Planar Heat Pipe Research Articles

Performance assessment of planar and non-planar cryogenic pulsating heat pipe with novel condenser

Pulsating heat pipe (PHP) is gaining popularity as a viable solution for thermal conductive links associated with a cryogenic cooling system. A competitive commercial environment requires PHP with a more compact configuration, high heat load removal capacity, and free of orientation dependency. Making orientation-free PHP, a planar PHP (2D layout), requires a large number of turns, increasing the overall size of the PHP device. A PHP with a non-planar (3D layout) layout is capable of accommodating a large number of turns in a short area to tackle both compactness and orientation-dependent issues. The present paper focuses on a comparative thermal performance assessment of planar and non-planar configurations of cryogenic PHP by developing a novel cylindrical shell-type condenser (CSTC). Liquid nitrogen is used as a working fluid. Two separate test setups are developed with 2D Four-Turn PHP (2D FT-PHP) and 3D Four-Turn PHP (3D FT-PHP) with a novel CSTC. The condenser, adiabatic, and evaporator lengths of PHP are kept as 60 mm, 120 mm, and 50 mm, respectively. The thermal performance is assessed by varying Filling Ratio (FR) (38−95 %) and heat load (1 W–130 W) with an evaporator at the bottom position. It is observed that, for a particular heat load, a 3D FT-PHP performed better than a 2D FT-PHP, irrespective of FR. The anti-dry-out ability of 3D FT-PHP is observed to be superior to 2D FT-PHP at a low FR. The frequency analysis results with Wavelet transform show that the working fluid oscillates with a more dominant frequency in 3D FT-PHP than in 2D FT-PHP configuration.

Molecular simulation of steady-state evaporation and condensation of water in air

It was shown in recent experiments and molecular dynamics (MD) simulations that Schrage equation predicts evaporation and condensation rates of water in the absence of a non-condensable gas with good accuracy. However, it is not clear whether Schrage equation is still accurate or even valid for quantifying water evaporation and condensation rates in air. In this work, we carry out MD simulations to study steady-state evaporation and condensation of water at a planar water-air interface. The simulation results show that the evaporation and condensation fluxes of water in the presence of air are still in a good agreement with the predictions from Schrage equation. From Schrage equation and Stefan's law of mass diffusion, we derive an analytical expression for the effective thermal conductivity of a planar heat pipe. The analytical prediction of the dependence of effective thermal conductivity on heat pipe length and density of non-condensable gas is corroborated by our MD simulation results and recent experimental data.

Thermal Performance of Vapour Chamber in Different Tilting Configurations for Battery Liquid Cooling System

Vapour chamber is a planar heat pipe and has been widely used in the thermal management of electronic device. In this work, a grooved aluminium vapour chamber was presented with a one-through cooling plate to develop a simplified and practicable battery liquid cooling system. The impact of filling ratio and tilting configuration on the thermal performance of vapour chamber was experimentally investigated. The results showed that the thermal resistance of the vapour chamber increased gradually with the increase in filling ratio, and there was a deterioration of the heat transfer in tilting configuration. However, the vapour chamber at 20% filling ratio had a better performance at positive tilting angle configuration compared with that at negative tilting angle configuration. Moreover, the temperature distribution of the heating surface is fairly uniform with the application of the vapour chamber. Despite the temperature rise of the cooling water in the cooling plate up to 3.8ºC, the minimum temperature difference of the heating surface could be kept within 1 ºC and the minimum thermal resistance could be kept within 0.075ºC/W. The results indicate that the temperature difference of the heating surface can be suppressed through the heat and mass transport of phases inside the vapour chamber.

Bubble Behavior in Pool Boiling Heat Transfer between Two Plates with a Narrow Gap

Abstract The pool boiling phenomena, which may occur at high heat flux in the planar heat pipe such as vapor chamber and thermal ground plane (TGP), were visually investigated in the vertical orientation by capturing the bubble behavior within a narrow gap between two plates using distilled water at 1 atm. The size of two copper plates is 80×80 mm2 and the 10×10 mm2 ceramic heater, which simulates the high power chip, was bonded to the backside of the bottom plate. The gap between two plates was adjusted from 3 mm to 0.5 mm using spacers. It was observed that the generated bubble spreads laterally throughout the channel between the plates as decreasing the gap. Thin liquid film forms beneath the flattened bubble and the thin film evaporation improves heat transfer at the narrow gaps of 3 and 1 mm. However, further reduction of the gap down to 0.5 mm brings about an increase of the dried region, resulting in the deterioration of boiling heat transfer. [This work was supported by the Civil-Military Technology Cooperation Program of the Institute of Civil-Military Technology Cooperation (ICMTC), with a grant funded by the Defense Acquisition Program Administration and the Ministry of Trade, Industry and Energy (Grant No. 18CM5017).]

