Modeling Transient Heat Transfer and Dry-Out Phenomena in Heat Pipes Using Finite Element Analysis

2020 ◽  
Author(s):  
Mustafa Özçatalbaş ◽  
Ramazan Aykut Sezmen
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Heat Flux ◽  
Finite Element ◽  
Heat Pipe ◽  
Heat Removal ◽  
Heat Pipes ◽  
Heat Transfer Analysis ◽  
Transient Heat Transfer ◽  
Dry Out

Abstract Heat pipes are passive two-phase heat transfer devices that used in various heat transport applications because of their high thermal conductance capacities with low temperature differences. One of these applications is aerospace avionics that heat pipes are exposed to transient heat loads. Although heat pipes have been one of the heat removal alternatives for compact electronic devices, they have some restrictions during the usage in such high heat flux areas. In order to use heat pipes as effective heat removal devices, operating heat load range should not be exceeded during the operation of avionics or electronic devices. Out of these operating range, heat pipes no longer perform as effective heat removal devices because of phenomena called dry-out. In this study, a novel Finite Element (FE) Analysis Method was developed to model transient heat transfer behavior in heat pipes including dry-out phenomenon. Transient heat transfer analysis using Finite Element Method (FEM) was conducted to investigate heat pipe thermal performance considering heat flux dependent thermal conductivity under randomly varying heat inputs, which were assumed as heat dissipation of an electronic device. Validation of the FE model was done by using the results given in the literature. Heat pipe was made of Al with a length of LHP = 200 mm. Heat flux and convective heat transfer boundary conditions were used at the evaporator and condenser sections, respectively. Effective thermal conductivity of heat pipe, keff, was calculated by using the heat input depended thermal resistance, Rth, values given in literature. Under transient heat loads, heat flux dependent effective thermal conductivity was defined using user defined subroutines to simulate the dry-out. The transient heat transfer analysis was conducted using ABAQUS commercially available software. Temperature differences between evaporator and condenser sections, ΔT = Te−Tc, and thermal resistance, Rth, values are calculated for varying heat input values and compared with the results that provided in literature.

scholarly journals The effect of operating parameters on the heat transfer in the heat pipe

2021 ◽  
Vol 2119 (1) ◽  
pp. 012088
Author(s):  
A. A. Litvintceva ◽  
N. I. Volkov ◽  
N. I. Vorogushina ◽  
V. A. Moskovskikh ◽  
V. V. Cheverda
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Heat Pipe ◽  
Temperature Difference ◽  
Heat Pipes ◽  
Working Fluid ◽  
Operating Parameters ◽  
Temperature Stabilization ◽  
Ir Camera ◽  
Fluid Properties

Abstract Heat pipes are a good solution for temperature stabilization, for example, of microelectronics, because these kinds of systems are without any moving parts. Experimental research of the effect of operating parameters on the heat transfer in a cylindrical heat pipe has been conducted. The effect of the working fluid properties and the porous layer thickness on the heat flux and temperature difference in the heat pipe has been investigated. The temperature field of the heat pipe has been investigated using the IR-camera and K-type thermocouples. The data obtained by IR-camera and K-type thermocouples have been compared. It is demonstrated the power transferred from the evaporator to the condenser is a linear function of the temperature difference between them.

Effect of Fluid Loading on the Performance of Wicked Heat Pipes

Volume 3 ◽  
2004 ◽  
Author(s):  
R. Kempers ◽  
A. Robinson ◽  
C. Ching ◽  
D. Ewing
Keyword(s):  
Heat Flux ◽  
Heat Pipe ◽  
Heat Transport ◽  
Heat Pipes ◽  
Working Fluid ◽  
Fluid Loading ◽  
Maximum Heat ◽  
Horizontal Orientation ◽  
Maximum Heat Flux ◽  
Dry Out

A study was performed to experimentally characterize the effect of fluid loading on the heat transport performance of wicked heat pipes. In particular, experiments were performed to characterize the performance of heat pipes with insufficient fluid to saturate the wick and excess fluid for a variety of orientations. It was found that excess working fluid in the heat pipe increased the thermal resistance of the heat pipe, but increased maximum heat flux through the pipe in a horizontal orientation. The thermal performance of the heat pipe was reduced when the amount of working fluid was less than required to saturate the wick, but the maximum heat flux through the heat pipe was significantly reduced at all orientations. It was also found in this case the performance of this heat pipe deteriorated once dry-out occurred.

scholarly journals Transport in Flat Heat Pipes at High Heat Fluxes From Multiple Discrete Sources

