Amir Faghri

Thermal Modeling of unlooped and looped pulsating heat pipes

by experimental results. The visualization test showed that the oscillatory flow formed waves that traveled among the turns of PHPs. Their theoretical model could be used to estimate the pressure and displacement of oscillatory flow. Kiseev and Zolkin @7# experimentally investigated the effects of acceleration and vibration on the performance of the unlooped PHP. Acetone was used as the working fluid and the filling charge ratio was 60 percent. Their results indicated that the PHP operates successfully by various acceleration effects. There was an increase in evaporator temperature, about 30 percent by increase of the acceleration from 26g to 112g. Dobson and Harms @8# presented simple mathematical models in which the behavior of PHPs was simulated. The mathematical model was applied to the open-ended PHP. They showed that the oscillating behavior would be different for different initial values. Wong et al. @9# proposed a theoretical model of PHPs based on a Lagrangian approach in which the flow was modeled under adiabatic conditions for the entire PHP. A sudden pressure pulse was applied to simulate local heat input into a vapor plug. They were able to show the pressure and velocity variations with time for the vapor plugs. In the present study, a very detailed theoretical model will be developed to accurately simulate the behavior of liquid slugs and vapor plugs in both unlooped and looped PHPs. Heat transfer due to the phase change is also considered.

Advances and Unsolved Issues in Pulsating Heat Pipes

Pulsating (or oscillating) heat pipes (PHP or OHP) are new two-phase heat transfer devices that rely on the oscillatory flow of liquid slug and vapor plug in a long miniature tube bent into many turns. The unique feature of PHPs, compared with conventional heat pipes, is that there is no wick structure to return the condensate to the heating section; thus, there is no countercurrent flow between the liquid and vapor. Significant experimental and theoretical efforts have been made related to PHPs in the last decade. While experimental studies have focused on either visualizing the flow pattern in PHPs or characterizing the heat transfer capability of PHPs, theoretical examinations attempt to analytically and numerically model the fluid dynamics and/or heat transfer associated with the oscillating two-phase flow. The existing experimental and theoretical research, including important features and parameters, is summarized in tabular form. Progresses in flow visualization, heat transfer characteristics, and theoretical modeling are thoroughly reviewed. Finally, unresolved issues on the mechanism of PHP operation, modeling, and application are discussed.

Thermal Analysis of a Micro Heat Pipe

A detailed mathematical model is developed in which the heat and mass transfer processes in a micro heat pipe (MHP) are examined. The model describes the distribution of the liquid in a MHP and its thermal characteristics depending upon the liquid charge and the applied heat load. The liquid flow in the triangular-shaped corners of a MHP with polygonal cross section is considered by accounting for the variation of the curvature of the free liquid surface and the interfacial shear stresses due to a liquid-vapor frictional interaction. The predicted results obtained are compared to existing experimental data. The importance of the liquid fill, minimum wetting contact angle, and the shear stresses at the liquid-vapor interface in predicting the maximum heat transfer capacity and thermal resistance of the MHP is demonstrated.

Heat transfer in a pulsating heat pipe with open end

Heat transfer in the evaporator and condenser sections of a pulsating heat pipe (PHP) with open end is modeled by analyzing thin film evaporation and condensation. The heat transfer solutions are applied to the thermal model of the pulsating heat pipe and a parametric study was performed. The results show that the heat transfer in a PHP is mainly due to the exchange of sensible heat. The frequency and amplitude of the oscillation is almost unaffected by surface tension after steady oscillation has been established. The amplitude of oscillation decreases with decreasing diameter. The amplitude of oscillation also decreases when the wall temperature of the heating section is decreased, but the frequency of oscillation is almost unchanged.

