13-6 |
Convective heat transfer measurements and flow visualization in air impingement systems |
A. SARKAR and R. P. Singh. Biological and Agricultural Engineering, University of California, Davis, One Shields Avenue, Davis, CA 95616 Heat transfer in impingement systems has complex airflow involved. Developments in experimental techniques for studying heat transfer and fluid flow and application of computational fluid dynamics (CFD) may help to resolve this complexity. The objective was to measure convective heat transfer and study airflow under impingement systems. Attempts were made to model it using CFD. A setup was developed to measure spatial variation of convective heat transfer (h-value) under impinging jets. Measurements were done for single, double and triple slot and circular jets impinging on a flat plate. The h-value variations under the jets were studied for various jet diameters (D), lengths (L) and nozzle to plate spacing (H). Flow under the jets was visualized using planar flow visualizations techniques. Heat transfer under the jets was modeled using Fluent 5 CFD solver with Gambit 1.3. Maximum h-values varied from 80 to 140 W/mēK at stagnation point for circular and slot jets for velocities from 15 to 30 m/s. h-values dropped by 50 to 75% of maximum at a radial distance of 4 cm for circular jets and 30 to 60% for slot jets. Results show good agreement with existing semi-empirical models for heated conditions, but poor agreement for frozen conditions. The experiments in conjunction with visualization and modeling indicate turbulent structures in the jet wake cause considerable energy dissipation for Reynolds number of 25000 and above for D/L ratios of 2 and D/H ratios of 8 and above. Maintaining jet velocities around 25m/s and D/H ratios of 8 and less for D/L ratio of 2 can achieve desirable heat transfer. The studies show that combined use of flow visualization, measurement of spatial variations of h-value and modeling can lead to a better understanding impingement processes. Similar studies may give significant insights into other convection situations.
Session 13, Food Engineering: Thermal processes
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