Loop Heat Pipe System Research Articles

A lean approach of modeling the transient thermal characteristics of Loop Heat Pipes based on experimental investigations

A mathematical model capturing the steady-state and transient thermal characteristics of a Loop Heat Pipe (LHP) is presented. It is intended to provide valuable insights into the static and dynamic LHP system behavior, to be used e.g. for the design of sophisticated thermal control strategies by LHP end users. The model is set up in such a way, that its parameters are both physically meaningful and accessible to a determination based on a non-destructive and pragmatic experimental characterization of the considered LHP. The transient behavior of the LHP is predicted by the model based on separate calculation and update of the internal mass flow rates and phase distributions. It has been applied successfully to two different LHPs, as used in common satellite applications. Numerical results of the steady-state characteristics show an excellent agreement with corresponding experimental results, proving the reliability and transferability of the approach proposed. An in-depth transient analysis of one selected LHP is presented using characteristic heat load profiles, common for LHP space applications.

Liquid–vapor phase behavior and operating characteristics of the capillary evaporator of a loop heat pipe at start-up

The liquid–vapor phase distribution and displacement in the capillary evaporator of a loop heat pipe (LHP) are key phenomena affecting the steady-state and transient operating behavior. This study conducts optical observation of liquid–vapor phase behavior in the evaporator using a transparent glass tube heated by far-infrared radiation during start-up of the LHP. A quartz wick-acetone LHP system is designed, fabricated, and operated successfully with a heat flux of 1 W/cm2 transported by the cylindrical evaporator. To investigate effect of the liquid–vapor phase distribution in the evaporator at the initial state, start-up experiments with eight phase patterns—which are either liquid-saturated or two-phase in each of the inlet of the vapor line, grooves, and compensation chamber (CC)—are performed. The phase distribution affects the temperature profile although it has no effect upon the steady-state characteristics, and nucleate boiling occurs in the wick or groove simultaneously with evaporator-temperature drop when both the grooves and the inlet of the vapor line are completely filled with liquid. Liquid–vapor displacements from start-up to the steady-state are classified into four transient behaviors, which include boiling in the CC and meniscus oscillation at the three phase contact line within the case, wick, and groove.

Energy-saving analysis of a case data center with a pump-driven loop heat pipe system in different climate regions in China

It is important to cool data centers for improving IT equipment reliability through indoor temperature control. The air conditioning energy consumption is almost 38% of the total energy consumption. Free cooling in cool days is a promising technology for energy conservation. A pump-driven loop heat pipe (PLHP) system for free cooling was developed and installed in a data center project, which has a high energy efficiency ratio (EER) compared to air conditioners. The running performance of the PLHP system in the case project was tested and studied. The fitting rule of EER and outdoor temperature was obtained. The hourly energy savings and payback periods of the PLHP system for different cities of different climate regions in China were analyzed basing on the fitting formula compared to air conditioners. A national distribution map was developed to show the annual energy saving ratio in China. Results show that the PLHP system provides remarkable energy savings and the indoor temperature is remained at 18°C to 25°C. About 74.2% of Chinese cities lie in regions suitable for applications, and the annual energy saving ratio is over 30%. The average payback period for most cities in China is about 3.9 years.

Thermal-fluidic transport characteristics of bi-porous wicks for potential loop heat pipe systems

With the miniaturization of electronic components, thermal management of electronic devices and systems becomes a crucial problem. A Loop Heat Pipe (LHP) is a promising solution for many contemporary and future thermal management problems. A LHP is a passive, flexible and robust two-phase heat transfer device which utilizes the latent heat of phase-change to transfer heat and the capillary force generated by porous wick to circulate the working fluid. The porous wick is the heart of LHP which strongly controls its thermal performance. The main aim of the present research is to develop a bi-porous wick and analyze its thermal performance. Bi-porous copper wicks are fabricated with cold press sintering using naphthalene as the pore former. The effect of pore former content and sintering temperature on porous characteristics has been studied. It is observed that porosity and permeability increase with an increase in pore former content and decreases with increase in sintering temperature. The local heat transfer characteristic of the porous wick is studied using Infrared Thermography. Both infrared thermography and digital videography show that the liquid-vapor interface in the porous wick moves away from the heated fin as heat load increases. A change in slope of temperature profile in the porous wick is observed at the liquid-vapor interface. The evaporative heat transfer coefficient at fin-wick interface decreases with increase in heat load.

