Timothy Ameel

• Ph.D.
• Chair at University of Utah

Gaseous flow in micro-scale systems comprised of moving components experience Fluid-Structure Interaction (FSI) and rarefaction when the length scales are small. While many commercial codes claim the ability to simulate FSI problems, such a combination is very challenging to model. Here we propose a momentum exchange coefficient model within the FS...

Spatially resolved temperature measurements and understanding the local heat-affected zone (HAZ) are of urgent importance in metal additive manufacturing (AM). Many experiments have used infrared (IR) temperature measurements at the build surface, but there is little experimental work measuring temperatures below the surface. Measurements below the...

Heat transfer and flow characteristics for a two-dimensional ellipse of different aspect ratios with single slot aligned with the principal flow direction is investigated numerically. In this study we quantify the change in drag force and heat transfer rate due to the slot. The ellipsoid aspect ratios are 1.0 (circle), 0.75, and 0.5 and the normali...

Omni-directional magnetic field generators (or Omnimagnets) are devices with several potential applications, such as capsule endoscope and magnetic guidance of a cochlear implant. An Omnimagnet heats due to the Joule effect and the resulting excessive temperatures may cause device failure. Thermal analysis of an Omnimagnet can provide operational l...

In traditional cochlear implant surgery, physical trauma may occur during electrode array insertion. Magnetic guidance of the electrode array has been proposed to mitigate this medical complication. After insertion, the guiding magnet attached to the tip of the electrode array must be detached via a heating process and removed. This heating process...

Magnetic guidance of cochlear implants is a promising technique to reduce the risk of physical trauma during surgery. In this approach, a magnet attached to the tip of the implant electrode array is guided within the scala tympani using a magnetic field. After surgery, the magnet must be detached from the implant electrode array via localized heati...

An Omnimagnet is an electromagnetic device that enables remote magnetic manipulation of devices such as medical implants and microrobots. It is comprised of three orthogonal nested solenoids with a ferromagnetic core at the center. Electrical current within the solenoids leads to undesired temperature increase within the Omnimagnet. If the temperat...

In traditional cochlear implant surgery, physical trauma may occur during electrode array insertion. Magnetic guidance of the electrode array has been proposed to mitigate this medical complication. After insertion, the guiding magnet attached to the tip of the electrode array must be detached via a heating process and removed. This heating process...

Magnetic guidance of cochlear implants is a promising technique to reduce the risk of physical trauma during surgery. In this approach, a magnet attached to the tip of the implant electrode array is guided within the scala tympani using a magnetic field. After surgery, the magnet must be detached from the implant electrode array via localized heati...

An Omnimagnet is an electromagnetic device that enables remote magnetic manipulation of devices such as medical implants and microrobots. It is comprised of three orthogonal nested solenoids with a ferromagnetic core at the center. Electrical current within the solenoids leads to Joule heating, resulting in undesired temperature increase within the...

Magnetic cochlear implant surgery requires removal of a magnet via a heating process after implant insertion, which may cause thermal trauma within the ear. Intra-cochlear heat transfer analysis is required to ensure that the magnet removal phase is thermally safe. The objective of this work is to determine the safe range of input power density to...

Magnetic cochlear implant surgery requires removal of a magnet via a heating process after implant insertion, which may cause thermal trauma within the ear. Intra-cochlear heat transfer analysis is required to ensure that the magnet removal phase is thermally safe. The objective of this work is to determine the safe input power density to detach th...

Heat exchangers with mini- and micro-channel components are capable of high energy exchange due to their incumbent large surface area to volume ratio. Concurrently, recent advances in additive manufacturing simplify the creation of metallic minichannels that incorporate turbulators for heat transfer enhancement. As part of the development of a mini...

An Omnimagnet is comprised of three orthogonal nested solenoids that enables magnetic guidance of devices such as medical implants. Electrical current within the solenoids leads to Joule heating resulting in undesired temperature increase within the Omnimagnet. If the temperature exceeds the melting point of the solenoid wire insulation, device fai...

