Maximum Heat Transport Capability Research Articles

Thermal performance of a cryogenic parallel heat pipe system

In this study, we experimentally investigated the thermal behavior of a cryogenic parallel heat pipe system, which comprises a parallel arrangement of an N2-heat pipe and Ar-heat pipe or an N2-heat pipe and Ne-heat pipe. The heat pipes in the parallel system have common evaporator and condenser sections. We conducted preliminary experiments to investigate the individual thermal performance of heat pipes considering three working fluids, which formed the basis for analyzing the parallel heat pipe system. The experiments were performed at several condenser temperatures between 27 and 110 K for a wide range of heat loads considering various heat pipe operating states, such as the normal heat pipe operation, local dry-out, and locally frozen states. Although the tested heat pipes, manufactured by Fujikura Electronics (Thailand) ltd., were commercial heat pipes for room temperature applications, which use water as a working fluid, the working fluid was replaced with nitrogen, argon, or neon in the present experiments. The heat pipes used a composite wick comprising axial grooves and sintered metal powders. The experimental results verify that the heat pipe can be utilized even in the local dry-out state or locally frozen state. Moreover, the parallel heat pipe system can enhance the thermal performance with respect to the maximum heat transport capability, which was 29 W at an operating temperature of 87 K, and expand the operating temperature range to 24–150 K. The proposed parallel heat pipe system can be used to improve the cooling system of superconducting magnets and in other cryogenic applications.

A study of thermal performance change of cryogenic heat pipes by wick structures for wide range of working fluid filling ratio

In this research the performances of nitrogen cryogenic heat pipes with different wick structures were compared under the condition of a wide range of heat load and several filling ratios. The heat pipes tested are commercially available ones, which were originally designed for room temperature applications using water as a working fluid. For the present research, working fluid, water, was replaced by liquid nitrogen. Heat pipe samples with three types of wicks, namely, axial grooves (G), sintered metal (S), and the combination of them (SG), were tested for comparing the thermal performance of the heat pipes with different wick structures, which is characterized by the effective thermal resistance Rth . The experimental data of the maximum heat transport capability Qmax was compared with theoretical predictions on the basis of the capillary limit for each wick structure. Under high filling ratio condition, wide range of heat load was supplied to investigate the variation of the thermal resistance including such operations as dry-out state and the state where liquid puddle was formed in the pipe. The result indicates that the thermal performances of heat pipes with three kinds of wick structures are ordered as follows in descending order; SG, S and G wick.

Experimental Investigation of Flat Plate Cryogenic Oscillating Heat Pipe

Recent developments in applications such as high-temperature superconducting magnets, infrared detectors, and electronics components have led to an alarming increase in heat dissipation rate, which now far exceeds the capability of conventional heat pipe cooling systems. This trend is responsible for a recent transition to flat plate oscillating heat pipes. A new flat plate cryogenic oscillating heat pipe (FPC-OHP) has been developed and validated through experimentations. The performance evaluation of FPC-OHP was investigated with temperature measurements. FPC-OHP consisted of evaporator, condenser, and adiabatic section with the dimensions of 93 × 70 × 8 mm3. The FPC-OHP was made of copper alloy and fabricated by a vertical milling machine, having square channels with a hydraulic radius of 0.66 mm. Liquid nitrogen was used as a working fluid with a charge ratio of 60%. Experimental results revealed the maximum heat transport capability up to 300 W. Moreover, the thermal resistance decreased from 0.25 to 0.11 K/W corresponding to an increase in the heat load from 25 to 300 W. The average temperature difference between evaporator section and condenser section reached up to 34 K for 300 W. The measured effective thermal conductivities were found to be 7353 W/m K.

Design and experimental investigation of a neon cryogenic loop heat pipe

Next generation space infrared sensor and detector have pressing requirement for cryogenic heat transport technology in the temperature range of 30–40 K. Cryogenic loop heat pipe (CLHP) has excellent thermal performance and particular characteristics such as high flexibility transport lines and no moving parts, thus it is regarded as an ideal thermal control solution. A neon CLHP referring to infrared point-to-point heat transfer element in future space application has been designed and experimented. And it could realize supercritical startup successfully. Experimental results show that the supercritical startup were realized successfully at cases of 1.5 W secondary evaporator power, but the startup was failed when 0.5 and 1 W heat load applied to secondary evaporator. The maximum heat transport capability of primary evaporator is between 4.5 and 5 W with proper auxiliary heat load. Before startup, even the heat sink temperature decreased to 35 K, the primary evaporator can still maintain at almost 290 K; and the primary evaporator temperature increased at once when the powers were cut off, which indicated the CLHP has a perfect function of thermal switch. The CLHP could adapt to sudden changes of the primary evaporator power, and reach a new steady-state quickly. Besides, some failure phenomena were observed during the test, which indicated that proper secondary evaporator power and heat sink temperature play important roles on the normal operation.

