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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 average heat transfer coefficient h for cookie-shaped objects in a hot-air jet impingement oven.

To obtain h, we used model cookies of aluminum (Biot No. <0.1). The experiments were carried out using a pilot-plant-scale hot-air jet impingement oven. The variation of h with position of the cookie in the oven, cookie size, air velocity and temperature, and bowl rotation was investigated. Correlations between the Nusselt number Nu and jet Reynolds number Re were generated. Experiments were also carried out with cookies made from wheat flour. The variation of temperature vs time within wheat flour cookies was compared with predictions using h values obtained from the correlations.

The h value varied between 100 and 250 W/m²°C for the range of air velocity (15-45 m/s), maximum air temperature (50-150°C), and cookie size (5-0.8 cm in diameter). The 5- and 3.5-cm-diameter cookies showed similar h values, while higher values were obtained for the 0.8-cm-diameter cookie. The Nu varied with Re_n, with n between 0.4 and 0.6. The experimentally obtained variation of temperature with time for wheat flour cookies showed good agreement with predictions.

This study has shown that h in impingement ovens is 3-8 times higher in than conventional ovens and that locally higher values of h exist in a small region right under the jets, which may cause localized heating.