In a refrigerated cabinet the thermostat measures one point. What the user experiences is a temperature distribution that varies shelf to shelf, and what matters for food safety is the warmest point.
Air distribution problems
Dead zones. Shelf corners, door shelves and the area around the crisper have low circulation. These regions run markedly above the mean.
Product blockage. The distribution measured in an empty cabinet does not hold when it is full. Products block the air paths, and their own thermal mass slows the system. Both an empty and a typical loaded scenario should be run.
Door openings. At every opening the cold air spills downward and warm humid room air takes its place. That is both a temperature shock and the main source of frosting.
Static and fan-assisted systems differ
In static (non-frost-free) cabinets the circulation is driven purely by buoyancy; the flow is very slow and strongly stratified. This is a natural convection problem and therefore demanding in terms of mesh and convergence.
In fan-assisted systems the distribution is set by duct and vent design; the problem approaches a classic distribution problem, though buoyancy still matters at the low velocities involved. See the mixed convection note.
Humidity and frosting
Moisture condensing on the evaporator surface turns to frost; the frost layer both reduces heat transfer and narrows the air path. A defrost cycle is therefore necessary, but every defrost adds heat to the cabinet.
Unless moisture transport is solved, where frosting will start cannot be seen. See the condensation note.
Insulation and thermal bridges
A significant part of the cabinet loss comes from the door seal and the body profiles. See the thermal bridge note.
References
- Churchill, S. W. & Chu, H. H. S. (1975). Correlating equations for laminar and turbulent free convection from a vertical plate. International Journal of Heat and Mass Transfer, 18(11), 1323–1329. DOI: 10.1016/0017-9310(75)90243-4
- Shah, R. K. & Sekulić, D. P. (2003). Fundamentals of Heat Exchanger Design. John Wiley & Sons. DOI: 10.1002/9780470172605