Passive Heat Transfer Device Research Articles

Experimental characterisation and visualisation of a novel two-phase flexible heat transfer device

The importance of passive Flexible Heat Transfer Device (FHTD) in electronic cooling or space applications lies in their ability to provide efficient, reliable, and adaptable thermal management solutions. Heat transfer enhancement studies pertaining to FHTD developed to meet the thermal management demands of futuristic flexible electronic devices are presented in the current work. The possibility of extending the heat transport limit of FHTD beyond 24 W by limiting the maximum operating temperature to 60 °C is discussed. The superior performance of FHTD at varying heat loads from 5 W to 40 W at 45°and 90°bending angles is brought out compared with existing heat transfer devices like Flexible Heat Pipe (FHP) and copper thermal straps. The performance is reported in thermal resistance and equivalent thermal conductivity. A commercial dielectric liquid is used as a working fluid in FHTD to enhance the heat transfer limit employing phase change. The evaporator and condenser sections of the flexible section are made transparent to visualise the boiling and condensation phenomenon. Under steady-state operation at a 45°bending angle, The FHTD demonstrates a minimum thermal resistance of 0.73 K/W and a peak effective thermal conductivity of 8172 W/m K. The heat transfer coefficient ranges from 200 to 700 W/m2 K, consistent with values reported for heat pipes in the literature. Additionally, the study explores the impact of modifying the external surface texture to create more nucleation sites, thereby enhancing the performance of the FHTD. The results show that the laser-textured surface on the heat pipe condenser effectively reduces the onset of nucleate boiling for dielectric fluid by 3.5 °C and decreases the maximum temperature of the evaporator by 4.5 °C. The present work dictates the fundamental baseline for possible design choices of futuristic passive flexible heat transfer devices.

Review of “cold shock” cases in operation of loop heat pipes and related thermal instabilities

Loop Heat Pipes (LHP) are passive two-phase heat transfer devices, driven by capillary pumps that source energy for circulation of working fluid solely from the heat source. They have a proven track record in spacecraft thermal control as well as terrestrial applications. However, LHP may experience partial or complete wick dryout and thermal instabilities at rapid power-up steps or (and) rapid condenser cooling conditions change due to a rapid inflow of subcooled liquid from the condenser into the evaporator. Such cases do not have very wide coverage in literature and are usually presented as a little nuisance in testing of a particular LHP, for which some design workaround was found. Oftentimes suggested solutions trade performance or other design merits for robustness of operation. This article aims to review such case studies where phenomena similar to the described “cold shock” affected LHP performance. The aforementioned instabilities become even more pronounced in grid-like multievaporator LHP having multiple parallel connections between capillary pumps and compensation chambers, as heat and mass transfer between those elements becomes less definite and can eventually increase the heat leak into compensation chambers. Vulnerability of LHP with distributed network of evaporators to seemingly “safe” transient regimes fosters deeper analysis of the “cold shock” phenomenon. With comprehensive definition to the problem and a list of research questions, technical measures for getting rid of it in perspective LHP designs can be foreseen.

On the hydrostatic limit for thin film flow with applications to thermosyphons

Heat pipes frequently encounter challenges in effectively returning condensed liquid from the condenser to the evaporator through capillary pumping, which limits their overall efficacy. When capillary pumping becomes irrelevant due to factors like the absence of wick, or overfilling, the heat pipe is more properly termed as thermosyphon. In a close-to-horizontal orientation, the driving force for liquid return in a thermosyphon relies on the difference in liquid pool depth between the evaporator and condenser. An excessively deep liquid pool in the condenser can hinder radial heat transfer, necessitating a design that favours intermediate-depth liquid pools. This study utilizes a theoretical approach based on the lubrication approximation to the Navier–Stokes equations to determine the fill ratio that maximizes thermosyphon performance. We explore the relationship between this fill ratio and factors such as the axial temperature difference along the thermosyphon. Our analysis thereby highlights the hydrostatic-driven flow limit in the thermosyphon as an analogue to the capillary limit in heat pipes. Regarding the hydrostatic limiting curve as an alternative to the capillary limiting curve offers valuable insights for enhancing thermosyphon performance and provides means of comparing the performance of one vs. the other passive heat transfer device.

