Comparison between Numerical Simulation and On-orbit Experiment of Oscillating Heat Pipes

With current complexity and diversity of space missions, on-board devices of spacecraft must be highly performing and densely mounted. These facts lead a remarkable increase in heat generation density. Therefore, the next generation of heat transfer devices are required to have higher heat transport capabilities and be more flexible. Recently, a new concept of heat pipe known as Oscillating Heat Pipes (OHPs) has attracted attention. OHPs consist only of narrow bended tubes containing a working fluid. The working fluid exists as vapor plugs and liquid slugs due to capillary force in OHPs. As heat is applied to the heating section, oscillation/circulation is driven by the pressure difference between vapor plugs. This way, the working fluid transports heat from the heating section to the cooling section by a combination of sensible and latent heat transfer. Since very thin pipes can be used, OHPs can transport heat from very narrow spaces with high efficiency. In Japan, JAXA has conducted on-orbit experiments of OHPs with check valves on a small satellite named SDS-4 since 2012, and the effectiveness has been verified. However, the operating characteristics of OHPs are not yet fully understood, and it has not yet been taken into practical use. The critical issue for the practical use is startup characteristics under some conditions, as noticed during the on-orbit experiments. The objective is to investigate the startup behavior of OHPs. In this paper, a one-dimensional mathematical OHP-model with check valves was developed. The model was validated by using laboratory experimental results and compared with on-orbit experiments. In a simulation, the initial vapor-liquid distribution, and the timing of heating regarding eclipse were varied. As results, the mathematical model showed good agreement with experimental results. Also, it was found that initial vapor-liquid distribution and the timing of heating affect the startup behavior of OHPs.