Flat Heat Pipe Research Articles

The efficiency of using simple heat pipes with a relatively low thermal conductivity for cooling transmit/receive modules

In this paper, we solved the problem of increasing the cooling efficiency of the basic version of the transmit/receive (T/R) module with a heat sink tilted in its working position while increasing its thermal power by a factor of 2 (from 112 W to 224 W). The purpose of the work is to confirm the correctness of the proposed strategy of cheap modernization of the T/R module through the use of inexpensive flat gravity heat pipes (HP) with a threaded evaporator and a relatively low effective thermal conductivity (3000 W/(m•°C)). The novelty of the work is the substantiation by the results of comparative numerical simulation of the possibility and efficiency of using such HPs in the basic version of the cooling system for T/R module and the determination of the critical geometric parameters and relationships necessary for this. It is shown that the integration of 16 flat HPs of a simplified design into the base of the radiator made it possible to reduce the maximum temperature non-uniformity of the mounting surface at an air speed of 1 m/s by more than two times and reduce the maximum temperature of the hottest transistor at 20.5 °C. The temperature non-uniformity of 8 transistors decreased from 10.5 °C to 1.6 °C. The minimum thermal impedance of the HP cooling system (0.06 °C/W) is half that of the base case. The correctness of the proposed strategy of cheap modernization of T/R modules is confirmed and the prospects of using HPs of a simplified design with working fluids with a freezing temperature below 0 °C are shown, which allows expanding the temperature range of operating conditions. The originality of the modernized cooling system and its world novelty are confirmed by the patent for the invention.

Cooling of power electronic devices using rectangular flat heat pipes with externally and internally cooled condenser regions

In traditional heat pipes employing external cooling, the internal flow pattern undergoes a transition to annular flow. In this stage, vapor moves through the central region of the heat pipe, while a liquid film develops on the inner wall surface. This liquid film acts as a thermal barrier, impeding the condensation of vapor within the vapor core area, which reduces the coolant's ability to condense vapor effectively. To address this issue, a rectangular flat heat pipe with an internal condenser (RFHP-IC) has been designed and tested for heat loads up to 360 W with different fill ratios. The RFHP-IC incorporates a tube-in-tube structure within the condenser section of the heat pipe, facilitating the condensation of the working fluid while maintaining a compact form factor. Results indicate that employing an internal condenser reduces thermal resistance and evaporator wall temperature by 67.5% and 20.1% respectively when compared to an external condenser. Furthermore, the RFHP-IC allows vapor to make direct contact with the cold surface of the internal condenser, increasing the condensation rate by 8.5% in comparison to an external condenser under the same heat load. The findings of this study can improve the performance, capacity, and reliability of RFHPs, making them more suitable and efficient solutions for thermal management applications.

Thermal performance comparison of flat plate pulsating heat pipes of different material thermal conductivity using ethanol-water mixtures

A flat plate pulsating heat pipe (FPPHP) consists of mini/micro capillary channels machined on a flat plate in a serpentine manner. The working fluid oscillates in the form of liquid slugs and vapor plugs transferring heat. The slug-plug oscillations, internal flow regimes, startup heat requirement, thermal resistance, and evaporator dry-out depend on the thermal conductivity of the flat plate. High thermal conductivity material has larger heat spreading through it with a possibility of pressure homogenizing in the system, low working fluid oscillations, and early dry-out of the working fluid. No study has compared the thermal conductivity of the different plate materials suitable with various working fluids in an FPPHP, and the present work fills this gap in the literature. In this work, the thermal characteristics of a high thermal conductivity copper and a comparatively low thermal conductivity steel FPPHPs are experimentally investigated and compared using different ethanol–water mixtures as the working fluid. Results are reported for pure water and ethanol and mixtures of ethanol:water in the ratio 1:1, 3:1, 5:1, 9:1, and 30:1 as the working fluids, filled at 60%. The power inputs are given from 40 to 200 W. The ethanol–water mixed in an ideal ratio compared to pure water or ethanol as the working fluid gives smaller thermal resistances, larger slug-plug oscillations, and lesser evaporator drying-out occurrences. The working fluid oscillations mainly influence the total thermal resistance in the steel FPPHP, and the fluid resistance is the minimum at all power inputs. In copper FPPHP, the spreading (dry) resistance through the plate primarily influences the total thermal resistance. At the optimum ethanol–water mixture ratio of 9:1, steel FPPHP has lower thermal resistance and larger slug velocities than copper FPPHP in the wide range of power inputs of 60 to 140 W. The present work on the effect of thermal conductivity and binary mixtures on the thermal performance of the FPPHP will help to design suitable FPPHP for different applications.

