Miniature Heat Pipe Research Articles

Impact of Microgroove Shape on Flat Miniature Heat Pipe Efficiency.

Miniature heat pipes are considered to be an innovative solution able to dissipate high heat with low working fluid fill charge, provide automatic temperature control, and operate with minimum energy consumption and low noise levels. A theoretical analysis on heat pipe thermal performance using Deionized water or n-pentane as the working fluid has been carried out. Analysis on the maximum heat and capillary limitation is conducted for three microgroove cross sections: rectangular, triangular, and trapezoidal. The effect of microgroove height and width, effective length, trapezoidal microgroove inclination angle, and microgroove shape on heat pipe performance is analysed. Theoretical and experimental investigations of the heat pipes’ heat transport limitations and thermal resistances are conducted.

Experimental investigation on the heat transfer performance of a latent thermal energy storage device based on flat miniature heat pipe arrays

An experimental latent-thermal energy storage device (LTESD) with a flat miniature heat pipe array (FMHPA) as a core heat transfer element is designed. Multihole flat pipes are utilized as a heat supply and heat removal loop for the passage of heat transfer fluid (HTF). Experiments are performed at different HTF volume flow rates and inlet temperatures to investigate the performance of the thermal storage unit consisting of FMHPA and vertical fins and to observe the change in the temperature of phase-change materials, such as lauric acid. The effects of heating/cooling section length and thermal resistance are also examined. Results indicate that LTESD works stably and efficiently and the respective storing and releasing power are 1299 and 1120 W under standard operating conditions of 2 L/min at 70 °C and 2 L/min at 15 °C.

Heat Transfer Performance of Heat Pipe Radiator with SiO2/Water Nanofluids

AbstractThe effect of nanofluids on thermal performance of the miniature heat pipe radiator which was assembled by two heat pipes containing 0.6 vol.% SiO2/water nanofluids and 30 pieces of rectangular aluminum fins was investigated experimentally and theoretically. The wall temperatures of the miniature heat pipe and fin surface temperatures were measured. Results showed that the utilization of SiO2/water nanofluids as a working fluid in the heat pipe enhanced the heat performance by reducing wall temperature differences. Compared with Deionized water (DI water), the thermal resistance of the miniature heat pipe with SiO2/water nanofluids decreased by about 23% to 40%. Furthermore, the theoretical calculation on a basis of one dimension found that the fin heat dissipation in the miniature heat pipe radiator charged SiO2/water nanofluids was about 1.17 times of that of the DI water radiator.

Modeling of the EHD effects on hydrodynamics and heat transfer within a flat miniature heat pipe including axial capillary grooves

A numerical model for an electrohydrodynamic (EHD) grooved Flat Miniature Heat Pipe (FMHP) is developed. Two microchannel shapes are considered as axial capillary structures: square and triangle grooves. For both groove shapes, the electric field affects the liquid-vapor radius of curvature which decreases in the condenser and increases in the evaporator under the action of the electric field. The liquid and vapor velocities are also affected by the EHD effects. The electric field effects on the velocities depend on the FMHP zone. It is also demonstrated that the electric field increases the vapor pressure drop; however, it decreases the liquid pressure drop. The liquid-wall and vapor-wall viscous forces as well as the shear liquid-vapor forces are affected by the electric field. The analysis of the electric forces shows that the dielectrophoretic forces which act on the liquid-vapor interface are predominant and their order of magnitude is much higher than the Coulomb forces. Finally, it is also demonstrated that the capillary limit increases with the electric field for both groove shapes.

Integration of Miniature Heat Pipes into a Proton Exchange Membrane Fuel Cell for Cooling Applications

ABSTRACTIn proton exchange membrane fuel cell (PEMFC) operations, the electrochemical reactions produce a rise in temperature. A fuel cell stack therefore requires an effective cooling system for optimum performance. In this study, miniature heat pipes were applied for cooling in PEMFC. Three alternatives were considered in tests: free convection, forced convection cooling with air, and also water. An analytical model was developed to show the possibility of evoking heat from inside a fuel cell stack with different numbers of miniature heat pipes. An experiment setup was designed and then used for further analysis. The proposed experiment setup consisted of a simulated fuel cell that produced heat and a number of thermosyphon miniature heat pipes to evoke heat from the simulated fuel cell. The experiment results reported in this paper present advantages and disadvantages of each tested cooling scenario. Results show that each cooling scenario, using a different number of heat pipes, provided different heat removal rates for PEMFC cooling.

