Heat Pipe Applications Research Articles

Experiment study on heat transfer enhancement of micro heat pipe PV/T by Reynolds number improvement

The rapid performance improvement of photovoltaic/thermal (PV/T) implies great potential application to various aspects of society. The previously developed micro heat pipe PV/T by our team has shown favorable cogeneration of electricity and heat in cold regions, while it is badly needed to improve its comprehensive performance by enhancement of heat transfer. For same working fluid as HCFC141b in micro heat pipe, higher Reynolds Number (Re) theoretically means better heat transfer. Therefore, two available micro heat pipes were developed with trapezoidal cross section and rectangular cross section respectively while with same cross section area and applied in two micro heat pipe PV/T systems. Subsequent experiment study testifies that the rectangular micro heat pipe PV/T of higher theoretical Re verily shows more favorable performance than the trapezoidal micro heat pipe PV/T when they are at 45° inclination angle. Deducting power consumption of circulating pump, the average electrical efficiency of rectangular micro heat pipe PV/T system is 12.4%, while that of trapezoidal micro heat pipe PV/T system is 11.9%. Moreover, the thermal efficiency, total energy power, and total energy efficiency of rectangular micro heat pipe PV/T system are respectively 28.1%, 472.3 W, and 40.5%, which are 10.1%, 119.8 W, and 10.8% higher than those of trapezoidal micro heat pipe PV/T system. The results are significant and valuable for the application of micro heat pipe PV/T in cold regions.

Numerical analysis of high temperature potassium heat pipe under marine condition

A heat pipe (HP) is a two-phase heat transmission device with very high effective thermal conductivity, extensively employed in multiple industrial fields. In the nuclear engineering area, numerous experimental and numerical studies are widely produced for its thermal behavior performance in high temperatures. In this paper, a HP that potassium as the working fluid with a capillary wick structure was taken into consideration under marine conditions through numerical simulations. The physical properties of potassium and the additional force method of rolling motion are utilized with user-defined functions (UDFs). This CFD model verified with available experimental results, is employed to investigate the influence of HP surface wall temperature distribution and thermal resistance by several parameters including inclination angle, filling ratio (FR), and rolling period. Besides, the charts of phase change and wall temperature distributions are visible with the help of CFD simulation under static and rolling condition. Therefore, the high temperature HP application of design and evaluation is provided as a reference in present work under marine condition.

Numerical analysis of transient heat and mass transfer for a water heat pipe under non-uniform heating conditions

Non-uniform loads are widely present in heat pipe applications and have significant effects on their performance. In this work, a transient 3D CFD model is developed to investigate the liquid–vapor flow and heat transfer for a water heat pipe under various non-uniform loads. The source terms are added to the governing equations using the User-Defined Function (UDF) to simulate the evaporation and condensation process. The model is verified using the experimental and simulated data under uniform and non-uniform loads in the literature. The effects of key parameters including adiabatic length, heating power, and heat distribution on the thermal performance of heat pipes are analyzed. The results show that the non-uniform conditions would shift the maximum axial velocity of vapor to the unheated side and induce a significant circumferential flow in the wick region, especially in the evaporator section. The short adiabatic length and high heating power could reduce the elapsed time of reaching the steady state. The heat distribution in the evaporator section exhibits little effect on the elapsed time reaching the steady-state and the average steady-state vapor temperature

Experimental study on the influence of surface modification methods of carbon fiber felt on the operation and heat transfer characteristics of loop heat pipes

Under high heat flux, the heat leakage rate of the compensation chamber in a small loop heat pipe is large, which affects the operational stability and heat transfer efficiency of the loop heat pipe. It was proposed for the first time to locally modify the capillary wick thermal conductivity through surface modification, thereby reducing the heat leakage from the compensation chamber. By spraying a high thermal conductivity metal coating on the surface of a low thermal conductivity carbon fiber felt, the contradiction between the heat dissipation rate of the heat source and the heat leakage of the compensation chamber was solved. Static wicking height showed that the metal coating increased the suction force of the carbon fiber felt. Start-up and operation experiments showed that the modified wick significantly improved the start-up stability of the loop heat pipe and reduced the temperature of the heat dissipation surface. The loop heat pipe with a single-sided-coated wick showed the lowest heat dissipation surface temperature, thermal resistance, and heat leakage rate. The loop heat pipe with an aluminum single-sided-coated wick exhibited a heat source temperature of 82.81 ℃, a thermal resistance of 0.26 K/W, and a heat transfer coefficient of 26.44 kW/(m2·K) under a load of 150 W (37.5 W/cm2). This study has significantly contributed to the future application of loop heat pipes in high heat flux electronic cooling.

