Three-Dimensional CFD Modelling and Analysis of the Thermal Entrainment in Open Refrigerated Display Cabinets

This study presents a three-dimensional Computational Fluid Dynamics (CFD) simulation of the air flow pattern and the temperature distribution in a refrigerated display cabinet. The thermal entrainment is evaluated by the variations of the mass flow rate and thermal power along and across the air curtain considering the numerical predictions of abovementioned properties. The evaluation on the ambient air velocity for the three-dimensional (3D) effects in the pattern of this type of turbulent air flow is obtained. Additionally, it is verified that the longitudinal air flow oscillations and the length extremity effects have a considerable influence in the overall thermal performance of the equipment. The non uniform distribution of the air temperature and velocity throughout the re-circulated air curtain determine the temperature differences in the linear display space and inside the food products, affecting the refrigeration power of display cabinets. The numerical predictions have been validated by comparison with experimental tests performed in accordance with the climatic class n.° 3 of EN 441 Standard (Tamb = 25 °C, φamb = 60%; νamb = 0,2 m s−1). These tests were conducted using the point measuring technique for the air temperature, air relative humidity and air velocity throughout the air curtain, the display area of conservation of food products and nearby the inlets/outlets of the air mass flow.

Entrainment by a Refrigerated Air Curtain Down a Wall

Thermal entrainment is important as it adversely affects energy consumption and evaporator humidity levels of refrigerated air curtain display cases, often at transitional Reynolds numbers. In order to get a more fundamental understanding of the mean and unsteady thermal entrainment processes, the shelf structure of a display case has been idealized to that of a plane, adiabatic wall subjected to refrigerated wall jets at laminar and transitional flow conditions. The wall jets are studied at different inflow profiles, Reynolds numbers, and Richardson numbers to investigate the effect on thermal entrainment rates. The primary simulation technique was direct numerical simulation of the Navier–Stokes equations in two dimensions for the low and moderate Reynolds numbers (though three-dimensional simulations were also conducted). At higher Reynolds numbers, a conventional Reynolds averaged Navier–Stokes approach was employed, which was found to give reasonable agreement with the above approach at a wall jet (early-transitional) Reynolds number of 2000. In general, the results yielded a significant variation in entrainment as a function of Reynolds number, with a minimum occurring at flow speeds immediately prior to transition. The entrainment rates were also sensitive to the initial velocity distribution, whereby a constant gradient profile (where any local velocity-gradient peaks were minimized) provided the least entrainment. Entrainment was also found to decrease with increasing Richardson number.