Numerical study of the electrohydrodynamic effects on the two-phase flow within an axially grooved flat miniature heat pipe

A model for the fluid flow and heat transfer in an electrohydrodynamic (EHD) miniature heat pipe is presented. Coulomb and dielectrophoretic forces have been considered in the model. The coupled non-linear governing equations are developed and solved numerically. The variations of the liquid and vapor velocities and pressures as well as the liquid and vapor distributions along the Flat Miniature Heat Pipe (FMHP) are obtained. The electric field affects the liquid and the vapor velocities. It is also demonstrated that the vapor pressure drop increases with the electric field intensity, however, the liquid pressure drop decreases as the electric field strength increases. Moreover, the higher is the electric field intensity; the lower is the capillary pumping required for the liquid flow. This results into a lower liquid–vapor radius of curvature at the condenser. Similarly, the electric field affects the liquid–vapor shear forces as well as the liquid-wall and vapor-wall viscous forces. The analysis of the electric forces shows that the dielectrophoretic forces which act on the liquid–vapor interface are predominant and their order of magnitude is much higher than the Coulomb forces. It is also demonstrated that the capillary limit increases with the electric field, and the electric field reduces the dry-out when the applied heat input is higher than the capillary limit.

Heat transfer characteristics of a parallel miniature heat pipe system

In this present study, the heat transfer characteristics of a parallel miniature heat pipes system (mHPs) intended for desktop computer processor cooling was experimentally investigated. The experimental system consisted of six single copper tube mHPs slotted into two copper blocks at the evaporator section and fifteen parallel copper sheets at the condenser section. The copper blocks were placed above the heat source (on the top of the computer processor) and the condenser section was provided with external fins perpendicular to the mHPs. Four working fluids, namely acetone, ethanol, methanol, and propanol-2 were used independently to probe the working fluids' effect on the thermal performance of the system. Heat transfer characteristics of the mHPs were determined based on the principle of phase change of the working fluids at different temperatures to evaluate the thermal performance. The thermal conductance and thermal resistance of the system were determined as the parameters of thermal performance. The results indicated that the thermal characteristics of the heat pipe varied significantly for different temperatures of the heat source at the evaporator and different working fluids. It was obtained that the methanol is the best working fluid to be used in a mHPs in comparison with other working fluids (acetone, ethanol, and propanol 2). Furthermore, the lowest evaporator surface temperature was achieved with the methanol based system.

Numerical analysis of latent heat thermal energy storage using miniature heat pipes: A potential thermal enhancement for CSP plant development

This paper presents a study, numerically and experimentally, on a new thermal enhancement method for improving the heat transfer performance of latent heat energy storage (LHTES) using miniature heat pipes (MHPs). As commonly known, heat pipes are passive heat transfer devices which are capable in transferring large amount of heat with a small temperature drop. The heat pipe used in this study is copper-water charged MHP and has physical dimensions of 2mm diameter and 100mm long. The phase change material (PCM) used in the LHTES is paraffin wax (RT60) which is an organic based PCM and has a melting point of 60°C. The attractive thermal features of using PCM as thermal mass are high heat capacity, exhibit constant temperature during phase change and poor thermal response. However, the poor thermal conductivity (∼0.2W/m·K) of the PCM has greatly limited its potential to be used as high heat storing materials for future thermal storage developments. One of the potential developments is the concentrated solar power (CSP) plant, where LHTES can play an important role for improving the power generating efficiency. Providing simple and yet effective thermal enhancement method is favourable for LHTES to be widely applicable. In this study, MHPs are randomly mixed in the PCM to provide better heat spreading and improve the effective thermal conductivity of the LHTES. The numerical method adopted is three-dimensional heat conduction and the numerical results are validated against experimental data. The results have shown that the effective thermal conductivity of MHP-PCM composition has improved exponentially with the increasing number of MHPs used.

Experimental testing of mini heat pipes under microgravity conditions aboard a suborbital rocket

Heat pipe technologies have numerous applications under microgravity conditions in aerospace, from miniature devices in laptops used aboard the International Space Station (ISS) to heat transport systems in satellites. Advances in understanding the behavior of two-phase flow systems under microgravity conditions could lead to higher-efficiency devices and improved heat-exchanger designs. Under normal gravity conditions, heat pipes can function properly without any kind of capillary structure (thermosyphons, vertical position) or they can be aided by a wick material in the horizontal position, allowing the return of the condensate from the condenser to the evaporator. However, gravitational acceleration can significantly alter the flow regime, masking some effects that are not observed under microgravity conditions. This paper presents an experimental analysis of three miniature heat pipe technologies developed in Brazil for thermal management and heat dissipation of electronic devices under gravity and microgravity conditions. Different geometries of heat pipes charged with methanol were tested under microgravity conditions aboard a sounding rocket. Results demonstrated that the mini heat pipes present low thermal resistances for both gravity and microgravity conditions.

