Micro-heat Pipe Research Articles

Experimental study on the new type of electrical storage heater based on flat micro-heat pipe arrays

A new type of electrical storage heater that utilizes latent heat storage and flat micro-heat pipe arrays (FMHPAs) was developed. The thermal characteristics of the heater were tested through experimentation. The structure and operating principle of the storage heater were expounded. Three rows of FMHPAs were applied (three rows with five assemblies each) with a mass of 28 kg of phase change material (PCM) in the heat storage tank. Electric power was supplied to the PCM in the range of 0.2‒2.04 kW, and air was used as heat transfer fluid, with the volume flow rate ranging from 40‒120 m 3 /h. The inlet temperature was in the range of 15‒24°C. The effects of heating power, air volume flow rate, and inlet temperature were investigated. The electrical storage heater exhibited efficiencies of 97% and 87% with 1.98 and 1.30 kW of power during charging and discharging, respectively. Application of the proposed storage heater can transfer electricity from peak periods to off-peak periods, and the excess energy generated by wind farms can be stored as heat and released when needed. Good economic and environmental benefits can be obtained.

A Comparative Investigation of Two Types of MHPA Flat-Plate Solar Air Collector Based on Exergy Analysis

In this study, a comparative investigation of two types of microheat pipe array (MHPA) flat-plate solar air collectors (FPSAC) based on exergy analysis has been conducted. The thermal performance of MHPA-type solar air collectors (SACs) with two different shaped fins is experimentally evaluated. A detailed parametric study is also conducted to examine the effects of various fins, operation parameters, and inlet air temperature at different mass flow rates on thermal and exergy efficiencies. Results indicated that using V-shaped slotted fins at the specified range of mass flow rates can enhance exergy efficiency. Exergy efficiency can be considered as the main criterion to evaluate the performance of MHPA FPSACs. Attaching V-shaped slotted fins on the condenser section of MHPA is more effective than attaching rectangular fins at high mass flow rates. By contrast, the latter is more effective than the former at low mass flow rates.

Experimental study on an integrated collector storage solar air heater based on flat micro-heat pipe arrays

This study investigated the performance of an integrated collector storage solar air heater based on latent heat storage and flat micro-heat pipe arrays. The structure and working principle of the device were explained in detail. The effective area of the collector is 0.93m2, and the mass of the phase change material (PCM) is 45.8kg. An integrated collector storage experimental system was set up, and outdoor experiments were conducted to demonstrate the feasibility of the device for energy storage and air heating. The temperature distribution of the PCM was recorded during charging and discharging to monitor the propagation of the melting and freezing front. Efficiency and power during charging and discharging were also calculated. The integrative device exhibited 59% and 91.6% efficiencies for solar air charging and discharging at 393 and 344W power, respectively.

Experimental study on the heat recovery characteristic of a heat exchanger based on a flat micro-heat pipe array for the ventilation of residential buildings

Heat recovery significantly and positively affects energy conservation and prevents global warming. A small flat heat pipe heat recovery device (SFHPHRD), which applies flat micro-heat pipe array (FMHPA) with welded, serrated and staggered fin on its surface, is designed as a core heat transfer component in the heat recovery systems of residential buildings. The air volume flow ratio between fresh air and exhaust air was maintained at a value of 1 in the experiment. Under simulated winter and summer conditions, the performance of SFHPHRD under varying indoor and outdoor air temperatures, air volume flows and rows of FMHPAs were separately investigated. Heat recovery efficiency, coefficient of performance (COP) and device volume were also analyzed. Important influencing factors, which affect the performance of SFHPHRD, were considered. Experimental data were combined actual data from buildings to analyze the energy conservation capacity and application of SFHPHRD. Results showed that efficient heat transfer of FMHPA with welded, serrated, and staggered fin and the independent components enables SFHPHRD to possess high heat recovery efficiency, high reliability, low resistance, and suitable volume. The maximum high heat recovery efficiency and COP can reach 78% and 91.9 under experimental conditions, respectively.

