91C-22 |
Air impingement heat transfer on a cylindrically shaped object |
B. A. ANDERSON and R. P. Singh. Biological and Agricultural Engineering Department, University of California, Davis, One Shields Avenue, Davis, CA 95616 One method for increasing the convective heat transfer to a food, and thereby decreasing the cooking time, is by using high velocity air. This principle is used in air impingement ovens. High velocity jets of hot air are directed onto a food product, which is usually traveling on a belt under the jets. Few studies have determined the spatial variation in convective heat transfer coefficient (h) under an impingement jet. The spatial variation on a flat plate has previously been studied; however, many food products are shaped very differently than a flat plate. The objective of this study was to determine the spatial variation in h under an impingement jet on a cylinder. In this study, h was determined at different points on a PVC tube, 9 cm in diameter. Small copper plates were connected to thermocouples and attached in holes drilled into the tube. The tube was placed under an impingement jet. Variables studied included distance from the jet, temperature of the hot air, orientation of the tube, and velocity of the air. Values for h at the stagnation point (directly under the impingement jet) were around 100 W/m2·K for air heated to 65.5°C at a velocity of 24 m/s. The h-value dropped to roughly 50 W/m2·K on the side of the tube at 90° from the stagnation point. Air temperature also varied as it leaves the unconfined jet. For air heated to 65.5°C at the jet nozzle, air temperature dropped to 46°C at the tube surface, 10 cm below the jet. The air temperature dropped further to 30°C at the side of the tube, another 4.5 cm lower. Determining how heat transfer parameters vary on a cylinder under an impingement jet can help lead to an understanding of how cylindrically shaped food product will behave under impingement heating.
Session 91C, Food Engineering: Food process engineering
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