Loop Heat Pipe Design Research Articles

Novel integration of dual evaporator loop heat pipe for improved electric vehicle battery thermal regulation

The Battery Thermal Management System (BTMS) plays a crucial role in ensuring the efficient and safe operation of Hybrid Electric Vehicles (HEVs) and/or Electric Vehicles (EVs). Among various heat transfer techniques, Loop Heat Pipes (LHPs) have demonstrated their potential for effective heat transfer from Li-ion batteries due to their high heat transfer rates over a longer distance. The conventional design of LHPs poses challenges for constructing robust battery pack modules, particularly in electric vehicles (EVs). This complexity arises from the separation of liquid and vapor lines at either the top or bottom of the evaporator, along with the inclusion of a compensation chamber. Furthermore, the evolving requirements of BTMS highlight the necessity for efficient heat removal from multiple locations. To address these requirements, a novel Dual Evaporator Loop Heat Pipe (DE-LHP) has been developed as a BTMS solution featuring separate vapor and two liquid lines positioned at the uppermost section of each cylindrical evaporator. The developed DE-LHP is tested as a BTMS solution with a 3S4P battery module (10Ah capacity) at different charging and discharging rates (1C, 1.5C, and 2C) in surrounding temperatures ranging from 30 °C to 45 °C. When the thermal performance of the DE-LHP-equipped BTMS was compared to a module without BTMS, a drop in maximum cell temperature was noted. At an ambient temperature of 35 °C, the DE-LHP BTMS achieved temperature reductions of 22.8 %, 22.6 %, and 18.8 % at charging rates of 1C, 1.5C, and 2C, respectively. In addition, at a threshold temperature of 60 °C, the module with DE-LHP BTMS demonstrated enhanced discharging capability from 41 % State of Charge (SOC) to 0 % SOC for a 1.5C discharging rate and from 59 % SOC to 0 % SOC for a 2C discharging rate compared to the module without BTMS. Furthermore, when the module was exposed to ambient temperatures ranging from 30 °C to 45 °C, the DE-LHP BTMS maintained a temperature difference across the battery module of less than 3.5 °C for all charging rates and 1C and 1.5C discharging rates.

Experimental thermal characterization of the in-wheel electric motor with loop heat pipe thermal management system

Current trends in technology toward the elimination of fossil fuels are increasing the value of electric vehicles (EVs). In turn, one of the main components of EVs is an electric motor (EM). The lifetime, reliability and output power of the EMs depend on the performance of their cooling systems. The cooling systems that are based on two-phase technologies of heat transfer, such as loop heat pipes (LHPs), have important advantages over traditional air- and liquid systems. On the other hand, LHP EM thermal control application is slowed down by the complication of their manufacturing and high production expenses. However, in recent years, new technologies are constantly being developed that can reduce the customization of LHP and, therefore, the cost of its production. In this paper the technology based on modular standardized approach for LHP design and manufacturing is applied to demonstrate the cooling performance of LHP in an in-wheel motor of a personal mobility vehicle (electric bike, wheelchair etc.) and compare it to the standard cooling solution. For this purpose, a double-capillary pump unit evaporator LHP was produced and coupled with an EM. EM thermal characteristics were studied and compared to the values obtained prior to LHP installation. Studies have shown that the use of two-phase cooling can increase motor performance up to 2.5 times.

Current Trends in Wick Structure Construction in Loop Heat Pipes Applications: A Review.

Thermal control systems have been introduced as an important part of electronic devices, enabling thermal management of their electronic components. Loop heat pipe (LHP) is a passive two-phase heat transfer device with significant potential for numerous applications, such as aerospace applications, high-power LEDs, and solar central receivers. Its advantages are high heat transfer capability, low thermal resistance, long-distance heat transfer, and compact structure. The essential role of wick structures on the performance of LHPs has already been highlighted, but no comprehensive review is available that deals with different parameters such as LHP design and wick size, which are largely decisive and effective in achieving a practical level of thermal transmission governed by wick structures. To rely on this necessity, this article summarizes, analyzes, and classifies advancements in the design and fabrication of wick structures. The main conclusion to be drawn after careful monitoring and weighing of the related literature is that LHPs with composites and additively manufactured wicks show a higher heat transfer coefficient than other conventional structures. Indeed, future works should be focused on the design of more structurally efficient wicks, which may allow us to optimize materials and geometrical parameters of wick structure for higher heat transfer through LHPs.

