Brandon Field

The purpose of this investigation is to visualize the air curtain on a refrigerator case in order to understand the flow physics of entrained air.

 Background:

Refrigerated display cases using air curtains are very popular in supermarkets to increase the throughput of product. This is a photo from one of the manufacturer's websites of a whole stretch of display cases.

The cases work by blowing a curtain of cold air down the front of the case to keep ambient air and to provide refrigeration. The air curtain forms a shear layer, which at low Reynolds numbers would be a very effective blockade. However the curtains can not be maintained at low Reynolds numbers, and the shear layer is actually in the transitional flow regime. The outside of the curtain mixes with the ambient air, entraining hot and humid air through into the refrigeration loop. As a result, the evaporator coils frost much more quickly than a closed door case and the defrost cycle must be run at a much higher frequency. To the right is a smoke visualization of the cold air curtain on a refrigerator case. Note the spillage from the bottom of the air curtain onto the floor. Reducing the curtain entrainment would increase the amount of cold air that is recirculated.

The experimental technique to be employed in this project is Particle Image Velocimetry (PIV). PIV consists of seeding the airflow with visible particles and then illuminating a sheet of them with a laser sheet. Digital photographs are then taken and correlated to detect direction and magnitude of the movement of the particles, producing a velocity vector field. This is a unobtrusive velocity measurement technique that is not subject to thermal biases and as such is ideal to measure air flow for a thermally stratified curtain. The data will provide a basis for understanding the entrainment physics and a validation for computational models of the air curtain that are being developed.

 A photo of the PIV Camera and laser in the lab