Comparison of the energy performance of novel dual-temperature cooling systems: From field testing to simulations

Air conditioners (AC) face the challenge of low energy efficiency in field operation. Considering that the performance of dual-temperature systems remains unexplored, especially in different climate zones, an AC with a flat heat pipe and different temperature controls was tested in an experimental chamber. The energy efficiency increased by 14% compared to that of an AC with a dehumidification mode. Furthermore, three novel systems that optimize the dual-temperature cooling process are proposed. These include an AC with a twin-cylinder compressor, AC with medium/low-pressure ejectors, and AC with high/low-pressure ejectors. The performance of these ACs was analyzed under different conditions. The maximum EER of the four ACs was 9.13, 9.91, 9.54, and 10.68, respectively, for an evaporation temperature of 10 °C and different condensation temperatures and part-load ratios. The nominal EER of the three dual-temperature systems increased by 9.4, 6.8, and 19.7%, respectively, with respect to the conventional system. The seasonal energy efficiency ratio of the three dual-temperature systems increased by approximately 5.8–18.8%, 6.3–19.9%, 6.0–18.8%, and 3.9–17.6% in four different climate zones, respectively. Our results indicate that the twin-cylinder compressor system has the highest potential for applications in high-efficiency cooling systems.

Design and Thermal Performance of Ultra-Thin Flat Plate Heat Pipes with Composite Wick for Electronic Cooling

The rapidly increasing heat load and high-density packaging of electronic devices demand heat dissipation devices that possess miniaturization and high thermal performance. A series of ultra-thin flat plate heat pipes (UPHPs) with thicknesses of 1.2 mm was prepared to satisfy this demand. Strengthening rib optimization and capillary optimization were processed to restrict the entrainment limit and capillary limit of the UPHPs. Furthermore, surface modification technology with composite nanostructure coated was introduced to improve the surface wettability of capillary wick. Finally, the heat transfer capacity of the UPHPs was experimentally tested. The influence of capillary structure and liquid filling ratio on the heat transfer characteristic was investigated in detail. This work provides insight into the design and application of UPHPs for the new generation of electronic devices.

Study on heat transfer characteristics of ultra-thin flat heat pipes at different liquid filling rates and heat loads

The boiling characteristics of flat heat pipes greatly affect their heat transfer performance, but the research on micro-scale ultra-thin steam chamber is still unclear. The heat transfer performance of ultra-thin plate heat pipe (UTFHP) was investigated by analyzing the boiling heat transfer characteristics under various operating conditions through both theoretical analysis and visualization experiments. The findings demonstrate that the heat transfer process of UTFHP can be classified into three stages based on changes in bubble behavior and heat transfer characteristics. Additionally, we conducted a comprehensive examination of the heat transfer mechanism. The study identifies key factors that affect bubble heat transfer in UTFHP, including temperature, pressure, working fluid properties, and gas–liquid interface area. The impact of heating power on the merging point and turning point of the UTFHP is discussed, while the liquid filling rate is found to have an insignificant effect. The start-up time of the UTFHP is delayed by the liquid bridge, which can be improved by introducing multiple air bubbles and liquid bridges. The study also investigates factors affecting the heat transfer capacity of UTFHP during stable operation, revealing that as heating power increases, the temperature of the evaporator and condenser increases linearly, while the thermal resistance initially increases, decreases, and then increases.

