Heat Pipe Applications Research Articles

Multiphase numerical analysis of heat pipe with different working fluids for solar applications

Energy crisis is a prognosis predicted in many cases with the indiscriminate encroachment of conventional energy sources for applications on a massive scale. This prediction, further emboldened by the marked surge in global average temperatures, attributed to climate change and global warming, the necessity to conserve the environment and explore alternate sources of energy is at an all-time high. Despite being among the lead candidates for such sources, solar energy is utilized far from its vast potential possibilities due to predominant economic constraints. Even while there is a growing need for solar panels at more affordable rates, the other options to harness better out of sun’s energy is to optimize and improvise existing technology. One such technology is the heat pipe used in Evacuated Tube Collectors (ETC). The applications of heat pipe have been gaining momentum in various fields since its inception and substantial volumes of research have explored optimizing and improving the technology which is proving effective in heat recovery and heat transfer better than conventional systems. This paper carries out a computational analysis on a comparative simulation between two working fluids within heat pipe of same geometry. It further endeavors to study the multiphase transitions within the heat pipe. The work is carried out using ANSYS Fluent with inputs taken from solar data for the location of Vellore, Tamil Nadu. A wickless, gravity-assisted heat pipe (GAHP) is taken for the simulation. Water and ammonia are used as the working fluids for comparative multiphase analysis to arrive at the difference in heat transfer at the condenser section. It is demonstrated that a heat pipe ETC with ammonia as working fluid showed higher heat exchange (temperature difference) as against that of water as working fluid. The multiphase model taken aided in study of phase transitions within both cases and supported the result of ammonia as fluid being a better candidate.

Prenošenje toplote u gravitacionoj toplotnoj cevi

Usled promene faze fluida, toplotna cev ima veoma visoku efektivnu toplotnu provodljivost i može da prenosi velike toplotne protoke i pri malim razlikama temperatura. U radu su date fizičke osnove funkcionisanja i prikazan je model prenošenja toplote u gravitacionoj toplotnoj cevi - termosifonu. Analiza dobijenih rezultata je izvršena za različite dimenzije toplotnih cevi, različite kombinacije radnog fluida i materijala cevi, kao i različite uslove rada. Dobijeni rezultati daju osnova za širu primenu toplotnih cevi u procesnoj tehnici.

Electrowetting-based microfluidic operations on rapid-manufactured devices for heat pipe applications

The heat transport capacity of traditional heat pipes is limited by the capillary pressure generated in the internal wick that pumps condensate to the evaporator. Recently, the authors conceptualized a novel heat pipe architecture, wherein wick-based pumping is replaced by electrowetting (EW)-based pumping of microliter droplets in the adiabatic section. An electrowetting heat pipe (EHP) can overcome the capillary limit to heat transport capacity and enable compact, planar, gravity-insensitive, and ultralow power consumption heat pipes that transport kiloWatt heat loads over extended distances. This work develops a novel technique for rapid, scalable fabrication of EW-based devices and studies critical microfluidic operations underlying the EHP, with the objective of predicting the key performance parameters of the EHP. Devices are fabricated on a printed circuit board (PCB) substrate with mechanically-milled electrodes, and a removable polyimide dielectric film. The first set of experiments uncovers the maximum channel gap (1 mm) for reliable EW-based pumping; this parameter determines the heat transport capacity of the EHP, which scales linearly with the channel gap. The second set of experiments uncovers the maximum channel gap (375 microns) at which EW voltages can successfully split droplets. This is an important consideration which ensures EHP operability in the event of unintentional droplet merging. The third set of experiments demonstrate and study EW-induced droplet generation from an open-to-air reservoir, which mimics the interface between the condenser and adiabatic sections of the EHP. The experimental findings predict that planar, water-based EHPs with a (10 cm by 4 mm) cross section can transport 1.6 kW over extended distances (>1 m), with a thermal resistance of 0.01 K W−1.

Heat dissipation in high-power semiconductor lasers with heat pipe cooling system

This study focuses on the application of heat pipes in thermal management for high-power semiconductor lasers. The heat pipe cooling systems are used for heat dissipation in high-power semiconductor lasers. These systems are used instead of water cooling machines to realize a compact and lightweight laser module. The n-shaped heat pipe cooling system, which consists of eight 6 mm copper heat pipes with sintered powder wicks, can easily handle a heat load of up to 73 W from a single-laser unit. The fabricated U-shaped heat pipe cooling system, which consists of ten 12 mm copper heat pipes with sintered powder wicks, can easily handle a heat load of up to 300 W from five laser units. The optical power of the multi-laser module cooled by the U-shaped heat pipe cooling system reaches 210 W. These results indicate that high-power semiconductor lasers can be cooled using heat pipe cooling systems instead of water cooling machines.

