Theoretical modelling of miniature loop heat pipe

Development in the design and thermal performance of the loop heat pipes (LHPs) demands the corresponding improvement in the theoretical modeling capabilities of these devices. In this paper, mathematical model for assessing the thermal performance of the miniature LHPs (mLHPs) on the basis of the operating temperature and thermal resistance of the loop has been discussed in detail. In order to validate the theoretical model, a mLHP with the flat disk shaped evaporator, 30 mm in diameter and 10 mm thick, was developed and tested with nickel and copper wick structure. By comparison with experimental results, it was found that the theoretical model was able to predict the evaporator temperature and loop thermal resistance very well and within the uncertainties imposed by the underlying assumptions. The mathematical model can be used to validate the design of the mLHP and verify whether the proposed design is consistent with the maximum heat load capacity required for the intended application. In addition to this, the model can assists in understanding and refining the outcomes of the experimental studies.     

Operational characteristics of miniature loop heat pipe with flat evaporator

Loop heat pipes are heat transfer devices whose operating principle is based on the evaporation and condensation of a working fluid, and which use the capillary pumping forces to ensure the fluid circulation. A series of tests have been carried out with a miniature loop heat pipe (mLHP) with flat evaporator and fin-and-tube type condenser. The loop is made of pure copper with stainless mesh wick and methanol as the working fluid. Detailed study is conducted on the start-up reliability of the mLHP at high as well as low heat loads. During the testing of mLHP under step power cycles, the thermal response presented by the loop to achieve steady state is very short. At low heat loads, temperature oscillations are observed throughout the loop. The amplitudes and frequencies of these fluctuations are large at evaporator wall and evaporator inlet. It is expected that the extent and nature of the oscillations occurrence is dependent on the thermal and hydrodynamic conditions inside the compensation chamber. The thermal resistance of the mLHP lies between 0.29 and 3.2°C/W. The effects of different liquid charging ratios and the tilt angles to the start-up and the temperature oscillation are studied in detail.     

Thermodynamic analytical model of a loop heat pipe

A thermodynamics analytical model is developed to explore different parameters effects on a loop heat pipe (LHP). The LHP is a two-phase device with extremely high effective thermal conductivity that utilizes the thermodynamic pressure difference to circulate a cooling fluid. The effects of pipe length, pipe diameter, condenser temperature, and heat load are reported. As pipe length increases and/or pipe diameter decreases, a higher temperature is expected in the evaporator.     

Comparison between oxide-reduced and water-atomized copper powders used in making sintered wicks of heat pipe

Oxide-reduced copper powder can be produced efficiently at low cost. The volume shrinkage, porosity, maximum pore size, permeability and thermal conductivity of wicks sintered from two oxide-reduced （OR） powders were compared with one from water-atomized （WA） powder. The green specimens were sintered at temperatures from 800 to 1000 ℃ in a tube furnace under a reduction stream of 10% hydrogen and 90% argon. The results show that the property variations of OR - 100 and WA wicks due to porosity changes have a similar tendency and range. Nine hundred degree celsius is a recommended sintering temperature for producing ideal wicks for use in heat pipes. A smaller maximum pore size can be obtained by increasing the green density.2007 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V.     

An experimental study on an oscillating loop heat pipe consisting of three interconnected columns

This paper presents some experimental results of an extensive research on a novel oscillating heat pipe. The heat pipe is formed of three interconnected columns as different from the pulsating heat pipe designs. The dimensions of the heat pipe considered in this study are large enough to neglect the effect of capillary forces. Thus, the self-oscillation of the system is driven by the gravitational force and the phase lag between the evaporation and condensation processes. The overall heat transfer coefficient is found to be approximately constant irrespective of heat load for the experimental cases considered. The results are also compared with the previously published data by other investigators for water as the working fluid and for the same heat input range. The experimental data for the time variation of the liquid column heights and the vapor pressure are correlated algebraically, convenient for practical uses.     

Heat transfer mechanism of miniature loop heat pipe with water-copper nanofluid: thermodynamics model and experimental study

In order to ensure the normal work of electronic product, the thermal management is of key importance. Miniature loop heat pipe (mLHP) is a promising device of heat transfer for electronic products. Cu-water nanofluid with different concentration is used as working material in mLHP. Experiments are conducted to investigate its heat transfer performance. The heat flux owing to thermal diffusion is calculated. It is found that this heat flux and the boiling temperature are non-monotonic function of concentration of nanoparticle. Turning concentration appears at about 1.5 wt%. Differential equation of thermal diffusion produced by micro movement of nanoparticle is established in this paper. Average speed formula for nanoparticles is derived and slope of the curve of phase equilibrium is obtained. Based on the theoretical research in this paper, enhanced heat transfer mechanism of nanofluid is analyzed. The facts that heat flux owing to thermal diffusion and boiling temperature are all associated with nanoparticle concentration are also well explained with the aid of the derived theory in this paper.     

