Three-dimensional CFD calculations for designing large food chillers
Résumé
In industry solid foods such as vegetables and meat are mostly air-chilled and stored in refrigerated rooms. Plant performance depends on heat and water exchanges between the air and the products and thus on the spatial distribution of the air parameters: velocity, turbulence intensity, temperature and relative humidity. In theory the transient behaviour of chillers could be calculated using computational fluid dynamics (CFD) codes, but current computer capacity and the inaccuracy of models for evaluating heat and mass transfer coefficients make them inapplicable to very large rooms containing hundreds of individual products. To overcome these limits a four-step calculation is proposed to assess how changes in chiller design and operating conditions affect chilling kinetics: (1) 3D calculation of the air velocity field with a CFD code, (2) deduction of an average air velocity map, (3) determination of heat and mass transfer coefficients in relation to the air velocity map using independent measurements, and (4) calculation of the product chilling kinetics using specific software. This procedure was applied to a pork chiller containing 290 carcasses. Two design cases differing in inlet air direction and flow rate, and two functioning modes, batch and continuous, were analysed. A fairly good agreement was observed between the calculated and measured air velocities. It was shown from temperature and weight loss kinetics that with this type of design, batch chilling can only be achieved overnight at the cost of overcooling for low weight carcasses, and increased weight losses.