A COMPREHENSIVE REVIEW STUDY ON HEAT TRANSFER IMPROVEMENT TECHNIQUES WITHIN TWISTED TUBES

Heat transfer equipment has been used in many different domestic and industrial applications. There has been a concentrated effort to create a heat exchanger design that will reduce energy requirements while saving materials and other costs. Increasing the effective heat transfer surface area or creating turbulence are two common ways to improve heat transfer and hence lower thermal resistance. The thermal Performance Factor is the ratio of the difference in heat transfer rate to the difference in friction factor and serves as a metric to assess the efficiency of heat transfer enhancement technologies. Different types of twisted tubes are used in many heat transfer improvement devices. geometrical parameters of the twisted tube encompassing the aspect ratio, twist ratio, twist direction, twist length, etc. impact the heat transfer. For Instance, oval pipes with unequal twist pitches have a thermal performance factor (1.75) and equal twist pitches have a thermal performance factor (1.98). furthermore, the thermal performance factor of the twisted tube with oval dimples is equal to 1.19 compared with the twisted tube without dimples. The thermal performance factor of the twisted tube with oval dimples is equal to 1.38 compared with the straight tube, with another improvement to the twisted tube that improves the heat transfer properties by disrupting the thermal boundary layer and destabilizing it. This paper presents a comprehensive investigation of passive heat transfer devices (twisted tubes) and their relative merits in a myriad of commercial applications.

Capillary limit of a sodium screen-wick heat pipe

High-temperature alkali-metal heat pipes are passive heat transfer devices, where the capillary pressure created by the menisci in the wick pumps the condensed fluid back to the evaporator. Thus, the heat pipe heat transfer capacity is limited by the maximum capillary pressure at high heat fluxes. This work presents a two-dimensional heat and mass transfer model for alkali metal heat pipes. In addition to the traditional wall heat conduction model, the model also incorporates gas–liquid flow heat transfer and capillary structure models, enabling the model to simulate both steady-state operation and transient capillary limit processes. The predictions of the model agreed well with experimental data. The predicted critical heat flux temperature has an error of less than 20℃ and a relative error of 6%. The predicted transient sequences are consistent with the measurements. The model shows that capillary limit is affected by inclination angle, heat flux, and wick structure. For inclinations, the critical heat flux at the capillary limit is linearly related to the inclination angle with the predicted temperature rising 2–10℃/s during the early stage of the capillary limit critical heat flux condition. The evaporator temperature oscillates before the capillary limit with positive inclinations. For heat fluxes, increasing the heat flux by 17% increases the temperature increase rate 10 fold with a longer dryout length. The evaporator exhibits hysteresis after the capillary limit due to partial dryout of evaporator. For wick structures, reducing the mesh screen pitch from 80 μm to 15 μm reduces the critical heat flux to 10% of the CHF at 80 μm. This model can accurately predict the heat pipe temperature variations at the capillary limit and can explain the physical mechanisms for the dryout oscillations and recovery hysteresis at the capillary limit. Thus, the measurements and model provide analytical guidelines for designing heat pipe cooling systems.

Capillary evaporating film model for a screen-wick heat pipe

Screen-wick heat pipes are efficient passive heat transfer devices. The heat transfer capacity is determined by the capillary pressure, which is related to the film curvature inside the screen wick. A capillary evaporating film model was developed for the screen wick that divided the film into a capillary pressure microlayer region and a macroscopic film. The film surface shape in the capillary pressure microlayer region is mainly influenced by the long-range molecular forces (disjoining pressure) and the surface tension, while the film surface shape in the macroscopic film is affected by the wick geometry. The numerical results show that the present model more accurately predicts the three-dimensional characteristics of the liquid film inside the screen wick than the classical model. The wire opening distance and the wire diameter affect the capillary pressure and the flow path in the wick. As the liquid film recedes into the wick, the capillary force first increases and then decreases. The film dynamics in the wick are related to the liquid height, which automatically adapts to the required evaporation and pressure balance for a given heat load. High operating temperatures can lead to a mismatch between the capillary head and the pressure loss, leading to dryout in the wick. Thus, this work provides a reference for operating alkali-metal wick heat pipes.

