29D-20 |
Determination of heat transfer coefficient during hot air jet impingement of cylindrical objects |
R. P. GADIRAJU, N. Nitin, and M. V. Karwe. Dept. of Food Science, Rutgers, The State Univ. of New Jersey, 65 Dudley Rd., New Brunswick, NJ 08901-8520 Jet impingement technology is used in industrial applications to heat, dry and cool products. The sweeping action of the jets over the product surface results in higher rate of heat transfer, rapid moisture removal, and quick crust formation. The food industry has used this technology to bake pizzas, pastries, cookies and toasted ready-to-eat cereals. Understanding transport phenomena associated with jet-impingement ovens is necessary to optimize the baking/cooking process. The objective of this research was to determine the average heat transfer coefficient (h) for hot dog shaped objects in a hot air jet impingement oven. Jetzone rotary dryer (Wolverine Corporation) was used for baking experiments. Hot dog shaped aluminum rods were used to obtain h using lumped mass analysis (Biot no. < 0.1). The variation of h with hot dog size, air velocity and temperature was investigated. Correlation between Nusselt number (Nu) and jet Reynolds number (Re) was obtained. Validation experiments were carried out using beef hot dogs. The measured time-temperature variations within real hot dogs were compared with predicted time-temperature variations from numerical simulation using experimentally obtained h values. For different air velocities (15-45 m/s), air temperature (65-150°C) and hot dog size (1, 1.5, and 2 cm diameter), the h value varied between 105-235 W/m2°C. The heat transfer coefficient was found to be a strong function of jet air velocity and hot dog diameter and a weak function of jet temperature. The measured time-temperature variation for real hot dogs closely matched with the predicted variation. The correlation relating Nusselt number as a function of Reynolds number and the ratio of diameter of hot dogs to diameter of jet was developed. These results can be used to accurately predict the processing times, center temperature, and for validation of predictive mathematical models for cylindrical objects like hot dogs, sausages, and breadsticks.
Session 29D, Food Engineering: Thermal processes
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