A continuum framework for modeling liquid-vapor interfaces out of local thermal equilibrium

Continuum numerical methods modeling liquid-vapor phase change processes typically assume local thermal equilibrium at the liquid-vapor interface – continuity of temperature at phase interfaces, and a relation between interfacial saturation pressure and temperature based on phase co-existence curves. Several standard macroscopic problems have been solved accurately by adhering to this assumption. However, in micro-scale and certain non-standard macro-scale applications, significant jumps in temperature are observed at liquid-vapor interfaces during phase change, and vapor pressure can be far from equilibrium values. In this paper, we present a locally discontinuous arbitrary Lagrangian Eulerian finite element formulation with temperature discontinuities at interfaces. A penalty-based approach is used to weakly enforce the temperature jump condition. Furthermore, we use kinetic-theory-based Schrage relationship to evaluate the rate of phase change. We apply our methodology to solve the problem of flowing vapor in a planar heat pipe. The flow rates predicted by our continuum simulations are in excellent agreement with recently published molecular dynamics simulation results of the same problem. Interestingly, accounting for temperature discontinuities leads to only a slight improvement in the prediction of mass fluxes as compared to the case when temperature continuity is assumed. This is in contrast to the large improvement in the prediction of temperature profiles and is a consequence of a weak dependence of the evaporation/condensation rates on the vapor temperature.

Benefits from heat pipe integration in H2/H2O fed SOFC systems

The Solid oxide fuel cell (SOFC) is an electrochemical energy-conversion device considered as a promising solution because of its high electrical efficiency, fuel flexibility and modularity with relevant environmental benefits. However, high temperatures and thermal gradients cause significant problems for the practical application of SOFC systems. SOFC operating conditions have to be maintained within specific bounds with limitation in the maximum achievable current density in order to avoid excessive degradation. Relevant air flow rate values are also required for a correct thermal management, dealing with the highly exothermicity of the SOFC operation. In this context, the integration of planar liquid metal heat pipes into the stack structure can contribute to reduce thermal gradients within the stack itself while lowering the parasitic consumption of the air blower since less convective cooling is required. In the present work, a model implementing different SOFC cells interposed between two planar heat pipe plates filled with sodium is developed. In particular, an SOFC system fed with a H2/H2O mixture is assessed (the most stringent thermal condition because of the absence of internal reforming). The maximum permissible current density is then evaluated by varying the air utilization factor (AU). The limiting operating conditions are identified by taking into account the following constraints: maximum cell temperature, maximum global and local temperature gradients and heat pipe transport limits. A comparison of the system with and without the presence of heat pipes is then carried out with the aim of highlighting the positive effects derived from their integration. Results from simulations show a remarkable improvement of the system performance with heat pipes, which allow to reach higher values of current densities (and so power densities) with a consequent economic benefit.

Prediction of thermal conductance at liquid-gas interfaces using molecular dynamics simulations

Using molecular dynamics (MD) simulations and theoretical calculations, we study heat transfer across liquid-gas interfaces within a planar heat pipe. To determine the thermal conductance (Kapitza conductance), GK, at the interface, two heat transfer mechanisms, namely, conduction and evaporation/condensation are considered. In the case of interfacial heat conduction, gas molecules, particularly non-condensable gas molecules, exchange heat with liquid surfaces through gas-liquid collisions, and the theoretical expression for GK is derived from the kinetic theory of gases. For interfacial heat transfer by evaporation or condensation, the theoretical expression for GK is derived from the Schrage relationships. To assess the accuracies of the theoretical expressions for GK, we compare these theoretical predictions to the GK obtained directly from MD simulations. For all cases studied, the theoretical predictions agree with the MD simulation results very well. If the density of non-condensable gas in the heat pipe is much higher than that of the working fluid in the gas phase, we find that the interfacial heat conduction could contribute significantly to the total heat flux across the liquid-gas interfaces. The effect of GK at liquid-gas interfaces on the overall heat transfer efficiency in a planar heat pipe is discussed.