Experiment study on heat transfer capability of an innovative gravity assisted ultra-thin looped heat pipe

This paper introduced a kind of gravity assisted ultra-thin looped heat pipe (ULHP) with 1.5 mm thick wickless flat evaporator that can be applied for power battery thermal management system, and investigated the heat transfer capability of the ULHP. Air cooling was used as the condensation method considering its advantage of low price and wide application in all walks of life. A series of experiments were conducted under different condensation powers during the same heating load range which started from 20 W to 80 W, to find out the best condensation condition of the ULHP. Further experiments were conducted under the best condensation condition to clarify the heat transfer capability of the ULHP, during which the start -up temperature, the minimum start-up heat load and the thermal resistance were discussed. The experiment results indicated that the ULHP can work most effectively as the fan voltage was 9 V, under which the ULHP can start up at minimum heat flux of 0.22 W/cm2, with the start-up temperature of 47 °C, and the minimum thermal resistance was just 0.097 W/K during the heat load range from 20 W to 120 W. Compared with vertical placed miniature looped heat pipe with wick structure flat evaporator (MFLHP) in literature, the ULHP can start under lower heat flux with lower temperature and lower thermal resistance, which showed an improved performance of ULHP than traditional MFLHP.

Experimental Investigation of Thermal Performance of Miniature Heat Pipe Using SiO2-Water Nanofluids.

The four miniature heat pipes filled with DI water and SiO2-water nanofluids containing different volume concentrations (0.2%, 0.6% and 1.0%) are experimentally measured on the condition of air and water cooling. The wall temperature and the thermal resistance are investigated for three inclination angles. At the same of inlet heat water temperature in the heat system, it is observed that the total wall temperatures on the evaporator section are almost retaining constant by air cooling and the wall temperatures at the front end of the evaporator section are slightly reduced by water cooling. However, the wall temperatures at the condenser section using SiO2-water nanofluids are all higher than that for DI water on the two cooling conditions. As compared with the heat pipe using DI water, the decreasing of the thermal resistance in heat pipe using nanofluids is about 43.10%-74.46% by air cooling and 51.43%-72.22% by water cooling. These indicate that the utilization of SiO2-water nanofluids as working fluids enhances the performance of the miniature heat pipe. When the four miniature heat pipes are cut to examine at the end of the experiment, a thin coating on the surface of the screen mesh of the heat pipe using SiO2-water nanofluids is found. This may be one reason for reinforcing the heat transfer performance of the miniature heat pipe.

Heat Pipe for Aerospace Applications—An Overview

The paper presents an overview of heat pipes, especially those used in different space missions. Historical perspectives, principles of operations, types of heat pipes are discussed. Several factors have contributed to the science and technology of the present state-of-Art heat pipe leading to the development of loop heat pipes, micro and miniature heat pipes and micro loop heat pipes. The paper highlights the advancement of heat pipe for hypersonic cruise vehicles, loop heat pipes with higher conductance in 10 K range, heat pipe switches for temperature control of the spacecraft electronics.

Transient and cyclic effects on a PCM-cooled mobile device

A mock handset with heat storage unit (HSU) has been designed, fabricated, and experimented under various conditions to examine the effect of external heat sink on the handset?s transient temperature distribution, performance of the individual HSU under different power level and orientation, as well as under the more realistic cyclic heating. The cooling of the handset is through using a phase change material (PCM), n-eicosane, stored in the external HSU connected to the handset through a miniature heat pipe. The heat pipe channels the internal heat dissipation to the HSU where it is absorbed by the PCM. Results show that the temperature is significantly lowered with the PCM-based HSU.

Thermal Performance of a Cylindrical Heat Pipe for Different Heat Inputs and Inclination Angles

An experimental study on thermal performance of a copper heat pipe operated with sintered powder wick has been performed. A miniature cylindrical heat pipe with small outer diameter, Ø10 mm and 300 mm in length, has been chosen in the study. An attempt is also made to design, fabricate and assemble a heat pipe test rig to be used for the experiment. The performance of the heat pipe is investigated at various heat inputs and inclination angles (orientations). The aim of this study is to evaluate and compare the thermal performance of the heat pipe based on temperature differential between the evaporator and condenser sections, thermal resistance and thermal conductivity. The results show that the orientation and heat input have significant effects on the thermal performance of the cylindrical heat pipe. The Ø10 mm heat pipe operates with good thermal performance at the heat input of 70 W - 80 W and at the inclination angle of 30° - 60°.

