Hydraulic modelling of a flat heat pipe with two different groove shapes and a small vapour section

This paper reports a hydraulic modelling of liquid and vapour flows occurring inside a working flat plate heat pipe. This specific grooved heat pipe, used for terrestrial applications, is composed of two kinds of grooves: triangular and drop-shape. It also presents the peculiarity of having a very thin vapour space. Based on classical conservation equations, the modelling approach is characterized by specific boundary conditions appropriate to such a system, which is highly sensitive to a variety of parameters (filling ratio, temperature, heat power applied, inclination…). Special attention was paid to resolution between the grooves in the upper part and grooves in the lower part. After comparison of the results with experimental data in terms of dry-out lengths, a parametric study is presented showing the evolution of pressure losses and liquid plug length as functions of working temperature for three working fluids: ethanol, acetone and n-pentane. Finally, due to the small dimensions of the vapour space, a specific study has been conducted on the liquid flow that may occur on both sides of the vapour channels. Even though flow assumptions are still poorly controlled, it is shown here that the length of the liquid plug is highly sensitive to the hydraulic behaviour occurring at the end of the condenser. This behaviour can have a significant impact on the thermal performances of heat pipes, especially when the length of the condenser is low, or of the same order of magnitude as the liquid plug.