Maximum Heat Transfer Capability Research Articles

Effect of heat load to a capillary starter pump on the thermal characteristics of a 2-m nitrogen cryogenic loop heat pipe

A thermal characteristic of a nitrogen-charged cryogenic loop heat pipe (CLHP) with an auxiliary loop and a capillary starter pump (CSP) was investigated in this study. One of the advantages of CLHPs is their high heat transfer capability over a long distance. However, there are only few studies on CLHPs with heat transfer lengths over 1 m. The effect of the CSP heat loads on the heat transfer characteristics has not been studied for the long-distance CLHPs, so far. To determine them, a 2-m nitrogen CLHP operating at 80 K–100 K was developed. The designed evaporator and CSP had stainless steel porous wicks with a pore radius of 1.0 µm and a permeability of 1.3 × 10−13 m2. The lengths of the vapor line and liquid line were 2000 mm. Experiments were conducted with CSP heat loads of 0, 2, and 4 W under horizontal condition. Based on the results, the CLHP successfully demonstrated a supercritical start-up when the heat loads to the evaporator and CSP were 4 W. The CLHP stably operated when evaporator heat loads of 4–24 W were applied while the operating temperature was within 80 K–86 K. The maximum heat transfer capability was 24 W for the CSP heat loads of 0, 2, and 4 W. On the other hand, the thermal resistance tended to be higher as the CSP heat load increased. The minimum CLHP thermal resistance was 0.064 K/W when the CSP heat load was 0 W. In addition, the effect of CSP heat load on the temperature difference between two compensation chambers and between the CSP and the condenser was discussed.

Fabrication and characterization of ultra-thin vapour chambers with printed copper powder wick

With the increasing heat flux required in portable electronic devices, ultra-thin vapour chambers (UTVCs) have attracted increasing attention as efficient phase-change heat-transfer components. In this work, a sintered copper powder wick fabricated through a screen-printing process is proposed, providing a novel method for manufacturing large-area and complex-shaped UTVCs. UTVCs with a thickness of 0.55-mm and printed copper powder wicks were fabricated, and the copper powder paste preparation and UTVC manufacturing processes are described in detail. The copper powder paste was made by mixing dipropylene glycol monomethyl ether and copper powder with a particle size of 65–75 μm in a 5:1 ratio. The thickness of the copper layer after sintering was 0.2 mm and its porosity was 59.7%. The heat transfer performance of UTVCs under different liquid-filling ratios and test states was investigated. The results showed that the optimal liquid-filling ratio of the presented UTVCs is 43.2% and that its maximum heat-transfer capability is 6.5 W with a thermal resistance of 0.46 °C/W in the horizontal state. The thermal resistance of the UTVC in the anti-gravity state is more than twice that of the UTVC in the horizontal state. The proposed manufacturing process provides a new solution for the research of the thinner VC and the patterned wick structures.

Optimization design with an advanced genetic algorithm for a compact air-air heat exchanger applied in aero engine

This work presents an optimal design method for a compact air-air serpentine tube heat exchanger (STHE), which is applied as the core component of the bearing sealing cooling system in the aero engine. A new optimization tool, coupling with the Logarithmic Mean Temperature Difference (LMTD) method and an advanced genetic algorithm (AGA) using Gray code, is proposed to optimize the STHE with unknown outlet parameters under variable working conditions. Under the given performance and structural constraints, six optimization design variables, including tube outer diameter, the transverse tube pitch, the longitudinal tube pitch, the transverse tube rows, the height and the number of elbows, are selected to obtain the maximum heat transfer capability KA per total weight. The optimization results show that, by using AGA, the target parameter distribution is narrowed from 88.31~93.86 kW/(K•kg) to 91.90~93.92 kW/(K•kg), and the proportion of the optimal solutions higher than 93 kW/(K•kg) increases from 22.62% to 94.32%. The optimization also improves the heat transfer capability KA per total weight to 93.92 kW/(K•kg), comparing with the original value of 70.08 kW/(K•kg). As the key limiting factors, the hot side maximum outlet temperature and pressure drop in the optimized STHE do not appear at the same operation mode and both of them are quite close to the given limits. It well demonstrates the validity and applicability of the proposed optimization design method using under multiple restrictions and conditions.