Estimation of the local instantaneous heat flux inside a pulsating heat pipe for space applications

Pulsating Heat Pipes (PHP) are promising and effective passive two-phase heat transfer devices in terms of high heat transfer capability, efficient thermal control, adaptability and low cost and therefore they have been extensively studied in the last years. Many authors have estimated the heat fluxes at the evaporator and at the condenser area only in terms of the average values based on first principle considerations. In the present study the application of an inverse analysis technique to experimental infrared temperature data is proposed to investigate the local convective heat flux for forced convection flow inside these devices along the adiabatic zone. A PHP specifically designed to be hosted on board the heat transfer host of the International Space Station was tested in microgravity during the 67th Parabolic Flight Campaign organized by the European Space Agency. The device consists of an aluminium tube closed in a loop with 14 turns in the evaporator section, 3 mm inner diameter, half filled with FC-72 fluid. The external temperatures of the device are measured in the adiabatic zone with a high-speed infrared camera (50 Hz, 1280x1024 pixels). The images are thereafter post-processed and adopted as input data for the solution of the inverse heat conduction problem in the pipe wall (Tikhonov regularisation method) to extrapolate time-varying local heat fluxes on the tube internal surface in contact with the fluid.

Polymer and Composite Materials in Two-Phase Passive Thermal Management Systems: A Review.

The application of polymeric and composite materials in two-phase passive heat transfer devices is reviewed critically, with a focus on advantages and disadvantages of these materials in thermal management systems. Recent technology developments led to an increase of the power density in several applications including portable electronics, space and deployable systems, etc., which require high-performance and compact thermal management systems. In this context, passive two-phase systems are the most promising heat transfer devices to dissipate large heat fluxes without external power supply. Usually, heat transfer systems are built with metals due to their excellent thermal properties. However, there is an increasing interest in replacing metallic materials with polymers and composites that can offer cost-effectiveness, light weight and high mechanical flexibility. The present work reviews state-of the-art applications of polymers and composites in two-phase passive thermal management systems, with an analysis of their limitations and technical challenges.

Experimental investigation on the start-up performance of a novel flat loop heat pipe with dual evaporators

Thermal management for multiple heat sources has been becoming increasingly important, especially under high heat flux conditions. As a passive heat transfer device, loop heat pipes (LHP) with multiple evaporators does not consume extra energy, demonstrating a great potential in thermal management. This paper proposed a flat plate LHP with dual evaporators for the first time. Two flat plate evaporators were adopted, and both of them had their own porous wick, compensation chamber (CC) and vapor line. The condensed liquid converged, then moved along the main liquid line. The CC of two evaporators was joined by a tube, which can ensure the sharing of the returning liquid. To verify the performance of novel flat plate LHP, the start-up test was investigated under different heat load arrangements, involving only a single evaporate was applied heat load, two evaporators were applied with the equal or unequal heat load. Meanwhile, the operation stability was determined by testing the performance with the successionally variable heat load. This system has been shown to operate successfully at heating source temperature below 100 °C with heat load ranging from 10 W–10 W to 130 W–130 W (corresponding to the maximum heat flux of 13.52 W/cm2).

Deployment of vapour chambers for electronic heat dissipation: state-of-the-art

Miniaturization and high performance are the two principal requirements of modern electronics. All commercial applications demand reliable performance at high operational efficiency for prolonged service life. In order to meet all these requirements, the cooling system should be able to handle large heat fluxes packed in compact spaces and take away the heat as soon as possible to avoid overheating of the chip. Phase change heat transfer devices can handle large heat fluxes on account of latent heating. Lately, vapour chambers (VCs) are found to be promising heat spreaders as they are passive two-phase heat transfer devices. They effectively eliminate the hotspots, transferring high heat fluxes with better efficiencies and offer lower thermal resistance compared to the conventional cooling techniques. A detailed review of the latest research and advancements in the field of VCs is presented in this paper, including the effect of different thermo-physical parameters on their performance. Finally, all the major findings are summarized, including the future scope of development in a systematic manner.

