Mathematical modeling of air-impingement cooling of finite slab shaped objects and effect of spatial variation of heat transfer coefficient

In industrial cooling systems, an important goal is to reduce the temperature of foods as quickly as possible. Heat transfer coefficient (h-value) is a parameter that to be increased to reduce the cooling time in convective cooling techniques. Higher h-values may be obtained by moving air at high velocities, such as those obtained with impingement systems. The objective of this research was to develop and validate a numerical model to predict the transient time-temperature change during cooling of slab shapes under impinged air conditions and to investigate the spatial variation of h-value over the surface. A three-dimensional explicit finite difference mathematical model was developed to handle the spatial variation, and a plexi-glass slab (16 × 12 × 1 cm), insulated on the bottom surface by polyurethane foam (4 cm in thickness) was used in the validation studies. Type-T 36 gauge thermocouples were inserted into the plexi-glass. Then, the slab was left in an oven until a uniform temperature distribution was obtained and placed under an impinged air jet for cooling. Model validation studies were accomplished at impingement nozzle (1.5 cm in diameter) exit air velocities of 14 and 28 m/s. For all experiments, excellent agreement was obtained between the predictions and the experimental results, and spatial variation of h-value was determined to be an important factor to handle in impingement modeling studies.