Experimental investigation of the operating characteristics of a hybrid loop heat pipe using pump assistance

A loop heat pipe (LHP) is one of the two-phase cooling technologies used in passive cooling systems. The LHP is an efficient heat transfer device, but its extreme power density can cause dry-out at the evaporator. Many researchers have predicted that passive devices will not be able to meet future cooling challenges because of this limitation. The objective of this research is to design a modified LHP that overcomes the dry-out problem by adding a diaphragm pump to accelerate fluid transportation (called a hybrid loop heat pipe, HLHP). The pump installed on the liquid line is coupled with a reservoir. The developed HLHP works passively using wick capillary pressure when there is no sign of dry-out. When dry-out occurs, the pump is activated via diaphragm pumping and has a temperature controller. Thus, the working fluid is circulated by both the capillary force and driving force of the diaphragm pump during the heat-transfer process. The operating characteristics of the HLHP under a variety of heat load supply and low-power start-up conditions have been investigated. The experimental results indicate that the installation of a diaphragm pump in a modified LHP system can prevent the occurrence of dry-out in the evaporator and significantly reduce the evaporator temperature.

Study of self-rewetting fluid applied to loop heat pipe with PTFE wick

It’s the first using self-rewetting fluid with PTFE wick structure applied to Loop Heat Pipe System. PTFE wick structure has low thermal conductivity, low sintering temperature thus low cost and high anti-oxidation production for long term using which can also overcome heat leakage when operating. It effectively improves the performance of traditional metal wick structure. However, with the hydrophobic of PTFE material, we can’t directly use water as working fluid. Therefore, this paper adds butanol in water to form self-rewetting fluid which not only easily goes through PTFE wick structure but spontaneously supplements the heating surface, making system operates successfully.After testing the surface tension and heat-transfer performances of LHP system, it shows that LHP system begins to work by adding 3wt% butanol aqueous solution. It has the maximum heat capacity of 400W, the minimum thermal resistance 0.32°C/W and heat flux 20W/cm2 when increasing to 6wt% butanol aqueous solution. It’s also the first point making LHP system operate successfully. Compared to the water-nickel system, butanol-PTFE system halving the total thermal resistance, heat flux increases over 50%, and the lower operating temperature and pressure, thus it can have higher stability and security.

A Passive and Remote Heat Transfer Solution for Avionics Thermal Management

A heat pipe utilizes liquid–vapor phase change mechanism to efficiently transfer heat. Among different heat pipes, loop heat pipe (LHP) and pulsating heat pipe (PHP) are known to be capable of high heat flux/high load heat transfer. In this article, LHP and PHP heat transfer systems are combined to achieve passive, reliable, and remote/long-distance heat transfer for thermal management of modern avionics systems. Aiming at this goal, a 2 m long LHP is developed to transport heat from the avionics chassis to the remote heat rejection site. To reduce inner saturation pressure and ensure structural safety at high operating temperature, water is used as the operating fluid in LHP. Within the avionics chassis, conduction heat transfer is enhanced by sandwiching a PHP with two printed circuit boards (PCBs) and solder-bonding them. Each PHP/PCB assembly is 20 cm long and 12.5 cm wide, with electrical heaters mounted on both sides to mimic electronic heat dissipation. Heat transfer demonstration of the LHP and PHP combo system is conducted in a lab environment with input power varying from 100 to 400 W. For all the three PHP/PCB assemblies set in the avionic chassis, heat source temperature is maintained below the required 150 °C even when heat dissipation is twice as high as the state-of-the-art (and coolant temperature is 50 °C). This combo heat transfer system reduces power consumption and increases reliability, enabling the avionic system operation in harsh environments.

