表面活性剂对水工质超长重力热管传热性能的影响

超长重力热管是近年来被提出的用于干热岩地热能开采的一种新技术.该技术方案通过工质的沸腾?冷凝相变来进行热量传输从而在地面获得地下数千米深的热量,突破了常规热管的热力输运距离.表面活性剂能降低液体的表面张力,从而改变液体工质的沸腾特性,能在一定程度上提升常规热管的热力性能,但在超长重力热管中的作用仍有待研究.本文在自行搭...     

Thermal performance of inclined screen mesh heat pipes using silver nanofluids

This study presents the effect of silver nanofluid on thermal performance of inclined screen mesh heat pipe in cooling applications. Four cylindrical copper heat pipes containing two layers of screen mesh were fabricated and tested with distilled water and water based silver nanofluids with mass concentrations of 0.25%, 0.5% and 0.75% as working fluids. The experiments were performed at four inclination angles of 0°, 30°, 6° and 90°. The main focus of this study is to investigate inclined heat pipe performance with nanofluid. Experimental results indicate that the thermal performance of heat pipes was improved with nanofluids compared to water and thermal resistance of the heat pipes decreased with the increase of nanoparticle concentration. Moreover, the thermal performance of the heat pipes at inclination angle of 60° is found to be higher than other tested inclination angles, which shows the effect of gravity on heat pipe performance.     

Characterization of evaporation and boiling from sintered powder wicks fed by capillary action

The thermal resistance to heat transfer into the evaporator section of heat pipes and vapor chambers plays a dominant role in governing their overall performance. It is therefore critical to quantify this resistance for commonly used sintered copper powder wick surfaces, both under evaporation and boiling conditions. The objective of the current study is to measure the dependence of thermal resistance on the thickness and particle size of such surfaces. A novel test facility is developed which feeds the test fluid, water, to the wick by capillary action. This simulates the feeding mechanism within an actual heat pipe, referred to as wicked evaporation or boiling. Experiments with multiple samples, with thicknesses ranging from 600 to 1200 μm and particle sizes from 45 to 355 μm, demonstrate that for a given wick thickness, an optimum particle size exists which maximizes the boiling heat transfer coefficient. The tests also show that monoporous sintered wicks are able to support local heat fluxes of greater than 500 W cm^?2 without the occurrence of dryout. Additionally, in situ visualization of the wick surfaces during evaporation and boiling allows the thermal performance to be correlated with the observed regimes. It is seen that nucleate boiling from the wick substrate leads to substantially increased performance as compared to evaporation from the liquid free surface at the top of the wick layer. The sharp reduction in overall thermal resistance upon transition to a boiling regime is primarily attributable to the conductive resistance through the saturated wick material being bypassed.     

Flat and U-Shaped Heat Spreaders for High-Power Electronics

To achieve a high heat-flux level and reduce manufacturing costs associated with conventional heat pipes, the concept of network heat spreaders employing a boiling heat-transfer mechanism in a narrow space had been proposed, and several flat-plate wickless heat spreaders had been designed and fabricated. The heat spreaders had been tested under different working conditions and orientations relative to gravity with very good results. The previously tested network heat spreaders, however, were based on plates with a relatively large size for general heat spreading purposes. In the present study, network heat spreaders with overall dimensions of 78 2 62 2 3.2 mm are designed and fabricated. Spreaders of this size are intended for use as heat sinks of high-power electronic components. External cooling fins are attached to enhance air-cooling heat transfer rate. The network heat spreaders are tested under various working conditions with water as the working fluid. The maximum heat input rate achieved is about 150 W with a corresponding heat flux of 60 W/cm 2 . Compared to the performance of a solid copper plate having the same overall size as the spreader, the maximum temperature difference over the surface is reduced from about 32°C to 3.3°C. The heat transfer performance of the spreader is also largely dependent on the filling ratio of the working fluid and the boiling heat transfer in the narrow space. For these reasons, boiling heat transfer mechanisms in a narrow space are analyzed, and a spreader design that would improve the performance in a horizontal position is described.     

