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Vacuum spray flash evaporation cooling (VSFEC) is considered as an alternative effective method for dissipating high heat flux in the field of space thermal control. In this paper, a VSFEC experiment system has been developed to investigate the influence of spray flow rate and spray height on the heat transfer characteristics of VSFEC. The experiment results show that the heat flux can be removed efficiently from the heat source in VSFEC system with a very small spray flow rate, which is only one third of that of conventional spray cooling under the same heat flux. And an optimal flow rate exists in VSFEC, at which the heat transfer approaches to a highest value, it is different from conventional spray cooling in which the heat flux increases with the increasing of spray flow rate all the time, in VSFEC, the heat flux firstly increases and then decreases with increasing of flow rate. Since VSFEC system is an open-loop heat dissipation system, it has very important significance that based on the optimal flow rate to reduce working medium consumption, especially for space thermal control system which has a strict limit on weight. In addition, spray height has an important effect on the heat transfer characteristics of VSFEC and an optimal spray height is also obtained at which the maximum value of heat flux can be achieved. [ABSTRACT FROM AUTHOR]

Methods to avoid the dynamic Leidenfrost effect are of great importance for high heat flux spray cooling which needs an efficient contact of liquid and superheated surface. In this paper, a novel method of increasing dynamic Leidenfrost temperature is proposed through addition of high-alcohol surfactant (HAS). Effects of 1-Octanol and 2-ethyl-hexanol surfactants on dynamic Leidenfrost temperature and spreading dynamic of droplets impacting on superheated surface were investigated for the first time using high-speed photography. Empirical correlations of dynamic Leidenfrost temperature and maximum spreading factor were obtained based on our experimental data. A possible explanation for the dynamic Leidenfrost temperature increase caused by HAS was proposed based on bubble bursting and bubble coalescence. The results show that dynamic Leidenfrost temperature is significantly increased by addition of HAS because of surface tension reduction. The empirical correlations of dynamic Leidenfrost temperature and maximum spreading factor show good agreement with maximum error of 5% and 10% respectively. These empirical correlations could provide reference value for the future research. [ABSTRACT FROM AUTHOR]

Piezoelectric atomizer could enhance heat transfer of spray cooling at low flow rate through improved atomization of liquid droplets. To optimize the heat transfer performance of piezoelectric atomizer based spray cooling, a novel piezoelectric atomizer was designed in this paper. The piezoelectric atomizer was composed of two piezoelectric ceramic films (110 kHz) and a stainless steel micropore disk with different outlet diameters of 5 μm, 7 μm, 9 μm, 20 μm and 25 μm (corresponding flow rates of 0.5 mL/min, 1.0 mL/min, 3.8–5.0 mL/min, 11.0–16.0 mL/min and 20.0–29.0 mL/min, respectively). The effects of micropore outlet diameter, volumetric flow rate and spray height on surface temperature distribution, heat flux and the spray cooling efficiency were studied. It was found that the volumetric flow rate increased with the increase of micropore outlet diameter. As the flow rate increased, the heat flux increased but the spray cooling efficiency decreased as a sacrifice. The correlation between spray cooling efficiency/heat flux and flow rate (range from 0.5 mL/min to 29.0 mL/min) of all atomizers could be generalized into one exponential decay/growth curve. An optimal diameter of 9 μm could achieve high heat flux of 123.8 W/cm 2 at a relatively low volumetric flow rate of 5.0 mL/min, and the corresponding spray cooling efficiency was as high as 53.8%. For each atomizer, there was an optimal spray height which differs with the micropore outlet diameter. [ABSTRACT FROM AUTHOR]

• Analytical solution of non-boiling heat transfer during droplet impact is obtained. • The analytical solutions are applicable to low velocity and low wall temperature. • Wall thermal effusivity can decide heat flux alone, but not the temperature drop. • Droplet property shows great and similar effect on heat flux and temperature drop. • Thermal effusivity is a decisive parameter and high thermal effusivity is preferred. Droplet impact process exhibits excellent heat and mass transfer characteristics. Analytical solution with simple explicit form and certain accuracy can clearly reflect the correlation of physical parameters, however the analytical study on droplet impact heat transfer is quite insufficient and limited to flow field analysis. In this paper, the analytical solutions of heat flux and temperature variation of droplet impact on heated wall in non-boiling regime were obtained based on boundary layer asymptotic matching method. Parameter analysis of thermophysical properties was conducted to explore the decisive properties of droplet and wall materials. The results showed that the analytical solutions are appropriate to predict non-boiling heat transfer at low impact velocity and low initial wall temperature. Parameter analysis showed that thermal effusivity is the decisive property rather than the thermal diffusivity. Wall thermal effusivity could independently determine heat flux, but should determine temperature variation together with wall thermal conductivity. Droplet thermal effusivity and Prandtl number were the decisive properties, however the effect of Prandtl number in reality can be ignored. Wall materials and droplet liquids with higher thermal effusivity showed higher heat transfer capacity. The importance of thermal effusivity was demonstrated and high thermal effusivity was preferred in future thermal management. [ABSTRACT FROM AUTHOR]