Evaporation heat transfer in thin biporous media

 This paper deals with the evaporation heat transfer mechanism in thin biporous media that have two characteristic capillary pore radii. The character of the two levels of pore sizes allows the liquid phase to easily occupy the void space of the small pores and vapor phase to occupy the void space of the big pores. Compared with mono-porous media, biporous media increase the number of small evaporating menisci with high heat transfer performance. Evaporation heat transfer in pores of porous media is analyzed in detail. The results indicate that the average heat transfer coefficient increases with the capillary pore size reduction. Under the assumption of the uniform structure of biporous media, a calculation method to predict heat transfer performance for the evaporation in thin biporous media is given. The preliminary results reflect the behavior of observed vaporization heat transfer in thin biporous media well. Received on 22 February 2000     

Thermal performance of an open loop closed end pulsating heat pipe

This paper presents an experimental study of an open loop pulsating heat pipe (OLPHP) of 0.9 mm inner diameter. The performance characterization has been done using four working fluids at vertical and horizontal orientations. Water, Methanol, 2-Propanol and Acetone has been employed as the working fluid with 50% fill ratio. The experimental results indicate a strong influence of gravity and thermo physical properties of the working fluids on the performance of OLPHP. Considering all the working fluids used, Water has shown better thermal performance in vertical orientation while Methanol has shown better performance in horizontal orientation. All the working fluids perform better at horizontal orientation.     

Effect of pore size distribution in bidisperse wick on heat transfer in a loop heat pipe

The purpose of this article is to experimentally investigate the effect of different pore size distributions in bidisperse wicks upon the heat transfer performance in a LHP. Three bidisperse wicks and one monoporous wick were tested in a loop heat pipe. The pore size distributions of the bidisperse wicks were measured, and the results reflected the three different large/small pore size ratios. The experiments showed that the maximum heat load of the monoporous wick reached about 400 W; and the three bidisperse wicks showed improvements on the maximum heat load up to 570 W. For the monoporous wick, the evaporator heat transfer coefficients of 10 kW/m² K and total thermal resistance of 0.19°C/W were achieved at a high heat load of 400 W. For the better bidisperse wick, the evaporator heat transfer coefficients could attain about 23 kW/m² K and total thermal resistance of 0.13°C/W. The results also indicated that a smaller cluster size in a bidisperse structure created a small pore size ratio. It was also found that the bidisperse wick with smaller clusters had a better enhancement in terms of the evaporator heat transfer coefficient.     

Two-phase flow patterns of a top heat mode closed loop oscillating heat pipe with check valves (THMCLOHP/CV)

This research is aimed at studying the two-phase flow pattern of a top heat mode closed loop oscillating heat pipe with check valves. The working fluids used are ethanol and R141b and R11 coolants with a filling ratio of 50% of the total volume. It is found that the maximum heat flux occurs for the R11 coolant used as the working fluid in the case with the inner diameter of 1.8 mm, inclination angle of ?90?, evaporator temperature of 125?C, and evaporator length of 50 mm. The internal flow patterns are found to be slug flow/disperse bubble flow/annular flow, slug flow/disperse bubble flow/churn flow, slug flow/bubble flow/annular flow, slug flow/disperse bubble flow, bubble flow/annular flow, and slug flow/annular flow.     

Turbulent heat transfer for pipe flow with uniform heat generation

An Analytic solution is presented of the problem of turbulent heat transfer in pipes with internal heat generation and insulated wall by applying a recently-developed eddy conductivity model. The results agree closely with available experimental data for a wide range of Prandtl number (0.02–10.5).     

Investigation of heat transfer on the Dowthern A gravity assisted heat pipe

An experimental study on heat transfer performance of a gravity-assisted heat pipe is described. Dowtherm A has been used as working fluid. The effect of fill ratio, inclination angle, heat length ratio, and thermal resistance on the rate of heat flow were investigated. Finding a dependence of the maximum rate of heat flow on the first two parameters, a corresponding numerical correlation has been formulated. The force of viscosity shear and the force of momentum transfer during the condensation have been accounted for, and, with a theoretical analysis and a numerical solution, a mean heat transfer coefficient in the condensation section is given.

