CFD multiphase modelling of the acetone condensation and evaporation process in a horizontal circular tube

With increasing demands on energy efficiency, the use of low grade waste heat using vapour absorption refrigeration systems (VARS) are receiving renewed interest. One idea is to use the combination of acetone and zinc bromide as the salt solution, which allows use of temperatures in the order of 10 s of degrees C above ambient conditions. This work numerically models acetone phase change in the evaporator and condenser in order to indicate how improvements can be made in these components of the system. ANSYS (R) Fluent finite volume method CFD is used to produce volume of fluid (VOF) and mixture multiphase flow models to investigate the evaporation and the condensation of acetone in a horizontal circular tube. Different velocities and temperatures were taken in each process to explore the effect of these variables in the system. A user defined function (UDF) is used to calculate the volume fraction of the phases. For the evaporation case, the heat transfer coefficient increases with increasing velocity and the temperature difference between the inlet flow and the wall, as expected. The mass transfer rate decreases with increasing the flow rate or decreasing the wall temperature, from 0.045 kg/m(3).s at 0.01 m/s to 0.016 kg/m(3).s at 0.06 m/s and it drops from 0.044 to 0.023 kg/m(3).s by reducing the temperature just from 300 to 298 K. There is a reduction in specific heat transfer to the liquid despite the higher wall heat transfer coefficient. In the condenser, vapour quality decreases along the tube as liquid acetone is created when the flow rate is reduced. Vapour volume fraction at the outlet section drops from 0.74 to 0.168 by increasing the ingoing velocity from 0.01 to 0.06 m/s. Increasing the rate of condensation will increase the liquid supplied to the evaporator, which increases the evaporation rate and then increases the performance of the VARS. This demonstrates the importance of controlling the temperature and the flow rate in the VARS for generating more refrigerant in the vapour generator. (C) 2019 Elsevier Ltd. All rights reserved.