Flat Plate Micro Research Articles

Pore size optimization and fluid channel analysis of composite wick and anti-gravity study

Flat plate micro heat pipes (FPMHPs) utilize the vapor–liquid two-phase circulation of the working medium to transfer heat and play an essential role in heat dissipation systems. But the effects of capillary force and porosity on the flow channel’s heat transfer characteristics and the working medium’s anti-gravity properties are not clarified. It is found in the heat transfer theory that balancing the capillary force and porosity of the composite wick can improve heat transfer efficiency, increase the maximum thermal power and anti-gravity of the FPMHP. FPMHPs with composite wicks of different pore sizes were developed and tested by the established test platform. The results indicated that the capillary force and porosity of the composite wick constrained each other, and the FPMHP made of the composite wick sintered with 80 mesh copper powder (CW80-FPMHP) had the best heat transfer characteristics and anti-gravity properties. The maximum thermal power of CW80-FPMHP is 38.6 W at 0°, which is 5.4%, 3.6%, 3.5% and 17% higher than that of CW60-FPMHP, CW70-FPMHP, CW90-FPMHP and CW100-FPMHP, respectively. At a tilt of 45°, the maximum thermal power of CW80-FPMHP is 26.27 W, which is 17.6%, 24.3%, 53.9%, 53.9%, and 51.8% higher than CW60-FPMHP, CW70-FPMHP, CW90-FPMHP, and CW100-FPMHP, respectively. In addition, the thermal resistance of the vapor channel in CW80-FPMHP is lower than that of the liquid flow channel. Gravity affects the thermal conductivity of the vapor and liquid flow channels, which in turn affects the performance of the FPMHP. Balancing the capillary force and porosity can improve the performance of the FPMHP under various operating angles.

Analysis of optimized combined microchannel and heat pipes for electronics cooling

In this work, innovative heat sink designs consisting of Microchannel Heat Sinks (MCHS) and Flat Plate Micro Heat Pipes (FPM-HP) are investigated to improve thermal management in electronic components. The three-dimensional finite volume method is used to analyze the fluid flow and heat transfer in the heat sinks. The validity of the numerical models is confirmed by comparison with pertinent experimental data. The thermal performances of the new heat sink designs are compared to that of double-layer microchannel heat sinks with parallel flow (DLPF MCHS) and counter flow (DLCF MCHS) configurations, as two of the more efficient heat sink designs in the industry. The investigated configuration of Silicon-Water Counter Dual MCHS with rectangular cross-section improves the temperature uniformity by 43–50 % and 26–31 % compared to DLPF and DLCF MCHSs, respectively, and the triangular cross-section configuration improves it by 38–47 % and 21–26 %, respectively. The thermal resistance of heat sinks is reduced by 10–15 % compared to DLPF MCHS and by 2–8 % compared to DLCF MCHS. Additionally, innovative hybrid heat sink designs combined with MCHS and FPM-HP are proposed to optimize hotspot management in electronics cooling applications that involve local high heat generation. The hybrid DLPF MCHS & FPM-HP reduces the maximum temperature and surface temperature gradient of the heat sink by 13–23 % and 20–49 %, while these are reduced by 16–40 % and 27–64 % for the hybrid DLCF MCHS & FPM-HP compared with the corresponding copper base (no heat pipe) designs. The hybrid Counter Dual Rectangular MCHS & FPM-HP reduces the maximum temperature and surface temperature gradient of the heat sink by 15–24 % and 29–65 % compared with the DLCF MCHS with copper base (no heat pipe) design, while these are reduced by 10–26 % and 24–63 % for the Counter Dual Triangular MCHS, respectively.

Effect of binary fluid on the heat transfer performance of flat plate micro heat pipe

A flat plate micro heat pipe (FMHP) is a passive thermal management device widely used in chip heat dissipation. In this paper, by analyzing the heat transfer principle of FMHP, we found that the heat transfer limit of FMHP is positively related to the latent heat of evaporation of the working medium, and the methanol binary fluid containing deionized water can increase the latent heat of the working medium. Therefore, four heat pipes with different concentrations of binary fluids of methanol and deionized water as the FMHP work medium and pure methanol as the working medium was fabricated, and a platform was built to test the five heat pipes. The results show that the 80% methanol content in binary fluid flat plate micro heat pipe(BF80-FMHP) has the maximum thermal power and exhibits better heat transfer characteristics than the 100% methanol content in binary fluid flat plate micro heat pipe (MF100-FMHP), with a 12.0% increase in maximum thermal power and a 15.1% decrease in dynamic response time. The 60% methanol content in binary fluid flat plate micro heat pipe(BF60-FMHP) has the fastest dynamic response time, 30.42% lower than the MF100-FMHP, but the maximum thermal power is 10.0 W lower than that of the MF100-FMHP. The proper concentration of binary fluid can effectively increase the maximum thermal power of FMHP, increase the working range, improve the heat transfer capability of FMHP at high temperatures, and enhance the dynamic response time at high power.