Boiling heat transfer on smooth and porous surfaces in the limitted space

When designing passive cooling systems for radioelectronic and computer equipment based on conventional and miniature heat pipes it is necessary to carry out a preliminary estimation of heat transfer characteristics. For calculating heat transfer capacity of a heat pipe it is necessary to possess information about heat transfer intensity at phase transitions in the zones of heating and condensing.The results of experimental studies of heat transfer coefficients when boiling on smooth and porous surfaces under conditions of a large and limited volume are given. There is a significant influence of a free space height above the heat transfer surface on a heat removal intensity rate under phase transition. The criterial dependencies, obtained from the analysis and synthesis of experimental data, are given. The given equations allow calculating heat transfer coefficients in relation to geometrical and operational parameters in the range of Reynolds numbers Re=400…2•105 and for free space heights, satisfying the condition of (h/lc)=1…10 for smooth surfaces, as well as in the range of Reynolds numbers from 150 to 3000 for metal-fiber capillary structures 0.7-1.0 mmthick and porosity within the range of 75-85 % of the spaces between the CPS (a capillary porous system) surface and the upper wall from 2 to20 mm. The error of the obtained result does not exceed 20 %.

EXPERIMENTAL AND THEORETICAL ANALYSIS OF FLOW AND HEAT TRANSFER WITHIN GROOVED FLAT MINIATURE HEAT PIPE

EXPERIMENTAL AND THEORETICAL ANALYSIS OF FLOW AND HEAT TRANSFER WITHIN GROOVED FLAT MINIATURE HEAT PIPE

Influence of capillary structure caracteristics on boilign intensity in heat pipe

The research results of the influence of capillary structure characteristics on the heat transfer intensity in the heating zone of miniature heat pipes are presented in the paper. Also, the research results of the influence of heat removal conditions in the condensation area on the internal characteristics of phase transition processes are given. Copper heat pipes, filled with methanol were selected as experimental samples. The capillary structure was made of copper fibers with diameters of 50 microns and 70 microns, and the length of3 mmand7 mm.The analysis of experimental data showed that the increase in fiber diameter by 1,4 times leads to the decrease in the heat transfer intensity more than by 15 %. At the same time, the essential influence of the length change on the vaporization process in the heating zone of heat pipe was not observed.The obtained results can be used in designing passive cooling systems of radio-electronic equipment, based on the miniature heat pipes.

Charging Station of a Planar Miniature Heat Pipe Thermal Ground Plane

Thermal ground planes (TGPs) are planar, thin (thickness of 3 mm or less) heat pipes which use two-phase heat transfer. The objective is to utilize TGPs as thermal spreaders in several microelectronic cooling applications. TGPs are innovative high-performance, integrated systems able to operate at a high power density with a reduced weight and temperature gradient. Moreover, being able to dissipate large amounts of heat, they have very high effective axial thermal conductivities and can operate in high adverse gravitational fields due to nanoporous wicks. A key factor in the design of the TGP is evacuation prior to filling and introduction of the proper amount of working fluid (water) into the device. The major challenge of this work is to fill heat pipes with a total liquid volume of less than 1 ml, without being able to see into the device. The current filling station is an improvement over the current state of the art as it allows for accurate filling of microliter sized volumes. Tests were performed to validate performance of the system and to verify that little to no noncondensable gasses were introduced to the system. Careful calibration of the amount of liquid introduced is important. Therefore, calibration of the burettes utilized for a liquid fill range of 0.01 ml to 100 ml was important. The magnitude of the pressure inside the TGP device is also an important factor. Charging station validation demonstrated the capability of charging TGPs with accuracy of ±1.64 μl. Calibration curves for the burettes and error characterization curves for a range of liquid charging volumes will be presented and discussed in this paper.

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.

Application of nanofluid in an inclined mesh wicked heat pipes

An experimental study was performed to investigate the thermal performance of an inclined miniature mesh heat pipe using water-based CuO nanofluid as the working fluid. The study focused mainly on the effects of the inclination angle and the operating temperature on the heat transfer performance of the heat pipe using the nanofluid with the mass concentration of CuO nanoparticles of 1.0wt%. The experiment was performed at three steady operating temperatures of 40°C, 50°C and 60°C. Experimental results show that the inclination angle has a strong effect on the heat transfer performance of heat pipes using both water and the nanofluid. The inclination angle of 45° corresponds to the best thermal performance for heat pipes using both water and the nanofluid. The present investigation indicates that the thermal performance of an inclined miniature mesh heat pipe can be strengthened by using CuO nanofluid.