Heat Transfer Studies of Flame Jet Impinging Over Wedge

Abstract

A transient inverse heat conduction problem concerning jet impingement heat transfer has been solved analytically in this work. Experimentally obtained transient temperature history at the non-impinging face, assumed to be the exposed surface in real practice, is the only input data. Towards developing and validate the experimental setup, a study on the effect of pressure on the volumetric flow rate of compressible gas flowing through a rotameter is undertaken. Both air rotameter (range 40 500 milliliters/ minute at STP) and methane rotameter (range 400 5000 milliliters/ minute at STP) are calibrated using a standard Soap Bubble Flow Meter (SBFM). The experimental observations towards change in the volumetric flow rate at STP with a change in gas pressure are in agreement with theoretical understanding. The predicted methane-air mixture flow rates are further verified using the blow-off flame stability concept, thus validating the experimental set up. This study aims to estimate two unknown parameters - heat transfer coefficient and adiabatic wall temperature - at the impinging face simultaneously. The Green's Function Approach to accommodate both the transient convective boundary conditions and radiation heat loss is used to derive the forward model, which is purely an analytical method. Levenberg Marquardt Algorithm, a fundamental approach to optimisation is used as a solution procedure to the inverse problem. An in-house computer code using MATLAB (version R2014a) is used for analysis. The method is applied for a case of a methane-air flame impinging on one face of a flat 3mm thick stainless steel plate. It keeps Reynolds number of the flame 1000, and dimensionless burner tip to impinging plate distance equals to 4 while maintaining the equivalence ratio one. Inclusion of both radiation and convection losses in the Green's function solution for the forward problem, enhances the accuracy in the forward model, thereby increasing the possibility of estimating the parameters with better accuracy. The results are found to be in good agreement with the literature. This methodology is independent of external fluid flow and heating conditions; and can be applied even to high-temperature applications. Heat transfer characteristics of impinging flame jet over a wedge-shaped structure similar to a deflector plate of a missile launch-pad are studied using the same analytical technique. The transient temperature of the non-impinging surface of the 4-mm-thick v test object made of stainless-steel is measured experimentally. Multiple experimental cases are considered in this work by varying methane-air gas mixture Reynolds number (800-1500), non-dimensional nozzle tip to test object distance (2-6), and wedge-angle (90o and 120o). The observations concerning heat transfer characteristics are discussed in detail. Uncertainty of estimation is evaluated using the Monte Carlo technique.