Start-up and heat transfer characteristics of oscillating heat pipe charged with binary partially miscible organic fluid mixtures

The development of highly-efficient, cost-effective and reliable cooling schemes is still a crucial need for prospective applications in energy- and thermal-related areas, and oscillating heat pipes become a promising option due to their simple structures, high performance and versatilities. In this work, an oscillating heat pipe fabricated by a copper flat-plate evaporator and a capillary copper-tube condenser has been developed, and the experimental study was conducted to investigate the start-up and heat transfer characteristics of the heat pipe device using partially miscible HFE7100-ethanol mixtures at mixing ratios in the range of 1:8 to 8:1 due to the advantage of complementary thermo-physical characteristics and the figure of merit with respect to an OHP. Results demonstrate that the oscillating heat pipe performance was mixing ratio dependent, and there existed optimal mixing ratios of 3.5:1 and 3:1 with respect to the start-up temperature/duration and thermal resistance, respectively. At the optimal mixing ratio of 3.5:1, the time and temperature constants related to the startup performance were reduced by 50.5 s and 5.9 °C, respectively, in comparison with that of ethanol. Besides, the thermal resistance of the oscillating heat pipe charged with HFE7100-ethanol mixture was maximally reduced by 35.5% as compared with that of ethanol at an optimal mixing ratio of 3:1. The complementary thermo-physical properties of partially miscible organic fluids as well as the phase transition suppression and easy transition to unidirectional flow circulation account for the performance improvement. Different from single organic mixtures, the binary organic mixtures exhibited distinctive capabilities to improve the oscillating heat pipe performance, and provides a promising candidate for oscillating heat pipe applications.

A Comprehensive Review and Experimental Investigation on Heat Pipes Application in Electrical Machines

The rapid development of transportation electrification brings unprecedented opportunities and challenges as well to the technological improvement of high-power density motors. As the major restriction of the motor performance, thermal issues attract more attentions recently and the heat pipe (HP), due to its high thermal conductivity and lightweight characteristics, is identified as a promising solution. Though HPs have been attempted in cooling systems of electrical machines, their thermal benefits still need to be fully understood. This article provides a comprehensive review and performs a series of experimental investigations to fill the technology gaps for the HP application in electrical machines. Firstly, the HP technology is briefly introduced, followed by a detailed review on its applications in different locations of electrical machines with corresponding thermal improvements reported. Then, experimental tests are conducted to investigate the correlation between the HPs’ parameters and their cooling effectiveness, including cross-sectional shape, bending angle, fins arrangement and inclination angle. The obtained results and conclusions could provide a guideline for the promotion of HPs in next generation electrical machines for future transportation.

Simulation study of nanomaterials in heat pipe enhanced high power LED heat sink

To facilitate the simulation of heat dissipation of high-power LED lamps, a set of simplified models for tubular heat pipes and carbon nanotube arrays were first established to estimate their thermal conductivity. Then several kinds of radiators were designed. The application of nanomaterials and heat pipes in the heat dissipation of lamps was studied by the finite element method. It is found that the radiator with vertical suspended fins strengthened by a heat pipe and heat dissipation coating has a better heat dissipation effect than other radiators. Finally, the simulation study of thermal interface materials and heat dissipation coatings for lamps and lanterns heat dissipation is carried out. The results show that the thermal interface materials of carbon nanotubes and carbon nano coatings can enhance the heat dissipation performance of lamps.

Thermal performance of an ammonia loop heat pipe using a rectangular evaporator with liquid-guiding channels

In order to enhance the heat transfer capacity of a stainless steel-ammonia loop heat pipe using a rectangular evaporator, liquid-guiding channels were adopted to significantly reduce the liquid flow resistacne in the evaporator wick. An experimental system was established, and extensive experimental studies have been implemented to investigate its startup characteristics, heat transfer capability and thermal resistance. To be specific, the effects of heat load, evaporator attitude and antigravity on the startup performance were evaluated. According to the experimental results, the loop heat pipe showed excellent startup performance, which could realize the startup at small heat loads for different attitudes of the evaporator. Both adverse evaporator attitude and anti-gravity can cause an increase of the operating temperature of the loop heat pipe, due to poor cooling effect of return liquid and increased heat leak from the evaporator respectively. The loop heat pipe could achieve a heat transfer capability up to 300 W, corresponding to a heat flux of 20 W/cm2. The minimum system thermal resistance reached as low as 0.048 K/W at the head load of 140 W. The present study contributes greatly to the future loop heat pipe applications in high heat flux electronics cooling.