Modeling film boiling within chimney-structured porous media and heat pipes

A porous medium with separated paths for liquid and vapor flows does not fail even after part of the porous medium is dried out. Instead, a vapor film resides within the porous medium, and it grows very slowly. This heat transfer regime was named as “confined film-boiling regime” in this study, and a heat transfer model for this regime was suggested in this paper. Especially, this paper mainly focuses on heat transfer of a CRUD (Chalk River Unidentified Deposit), which is a naturally occurring porous medium found in nuclear reactors. In the present model, the balance between capillary pressure and pressure drops of liquid and vapor flows determined thickness of the vapor film. In addition, we assumed that capillary pressure was changed with applied heat flux. This assumption was validated with a planar heat pipe case: the root-mean-square-error (RMSE) was 16% for the model with the heat flux dependent capillary pressure, while one for a model with the constant capillary pressure was 790%. Furthermore, this approach also turned out to be valid for the case of the CRUD: the model predicted the wall superheat during the film boiling of the CRUD, and its relative error was only within 20% when it was compared with the measured wall superheats. Finally, sensitivity analysis for CRUD parameters found that the heat transfer performance of the CRUD is largely sensitive to chimney density and pore size.

The influence of non-condensable gas on an integral planar heat pipe radiators for space applications

A unique heat rejection system for space applications is proposed herein by integrating a variable conductance planar heat pipe with a radiator panel. The working temperature of the planar heat pipe charged with a small amount of non-condensable gas (NCG) can be maintained at a desired level when it is subjected to a sudden heat load or an environmental temperature variation. This advantage makes the variable conductance planar heat pipe (VCPHP) technology appealing to space heat rejection system designers. In the present work, a comprehensive study of the effect of a non-condensable gas on the planar heat pipe performance is conducted. A simple 2D mathematical model was derived from the ideal gas law and flat front assumption. Through this model, the effects of factors such as the NCG inventory, the reservoir size and the sink temperature on the behavior of non-condensable gas as well as on the performance of the planar heat pipe radiator can be analyzed mathematically. To validate this model, a heat rejection experiment was performed. In the experiment, different amounts of nitrogen were carefully measured and injected into the prototype brass planar heat pipe. Temperature profiles were measured and compared with the results calculated from the Steady-State Heat Pipe Operation Model combined with the NCG expansion model. The results indicate that the NCG expansion model is able to predict the location of the vapor-NCG interface as well as the working temperature accurately. In addition, the self-adjustment mechanism of the integral VCPHP is identified to support the applicability of the flat shape Vapor-NCG interface assumption when a non-uniform heat input is applied.

Compact Heat Rejection System Utilizing Integral Variable Conductance Planar Heat Pipe Radiator for Space Application

ABSTRACT In order to meet heat rejection requirements for future NASA exploration, scientific, and discovery missions, a study is being conducted for the feasibility of integral variable conductance planar heat pipe (VCPHP) technology. This represents a novel, low technology readiness level (TRL) heat rejection technology that, when developed, could operate efficiently and reliably across a wide range of thermal environments. The concept consists of a planar heat pipe whose evaporator acquires the excess thermal energy from the thermal control system and rejects it at its condenser whose outer surface acts as a radiating surface. The heat pipe is made from thermally conductive polymers in order to minimize its mass. It has a non-condensable gas that changes the active radiator surface depending on the heat load. A mathematical model of steady-state variable conductance heat pipe is developed. Two planar heat pipes are designed, fabricated, and tested to validate the theoretical model. The feasibility of the proposed VCPHP working in a space environment is discussed, based on the model.