Effect of evaporation and condensation section length ratio on thermal performance of aluminum flat plate heat pipe with different micro grooved wicks

The length of the evaporation or condensation section has a significant impact on conventional heat pipe or copper flat heat pipe heat transfer performance. Based on this demand, a novel concept of the length ratio of evaporation and condensation section (LREC) of the flat plate micro grooved heat pipe (FPMHP) was proposed and the effect of the LRECs on the thermal performance of aluminum-based FPMHP with different micro grooved wicks was experimentally investigated for the first time. The experiment explored the effects of nine different LRECs on the thermal performance of FPMHP with the same section size, filling mass, rectangular and triangular micro grooved wicks installed at 90°, a constantly increasing input heat flux, and forced air cooling. The findings demonstrate that rectangular micro grooved wick flat heat pipe (FPMHP-r) and triangular grooved wick flat heat pipe (FPMHP-t) have the shortest startup times when LREC is 1.0, with FPMHP-r starting 70 s sooner than FPMHP-t. When 0.8 ≤ LREC ≤ 1.25, the heat transfer performance of FPMHP-t and FPMHP-r is excellent, better than performance when LREC < 0.8 or LREC > 1.25, and the LREC is recommended to be 0.8 ∼ 1.25 in practical applications. The optimal LREC value of aluminum-based FPMHP is 1.0, which can maximizes the heat transfer performance. Under the same conditions, the heat transfer performance of FPMHP-r is better than FPMHP-t, flat heat pipe wicks with high permeability, low porosity and effective pore radius should be designed in practice. The results can provide a theoretical basis for the design and optimization of aluminum-based FPMHP application.

Experimental study of flat plate micro heat pipe with multi-layer structured wick

With the increased requirement of heat dissipation in the microelectronics field, the study of Flat plate Micro-Heat Pipe (FMHP) has become a hot topic. In this work, by analyzing the characteristics of the gas–liquid two-phase flow cycle within the FMHP, a Multi-Layer structured wick FMHP (MLHP) was prepared and used to improve the heat dissipation efficiency of microelectronic devices. The performance test system was set upto measure the heat transfer performance of the MLHP at different thermal powers and different liquid filling rates. The experimental results show that the MLHP of 150% filling rate can operate stably under 22 W thermal power, and it has an operating thermal resistance of 0.44[Formula: see text]C/W. The liquid filling rate has a significant effect on the thermal performance of the MLHP. Around the suitable filling rate, the MLHP has sufficient working fluid inside and low resistance to vapor flow. Under the high thermal power, the advantages are more significant.

Thermal Transfer Characteristics of Flat Plate Micro Heat Pipe with Copper Spiral Woven Mesh and a Copper Foam Composite Wick.

The thermal efficiency limitation of the Flat-plate Micro Heat Pipe (FMHP) is a major challenge in the development of the FMHP, where the effect of wick structure and wettability on its thermal performance is studied to improve the thermal efficiency of the FMHP. In this work, a copper spiral woven mesh and copper foam Composite Wick FMHP (CW-FMHP) is designed based on the conventional Copper Foam Wick FMHP (CFW-FMHP), and its thermal performance is analyzed regarding the wick structure and internal gas–liquid two-phase flow characteristics. An oxidized copper spiral woven mesh and copper foam Composite Wick FMHP (OCW-FMHP) has been further developed through the modification of composite wick wettability. The performance tests are carried out with the thermal transfer characteristics of CW-FMHP, OCW-FMHP, and CFW-FMHP under different filling rates and different thermal powers. The experimental results show that the thermal transfer performance of CW-FMHP reaches the optimal under a liquid filling rate of 150%, where the maximum thermal power is 15.7 W, 35.3% higher than that of the CFW-FMHP under the same filling rate. Moreover, the dynamic response characteristics of the CW-FMHP are significantly improved. The thermal resistance of the CW-FMHP is 0.48 °C/W under the filling rate of 150% at the thermal power of 10 W with a reduction of 9.4% compared to the CFW-FMHP under the same condition. Furthermore, the optimal filling rate for OCW-FMHP is lower compared with the CW-FMHP. The maximum thermal power of OCW-FMHP increases to 17.8 W while the thermal resistance reduces to 0.34 °C/W under the liquid filling rate of 140%. This implies that the composite wick structure designed in this work can improve the thermal transfer performance of the FMHP, and the composite wick with wettability modification is more effective regarding both thermal resistance and maximum thermal power.

