Abstract
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 °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® 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/m3·s at 0.01 m/s to 0.016 kg/m3·s at 0.06 m/s and it drops from 0.044 to 0.023 kg/m3·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.
Highlights
Low grade waste heat from small industrial and micro-generation plants can be used to generate cooling using vapour absorption refrigeration systems which are suitable for use with low temperature difference between waste heat and ambient temperature
vapour absorption refrigeration systems (VARS) rely on effective evaporation and condensation processes to enhance performance
The simulations were conducted with 28000 Pa for condensation and 22000 Pa for evaporation with corresponding saturation temperature point of 310 and 296 K respectively; these conditions reflect the exchange of heat with ambient heat exchangers
Summary
Low grade waste heat (heat produced by system due to a mechanical or chemical processes that use energy) from small industrial and micro-generation plants can be used to generate cooling using vapour absorption refrigeration systems which are suitable for use with low temperature difference between waste heat and ambient temperature. Where SM is the source term in mass conservation equation (kg/m3 s), SE source term in the energy equation (J/m3 s), ε is a time relaxation factor with unit s-1, which is set equal to 0.1 s−1 (to keep the interface temperature close to the saturation temperature) in both of evaporation and condensation cases, and φl and φv are the volume fraction of the liquid and vapour respectively.
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