Experimental investigation on thermal characteristics of a capillary pumped loop with different reservoir locations

This paper presents an effect of reservoir location on heat transfer characteristics of a capillary pumped loop (CPL). The designed CPL has a cylindrical evaporator with a diameter of 12 mm and a length of 65 mm. A stainless steel porous medium that has a pore radius of 4.4 μm is used as a wick. Heat transport length is designed as 1000 mm. The experiments were conducted with four types of reservoir location such as (1) Distance from the evaporator, L = 150 mm, (2) L = 250 mm, (3) L = 750 mm, (4) L = 1450 mm. As results, it was found that the CPL of case (1) showed high operating temperature and much smaller heat transfer capability such as 10 W. For cases of (2) – (4), the maximum heat transfer capability reached 140 – 160 W and showed the good controllability of the operating temperatures ranging from 60 to 100°C. The reason for the low heat transfer performance of case (1) and the difference in operational characteristics are discussed. It was found that there is a design limitation of the reservoir location of CPLs. In addition, the operation of the CPL that reservoir was located much far from the evaporator was successfully demonstrated.

Liquid phase enhanced sintering of porous aluminum for cylindrical Al-acetone heat pipe

In this study, porous aluminum is successfully sintered on the inner surface of a slim aluminum pipe utilizing liquid phase enhanced sintering method, and cylindrical heat pipe is successfully fabricated using acetone as the working fluid. The process mainly includes sintering, acetone filling, vacuuming, secondary degassing and argon welding. The sintering experiments indicate that pure aluminum powders are poorly sintered; however apparent sintering necks between aluminum powders are formed with copper particles added. Moreover, aluminum powders agglomerate and average pore size increases obviously when copper mass fraction reaches 4.5 wt%. Thermal performance test results of Al-acetone heat pipe demonstrate that the temperature nonuniformity is small and the maximum heat transfer capability is 7 W in horizontal orientation. The total thermal resistance of the heat pipe is 0.6 °C/W within the heat load range of 3–7 W.

Experimental study on the effect of various heat source configurations on the thermal performance of an axial swallow-tailed micro-grooved heat pipe

An experimental testing station with the constant circular water-cooling system is assembled to study the heat transfer characteristics of an axial swallow-tailed micro-grooved heat pipe subjected to heating with discrete heat sources. Influence of different heat source arrangement under single and double heat source working condition on performance parameter of heat transfer are investigated. Experimental results indicated that the temperature of the existing heat source is apparently higher than the temperature of the non-existent heat source. The condensing section length affects the transient thermal response of heat pipe, and the steady wall temperature decreases with an increase in condensing section length. Different configurations of heat source will affect the maximum heat transfer capability of the heat pipe, and the maximum heat transfer capability is relevant to the distance between the heat source and heat sink. The smaller the distance between the heat source and the heat sink, the greater the maximum heat transfer capability of the heat pipe. If the heat pipe is not heated from the starting point, the heat pipe can enter a steady state when it reaches maximum heat transfer capability, but the temperature at the heat source is much higher than the normal working temperature. The temperature response in front of heat source is slower than that in other position when heat pipe is not heated from the initial end of evaporating section.

Thermal performance enhancement of micro-grooved aluminum flat plate heat pipes applied in solar collectors

Micro-grooved aluminum flat plate heat pipes (MFPHPs), fabricated by hot extrusion and subsequent inner surface treatment via chemical corrosion, were developed to improve the thermal efficiency and reduce the costs of solar collectors. Thermal performances of MFPHPs, including temperature distribution, maximum heat transfer capability, and thermal resistance, were experimentally conducted. The effects of treatment time and solution concentration on the thermal performance enhancement of MFPHPs were also investigated. The experimental results show that inner surface treatment can substantially enhance the thermal performance of MFPHPs, and different treatment morphologies of inner surfaces result in differences in thermal performance enhancement. The optimal treatment parameters were determined to be a treatment time of 10 min with a solution concentration of 1.5 moL/l. This resulted in the optimal thermal performance enhancement: an increase of approximately 80% in heat transfer capability and a decrease of more than 44% in thermal resistance, compared to the untreated MFPHP. This study provides a convenient, effective, and low-cost method to enhance the thermal performance of MFPHPs applied in solar collectors.