Effective passive phase-change light-emitting diode cooling system using graphene nanoplatelets coatings

An effective passive two-phase light-emitting diode (LED) cooling system using thermally cured graphene nanoplatelets (GNPs) coatings is introduced. The GNPs-coated surfaces demonstrate significant reduction in the static contact angle as compared to the uncoated aluminium surface. The ultrafast water permeation property of the micro/nano-capillaries-structured GNPs coatings engenders significant cooling performance enhancement of the LED. The water is absorbed by the GNPs coatings and permeating upwards against the gravitation force. The water molecules are propelled through the nanocapillaries formed by the carbon walls and the capillary force dominates over the gravitational force. This induces the filmwise evaporation on the GNPs-coated surfaces and significantly enhances the LED cooling. As compared to the uncoated case, a maximum heat transfer coefficient enhancement of 73% is achieved. The GNPs coatings also significantly lower the operating light emitting surface temperature up to 10 °C reduction, effectively removes potential hotspots which in turn optimise the LED functionality and durability. This study paves the way for the promising employment of thermally cured GNPs coatings in developing a passive phase-change heat transfer device for LED cooling.

Effect of wick oxidation on the thermal performance of a copper-acetone loop heat pipe

Loop Heat Pipe (LHP) is a passive phase-change heat transfer device. Among other factors, its performance largely depends on the heat leak from the evaporator to the compensation chamber (CC). For copper wick LHP, a high heat leak creates a problem in successful LHP start-up and can adversely affect its operation. The present study focuses on reducing the heat leak through manipulation of the copper wick properties. The effect of fluid charging, overall pressure drop, and wick oxidation on thermal performance is investigated. A cylindrical copper-acetone LHP is tested in favorable orientation for two cases with (1) oxygen-free, or pure copper wick and (2) oxidized copper wick in its evaporator. For Case #1, LHP is filled with acetone at different Charging Ratios (CR) of 50%, 60%, and 70%. For Case #2, the charging ratio is kept constant at 50%. For the pure copper wick, a charging ratio of 50% provides the best thermal performance, and it decreases with an increase in charging ratios. At CR = 50%, LHP can transfer a heat load of 90 W (hEvp = ∼900 W/m2K) and 180 W (hEvp = ∼2500 W/m2K) at the evaporator temperature below 100 °C for the pure copper and oxidized wick, respectively. The significant improvement in LHP thermal performance of the latter is attributed to a decrease in heat leak because of the low thermal conductivity of the oxidized porous wick. The heat leak for pure and oxidized wick is found ∼18% and ∼7% of input heat loads, respectively. This can be an effective methodology to pave the way for the usage of commercial copper-based LHPs for the thermal management of terrestrial devices.

Development of two-phase flow regime map for thermally stimulated flows using deep learning and image segmentation technique

Accurate prediction of the flow patterns is important for understanding the behavior and operation of thermo-fluidic phenomena in two-phase systems. Considering the importance of thermally induced flows in capillaries and their use in various processes, the thermo-hydrodynamics of two-phase flows in pulsating heat pipes (PHPs) have been studied. In the present research, flow regimes were identified in capillaries where the mass flow rate is difficult to deal with. A deep learning technique has been carried out to classify the flow patterns and extract their attributes in a particular capillary using multi-spectral segmentation. The modified Weber, Froude, and Bond numbers are used by considering the absolute velocities, accelerations, and bubble lengths for classifying the flow regime. Flow regime maps were plotted based on the proposed non-dimensional numbers by analyzing experimentally measured data sets and visual observation for methanol as a working fluid at six different heat levels (18,38,44,50,60, and 70 W). Four different flow regimes have been identified and classified as a bubbly flow, a slug flow, an elongated plug flow (transitional flow), and an annular/semi-annular flow, and a new flow regime map for thermally induced two-phase flows in a PHP is presented. The flow regime map gives quantifiable criteria for the calculation of bubbly, slug, and annular flow patterns. The present study will be useful for researchers analyzing a large number of two-phase flow images and to design two-phase passive heat transfer devices based on the flow regime map.