Thermal-gradient Continuous Flow Polymerase Chain Reaction (CF-PCR) is one promising way to amplify DNA on a microchip, a key technology in point-of-care diagnostics. But questions remain about its efficacy in the field, where DNA sample temperature, operating environment, and volumetric flow rate vary. This paper investigates the effects of these...

Polymerase Chain Reaction (PCR) is used to amplify a specific segment of DNA through a thermal cycling protocol. The PCR industry is shifting its focus away from macro-scale systems and towards micro-scale devices because: micro-scale sample sizes require less blood from patients, total reaction times are on the order of minutes opposed to hours, a...

A numerical study has been conducted to determine which of eight fundamental design parameters have the greatest impact on the performance of a realistic, thermal gradient, continuous-flow polymerase chain reaction (CF-PCR) device. The 25-pass CF-PCR device considered in this study is practical, effective, easy to fabricate, and circumvents earlier...

Flow around a rigid, truncated, wall-mounted cylinder with an aspect ratio of 5 is examined computationally at various Reynolds numbers Re to determine how the end effects impact the vortex shedding frequency. The existence of the wall and free end cause a dampening of the classical shedding frequency found for a semi-infinite, two-dimensional cyli...

An experimental investigation of water flow in a rectangular T-channel with inner dimensions of 20 × 20 mm (inlet channel) and 20 × 40 mm (outlet mixing channel) has been conducted. The inlet Reynolds number Re, based on inlet hydraulic diameter, ranged from Re1 = 90 to 775. Inlet flow conditions were asymmetric, and inlet Re ratios of Re1/Re2 = 1....

While many microscale systems are subject to both rarefaction and fluid-structure-interaction (FSI) effects, most commercial algorithms cannot model both, if either, of these for general applications. This study modifies the momentum and thermal energy exchange models of an existing, continuum based, multifield, compressible, unsteady, Eulerian-Lag...

A cost-effective characterization technique for the measurement of depth in fully enclosed, optically transparent microchannels is described. The technique is suitable for depth measurement in xurographic microchannels, which are increasingly being used in microfluidic and bioengineering applications. The principle of interference is utilized in a...

The redistribution of mean momentum and vorticity, along with the mechanisms underlying these redistribution processes, is explored for post-laminar flow in fully developed, pressure driven, channel flow. These flows, generically referred to as transitional, include an instability stage and a nonlinear development stage. The central focus is on the...

Xurography is an inexpensive rapid prototyping technology for the development of microfluidic systems. Imprecision in the xurographic tape cutting process can result in undesired changes in channel dimensions near features that require a change in cutting direction, such as 90° miter bends. An experimental study of water flow in rectangular xurogra...

An experimental investigation of water flow in a T-shaped channel with rectangular cross section (20×20mm inlet ID and 20×40mm outlet ID) has been conducted for a Reynolds number Re range of 56–422, based on inlet diameter. Dynamical conditions and the T-channel geometry of the current study are applicable to the microscale. 2-D planar particle ima...

An experimental investigation of water flow through a high aspect ratio rectangular microchannel was conducted to further understand fluid dynamic characteristics in microchannels and to test the validity of macroscale theories that are commonly utilized at the microscale. A rectangular microchannel with nominal dimensions of 500 μm in height, 6 mm...

The redistribution of mean momentum and vorticity are explored for laminar-to-turbulent transition in fully developed channel flow. A central aim is to better understand how the mean dynamical mechanisms representative of the fully turbulent regime are first established. Specifically, the mean dynamics of channel flow transition evolve from a simpl...

An experimental study of water flow in a T-shaped channel with rectangular cross section (20×20 mm2 inlet ID and 20×40 mm2 outlet ID) has been conducted for a Reynolds number Re range based on inlet geometry of Re=56-422. Dynamical conditions and T-channel geometry of the current study are applicable to the microscale. This study supports a large b...