Experimental study on effect of wick structures on thermal performance enhancement of cylindrical heat pipes

The effect of varying wick structures viz. mesh, sintered and composite wick (sintered-mesh) on the thermal enhancement of cylindrical heat pipes is experimentally investigated. In addition, the investigation focused on the effect of inclination angle and heat input of heat pipe. Surfactant free CuO nano-fluid with a mass concentration of 1.0% is used as a working fluid. The energy and exergy analysis of heat pipe was also conducted at various conditions. To analyze the distinctive performance of composite heat pipe, a heat pipe is filled with DI water and the obtained results are compared with nanofluid results. The maximum heat transfer capability of composite heat pipe is improved by 35.71% and 18.75% compared with mesh and sintered wicks. The composite heat pipe with CuO nanofluid as working fluid instead of DI water improves the heat transport capacity by 11.76%. Surface temperature of heat pipe significantly reduces by varying the wick structure viz. mesh, sintered and composite wick. The composite heat pipe with 1.0 mass% of CuO nanofluid obtained 3.7 °C reduction in surface temperature at evaporator section compared with DI water. Thermal resistance of heat pipe is gradually reduces with increasing inclination angle. The maximum reduction is observed for composite wick, sintered and mesh wick heat pipes are 47.50, 43.70 and 24.39% respectively at 45° inclination angle compared with horizontal axis.

A novel oxidized composite braided wires wick structure applicable for ultra-thin flattened heat pipes

This present study proposed a novel low-cost and high performance braided wire wick structure having superhydrophilic treatment applicable for ultra-thin heat pipe. The test wick structures include mono and composite wick structure. The composite braided wire adopts two different diameter wire design, using a large diameter (0.1mm) in the core while the exterior of the core is covered by smaller diameter copper wire (0.05mm). The mono design contains only one uniform wire diameter (0.1mm) in core region. Test results indicate that via oxidizing the surface of the braided wires can largely enhance the surface roughness and result in a much higher capillary force as comparing to the un-oxidized one. In addition, both mono (m-FHP) and composite braided wire (c-FHP) with oxidization could possess a maximum heat transfer capability over 15W under horizontal operation, and test results showed that the heat transfer capability of composite design outperforms mono one by more than 32.5% in all orientations. It is also found that the filling loading ratio plays essential role on the overall performance of FHP. Either a low filling loading ratio or a high filling loading ratio may yield a lower maximum heat transfer capability and a higher thermal resistance. An optimum filling loading ratio of 125% yields the lowest thermal resistance around 0.12K/W with the best heat transfer capability of 20W.

Multi-objective optimization of geometrical parameters of corrugated-undulated heat transfer surfaces

To achieve a maximum heat transfer capability and a minimum pumping power for corrugated-undulated (CU) heat transfer surfaces, a multi-objectives genetic algorithm was used to obtain the optimal values of the pitch and height of undulated plate (U-plate) and the height of the corrugated plate (C-plate) by using the Pareto optimal strategy. For this purpose, computational fluid dynamics (CFD) simulation, support vector machine (SVM) and the fast non-dominated sorting genetic algorithm were combined together and used for the optimization process. Three dimensional numerical simulations were performed to investigate the effect of geometrical parameters on the thermos-hydraulic performance of CU heat transfer surface. The maximum deviation for the Nusselt number and friction factor between the simulation and the published data were 8.81% and 13.1% respectively when the Reynolds number ranged from 1500 to 10,000. The flow and temperature profile in the CU passage were analyzed. Intensive secondary flows occurred in the C-plate channel and the U-plate channel due to the drag effect between the main flows in the two channels. And the effects of Reynolds number and structure parameters were studied. The change of U-plate height rather than that of U-plate pitch would have a dominant effect on the disturbance influence of U-plate. Besides, two SVM models were trained by the CFD results to predict the Nusselt number and friction factor of flow in CU passages with different geometrical and operational parameters. The comparison between the SVM predictions and the CFD results showed that the SVM models could predict the numerical data with a good accuracy. In addition, two evaluation criteria were proposed from perspectives of the manufacturers and the users, respectively. Finally, a set of optimized solutions were obtained. The optimal values of pumping power ratio and heat transfer area ratio between different CU passages and the standard one were in the range of 0.8–3.1 and 0.5–1.2, respectively. The manufacturers and the users can select the best design points according to their considerations.