Design and analysis of grooved heat pipe

AbstractA heat pipe is a heat conduction program that utilizes both heat permeability and regime shift concepts to transport heat effectively between 2 different lines. A heat pipe is made up of a pipe or tube and a base fluid. In practice, the heat pipe is poured into a mould that is compatible with the cooling media. These devices have found uses in a variety of fields, including space apparatus, solar energy systems, electronic equipment, and air conditioning systems, due to their simplicity of design and ease of manufacture and maintenance. Thermal performance improvement being the major concern in our project we researched different techniques. The heating surface area has a direct impact on heat transfer. Therefore, we have focused on heat enhancement by introducing grooves. Alongside we also considered using different materials for the pipe. At the end of our research, we are going to produce groove structure models with different materials and analyze them using ANSYS software and propose the best structures with highest thermal efficiency for different applications of heat pipes. This is an attempt to increase heat transmission in response to various material and structural changes. Heat transmission is improved with grooved heat pipes as well as heat transmission various with different types materials used in heat pipe.

Design and application of multi-loop capillary heat pipes to cool GAPS silicon detectors

This paper presents the basic design of a thermal control system that can cool the >1000 semiconducting silicon detectors for the General Anti-Particle Spectrometer (GAPS) balloon-borne cosmic antiparticle mission. To identify antiparticles with sufficient energy resolution, it is essential to cool the detectors to operating temperatures below −40°C. To provide such cooling, we developed a novel multi-loop capillary heat pipe (MCHP) which merges gravitational force, capillary force, and self-oscillating driving force to flow a refrigerant effectively in thin tubes. The MCHP is suitable to cool a number of widely distributed detectors with a minimum mass, minimum power consumption, and large thermal conductivity, compared to any other techniques such as traditional pumped circulating systems. Experimental results using a two-meter-scale full-size model verified that the MCHP can meet all the requirements. The unique MCHP technique can be applied to thermally controlling other large particle/nuclear instruments.

Implementation of Artificial Intelligence in Modeling and Control of Heat Pipes: A Review

Heat pipe systems have attracted increasing attention recently for application in various heat transfer-involving systems and processes. One of the obstacles in implementing heat pipes in many applications is their difficult-to-model operation due to the many parameters that affect their performance. A promising alternative to classical modeling that emerges to perform accurate modeling of heat pipe systems is artificial intelligence (AI)-based modeling. This research reviews the applications of AI techniques for the modeling and control of heat pipe systems. This work discusses the AI-based modeling of heat pipes focusing on the influence of chosen input parameters and the utilized prediction models in heat pipe applications. The article also highlights various important aspects related to the application of AI models for modeling heat pipe systems, such as the optimal AI model structure, the models overfitting under small datasets conditions, and the use of dimensionless numbers as inputs to the AI models. Also, the application of hybrid AI algorithms (such as metaheuristic optimization algorithms with artificial neural networks) was reviewed and discussed. Next, intelligent control methods for heat pipe systems are investigated and discussed. Finally, future research directions are included for further improving this technology. It was concluded that AI algorithms and models could predict the performance of heat pipe systems accurately and improve their performance substantially.

Optimization of Process Parameters of Composite Wicked Heat Pipe

Heat pipe is used to extract heat generated from a surface and applicable to cooling of the many components. Some of the main applications of heat pipes are space crafts, computer systems, permafrost cooling, heat exchangers and thermal storage sub systems. In this work, an experiment will be carried out with an indirect heat composite pipe located with different orientation, heat input and mass flow. Thermal resistance, heat transfer coefficient and thermal efficiency are determined, which are applicable for laptop cooling and solar heating. Heat input 25 to 150 watts, with orientation angles 0, 30, 45, 90 degrees, mass flow rate are 0.01kg/sec, 0.02kg/sec and 0.03kg/sec are considered.