Copper wick based loop heat pipe for thermal management of a high-power LED module

Contemporary high-power LED modules are subjected to severe thermal management challenges due to continuing miniaturization and increased heat flux levels. The thermal loads need to be effectively dissipated to improve module reliability by maintaining the junction temperature below 120 °C. The viability of a Loop Heat Pipe (LHP), with copper as the wick material, as a potential thermal management solution for a 200 W high-power LED module is demonstrated in this work. The investigation is conducted in two stages. Firstly, the thermal performance of the LHP is characterized by using different working fluids, and sink temperatures, to obtain the optimum working conditions, using a heater assembly mimicking the LED module. Subsequently, the LHP is integrated with an actual high-power LED module, and this thermo-mechanical demonstrator is then characterized under different real-time operating conditions. Complimentary 3D computational heat transfer simulations are carried out to estimate the LED junction temperature and visualize the thermal fields generated in the LED module. The results demonstrate the efficacy of the copper-methanol LHP design for successful thermal management of the high-power LED module dissipating over 100 W/cm2 (maintaining junction temperature below 120 °C for the 200 W nominal power LED module).

Loop Heat Pipe Design: An Evaluation of Recent Research on the Selection of Evaporator, Wick, and Working Fluid

Abstract Since its invention nearly five decades ago, the loop heat pipe has revolutionized every application requiring cooling or maintaining a constant temperature environment. In this article, its various designs aspects are explored, which include design of the evaporator, wick, and selection of working fluid. Factors such as design guidelines and how they affect the physics of the overall system are surveyed. For the evaporator part, its various designs and their respective applications/operating ranges are reviewed. In the wick section, recent trends on its fabrication and performance enhancement are shown. A special section on how the wick functions is added, with a focus on the study of liquid–vapor meniscus using the thin-film evaporation theory. Attention is also given to the investigations on the various figures of merit used for the selection of the working fluid. For the first time, these figures of merit are categorized with respect to the device physics they represent/simulate. In the end, this review article also touches upon the various creative designs and ideas used to enhance the loop heat pipe performance.

Loop Heat Pipes for Thermal Management of Electric Vehicles

Abstract The present paper investigates the potential of loop heat pipe (LHP), with respect to technological merits and application niche, in automotive thermal management. Broadly, LHP design and applicability for hot spot cooling in electronics (local dissipation), and for heat transport over longer distances (remote dissipation) has been proposed and discussed in detail. The basic module in these applications includes loop heat pipe with different shapes and sizing factors. Two types of LHP designs have being tested and results discussed. The miniature version, with 10 mm thick and flat disk evaporator, for cooling electronic control unit (ECU) with 70 W chipset while keeping source temperature below 100 °C limit was evaluated. Two larger versions with cylindrical evaporator, 25 mm diameter and 150 mm length, and heat transfer distances of 250 mm and 1000 mm, respectively, were tested for power electronics and battery cooling, with more than 500 W transport capabilities in gravity field. In conclusions, loop heat pipes will provide an energy efficient passive thermal control solution for next-generation low emission automotive, particularly for electric vehicles (EVs), which have high-level electrifications and more definitive cooling requirements.

Experimental Investigation and Visualization Study of the Condensation Characteristics in a Propylene Loop Heat Pipe

At present, the structural design of a loop heat pipe (LHP) condenser mostly relies on experience. To analyze the condensation flow pattern in an LHP and to find a more accurate empirical correlation that can better guide condenser design in existing empirical correlations, a test system was designed and fabricated to observe the condensation phenomenon in the condenser and to evaluate the LHP’s heat transfer performance. Since propylene has a wide temperature range to different temperature control requirements, it was chosen as the working fluid. In this paper, the condensation process in the condenser was observed, and the operating parameters of a propylene LHP were measured at 283 K. The condensation flow patterns in the 2.5 mm ×2.5 mm channel mainly included stratified flow, wavy flow and intermittent flow. By comparing the experimental results with four typical condensation flow regime maps, it was found that the Breber and Cavallini flow regime maps can successfully predict most of the condensation flow patterns of the LHP even with ultra-small mass flux. In addition, four condensation heat transfer coefficient correlations were selected to compare the predicted values of heat transfer coefficient with the experimental results. The study showed that the predicted values from the correlation by Cavallini et al. matched 90% of the experimental values well with an error of less than 20%, and this correlation is recommended for highly accurate LHP design.