Xurography is a novel manufacturing process for microfluidic systems, providing rapid prototyping capability at low cost compared to traditional microfabrication technologies. An experimental study of flow characteristics in rectangular xurographic microchannels is reported. Mean microchannel depth, defined by the thickness of the double-sided adhe...

Developments in manufacturing have led to progressively smaller and more complex micro-elctro-mechanical systems (MEMS), many of which employ heat exchangers to enhance performance. Due to the size constraints on these devices, adiabatic surfaces are difficult to create, and thus, ambient thermal interaction becomes an import factor in heat exchang...

While many microscale systems are subject to both rarefaction and fluid-structure-interaction (FSI) effects, most commercial algorithms cannot model both, if either, of these for general applications. This study modifies the momentum and thermal energy exchange models of an existing, continuum based, multifield, compressible, unsteady, Eulerian-Lag...

An experimental investigation of water flow in a T-shaped channel with rectangular cross section (20 × 20 mm inlet ID and 20 × 40 mm outlet ID) has been conducted for a Reynolds number Re range of 56 to 422, based on inlet diameter. Dynamical conditions and the T-channel geometry of the current study are applicable to the microscale. This study sup...

The frictional and convective heat transfer characteristics of rarified flows in rectangular microchannels, with either isoflux or isothermal boundary conditions, are evaluated subject to second-order slip boundary conditions, creep flow, viscous dissipation, and axial conduction effects. Numerical results are obtained using a continuum based, thre...

The effect of viscous dissipation and rarefaction on rectangular microchannel convective heat transfer rates, as given by the Nusselt number, is numerically evaluated subject to constant wall heat flux (H2) and constant wall temperature (T) thermal boundary conditions. Numerical results are obtained using a continuum based, three-dimensional, compr...

Second-order slip flow and temperature jump boundary conditions are applied to solve the momentum and energy equations in a microtube for an isoflux thermal boundary condition. The flow is assumed to be hydrodynamically fully developed, and the thermal field is either fully developed or developing from the tube entrance. In general, second-order bo...

Continuous-flow temperature gradient microfluidics can be used to perform spatial DNA melting analysis. To accurately characterize the melting behavior of PCR amplicon across a spatial temperature gradient, the temperature distribution along the microfluidic channel must be both stable and known. Although temperature change created by micro-flows i...

This paper reports heat transfer results obtained during condensation of refrigerant propane inside a minichannel aluminium heat exchanger vertically mounted in an experimental setup simulating a water-to-water heat pump. The condenser was constructed of multiport minichannel aluminium tubes assembled as a shell-and-tube heat exchanger. Propane vap...

This paper presents heat transfer data for a multiport minichannel heat exchanger vertically mounted as an evaporator in a test-rig simulating a small water-to-water heat pump. The multiport minichannel heat exchanger was designed similar to a shell-and-tube type heat exchanger, with a six-channel tube of 1.42 mm hydraulic diameter, a tube-side hea...

A prototype liquid-to-refrigerant heat exchanger was developed with the aim of minimizing the refrigerant charge in small systems. To allow correct calculation of the refrigerant side heat transfer, the heat exchanger was first tested for liquid-to-liquid (water-to-water) operation in order to determine the single-phase heat transfer performance. T...

An experimental investigation of water flow through an aluminum rectangular microchannel with a hydraulic diameter of 169μm was conducted over a Reynolds number (based upon mean velocity and hydraulic diameter) range from 230 to 4,740. Pressure measurements were simultaneously acquired at eight different axial locations within the channel along wit...

Microchannel convective heat transfer and friction loss characteristics are numerically evaluated for gaseous, two-dimensional, steady state, laminar, constant wall heat flux flows. The effects of Knudsen number, accommodation coefficients, second-order slip boundary conditions, creep flow, and hydrodynamically/thermally developing flow are conside...

Flow in a 20mm, square T-channel geometry was experimentally investigated and characterized for Reynolds numbers from fifty to six-hundred. The dynamic conditions and square channel geometry are relevant to microscale mixing. A T-shaped geometry is especially attractive because it is easy to fabricate and integrate into complex systems. In this stu...