Opportunities and Challenges in Passive Thermal-Fluid and Energy Systems

Abstract This article focuses on passive systems that are used in energy and thermal-fluid applications. These passive systems do not have moving parts and are reliable and cost-effective. Fluid motion in these passive devices could be driven by capillary force, gravity, osmotic pressure, and/or concentration gradient. The fundamental mechanisms and limitations of transport phenomena for passive systems are highlighted, followed by their applications in heat pipes, fuel cells, thermal energy storage, and desalination systems. The capabilities of the passive systems are limited by the balance between the driving force and transport resistance. Based on the fundamental understanding of fluid flow and phase change in passive systems, this study proposes associated transport phenomena and quantitative criteria to determine the maximum heat transfer rate, the transport distance, and minimum pore size of wick structures (when relevant) in these passive devices. This article concludes with the discussion of challenges and future opportunities of passive systems.

Numerical simulation of heat pipes in different applications

• Background about the main components of HPs is presented. • Fluid flow and thermal performance of HPs are discussed. • Numerical simulations of HPs in various applications are elaborated. • Critical challenges and recommendations are given. Nowadays heat pipes are considered to be popular passive heat transfer technologies due to their high thermal performance. The heat pipe is a superior heat transfer apparatus in which latent heat of vaporization is employed to transfer heat for an extended distance under a limited operating temperature difference. Numerical simulation of heat transfer devices is a principal step before implementing in real-life applications as many parameters can be tested in cost-and time-effective behaviors. The present study provides a review of the numerical simulations of various heat pipes in different applications such as cooling of electronic components, heating, ventilation, and air conditioning (HVAC), nuclear reactors, solar energy systems, electric vehicles, waste heat recovery systems, cryogenic, etc. Firstly, this work introduces a background about the main components of heat pipes such as an evacuated tube, wick, and working fluid. The fluid flow and thermal performance characteristics of heat pips are discussed, considering the optimum parameters. Finally, the critical challenges and recommendations for future work encountering the broad application of heat pipes are thoroughly studied.

Wicking characteristics of porous media using short copper micro/nanofibers

• Chemical reduction with surface modification enhanced wicking characteristics. • Short copper fibers with 150 nm diameter were used to create porous media. • Combination of two methods estimated contact angle hysteresis in porous medium. • Critical saturation in drying characteristics was high for small pore porous media. Porous media using short copper micro/nanofibers were created and their wicking characteristics of permeability K, capillary performance K/r eff , effective pore radius r eff , and porosity and drying characteristics were evaluated for loop heat pipe (LHP) applications. Different measurement methods were employed to evaluate the characteristics, and the features of each measurement method were discussed. Fibers with diameters of 120 µm, 15 µm, and 150 nm were used. For the wicking characteristics of the 15-µm porous sample, the optimal sintering conditions with respect to the temperature increasing rate, sintering temperature, and holding time were specified. The wicking characteristics were enhanced via chemical reduction, inducing a mechanical surface modification. The differences in the drying characteristics of the prepared porous samples were demonstrated. The wicking characteristics were plotted using K/r eff –1/r eff maps, and the obtained results were compared with those reported in the literature. Porous media with a submicron-scale pore radius were fabricated; these media could transfer heat to more than several meters using an LHP.

Research on the efficiency evaluation of heat pipe PV/T systems and its applicability in different regions of China

As a passive heat transfer system, the heat pipe is becoming an important way to solve the problem of heat loss and heat accumulation in PV/T systems. However, the evaluation of the annual average efficiency of heat pipe PV/T systems in the existing studies does not consider the efficiency under different solar radiation and the difference in the annual solar radiation distribution in different regions. The purpose of this study is to propose a method for the annual average efficiency evaluation of heat pipe PV/T systems (AEHP method), which comprehensively considers the efficiency differences under different radiation and the distribution of solar radiation of different intensities in the typical year. The calculation results are more accurate, which will promote the reasonable evaluation and engineering application of heat pipe PV/T systems. Firstly, the dimensionless models of different radiation and their performance (photovoltaic efficiency, photothermal efficiency, and comprehensive efficiency) are established by experimental tests on the heat pipe PV/T system in this paper. Secondly, the frequencies and frequency of solar radiation in different intervals of the typical year in major cities of China are analyzed statistically, and the radiation frequency method is proposed. Thirdly, the average efficiencies of heat pipe PV/T systems in the existing literature are obtained by statistical analysis. Combined with the radiation frequency method, an annual average efficiency evaluation method of heat pipe PV/T systems (AEHP method) is proposed. The annual average efficiency of heat pipe PV/T systems in different regions is obtained. Finally, the annual heat collection and power generation of heat pipe PV/T systems are calculated based on the AEHP method, and provide a scientific and reasonable evaluation method for the promotion and application of heat pipe PV/T systems.

