Impact of particles on thermal performance of closed-loop oscillating heat pipe

Abstract

The need to save and recover thermal energy requires new technological solutions. Passive phase change probes like closed-loop oscillating heat pipes (CLOHP) can help to meet these requirements. A current approach to improving the thermal performance of such devices is to modify the working fluid with particles. However, the working principle of such suspensions as heat transfer mediums in phase change probes is still not well understood. The aim of the present study is to investigate whether particles accumulate on the evaporator surface and improve the heat transfer coefficient there, rather than affecting the bubble-slug interaction. In addition, we investigate whether statistical measures help distinguish between different working regimes of CLOHPs.Experiments are carried out with a glass CLOHP with four loops and cooling volume fluxes of 6 and 40 l/h. The heat imposed on the evaporator ranges from 10 to 55 W. The results show that an increased number of nucleation sites on the coated evaporator surface improves thermal performance. Statistical parameters, namely skewness and kurtosis, adequately characterise the operating regimes. The achievable increase in transferred heat is between 5 and 12 %, depending on the heat supplied at the evaporator and the condenser cooling. Furthermore, such a coating suppresses the usual start-up behaviour and lowers the value of heat input at which stable operating conditions occur. Skewness and kurtosis of the condenser temperature difference indicate that this lowering is from 40 W down to 30 W at 6 l/h and from 40 W down to 10 W at 40 l/h.The study paves the way for a cost-effective in-situ manufacturing step - evaporator coating during the first minutes/hours of operation - leading to an improvement in the thermal performance of CLOHPs. Further studies are needed to stabilise this improvement in the long term.