Heat transfer enhancement in latent heat thermal energy storage system by using the internally finned tube

The heat transfer enhancement in the latent heat thermal energy storage system by using an internally finned tube is presented in this paper. The phase change material fills the annular shell space around the tube, while the transfer fluid flows within the internally finned tube. The melting of the phase change material is described by a temperature transforming model coupled to the heat transfer from the transfer fluid. The heat conduction in the internal fins is an unsteady two-dimensional heat conduction problem and is solved by a finite difference method. The results showed that adding internal fins is an efficient way to enhance the heat transfer in thermal energy storage systems when a fluid with a low thermal conductivity is used as the transfer fluid.

A numerical analysis of Stefan problems for generalized multi-dimensional phase-change structures using the enthalpy transforming model

Abstract An enthalpy transforming scheme is proposed to convert the energy equation into a non-linear equation with the enthalpy, E, being the single dependent variable. The existing control-volume finitedifference approach is modified so it can be applied to the numerical performance of Stefan problems. The model is tested by applying it to a three-dimensional freezing problem. The numerical results are in agreement with those existing in the literature. The model and its algorithm are further applied to a threedimensional moving heat source problem showing that the methodology is capable of handling complicated phase-change problems with fixed grids.

Review and Advances in Heat Pipe Science and Technology

Over the last several decades, several factors have contributed to a major transformation in heat pipe science and technology applications. The first major contribution was the development and advances of new heat pipes, such as loop heat pipes (LHPs), micro and miniature heat pipes, and pulsating heat pipes (PHPs). In addition, there are now many commercial applications that have helped contribute to the recent interest in heat pipes. For example, several million heat pipes are manufactured each month for applications in CPU cooling and laptop computers. Numerical modeling, analysis, and experimental simulation of heat pipes have significantly progressed due to a much greater understanding of various physical phenomena in heat pipes as well as advances in computational and experimental methodologies. A review is presented hereafter concerning the types of heat pipes, heat pipe analysis, and simulations.

Heat transfer during evaporation on capillary-grooved structures of heat pipes

A detailed mathematical model is developed that describes heat transfer through this liquid films in the evaporator of heat pipes with capillary grooves. The model accounts for the effects of interfacial thermal resistance, disjoining pressure, and surface roughness for a given meniscus contact angle. The free surface temperature of the liquid film is determined using the extended Kelvin equation and the expression for interfacial resistance given by the kinetic theory. The numerical results obtained are compared to existing experimental data. The importance of the surface roughness and interfacial thermal resistance in predicting the heat transfer coefficient in the grooved evaporator is demonstrated. 17 refs., 5 figs., 2 tabs.

Analysis of Forced Convection Heat Transfer in Microencapsulated Phase Change Material Suspensions

B = Bi, = C = C* = C = E= = e = Fo = h, = K, = k = L = rn = Nu, = Pe = 4 = R = Rd = Re = r = r* = r,. = r,, = Is1. = Ste = S = T= t = u = 47, = X = X = a = A numerical solution of laminar forced convection heat transfer of a microencapsulated phase change material suspension in a circular tuhe with constant heat flux has been presented in this article. Melting in the microcapsule was solved by a temperature transforming model instead of a quasisteady model. The effects of the microcapsules crust, the initial subcooling, and the width of the phase change temperature range on the variation of the dimensionless tuhe wall temperatures, along the axial direction, were also considered in the present model. The agreement between the present numerical results and the experimental results is very good.

A generalized thermal modeling for laser drilling process—I. Mathematical modeling and numerical methodology

Abstract Conduction and advection heat transfer in the solid and liquid metal, respectively, the free surface flow of the liquid melt and its expulsion, the tracking of the solid-liquid and liquid-vapor interfaces with different thermo-physical properties in the two phases and the evolution of latent heat of fusion over a temperature range are mathematically modeled for the two-dimensional axisymmetric case in the transient development of a laser drilled hole where the impressed pressure and temperature on the melt surface is provided by a one-dimensional gas dynamics model. Significant improvement made to our earlier melting and solidification submodel is discussed that comprises a temperature transforming model on a fixed grid system. The resulting advection-diffusion equations compatibility with the present fluid flow simulation model is described. The mathematical formulation of the submodels and the numerical methodology is presented.