Separation of Radiative and Convective Wall Heat Fluxes Using Thermal Infrared Measurements Applied to Flame Impingement

Abstract

Flame impingement is critical for the processing and energy industries. The high heat transfer rates obtained with impinging flames are relevant in metal flame cutting, welding, and brazing; in fire research to understand the effects of flames on the structures of buildings; and in the design of high temperature combustion systems. Most of the studies on flame impingement are limited to surfaces perpendicular to the flame, and measurements are often performed using heat flux sensors (such as Schmidt-Boelter heat flux transducers) at discrete locations along the target surface. The use of in-situ probes provides high accuracy but heavily limits the spatial resolution of the measurement. Moreover, flame radiation effects are often neglected, due to the small contribution in non-luminous flames, and the entire heat flux to the target is assumed to be due to convection. Depending on the character of the flame and the impingement surface, local radiative heat transfer can be significant, and the contribution of radiation effects has not been fully quantified. This study presents a novel non-intrusive method with high spatial resolution to simultaneously determine the convective and radiative heat fluxes at a wall interacting with a flame or other high temperature environment. Two initial proof of concept experiments were conducted to evaluate the viability of the technique: one consisting of a flame impinging normal to a target and another with a flame parallel to the target surface. Application of the methodology to the former case yielded a stagnation convective heat flux in the order of 106kWm−2 that decreased radially away from the stagnation point. The radiation field for the direct impingement case accounted on average for 4.4% of the overall mean heat flux. The latter experiment exemplified a case with low convective heat fluxes, which was correctly predicted by the measurement. The radiative heat fluxes were consistent between the parallel and perpendicular cases.