the interface, is it correct to write:
and
(ii) Tsolid=Tfluid
Please have a eye on this.
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How to study convection in fluid flow through composite porous media
An example of such a phenomenon is the flow of species (both gas phase and liquid phase) through GDL (Gas Diffusion Layer) in PEM fuel cells. You can find corresponding governing equations in simulation/modeling papers in this record.
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I search for information about thunderstorm occurrences in a region of Romania. Can I use a parameter like "convective precipitation accumulation" from ESRL or Era -Interim?
You are welcome, dear Istrate Vasilică.
Regards,
Vaid, B. H.
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Hi,
I am trying to model a laser melting simulation, where a solid is being heated by a laser source (refereed as a Gaussian heat source). For that, I took a domain (Grey color in the attached image) set it as Air (to incorporate convection effects) and then I patched a domain (Blue region in the attached image) with solid properties. But the issue is, I am not able to set any boundary condition to the patched area.
Any suggestion / answers how can I do that.
as Akter Hossain already mentioned, you will need a UDF for the heat source, which needs to be compiled and hooked and then taken as a BC under Boundary Conditions -> Type Wall -> Thermal -> Heat Flux -> udf_profile.
However, I have a remark on your numerical setup. You do not really need to have air in order to account for the convective cooling. You can simply define it by setting a Heat Transfer Coefficient under Boundary Conditions -> Type Wall -> Thermal -> Convection -> Heat Transfer Coefficient. So you will also not need to patch any material properties to the solid domain since this will be the only domain to consider. I hope this helps ;)!
Best regards,
Antoni Artinov
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I am working in forced convection flow using nanofluid in euler euler model but my problem is not converging. Can any one help me out regarding the same.
Divergence can be due to different issues. If I were you I begin by giving some details about your case study in order to help you.
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I am working on a problem
It depends on Reynolds number
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mixed convection means natural and forced
The equation can be extracted from the excel program
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I am trying to perform an energy balance on surface 1 as shown in the attached figure. The main difference is that I am also trying to account for an airflow through the duct. Could someone give me some guidance as to how to account for all the radiation, convection and conduction terms?
Franklin Uriel Parás Hernández I am building a solar collector similar to the one in the example, I want to be able to determine the heat gain of the air as it leaves the collector and also the temperature of the surface T1.
I am assuming steady state but I feel like I am missing some terms as the outlet temperature is coming out greater than T1, which shouldn't be the case.
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Hi guys,
I am facing some problem in the thermal transient analysis of forced convection.
There is a flat plate (Material: Aluminium) with a heating element(2000W power, Material: Nichrome) attached to it at one end of plate. I have given both convection and radiation. Convection coefficient is a variable which changes with temperature.. I have imported these values in a tabular format. In the analysis settings I have used 6 steps of time (600, 1200, 1800, 2400, 3000, 3600).
Problem:
• The temperature distribution of the plate is varying till the 600s mark, reaching a max temperature of 625 C, and then after that it just remains constant for the remainder of the time.
• Moreover the temperature of plate at the far end of heating element is just 35 C (Plate is 40cm in length). With 2000W of power and 3600s of time, I don't think the temperature distribution of the plate is accurate since aluminium is a good conductor of heat (referrence picture is attached).
Can someone explain me why this problem is occurring.. Is it because of some fault in my simulation? Any help would be much appreciated.
Ruben Borrajo The heating element covers only initial part of the plate as shown in the image (the golden yellow part)
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I want to model the Rayleigh Benard convection using Lattice Boltzmann method. Relevant papers will be helpful to me.
At www.openlb.net you will find several examples with the open source code.
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To understand the the mechanism of the dynamic interfacial tension (DIFT), people always consider the diffusion process. For example, The Ward and Tordai equation is a well-known model to fit the DIFT data measured by the maximum bubble pressure method. But I am curious about whether people have considered the convection process as well? I searched for a while but couldn't find any literature discussing on it. By talking about the "convection", I mean, for example, when we do the maximum bubble pressure test, the bubble will be pushed out of a capillary tube, and this process pushes the solution around the bubble as well, and thus the convective flow occurs. In this case, won't the surfactant been carried by the convective flow as well? In this case, the drifting of the surfactant in the solution should be governed by the diffusion and the convection together. Similarly, the convective flow may also occur in the pendant drop, du nouy ring or any other surface tension measurement methods.
I wonder why people don't care about the convection? Or it's indeed negligible?
Thanks a lot in advance for any discussion!