2004 ◽  
Vol 126 (3) ◽  
pp. 347-354 ◽  
Author(s):  
Unnikrishnan Vadakkan ◽  
Suresh V. Garimella ◽  
Jayathi Y. Murthy
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Fluid Flow ◽  
Heat Pipe ◽  
Three Dimensional ◽  
Heat Pipes ◽  
Density Change ◽  
Heat Sources ◽  
Discrete Heat Sources ◽  
Energy Equations

A three-dimensional model has been developed to analyze the transient and steady-state performance of flat heat pipes subjected to heating with multiple discrete heat sources. Three-dimensional flow and energy equations are solved in the vapor and liquid regions, along with conduction in the wall. Saturated flow models are used for heat transfer and fluid flow through the wick. In the wick region, the analysis uses an equilibrium model for heat transfer and a Brinkman-Forchheimer extended Darcy model for fluid flow. Averaged properties weighted with the porosity are used for the wick analysis. The state equation is used in the vapor core to relate density change to the operating pressure. The density change due to pressurization of the vapor core is accounted for in the continuity equation. Vapor flow, temperature and hydrodynamic pressure fields are computed at each time step from coupled continuity/momentum and energy equations in the wick and vapor regions. The mass flow rate at the interface is obtained from the application of kinetic theory. Predictions are made for the magnitude of heat flux at which dryout would occur in a flat heat pipe. The input heat flux and the spacing between the discrete heat sources are studied as parameters. The location in the heat pipe at which dryout is initiated is found to be different from that of the maximum temperature. The location where the maximum capillary pressure head is realized also changes during the transient. Axial conduction through the wall and wick are seen to play a significant role in determining the axial temperature variation.

scholarly journals Fluid flow and heat transfer in a dual-wet micro heat pipe

2007 ◽  
Vol 589 ◽  
pp. 1-31 ◽  
Author(s):  
JIN ZHANG ◽  
STEPHEN J. WATSON ◽  
HARRIS WONG
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Heat Flux ◽  
Fluid Flow ◽  
Temperature Distribution ◽  
Heat Pipe ◽  
Contact Line ◽  
Heat Pipes ◽  
Dimensionless Parameters ◽  
Micro Heat Pipes

Micro heat pipes have been used to cool micro electronic devices, but their heat transfer coefficients are low compared with those of conventional heat pipes. In this work, a dual-wet pipe is proposed as a model to study heat transfer in micro heat pipes. The dual-wet pipe has a long and narrow cavity of rectangular cross-section. The bottom-half of the horizontal pipe is made of a wetting material, and the top-half of a non-wetting material. A wetting liquid fills the bottom half of the cavity, while its vapour fills the rest. This configuration ensures that the liquid–vapour interface is pinned at the contact line. As one end of the pipe is heated, the liquid evaporates and increases the vapour pressure. The higher pressure drives the vapour to the cold end where the vapour condenses and releases the latent heat. The condensate moves along the bottom half of the pipe back to the hot end to complete the cycle. We solve the steady-flow problem assuming a small imposed temperature difference between the two ends of the pipe. This leads to skew-symmetric fluid flow and temperature distribution along the pipe so that we only need to focus on the evaporative half of the pipe. Since the pipe is slender, the axial flow gradients are much smaller than the cross-stream gradients. Thus, we can treat the evaporative flow in a cross-sectional plane as two-dimensional. This evaporative motion is governed by two dimensionless parameters: an evaporation number E defined as the ratio of the evaporative heat flux at the interface to the conductive heat flux in the liquid, and a Marangoni number M. The motion is solved in the limit E→∞ and M→∞. It is found that evaporation occurs mainly near the contact line in a small region of size E−1W, where W is the half-width of the pipe. The non-dimensional evaporation rate Q* ~ E−1 ln E as determined by matched asymptotic expansions. We use this result to derive analytical solutions for the temperature distribution Tp and vapour and liquid flows along the pipe. The solutions depend on three dimensionless parameters: the heat-pipe number H, which is the ratio of heat transfer by vapour flow to that by conduction in the pipe wall and liquid, the ratio R of viscous resistance of vapour flow to interfacial evaporation resistance, and the aspect ratio S. If HR≫1, a thermal boundary layer appears near the pipe end, the width of which scales as (HR)−1/2L, where L is the half-length of the pipe. A similar boundary layer exists at the cold end. Outside the boundary layers, Tp varies linearly with a gradual slope. Thus, these regions correspond to the evaporative, adiabatic and condensing regions commonly observed in conventional heat pipes. This is the first time that the distinct regions have been captured by a single solution, without prior assumptions of their existence. If HR ~ 1 or less, then Tp is linear almost everywhere. This is the case found in most micro-heat-pipe experiments. Our analysis of the dual-wet pipe provides an explanation for the comparatively low effective thermal conductivity in micro heat pipes, and points to ways of improving their heat transfer capabilities.