Heat Transfer Properties of a Latent Thermal Storage Unit with Flat Microheat Pipe Arrays

AbstractA thermal storage unit (TSU) with flat microheat pipe arrays (FMHPAs) as heat transfer core is experimentally investigated in this study. A comprehensive evaluation parameter of the TSU def...

Improve the Performance of Micro Heat Pipe by Surface Modification

The appearance of high power LED has put forward higher requirements for thermal management. The micro heat pipe (MHP) has high heat transfer ability and plays an important role in high power LED and other high heat flux device cooling. In this paper, we designed and fabricated a micro heat pipe with fluoroalkyl silane (FAS) surface modified glass cover. The contact angle of the working area of glass cover reached 95.49° and made working fluid drops fall back to micro groove of silicon substrate more quickly. Thus the new glass cover can speed up the circulation of working fluid and enhance the heat transfer. The experimental results also proved that hydrophobic glass cover has a better heat transfer capability. Besides, this novel MHP reached the stable working status faster. When the input heat power was 10 W, the balance temperature of MHP with hydrophobic glass cover was 22 oC lower than traditional MHP, while the balance time is 58 seconds less. The work presented in this paper provides a new direction for optimize the MHP, not only the wick structure in substrate, but also the wettability of cover plate.

Influence of Parallel and Trapezoidal Wick Structures to the Heat Transfer Capability of MHPs

Heat transfer capability of micro heat pipe (MHP) is relied on the thermal resistance of material, the specific phase change latent heat of working fluid, and the pattern of micro structures which is served as wick. In this paper, parallel and trapezoidal micro Cu structures were designed and fabricated by UV-LIGA. The capillary pressure and the effective coefficient of heat transfer conductivity of the micro grooves were calculated and results showed that trapezoidal grooves with small dimension in evaporator and large dimension in condenser (forward trapezoidal) have the best performance, because this structure can generate larger capillary force of working fluid and enhance the heat transfer. Two MHPs based on the calculations were fabricated and tested, results demonstrated that forward trapezoidal groove had the lowest equilibrium temperature while the inversed trapezoidal groove had the highest equilibrium temperature, approved the numerical calculation results. When the input power was 10W, the equilibrium temperature of forward trapezoidal, parallel and backward trapezoidal grooved MHP was 67.2 oC, 73.4°C and 89.1 oC, respectively. The forward trapezoidal grooves enlarge the capillary pressure and benefit the heat transfer of MHP.

New encapsulation method using low-melting-point alloy for sealing micro heat pipes

This study proposed a method using Low-melting-point alloy (LMPA) to seal Micro heat pipes (MHPs), which were made of Si substrates and glass covers. Corresponding MHP structures with charging and sealing channels were designed. Three different auxiliary structures were investigated to study the sealability of MHPs with LMPA. One structure is rectangular and the others are triangular with corner angles of 30° and 45°, respectively. Each auxiliary channel for LMPA is 0.5 mm wide and 135 μm deep. LMPA was heated to molten state, injected to channels, and then cooled to room temperature. According to the material characteristic of LMPA, the alloy should swell in the following 12 hours to form strong interaction force between LMPA and Si walls. Experimental results show that the flow speed of liquid LMPA in channels plays an important role in sealing MHPs, and the sealing performance of triangular structures is always better than that of rectangular structure. Therefore, triangular structures are more suitable in sealing MHPs than rectangular ones. LMPA sealing is a plane packaging method that can be applied in the thermal management of high-power IC device and LEDs. Meanwhile, implanting in commercialized fabrication of MHP is easy.

Thermal performance of solar air collection-storage system with phase change material based on flat micro-heat pipe arrays

In this study, a new type of solar air collection-storage thermal system (ACSTS) with phase change material (PCM) is designed using flat micro-heat pipe arrays (FMHPA) as the heat transfer core element. The solar air collector comprises FMHPA and vacuum tubes. The latent thermal storage device (LTSD) utilizes lauric acid, which is a type of fatty acid, as PCM. The experiments test the performance of collector efficiency and charging and discharging time of thermal storage device through different air volume flow rates. After a range of tests, high air volume flow rate is concluded to contribute to high collector efficiency and short charging and discharging time and enhance instantaneous heat transfer, whereas an air volume flow rate of 60m3/h during discharging provides a steady outlet temperature. The cumulative heat transfer during discharging is between 4210 and 4300kJ.