Energy saving potentials from the application of heat pipes on geothermal heat pump system

A ground-loop heat exchanger is one of the key components to collect low grade ground heat for a ground source heat pump system. A typical ground source heat pump consists of a refrigerant loop operated by a compressor and a geothermal brine loop operated by a circulation pump. Although ground source heat pumps are known as one of the most efficient HVAC devices, their performance still need to be improved since the energy consumption by the peripherals like the circulation pump are considerable and estimated more than 10% of the total electricity input. In order to reduce the ground circulation energy, a variety of researches such as direct expansion geothermal heat pumps are studied, but no apparent performance improvement was found due to other negative effects like the increase of pressure drop in the ground loop. Geothermal heat pipe can be a good alternative to replace ground-loop heat exchanger for heating purpose. The energy saving potential using heat pipe as ground heat exchanger was investigated by comparing the energy consumption and COP of three types ground source heat pump. As a result, energy consumption for heating in Seoul by 10.3% than direct expansion ground source heat pump and 21.1% than secondary loop ground source heat pump. Therefore, ground source heat pump with geothermal heat pipe can be a suitable choice to save energy in a cold region.

A REVIEW OF WASTE HEAT RECOVERY BY USING THE HEAT PIPE AND THERMO-ELECTRIC GENERATOR

This paper continued a detail review of heat pipe and thermoelectric generator application in industrial, automobile and solar waste heat recovery. Each day new greenhouse gasemission further accelerates global warming. The exhaust heat recovery system by usingheat pipe and TEGsis topotentially reduce the greenhouse gases emission. The use of two specific technologies heat pipe and TEGs make passivesolid state system design. Heat pipe used in system consents more flexible design as TEG location not inadequate to exhaust pipe surface. The conversion efficiency of thermoelectric generator is function of temperature difference Heat source and heat sink temperature. The thermal performance of thermoelectric generator will be improved or enhanced by using heat pipe as a heat source in thermoelectric generator. Heat pipes do have limitations such as maximum rates of heat transfer and temperature limits. TEGs do have limitations such as low temperature limits and relatively low efficiency. When used in conjunction, these technologies have the potential to create a completely solid state and passive waste heat recovery system.

Effect of non-condensable gas on the startup of a loop heat pipe

It is essential to address the startup issues prior to the wide application of loop heat pipes (LHPs) in both space and terrestrial surroundings. As non-condensable gas (NCG) is an important factor affecting the startup behavior, its effects on the startup performance of an ammonia-stainless steel LHP with and without preconditioning were experimentally investigated in this work. Nitrogen with controlled amounts was used to simulate the NCG, and the temperature overshoot, liquid superheat and startup time were employed as the evaluation criteria. Four situations relating to initial liquid/vapor distribution in the evaporator were examined: (1) both evaporator core and vapor grooves are filled with liquid, (2) vapor exists in vapor grooves and the evaporator core is filled by liquid, (3) vapor grooves are filled by liquid and vapor exists in the evaporator core, and (4) vapor exists in both evaporator core and vapor grooves. Experimental results showed that with NCG presence in the LHP, the startup could only proceed in situation 1 with preconditioning, while it could proceed in situations 1, 3 or 4 without preconditioning. For the startup in situation 1, a larger NCG inventory led to much degraded startup performance, and a higher startup heat load could benefit the startup. For the startup in situation 3, the most difficult startup situation, NCG resulted in a very high temperature overshoot, which may even exceed the maximum allowable value. For the startup in situation 4, the existence of NCG in the vapor grooves could facilitate the evaporation there, leading to a very desirable startup.

Accident analysis of heat pipe cooled and AMTEC conversion space reactor system

A space power with high power density, light weight, low cost and high reliability is of crucial importance to future exploration of deep space. Space reactor is an excellent candidate because of its unique characteristics of high specific power, low cost, strong environment adaptability and so on. Among all types of space reactors, heat pipe cooled space reactor, which adopts the passive heat pipe (HP) as core cooling component, is considered as one of the most promising choices and is widely studied all over the world.This paper develops a transient analysis code (TAPIRS) for heat pipe cooled space reactor power system (HPS) based on point reactor kinetics model, lumped parameter core heat transfer model, combined HP model (self-diffusion model, flat-front startup model and network model), energy conversion model of Alkali Metal Thermal-to-Electric Conversion units (AMTEC), and HP radiator model. Three typical accidents, i.e., control drum failure, AMTEC failure and partial loss of the heat transfer area of radiator are then analyzed using TAPIRS. By comparing the simulation results of the models and steady state with those in the references, the rationality of the models and the solution method is validated. The results show the following. (1) After the failure of one set of control drums, the reactor power finally reaches a stable value after two local peaks under the temperature feedback. The fuel temperature rises rapidly, however it is still under safe limit. (2) The fuel temperature is below a safe limit under the AMTEC failure and partial loss of the heat transfer area of radiator. This demonstrates the rationality of the system design and the potential applicability of the TAPIRS code for the future engineering application of heat pipe cooled space reactor.

