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To solve the heat dissipation problem of high heat flux components on spacecraft, experiments were carried out to investigate the flow boiling heat transfer characteristics of ammonia in a microchannel heat sink. Verified by gravity-independent criteria, the experimental results can be used to predict the performance of flow boiling heat transfer in microgravity environment. The heat sink consisted of 37 V-shaped microchannels with hydraulic diameters of 280 μm and channel lengths of 45 mm. Saturated flow boiling experiments were conducted with heat fluxes of 60.2~134.3 W/cm2, mass fluxes of 165~883 kg/m2s and saturation temperatures of 25 and 35 °C, as well as subcooled flow boiling experiments with inlet subcooling of 5 °C as a comparison. According to the experimental results, the following conclusions can be drawn. (1) As the mass flow rate increases, higher wall superheat is needed to trigger nucleate boiling. (2) In a lower mass flux range, the heat transfer coefficient changes drastically with mass flux, showing convective boiling characteristics, while the value varies slightly in a higher mass flux range, indicating that the nucleate boiling mechanism is dominant. (3) Under the experimental conditions in this work, the average heat transfer coefficient of ammonia reaches its peak when the outlet vapor quality is between 0.21 and 0.31. (4) A subcooled inlet condition has a suppression effect on the average heat transfer capacity of the working fluid at high mass flux, and increasing the saturation temperature reduces the average heat transfer coefficient. In addition, a comparison between the experimental results and existing correlations demonstrated that the correlation proposed by Fang predicted the experimental results well, and then a modified Fang correlation was proposed based on the experimental data obtained in this study. The results could provide references for the prediction of flow boiling heat transfer performance in aerospace environments and the design of heat dissipation systems.

To solve the heat dissipation problem of electronic devices with high heat flux hotspots, a diamond microchannel heat sink consisting of 37 parallel triangular microchannels with channel lengths of 45 mm and hydraulic diameters of 280 μm was designed. The flow boiling heat transfer characteristics of ammonia in the microchannels were investigated under high heat fluxes of 473.9-1000.4 W/cm2. Saturated flow boiling experiments with saturation temperatures of 25, 30, and 35°C and mass fluxes of 98–1200 kg/m2s were conducted, as well as subcooled flow boiling with inlet subcooling of 5°C as a comparison. The temperature and pressure drop measurements were analyzed, and the main conclusions below can be drawn. (1) At a given heat flux, the heat source temperature first decreased and then increased with the mass flux, and there existed an optimum mass flux to optimize the cooling performance of the heat sink. (2) The heat transfer performance under the saturated inlet condition was obviously better than that under the subcooled inlet condition. (3) A larger saturation temperature leaded to weakening of both the heat transfer capacity and the stability of the microchannel heat sink. Notably, with the high heat diffusion ability of the diamond substrate and the great heat transfer capacity of ammonia flow boiling in microchannels, the heat sink can achieve a heat removal capacity of up to 1000.4 W/cm2.

Flow boiling in microchannels is a promising technique for cooling high power-density electronic devices. In this study, a microchannel heat sink using ammonia as working fluid is developed, and its cooling efficiency is experimentally investigated under nonuniform high-density heat flux, which well simulates a practical heat dissipation scenario for microthermal systems. Diamond with high thermal conductivity of 1500 W/m·K is selected as the microchannel heat sink material. A total of 37 parallel triangular microchannels with aspect ratio of 5, channel length of 45 mm and hydraulic diameter of 280 μm are uniformly engineered on the diamond film by laser ablation processing. The significance of diamond substrate as a heat spreader to minimize the nonuniformity of heat flux imposed by a central hotspot is verified. The influences of heat flux and mass flux on the cooling efficiency are experimentally investigated. An optimal range of outlet vapor quality from 0.10 to 0.13 is found, within which the minimum heat source temperature can be achieved. Notably, the microchannel heat sink is capable of managing a central hotspot with heat flux of 267 W/cm2 while maintaining the heat source temperature at 53.3 °C for a mass flux of 320 kg/m2s.

A continuous damage theory is employed to study mechanics behavior of concrete material characterized by strain–softening. The influence of the initiation and growth of microdefects (damage) between the interphase interface is studied by analyzing the condition of interphase.Problems are considered in terms of elastic–damage theory for brittle materials. Basic elements of continuous damage mechanics are given, and the material model of dominant microdefect is proposed. In particuler, in order to evaluate the accuracy of the theory model, results obtained through the analytical 1solution are compared with the experimental results and found to be in excellent agreement.