A study has been carried out to investigate the operating characteristics of two phase Thermosiphon loops. These are devices which may be utilized to transport thermal energy from air in one region to cooler air in a remote region through the two-phase natural circulation flow of a working fluid within a closed loop which passes through both source and sink regions. Under normal operation, the warm portion of the loop acts as an evaporator and the cool portion as a condenser. The interconnecting top portion of the loop acts as a vapour header and the bottom acts as a condensate return line. Such loops may transfer heat equally well in either direction or may be capable of heat transfer in one direction only depending upon whether or not liquid is present in both source and sink regions when the system is not operating. This can be controlled through the amount of charge as well as the geometric configuration of the loop.

A computer program has been developed, utilizing correlations and data available in the literature, which is capable of simulating the behaviour of the working fluid in any thermosiphon loop made up of four straight tubes of any physically possible lengths, diameters and orientation in a gravity field. The program also provides a measure of the overall performance of the loop through a "loop conductance", the heat transfer rate per unit of evaporator surface area and temperature difference between the evaporator and condenser tube surfaces.