Fabrication of Lotus-Type Porous Copper Using Slip Casting and Sintering Techniques for Heat Pipe Applications

A lotus type-porous copper will be used to substitute wick materials from tabulate coral capillary for heat pipe applications. The lotus type-porous material are normally fabricated using Gasar process, in which unidirectional solidification was applied to the metal-eutectic systems to obtain long cylindrical pores in the direction of solidification. A new process is proposed to fabricate the lotus type-porous material using slip casting and sintering. As an initial work, this paper is aimed to obtain appropriate processing parameters of fabrication of lotus-type porous copper using slip casting and sintering techniques. Nylon strings were used as pore formers to form cylindrical pores. They were coated with copper slurry consisting of copper powder and binding agent. The coated nylon strings were arranged in a mold and the slurry were poured. After drying, these castings were sintered at various sintering temperatures and sintering times. The results showed that starch as the binding agent is superior to PVA. The mixture of starch-water with starch content of 97%, the copper slurry with copper content of 50% by volume, the copper powder size of 200 µm, the sintering temperatur of 900°C, and the sintering time of 60 minutes are able to produce lotus-type porous copper similar to that produced through Gasar process.

Research on Heat Recovery Technology for Reducing the Energy Consumption of Dedicated Ventilation Systems: An Application to the Operating Model of a Laboratory

In this research, the application of heat pipes in the air handler dedicated to decoupling dehumidification from cooling to reduce energy consumption was simulated and investigated by simulations and experimental studies. The cooling load profiles and heat pipes with effectiveness of 0.45 and 0.6, respectively, were evaluated in achieving the desired space conditions and calculated hour by hour. The results demonstrated that for all examined cases, a heat pipe heat exchanger (HPHX) can be used to save over 80% of the energy during the hours of operation of air conditioning. The overall energy reduction rate was from 3.2% to 4.5% under air conditioning system conditions. It was found that the energy saving potential of a laboratory was higher than for other kinds of buildings. Therefore, the dedicated ventilation system combined with heat recovery technology can be efficiently applied to buildings, especially for laboratories in subtropical areas.

Heat pipes application to solar energy systems

Today widespread application of energy-saving equipment based on heat pipes makes a significant contribution to the task of resources saving. Using the heat pipes as heat transfer and heat exchange design elements allows creating new effective equipment generation for solar energy systems. Heat pipes are widely used both to improve the outdated equipment, increase its efficiency, reliability and lifetime and in the creation of new high-quality and economic technology samples. Up to the present day there are the following systems and solar energy equipment where heat pipes are widely used: photovoltaic-thermal solar collectors, solar thermal collectors, concentrating photovoltaic and concentrating solar plant. The article presents an analysis of the current state and prospects of heat pipes using in solar energy systems.

Application of pump-assisted separate heat pipe on dehumidifying enhancement in air conditioning system

A study was carried out to investigate the effect of pump-assisted separate heat pipe (PASHP) on energy consumption and dehumidifying enhancement in the air conditioning system. In the experiment, four sets of PASHP filled into R134a as the working fluid were constructed. when the number of PASHP employed in the system changes from 0 to 4 sets at the constant inlet air state of 28.5 °C and 60% relative humidity, the apparatus dew point falls from 11.7 °C to 8.2 °C, the supply air temperature rises from 11.7 °C to 24.1 °C, while dehumidifying capacity of the system has ultimately increased by 29.5%. The index β (ratio between the cooling energy saved by PASHP and the total cooling capacity of the air cooling system with PASHP) is 40.4%, and the index δ (ratio between the cooling energy saved by PASHP and the cooling capacity of the air conditioner's evaporator) is 66.0%. The study indicates that the air conditioning system employed with PASHP can significantly reduce its energy consumption and improve its dehumidifying capacity.

Porous Cu Nanowire Aerosponges from One-Step Assembly and their Applications in Heat Dissipation.

Highly porous metal nanowire aerosponges are produced by direct assembly of the Cu nanowire in situ during their synthesis. Such a method offers not only great simplicity, but also excellent properties such as extremely low densities, high electrical conductivities, and remarkable mechanical properties. Furthermore, these Cu aerosponges exhibit excellent wicking behavior, suggesting their potential for heat-exchange applications in heat pipes.

Analytical expression for velocity profiles and flow resistance in channels with a general class of noncircular cross sections

This article discusses how to derive a closed-form analytical solution describing the profile of unidirectional flow through a class of noncircular tubes. The cross sections of the conduits are a slightly distorted version of regular shapes, like triangles or trapeziums or rectangles, that are commonly seen in microfluidic or heat pipe applications. Hence, the study leads to approximate but useful expression for velocity profiles as well as resistivity for the aforementioned channels. Moreover, the paper shows how to extend the fundamental idea to other arbitrary geometries, providing simple expressions for flow fields that can be used subsequently in various hydrodynamic calculations. The analysis also gives an estimate of error in such a mathematical treatment that is found to be adequately accurate. Thus, the findings will be especially relevant in a number of fields, including microfluidic separation, heat pipe design, and capillary transport optimization.