Numerical study on heat-transfer characteristics of loop heat pipe evaporator using three-dimensional pore network model

An important consideration while designing the shape of a capillary evaporator is the phase distribution in the wick. However, the distribution depends on the working fluids and porous materials. This study investigates the heat-transfer characteristics of a loop heat pipe (LHP) evaporator by using a three-dimensional pore network model with a dispersed pore size wick. The simulation considers saturated and unsaturated wicks. A stainless steel (SS)-ammonia, polytetrafluoroethylene (PTFE)-ammonia, and copper-water LHP are simulated. The copper-water LHP has the highest transition heat flux to the unsaturated wick. When the optimum evaporator shape of the copper-water LHP is designed, it is reasonable to assume that the phase state is saturated. On the other hand, for the ammonia LHP design, the state is assumed to be unsaturated. Simulation results show the heat-transfer structure in the evaporator and indicate that the applied heat flux concentrates on the three-phase contact line (TPCL) within the case, wick, and grooves. An evaporator configuration with circumferential and axial grooves is simulated to investigate the effect of the TPCL length. Results indicate that the optimum shape can be realized by varying the TPCL length. The proposed method is expected to serve as a simple approach to design an evaporator.

Effect of evaporator tilt on the operating temperature of a loop heat pipe without a secondary wick

The effect of evaporator tilt on the operating temperature of a loop heat pipe (LHP) without a secondary wick under terrestrial surroundings was investigated both experimentally and theoretically in this work. The experiments were conducted with the evaporator placed at three different tilts: (a) the evaporator was horizontal with the compensation chamber (CC), (b) the evaporator was vertically below the CC, and (c) the evaporator was a little higher than the CC with a tilt angle of 1.80°. Experimental results show that the evaporator tilt has significant effect on the operating temperature of the LHP: the operating temperature at evaporator tilt (c) was much higher than those at two other evaporator tilts. Theoretical analysis based on a two-zoned evaporator wick, i.e. a subcooled zone and a saturated zone, is conducted to explain the experimental results, indicating that the cooling effect of the returning liquid on the vapor region in the CC or the evaporator core is crucial in determining the operating temperature of LHPs, which should be well considered in the LHP design and applications.

Effects of non condensable gas in an ammonia loop heat pipe operating up to 125 °C

A Loop Heat Pipe (LHP) able to operate efficiently up to 125 °C has been developed, which is in line with the current tendency of satellite on-board equipment to increase their operating temperatures. In selecting the optimal LHP design for the elevated temperature application, the compatibility between the materials and working fluid has been identified as one of the main drivers. Incompatibility is linked to Non Condensable Gas (NCG) generation. In this paper, the behavior of the LHP developed with different amounts of nitrogen injected in its compensation chamber to simulate NCG generation, is analyzed. The LHP performance is studied by analysis of the test results at different temperatures and power levels. The test results are also compared to simulation in EcosimPro. Two main conclusions can be drawn from the study. The first is that the effects of an amount of more than two times the expected NCG at the End of Life of a typical telecommunications satellite (15 years) is almost negligible on the LHP operation. The second is that the main effect of the NCG is a decrease in the LHP thermal conductance, especially at low temperatures and low power levels. This decrease is more significant with the progressive addition of NCG.

Heat removal from bipolar transistor by loop heat pipe with nickel and copper porous structures.

