Experimental investigation on heat transfer characteristics of hexamethyldisiloxane (MM) at supercritical pressures for medium/high temperature ORC applications

Abstract

The supercritical organic Rankine cycle (ORC), with favorable thermo-economy, has been considered as one of the most promising solutions to convert medium/high temperature renewable energy or waste heat into electricity. Hexamethyldisiloxane (MM) has relatively high thermostability and low critical parameters, rendering it suitable for the medium/high temperature supercritical ORC system. Since the abnormal heat transfer phenomenon caused by the sharp change of the thermal properties around the pseudo-critical temperature at supercritical pressures of MM in the heating tube, the heat exchanger design becomes one of the most challenging aspects related to the supercritical ORC modeling. To fill this gap, the present work experimentally investigates the heat transfer to MM at supercritical pressures in a smooth vertically upward tube with an inner diameter of 2.4 mm. The effects of key heat transfer conditions, predictions of heat transfer deterioration (HTD), applicability of existing heat transfer correlations and the development of a new correlation are analyzed based on the experimental data. The results show that the heat transfer process at supercritical pressures can be divided into three stages according to the wall temperature, bulk temperature and pseudo-critical temperature. The higher heat flux, lower pressure and mass flux lead to the more serious HTD. The non-dimensional heat flux gives a better prediction of HTD than the buoyancy parameter and flow acceleration parameter. Among the existing correlations, Bishop correlation can get a more accurate predicted result with the mean absolute relative deviation (MAD) of 8.5%. Fitted by 900 experimental points of MM, the proposed correlation considering the non-dimensional heat flux shows a lower MAD of 4.6%.