Isothermal air jet and premixed flame jet impingement: Heat transfer characterisation and comparison

Abstract

The jet impingement heat transfer is a well-established technique for obtaining high heat transfer rates for many cooling and heating applications. Impingement heat transfer methods are widely used in domestic and industrial appliances. The impinging isothermal gas jets are used for both heating and cooling. However, premixed flame jets are used for heating the target surface. Present study is an attempt to compare the heat transfer characteristics of the isothermal air jets and premixed flame jets. The isothermal air jet having jet temperature of 30 °C (cold jet or ambient temperature jet), 100 °C (hot jet), and premixed methane–air flame jet (stoichiometric mixture) with Reynolds numbers of 500, 750, 1000, 1250 and 1500 and nozzle or burner to plate spacings of 2d, 4d and 6d are experimentally investigated for heat transfer characterisation. Thin metal foil technique is used for characterising isothermal air jets. For a fixed jet Reynolds number, the Nusselt number of isothermal air jet is found independent of the temperature difference between jet and surrounding fluid. A steady state technique is proposed for characterisation of premixed flame jets. The outcome of the present study reveals that the heat transfer characteristics of isothermal air jet and the premixed flame jet are almost same. The higher thermal entrainment in premixed flame jet in comparison with isothermal air jet results in a lower effectiveness. The present heat transfer data can be directly utilised for many practical applications.