Loop heat pipes (LHPs) are used in many branches of industry, mainly for cooling of electrical elements and systems. The loop heat pipe is a vapour-liquid phase-change device that transfers heat from evaporator to condenser. One of the most important parts of the LHP is the porous wick structure. The wick structure provides capillary force to circulate the working fluid. To achieve good thermal performance of LHP, capillary wicks with high permeability and porosity and fine pore radius are expected. The aim of this work was to develop porous structures from copper and nickel powder with different grain sizes. For experiment copper powder with grain size of 50 and 100 μm and nickel powder with grain size of 10 and 25 μm were used. Analysis of these porous structures and LHP design are described in the paper. And the measurements' influences of porous structures in LHP on heat removal from the insulated gate bipolar transistor (IGBT) have been made.

Testing of a Low-Cost Loop Heat Pipe Design

This paper presents and describes the test campaign of a low-cost Loop Heat Pipes (LHP) design. LHP have been around for many decades now. Their potential as passive heat transfer devices has been widely demonstrated in numerous both ground- and space-based applications. One of the major disadvantages of LHP is their inherent high manufacturing cost; this is the main factor why LHP are still confined to niche/high end applications such as thermal management of spacecrafts. This paper proposes to use an alternative manufacturing design for the LHP evaporator, which is the main contributor to the overall LHP cost. Preliminary thermal results are also reported and briefly explained. Future work is needed to confirm the promising results discussed in this paper and address fully other issues such as tolerance of this LHP design to vibrations and accelerations typical of space missions.

Development and Characterization of an Air-Cooled Loop Heat Pipe With a Wick in the Condenser

Thermal management of modern electronics is rapidly becoming a critical bottleneck of their computational performance. Air-cooled heat sinks offer ease and flexibility in installation and are currently the most widely used solution for cooling electronics. We report the characterization of a novel loop heat pipe (LHP) with a wick in the condenser, developed for the integration into an air-cooled heat sink. The evaporator and condenser are planar (102 mm × 102 mm footprint) and allow for potential integration of multiple, stacked condensers. The condenser wick is used to separate the liquid and vapor phases during condensation by capillary menisci and enables the use of multiple condensers with equal condensation behavior and performance. In this paper, the thermal–fluidic cycle is outlined, and the requirements to generate capillary pressure in the condenser are discussed. The LHP design to fulfill the requirements is then described, and the experimental characterization of a single-condenser version of the LHP is reported. The thermal performance was dependent on the fan speed and the volume of the working fluid; a thermal resistance of 0.177 °C/W was demonstrated at a heat load of 200 W, fan speed of 5000 rpm and fluid volume of 67 mL. When the LHP was filled with the working fluid to the proper volume, capillary pressure in the condenser was confirmed for all heat loads tested, with a maximum of 3.5 kPa at 200 W. When overfilled with the working fluid, the condenser was flooded with liquid, preventing the formation of capillary pressure and significantly increasing the LHP thermal resistance. This study provides the detailed thermal–fluidic considerations needed to generate capillary pressure in the condenser for controlling the condensation behavior and serves as the basis of developing multiple-condenser LHPs with low thermal resistance.

Evaluation of capillary performance of sintered porous wicks for loop heat pipe

Thermal management for various applications has promoted the development and in-depth investigations of loop heat pipes (LHP) with sintered porous wicks. The capillary performance of sintered porous wicks, which integrates both capillary pressure and permeability, determines the operation and heat transfer performance of LHP. In this study, the capillary performance was accessed using the capillary rate-of-rise experiments. An infrared (IR) thermal imaging method was employed to monitor the capillary rise processes. Four kinds of sintered porous wicks, i.e., two Inco type 255 and 123 nickel wicks with powder size of 2.2–2.8μm and 3–7μm respectively, and two spherical and irregular copper wicks of 75–110μm, were characterized for the purpose of wick design of LHP. Two working liquids tests, ethanol and acetone, were conduced. Tests results show that the capillary performance determined by both liquids tests showed good agreement. It suggests that the capillary rate-of-rise experiments coupled with the IR thermal imaging method can provide an easy and accurate way to characterize the hydraulic properties of porous wicks of LHP. The copper porous wicks presented both larger permeability and capillary performance than the nickel wicks. The type 255 nickel porous wick outperformed the type 123 in both permeability and capillary performance.

