Utilization of the widely available low grade heat including the solar thermal energy, waste heat and even the ambient heat offers great opportunities for environmentally friendly energy harvesting. Despite the efforts on developing new low grade heat conversion technologies, heat pump is still one of the most effective ways to utilize low grade heat in the current stage. Regarding the heat pump for efficient low grade heat uses, this paper unrolls a comprehensive view from the thermodynamic cycle, working medium, as well as heat and mass transfer enhancement, to the low grade heat utilization systems. Different heat pumps including vapor compression heat pump, absorption heat pump, and adsorption heat pump are included. The aim of this paper is to share our experience in the evolution from heat pump concept to low grade heat utilization. It is also helpful for researchers who try to build a bridge between thermal science and thermal engineering.

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In steel manufacturing processes such as hot strip, a hot-steel at a temperature higher than the Leidenfrost temperature is quenched with water to improve mechanical properties. The quenching process is governed by transient transition boiling, which includes inherently complicated intermittent wetting coupled with transient heat conduction in the hot workpiece. Existing experimental studies on quenching generally applied an exact solution of heat conduction problem during sudden contact of semi-infinite liquid and solid to assess very earlier liquid-solid interface temperature and whether the hot surface is able to be wetted or not. The prediction of liquid-solid interface at a very early stage before the inception of homogeneous or boiling nucleation is governed by the pure transient heat conduction between the liquid and solid. However, a thin porous oxide layer with poor thermal conductivity deposited on the surface may have a major effect on the earlier heat transfer. Recent progress in research on thermal properties of oxide layers enables more accurate assessment of the interface temperature characteristics. In this study, we attempted to derive an exact solution of liquid-solid interface temperature on the oxidized surface. Besides, the effects of oxide layer thickness as well as the interfacial thermal resistance between layer and base metal have been evaluated analytically.

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A loop heat pipes is a highly efficient two-phase fluid loop utilizing latent heat. In recent years, due to the miniaturization and high performance of electronics, the heat density of the equipment has been increasing remarkably, so thermal demand is increasing in the terrestrial field. In this study, we developed a high heat flux loop heat pipe aiming to be mounted in electronics and conducted experiments under an atmospheric environment. It shows that heat transport is performed up to a maximum heat flux of 42.5 W/cm2 at a heat transport distance of 400 mm, and it is confirmed that it operates normally in three positions of horizontal, top heat, and bottom heat.

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Considering application to thermal management of space satellites, heat transport characteristics of an ammoniacharged oscillating heat pipe (OHP) in a low temperature region are investigated experimentally. A 6-turn looped-OHP of 400 mm in length is set in a horizontal position with uniform heat flux heating and a cooled aluminum block. Temperatures are measured at about 100 points with thermocouples to evaluate its heat transport characteristics. In the adiabatic section, the adjacent pipes show different temperature distributions along the axes. One is a uniform distribution and the other is a linearly-decreasing distribution from the heating to cooling section. In the former pipe the fluid flows from the heating to cooling section and latent heat transport may be dominant. The flow direction in the latter pipe is opposite and sensible heat transport by oscillatory flows may be dominant. By changing both the cooling section temperature from -60 °C to -20 °C and the charge ratio from 25 wt% to 73 wt%, we make two heat transfer performance maps: a map on the maximum heat transport rate and a map on the maximum effective thermal conductivity. The peak values of about 60 W and 43 kW/(m∙K) are obtained at around the same conditions: the cooling section temperature of -30 °C and the charge ratio of 54 wt%.

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In order to realize a liquid droplet radiator (LDR), which is an equipment used for waste heat rejection in large space structures, the exhaust heat characteristics of a single liquid droplet stream in vacuum are required. In this study, these characteristics were obtained by a combination of experiments and numerical analyses. Experiments were conducted for measuring the amount of waste heat coming from a single liquid droplet stream of silicone oil as the working fluid, using a radiant flux sensor (RFS). The emissivity of the RFS at low temperature was measured, using a black radiation ball with known emissivity at 26°C. Numerical analyses were also performed to separate the radiation from the background of the experimental apparatus included in the radiation measured by the RFS. It was determined that at approximately -180°C, the emissivity of the RFS falls from the catalog value of 0.8 to 0.5. Moreover, the emissivity of the liquid droplet was found to be approximately in the range 0.42–0.51 and the effective emissivity of the single droplet stream was approximately 0.10. The application of these results can improve the feasibility of LDRs.

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