Numerical Model of Flame Spread over Solids in Microgravity: A Supplementary Tool for Designing a Space Experiment

Department of Mechanical and Aerospace EngineeringCase Western Reserve UniversityCleveland, Ohio 44106-7222, USAPICAST4ABSTRACT: The recently developed numerical model of concurrent-flow flame spread over thin solids has been used as a simulationtool to help the designs of a space experiment. The two-dimensional and three-dimensional, steady form of the compressibleNavier-Stokes equations with chemical reactions are solved. With the coupled multi-dimensional solver of the radiative heat transfer,the model is capable of answering a number of questions regarding the experiment concept and the hardware designs. In this paper, thecapabilities of the numerical model are demonstrated by providing the guidance for several experimental designing issues. The testmatrix and operating conditions of the experiment are estimated through the modeling results. The three-dimensional calculations aremade to simulate the flame-spreading experiment with realistic hardware configuration. The computed detailed flame structures providethe insight to the data collection. In addition, the heating load and the requirements of the product exhaust cleanup for the flow tunnel areestimated with the model. We anticipate that using this simulation tool will enable a more efficient and successful space experiment tobe conducted.INTRODUCTION: SPACE EXPERIMENT SIBAL THE NUMERICAL MODELFlame spread over solid fuels is a classic combustionphenomenon involving the interaction among fluid dynamics,heat transfer and chemical reaction for a complex two-phasenon-premixed flame. The recent modeling results [1,2] showedthat flame spread and extinction phenomena in low-speed flow(less than 20 cm/s) are fundamentally different from those inhigher-speed flow typically encountered on earth. Therefore, thestudies of flame spreading and flammability in a microgravityenvironment are of scientific interest and also essential for theimprovement of fire safety in spacecraft and space stations.In a microgravity experiment called SIBAL [3] (SolidInflammability Boundary At Low-speed), proposed to beconducted in the International Space Station, longermicrogravity duration will be available to determine theflammability of thin combusting solids and the steady flamestructure in low-speed, forced-concurrent flow. A novel devicethat facilitates the tedious process of finding the flammabilityboundary for a solid material has been developed and testedsuccessfully [4]. The SIBAL experiment will validate thetheoretical prediction, fill the void in experimental data andcontribute to the scientific understanding of the flame atlow-speed flows.However, the limitations of space and materials, the stringentrequirements of exhaust gases in the space station and the lack ofopportunity to do trial-and-error testing post many challenges forthe experimental design. Although not originally intended, therecently developed numerical model [2,5] for flame spreading inmicrogravity is being used as a supplementary tool for designingthe space experiment. In the following, the essence of the modelwill first be described and several examples of using the model toDescription of the modelThe simulation . tool includes two-dimensional andthree-dimensional codes, which solves the laminar, steady, fullNavier-Stokes equations for the conservation of mass,momentum, energy and species. The two-dimensional (2-D)model predicts the limiting situation and gives the qualitativetrends of the flame behaviors. The three-dimensional (3-D)model can simulate the real experiment becausethree-dimensional effects cannot be avoided due to the limitedavailable space of the experiment hardware in spacecraftfacilities. The solid is assumed to be a thermally thin solid sheetand the solid model consists of continuity and energy equationswhose solutions provide boundary conditions for the gas phase.The gas-phase reaction is represented by a one-step,second-order finite-rate Arrhenius kinetics and the solidpyrolysis is approximated by a one-step, zeroth-orderdecomposition obeying an Arrhenius law. The detailedmathematical formulations, thermal and transport properties canbe found in [2,5].The SIMPLER algorithm [6] is used for the fluid flow andcombustion equations and the gas-phase radiation is solved usingS-N discrete ordinates method [7,8], which is capable of treatingmulti-dlmensional radiative transfer. Radiation (gaseous and/orsurface) plays art important role onthemicrogravity flames andakey part in the model. It is responsible for the existence of thelow-speed quenching limit [91 and also a heat transfermechanism besides conduction/convection in combustionsystems. These models (two.diraensional, three-dimensionalcodes and radiation solver) can provide some guidance for theexperiment during the designing stages.help the experimental design will be presented.This is a prepdnt or reprint of a paper intended for presentation at aconference. Because changes may be made before formalpublication, this is made available with the understanding that it willnotbe cited or reproduced without the permission of the author.