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A three dimensional computational fluid dynamic investigation is carried out to predict the turbulent flow and surface heat transfer under an impinging air jet issuing normally from a single noncircular orifice in a plate held parallel to the target surface. Static pressure distributions, velocity fields and local as well as average Nusselt number on the impinged surface are presented for square, elliptic, and rectangular orifices and compared with those for a circular orifice. Effects of jet Reynolds number as well as spacing between the nozzle plate and the impinged surface are examined using a two-layerκ–η turbulence model. Results show flow structure similarities between the characteristics of rectangular and elliptic jets of equal aspect ratio. Further, it is observed that noncircular impinging jets can provide higher average heat transfer rates than corresponding circular jets for certain geometric parameters viz. nozzle-to-plate spacing and the size of the averaging area used to compute the average Nusselt number. [ABSTRACT FROM AUTHOR]

Based on the earlier studies the most common empirical correlations in the literature predict the impingement heat transfer coefficients rather well at low (close to 100°C) temperatures, but get inaccurate at higher temperatures. Impingement temperatures used in paper drying applications are typically 300 to 700°C, and there has been a need to improve the existing heat transfer correlations at high temperature area. This study presents experimental results of impingement heat transfer measurements with a laboratory-scale heat transfer test rig. Five nozzle configurations were measured. The parameters varied in the investigation were nozzle-to-plate distance, nozzle open area, nozzle diameter, impingement velocity and impingement air temperature. Regression model of heat transfer was developed based on the measurement data. A correction factor including the impingement and heat receiving surface temperatures is proposed. The correction factor can be used to improve the existing correlations at large jet to heat receiving surface temperature differences. [ABSTRACT FROM AUTHOR]