Slip flow convective heat transfer and friction loss characteristics are numerically evaluated for constant wall temperature rectangular microchannels. The effects of rarefaction, accommodation coefficients, aspect ratio, second-order slip boundary conditions, axial conduction, and viscous dissipation with flow work are each considered. Second-orde...

The effects of rarefaction on convective heat transfer and pressure drop characteristics are numerically evaluated for uniform wall heat flux rectangular microchannels. Results are obtained by numerically solving the momentum and energy equations with both first- and second-order slip velocity and temperature jump boundary conditions. The resulting...

Microchannel convective heat transfer characteristics in the slip flow regime are numerically evaluated for two-dimensional, steady state, laminar, constant wall heat flux and constant wall temperature flows. The effects of Knudsen number, accommodation coefficients, viscous dissipation, pressure work, second-order slip boundary conditions, axial c...

Two analytical models are presented in which the continuum momentum and energy equations, coupled with second-order slip flow and temperature jump boundary conditions, are solved. An isothermal boundary condition is applied to a microchannel with a circular cross section. The flow is assumed to be hydrodynamically fully developed and thermal field...

Deionized water flows in fused silica microtubes, with nominal diameters of 100, 320, and 1000-μm were studied. Flow conditions produced Reynolds numbers in the range 150 < Re < 11,000. Critical Reynolds number was found to be in the range of 2180 < Recr < 2450 for the three tube sizes. The average Poiseuille number was 76.9 ± 5%. Experimental turb...

Six different effectiveness parameters or figures of merit are defined based on the five identified engineering goals of three-fluid heat exchangers having three thermal communications. The effect of six non-dimensional design parameters on the defined figures of merit is discussed. It is shown that various effectivenesses of this class of heat exc...

A general, simple and easy to implement analytical model that enables design and analysis of three-fluid heat exchangers with three thermal communications for all flow arrangements is developed. The model is shown to reduce to either a three-fluid, two thermal communications model or the traditional two-fluid heat exchanger model under proper condi...

This paper presents a case-study of some University/Government/Industry interactions at the University of Utah that build research and academic programs and create opportunities for economic growth in the areas of micro and nano science and engineering.

Fluid flow and heat transfer characteristics of single-phase flows in microchannels for refrigerant R-134a were experimentally investigated. Experiments were conducted using rectangular channels micromilled in aluminum with hydraulic diameters ranging from approximately 112 to 210 m and aspect ratios that varied from 1.0 to 1.5. Using overall temp...

Laminar forced convection in thermally developing slip flow through isoflux rectangular microchannels is analytically investigated. Local and fully developed Nusselt numbers, fluid temperatures, and wall temperatures are obtained by solving the continuum energy equation tor hydrodynamically fully developed slip flow with the velocity slip and tempe...

A general analytical development, based on the first and second laws of thermodynamics, is used to define the maximum possible heat transfer in an ideal two-fluid exchanger as well as the maximum possible temperature differences for both fluid streams. It is shown that the conventional expression for the maximum possible heat transfer in ideal two-...

Microfluidics plays a major role in the development of many innovative research activities aimed at the development of miniaturized devices and systems, and new applications related to microscale handling of fluids. As the field of microfluidics continues to grow, it is becoming increasingly important to understand the mechanisms and fundamental di...

Laminar slip-flow forced convection in rectangular microchannels is studied analytically by applying a modified generalized integral transform technique to solve the energy equation, assuming hydrodynamically fully developed flow. Results are given in terms of the fluid mixed mean temperature, and both local and fully developed mean Nusselt numbers...

A universal finite Nusselt number is found for laminar slip flow heat transfer at the entrance of a conduit. The Nusselt number expression is valid for both isothermal and isoflux thermal boundary conditions and for any conduit geometry. The value of the entrance Nusselt number is found to be dependent on two nondimensional parameters that include...

Fluid flow and heat transfer characteristics of single-phase flows in microchannels for refrigerant R-134a were experimentally investigated. Experiments were conducted using rectangular channels micro-milled in aluminum with hydraulic diameters ranging from approximately 112-mm to 210-mm and aspect ratios that varied from 1.0 to 1.5. Using overall...

