The application of phase change cooling of electronic components has been used by GEC ALSTHOM for several years in railway traction equipment . The basic knowledge of electronic cooling has been reviewed by Bergles . More particularly, Incropera  has presented a good survey on immersion cooling problems. An other way to ensure the cooling of electronic components is the use of two-phase closed thermosyphons or gravity assisted heat pipes as cooling devices. There are several possible technologies of thermosyphons. The main aim in the development of these systems, is the design of more and more compact and reliable devices, able to handle higher power levels. In a first configuration, each power component is cooled by its own thermosyphon in order to avoid maintenance problems . A two-phase thermosyphon consists of a closed device operating in a gravity field. Thus, the liquid at the bottom of the device (evaporator, usually a tube), evaporates and the vapor rises to the top of the tube where it condenses in a condenser cooled by natural or forced convection air. Then, the condensate returns to the evaporator in the same tube, by gravity. In this case, all coolers are independent from one another. The electronic components can be pressed alternatively with the heat sinks (evaporators) set in a sandwich pattern. An other configuration, which is already in use, is, like in the last case, a thermosyphon with the same independent evaporators pressed together with the components. The vapor evaporated in these heat sinks is collected in a head tube and carried to a common condenser [1, 5]. Such a device, called SMF (Systeme Modulaire Fluide) and developped by GEC ALSTHOM, comes in the form of an interchangeable panel. The advantage of this device is that the condenser dissipates only the mean heat losses from all the power components in a specific operating cycle.
The primary objective of the present study is the model development of the transient operating characteristics of a two-phase thermosyphon in order to simulate the performance of the lastly described device, using FC72 as a working fluid. It is particularly interesting to determine theoretically the various thermal resistances of the thermosyphon in order to analyze the different possibilities of coupling the evaporators and to qualify the chosen method for variable operating conditions. The theoretical results are compared with the experimental ones. These data are also compared with those obtained for the R113.