Development of LHPs with ceramic wick

Abstract The ceramic porous wick is proposed as an alternative to wicks made of metal and plastic for Loop Heat Pipes (LHP) applications. Performance tests of two LHPs using acetone and water as working fluids were carried out for power inputs up to 25 W. The evaporators have a stainless steel cylindrical assembly, with 80 mm of length and 10 mm of inner diameter. The ceramic porous wick has 50% of porosity, 1–3 μm of pore radius distribution and a permeability of about 35 × 10 −15 m 2 . The active zone length of the evaporators is 25 mm. For a limited operation temperature of 100 °C, the LHPs were able to transfer up to 25 and 15 W using acetone and water, respectively, at steady state condition. An operation analysis concerning the effects of the LHP inclination, the heat sink temperature and LHP thermal resistances is also presented. Despite the relative good performance of the LHPs, further researches are still required taking into account changes related to properties of the ceramic porous wick in particular and improvements in the whole LHP design in general.

Analytical Model for Characterization of Loop Heat Pipes

The present paper proposes general equations for predicting the steady-state behavior of a loop heat pipe, linking its operating temperature to various fluidic and geometrical parameters. The closed-form solutions determined for variousloop-heat-pipe operatingmodesarededucedfromtheequationsofapreviouslydevelopednumericalmodel. This new approach of the loop-heat-pipe modeling facilitates the identification of the physical mechanisms that influence its operating behavior. In addition, the transition heat flux between variable and fixed conductance modes can also be estimated. This simplified model has been validated for each loop-heat-pipe operating mode for various geometries and operating conditions. The present model could be a useful tool for the design of loop heat pipes. Nomenclature A = cross-sectional area cp = specific heat D = diameter e = thickness f = friction factor g = gravitational acceleration h = convective heat transfer coefficient hC = condensation heat transfer coefficient K = overall heat transfer coefficient or permeability k = thermal conductivity L = length lv = latent heat of vaporization _ m = mass flow rate P = pressure Q = heat transfer rate R = thermal resistance Rp = pore radius Re = Reynolds number S = heat exchange surface area T = temperature � H = relative elevation � = wetting angle � = dynamic viscosity � = density � = surface tension Subscripts

Applying manufacturing procedures to improve Loop Heat Pipes performance

The Loop Heat Pipe (LHP) technology has been developed using acetone as working fluid. As the thermal performance obtained by using this fluid decreased, changes to the LHP design were made and implemented. Manufacture of capillary evaporators with internal microgrooves and circumferential grooves on the primary wick structure were done using procedures applied for this specific device. The LHPs with modifications presented better results when compared to previous designs; evaporator temperatures are up to 50% lower than the previous design, proving that the features implemented have been effective in increasing the device's performance even using a less efficient working fluid.

Model-Based Method of Theoretical Design Analysis of a Loop Heat Pipe

Mathematical models accepted so far for loop heat pipes (LHPs) are mentioned, with a brief account of the operating characteristics. The necessity of an analytical model formed in a consistent way is stressed from the viewpoint of design practices. An ordinary differential equation, expressing the heat and mass transfer in a thick-walled porous cylinder, is solved to give a radial temperature distribution of the cylindrical wick, from which the outside-to-inside wick diameter ratio is derived. This ratio depends on the number of transfer units and is finally expressed in terms of the evaporator temperature effectiveness, which serves as a performance index. Positive-powered expressions, including the specified critical Bond number and specified linear pressure loss gradients, are then given to determine the wick inside, vapor line, and liquid line diameters. A pressure-loss model, consisting of theoretically or empirically obtained practical expressions, is presented to specify the wick pore radius. Derived expressions for the wick permeability and conductivity are combined to find an optimal wick porosity. The degree of subcooling, determined from the heat leak and the heat loss or gain, is given in a binominal expression dependent on the heat load, operating temperature, ambient temperature, and modeled coupling conductances. That degree is then converted into the pump efficiency, serving as another performance index. A reasonable method for reservoir sizing is proposed, considering both hot and cold startups. A two-region model representing the surface activity of a condensing radiator is introduced to determine the condenser/subcooler tube diameter, the total tube length, and the radiator half feeder spacing. All the expressions are arranged to develop into an LHP design code of convenience. Results of numerical computations done over a possibly wide range of parameters are graphically shown in the figures with a view to offering LHP design curves of interest.