Fluid flow and heat transfer characteristics of single-phase flows in microchannels for refrigerant R-134a were experimentally investigated. Experiments were conducted using rectangular channels micro-milled in aluminum with hydraulic diameters ranging from approximately 112-mm to 210-mm and aspect ratios that varied from 1.0 to 1.5. Using overall...

Microfluidics plays a major role in the development of many innovative research activities aimed at the development of miniaturized devices and systems, and new applications related to microscale handling of fluids. As the field of microfluidics continues to grow, it is becoming increasingly important to understand the mechanisms and fundamental di...

A mathematical model is developed to study the performance of a parallel-flow heat exchanger in which both fluid streams are interacting thermally with the surroundings. The fluid temperatures are found to be dependent on the magnitude of the ambient temperature relative to fluid inlet temperatures, the ratios of conductances between the fluids and...

Microfabrication technologies have made possible the development of meso-scale energy conversion and chemical processing systems with microscale features, Scaling effects, such as the linear increase in surface-area-to-volume ratio that affects surface processes such as convection heat transfer, adsorption, and catalytic chemical conversion process...

Design of meso-scale energy systems, either for power production or heating/cooling, will require understanding of the thermodynamics of the proposed system as well as knowledge of the heat transfer and fluid dynamic characteristics associated with flow in microchannels. These proposed thermal energy systems require the exchange of significant amou...

In this paper, the effects of rectangular microchannel aspect ratio on laminar friction constant are described. The behavior of fluids was studied using surface micromachined rectangular metallic pipette arrays. Each array consisted of 5 or 7 pipettes with widths varying from 150 micrometers to 600 micrometers and heights ranging from 22.71 microme...

A mathematical model was developed to study the performance of a counterflow heat exchanger in which both fluid streams are exposed to external heating. It was found that under the external heat transfer condition, the effectiveness of the heat exchanger was drastically reduced. The hot fluid temperature effectiveness increased as the external ther...

In this paper, we describe microchannel fluid behavior using a numerical model based on micropolar fluid theory and experimentally verify the model using micromachined channels. The micropolar fluid theory augments the laws of classical continuum mechanics by incorporating the effects of fluid molecules on the continuum. The behavior of fluids was...

A three-dimensional transient numerical model is developed based on the Douglas alternating direction implicit (ADD) scheme and the parallel Gaussian elimination technique, to analyze the temperature distribution and temperature rise of an X-ray lithography process. The numerical model includes four layers commonly found in an X-ray lithography pro...

An analytical solution to the problem of flow in a tube with a diameter of the same order as the mean free path of the fluid is presented. Under these conditions, the Knudsen number becomes an important parameter and the fluid may exhibit slip flow and thermal nonequilibrium at the tube walls. A power-law fluid is considered to be hydrodynamically...

One of the challenges of future miniaturized biochemical analysis laboratories is manipulation of sub-(mu) L samples on a macro-scale in a parallel fashion. Today's commercially available sample handling systems for biochemical analysis are limited to wide center-to-center spacing (3.0 mm) and cannot handle sample volumes less than approximately 0....

One of the challenges of future miniaturized biochemical analysis laboratories is manipulation of sub-(mu) L samples on a macro-scale in a parallel fashion. Today's commercially available sample handling systems for biochemical analysis are limited to wide center-to-center spacing (3.0 mm) and cannot handle sample volumes less than approximately 0....

In this paper, we describe microchannel fluid behavior using a numerical model based on micropolar fluid theory and experimentally verify the model using micromachined channels. The micropolar fluid theory augments the laws of classical continuum mechanics by incorporating the effects of fluid molecules on the continuum. The behavior of fluids was...

Parallel flow over a flat plate with an unheated starting length is a good approximation of actual conditions in many engineering applications. A single general closed-form analytical solution for the average heat transfer coefficient for either an isothermal or isoflux boundary condition and for either laminar or turbulent flow is presented. For a...

