Confinement and vapour production rate influences in closed two-phase reflux thermosyphons Part B: Heat transfer

This work investigates boiling heat transfer for a small scale thermosyphon. Heat transfer results were analysed for three fluids; water, ethanol and HFE-7000. The influence of confinement and vapour mass flux on thermosyphon heat transfer was studied using a fully transparent two-phase closed thermosyphon. By considering the thermal performance of the thermosyphon together with the observed flow regimes, diabatic flow maps were developed. Of particular importance is the reduced pressure of the system which influences the size of the generated vapour bubbles in the evaporator. At low reduced pressures, bubble diameters may be large enough to span the tube diameter and the thermosyphon exhibits confined flow behaviour similar to that of micro-scale convective two-phase flows. Flows of this nature result in oscillatory behaviour and the boiling heat transfer is less effective. The phenomenon can be avoided by increasing the reduced pressure or tube diameter of the system. The most favourable flow regime in the context of thermosyphon heat transfer was found to be churn-type flow. It was found that slug/plug-type flow was not conducive to heat transfer where vapour production rates, and thus latent heat transfer, were relatively low. The flow regime maps relating phase superficial momentum flux the heat transfer coefficient may be used as an aid to the design of thermosyphons for specific applications.