33-4 |
Heat transfer in air impingement systems: Design considerations |
R. P. SINGH1, A. Sarkar1, and B. A. Anderson. (1) Dept. of Biological & Agricultural Engineering, Univ. of California, Davis, 1 Shields Ave., Davis, CA 95616-5200 Air impingement systems have been used in various industrial applications such as textile and paper drying, electronic cooling, glass quenching etc. More recently, air impingement systems are developing as a promising thermal processing technique for industrial food processing operations. Impingement systems have been applied to food processes such as drying, baking, and freezing. Considerable reduction in process times and added improvement in product quality can be obtained by the application of air impingement systems. A point of concern for application of air impingement systems is the variation of heat transfer coefficient on the surface of the product that is being processed, resulting in quality concerns. Laser doppler studies have indicated that air impingement systems result in high convection coefficient at concentrated points resulting in localized hot and cold spots. In the present work, a comparative study of flow distribution patterns from an array of four circular-opening nozzles and a two- dimensional jet was conducted using flow visualization techniques. A Helium-bubble generation device in conjunction with a CCD camera system was used for the purpose of visualization. The relative spatial distribution of convective heat transfer from the two types of jets was studied at jet exit with airflow velocities of 15, 20 and 25 m/s. A previously described setup for measurement of spatial heat transfer coefficient was used for the temperature ranges of 25 C to 80 C. For the same exit air velocities, the laminar jets show a slightly lower convective heat transfer coefficient (averaged over the entire area) but a more uniform heat transfer coefficient distribution pattern as compared to the array of circular jets. The lower convective heat transfer may be attributed to the reduced turbulence in the boundary layer while using the two-dimensional jet as compared to the circular jets where the jets interact with each other to increase the turbulence effect.
Session 33, Food processing with air impingement systems: Innovations and opportunities
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