A three-dimensional thermal-fluid analysis of flat heat pipes

A detailed, three-dimensional model has been developed to analyze the thermal hydrodynamic behaviors of flat heat pipes without empirical correlations. The model accounts for the heat conduction in the wall, fluid flow in the vapor chambers and porous wicks, and the coupled heat and mass transfer at the liquid/vapor interface. The flat pipes with and without vertical wick columns in the vapor channel are intensively investigated in the model. Parametric effects, including evaporative heat input and size on the thermal and hydrodynamic behavior in the heat pipes, are investigated. The results show that, the vertical wick columns in the vapor core can improve the thermal and hydrodynamic performance of the heat pipes, including thermal resistance, capillary limit, wall temperature, pressure drop, and fluid velocities due to the enhancement of the fluid/heat mechanism form the bottom condenser to the top evaporator. The results predict that higher evaporative heat input improves the thermal and hydrodynamic performance of the heat pipe, and shortening the size of heat pipe degrades the thermal performance of the heat pipe.     

自激振荡流热管内Cu-水纳米流体的传热特性

对工质为Cu-水纳米流体的自激振荡流热管在不同激光加热功率下的热传输特性进行了实验研究,并对工质为蒸馏水的自激振荡流热管的传热性能进行了比较.通过对不同的充液率、Cu纳米颗粒份额的Cu-水纳米流体自激振荡流热管实验结果分析发现:自激振荡流热管内Cu-水纳米流体的热传输具有一定的特殊性,在一定条件下纳米流体可以起到强化传热的作用,但决定纳米流体自激振荡流热管热传输性能的参数应是充液率.     

Superposition method to investigate the thermal performance of heat sink with embedded heat pipes

This article utilizes the thermal performance experiment with superposition method to investigate the thermal performance of heat sinks with one and two pairs of embedded heat pipes. A heat sink with embedded heat pipes transfers the total heat capacity from the heat source to both the base plate and heat pipes, and then disperses heat into the surrounding air via the forced convection. The heat capacity carried by embedded heat pipes can be found using the thermal resistance analytical approach stated in this article. The results show that two and four heat pipes embedded in the base plate carry 36% and 48% of the total dissipated heat respectively; in addition, when the total heating power of the heat sink with two embedded heat pipes is 140 W, the total thermal resistance reaches its minimum value of 0.27 °C/W, while for the heat sink with four embedded heat pipes, when the total heating power is between 40 W and 240 W, the total thermal resistance is 0.24 °C/W, meaning that the thermal performance is better than that of heat sink with two embedded heat pipes.     

Assessment of heat transfer and flow characteristics of a two-phase closed thermosiphon

In this study, comprehensive modeling and simulations were developed and carried out to perform the investigation of the thermal performance of the enclosed thermosiphon through pool boiling in the evaporator sector and the condensation of the liquid film in the condenser part. To simulate these phenomena, the volume of fluid model was utilized. The simulation modeling using the computational fluid dynamics (CFD) technique was validated with existing experimental results, and a good agreement was reached. The simulation results were presented and evaluated in terms of temperature profiles and contours, the volume of fraction contours, and velocity vector distribution. Moreover, the thermal performance (ie, the heat transfer coefficient and thermal resistance) through the thermosiphon operation was analyzed. From the simulation results, it is found that the thermosiphon performance can be improved by the tilt angle and fill ratio. The results indicated that the optimal performance (ie, a high heat transfer coefficient and a low thermal resistance) was attained at a power input of 250 W, tilt angle of 90°, and fill ratio of 0.5. The established CFD simulations effectively predicted the formation of two-phase flow pattern and boiling and condensation zones with water at a low power input, termed as geyser boiling.     