A channel layout of a micro pulsating heat pipe for an excessively localized heating condition

A channel layout of a flat-plate micro pulsating heat pipe (MPHP) for an excessively localized heating condition is proposed. The excessively localized heating condition is implemented using a small heater, which covers only about 2% of the whole area, in the middle of the bottom part. To simplify the heat transfer characteristics in MPHPs, the channel regions are modeled as a region with the high effective thermal conductivity, and the topology optimization method is employed to determine a distribution of the channel regions of the MPHP. The constraints on the maximum length scale and the minimum solid volume fraction in local regions are imposed to take into account the operational characteristics of MPHPs. In the proposed channel layout, which is inspired by the topology optimization result, the channels are laid out in such a way that some channels are merged to reduce the number of channels. Due to a decrease in the number of channels, all channels are able to overlap the localized heating section without any thermally-isolated channel. To verify the effectiveness of the proposed method, the MPHPs with the channel layout designed by topology optimization and two kinds of channel layouts for the references are fabricated, and their flow characteristics and thermal performances are experimentally compared. In case of the MPHP with the uniform channel layout, only 4 channels out of 18 channels overlap the heating section, and the stagnant liquid slugs in the thermally-isolated channels are observed. On the other hand, in the MPHP with the channel layout designed by topology optimization, all channels overlap the heating section, and the active oscillating motion of the liquid slugs occurs in all channels. This allows the MPHP with the channel layout designed by topology optimization to have about two times higher thermal performance than the MPHP with the uniform channel layout. In this study, the channel layout of the MPHP for the excessively localized heating condition is firstly suggested, and the experimental results show that the proposed channel layout can effectively overcome the limitation associated with the localized heating condition.

Experimental Investigation on the Operating Characteristics of Flat-Plate CLPHP for PEMFC Cooling

Proton exchange membrane fuel cell (PEMFC) generates electricity through hydrogen and oxygen chemical reaction with the generation of much heat. According to the working temperature of PEMFC, the thermal resistance and internal relative pressure change of the flat-plate micro closed-loop pulsating heat pipe (CLPHP) are tested and analyzed at different filling ratios, vacuum degrees, and inclination angles, whose working medium is binary methanol-deionized water with a mass ratio of 5:1. The experiment results show that the higher the vacuum degree is, the better the startup and heat transfer performance of CLPHP become; the thermal resistance is less sensitive to inclination angle except for horizontal placement; the 40% volume-filling ratio can effectively avoid dry-out at high temperature. When 80 °C is the ideal working temperature of PEMFC, the thermal resistance of CLPHP is 0.093°C/W; the heat transfer heat flux is 1.59 W/cm2, and the temperature fluctuation is small. Therefore, flat-plate CLPHP has great application potential for PEMFC cooling.

Performance investigation of flat-plate CLPHP with pure and binary working fluids for PEMFC cooling

While proton exchange membrane fuel cell (PEMFC) generates electricity, about half of the energy is converted into heat. According to structural characteristics and heat dissipation requirements of PEMFC, a flat-plate micro closed-loop pulsating heat pipe (CLPHP) cooling method is designed. The flat-plate CLPHP is an aluminum alloy plate with a thickness of 2.4 mm, and the inside is a 2.3 mm × 1.4 mm rectangular flow channel, which transfers heat mainly through the internal working fluid's vapor-liquid phase change and forced convection. The experiment tested the heat transfer performance and the internal pressure of pure working fluids methanol, ethanol, isopropanol, deionized water, and methanol-deionized water with different mass ratios. By comparison, it is found that the binary working medium methanol-deionized water with a mass ratio of 5:1 has the best startup performance, lower internal pressure, and less temperature fluctuation, which has great potential in the application of PEMFC. Through the dimensionless number correlation analysis of the internal working fluid's thermophysical parameters, a CLPHP heat flux prediction equation with an average deviation of 15.0% is fitted.

Effect of geometries on the heat transfer characteristics of flat-plate micro heat pipes

The geometries of wicking structure of flat-plate micro heat pipes (FPMHPs) determine the capillary force and quantity of working fluid, which remarkably affect thermal performance. In this study, flat-plate micro heat pipes (FPMHPs) with different geometries were designed and fabricated, and their heat transfer characteristics were compared. Results showed that the thermal performance of the FPMHPs was relatively weak at α = 0° because the condensate backflow can only be driven by capillary force. At α = 90°, the A-4-type FPMHP presented the best performance under the common joint effect of capillary force, gravitational force, and the quantity of working fluid. The minimum thermal resistance and the maximum thermal conductivity of this FPMHP were 0.89 K/W and 1739.98 W/(m·K), respectively. A visualization study was also conducted to validate and investigate the heat transfer process of the vapor–liquid phase–change and the heat transfer limit in the FPMHP. We found that a slug-train unit formed when the heating power was sufficiently large, causing a local dryout phenomenon. The evaporation section presented a complete dryout with the further increase in heating power. Therefore, the heat transfer limit in the mini/micro FPMHP was dependent on the dryout limit.

