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Computational Study on the Design for Fluid Flow in High Angular Velocity Screw Pumps
Abstract
In this study, a multiphase CFD method is used to analyse fluid flow in a screw pump which rotates at very high angular velocity under cavitating conditions. This model utilizes a homogeneous phase approach, based on volume-scalar-equations and a truncated Rayleigh-Plesset equation for bubble dynamics. The model is implemented in the CFD software CFX. Three variants of screw pumps with different combinations of plain and threaded shrouds are studied for their Net Positive Suction Head (NPSH) required and compared. The three variants are studied under similar conditions, and the pump with maximum available NPSH is found out.
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A new multi-phase model for low speed gas/liquid mixtures is presented; it does not require ad-hoc closure models for the variation of mixture density with pressure and yields thermodynamically correct acoustic propagation for multi-phase mixtures. The solution procedure has an interface-capturing scheme that incorporates an additional scalar transport equation for the gas void fraction. Cavitation is modeled via a finite rate source term that initiates phase change when liquid pressure drops below its saturation value. The numerical procedure has been implemented within a multi-element unstructured framework CRUNCH that permits the grid to be locally refined in the interface region. The solution technique incorporates a parallel, domain decomposition strategy for efficient 3D computations. Detailed results are presented for sheet cavitation over a cylindrical head form and a NACA 66 hydrofoil.
How fitting clearance between screw and sleeve of labyrinth screw pump(LSP) effects the pump performance and what law it follows are still unsolved problems for LSP design. So the fluid quasi-steady flow in triangular thread LSP with different clearances is approximately modelled by the shear-stress transport k-ω model (SST k-ω model) in multiple reference frame (MRF) of CFD, and the pump characteristics experiments are also carried out. In analysis, fluid flow is simplified as a linear superposition of Couette flow dragged by rotor screw and pressure flow driven by differential pressure of LSP. The CFD simulated results reveal that the clearance has scarcely effect on drag flow, but the average axial pressure flow velocity is the maximal in clearance and it increases with increase of clearance at the same differential pressure; Leakage mainly occurring between the lands of screw and sleeve can well be estimated by leakage formula for a narrow slot with coefficient ζ of 1.62. The CFD simulated lift-capacity curve of triangular thread LSP becomes more flat with clearance increase, which is confirmed by the pump performance experiments. The experimental results also indicate that with increase of clearance, the pump efficiency obviously decreases especially at the vicinity of the highest efficiency point while the power decreases a little in a whole range of capacity. Finally, the paper presented a set of simple and tidy conversion expressions to predict the capacity, head and efficiency changing with clearance for triangular thread LSP, which has made some applications in engineering.
In the present study a numerical model of 3D cavitating
flows is proposed. It is applied to investigate the behavior of a
spatial turbopump inducer in noncavitating and cavitating
conditions. Experimental and numerical results concerning inducer
characteristics and performance breakdown are compared at
different flow conditions. The cavitation development and the
spatial distribution of vapor structures within the inducer are
also analyzed. The results show the ability of the code to
simulate the quasi-steady cavitating behavior of such a complex
geometry. Discrepancies concerning the breakdown prediction are
also discussed.
A robust CFD model is described, suitable for general three-dimensional flows with extensive cavitation at large density ratios. The model utilizes a multiphase approach, based on volume-scalar-equations, a truncated Rayleigh-Plesset equation for bubble dynamics, and specific numerical modifications (in a finite-volume solution approach) to promote robust solutions when cavitation is present. The model
is implemented in the CFD software CFX TASCflow 2.12. The validation of the model was done on an inducer designed and tested at LEMFI. First, The physical model and
the numerical aspects are described. Then, the experimental and numerical methodologies, at cavitating regime, are presented. Finally, for several flow rates, the comparisons between experimental and simulated results on the overall performances,
head drop and cavitation figures, are discussed. For a range of flow rates, good agreement between experiment and prediction was found.
Fluid flow in a screw pump which rotates at very high angular velocity is numerically analyzed. In the present study, fluid flow in screw pumps under high Reynolds number, of the order of 10⁵, is considered. Screw pump has two major elements, a plain shroud which is a stationary element and a rotating hub with helical grooves contained within the shroud. In this paper, three variants of hubs with different number of thread starts numbering six, eight and twelve in combination with a plain shroud is studied. Each of the three possible combinations are analyzed on the basis of pressure rise developed, efficiency and shaft power. It was seen that pressure rise, efficiency and shaft power increases as the number of threads increases in the range of mass flow rates studied.
A new screw pump/extruder is analyzed with regard to the effect of the pump components on the pump output. The pump is constructed such that the barrel, screw core, and screw helix can all be rotated independently. Contrary to conventional wisdom, it is demonstrated that the major physical component of the pump which contributes to the output is the screw helix. A new theory is developed for pumping characteristics of this new pump and the theoretical analysis is compared with laboratory data. The theory compares quite well with the data.
