Separation of heat transfer components from impinging methane diffusion flames

The high heat transfer rates from impinging flame jets and plumes are extensively used in many industrial applications. Comprehensive literature review suggests that the characterization of flame impingement is done primarily by the measurement of heat flux onto the target surface. The use of in-situ probes limits the spatial resolution of the measurement. For a diffusion flame, radiation cannot be neglected and it is therefore necessary to determine the convective and radiative heat flux components to quantify the thermal boundary condition to the impingement surface. For determining the heat transfer characteristics of impinging diffusion flames, the target surface is impinged by a methane diffusion flame from the bottom and is simultaneously cooled from the top by air jets of different Reynolds number. At steady state, one dimensional energy balance across the impingement surface provides an equation with the three unknowns being the heat transfer coefficient of the flame jet, the reference temperature and the emissivity of the gas/flame. By keeping the flame jet impingement conditions same and varying the air jet impingement on the top surface, five different forms of the energy balance equation is obtained. A minimization technique, that makes use of the Nelder-Mead algorithm, is developed to solve for the over-determined system of equations. The obtained results are compared with the slope method that determines the effective heat transfer coefficient and the reference temperature. The impingement surface is modeled in FLUENT and the experimentally obtained heat transfer coefficient of the flame jet, the reference temperature and the emissivity of the gas/flame is provided as the boundary condition to numerically determine the surface temperature. For validation purpose, the impingement surface material and thickness is changed and the experimentally obtained and numerically determined wall temperatures are compared. It is demonstrated that the minimization technique is capable of separating the convective and radiative heat transfer components from impinging diffusion flames. (C) 2018 Elsevier Ltd. All rights reserved.