Effect of binary fluid on the heat transfer performance of flat plate micro heat pipe

A flat plate micro heat pipe (FMHP) is a passive thermal management device widely used in chip heat dissipation. In this paper, by analyzing the heat transfer principle of FMHP, we found that the heat transfer limit of FMHP is positively related to the latent heat of evaporation of the working medium, and the methanol binary fluid containing deionized water can increase the latent heat of the working medium. Therefore, four heat pipes with different concentrations of binary fluids of methanol and deionized water as the FMHP work medium and pure methanol as the working medium was fabricated, and a platform was built to test the five heat pipes. The results show that the 80% methanol content in binary fluid flat plate micro heat pipe(BF80-FMHP) has the maximum thermal power and exhibits better heat transfer characteristics than the 100% methanol content in binary fluid flat plate micro heat pipe (MF100-FMHP), with a 12.0% increase in maximum thermal power and a 15.1% decrease in dynamic response time. The 60% methanol content in binary fluid flat plate micro heat pipe(BF60-FMHP) has the fastest dynamic response time, 30.42% lower than the MF100-FMHP, but the maximum thermal power is 10.0 W lower than that of the MF100-FMHP. The proper concentration of binary fluid can effectively increase the maximum thermal power of FMHP, increase the working range, improve the heat transfer capability of FMHP at high temperatures, and enhance the dynamic response time at high power.

Experimental investigation of the thermal performance of high-temperature flat heat pipe with a single heat source and a multi heat source

This study proposed a sodium high-temperature flat heat pipe (HTFHP) and conducted a series experimental research. The experiment compared the impact of different heating power and heat source arrangement on the startup performance and temperature distribution of the HTFHP. The experimental conditions involved varying heating power and different heat source layouts, including the use of either one or two heat sources. Results indicate that the HTFHP could start stably when the input power was 450-600 W. The single heat source started the fastest at an input power of 600W, with a startup time of 893 s. The multiple heat source started the fastest, with a start time of 533 s, when both sides had an input power of 600 W. The startup time shortened as the input power increased. At high input power, the temperature distribution of the HTFHP was more uniform at different positions, and the minimum temperature difference between the evaporating section and the condensing section was 31℃. In multiple heat source condition, the temperature at the evaporating section fluctuated slightly except at the heat source points. The findings indicated that HTFHP has good startup performance, can quickly dissipate heat, and has great potential for application.

Experimental study on the characteristics of loop heat pipe with modified carbon fiber felt wick

In order to achieve accurate and effective thermal management of high-performance electronic components, the performance of a small flat loop heat pipe was investigated. A flexible composite capillary wick material has been applied in a flat loop heat pipe. Surface modification of carbon fiber felt was carried out using arc spraying technology to make it a more suitable material for producing capillary wicks. The physical properties of the modified carbon fiber felts were determined by measuring their wettability, porosity, capillary force, and thermal conductivity. A small flat loop heat pipe assembled with carbon fiber felt wick was designed and fabricated, and experimental testing was conducted. The experimental data indicated that the performance of methanol was better than that of acetone and water. The loop heat pipe with a 60% filling ratio maintained the heat source temperature at 84.47 °C under a load of 120 W (30 W/cm2). Conditions where the heat source starts before the cooling system are more favorable for the quick and stable startup of the loop heat pipe. The experimental results demonstrated that the loop heat pipe with a modified carbon fiber felt wick exhibited excellent operational and heat transfer performance.

Effect of Working Fluid, Orientation, and Cooling Mode on Thermal Performance of Miniature Flat Heat Pipe