Effect of a channel layout on the thermal performance of a flat plate micro pulsating heat pipe under the local heating condition

The effect of a channel layout on the thermal performance of a flat plate micro pulsating heat pipe (MPHP) under the local heating condition was investigated through a series of experiments. The thermal characterization was investigated, and flow visualization was conducted. The MPHPs with randomly-arranged channel layouts were fabricated to have the various effective dissimilarities. HFE-7000 was used as the working fluid, and a filling ratio was fixed at 50% in the present study. The effective dissimilarity was first introduced in this study as an attempt to quantify the effect of a channel layout on the thermal performance of a PHP under the local heating condition, and it was evaluated using the non-uniformity of a channel layout in the heating section. In the case of the MPHP with a uniformly-arranged channel layout, it had non-oscillating liquid slugs, which aggravates the thermal performance and operating range of the MPHP. However, liquid slugs in the MPHPs with randomly-arranged channel layouts oscillated with large-amplitudes in all the channels. Consequently, they had up to 32% higher thermal performance and two times wider operating range than those of the MPHP with a uniformly-arranged channel layout. Finally, this study experimentally found that the thermal performance of the MPHP increases as the effective dissimilarity increases. Therefore, the channels of the MPHP under the local heating condition should be arranged to have the large effective dissimilarity as much as possible to enhance the thermal performance and to extend the operating range simultaneously.

Performance of a flat-plate micro heat pipe at different filling ratios and working fluids

An experiment was carried out to study the effect of liquid filling ratio, inclination angle, and type of working fluid on the performance of a novel flat-plate micro heat pipe (FPMHP). The FPMHP is developed based on a multiport micro/minichannel flat tube, capable of the advantages such as the simple manufacturing process, flexible structure, good isothermal performance and heat transport with a long distance. Results shows that a filling ratio has a significant effect on thermal performance, and a FPMHP with a filling ratio of 20% has the best performance. The temperature distribution along the adiabatic section showed nearly no difference between two measuring points, indicating that the FPMHP is able to employed as a high-efficiency heat transfer equipment of phase change. FPMHP performance was improved remarkably when the inclination angle increased from 0° to 20° due to the effect of gravitational and buoyancy force, but the performance difference was negligible when the inclination angle increased from 20° to 90°. For different working fluids, the higher transmission factor, the better working fluids thermal performance. Thus, the FPMHP using methanol has the best performance, with a minimum thermal resistance of 0.18 K/W and a maximum thermal conductivity of 8539 W/mK, respectively.

Numerical investigations on effects of bluff body in flat plate micro thermo photovoltaic combustor with sudden expansion

In order to reveal combustion characteristics of H2/air mixture in a micro-combustor with and without bluff body, the effects of inlet velocities, equivalence ratios and bluff body’s blockage ratios on the temperature field, pressure of the combustor wall, combustion efficiency and blow-off limit were investigated. The numerical results indicate that the sudden expansion plate micro combustor with bluff body could enhance the turbulent disturbance of the mixed gas in the combustion chamber and the combustion condition is improved. Moreover, a low-speed and high temperature recirculation region was formed between the sudden expansion step and the bluff body so that the high and uniform wall temperature (>1000 K) could be gotten. As a result, it could strengthen the mixing process, prolong the residence time of gas, control the flame position effectively and widen the operation range by the synergistic effect of the bluff body and steps. When the blockage ratio ranged from 0.3 to 0.6, it could be found that the bluff body could play a stabilizing effect and expand combustion blow burning limit, and combustion efficiency firstly was increased with the inlet velocity and equivalence ratio, and then was decreased.