Experimental study on the supercritical startup and heat transport capability of a neon-charged cryogenic loop heat pipe

Neon-charged cryogenic loop heat pipe (CLHP) can realize efficient cryogenic heat transport in the temperature range of 30–40K, and promises great application potential in the thermal control of future space infrared exploration system. In this work, extensive experimental studies on the supercritical startup and heat transport capability of a neon-charged CLHP integrated with a G-M cryocooler were carried out, where the effects of the auxiliary heat load applied to the secondary evaporator and charged pressure of the working fluid were investigated. Experimental results showed that the CLHP could successfully realize the supercritical startup with an auxiliary heat load of 1.5W, and there existed an optimum auxiliary heat load and charged pressure of the working fluid respectively, to achieve the maximum temperature drop rate of the primary evaporator during the supercritical startup. The CLHP could reach a maximum heat transport capability of 4.5W over a distance of 0.6m corresponding to the optimum charged pressure of the working fluid; however, the heat transport capability decreased with the increase of the auxiliary heat load. Furthermore, the inherent mechanisms responsible for the phenomena observed in the experiments were analyzed and discussed, to provide a better understanding from the theoretical view.

Dryout analysis of overloaded microscale capillary-driven two-phase heat transfer devices

Dryout occurrence at high heat input is one of the detrimental factors that limit the thermal efficiency of a phase-change heat transfer device. In this work, we demonstrate that by employing visualization method, the dryout occurrence of an elongated liquid droplet in a transparent evacuated microscale two-phase flow device can be scrutinized. The circulation of liquid from the condenser to the evaporator is driven by the capillary action which is the primary limitation that governs the maximum heat transport capability of the device. When the evaporation rate exceeds the circulation rate of condensate, dryout will take place in the evaporator end. The propagation of dryout lengths can be accurately determined directly from visualization and a more accurate evaluation of the dryout length compared to the conventional method by measuring the axial temperatures has been developed. By quantifying the performance indicators of the cooling device over a wide range of operating conditions, including the underloaded and overloaded operations, the observation of dryout occurrence in this study correlates highly with the anticipated heat transfer characteristics of a phase-change heat transfer device. This study provides essential insights, particularly on the overloaded conditions, to the design of a microscale two-phase heat transfer device.

Thermofluid Characteristics in Microporous Structure With Different Flow Channels for Loop Heat Pipe

The use of two-phase heat transfer devices using capillary action in a microscale porous structure such as a loop heat pipe (LHP) is a promising heat transport technology. This is because they have characteristics of higher heat transfer power and longer heat transport distances with no electrical power compared with conventional heat pipes. The thermal performance of an LHP is governed by the thermofluid behavior in a microscale porous structure called the wick. In this research, high-performance wicks made of polymer have been developed, and their pore distribution and permeability were evaluated. The effects of the vapor channel's shape on the loop's thermal performance have been investigated by calculation and experiment to enhance evaporator performance. A mathematical model of the evaporator considering super heat in the channel, pressure drop across the wick, and two-phase pressure loss on the boundary face between the wick and the evaporator case was newly developed. The experiment was also conducted as a function of the groove shapes. From calculations and test results, it was found that in order to increase the maximum heat transport capability and decrease the operating temperature, the groove should be well distributed.

Operation limitation of an oscillating heat pipe

The oscillating heat pipe (OHP) has been widely studied as a promising heat transfer device, which can efficiently transport heat by thermally exciting oscillating motions. In the current investigation, the operating limitation of an OHP is theoretically studied to determine its maximum heat transport capability. When the heat transfer rate added to the evaporator section reaches a critical point, the vapor generation rate increases resulting in a significant increase in momentum, which can penetrate the liquid plug causing the slug flow to become annular flow. When this phenomenon takes place, the spring–mass system consisting of a train of liquid plugs and vapor bubbles disappears, and the OHP reaches the maximum heat transport capability. Based on this mechanism, a new mathematical model to predict the OHP operating limitation is developed. The results indicate that the operation limitation of an OHP depends on its working fluid, latent heat, operating temperature, turn number, filling ratio, heat pipe dimensions, and liquid plug length.