Abstract Flat heat pipes (FHPs) are commonly used as a passive cooling system in portable electronic gadgets due to their compact profile. The present study investigates the effect of different working fluids on the thermal performance of a miniature FHP under different orientations and condenser cooling mechanisms and the start-up performance of FHP. Deionized water, acetone, ethanol, and methanol are chosen as working fluids in the FHP. Five different inclinations (0 deg (horizontal), 30 deg, 45 deg, 60 deg, and 90 deg (vertical)) and two different condenser cooling methods (natural convection and forced convection with fan cooling) are considered in this experimental study. The FHP thermal performance is quantified in terms of overall temperature difference, thermal resistance, and effective thermal conductivity. The results indicate that comparatively higher effective thermal conductivity values are obtained for methanol and acetone heat pipes at low heat loads and under natural convection. At higher heat loads, the ethanol heat pipe had higher effective thermal conductivity values for the same condenser cooling method. For the case of the forced convection cooling mode, the methanol heat pipe had enhanced thermal performance as compared to the other three fluids for all heat load ranges and different inclinations. Due to the higher boiling point of water, as a working fluid water is not suitable in most of the experimental trials except at high heat load under forced convection cooling and in a horizontal orientation. The maximum effective thermal conductivity of 7846 W/mK is obtained for FHP filled with methanol at 24 W heat load and 90 deg orientation under forced convection condenser cooling.

Effect of evaporation and condensation section length ratio on thermal performance of aluminum flat plate heat pipe with different micro grooved wicks

The length of the evaporation or condensation section has a significant impact on conventional heat pipe or copper flat heat pipe heat transfer performance. Based on this demand, a novel concept of the length ratio of evaporation and condensation section (LREC) of the flat plate micro grooved heat pipe (FPMHP) was proposed and the effect of the LRECs on the thermal performance of aluminum-based FPMHP with different micro grooved wicks was experimentally investigated for the first time. The experiment explored the effects of nine different LRECs on the thermal performance of FPMHP with the same section size, filling mass, rectangular and triangular micro grooved wicks installed at 90°, a constantly increasing input heat flux, and forced air cooling. The findings demonstrate that rectangular micro grooved wick flat heat pipe (FPMHP-r) and triangular grooved wick flat heat pipe (FPMHP-t) have the shortest startup times when LREC is 1.0, with FPMHP-r starting 70 s sooner than FPMHP-t. When 0.8 ≤ LREC ≤ 1.25, the heat transfer performance of FPMHP-t and FPMHP-r is excellent, better than performance when LREC < 0.8 or LREC > 1.25, and the LREC is recommended to be 0.8 ∼ 1.25 in practical applications. The optimal LREC value of aluminum-based FPMHP is 1.0, which can maximizes the heat transfer performance. Under the same conditions, the heat transfer performance of FPMHP-r is better than FPMHP-t, flat heat pipe wicks with high permeability, low porosity and effective pore radius should be designed in practice. The results can provide a theoretical basis for the design and optimization of aluminum-based FPMHP application.

Heat transfer performance of the L-shaped flat gravity heat pipe used for zero-carbon heating houses

The L-shaped flat gravity heat pipe (LFGHP) is applied to the building envelope to effectively improve solar heat gain and achieve zero-carbon heating due to its ultrahigh equivalent thermal conductivity (ke) and thermal diode effect. In this paper, the forward and reverse heat transfer models of LFGHP are developed and experimentally verified. An experimental platform is built to experimentally study the thermal start-up time and the key factors on forward heat transfer, including the working fluid and filling ratio, the bending angle, the inclination angle, the length ratio of the condenser to the evaporator, the cooling temperature, and the heat flux. The ke of an LFGHP in practical use during a whole day is obtained. The results show that: (1) The heat transfer model of LFGHP can be regarded as three series thermal resistances in forward heat transfer and three parallel thermal resistances in reverse heat transfer, and the thermal diode effect is experimentally verified, with a rectification ratio over 170. (2) The acetone heat pipe with a volume filling ratio of 10% is of the highest ke and is the optimum combination of working fluid and filling ratio. (3) The ke in forward heat transfer increases with the decrease of bending angle and inclination angle, and increases with the increase of heat flux, cooling temperature, and length ratio of the condenser to the evaporator. (4) The thermal start-up time shortens with the increase of heat flux and inclination angle and can be neglected in applications. This study is helpful for the selection and design of the LFGHP used in solar houses and has guiding significance for the improvement and optimization of the practical applications of zero-carbon heating houses.

