•Propose the best PTFE wick thickness for Loop Heat Pipe application that minimizes both flow drag and thermal resistance.•Present the permeability of PTFE wick as an important reference for flow ...drag.•Self-rewetting fluid can overcome the flow drag and hydrophobicity of PTFE wick.
Heat leakage has long been a problem in the development of Loop heat pipe (LHP). Among the strategies to suppress heat leakage, the use of low thermal conductivity polymer materials and adjustment of wick thickness are considered as one of the effective methods. However, although there are many recent reports on the effect of metal wick thickness on performance, there are few studies on the effect of polymer wick thickness such as PTFE wick. Therefore, this paper hopes to integrate the two factors and present the best results. This paper investigates the effect of PTFE wick thickness (1 to 2.75 mm) on LHP performance and proposes the thickness effects on both thermal resistance and flow drag. In addition, since PTFE is hydrophobic, water, with its high surface tension, cannot be used as working fluid. Therefore, in this study, a tiny amount of Butanol (6 wt%) is added to water to create self-rewetting fluid to be used as working fluid, overcoming the flow drag of PTFE wick and allowing the system to operate smoothly.
Regarding the wick’s porosity and permeability, the best capillary effect and flow drag occurs at wick thickness of 2 mm. The LHP performance result shows, compared to using PTFE wick with water as working fluid where the system was unable to start up, the system using self-rewetting fluid can smoothly operate, overcome heat leakage and enhance performance. The best performance occurs at wick thickness of 2 mm, the maximum heat load reaches 600 W and heat flux 30 W/cm2; the results are all consistent with the performance predictions based on the wick parameters.
•Intermittent spray cooling with extremely low flow rate is realized.•Inverse Marangoni convection can be used to improve cooling of intermittent spray.•Strong evaporation and intensive boiling are ...the main heat transfer mechanisms.•For multiple nozzles, liquid accumulation helps to form an unbroken liquid film.•Nozzle density can be as low as 0.10 cm−2 for large cooling area.
An economical approach is required for solving the portability and maintenance problems of a spray cooling system for facilities stationed in remote areas. To reduce liquid consumption and decrease the number of nozzles required for large-area cooling, we propose the use of a self-rewetting fluid coupled with an intermittent spray that induces a surface-tension-driven flow to extend film evaporation. With an extremely low flow rate of 0.325 ml s−1, the working fluid can be effectively pulled toward a hot region through the inverse Marangoni convection, which promotes thermal uniformity and postpones dryout. Consequently, the cooling rate is considerably improved and the temperature fluctuation over time is reduced. Because of the extremely low liquid consumption, strong film evaporation and intensive nucleation, as opposed to impingement momentum, play major roles in cooling. Although multiple-nozzle configurations outperform single-nozzle configurations at high input power, the performance differentiation between double and quadruple nozzles is small, and excellent cooling can be achieved with a nozzle density as low as 0.10 cm−2. By using double or quadruple nozzles in the short-spray mode, the highest heat flux of 7.4 W cm−2 can be achieved with a temperature fluctuation of ±0.5 °C and spatial deviation smaller than 4%.
•An OHP with external expansion structure and compact layout was designed.•The OHP filled with self-rewetting fluid as working fluid was studied.•Continuous, pulsing and alternate heating mode were ...investigated comparatively.•The temperature fluctuation characteristic and thermal resistance were analyzed.
In this paper, an oscillating heat pipe (OHP) with external expansion structure and compact layout for thermal energy recovery and storage was designed and fabricated. The thermal performance of the OHP with different filling ratios (FRs) of self-rewetting fluid under continuous heating mode was tested at first. Then pulsing heating mode and alternate heating mode were explored to enhance the heat transport performance. The temperature fluctuation characteristic and the thermal resistance under different heating modes and heating periods were compared and analyzed. The result showed that the OHP can function well and sustain large heating power under appropriate range of FR. The pulsing mode and alternate mode are both helpful to increase the fluctuation frequency and amplitude of evaporator temperature. For the condition of relative low heating power and short heating period, the enhancement effect is more obvious. The thermal resistance at 100 W can be reduced by more than 10% when the heating period varies from 2 s to 10 s. Under short heating period of 0.4 s, the thermal resistance of the OHP under pulsing heating mode at 200 W can be even reduced by over 30%.