Improved Thermal Switch Based on an Adsorption Material in a Heat Pipe

For many applications, the possibility of controlling heat flow by “thermal switching” can be very beneficial. In previous work, we presented a novel approach for thermal switching using a water-loaded adsorbent as part of the evaporator of a heat pipe. The basic idea is that the adsorbent releases water upon exceeding a certain evaporator temperature and thus “activates” the heat pipe by providing the working fluid for thermal transport. In this work, we present an improved version of the heat switch. We found that an ordinary copper heat pipe (i.e., thermosyphon) with outer diameters common in heat pipe applications (10 mm) can be used as a base of the heat switch. The reversibility of the switching effect was proven. The location of heat input relative to the adsorbent position was optimized, leading to improved switching ratios of up to 36 as calculated from thermal resistance change between evaporator and condenser. Furthermore, the role of the insulation on thermal resistance and switching ratio was examined and found to be significant.

Performance characterisation and printing parameter modelling of selective laser melting printed capillary wicks

PurposeThe objective of this study is to investigate the feasibility of using selective laser melting (SLM) process to print fine capillary wick porous structures for heat pipe applications and clarify the interrelations between the printing parameters and the structure functional performance to form guidelines for design and printing preparation.Design/methodology/approachA new toolpath-based construction method is adopted to prepare the printing of capillary wick with fine pores in SLM process. This method uses physical melting toolpath profile with associated printing parameters to directly define slices and assemble them into a printing data model to ensure manufacturability and reduce precision loss of data model transformation in the printing preparation stage. The performance of the sample was characterised by a set of standard experiments and the relationship between the printing parameters and the structure performance is modeled.FindingsThe results show that SLM-printed capillary wick porous structures exhibit better performance in terms of pore diameter and related permeability than that of structures formed using traditional sintering methods, generally 15 times greater. The print hatching space and infilling pattern have a critical impact on functional porosity and permeability. An empirical formula was obtained to describe this impact and can serve as a reference for the design and printing of capillary wicks in future applications.Originality/valueThis research proves the feasibility of using SLM process to printing functional capillary wicks in extremely fine pores with improved functional performance. It is the first time to reveal the relations among the pore shapes, printing parameters and functional performance. The research results can be used as a reference for heat pipe design and printing in future industrial applications.

Investigation of a novel natural convection heat sink for LEDs based on U-shaped mini-heat pipe arrays

• A novel heat sink based on mini-heat pipe array is proposed and tested. • Substrate temperature of LED can be reduced below 70 °C when the input power is 100 W. • The effect of heat sink structure on thermal performance is discussed. The increasing use of light-emitting diodes (LEDs) requires advances in heat dissipation. Previous applications of heat pipes for LED cooling faced the challenges of contact thermal resistance and insufficient heat transfer areas. In this study, a novel heat sink based on a mini-heat pipe array (MHPA) is proposed, which can overcome these challenges. The thermal performance of the heat sink was experimentally tested. The substrate temperature can be reduced below 70 °C when the input power is 100 W. MHPA shows good temperature uniformity. When the input power is 100 W and 200 W, the maximum temperature drops of MHPA in the vertical direction are 0.6 °C and 1.1 °C, respectively. A numerical model was established and validated. The effect of the heat sink structure was investigated using numerical methods. A higher fin height and more cross-cuts can enhance heat transfer. When the fin pitch is less than 10 mm, increasing the pitch is beneficial to heat transfer. Appropriate use of variable fins can enhance heat transfer. Orthogonal tests are designed to find the optimal heat sink structure. The optimal structure has a fin pitch of 8 mm, a fin height of 43 mm, a fin thickness of 0.5 mm, 40% of the variable fins, and 6 cross-cuts.

Analysis of working fluids applicable for high-temperature loop heat pipe applications

An objective of this study was to perform an analysis of available working fluids and select those one(s) that will be able to comply with the specific requirements of the ultra-high bypass ratio (UHBR) engine air bleed system and ensure efficient LHP operation. A multi-step approach was applied to analyse more than 700 working fluids and select four potential candidates, taking into account (1) working fluids compliance with EU regulations; (2) working fluids freezing, boiling, and critical points for the operating temperature range; (3) working fluids specific properties that influence the LHP performances. Selected fluids (toluene, acetone, methanol, 1,2-dichlorobenzene) were subjected to accelerated life tests to check their chemical compatibility with AISI 316 stainless steel to be used as the LHP material. Based on the results obtained, the toluene was selected as the working fluid for application in the innovative LHP-based thermal management solution for the UHBR engine air bleed system.