Comprehensive optimization design of ultra-thin flat heat pipe based on pressure-bearing and heat transfer performance

With the development of high-power consumption and miniaturization of electronic devices, the high heat flux in limited space requires thinner thickness and better heat transfer performance of ultra-thin flat heat pipe (UFHP). Due to the decrease of UFHP thickness, the thickness of wall and vapor chamber, and the arrangement of support structure have completely different effects on pressure-bearing and heat transfer performance. Therefore, in order to reasonably design the structure of UFHP, it is necessary to consider both the pressure-bearing and heat transfer capacity. In this study, the deformation, thermal resistance and pressure drop of UFHP with support strips are analyzed. It is found that with the decrease of vapor chamber thickness, under the constraint of the maximum wall deformation, the rapid increase of thermal resistance and pressure drop caused by thin thickness could be reduced by optimizing the vapor chamber size. The size optimization model of UFHP based on pressure-bearing and heat transfer performance is established to guide the design of UFHP with support strips.

A dumbbell-shaped 3D flat plate pulsating heat pipe module augmented by capillarity gradient for high-power server CPU onsite cooling

With the development of the data center industry and the increasing power density of servers, it is crucial to remove waste heat from small-area heat sources to ensure the safe and efficient operation of data processing chips. To complement existing studies, in this paper, a novel three-dimensional flat plate aluminum pulsating heat pipe (3D-FPPHP) with four different channel structures for radial heat dissipation of high-power server chips was proposed. The results showed that under the favorable influence of the additional capillarity force provided by the variable diameter channels, the FPPHP filled with acetone can start up easily and has good robustness during variable power operation. When the input power was 150 W, the junction temperature of the heating source could be reduced by 42.49 °C (48.67%) compared to an identical aluminum plate. In addition, a heat sink module based on the proposed 3D-FPPHP with optimal charging ratio was tested in a 1U server chassis. The results showed that it can manage an input power of 300 W (corresponding to a heat flux of 41.15 W/cm2) and achieve a total thermal resistance of 0.2053 °C/W under the premise of ensuring a safe junction temperature of chips below 85 °C.

A coupled model and thermo-electrical performance analysis for flat heat pipe-based battery thermal management system

Power battery is one of the key components of the electric vehicles, and an efficient battery thermal management system (BTMS) is significant for improving power battery performance. Flat heat pipe (FHP) has lightweight structure and high thermal conductivity, receiving much more focuses in the field of BTMS. In this paper, an FHP-based BTMS configuration is proposed. A coupled model of FHP and batteries was established considering the vapor flow effect of the FHP working medium, which can calculate the battery electrochemical parameters, battery heat generation rate, FHP heat transfer capacity in real time. The coupled model was verified by experiment. Then, the battery thermo-electrical performance at different discharging conditions is simulated and analyzed, and the relationship between battery electrochemical parameters and discharge rate is reflected. Under 3C discharge rate, the battery maximum temperature can be limited under 50 °C, while the maximum temperature difference can be kept below 2.26 °C. Finally, the influence mechanism of different FHP structural parameters including vapor chamber thickness, FHP total thickness, FHP total length on battery thermo-electrical characteristics is investigated. The results show that the decrease of FHP vapor cavity thickness or FHP total thickness will not only increase the vapor thermal resistance, but also aggravate the uneven distribution of FHP heat transfer capability, worsening the battery thermal performance on this occasion. The uneven battery module temperature distribution will further lead to non-uniformity of battery electrical performance. The change of FHP total length will result in the change of the total FHP heat transfer thermal resistance and affect the battery overall performance.

Experimental and numerical studies of thermal transport in a latent heat storage unit with a plate fin and a flat heat pipe

Experimental and numerical studies of thermal transport in a latent heat storage unit with a plate fin and a flat heat pipe

Comparative experimental study on series–parallel heat transfer characteristics of flat heat pipes

