Numerical Analysis of Mechanism on Heat Transfer Deterioration of Hexamethyldisiloxane in a Vertical Upward Tube at Supercritical Pressures

The working fluids at supercritical pressures will experience abnormal heat transfer compared with those in a sub-critical state. In particular, the heat transfer deterioration (HTD) can make the wall temperature increase sharply in the tube, posing a challenge for the design of heat exchangers in the supercritical organic Rankine cycle (SORC). It is generally acknowledged that the effects of buoyancy and flow acceleration lead to abnormal heat transfer. However, a clear understanding of the interactions between the turbulent flow and heat transfer characteristics still needs to be further improved by obtaining the internal flow mechanism. The current study analyses the contours of the turbulent flow information under the different boundary conditions by means of validated CFD numerical simulation based on the previous experimental data and reveals the main causes of HTD and the impact mechanism of boundary conditions. The results reveal that two deteriorated extreme points are generated in a vertical upward tube with uniform heat flux for hexamethyldisiloxane at supercritical pressures. The buoyancy and flow acceleration effects caused by the drastic variation in fluid density near the pseudo-critical temperature can deform the velocity profile, thus reducing the local shear stress and turbulence intensity, and leading to the HTD. Moreover, HTD gets worse with the increase in heat flux and moderate with the rise in supercritical pressure. This study should support the data and theory for the refined design of heaters applied to the SORC in the future.