Local Burning Rates and Heat Flux for Forced Flow Boundary-Layer Diffusion Flames

Abstract

A methodology based on the Reynolds analogy was developed earlier that allowed for the estimation of local mass burning rates and heat fluxes in free-convection laminar boundary-layer diffusion flames. In this study, the relationship was examined in a forced-convective environment using methanol as a liquid fuel. The gas-phase temperature profiles across the laminar boundary layer with a methanol diffusion flame established over it were measured with the freestream air flowing parallel to the condensed fuel surface. Local and averaged mass burning rates were measured along with shear stresses at the fuel surface. The fuel consumption rate and flame lengths were observed to increase monotonically with an increase in the freestream velocity. Although the initial study was taken in the laminar regime, further extensions of the technique could be applicable to turbulent boundary-layer combustion in propulsion-oriented research.

Experimental Methodology for Estimation of Local Heat Fluxes and Burning Rates in Steady Laminar Boundary Layer Diffusion Flames

“…Fit the non-dimensional temperature data at the fuel surface with an appropriate higher-order polynomial fit using a curve fit algorithm in Matlab or other dedicated software. 4 to 6 points near the surface were found to work well in previous studies [4][5][6] . 7.…”

“…The correlation of Collis and Williams was applied for heat losses from the sample [5][6]8 , (1) where Nu is the Nusselt number and Re = Ud w / v is the Reynolds number, which was obtained for 0.02 < Re < 44, with properties evaluated at the film temperature, Τ m , an average of the gas, Τ g, and thermocouple, Τ tc temperatures. Here, the Reynolds number Re is defined as indicated for the local gas flow velocity U and kinematic viscosity v. d w in Eq.…”

“…Estimation of local temperature gradients using micro thermocouples is a particularly useful technique for the estimation of local mass burning rates and heat fluxes in these flames [4][5][6] . An analysis of literature data shows the difficulty of determining local heat transfer, combustion and friction coefficients at the condensed fuel surface, which are important for understanding the physics and the underlying mechanisms that drive a particular fire and its spread 6.…”

“…Set the temperature mapping interval based on the steady burning time interval of liquid and solid fuels. A recommended step size near the surface is 0.25 mm [4][5][6] . 14.…”

“…Using a data acquisition program on a computer, acquire data and write it to a measurement file. Note, sampling rates of 100 [4][5] to 500 6 Hz were used in past experiments. 18.…”