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Untersuchung der Wärmeübertragung in einem Gravitationswärmerohr mit Dowtherm A als Arbeitsfluid

Zusammenfassung Es wird eine experimentelle Untersuchung des Wärmeübertragungsvermögens eines Gravitationswärmerohres beschrieben. Als Arbeitsfluid diente Dowtherm A. Der Einfluß der eingefüllten Menge, des Neigungswinkels, der beheizten Länge und des Wärmewiderstandes auf den Wärmestrom wurde untersucht. Hierbei zeigte sich eine Abhängigkeit des maximalen Wärmestroms von den ersten beiden Parametern, die sich als Gleichung formulieren ließ. Unter Berücksichtigung der viskosen Scherkräfte und der Kräfte infolge Impulsübertragung während der Kondensation wird neben einer theoretischen Analyse und der numerischen Lösung der mittlere Wärmeübergangskoeffizient im Kondensationsgebiet angegeben.

     

Thermal performance of heat pipe with suspended nano-particles

Nanofluids are employed as the working medium for a conventional cylindrical heat pipe. A cylindrical copper heat pipe of 19.5?mm outer diameter and 400?mm length was fabricated and tested with two different working fluids. The working fluids used in this study are DI-water and Nano-particles suspension (mixture of copper nano particle and DI-water). The overall heat transfer coefficient of the heat pipe was calculated based on the lumped thermal resistance network and compared with the heat transfer coefficient of base fluid filled heat pipe. There is a quantitative improvement in the heat transfer coefficient using nano-particles suspension as the working medium. A heat transfer correlation was also developed based on multiple regression least square method and the results were compared with that obtained by the experiment.     

Comparative study on heat pipe performance using aqueous solutions of alcohols

This paper deals with the performance characterization of heat pipes using an aqueous solution of long chain alcohols like n-Butanol, n-Pentanol, n-Hexanol and n-Heptanol as working mediums. These solutions are called as self-rewetting fluids, since these fluid mixtures possess a non-linear dependence of the surface tension with temperature. A cylindrical heat pipe made up of copper with two layers of wrapped screen is used as a wick material and partially filled with the self-rewetting fluid water mixture and tested for its heat transport capability like thermal efficiency and thermal resistance at different inclinations and input power levels. A number of tests have been performed with heat pipes, filled with various aqueous solutions of alcohols with a concentration of 2?ml/l in de-ionized water (DI water) on volume basis. The results obtained for heat pipes using self rewetting fluids show improved performances, when compared to DI water heat pipes.     

The experimental study on flat plate heat pipe of magnetic working fluid

An effective thermal spreader can achieve uniform heat flux distribution and thus enhance heat dissipation of heat sinks. Flat plate heat pipe is one of the highly effective thermal spreaders. Magnetic fluid is liquid and can be moved by the force of magnetic field. Therefore, the magnetic fluid is suitable to be used as the working fluid of flat plate heat pipes which have a very small gap between evaporation and condensation surfaces. We prepared a disk-shaped wickless flat plate heat pipe, and the distance between evaporation and condensation surfaces is only 1 mm. From experimental study, the effect of heat flux and working fluid ratio on the performance of flat plate heat pipe is presented. Also we compared the experimental results between the performance of water and magnetic fluid as working fluids.     

Advanced numerical method for a thermally induced slug flow: application to a capillary closed loop pulsating heat pipe

An advanced hybrid lumped parameter code for the simulation of Pulsating Heat Pipes is developed. Being able to simulate transient operative conditions and removing common physical simplified assumptions, it represents a step forward with respect to the present models of passive two‐phase systems. Mass, momentum and energy balances account for the thermal and fluid‐dynamics phenomena. Heterogeneous and homogeneous phase changes are directly integrated. In addition, a fitting correlation for the wall/vapour heat transfer coefficient is implemented and tuned against experimental data in order to evaluate the influence of the liquid film on conjugate heat transfer. The resulting numerical tool have been validated against experimental data achieved testing a copper pulsating heat pipe during the 58th ESA Parabolic Flight Campaign in several operative conditions and transient gravity levels. The predicted results show very good matching with the actual thermo‐physical behaviour of the system. Copyright © 2016 John Wiley & Sons, Ltd.