Startup characteristics of an ammonia loop heat pipe with a rectangular evaporator

Flat-plate loop heat pipe (FLHP) is a passive two-phase heat transfer device. Comparing with traditional LHP with a cylindrical evaporator, it can be directly connected to a flat heat source without the employment of a saddle, which can effectively reduce the system thermal resistance and enhance the temperature uniformity. In this work, a stainless steel-ammonia FLHP was developed, and extensive experiments have been conducted to investigate its startup characteristics with the evaporator in the horizontal and vertical positions. Experimental results show that the FLHP exhibits excellent startup performance. It can successfully start up at a small heat load as low as 2 W with no obvious temperature overshoot. In a wide power range of 5–35 W, the FLHP generally starts up in only one situation, much simpler than the startup of a LHP with a cylindrical evaporator. For this rectangular evaporator, the heat leak from the evaporator to the compensation chamber (CC) becomes very small. As a result, the vapor can easily exit the condenser in most cases in the power range of 5–35 W, leading to a 100% utilization efficiency of the condenser and the resultant satisfactory thermal performance of the FLHP. In addition, the startup performance and the system thermal resistance of the FLHP are insensitive to the evaporator orientation, promising great application potential in future electronics cooling.

Flat plate pulsating heat pipes: A review on the thermohydraulic principles, thermal performances and open issues

Thermal management of modern micro- and high-power electronic systems has become a progressively challenging problem due to current trends in miniaturization and increased heat generation. Conventional cooling methods, such as natural and force convection, have become insufficient to meet new challenges in electronics industries, transport and space applications where thermal management system size, mass, autonomy, high density and overall system reliability play a crucial role. Unlike active technologies, heat pipes are passive heat transfer devices without moving mechanisms which lead to greater reliability, even though they are limited by capillary, viscous and/or gravity factors. In contrast, the Pulsating Heat Pipe (also called Oscillating Heat Pipe) has recently become a source of increasing interest: first, due to fascination for the complexity of the internal two-phase phenomena; and secondly, due to novel applications, which are rapidly increasing the Technology Readiness Level of this device, for both ground and space environments. Pulsating heat pipe manufacturing can be divided into two categories: the classical tubular pulsating heat pipe, and the flat plate pulsating heat pipe. While the former is manufactured and developed from a single tube enrolled like a serpentine around heat and cold sources, the latter is obtained from a metallic plate including an engraved serpentine single channel, and hermetically closed on its top side by a second plate. These two manufacturing concepts lead to differences in operating behavior that are worth noting in order to better understand the functioning of the devices.

Physical principles and state-of-the-art of modeling of the pulsating heat pipe: A review

The Pulsating (called also oscillating) Heat Pipe (PHP) is a high performance passive heat transfer device. It consists in a closed capillary channel folded into several meanders, evacuated, and partially filled with a liquid and its vapor. One side is in thermal contact with a hot spot, the other with a cold spot. The oscillation of the liquid plugs and vapor bubbles spontaneously occurs after the heating beginning. The heat exchange takes place not only by latent heat transfer, but also by liquid convection. Both this advantage and the PHP structural simplicity make it competitive with respect to other kinds of heat pipes. However, the PHP operation is non-stationary and depends on many physical and material parameters. As a result, application of empirical correlations is quite unsuccessful. More sophisticated direct modeling is thus required. In this review, we present the past, present, and future perspectives of the theoretical PHP studies. The physical phenomena on the level of a single bubble and single liquid plug is addressed first. In this context, the modeling of the simplest, single branch PHP is described as a necessary first step toward the PHP understanding and model validation. The modeling of interaction between the bubbles and plugs (bubble generation and disappearance, plug evaporation) is discussed next. Finally, the state of the art of the physical modeling and simulation of the multi-turn PHP is reviewed and the difference between existing approaches is explained. • Pulsating heat pipe (PHP) numerical simulations are reviewed. • 1D PHP simulations begin to emerge as a main PHP design tool. • 2D and 3D simulations are not mature enough for PHP designing but can be used to improve the flow models for 1D simulations. • Improvement of the background models (in particular, of the liquid film model) is necessary. • The components of the 1D PHP model should be validated against experiment for simple PHP geometries, in particular single branch and U-turn PHP. More U-turn PHP experiments are necessary.