Performance evaluation of a novel flat-plate solar air collector with micro-heat pipe arrays (MHPA)

This paper proposes a new type of a flat-plate solar air collector (SAC) with micro-heat pipe arrays (MHPA), which consist of a heat collection and transfer section and an air ventilation and heat exchange section. In the unique structure, the heat exchange of air is separated from heat collection and transmission. This work experimentally and theoretically investigates the heat transfer and friction factor characteristics of the proposed MHPA-flat plate solar air collector. Numerical simulation and experimental results are compared to show the instantaneous performance of the collector under quasi-steady state conditions. Efficiency, time constant, inlet temperature, outlet temperature, absorber temperature, and glass cover temperature are obtained. The effects of weather condition, and operational parameters are evaluated on the basis of thermal efficiency, outlet temperature, heat loss, and heat transfer. Results reveal that the average efficiency is approximately 69% at a flow rate of 290m3/h. In the testing range of the air flow rate, the frictional coefficient varies from 0.05 to 0.18.

Non-linear drag induced entropy generation analysis in a microporous channel: The effect of conjugate heat transfer

The entropy generation analysis in a viscous dissipative flow of a Newtonian fluid through a hyper-porous microchannel formed between two heated parallel plates is conferred. Employing an analytical method, which is consistent with the perturbation analysis, the transport equations governing the thermo-hydrodynamics are studied. The effects of nonlinear Forchheimer drag and conjugate heat transfer on the thermal transport characteristics of heat are considered, while the thermal boundary conditions of third kind have been employed at the outer boundaries of channel for the conjugate heat transfer analysis. The explicit alterations are made in the thermal transport of heat in the system as attributable to the effect of dissipative heat generated due to the non-linear effect Forchheimer drag, Darcy frictional effect and the viscous shearing stress in the flow field. To account these effects, the explicit variation of Forchhiemer constant, Darcy number, porosity, thickness and conductivity of upper wall and Biot number of upper wall are carried out to shows the changes in the available energy of the system. Also, it is shown that the effect of non-linear drag mainly stemming from the presence of complex porous structure in the flow field and its interaction with the conjugate transport of heat alters the heat transfer rate in the system non-trivially, which, in turn, gives rise to the entropy generation in the system. The individual contribution of two different effects viz., the heat transfer and viscous dissipation on the system entropy generation rate for different cases are studied. It is believed that the implications of the present analysis may have direct bearing on the design of micro devices/systems typically used in electronic cooling, micro-heat pipes.

Effect of Conjugate Heat Transfer on Entropy Generation in Slip-Driven Microflow of Power Law Fluids

ABSTRACTWe analyze the entropy generation characteristics in a non-Newtonian microflow under the influence of interfacial slip as modulated by the conjugate transport of heat. We consider power law model to represent the constitutive behavior of the non-Newtonian fluid. In this analysis, we analytically solve the transport equations employing the thermal boundary condition of the third kind at the exterior wall surface accounting for the effect of conjugate heat transfer. We demonstrate that the slip flow–driven alteration in convective transport of heat and its nonlinear interaction with viscous dissipation, as modulated by fluid rheology and conjugate transport of heat, gives rise to a minimum entropy generation rate of the system. We determine the optimum value of the geometrical parameter—that is, the channel wall thickness and the thermophysical parameters, such as the Biot number and Peclet number—leading to a minimum entropy generation rate in the system. The results of this analysis could be of helpful in designing microsystems/devices typically used for electronic cooling, micro-heat pipes, and micro-heat exchangers.