Experimental investigation on the thermal performance of a micro pulsating heat pipe with a dual-diameter channel

Experiments were performed to investigate the effect of a dual-diameter channel on the flow and heat transfer characteristics of flat plate micro pulsating heat pipes (MPHPs). Using MEMS techniques, the MPHPs were fabricated: rectangular channels with dual hydraulic diameters were engraved on a silicon wafer to form a meandering closed loop with 5 turns. The Pyrex glass was used as a cover which enables flow visualization in the MPHP. Experimental results show that the MPHPs with a dual-diameter channel can operate stably even in a horizontal position and have uniform thermal performance regardless of the orientation when a proper condition is met. It is postulated that the orientation-independent performance occurs when the capillary pressure generated by the difference in channel diameters is larger than the viscous pressure drop. This postulate was experimentally confirmed using Ethanol and FC-72 as working fluids at various input powers and inclination angles. To provide a guideline for orientation-independent operation of the MPHP with a dual-diameter channel a figure of merit is proposed: the figure of merit is defined as the ratio of capillary pressure difference to the viscous pressure drop, which is similar to the figure of merit for a conventional heat pipe. According to experimental data, the figure of merit has to be larger than 2×105 for orientation-independent operation of the MPHP.

Heat Transfer Characteristic of Flat Trapezoid Grooved Micro Heat Pipes

To study the heat transfer performance of micro heat pipe, theoretical analysis of flat plate micro heat pipe with trapezoid cross section are presented in this paper. A one-dimensional stationary mathematical model for micro heat pipe grooved capillary flow using finite volume method (FVM) was established. The micro heat pipe had vapor space connect with each other and the influences of shear stress between vapor and fluid in the working process were described in the model which made the model more precisely. The axial variation of working fluid distribution in the heat pipe, pressure difference between vapor and liquid, and velocity of vapor and liquid were analyzed. In addition, the maximum heat transport capacity of micro heat pipe was calculated. The simulation results showed good agreement with the experiment results, and it could predict the heat transfer performance accurately, which was useful to micro heat pipe structural design.

Experimental Study of Solar-Air Composite Heat Pump System

In the light of low efficiency of photovoltaic power generation and the problems of air source heat pumps application in the cold regions, this paper developed a composite evaporator and designed a new type of solar-air composite heat pump system, which was comprised of independently developed solar photovoltaic-thermal collector based on flat plate micro heat pipe and air source heat pump. The performances of heat pump system were studied experimentally, including water temperature of the heating tank, suction and discharge pressure, and performance of heat pump. When 73L water in the heating tank was heated by 1 HP heat pump from 15°C to 50°C at the ambient temperature of 5°C, the running time of dual heat source operation conditions was 5.14% shorter than that of separate air heat source operation conditions. Meanwhile, COP increased by 5.99%.

Heat Transfer Steady and Transient Characteristics in Flat Plate Micro Heat Pipe

Heat Transfer Steady and Transient Characteristics in Flat Plate Micro Heat Pipe

Performance characterization of a novel flat plate micro heat pipe spreader

A combined numerical and experimental investigation was conducted on a novel flat plate micro heat pipe spreader, to better understand the effect of primary operating parameters governing the performance of such devices. A numerical thermal model was developed to predict the temperature response with variation in the leading geometrical, material and boundary parameters of the spreader, viz. wall thickness, thermal conductivity, power input and heat source size. The results showed that, unlike conventional heat pipes, wall thermal conductivity is a major factor in such thin, flat spreaders. The spreader performance also degrades with decrease in heat source size. Visualization experiments have been conducted to qualitatively understand the heat transfer phenomena taking place on these devices. These confirmed that the primary limitation to heat transfer from these devices was due to the capillary limitation of the wick structures.

Optimization of conduits' shape in micro heat exchangers

An approximate theory is derived to compute the thermal resistances of flat plate micro heat exchangers whose surfaces are heated with uniform flux. It is demonstrated that the thermal resistance can be minimized by proper selection of uniform conduit geometry. Further reductions in the maximum heated surface temperature and in the heated surface temperature gradients can be achieved by varying the conduit's cross-sectional dimensions as a function of the axial coordinate. This paper illustrates that a conduit's shape can be customized so as to achieve desired objectives.

Modeling, Fabrication, and Testing of Micro Heat Pipes: An Update

In 1992, an overview was presented which summarized the status and progress made in the development of very small, “micro” heat pipes, manufactured as stand alone devices or fabricated as an integral part of silicon wafers. Since that initial review, significant advances have been made in the analysis, fabrication and testing of these devices, for use in a wide variety of applications. Following, is a review of the more recent work in this rapidly emerging field. Included is a summary of the analytical techniques developed, the various proposed methods of fabrication, and a summary of the most current test results achieved to date. Because the fundamental operating characteristics of micro heat pipes larger than 1 mm in diameter are similar to that of conventional heat pipes, this review focuses on the analysis, fabrication, and testing of micro heat pipes with characteristic dimensions of less than 500 μm. Particular emphasis is placed on research, related to the development of arrays of micro heat pipes and flat plate micro heat pipes fabricated as an integral part of semiconductor devices.