Self-rewetting fluid (SRWF) is believed to be a promising and useful working liquid for the application of boiling to the development of high efficient cooling devices with micro structure. To ...clarify the fundamental heat transfer characteristic and heat transfer enhancement mechanism of pool boiling with SRWF, by employing dilute heptanol aqueous solution as SRWF, a series of boiling experiments have been carried out. In pool boiling tests, a boiling system using a horizontal heated wire was employed. The experimental results show that, the critical heat flux (CHF) of the SRWF increased up to 2.52 times the CHF of water, and the heat transfer enhancement mechanism was discussed. With a high speed video camera, the nucleation boiling process on the heated wire has been recorded. It is found out that, the bubble size of the SRWF is much smaller than that of pure water, and the bubbles of SRWF were hard to coalesce, which is beneficial for the application in small thermal devices. Furthermore, when the heat flux was up to a certain value, the micro-bubble emission boiling (MEB) appeared in the SRWF. It can be concluded that the Marangoni convection induced by surface tension gradient of SRWF is probably one of the key factors causing the formation of MEB.
Recently, many countries have become increasingly interested in unused but possibly useful energy resources. Among these unused resources, the thermal energy produced around us can be used as a ...potential energy source for heating, cooling and power generation. This thermal energy is relatively stable on the supply side as waste heat in the industrial field. Heat transport devices are one of the important technology for the effective use of unused heat energy. This paper conducts basic research on devices that effectively transport heat below 200ºC. A pulsating heat pipe (PHP) is an excellent heat transport device based on the phase change of a working fluid. Experiments are performed to investigate the thermal performance of a PHP using different working fluids. The PHP consists of 20 parallel channels made of a copper capillary tube with an internal diameter of 1.8 mm. The PHP is filled with deionized water and an aqueous solution of 1-butanol as working fluids, with different filling ratios (FRs) in the range 50-60 vol.%. The 1-butanol aqueous solution is known as a self-rewetting fluid. The experimental results indicate that, in the case of self-rewetting fluid, stable oscillating motion in the PHP arises at the heat load regime lower than that with water. In addition, the effective thermal conductivity of the PHP with the highest concentration of self-rewetting fluid is higher than that with other fluids in the high heat load regime.
•Evaporative spray cooling is drastically enhanced by the ‘heat chasing’ effect.•The term ‘heat chasing’ effect is coined to describe inverse Marangoni convection.•Using self-rewetting fluid and ...superhydrophilic surface are both vital.•Surface-tension-driven flow prevents dryout by replenishing the heated region.•Enormous heat transfer can be achieved by a single shot of spray.
In the study, we successfully utilize the self-rewetting fluid to enhance the evaporative cooling of a pulsed spray system by inducing the inverse Marangoni convection. We find that the superhydrophilicity of the surface is essential to promote a continuous surface-tension-driven fluid flow, which helps to replenish the hot region with the working fluid, and prevent dryout from happening. As a result, cooling lasts much longer beyond the discharge and a significant amount of heat can be removed by a single shot of spray. We coin a term ‘heat chasing’ effect to describe the excellent cooling caused by the inverse Marangoni convection. Besides the surface superhydrophilicity, the spray height should be confined to a certain range so that a liquid film is formed. Once the inverse Marangoni convection commences, the total heat transfer can be augmented three to seven times for a given spray amount. Although long spray always results in better heat transfer, enhancement with the inverse Marangoni convection is still present under the condition of the short spray for a surface temperature as high as 255 °C.
•Heat transfer enhancement was investigated in a micro-oscillating heat pipe.•Property of hexanol in water changes the surface tension behavior of the fluid.•High filling ratio achieves the SRWF ...behavior at the lower heat load.•Self-driven phenomena of fluid occur due to Marangoni effect and capillary action.•SRWF can eliminate the occurrence of dry out in a heating portion of electronic devices.