Heat Transfer in a Loop Heat Pipe Using Fe2NiO4-H2O Nanofluid

Nanofluids are stable suspensions of nano fibers and particles in fluids. Recent investigations show that thermal behavior of these fluids, such as improved thermal conductivity and convection coefficients are superior to those of pure fluid or fluid suspension containing larger size particles. The use of enhanced thermal properties of nanofluids in a loop heat pipe (LHP) for the cooling of computer microchips is the main aim of this study. Thus, the Fe 2 NiO 4 -H 2 O served as the working fluid with nanoparticle mass concentrations ranged from 0 to 3 % in LHP for the heat input range from 20W to 60W was employed. Experimental apparatus and procedures were designed and implemented for measurements of the surface temperature of LHP. Then, a commercial liquid cooling kit of LHP system similar as used in experimental study was installed in real desktop PC CPU cooling system. The test results of the proposed system indicate that the average decrease of 5.75oC (14%) was achieved in core temperatures of desktop PC CPU charged with Fe 2 NiO 4 -H 2 O as compared with pure water under the same operating conditions. The results from this study should find it’s used in many industrial processes in which the knowledge on the heat transfer behavior in nanofluids charged LHP is of uttermost importance.

TEMPERATURE CONTROL WITH TWO PARALLEL SMALL LOOP HEAT PIPES FOR GLM PROGRAM

The concept of temperature control of an electronic component using a single Loop Heat Pipe (LHP) is well established for Aerospace applications. Using two LHPs is often desirable for redundancy/reliability reasons or for increasing the overall heat source-sink thermal conductance. This effort elaborates on temperature controlling operation of a thermal system that includes two small ammonia LHPs thermally coupled together at the evaporator end as well as at the condenser end and operating parallel. A transient model of the LHP system was developed on the Thermal Desktop (TradeMark) platform to understand some fundamental details of such parallel operation of the two LHPs. Extensive thermal-vacuum testing was conducted with two thermally coupled LHPs operating simultaneously as well as with only one LHP operating at a time. This paper outlines the temperature control procedures for two LHPs operating simultaneously with widely varying sink temperatures. The test data obtained during the thermal-vacuum testing, with both LHPs running simultaneously in comparison with only one LHP operating at a time, are presented with detailed explanations.

Interface engineering to enhance thermal contact conductance of evaporators in miniature loop heat pipe systems

While miniature loop heat pipes (mLHP) have significant potential for electronic cooling, they are only used in a narrow niche of applications, such as space or military. Complicated fabrication and system integration leading to high cost devices are the main culprit. To this end, this paper explores a low-cost sintering method for fabricating evaporators for mLHP that have increased heat transfer performance. Through this method, the porous wick of the evaporator is fabricated to partially fill the vapor collection channels embedded in the base plate of the evaporator. The sintering method employs an organic material used to define the vapor collection channels, which is sublimated at the end of the sintering process. Interpenetrating these two, otherwise distinctive, parts of the evaporator results in an increased contact area and thermal conductance. The heat transfer performance of an mLHP employing the new evaporator is compared to that of a system using a standard evaporator configuration, where the porous wick is rested against a flat base plate. It is found that the thermal contact conductance increases about 25%, depending on the applied heat load, while the total thermal resistance of the mLHP with the new evaporator decreases approximately by a factor of two.

ADVANCED ANALYSIS OF AN AMMONIA LOOP HEAT PIPE FOR SPACE APPLICATIONS. PART I: THE NUMERICAL MODEL

A novel and advanced thermal model is proposed for nonstationary simulations of a loop heat pipe (LHP) with varying the heat power loads and the boundary heat flux conditions. Integrated with the time-varying conditions and thermal coupling between the LHP systems and the orbit environment, a system-level model was built in order to understand the temperature characteristics of LHP systems during the mission time. Since one critical problem for the thermal simulations of space components is linked to the long computational time, which has the effect that the numerical design and virtual prototyping for the thermal optimization is restricted to very few operating conditions, the main purpose of the research is to build an adaptive and fast model for an LHP, based on a thermal model built using the lumped parameter code SINDA/FLUINT, in order to get CPU times lower than the real times of the running device cooled by the given LHP. The procedure may be easily extended to future implementation of LHP models in other space-dedicated software, such as ESATAN.