By using Prandtl's mixing length theory to model two-dimensional Reynolds stress equations, the pumping performance of a labyrinth screw pump (LSP) is studied and several key parameters are empirically determined. As a result, two innovative concepts, a cell head coefficient K f and a pump total head coefficient K b, are proposed. A simple empirical equation quantifying the effects of the main geometric parameters of the threads on the pump performance is obtained and compared with Golubiev's experimental results (1965, "Studies on Seal for Rotating Shafts of High- Pressure Pumps," Wear, 8, pp. 270-288; 1981, Labyrinth-Screw Pumps and Seals for Corrosive Media, 2nd ed., Mashinostroenie, Moscow, pp. 34-49). Both theoretical study and Golubiev's results indicate that with an increase in screw lead, K f increases while K b decreases. K f is inversely proportional to power of screw-sleeve relative diametrical clearance, and the power exponent varies with different shapes of thread. Finally, K f decreases with an increase in the relative depth of the thread groove over a wide range. Furthermore, some empirical relations between K f and screw lead, the screw-sleeve relative diametrical clearance and the relative depth of thread groove are fitted, respectively, based on the derived relation between K f and thread geometric parameters and Golubiev's experimental data, which would provide a theoretical basis for LSP design.
A nonlinear numerical model has been developed to assess nonequilibrium effects in cavitating flows. The numerical implementation involves a two-phase treatment with the use of a pseudo-density which varies between the liquid and gas/vapor extremes. A new constitutive equation for the pseudo-density is derived based on the bubble response described by a modified form of the Rayleigh-Plesset equation. Use of this constitutive equation in a numerical procedure permits the assessment of nonequilibrium effects. This scheme provides a quantitative description of scaling effects in cavitated flows. With minimal modifications, the model can also be used for bubbly two-phase flows.
The purpose of this work is to advance an alternative analytical solution of the pure drag flow in single screw extruders which is also applicable to intermediate values of the screw channel cross-section aspect ratio. The model is based on that of Li and Hsieh [Li, Y., Hsieh, F., 1996. Modeling of flow in a single screw extruder. Journal of Food Engineering 17, 353–375] for the isothermal flow of a Newtonian fluid in a small curvature screw channel where the motion of the screw flights are taken into account in the boundary conditions. The resulting boundary value problem was solved analytically via the generalized integral transform technique (GITT). The model was validated against other models available in the open literature. The effect of parameters related with the screw geometry, i.e., the aspect ratio, the curvature ratio and the helix ratio, on the down channel flow rate is also explored in the manuscript.
A discussion of the stability of the spherical shape of a bubble is given in view of the familiar Rayleigh-Taylor instability phenomenon. In the collapse of cavitation bubbles the effects of compressibility are briefly reviewed, and the effects of adjacent solid boundaries on the collapse are shown. The relevance of these analyses for cavitation damage is also considered. Experimental observations of the variation of cavitation damage with the temperature of the cavitating liquid are described briefly, and a physical interpretation of the findings is presented.
A new analytical solution of an isothermal, Newtonian flow in a single screw extruder with a finite channel is developed with the actual boundary conditions encountered. Down channel velocity distributions are presented in three-dimensional plots. The boundary conditions, velocity distributions, and screw characteristics predicted by the new solution are tested using the experimental data from published literature (Choo et al., 1980, Polymer Engineering and Science, 20, 349-56; Griffith, 1962, Industrial and Engineering Chemistry Fundamentals, 1, 180-7). The results are found to be more accurate than existing theories (Rowell & Finlayson, 1922, Engineering, 126, 249-87; Tadmor & Gogos, 1979, Principles of Polymer Processing, Wiley; Rauwendaal, 1986, Polymer Extrusion, Hanser).
This work presents a mathematical analysis of an oil supply system for reciprocating compressors. The system is based on a single screw pump attached to the bottom end of the vertical rotating shaft immersed in the oil sump. The fluid flow in the pump was modeled with a semi-analytical approach based on the solution for the laminar fully developed oil flow in a screw extruder via the Generalized Integral Transform Technique. The screw pump model is coupled with that for the flow in the shaft region so as to provide an estimate of the oil flow rate and of the so-called ‘climbing-time’, i.e., the amount of time needed for a fluid particle to travel from the oil sump to the top of the shaft. The calculation method was verified against experimental data and Computational Fluid Dynamics modeling results.
Two new two-equation eddy-viscosity turbulence models will be presented. They combine different elements of existing models that are considered superior to their alternatives. The first model, referred to as the baseline (BSL) model, utilizes the original k-omega model of Wilcox In the inner region of the boundary layer and switches to the standard k -epsilon model in the outer region and in free shear flows. It has a performance similar to the Wilcox model, but avoids that model's strong freestream sensitivity. The second model results from a modification to the definition of the eddy-viscosity in the BSL model, which accounts for the effect of the transport of the principal turbulent shear stress. The new model is called the shear-stress transport-model and leads to major improvements in the prediction of adverse pressure gradient flows.
Application of the full cavitation model to pumps and inducers
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Flow analysis of a screw pump in the turbo pump of a semi-cryogenic engine
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