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N. NITIN and M. V. Karwe. Department of Food Science, Rutgers University, 65 Dudley Road, New Brunswick, NJ 08901-8520

Jet impingement technology is used to heat or cool products because of its higher rates of heat transfer. Hot-air jet impingement ovens are used to bake pizza shells, crackers, and cookies and to toast ready-to-eat cereals. To characterize and optimize the baking process in these ovens, we need to study the associated transport phenomena.

The objective of this research was to determine the effect of spacing and cookie diameter on the heat transfer coefficient h in a hot-air jet impingement oven.

The experiments were carried out using a pilot-plant-scale hot-air jet impingement oven. The model cookies of aluminum (Biot No. <0.1) were 5, 3.5, and 2 cm in diameter and 1 cm in height. A batch of 5 cookies was placed in the jet impingement oven. The study was carried out at different conditions of temperature and air velocity.

The h value for all cookies varied between 100 and 250 W/m²°C for the range of air velocity (15-45 m/s) and the maximum air temperature (65-150°C). For 5-cm-diameter cookies, the variation of h from cookie to cookie was small, and the value of h was close that for a single cookie. For 3.5- and 2-cm-diameter cookies, the h value for the center cookie was always lower than for the surrounding cookies. For the 5-cookie configuration, the center cookie with 3.5 cm diameter had an h value lower than the value for 5- or 2-cm-diameter center cookies at lower temperature (65°C), whereas no particular pattern was observed at higher temperature (150°C).

This study has shown that the variation of heat transfer coefficient from jet to jet and cookie to cookie is small. The small cookies do experience a higher heat transfer coefficient right under the jets, but the effect of the surrounding cookies is small.