Jet Impingement and Infrared Heating of Cylindrical Foods. Flow and Heat Transfer Studies

University dissertation from Department of Food Technology, Nutrition and Engineering, Lund Institute of Technology, Lund University

Abstract: Rapid heat transfer methods such as jet impingement and infrared radiation can be used to speed up thermal processing in the food industry. The heating rate affects important food characteristics, such as colour. Jet impingement consists of directed jets of air with a high velocity that impinge on the food surface, which reduces the velocity boundary layer between the air and surface and gives a fast heat transfer. Infrared heating is a radiative heat transfer method with instant and effective heating of the food product, without heating the surrounding air. Flow and heat transfer from single and multiple jets impinging on a circular cylinder placed on a flat surface were studied numerically by computational fluid dynamics (CFD) simulations using the k-? SST and RSM models. The results concerning velocity obtained by CFD were generally in good agreement with experimental measurements using particle image velocimetry (PIV). The heat transfer distribution on the cylinder surface, expressed as local heat transfer coefficients, was non-uniform due to air flow characteristics that influenced the boundary layer thickness. The heat transfer was highest on the top of the cylinder, and lower close to the separation point and on the back of the cylinder. In addition to making CFD simulations, the local heat transfer coefficients were determined by an inverse heat transfer method, including a sensitivity analysis, using temperature profiles in the cylinder at different positions. The heat transfer coefficients were generally higher than what was predicted by CFD. The interaction between multiple jets strongly affected the heat transfer around the cylinder. The distance and opening between the jets were found to be important to achieving a high and even heat transfer rate using multiple jets. Jet impingement and infrared heating simultaneously was studied using CFD, which showed that there is a synergistic effect when the two heating methods are combined. A higher heat transfer was achieved by combining the methods than by using each method separately. The effect of impingement and infrared heating on colour and crust formation on partially baked baguettes was investigated experimentally. Impingement and infrared heating gave different temperature profiles in the crust, which resulted in different colour development and crust formation. A rapid heating increased the rate of colour development and shortened the total heating time, which resulted in a thin crust and reduction in the total moisture loss. The combination of jet impingement and infrared made it possible to create product properties that cannot be achieved by use of the heating methods alone. A thicker crust and better heating in the centre of the bread could only be achieved by combining infrared and impingement. For industrial thermal processes, prediction of flow and heat transfer with CFD can be used to design equipment and processes that offer optimal heat transfer to the product, thus resulting in high and even product quality.

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