29D-21 |
Numerical simulation and experimental investigation of flow field and surface heat transfer coefficient for a turbulent hot air jet impinging on a cookie |
M. V. KARWE and N. Nitin. Dept. of Food Science, Rutgers, The State Univ. of New Jersey, 65 Dudley Rd., New Brunswick, NJ 08901-8520 Jet impingement technology is used to heat or cool products because of higher rates of heat transfer. Hot air jet impingement ovens are used to bake pizza shells, crackers, cookies and to toast ready-to-eat cereals. To characterize and optimize the design and baking process in these ovens we need to study the flow field and the associated transport phenomena during baking. The objective of this research was to use numerical simulation to calculate surface heat transfer coefficient for a single hot air jet impinging on a cookie-like product and to compare the results with experimental measurements. Flow and temperature fields for a single, axisymmetric, turbulent jet impinging on a model cookie were obtained using a commercial CFD code (Fluent, NH). Heating of the model cookie placed on an insulating plate was modeled as a conjugate heat transfer problem. The surface heat transfer coefficient (h) values were calculated at surface grid points using the difference between hot air jet temperature and model cookie surface temperature. Simulations were carried out at nozzle to plate spacing (z/d) values of 2, 3, and 5, and jet velocities of 10, 20, 30, and 40 m/s. Average and local maximum values of h calculated from the numerical simulation were compared with the experimental data obtained using a pilot scale jet impingement oven (Jetzone, Wolverine Corp.). Numerical simulations clearly showed a local maximum for h near the stagnation point on the surface of the model cookie. Both the numerical simulation and experiments showed increasing surface heat flux with increasing jet velocity and decreasing z/d. From the simulation, regions of potential core, mixing zone, stagnation point and radial acceleration in the impinging jet could be identified. This study has shown that numerical simulation can be effectively used to simulate the thermal transport during hot air jet impingement processing.
Session 29D, Food Engineering: Thermal processes
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