In the present paper, the heat transfer enhancement parameter is investigated experimentally for a four turns pulsating heat pipe (PHP) designed in a vertical orientation and made of copper (ID 1.30 mm, OD 2.20 mm and total length 1940 mm) for working fluids selected as deionized water (DIW) and 0.1 wt% hexanol in place of self- rewetting fluid (SRWF) at filling ratios (FRs) 10, 50, and 90% and heat input variation 20–140 W. Results indicate the resistance starts decreasing with the further increase in heat input as compared to DIW of SRWF after heat input 80, 100, and 120 W for FRs 10, 50, and 90%, respectively. The higher FR achieves the SRWF behavior at the lower heat input in comparison to the lower FR. Higher FR favors the sinking of heat from hot to cold region in comparison to lower FR for both SRWF and DI. The thermal performance of PHP in the case of SRWFs is explained based on the combined effect of phenomena such as the Marangoni effect, capillary effect, and wettability. The effect of temperature on properties of the working fluid, such as contact angle, electrical resistance, and surface tension of the working fluid is also studied. The occurrence of pulsating action is clarified based on the temperature profile of thermocouples installed in the condensation section at an optimum high 90% FR.
•Using low manufacturing costs PTFE wick structure applied to loop heat pipe.•First using butanol aqueous solution to penetrate hydrophobic PTFE wick structure.•Find out optimum concentration of ...butanol used for PTFE wick structure.•The performance of butanol-PTFE loop heat pipe is better than that using water-nickel.
It’s the first using self-rewetting fluid with PTFE wick structure applied to Loop Heat Pipe System. PTFE wick structure has low thermal conductivity, low sintering temperature thus low cost and high anti-oxidation production for long term using which can also overcome heat leakage when operating. It effectively improves the performance of traditional metal wick structure. However, with the hydrophobic of PTFE material, we can’t directly use water as working fluid. Therefore, this paper adds butanol in water to form self-rewetting fluid which not only easily goes through PTFE wick structure but spontaneously supplements the heating surface, making system operates successfully.
After testing the surface tension and heat-transfer performances of LHP system, it shows that LHP system begins to work by adding 3wt% butanol aqueous solution. It has the maximum heat capacity of 400W, the minimum thermal resistance 0.32°C/W and heat flux 20W/cm2 when increasing to 6wt% butanol aqueous solution. It’s also the first point making LHP system operate successfully. Compared to the water-nickel system, butanol-PTFE system halving the total thermal resistance, heat flux increases over 50%, and the lower operating temperature and pressure, thus it can have higher stability and security.
•Unique bubble dynamics is observed in the glass beads porous structures.•Heat transfer during nucleate boiling can be enhanced by the porous structures.•Heat transfer can also be enhanced by ...increasing the number of carbon atoms.•Effect of self-rewetting fluids is shown during boiling with the porous medium.
Enhancing the heat transfer of boiling has been extensively investigated by either surface modification technique or alteration of the properties of working fluids. In this paper, an experimental investigation on the nucleate boiling characteristics of deionized water and some typical self-rewetting fluids (aqueous n-butanol, n-pentanol and n-hexanol solutions) in spherical glass beads packed bed porous structures was performed. Unique bubble dynamics was observed in the porous structures for these working fluids under different heat flux conditions. The results show that the heat transfer during nucleate boiling can not only be enhanced by the porous structures, but also be enhanced by increasing the number of carbon atoms of the alcohols. This work provides fundamental data for further development of theoretical models of pool boiling heat transfer in porous structures. Extended investigation can be performed by improving the superheats, to fully understand the effect of interfacial property on the heat transfer characteristics of various working fluids in the glass beads packed porous structures and in related applications.
The purpose of this paper is to develop SRF formulations for LHP performance enhancement. In this paper, the solute of SRF is prepared, and butanol and its isomer, 2-butanol, are selected. In this ...paper, concentrations of the 2-butanol aqueous solution (10%, 15%, and 20%) plus the butanol 6% aqueous solution were used to measure the surface tension of four different formulations of SRF and water. It was found that the higher the solute concentration became, the stronger the Marangoni effect was, and the more obvious the surface tension change of the 2-butanol 20% aqueous solution was. Water, the butanol 6% aqueous solution and the 2-butanol 20% aqueous solution were filled into LHP respectively, and the heat transfer performance was measured. The 2-Butanol 20% aqueous solution improved LHP performance by about three times compared with water, and the lowest total thermal resistance was only 1/4 that of water. Therefore, the 2-butanol 20% SRF aqueous solution is an ideal formula for improving the LHP heat transfer performance.
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