Theoretical analysis of screened heat pipes for medium and high temperature solar applications

A mathematical model is applied to study the cylindrical heat pipes (HPs) behaviour when it is exposed to higher heat input at the evaporator for solar collector applications. The steady state analytical model includes two-dimensional heat conduction in the wall, the liquid flow in the wick and vapour hydrodynamics, and can be used to evaluate the working limits and to optimize the HP. The results of the analytical model are compared with numerical and experimental results available in literature, with good agreement. The effects of heat transfer coefficient, power input, evaporator length, pipe diameter, wick thickness and effective pore radius on the vapour temperature, maximum pressure drop and maximum heat transfer capability (HTC) of the HP are studied. The analysis shows that wick thickness plays an important role in the enhancement of HTC. Results show that it is possible to improve HTC of a HP by selecting the appropriate wick thickness, effective pore radius, and evaporator length. The parametric investigations are aimed to determine working limits and thermal performance of HP for medium temperature solar collector application.

Fabrication and thermal performance of grooved-sintered wick heat pipe

Some novel grooved-sintered composite wick heat pipes (GSHP) were developed for the electronic device cooling. The grooved-sintered wicks of GSHP were fabricated by the processes of oil-filled high-speed spin forming and solid state sintering. The wick could be divided into two parts for liquid capillary pumping flow: groove sintered zone and uniform sintered zone. Both of the thermal resistance network model and the maximum heat transfer capability model of GSHP were built. Compared with the theoretical values, the heat transfer limit and thermal resistance of GSHP were measured from three aspects: copper powder size, wick thickness and number of micro grooves. The results show that the wick thickness has the greatest effect on the thermal resistance of GSHP while the copper powder size has the most important influence on the heat transfer limit. Given certain copper powder size and wick thickness, the thermal resistance of GSHP can be the lowest when micro-groove number is about 55.

H143 ファイバーウィッグを用いた高性能薄型ピートパイプの開発(OS-8: 電子機器・デバイスの熱課題)

In the present experimental and theoretical study, main purpose is to find out the performance limiting factors of thin heat pipe with new fiber wick structure. As the limiting factors we mainly focused on Cu pipe structure, wettability, capillary force, liquid flow area, vapor flow area and mechanical damage. In this paper, relation between the thermal performance of heat pipes with different thickness and width are showed. With the effective length of heat pipe is 125mm; maximum heat transfer capability of 0.8, 1.2, 1.5 and 2.0mm thick heat pipes are 7W, 15W, 25W, 34W and 42W respectively. Also, the theoretically achieved relation between the total pressure drop, liquid flow area and vapor flow area with different Cu pipe structure is shown.

Application of high temperature heat pipe in hypersonic vehicles thermal protection

In order to develop further the application of high temperature heat pipe in hypersonic vehicles thermal protection, the principles and characteristics of high temperature heat pipe used in hypersonic vehicles thermal protection were introduced. The methods of numerical simulation, theory analysis and experiment research were utilized to analyze the frozen start-up and steady state characteristic of the heat pipe as well as the machining improvement for fabricating irregularly shaped heat pipe which is suitable for leading edge of hypersonic vehicles. The results indicate that the frozen start-up time of heat pipe is long (10 min) and there exists large temperature difference along the heat pipe (47 °C/cm), but the heat pipe can reduce the temperature in stagnation area of hypersonic vehicles from 1 926 to 982 °C and work normally during 1 000–1 200°C. How to improve the maximum heat transfer capability and reduce the time needed for start-up from frozen state of the heat pipe by optimizing thermostructure such as designing of a novel wick with high performance is the key point in hypersonic vehicles thermal protection of heat pipe.