Laminar mixed convection heat transfer across five in-line microchip-sized heaters, surface mounted on printed circuit board (PCB), was investigated by the weighted residual finite element method. The effects of axial heat conduction within the PCB for both mixed convection and pure forced convection are reported. The flow regime considered was 200...

As the field of microfluidics continues to grow, it is becoming increasingly important to understand the mechanisms involved with heat transfer in two-phase flow through microgeometries. This importance is reiterated by the increasing number of applications which use phase change as the principal mechanism to conduct or remove heat. The purpose of...

An analytical solution to the problem of laminar gas flow in microtubes with a constant heat flux boundary condition at the wall is presented. The microtube diameter is small enough that the Knudsen number becomes an important parameter; thus, both a slip flow hydrodynamic condition and a temperature jump thermal condition are assumed at the wall....

The objective of this research was to develop an analytical solution to the heat transfer problem in microchannels with slip-flow and an isothermal wall, a heat transfer problem for gases at low pressures or in extremely small geometries, and to verify this solution experimentally. In this investigation, an analytical expression for the velocity di...

An experimental apparatus was designed to test friction and heat transfer characteristics of microchannels (diameters of 500 micrometers or less). Two sets of microchannels were tested: a set of nine 508 micrometers square channels and a single 254 micrometers square channel. The laminar friction factor for the 508 mm channels was lower than expect...

LIGA is a relatively new technology used to pattern high aspect ratio microelectromechanical systems (HARMEMS) in a resist material, using X-ray radiation. Resist materials used in LIGA typically have conduction and thermal expansion properties that are very different from those of the substrate that supports them during the exposure process, and t...

As the field of microfluidics continues to grow, it is becoming increasingly important to understand the mechanisms involved with heat transfer in two-phase flow through microgeometries. This importance is reiterated by the increasing number of applications which use phase change as the principal mechanism to conduct or remove heat. The purpose of...

An overview of the miniaturization technologies and their application to energy systems is presented. Based on the technologies referred to as MEMS (microelectromechanical systems) or MST (micro systems technologies), silicon based micromachining, deep X-ray lithography and the micro mechanical machining processes (micro drilling, milling, cutting,...

Absorption of water vapor in a liquid film is an important process in a proposed solar cooling system. Film absorption involves simultaneous heat and mass transfer in the gas—liquid system. The heat of absorption gives rise to temperature gradients leading to the transfer of heat while temperature affects the vapor pressure-composition equilibrium...

The original problem for thermally developing heat transfer in laminar flow through a circular tube, as formulated by Graetz, did not consider the ‘slip-flow’ condition. This paper extends the original work of Graetz to include the effect of slip-flow, which occurs in gases at low pressures or in microtubes at ordinary pressures. A special techniqu...

Both LiCl and LiBr solutions were considered for potential use in a solar-driven Open-Cycle Absorption Refrigeration (OCAR) system. A vertical falling film absorber was proposed and built to evaluate the performance of LiCl and LiBr as an absorbent. Absorption experiments were performed and the results are reported for typical operating conditions...

A technique was developed for evaluation of the eigenvalues for the Graetz problem extended to slip-flow. The first four eigenvalues for Knudsen numbers of 0.02, 0.04, …, 0.12 were found. By using a least square curve-fit method, simplified relationships between the eigenvalues and Knudsen number were obtained. The efficient and accurate determinat...

An analytical solution is presented for the effect of air (nonabsorbable gas) on the heat and mass transfer rates during the absorption of water vapor (absorbate) by a falling laminar film of aqueous lithium bromide (absorbent), an important process in a proposed open-cycle solar absorption cooling system. The analysis was restricted to the entranc...

The weighted residuals method is applied to analyze mixed convection heal transfer in 3 × 3 in-line horizontal tube bundles placed between two vertical parallel plates. The flow regime of Reynolds numbers up to 500 and Grashof number up to 53,000 is investigated. The local data of different geometries (nondimensional streamwise and transverse cylin...