A Numerical Model to Investigate Evaporative Condenser Behaviour at Tube Scale

Evaporative condensers operate at lower temperatures and with a higher efficiency compared to air condensers, as heat rejection is limited by air wet bulb temperature and mainly caused by water vaporization. This reduces the compressor pressure-lift and improves refrigeration cycle performance. Due to complex phenomena of heat and mass transfer on the tube bundles, modeling the evaporative condensers is a hard task and fine grids in numerical simulations are requested to reach acceptable results. A two-dimensional steady state numerical model at the single tube scale has been developed in Ansys-Fluent (release- 14.5), adopting the VOF multiphase model. Moist air has been treated as a mixture of air and water vapor species, while water vaporization and latent heat have been modeled with a C++ User Defined Function. The tube wall temperature has been assumed constant. The aim of this work is to describe the developed numerical model and to validate it by comparing results obtained at different operating conditions with empirical relationships found in the literature in terms of combined and overall heat transfer coefficients. Combined heat transfer coefficient variation along the tube surface has been analyzed, observing that the heat transfer coefficient is higher in the impingement zone, becomes approximately uniform and rises approaching the trailing edge. Moisture content distributions at different sections through the heat exchanger have been examined in detail as well. This study will be the basis to investigate the performance of the whole condenser taking into account the real evolution of the operating conditions of each single tube in the bundle, whatever its arrangement.