scholarly journals Study of Thermal Performance of Closed Loop Pulsating Heat Pipe using Computational Fluid Dynamics

2021 ◽  
Vol 9 (9) ◽  
pp. 1384-1388
Author(s):  
K. C. Giri
Keyword(s):  
Heat Transfer ◽  
Fluid Dynamics ◽  
Thermal Conductivity ◽  
Heat Flux ◽  
Heat Pipe ◽  
Closed Loop ◽  
Filling Ratio ◽  
Pulsating Heat Pipe ◽  
Heat Transfer Characteristics ◽  
Different Temperatures

Abstract: Pulsating heat pipe is a heat transfer device which works on two principles that is phase transition and thermal conductivity which transfer heat effectively at different temperatures. Different factors affect the thermal performance of pulsating heat pipe. So, various researchers tried to enhance thermal conductivity by changing parameters such as working fluids, filling ratio, etc. Analysis of heat transfer characteristics of closed loop pulsating heat pipe (CLPHP) is to be carried out by using Computational Fluid Dynamics. The CLPHP is to be modelled on ANSYS Workbench, the flow of CLPHP is to be observed under specific boundary conditions by using ANSYS Fluent software. Acetone and Water are taken as the working fluid with 70% filling ratio at ambient temperature 30° C and the heat flux of 200 W is supplied at evaporator. Also, the analysis has been done to know the behaviour of PHPs under varying supply of heat flux at evaporator (inlet), the output heat flux is obtained at condenser (outlet) and find out how the heat flux is varying at different temperatures. CFD results shows the heat transfer characteristics observing the performance of CLPHP is a numerical manner. The obtained CFD results are compared with the experimental. The outputs of the simulations are plotted in graphs and outlines. Keywords: Closed Loop Pulsating Heat Pipe, CFD, Heat Transfer, ANSYS.

A Novel Quick Qmax Test Method and Experimental Investigation of Heat Pipes

2005 ◽  
Author(s):  
Yao-Chen Chan ◽  
Wei-Keng Lin
Keyword(s):  
Heat Transfer ◽  
Heat Pipe ◽  
Wall Temperature ◽  
Transfer Rate ◽  
Heat Pipes ◽  
Adiabatic Temperature ◽  
Test Method ◽  
Maximum Heat ◽  
Maximum Heat Transfer ◽  
Dry Out

In traditional heat pipe performance test, to keep an adiabatic temperature at a constant value, the evaporator wall temperature would be slowly increased when the thermal power was step input to the evaporator of the heat pipe. The maximum heat transfer rate (Qmax) was then defined that when the evaporator wall temperature rapidly increased at a certain amount of power input to the heat pipe. However, it is not easy to distinguish this sharp increased curve and sometimes result in the wrong Qmax data. In addition, it took too long for waiting the evaporator temperature approach to a steady state, thus this process could not use be for the fully check Qmax of the heat pipe. In this paper, we propose a novel quick test method to predict the maximum heat dissipation of the heat pipes namely Dynamic-Temperature-Tracing (D.T.T). The concept of the D.T.T was when we tracing the evaporator and the adiabatic wall temperature, these two temperature curves should be the same trend before the dry-out phenomena was occurred. Theoretically, when the dry-out start to occur in the heat pipe, the adiabatic temperature profile was no longer kept the same temperature profile as that of the evaporator. Hence, the maximum heat dissipate ability of the heat pipe was then easy to obtained at this measuring adiabatic temperature. The data were also compared with those obtained from the traditional standard method at the same equivalent evaporator length, condenser length and adiabatic temperature. In this experiments, sinter powder and groove heat pipes with diameter 6mm 8mm and 200mm length were selected as the capillary wick structure. Comparing with traditional method results, the errors of maximum heat transfer rate are less than 15%. The results also shown D.T.T. method is much fast and reliable compare with the traditional test method.   This paper was also originally published as part of the Proceedings of the ASME 2005 Pacific Rim Technical Conference and Exhibition on Integration and Packaging of MEMS, NEMS, and Electronic Systems.

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