Numerical analysis of heat transfer characteristics of hexamethyldisiloxane (MM) at supercritical pressures

Organic Rankine cycle (ORC) is one of the most promising solutions to utilize low-grade thermal energy for the worldwide energy crisis, environment deterioration, and climate change. Organic fluids, commonly with relatively low critical temperature and pressure, can be heated and compressed directly to the supercritical state in order to obtain better match with the heat source temperature and lower corresponding exergy destruction. Supercritical ORC has therefore attracted increasing attention in recent years. Supercritical fluids in the heated channels experience sharp changes in thermal properties during the pseudocritical temperature range, leading to abnormal supercritical heat transfer characteristics. However, to the best of our knowledge, as one of the most challenging aspects related to the ORC modeling, heat transfer mechanisms for supercritical organic fluids have not been completely explained. To fill this gap, this work numerically analyzes the heat transfer to supercritical hexamethyldisiloxane (MM) with characteristics of high thermal stability and low critical parameters and therefore it is applicable for high temperature supercritical ORC system. In the numerical analysis, the shear stress transport k-σ turbulence model is employed to simulate the supercritical heat transfer process in a vertical upward tube under different boundary conditions of pressure, mass flux, and heat flux. Further insight is provided about the physical mechanisms of heat transfer deterioration with numerical results. The results show that the distributions of specific heat and turbulent kinetic energy are the key factors in determining the deterioration degree of heat transfer.