Thermal Characteristics of Heat Pipe with Axially Swallow-tailed Microgrooves

A thermal model for a heat pipe with axially swallow-tailed microgrooves is developed and analyzed numerically to predict the heat transfer capacity and total thermal resistance. The effect of heat load on the axial distribution of capillary radius, and the effect of working temperature and wick structure on the maximum heat transfer capability, as well as the effect of the heat load and working temperature on the total thermal resistance are all investigated and discussed. It is indicated that the meniscus radius increases non-linearly and slowly at the evaporator and adiabatic section along the axial direction, while increasing drastically at the beginning of the condenser section. The pressure difference in the vapor phase along the axial direction is much smaller than that in the liquid phase. In addition, the heat transfer capacity is deeply affected by the working temperature and the size of the wick. A groove wick structure with a wider groove base width and higher groove depth can enhance the heat transfer capability. The effect of the working temperature on the total thermal resistance is insignificant; however, the total thermal resistance shows dependence upon the heat load. In addition, the accuracy of the model is also verified by the experiment in this paper.

Experimental study on the performance limitation of micro heat pipes of non circular cross-sections

An experimental study of three different cross-sections (circular, semicircular and rectangular) of micro heat pipes having same hydraulic diameter (D= 3mm) is carried out at three different inclination angles (0?, 45?, 90?) using water as the working fluid. Evaporator section of the pipe is heated by an electric heater and the condenser section is cooled by water circulation in an annular space between the condenser section and the water jacket. Temperatures at different locations of the pipe are measured using five calibrated K type thermocouples. Heat supply is varied using a voltage regulator which is measured by a precision ammeter and a voltmeter. It is found that thermal performance tends to deteriorate as the micro heat pipe is flattened. Thus among all cross-sections of the pipes circular cross-section exhibits the best thermal performance followed by semicircular and rectangular cross-sections. Moreover maximum heat transfer capability of the pipes also decreases with decreasing of its inclination angle. A correlation is developed using all the gathered data of the present study to predict the heat transfer coefficient of micro heat pipes of different cross-sections placed at different inclination angles.

Prediction of the maximum heat transfer capability of two-phase heat spreaders – Experimental validation

The present paper is devoted to an experimental study to determine the thermal behaviour of a two-phase heat spreader (TPHS) with micro-grooves. The proposed application is the cooling of fuel cell systems. This TPHS aims at reducing the volume of actual cooling systems and to homogenize the temperature in the hearth of fuel cells. The TPHS is flat with a wide evaporating area (190 × 90 mm 2) compared to the condenser area (30 × 90 mm 2). It has been tested with three working fluids: water, methanol and n-pentane. Experimental results obtained with methanol show a temperature difference lower than 1.6 K on the entire evaporator area for a heat transfer rate equal to 85 W and a working temperature equal to 70 °C. The TPHS has been tested in both horizontal and vertical favourable orientation (thermosyphon orientation). The temperature field is similar in both cases for heat transfer rates lower than 155 W. In horizontal orientation, a confocal microscope is used to measure the meniscus curvature radius along the grooves. A two-phase flow model allowing the calculation of the meniscus radius, the liquid and vapour pressures and the liquid and vapour velocities along the TPHS is developed. The comparison between experimental and model results shows the good ability of the numerical model to predict the meniscus curvature radii from which the maximum heat transfer capability of the TPHS is depending.

Coupled thermal and hydrodynamic models of flat micro heat pipes for the cooling of multiple electronic components

An analytical solution for both the liquid and vapour flows inside a flat micro heat pipe (MHP) coupled to an analytical solution for the temperature inside the MHP wall is presented. The maximum heat transfer capability of a flat MHP, on which several heat sources and heat sinks are located, is calculated. The capillary structure inside the MHP is modeled by considering a porous medium, which allows to take into account capillary structures such as meshes or sintered powder wicks. The thermal model is able to calculate the part of heat flux transferred only by heat conduction in the MHP wall from the heat transferred by change of phase.