Circulation Effectiveness of Working Fluid in Inclined Micro Heat Pipes

Micro heat pipe is a two-phase heat transfer device offering effective high heat-flux removal in electronics cooling. Essentially, micro heat pipe relies on the phase change processes, namely evaporation and condensation, and the circulation of working fluid to function as heat transfer equipment. The vast applications of micro heat pipe in portable appliances necessitate its functionality under different orientations with respect to gravity. Therefore, its thermal performance is strongly related to its orientation. By incorporating solid wall conduction, together with the continuity, momentum, and energy equations of the working fluid, a mathematical model is developed to investigate the heat and fluid flow characteristics of inclined micro heat pipes. We investigate both the favorable and adverse effects of gravity on the circulation rate which is intimately related to the thermal performance of micro heat pipes. The effects of gravity, through the angle of inclination, on the circulation strength and heat transport capacity are analysed. This study serves as a useful analytical tool in the micro heat pipe design and performance analysis, associated with different inclinations and operating conditions.

Two-Phase Flow With Phase Change in Porous Channels

Phase change heat transfer in porous media finds applications in various geological flows and modern heat pipes. We present a study to show the effect of phase change on heat transfer in a porous channel. We show that the ratio of Jakob numbers based on wall superheat and inlet fluid subcooling governs the liquid–vapor interface location in the porous channel and below a critical value of the ratio, the liquid penetrates all the way to the extent of the channel in the flow direction. In such cases, the Nusselt number is higher due to the proximity of the liquid–vapor interface to the heat loads. For higher heat loads or lower subcooling of the liquid, the liquid–vapor interface is pushed toward the inlet, and heat transfer occurs through a wider vapor region thus resulting in a lower Nusselt number. This study is relevant in the designing of efficient two-phase heat exchangers such as capillary suction based heat pipes where a prior estimation of the interface location for the maximum heat load is required to ensure that the liquid–vapor interface is always inside the porous block for its operation.

More about Thermosyphone Rankine Cycle Performance Enhancement

The heat pipe applications have been coupled with the renewable energy sources such as solar energy, waste heat and geothermal energy. Thermosyphon Rankine Cycle (TRC) is a vertical wickless heat pipe engine. In this engine, the turbine is installed between the insulated section and a condenser section of thermosyphon. The mechanical energy developed by the turbine can be converted to electricity by direct coupling to an electrical generator. Our simulation results showed that the enhanced TRC model is able to increase the efficiency of the TRC system. This paper introduces the miscellanies new ways in order to improve the performance of a TRC system for supplying the sustainable electricity. For example, a0.78%increase in the turbine useful efficiency due to the superheating process was obtained.

Experimental Application of Heat Pipes in Hydraulic Oil Cooler

Experimental Application of Heat Pipes in Hydraulic Oil Cooler

Sintered Nickel Powder Wicks for Flat Vertical Heat Pipes

The fabrication and performance of wicks for flat heat pipe applications produced by sintering a filamentary nickel powder has been investigated. Tape casting was used as an intermediate step in the wick production process. Thermogravimetric analysis was used to study the burn-off of the organic binder used and to study the oxidation and reduction processes of the nickel. The wicks produced were flat, rectangular and intended for liquid transport in the upwards vertical direction. Rate-of-rise experiments using heptane were used to test the flow characteristics of the wicks. The wick porosities were measured using isopropanol. The heat transfer limitation constituted by the vapour static pressure and the capillary pressure was discussed. The influence on wick performance by using pore former in the manufacturing was studied. When Pcap/Psat > 1, the use of a pore former to increase the wick permeability will always improve the wick performance. When Pcap/Psat < 1, it was shown that if the effective pore radius and the permeability increase with an equal percentage the overall influence on the wick capacity is negative. A criterion for a successful pore former introduction is proposed and the concept of a pore former evaluation plot is presented.

Performance of compressed nickel foam wicks for flat vertical heat pipes

The fabrication and performance of wicks for vertical flat heat pipe applications produced by compression of nickel foams has been investigated. The permeabilities and the effective pore radii for the wicks were estimated from rate-of-rise experiments, using the model fluid heptane. The porosities of the wicks were measured using isopropanol. The results, which are new and of vital importance for optimum use of such wicks, show that the permeabilities and the effective pore radii are in the upper range for heat pipe use. The joining pressure required during the sintering of the wicks was determined, and it was discovered that the nickel foams turned hydrophilic during the sintering.