Visualization and performance measurement of operating mesh-wicked heat pipes

This work presents visualization of the evaporation/boiling process and thermal measurements of operating horizontal transparent heat pipes. The heat pipes consisted of a two-layered copper mesh wick consisting of 100 and/or 200 mesh screens, a glass tube and water as the working fluid. Experimental results indicated that nucleate boiling was prompted for a wick having a fine 200-mesh bottom layer. When the fluid charge approximately equaled the pore volume in the wick, the water–vapor interface receded into more curved menisci with increasing heat load Q. Thus, larger capillary forces and evaporation areas were attained to meet the increasing need of liquid supply and evaporation rate at the water–vapor interface. At Q = 40 and 45 W, the water film became less than 100 μm and the nucleate boiling observed at lower heat loads disappeared. Optimal thermal characteristics with smallest thermal resistances across the evaporator and lowest overall temperature distributions were found for such a wick/charge combination. Under a smaller charge, partial dry-out was observed in the evaporator. Under a larger charge, liquid recession with increasing heat load was limited and bubbles grew and burst violently at high heat loads. The effects of different wicks and fluid charges on the evaporation/boiling characteristics were discussed.     

Thermal performance enhancement studies using graphite nanofluid for heat transfer applications

Enhanced boiling heat transfer using nanofluids is highly relevant due to its potential applications in thermal management of systems producing large heat fluxes. However, the sedimentation of nanoparticles limits their application in heat transfer systems. So, the preparation of a stable nanofluid remains a big research challenge. The stability issues arise due to the large difference in the density of nanoparticle and the base fluid. Graphite nanoparticle is selected in this study, as it has 4.5 times lower density than copper and comparable thermal conductivity. An experimental study is conducted to evaluate the suitability of graphite nanofluid in mesh wick heat pipes, which are devices that utilize boiling and condensation principles to transfer high heat fluxes. Thermal transport properties and boiling heat transfer characteristics showed enhancement and the effect of nanofluid on the device level thermal performance is thoroughly assessed. Experimental results are compared with the published literature. A reduction in thermal resistance by 32.5% and an improvement in the heat transfer coefficient by 48.02% in comparison with base fluid clearly indicate the superiority of the graphite nanofluid for heat transfer applications.     

Experimental comparative study of heat pipe performance using CuO and TiO2 nanofluids

In recent years, developing an energy efficient conventional heat pipe is more important because of the development of electronics and computer industries. To enhance the thermal performance of heat pipe, different nanofluids have been widely used. In this paper, an experimental investigation of heat transfer performance of heat pipe has been conducted using three different working fluids such as DI water, CuO nanofluid and TiO₂ nanofluid. The heat pipe used in this study is made up of copper layered with two layers of screen mesh wick for better capillary action. The Parameters considered in this study are heat input, angle of inclination and evaporator fill ratio. The concentration of nanoparticle used in this study is of 1.0 wt.%. From the experimental results, comparisons of thermal performance were made between the heat pipes using various working fluids. Among various fill ratio charged, the heat pipe shows good thermal performance when it is operated at 75% fill ratio for all working fluids. However, the heat pipe operated with CuO nanofluid showed higher results compared with TiO₂ nanofluid and DI water. Copyright © 2013 John Wiley & Sons, Ltd.     

Experimental Investigation of Pulsating Heat Pipes and a Proposed Correlation

Pulsating heat pipes are complex heat transfer devices, and their optimum thermal performance is largely dependent on different parameters. In this paper, in order to investigate these parameters, first a closed-loop pulsating heat pipe (CLPHP) was designed and manufactured. The CLPHP was made of copper tubes with internal diameters of 1.8 mm. The lengths of the evaporator, adiabatic, and condenser sections were 60, 150, and 60 mm, respectively. Afterward, the effect of various parameters, including the working fluid (water and ethanol), the volumetric filling ratio (30%, 40%, 50%, 70%, 80%), and the input heat power (5 to 70 W), on the thermal performance of the CLPHP was investigated experimentally. The results showed that the manufactured CLPHP has the best thermal performance for water and ethanol as working fluids when the corresponding filling ratios are 40% and 50%, respectively. Finally, with the available experimental data set of CLPHPs, a power-law correlation based on dimensionless groups was established to predict their input heat flux. Compared with the experimental data, the root-mean-square deviation of the correlation prediction was 19.7%, and 88.6% of the deviations were within ± 30%.     