Electrowetting Heat Pipes for Heat Transport Over Extended Distances

The heat transport capacity of the conventional heat pipes over extended distances is limited by the maximum condensate flow that their wicks can sustain. This paper introduces and analyzes a new planar heat pipe technology, which can transport ultrahigh heat loads (>1 kW) over extended distances (>1 m). The concept of an electrowetting heat pipe (EHP) is based on replacing the adiabatic wick section of a conventional heat pipe with EW-based pumping. EW is a well-developed fluid handling technique to carry out various microfluidic operations, such as droplet pumping, and is based on the electrical modulation of surface tension. The EHP relies on EW pumping of the condensate to the evaporator in the form of discrete droplets. EHPs are not subject to the capillary limit on heat transport capacity. The first-order models predict that EHPs with a cross section footprint of 10 cm $\times 4$ mm (width $\times $ thickness) can transport 1.5 kW over extended distances (>1 m) using water as the working fluid. Other benefits include ultralow power consumption (microwatts), opportunities for active control of heat pipes, selection of a wide range of working fluids, and the absence of moving parts. The heat transport capacity and the thermal resistance of an EHP are compared with four other devices of similar dimensions: 1) a sintered powder heat pipe; 2) an axial groove heat pipe; 3) a loop heat pipe; and 4) a thermosyphon. The EHP can transport more than twice the maximum heat capacity of other devices while offering a low thermal resistance of 0.01 K/W. Thus, EW is an enabling technology for the development of long, high-capacity heat pipes.

Integration of Planar Heat Pipes to Solid Oxide Cell Short Stacks

AbstractThis work presents the results of a thermal balancing, heat removal and supply mechanism via high temperature heat spreaders integrated to the solid oxide cell (SOC) stacks. Planar alkali metal heat pipes for almost isothermal two‐directional heat transfer at high power densities are incorporated into the metal interconnector structure of SOC‐short stacks. The objective of this concept is a temperature gradient shrinking within the stacks and a thermal control of SOFC system with a largely reduced additional air cooling. Subsequent to the development of planar heat pipe interconnectors with thickness down to 4 mm and heat transfer rates of up to 150 W per cm2 cross sectional area at isothermal operation, this paper describes their integration into the SOC stack structure. Therefore, short stacks with incorporated planar heat pipes are designed and built up for evaluation. The experimental study focuses on the influence of planar heat pipe interconnectors on temperature distributions within short stacks under various thermal load conditions. The experiments demonstrate the beneficial effects on thermal control of the stack structure and allow a first estimation of their amplitudes.

Thermal-Fluid Modeling For High Thermal Conductivity Heat Pipe Thermal Ground Planes

The thermal ground plane is an advanced planar heat pipe designed for cooling microelectronics in high gravitational fields. A thermal resistance model is developed to predict the thermal performance of the thermal ground plane, including the effects of the presence of noncondensable gases. Viscous laminar flow pressure losses are predicted to determine the maximum heat load when the capillary limit is reached. This paper shows that the axial effective thermal conductivity of the thermal ground plane decreases when the substrate and/or wick are thicker and/or with the presence of noncondensable gases. Moreover, it was demonstrated that the thermal-fluid model may be used to optimize the performance of the thermal ground plane by estimating the limits of wick thickness and vapor space thickness for a recognized internal volume of the thermal ground plane. The wick porosity plays a significant role in maximum heat transport capability. A large adverse gravitational field strongly decreases the maximum heat transport capability of the thermal ground plane. Axial effective thermal conductivity is mostly unaffected by the gravitational field. The maximum length of the thermal ground plane prior to reaching the capillary limit is inversely proportional to input power.

Ceramic-Based Planar Heat Pipe (Plate) for Passive Electronics Cooling

Heat dissipation has become a major hurdle for the electronics industry, especially as higher performance integrated circuits are being developed for the power industry. Two of the primary hurdles in dissipating this heat are:The thermal contact resistance between the IC and the cooling device.The ability to effectively spread the heat, such that traditional cooling technologies can be effective.By selecting ceramic materials that are thermo-mechanically matched (CTE) to IC materials, the proposed heat plate can be directly bonded by typical solder or braze techniques to the back-side of the IC. This eliminates thermal resistances due to contact and thermal interface materials. Within these heat plates, a three dimensional network of gas channels and fluid wicks spread the high-flux heat loads from localized hot spots to the surrounding regions via phase change fluids and mass transport. Like traditional heat pipes, these heat plates operate at nearly uniform temperature due to the phase change. The internal networks provide for multidimensional heat and mass flow, increasing their dissipating capability. By using matched ceramic materials, and the inclusion of a heat plate, these primary hurdles for heat dissipation can be mitigated. The performance of prototypical planar heat plates will be presented.