Thermal Performance of Cooling Enhancement of Miniature Flat Plate Heat Pipe Under Different Angle

AbstractThe possibility of cooling enhancement of flat plate heat pipes (FPHPs) by tilting was examined experimentally in this study. All of the FPHPs were made of Al and were partially filled with acetone. They had the same size of 120 mm (length) by 36 mm (width) by 2.5 mm (thickness) and the same liquid filling ratio of 25.1%. The effects of six tilting angles of -30°, -15°, -10°, 0°, 45°, and 90° were explored. The results showed that the thermal resistance decreased and the effective thermal conductivity increased when the tilting angle was increased. By increasing the tilting angle from 0° to 45° and further to 90°, the maximum effective thermal conductivity increased by a factor of 1.205 from 4561 W/mK to 5497 W/mK and of 1.212 to 5530 W/mK, respectively. The corresponding maximum heat transport capability increased by a factor of 2.89 from 39.8 W to 115 W and of 3.27 to 130 W. Hence, by proper tilting into positve angles, cooling enhancement of the FPHPs can be greatly achieved.

The length and bending angle effects on the cooling performance of flat plate heat pipes

The effects of length and bending angle on the cooling performance of flat plate heat pipes (FPHPs) were examined experimentally in this study. The length effect was explored with four lengths of 80, 150, 200, and 300mm; each FPHP was filled by acetone in cold condition to its optimum filling ratio. The bending angle effect was examined with four angles of 0°, 30°, 60°, and 90°, using the length of 200mm and the volumetric filling ratio of 31.4%. All FPHPs had the same cross sectional area of 50mm (width) by 2.5mm (thickness). Experimental results showed that by increasing the length from 80 to 150mm, to 200mm, and to 300mm, the minimum thermal resistance, Rth(min), increased by the factors of 2.4, 6.0, and 17.9, respectively from that of 0.103K/W of the 80mm FPHP. A rapid increase in Rth(min) occurred around the length of 150mm. For the FPHPs with lengths smaller than 150mm, Rth(min) could be smaller than 0.252K/W. The maximum heat transport capability Qmax decreased quickly from 109.5 to 49.6W (a factor of about 0.452) when the length was increased from 80 to 150mm, and then slowly decreased to the minimum value of 35W (a factor of about 0.318) for the length of 300mm. In contrast, the results of bending angles showed that by increasing the bending angle, the thermal resistance decreased; Rth(min) reduced by a factor of about 3.3 from 0.6207K/W of 0° bending to 0.1885K/W of 90° bending. The corresponding maximum effective thermal conductivity, Keff(max), increased from 1933.4 to 6365.6W/mK and Qmax increased from 45 to 85W. That is, a short FPHP preformed better than those of longer ones, and the thermal performance of FPHPs could be enhanced by proper bending.

Investigation of ultra-thin flattened heat pipes with sintered wick structure

This study proposes a novel sintered wick structure called bilateral arch-shaped sintered wick (BASSW) for the improvement of the ultra-thin heat pipes (UTHPs). The bilateral arch-shaped wick was sintered at the middle of its copper container and the vapor flow channels are located on both sides. The sintered wick was manufactured with several paramount parameters including maximum wick thickness, flattened thickness, and copper powder particle size fully controlled. An experimental apparatus was set up to investigate the thermal performance of the UTHP samples under the impacts of incremental heat loads. The effects of each processing parameter on the thermal performance of the UTHP samples were analyzed and compared with a mathematical model incorporating effects of the evaporation and condensation heat transfer in a copper-water wick. Results indicate that the most critical factor for thermal performance of UTHP is flattened thickness, as it decreases, the heat transport capability drastically decreases and the thermal resistance increases. The maximum wick thickness affects the evaporation thermal resistances by the variation of evaporation area of the liquid–vapor interface, and particle size affects the heat transport capability by the variation of the porosity of the wick structure. The thermal resistances of the evaporator and condenser sections are consistent with the mathematical model before dry out occurs. The total thermal resistances of the UTHP samples range from 0.02 K/W to 0.60 K/W, and the maximum heat transport capability can reach as high as 25 W.