Multilayer fabric stratification pipes for solar tanks

The thermal performance of solar heating systems is strongly influenced by the thermal stratification in the heat storage. The higher the degree of thermal stratification is, the higher the thermal performance of the solar heating systems. Thermal stratification in water storage can for instance be achieved by use of inlet stratifiers combined with low flow operation in the solar collector loop. In this paper, investigation of a number of different fabric stratification pipes is presented and compared to a non-flexible inlet stratifier. Additional, detailed investigation of the flow structure close to two fabric stratification pipes is presented for one set of operating conditions by means of the optical PIV (Particle Image Velocimetry) method.     

Long-term thermal performance of a two-phase thermosyphon solar water heater

This article investigates experimentally the long-term thermal performance of a two-phase thermosyphon solar water heater and compares the results with the conventional systems. Experimental investigations are conducted to obtain the system thermal efficiencies from the hourly, daily and long-term performance tests. Different heat transfer mechanisms, including natural convection, geyser boiling, nucleate boiling and film-wise condensation, are observed in the two-phase thermosyphon solar water heater while solar radiation varies. The thermal performance of the proposed system is compared with that of four conventional solar water heaters. Results show that the proposed system achieves system characteristic efficiency 18% higher than that of the conventional systems by reducing heat loss for the two-phase thermosyphon solar water heater.     

Numerical and experimental analysis of the flat plate solar water heater systems' thermal performance

In this paper, improving the thermal performance of flat plate solar water heater systems by inserting different tube configurations inside the riser pipes has been numerically and experimentally studied. This study is focused on increasing the moving of energy from riser pipes to the operating fluid within the riser pipes. To achieve that, the diameter of the riser pipes was increased along with the insertion of different tube configurations within them, namely, smooth, helical, and wavy tubes, keeping the same amount of the operating liquid. A comparison was performed to determine the best in terms of coefficient of heat transfer of the operating liquid, mass flow rate of the operating liquid, pressure drop, and water temperature in the storage tank, as a thermal performance indication of the system under study. The findings show the model consisting of a straight tube inside the riser pipe provides the best thermal performance. In terms of thermal performance, the straight model outperforms the conventional model by 12.3%. An experimental and numerical comparison between the optimum model (straight model) was conducted. The study proves that the average difference between numerical results and experimental findings is 7.2%.     

Thermal performance of inclined grooved heat pipes using nanofluids

An experimental study was performed to investigate the thermal performance of an inclined miniature grooved heat pipe using water-based CuO nanofluid as the working fluid. This study focused mainly on the effects of the inclination angle and the operating pressure on the heat transfer of the heat pipe using the nanofluid with the mass concentration of CuO nanoparticles of 1.0 wt%. The experiment was performed at three steady sub-atmospheric pressures. Experimental results show that the inclination angle has a strong effect on the heat transfer performance of heat pipes using both water and the nanofluid. The inclination angle of 45° corresponds to the best thermal performance for heat pipes using both water and the nanofluid. The present investigation indicates that the thermal performance of an inclined miniature grooved heat pipe can be strengthened by using CuO nanofluid.     

Experimental investigation on performance of a rotating closed loop pulsating heat pipe

Pulsating heat pipes (PHPs) are interesting heat transfer devices. Their simple, high maintaining, and cheap arrangement has made PHPs very efficient compared to conventional heat pipes. Rotating closed loop PHP (RCLPHP) is a novel kind of them, in which the thermodynamic principles of PHP are combined with rotation. In this paper, effect of rotational speed on thermal performance of a RCLPHP is investigated experimentally. The research was carried out by changing input power (from 25 W to 100 W, with 15 W steps) and filling ratio (25%, 50%, and 75%) for different rotational speeds (from 50 rpm to 800 rpm with an increment of 125 rpm). The results presented that at a fixed filling ratio, thermal resistance of RCLPHP decreased with increasing heat input applied to evaporator. Above a certain range of heat input, probability of partial dry-out of evaporator existed, which led into thermal performance deterioration of RCLPHP. Moreover, thermal resistance of RCLPHP decreased with increasing rotational speed and probability of partial dry-out in the evaporating section reached to its least amount.     

