Convective Heat Transfer in Jet Interaction with a Boundary Surface

Abstract

A method is proposed for calculating the convective heat transfer when a single circular jet interacts with a plane surface. The differences between this method and those currently existing are noted. The energy-dynamic potential of a flux and the energy-dynamic power of a flux are introduced. These concepts may be used to determine the rate of convective heat transfer at the gas–solid boundary. The differences between the proposed concepts and the existing concepts of the heat-flux density and heat flux are noted. The fundamental difference between the heat-flux density q and the energy-dynamic potential q_e is as follows. The heat-flux density q for convective heat transfer is the heat transferred from a liquid to a solid surface (or conversely) in unit time through unit area of heat-transfer surface. Thus, q characterizes the rate of convective heat transfer at a phase boundary. By contrast, the energy-dynamic potential q_e characterizes a property of the flux as a source or carrier of heat. Specifically, q_e characterizes the unit power of the liquid flux. In calculating the heat transfer, the jet interacting with the surface may be divided into two parts: before interaction, a simple jet; after interaction, a fan-like flux. It is not entirely correct to calculate the convective heat transfer in jet heating on the basis of the Reynolds number calculated from the characteristics of the gas leaving the nozzle. Instead, characteristics of the fan-like flux must be used: specifically, the initial mean velocity U_fan of the fan-like flux; and the distance from the critical point of the jet (the point of intersection of the jet’s vertical axis and the surface) to the current radius downstream. To assess the change in the basic characteristics of the free jets with different distances from the nozzle outlet to the boundary surface, we present expressions for the expansion coefficient of the jet; the injection coefficient of the jet; the velocity coefficient for any cross section of the jet (including or excluding the nozzle’s outlet cross section); and the ratio of the Reynolds numbers. Our findings confirm the need to calculate the heat transfer on the basis of the characteristics of the fan-like flux.