Feasibility of Metallic Structural Heat Pipes as Sharp Leading Edges for Hypersonic Vehicles

Hypersonic flight with hydrocarbon-fueled airbreathing propulsion requires sharp leading edges. This generates high temperatures at the leading edge surface, which cannot be sustained by most materials. By integrating a planar heat pipe into the structure of the leading edge, the heat can be conducted to large flat surfaces from which it can be radiated out to the environment, significantly reducing the temperatures at the leading edge and making metals feasible materials. This paper describes a method by which the leading edge thermal boundary conditions can be ascertained from standard hypersonic correlations, and then uses these boundary conditions along with a set of analytical approximations to predict the behavior of a planar leading edge heat pipe. The analytical predictions of the thermostructural performance are verified by finite element calculations. Given the results of the analysis, possible heat pipe fluid systems are assessed, and their applicability to the relevant conditions determined. The results indicate that the niobium alloy Cb-752, with lithium as the working fluid, is a feasible combination for Mach 6–8 flight with a 3 mm leading edge radius.

A device and technique to measure the heat transfer limit of a planar heat pipe wick

Heat pipes are increasingly being utilized in applications for cooling electronics such as micro-processors. Heat pipes afford a very high heat transfer rate with only a small temperature difference between the heat source and heat sink. However, heat pipes have several operating limits, known as heat transfer limits, that can cause a severe reduction in the heat transfer rate and lead to catastrophic failure of the micro-processor. Two of these limits, known and the capillary limit and boiling limit are primarily a function of the heat pipe wick. In the present work, we developed a test apparatus and technique to determine the heat transfer limit of a planar heat pipe wick. The advantage of this apparatus and technique is a quantifiable measurement of the wick’s heat transfer limit without being confounded by other operating limit mechanisms such as the viscous or sonic limits. The apparatus and technique also yields the wick’s effective thermal conductivity. The apparatus is compatible with many of the low temperature heat pipe working fluids such as water or methanol, can be operated in any orientation with respect to the gravity vector, accounts for parasitic heat losses, provides for flow visualization studies, and is adaptable to a wide range of wick thicknesses.

Fin efficiency enhancement using a gravity assisted planar heat pipe

Abstract An experimental and analytical study was conducted to evaluate a planar heat pipe used as a cooling fin for high density heat dissipation. Both natural and forced convection configurations were investigated. For forced convection, the thermal resistance of the heat pipe, 100 mm × 90 mm × 5 mm in dimensions, was more than three times smaller than that for a solid fin with the same dimensions when the heat dissipation rate exceeded 10 W cm 2 and the heating temperature was under 100°C. The improved performance makes planar heat pipes viable alternatives to conventional solid fins, for applications requiring efficient and reliable heat dissipation.

Physical mechanisms for heat and momentum transfer in a short low-temperature heat pipe. II. Vapor flow structure

The authors present results of an experimental investigation of the structure of flow of a condensible vapor flow in a planar heat pipe. They have examined the influence of volume condensation in the vapor stream and in the vapor-gas front region on heat and mass transfer in the heat pipe.

Physical mechanisms for transfer of heat, mass, and momentum in a short low-temperature heat pipe. I. Hydrodynamics of vapor flows

The authors present results of an experimental investigation of vapor flow hydrodynamics under positive pressure gradient conditions leading to boundary layer separation and the occurrence of a reverse vortex flow region of vapor in the condensation zone of a planar heat pipe. The influence of a noncondensible gas on heat and mass transfer and the configuration of the vapor gas front is examined.

DEVELOPMENT AND EVALUATION OF A PLANAR HEAT PIPE FOR COOLING ELECTRONIC SYSTEMS

A planar heat pipe (;rectangular cross section) was designed, built and tested using flexible electrical rubber heaters to provide the necessary heat. The device was constructed from 0.043” (0.1092 cm) copper sheet, with three layers of 100-mesh copper screen as a capillary wick. The dimensions of the planar heat pipe were 6 inches ( 15.24 cm) by 12 inches (30.48 cm) by 3/4 inches (1.905 cm). Water was used as the working fluid. Steady state, and in once case, transient responses, were investigated. External axial temperature profiles for different applied powers and operating temperatures were measured. Operation of the device was stable and repeatable within a temperature range of 30° to 95°C; no temperature or pressure fluctuations were noted. The optimum amount of working fluid for a typical situation was obtained experimentally. Effects of air and amount of working fluid inside the planar heat pipe were investigated. The optimum amount of working fluid which was obtained experimentally was used throu...