Enhancement of thermal performance of micro heat pipes using wettability gradients

We present a methodology for enhancing thermal performance of micro heat pipes (MHPs) by creating wettability gradients on the inner surface of MHPs. We analyse the effect of wettability gradients on MHP performance using a quasi one-dimensional mathematical model that accounts for axially-varying solid–liquid contact angle. For our analysis, we consider MHPs with various wettability schemes, such as uniform, step-variation, and linear variation in the contact angle. Our model predictions show that increasing the wettability of the evaporator surface and reducing the wettability of the condenser surface of MHP can lead to an increase in the heat transfer capacity of MHP by over 35%. We demonstrate the favourable effect of wettability gradients on MHP performance for different working fluids over a wide range of operating temperature and fluid charge. We also discuss the underlying physical mechanism that leads to enhanced thermal performance of MHP with mixed wetting surfaces. We show that the optimal choice of wettability gradient in MHP is governed by the competing effects of high liquid flow resistance in the lower wetting condenser and high liquid mass in the higher wetting evaporator.

A Novel Flat Micro Heat Pipe With a Patterned Glass Cover

The light-emitting diode (LED) is a promising lighting source because of its energy-saving and environment friendly features. High-power LEDs have specific wide applications, such as car headlights and street lamps. However, heat transfer is still a critical issue that limits the usage of LEDs. The flat microheat pipe (FMHP) is a suitable passive heat transfer device for LEDs. Silicon substrate was selected to fabricate LED electrodes and the wick structure of the FMHP, and thus, LEDs’ mount base and the FMHP were integrated together and the contact thermal resistance could be reduced. Meanwhile, Pyrex 7740, which could be anodic bonded to the silicon substrate and sustained relatively high temperature and pressure, was used to fabricate its cover plate. In this paper, a new type of FMHP is proposed. The cover plate had circles patterned on it, and the etched structure was shell shaped. The contact angle of it was 81.46°, whereas that of the normal cover was 67.97°, which meant that the new cover was more hydrophobic. Theoretically, the new cover could make working fluid drops fall down to the grooves in a relatively shorter time and speed up heat transfer. The evaporator temperatures of the novel FMHP and that of the normal one were 71.6 °C and 79.4 °C, respectively. In addition, the new designed FMHP also had better temperature uniformity. The experimental results proved that the patterns on the cover plate of the FMHP could promote the heat transfer capability of the LED module.

Experimental study on the thermal performance of a new type of thermal energy storage based on flat micro-heat pipe array

The thermal performance of an air-based phase change storage unit is analyzed and discussed in this study. The thermal energy storage uses flat micro-heat pipe array (FMHPA) as the core heat transfer component and lauric acid as phase change material (PCM). An experimental system is devised to test the heat storage–release property of the storage unit under different inlet temperatures and flow rates of the heat transfer medium. The performance of the storage unit and the melting/solidification curves of the phase change material are obtained based on extensive experimental data. Experimental results indicate that the flat micro-heat pipe array exhibits excellent temperature uniformity in the heat storage–release process, and the performance of the storage unit is efficient and steady.

Thermal performance of a new CPC solar air collector with flat micro-heat pipe arrays

A new type of compound parabolic concentrator (CPC) solar air collector (SAC) with flat micro-heat pipe arrays (FMHPA) was investigated in this study. A cylindrical absorber was constructed by inserting the FMHPA into an evacuated glass tube to transport heat during the working process. The new FMHPA-CPC SAC is an edge ray collector with a concentration ratio of 1.8. The core concept is the integration of the FMHPA, an absorber tube, and a CPC reflector as a heat-collecting unit. Thermal performance investigation was conducted theoretically and experimentally. This study mainly analyzed the effect of different factors on the thermal performance of the collector. The efficiencies and heat loss coefficients of the new collector were determined. Experimental curves with different testing conditions are presented. The optimal value of efficiency was determined for the SAC. The aperture area of each CPC collecting unit is approximately 0.4 m2. The new FMHPA-CPC SAC was tested in Beijing, China. The average efficiency was approximately 61% at a volume flow rate of 320 m3/h, with a radiation of 799 W/m2 (ambient temperature of 28.8 °C). The instantaneous efficiency reached 68%, depending on the solar radiation, air volume flow rate, and ambient temperature. In addition, the time constant of the collector was approximately 14.8 min, with an air volume flow rate of 260 m3/h.