In this study, a thermal performance test rig for heatpipes with different connection modes is independently designed to explore the steady-state and transient thermal performance of series–parallel flat heat pipes under different heat source powers. The dynamic thermal performance changes of three parallel modules, 1A2,2A1,3A2, and three series modules, 12A,21A,23A, under different power conditions are compared and analysed. The experimental results show that the total thermal resistance of the parallel modules is the lowest, followed by the total thermal resistance of the traditional module, and the total thermal resistance of the series modules is the highest. The minimum total thermal resistance of the parallel module is 0.02 K/W. At the 23.6 W input power, the parallel heat dissipation has the lowest heat source temperature of 74 °C and the slowest heat source heating rate of 3.2 °C/min in comparison with other heat dissipation methods. Further, under the parallel connection condition, the standard deviation of condenser and evaporator of the No. 2 heat pipe is reduced by 0.45 and 6 respectively, indicating that the sintered flat heat pipe has better temperature equalization than the grooved flat heat pipe. Different from the other two dissipation methods, the maximum temperature of the heat pipe in parallel heat dissipation is only 57.2 °C, which can satisfy the heat dissipation requirements of the heat source. It is important to mention that the parallel heat dissipation method of the flat heat pipe proposed in this investigation can realize the double-sided arrangement of heat sources of multiple components and accomplish the rapid heat transfer process while increasing the heat flux exponentially, which provides technical support for solving the problem of high heat flux concentration of multi-chip components.

A novel flat heat pipe for anti-gravity orientations: Leaf fractal evaporator and bi-directional transport capillary channel

Flat heat pipes (FHPs) could be an innovative solution for thermal management in aerospace applications; without the aid of gravity, FHPs could still transport high density energy fluxes with superior temperature control and almost zero energy consumption. Inspired by the powerful transpiration and liquid transport ability of leaf vein structure of plants, a novel FHP design with bionic grading evaporator structured surface was proposed. In addition, a bi-directional transport capillary structure was built inside this FHP, reducing the vapor–liquid flow path and enhancing condensed liquid return simultaneously, and thereafter promoting phase change intensity of FHPs. In the present research, heat transfer performance of FHP with self-wetting fluid as a coolant has been experimentally investigated under anti-gravity, concerning on heat inputs, filling ratios, diameter ratios, porosities of hybrid capillary wick, and two representative tree-like evaporator plate structures (H type and Y type). Our experimental results indicated that heat transfer capability of FHP showed a minimal temperature difference, which was suitable for multi-gravity working conditions. Interestingly, minimal thermal resistance value of 0.45 ℃/W and average enhancement ratio in heat transfer coefficient of 56.39% were achieved simultaneously at anti-gravity orientations for a novel FHP with the 70 PPI and αMF = 0.2 of hybrid capillary wick. Superior vapor flow and diffusion and permeability capability of FHP were further obtained as filling ratio of 30%. Moreover, due to the Marangoni effect, the addition of SRWFs could reduce the axial and radial thermal resistance of the FHP by 5–7 % under anti-gravity condition. Compared with traditional heat sink, the novelty of this passive heat transfer application was capable for realizing high heat transport performance for aerospace flights and applications.

Study of the effect of tilt angle on the vaporization processes in a flat gravity heat pipe with a threaded evaporator

Study of the effect of tilt angle on the vaporization processes in a flat gravity heat pipe with a threaded evaporator

Simulation and Optimization of Lithium-Ion Battery Thermal Management System Integrating Composite Phase Change Material, Flat Heat Pipe and Liquid Cooling

A large-capacity prismatic lithium-ion battery thermal management system (BTMS) combining composite phase change material (CPCM), a flat heat pipe (FHP), and liquid cooling is proposed. The three conventional configurations analyzed in this study are the BTMSs using only CPCM, CPCM with aluminum thermal diffusion plates, and CPCM with FHPs. In addition, a CPCM–FHP assisted with liquid cooling at the lateral sides is established to enhance the thermal performance of large-capacity batteries. Moreover, the influences of coolant temperature, the number of FHPs and cooling pipes, and the coolant direction on the temperature field of a BTMS are discussed. Finally, the orthogonal design method is used for the multi-level analysis of multiple factors to improve the light weight of the system. The optimal parameter combination is obtained to achieve the best thermal performance of the BTMS, with the maximum temperature and the temperature difference at 43.17 °C and 3.36 °C, respectively, under a maximum discharge rate of 2C and a high-temperature environment of 37 °C. The optimal scheme is further analyzed and affirmed through the comprehensive balance method.