Condenser design optimization and operation characteristics of a novel miniature loop heat pipe

Loop heat pipe (LHP) is a promising means for electronics cooling since LHP is a exceptionally efficient heat transfer device. In this paper, a novel miniature LHP (mLHP) system is presented and optimal design of condenser is considered seeing that evaporators have been able to handle very high-heat fluxes with low-heat transfer resistances since most of the previous researchers focused on the evaporator of mLHP. The arrayed pins were designed and machined out on the bottom of condenser to enhance condensation heat transfer. The parameters of the arrayed pins, including layout, cross-section shape and area, were optimized by finite element analysis. Tests were carried out on the mLHP with a CPU thermal simulator using forced air convection condenser cooling to validate the optimization. The operation characteristics of the mLHP with optimal design parameters of condenser were investigated experimentally. The experimental results show that the mLHP can reject head load 200W while maintaining the cooled object temperatures below 100°C, and for a variable power applied to the evaporator, the system presents reliable startups and continuous operation.

Dynamic performance of a façade-based solar loop heat pipe water heating system

This paper reported a dedicated study of a novel façade-based solar loop heat pipe (LHP) water heating system using both theoretical and experimental methods. This system employs a modular panel incorporating a unique loop heat pipe that is able to serve as part of the building façade or a decoration layer of the façade, thus creating a façade integrated, low cost, highly efficient and aesthetically appealing solar water heating structure. Taking into account heat balances occurring in different parts of the system, e.g., solar absorber, heat pipes loop, heat exchanger and storage tank, a dedicated computer model was developed to investigate the dynamic performance of the system. An experimental rig was also established to evaluate the performance of such a prototype system through measurement of various operational parameters, e.g., solar radiation, temperatures and flow rates of the heat pipe fluid and water. Through comparison between the testing and modelling results, the model has been approved to be able to give a reasonable accuracy for predicting the performance of the LHP system. Two types of glass covers, i.e., double glazed/evacuated tubes and single-glazing plate, were applied to the prototype. It was found that for both covers, the heat pipe fluid temperature rose dramatically at the start-up operation and afterwards remained a slow but steady growth; while the water temperature remained a steadily growing trend throughout the operational day. The temperature rise of the circulated water at 1.6l/min of flow rate was around 13.5°C in the double-glazed/evacuated tubes based system and 10°C in the single-glazing based system; correspondingly, their average solar conversion efficiencies were 48.8% and 36%, and the COPs were 14 and 10.5 respectively. In overall, the double-glazed/evacuated tubes based system presented a better performance than the single glazing based one.

Analytical study of the heat transfer limits of a novel loop heat pipe system

A novel loop heat pipe system was designed for use in solar hot water heating and an analytical model was developed to investigate its thermal performance and determine six major limits to system operation, i.e. capillary limit, entrainment limit, viscous limit, boiling limit, sonic limit, and filled liquid mass limit. Relations among the limits and several associated parameters, i.e. the heat pipe operating temperature, wicks type, heat pipe diameter, and height difference between the absorbing pipes array and condenser (heat exchanger), were established through a comprehensive analyses. It was found that the levels of capillary, entrainment, viscous, sonic, and filled liquid mass limits increased with the increasing temperature; however, the boiling limit was in the adverse trend. It was also found that the mesh screen wicks were able to obtain a higher capillary limit than sintered powder wicks, whilst other limits remained same. Larger pipe diameters would lead to higher operating limits. The height difference between the condenser (heat exchanger) and absorbing pipes (absorber) was the most important factor impacting on heat transfer capacities of the system, and largely affected the capillary limit of the system. It was noted when the pipe (inner) diameter increased to 5.6 mm or above, the governing limit of the system switched from entrainment to capillary. Relationship between the system governing limit, i.e. capillary limit, and the above addressed parameters were analysed. Adequate system configuration and operating conditions were suggested, which were summarized as follows: 6 mm of pipe inner diameter with mesh screen wicks, 58°C of heat pipe operating temperature, and 1.3 m height difference between absorber and condenser (heat exchanger). Copyright © 2010 John Wiley & Sons, Ltd.

