Development and analysis of a Burning Rate Emulator (BRE) for study in microgravity

Abstract

The Burning Rate Emulator (BRE) is a gas-fueled burner that emulates real condensed fuel flames. This is accomplished by matching four fundamental properties: heat of gasification, heat of combustion, surface temperature, and smoke point. The aim of the current study is to establish immediate sustained BRE flames in a calm microgravity environment. This study presents 49 tests at NASA Glenn's 5.18-s Zero Gravity Research Facility for two burner diameters (25 mm and 50 mm). The burner sizes and test parameters are chosen to emulate small laminar pool fires. The experiments show that the flames are nearly hemispherical at the end of the 5-s experiment, with the flame height still increasing. The heat flux initially falls quickly and then becomes steadier. Steady-state theory correlates the end-of-drop experimental data for the flame heat flux, and therefore the fuel burning rate. The apparent lack of correlation of the burning rate for the larger burner is attributed to gas radiation.

The burner's perforated copper plate, which has two embedded heat flux thermopile sensors, is calibrated as a slug calorimeter. The calorimeter provides the average heat flux over the burner surface as a function of time. During the 5-s microgravity experiments, average heat fluxes measured with the calorimeter agree with the locally measured heat fluxes through a theoretical distribution function. The results show that the average calorimeter heat flux and the two local heat flux measurements are in harmony over a wide range of microgravity flame fluxes ranging from 5-20 kW/m^2, with the edge heat flux much higher.

The transient combustion model formulated in oblate ellipsoidal coordinates is developed to analyze the behavior of the microgravity BRE flames. The model is axially symmetric and considers the burning of gaseous fuel leaving the surface of a porous ellipsoidal burner in microgravity. A composite solution is generated as a product of an exact steady state solution and an asymptotic transient solution that becomes exact far from the burner. The transient combustion model predicts that quasi-steady microgravity BRE flames will require much longer than 5 seconds.