Cooling performance of flat plate heat pipes with different liquid filling ratios

The effects of liquid filling ratios and leakage on the cooling performance of flat plate heat pipes (FPHPs) were examined experimentally in this study. With the size of 150mm×50mm×2.5mm for all Al 6061 FPHPs filled with acetone (99.87% pure), the results showed that the one with the liquid filling ratio of 25% performed thermally the best. The corresponding maximum heat transport capability, minimum thermal resistance, and maximum effective thermal conductivity were about 47W, 0.254K/W, and 3150W/mK, respectively. In contrast, improper vacuum and leakage would decrease the maximum thermal conductivities greatly to about 200–306W/mK and 164W/mK, respectively. The latter was similar to that of an aluminum block and performed the worst among all FPHPs. This situation should be avoided or carefully assessed as the maximum effective thermal conductivity decreased from the best by a factor of about 19.2.

AN EXPERIMENTAL STUDY ON HEAT TRANSPORT CAPABILITY OF A TWO PHASE THERMOSYPHON CHARGED WITH DIFFERENT WORKING FLUIDS

In the present investigation a two phase thermosyphon has been fabricated to investigate the effect of operating parameters on the heat transport capability. The system consists of evaporator section, adiabatic section and condenser section with thermocouples located on the wall of thermosyphon. Electric heater was fixed on the bottom of the evaporator section and water jacket for cooling the condenser was placed on the top of the condenser section of the thermosyphon. The experiments were conducted with three different thermosyphons with inner diameters of 6.7, 9.5 and 12 mm. The variation of heat transport capability of the thermosyphon was studied for the input heat transfer rate ranging from 0 to 1200 W for various filling ratios and with operating temperature from 30 to 70°C. Water, methanol, ethanol and acetone were used as working fluids. The maximum heat transport capability was found to be high for water compared to other fluids such as ethanol, methanol and acetone at the operating temperatures higher than 40°C.

Thermal-Fluid Modeling For High Thermal Conductivity Heat Pipe Thermal Ground Planes

The thermal ground plane is an advanced planar heat pipe designed for cooling microelectronics in high gravitational fields. A thermal resistance model is developed to predict the thermal performance of the thermal ground plane, including the effects of the presence of noncondensable gases. Viscous laminar flow pressure losses are predicted to determine the maximum heat load when the capillary limit is reached. This paper shows that the axial effective thermal conductivity of the thermal ground plane decreases when the substrate and/or wick are thicker and/or with the presence of noncondensable gases. Moreover, it was demonstrated that the thermal-fluid model may be used to optimize the performance of the thermal ground plane by estimating the limits of wick thickness and vapor space thickness for a recognized internal volume of the thermal ground plane. The wick porosity plays a significant role in maximum heat transport capability. A large adverse gravitational field strongly decreases the maximum heat transport capability of the thermal ground plane. Axial effective thermal conductivity is mostly unaffected by the gravitational field. The maximum length of the thermal ground plane prior to reaching the capillary limit is inversely proportional to input power.

Performance and Heat Transfer Characteristics in Loop Thermosyphon (An Experimental Study)

A Loop Thermosyphon has been designed, fabricated and tested to investigate the heat transfer characteristics by two-phase natural convection. The rig was made from copper tubes connected with each other in closed loop including four main parts: vertical evaporator located in the bottom of the device and surrounded by an electrical coil heater as a heat source ,a horizontal condenser located in the upper part of the device cooled by water, vapor rising adiabatic tube and condensate return adiabatic tube. In this research the effect of heat input and filling ratio of the evaporator on the heat transfer coefficient and temperature distribution on the loop surface was verified experimentally using distilled water as working fluid at operating pressure near-atmospheric pressure. The maximum heat transport capability for evaporator was found to be equal to (3424 W/m2 oC). A comparison between the present work results with empirical and theoretical correlations of other researchers showed good agreement.

Fabrication and evaluation of a slicon-based micro pulsating heat spreader

A silicon-based micro pulsating heat spreader (MPHS) was fabricated and its thermal performance was experimentally evaluated. The width and height of the channel are 1 mm and 0.4 mm, respectively. The MPHS has length of 50 mm, width of 15.5 mm, and thickness of 1.5 mm. Ethanol was used as a working fluid. The MPHS achieved maximum effective thermal conductivity of 600 W/m K and a maximum heat transport capability of 4 W. In order to investigate the oscillating flow characteristics, the frequency and the amplitude of the liquid oscillation were measured. As the input power increases, the frequency and the amplitude are increased, which results in an increase in the thermal performance of the MPHS. The effective thermal conductivity was evaluated from a theoretical model in which the thermal performance of the MPHS is assumed to be similar to that of oscillating flow in a channel completely filled with liquid. When the input power is low, the calculated values are in close agreement with the measured values. When the input power is high beyond 2 W, the measured values are slightly higher than the calculated values, which is due to the contribution of heat transport by phase change. The MPHS transports heat mainly by liquid oscillation and the contribution of latent heat transport increases with increasing input power; however, the effect of the latent heat transport is not significant.