Experimental study on high-power LEDs integrated with micro heat pipe

Micro heat pipe (MHP) is applied to implement the efficient heat transfer of light emitting diode (LED) device. The fabrication of MHP is based on micro-electro-mechanical-system (MEMS) technique, 15 micro grooves were etched on one side of silicon (Si) substrate, which was then packaged with aluminum heat sink to form an MHP. On the other side of Si substrate, three LED chips were fixed by die bonding. Then experiments were performed to study the thermal performance of this LED device. The results show that the LED device with higher filling ratio is better when the input power is 1.0 W; with the increase of input power, the optimum filling ratio changes from 30% to 48%, and the time reaching stable state is reduced; when the input power is equal to 2.5 W, only the LED device with filling ratio of 48% can work normally. So integrating MHP into high-power LED device can implement the effective control of junction temperature.

Charging method of micro heat pipe for high‐power light‐emitting diode

Since high precision of working fluid charging is key to the evaluation of thermal performance, a novel perfusion method of a micro heat pipe (MHP) is presented. The MHP has a length of 26 mm, width of 20 mm and a thickness of ∼2.2 mm. The predetermined quantity of perfusion is 25 or 35 μl. Small volume and large capillary force render conventional vacuum perfusion methods quite impractical. To realise microscale and high precision of perfusion, the method of combining vacuum perfusion using a peristaltic pump and weight comparison before and after working fluid charging was used. The charging deviation of the method was <2 μl. After perfusion and sealing, the thermal performance testing of a MHP, which is engineered in light-emitting diode (LED) heat dissipation, was conducted and the input power varied from 1 to 7 W. The results show that high-power LEDs can reach the status of heat balance and can work steadily, and the maximum deviations of actual and simulated temperatures are 4.5 and 5.1°C, respectively, and the relative errors are 6.1 and 7.3%. Therefore, this perfusion method can be used for the working fluid perfusion of the MHP and makes it feasible for use in packaging manufacture of heat pipes.

Experimental study of the heat transfer characteristics of a new-type flat micro-heat pipe thermal storage unit

In this study, the heat transfer characteristics of a new-type flat micro-heat pipe thermal storage unit, which uses a moderate-temperature paraffin as heat storage phase change material, were investigated. The basic structure, working principle, and design concept of the thermal storage unit were discussed. Experiments were conducted to study the charging and discharging heat transfer processes of the thermal storage unit under different inlet temperatures and flow rates for the cold/hot heat transfer fluid (HTF). Experimental results show that the performance of the thermal storage unit was steady and efficient during heat charging and discharging, and the average charging and discharging powers of the thermal storage unit were approximately 658 and 894 W, respectively, under specific experimental parameters.

Forming method of micro heat pipe with compound structure of sintered wick on grooved substrate

Micro heat pipes (MHPs) with excellent heat transfer performance have been the ideal radiating components to meet increasingly higher requirements posed by high heat-flux products. Based on MHPs’ working principle, this study deduced capillary limit of a novel MHP with compound structure of sintered wick on grooved substrate, and probed into its forming mechanism: first, high-speed oil-filled spinning was applied to fabricating micro grooves, with optimal spinning and drawing speeds determined; then a mini-type vibration machine was used to help fill copper powders fast and uniformly, with appropriate sintering temperature and time fixed; the manufacturing method that integrates vacuum-pumping–cold-welding with secondary-degassing–cold-welding to increase vacuumizing efficiency. The results of experiments on its heat transfer performance show that the MHPs with sintered-wick-on-grooved-substrate structure fabricated through the proposed forming method can not only acquire much better heat transfer performance, but have advantages such as higher productivity and lower cost.