The thermal control system for a network mission on Mars: The experience of the Netlander mission

The Netlander mission wants to establish an operating network of stations on the surface of Mars. Each one of four identical landers is equipped with science payloads dedicated to study the atmosphere and geosphere of Mars; operating together their objective is to investigate the Martian meteorology, ionosphere, ground and subsurface. Landing locations spread over two hemispheres and a mission duration of one Martian year, expose the surface modules and its sensitive electronics to a wide range of hostile conditions. Additional constraints come from the transporting spacecraft, where heat can be exchanged only across small interfaces. The purpose of the thermal control system is to maintain nevertheless the electronics and battery temperatures within a narrow band. Contrasting demands of reduced heat leaks and effective dump of surplus heat require new technologies and advanced design concepts to be satisfied under strict mass limits imposed. The paper describes the design, development and testing activities of a thermal control concept including high-performance insulation combined with an innovative loop heat pipe system. Extensive thermal analyses have been run and hardware has been built, qualified and tested. Results of the test are fairly positive, even in presence of some problematic issues. A post-fit numerical simulation has been initiated but further developments are needed.

NETLANDER thermal control

The intention of the N ETL ANDER mission, is to establish for the first time a Network of stations on the surface of Mars. Four identical surface modules are equipped with science payloads dedicated to study the atmosphere and geosphere of Mars at four different landing locations spread over the two hemispheres. The mission duration will be one Martian year. The surface modules and their sensitive electronics compartments have to withstand a wide range of hostile conditions on Mars. Further constraints are given during flight, where heat can be exchanged only across small interfaces. The purpose of the N ET L ANDER thermal control system is to maintain the electronics and battery temperatures within a narrow band. Contrasting demands of reduced heat leaks and effective dump of surplus heat require new technologies and advanced design concepts to be satisfied under strict mass limits imposed. Recently, the first thermal test model with the original thermal equipment has been completed and tested. The model includes a high performance insulation combined with an innovative loop heat pipe system integrated into a one-to-one lander-structure. The paper describes the design and development activities as well as the ground test campaign performed in simulated Martian environment.

Heat Transfer Performance of a Flexible Looped Heat Pipe Using R134a as a Working Fluid. A Proposal of a Method for Predicting the Maximum Heat Transfer Rate of FLHP.

This paper presents the heat transfer performance of a Flexible Looped Heat Pipe (FLHP) using R 134 a as a working fluid. In our evaluation system, an evaporator and a condenser are connected by long flexible tubes with diameter of 3 mm, and the total piping length of this loop heat pipe system is approximately 7500 mm. We selected porous teflon with effective pore diameter of 2γcw=1.2μm to overcome high gravitational heads. Elevation of the evaporator above the condenser (△H=He-Hc) were changed in 3 conditions ((1) Top heat mode (△H=+1m), (2)Horizontal mode (△H=0), (3)Bottom heat mode ((△H=-1m) considering the terrestrial application and the influence of gravity on the FLHP performance was investigated. FLHP provided high thermal transport capacities over long distances through small cross-sectional flexible tubes comparing with conventional heat pipes.

95/03996 Heat pipe science and technology

Preface Nomenclature 1.Introduction 2.Solid-Liquid-Vapor Phenomena, Driving Forces and Interfacial Heat and Mass Transfer 3.Steady Hydrodynamic and Thermal Characteristics 4.Heat Transfer Limitations 5.Continuum Transient and Frozen Startup Behavior of Heat Pipes 6.Two-Phase Closed Thermosyphons 7.Rotating and Revolving Heat Pipes 8.Variable Conductance Heat Pipes 9.Capillary Pumped Loop and Loop Heat Pipe Systems 10.Micro/Miniature Heat Pipe Characteristics and Operating Limitations 11.Heat Pipe Heat Exchangers 12.Analysis of Nonconventional Heat Pipes 13.Special Effects on Heat Pipes 14.Heat Pipe Fabrication, Processing, and Testing Appendix A:Thermophysical Properties Appedix B:Experimental Heat Pipe Results Index