Wicking and thermal characteristics of micropillared structures for use in passive heat spreaders

The thermal and hydrodynamic performance of passive two-phase cooling devices such as heat pipes and vapor chambers is limited by the capabilities of the capillary wick structures employed. The desired characteristics of wick microstructures are high permeability, high wicking capability and large extended meniscus area that sustains thin-film evaporation. Choices of scale and porosity of wick structures lead to trade-offs between the desired characteristics. In the present work, models are developed to predict the capillary pressure, permeability and thin-film evaporation rates of various micropillared geometries. Novel wicking geometries such as conical and pyramidal pillars on a surface are proposed which provide high permeability, good thermal contact with the substrate and large thin-film evaporation rates. A comparison between three different micropillared geometries – cylindrical, conical and pyramidal – is presented and compared to the performance of conventional sintered particle wicks. The employment of micropillared wick structure leads to a 10-fold enhancement in the maximum heat transport capability of the device. The present work also demonstrates a basis for reverse-engineering wick microstructures that can provide superior performance in phase-change cooling devices.

Study on a Pulsating Heat Pipe With Self-Rewetting Fluid

This paper discusses a pulsating heat pipe (PHP) using a self-rewetting fluid. Unlike other common liquids, self-rewetting fluids have the property that the surface tension increases with temperature. The increasing surface tension at a higher temperature can cause the liquid to be drawn toward a heated surface if a dry spot appears and thus to improve boiling heat transfer. In experiments, 1-butanol and 1-pentanol were added to water at a concentration of less than 1 wt % to make self-rewetting fluid. A pulsating heat pipe made from an extruded multiport tube was partially filled with the self-rewetting fluid water mixture and tested for its heat transport capability at different input power levels. The experiments showed that the maximum heat transport capability was enhanced by a factor of 4 when the maximum heater temperature was limited to 110°C. Thus, the use of a self-rewetting fluid in a PHP was shown to be highly effective in improving the heat transport capability of pulsating heat pipes.

Experimental investigation of cryogenic oscillating heat pipes

A novel cryogenic heat pipe, oscillating heat pipe (OHP), which consists of an 4 × 18.5 cm evaporator, a 6 × 18.5 cm condenser, and 10 cm length of adiabatic section, has been developed and experimental characterization conducted. Experimental results show that the maximum heat transport capability of the OHP reached 380 W with average temperature difference of 49 °C between the evaporator and condenser when the cryogenic OHP was charged with liquid nitrogen at 48% (v/v) and operated in a horizontal direction. The thermal resistance decreased from 0.256 to 0.112 while the heat load increased from 22.5 to 321.8 W. When the OHP was operated at a steady state and an incremental heat load was added to it, the OHP operation changed from a steady state to an unsteady state until a new steady state was reached. This process can be divided into three regions: (I) unsteady state; (II) transient state; and (III) new steady state. In the steady state, the amplitude of temperature change in the evaporator is smaller than that of the condenser while the temperature response keeps the same frequency both in the evaporator and the condenser. The experimental results also showed that the amplitude of temperature difference between the evaporator and the condenser decreased when the heat load increased.

Study on flow and heat transfer characteristics of heat pipe with axial “ Ω”-shaped microgrooves

A theoretical model of fluid flow and heat transfer in a heat pipe with axial “ Ω”-shaped grooves has been conducted to study the maximum heat transport capability of these types of heat pipes. The influence of variations in the capillary radius, liquid–vapor interfacial shear stress and the contact angle are all considered and analyzed. The effect of vapor core and wick structure on the fluid flow characteristics and the effect of the heat load on the capillary radius at the evaporator end cap, as well as the effect of the wick structure on the heat transfer performance are all analyzed numerically and discussed. The axial distribution of the capillary radius, fluid pressure and mean velocity are obtained. In addition, the calculated maximum heat transport capability of the heat pipe at different working temperatures is compared with that obtained from a traditional capillary pressure balance model, in which the interfacial shear stress is neglected. The accuracy of the present model is verified by experimental data obtained in this paper.