An advanced conduction based heat pipe model accounting for vapor pressure drop

Heat pipes are passive heat transfer devices which play an increasingly important role in state-of-the-art cooling solutions for various technologies like consumer electronics or electric machines. During design phase, reliable simulation results are the key for optimal performance and efficiency. In system level simulations, heat conduction models, characterized by low computational effort, are widely used to predict heat pipe thermal resistance. In order to accurately calculate overall thermal characteristics, a precise prediction of the vapor core’s temperature drop is crucial. In this study, a novel conduction-based method for calculating heat pipe performance is presented. Taking velocity components perpendicular to the main vapor flow into account, an analytic expression for calculating the axial pressure gradient is developed and validated through detailed CFD simulations. Assuming saturation conditions, results are used to determine the vapor core’s local effective thermal conductivity. Parameters needed for calculation are provided for both circular and flat heat pipes under various operating conditions. The final model accounts for phase change thermal resistance and temperature dependent fluid properties. The presented method yields significantly different results compared to state-of-the-art approaches in which thermal vapor resistance is either assumed to be zero or calculated assuming (Hagen-)Poiseuille flow. Simulation results suggest a strong dependence of vapor thermal resistance on evaporation and condensation mass fluxes, thermophysical fluid properties and local vapor temperature.

Experimental study on thermal performance of ethanol loop heat pipe with flat evaporator under gravity – assisted condition

Nowadays, extreme growing of telecommunication and information technology causes the significant changes in the electronics belonging to the high-performance computer, data centers that are miniaturizing in their size and increasing the heat power dissipation. As a result, it is important to study on new cooling methods rather than conventional air cooling to warrant electronic device operate durably and stably. Besides, saving electricity power consumed by cooling systems is another considerable concern. Loop heat pipe, a passive heat transfer device which functions based on phase change processes and natural forces like gravity or capillary force, has been being one of potential solutions for the above challenges. In this paper, a loop heat pipe with flat evaporator was fabricated to investigate its cooling characteristics during start-up, stable operating period at different heat loads under gravity–assisted condition. This loop heat pipe was charged with ethanol whereas sintered stainless steel wick was the capillary structure. Time for successful start-up shortened from 13 minutes to 4 minutes as heat load increased from 30 W to 225 W. When heating power supplied to loop heat pipe’s evaporator was adjusted from 30 W to 400 W (14.8 W/cm2), the heating block surface’s temperature increased from 33oC to 133oC. This temperature could be maintained below 85oC, electronics limitation temperature in industry, if heat power released from the heating block is smaller than 220 W (8.14 W/cm2). Besides, the change of different types of thermal resistance such as total thermal resistance, evaporator and condenser thermal resistances with heating power is discussed detail in this paper.

Use of loop thermosyphon as an efficient way of heat dissipation in transport facilities

The possibilities of using heat pipes have an irreplaceable place also in the field of technologies used in transport. The use of these passive heat transfer devices is becoming an increasingly topical, applied and economically advantageous area. At a time of significant electronic transport, each electrical element produces a certain amount of heat loss. An efficient way of using these energy losses is also possible through heat pipes, which work on the principle of phase changes. Precisely for this reason, this scientific work is also devoted to a more detailed study of the processes taking place in a heat pipe, which is located in a space with electrical equipment. The main area of research is concentrated on the evaporation process, which depends on the amount of filling the evaporating part of the heat pipe with the working substance with water. The amount of heat transferred was also monitored. The results of experimental measurements provide specific information on the course of the evaporation itself and will be used in further research with heat pipes.

Nanoscale silica coating of porous stainless steel and its impact on water wettability

The water wettability of porous stainless steel specimens was enhanced via a nanoscaled silica coating for application to passive two-phase heat transfer devices. A combination of porous stainless steel and water has attracted attention in the area of the heat transfer devices. However, the water wettability of stainless steel is poor, limiting the performance of the devices. In the present study, the silica coating of the specimens was conducted via a reaction of tetraethyl orthosilicate (TEOS) after pretreatment using NaOH aq. or HCl aq. Energy-dispersive X-ray spectrometry revealed that the amount of silica deposited on the surface was dependent on the pretreatment conditions and the composition of the silica-coating solutions. Measurement of porous properties indicated that the silica coating did not affect pore diameter and gas permeability since the coating was in nanoscale. Microscale contact angles were directly evaluated using an environmental SEM. The specimens showed excellent water wettability when they were covered with numerous tiny silica bumps. When the specimen was pretreated with 2 M NaOH aq. and coated in weak alkaline TEOS solution, the contact angle of the porous stainless steel decreased from 87° to 54° after silica coating. The excellent water wettability originated from the relatively smooth surface and a sufficient coverage ratio, which resulted from the moderate strength of chemical etching of the NaOH aq. and the mild silica-coating condition in the weak alkaline solution.