一种新型微热管传热性能的实验研究

对一种新型的平板式微热管一零切角曲面微热管进行了实验研究。以热阻为基础,研究不同倾角、工质、充液比下微热管的热性能。为便于分析,将热管总热阻分解为4个部分：加热热阻、蒸发段热阻、冷凝段热阻和热沉热阻。通过实验得出如下结论：微热管总热阻的主要变化因素是冷凝段热阻和蒸发段热阻;与相应的无工质平板式换热器相比,实验件主要热阻变为热沉热阻．蒸发段和冷凝段热阻所占比例较低。根据不同的充液比和倾角。微热管传热极限分别由局部干烧和核态沸腾向膜态沸腾转化引起。实验表明。这种新型的微热管具有良好的应用前景,但是对于其机理还需要更深入的研究。     

Visualization of vapor formation regimes during capillary-fed boiling in sintered-powder heat pipe wicks

The current study investigates capillary-fed boiling of water from porous sintered powder wicks used in emerging high-effective-conductivity vapor chamber heat spreaders intended for management of hot spots with heat fluxes exceeding 500 W cm^?2. Characterization of 1 mm thick wicks composed of 100 μm sintered copper particles is performed in a test facility which replicates the capillary feeding conditions that occur in such devices. Boiling curves are obtained for a 5 mm × 5 mm heated input area, along with high-speed in-situ visualization of the evaporation/boiling processes. Understanding the vapor formation regimes is essential to predictive modeling of the observed characteristics. Schematic representations of such regimes along the boiling curves are presented for homogeneous and modified wick structures. In general, incipience of boiling in sintered-powder wicks reduces the effective thermal resistance and, for small heat input areas, does not cause liquid starvation due to a capillary limitation. The thermal performance enhancement provided by two different augmentation methods is quantified and explained in terms of the observed vapor formation characteristics. Patterns fabricated within the sintered powder create multi-scale wicks with regions of different pore size. These patterns reduce thermal resistance throughout the boiling regime by increasing the permeability to vapor exiting the wick, as confirmed by visualization of the preferential vapor venting from the surface. At the highest heat fluxes investigated prior to dryout, a thin liquid film is observed to form in the recessed patterned areas at the base of the wick. Integration of copper-coated carbon nanotubes on to the sintered powder reduces the required superheat for boiling incipience, thus reducing the overall thermal resistance at low heat fluxes. Evaporation and boiling regime heat transfer predictions from several available correlations are compared to the current results, and are shown to corroborate the conclusions regarding vapor permeability.     

Experimental study of closed‐loop thermosyphon with a different evaporator geometry

In this study, the effect of evaporator geometry on the loop thermosyphon's heat transfer coefficient is experimentally verified by using water as a working fluid with three filling ratios (50%, 70%, 90%), constant heat input (185 W), and condenser cooling water flow rate remaining constant at 2 Lpm. Three evaporator pipes are used (I: straight; II: helical coil evaporator with a diameter of 100‐mm coil and two turns; III: helical coil evaporator with a diameter of 50‐mm coil and four turns). From the experimental results, it can be observed that the performance of evaporator III is higher than the two other forms. A greater heat transfer coefficient value is found in case of type III evaporator and is equivalent to 2456 W/m²·°C. The maximum thermal resistance reduction occurs in the type III evaporator (37.32%), and the highest effective thermal conductivity for the same type is 6.123e + 05 W/m·°C. The experimental results demonstrate good agreement with the empirical equations.