Flame spread over an array of fuel sheets of finite width size has been modeled and numerically investigated for opposed, low convective flows in microgravity. As opposed to the previous studies based on 2D models, steady flame spread rates were observed for all separation distances up to the separation distance of flame extinction. The flame spread rate increased with decrease in separation distance up to a point where it was maximum, further reduction in separation distance, reduced the flame spread rate. The flammability map as a function of separation distance was also obtained for different fuel widths. While the extinction map qualitatively matches with the flammability map obtained from the 2D model, the flame extinguished at higher oxygen levels with the decrease in fuel width due to radiation heat losses. © Springer India 2017.

Amit Kumar

Department of Aerospace Engineering

Mehul N.Kiran Kumar

IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

A 3D Numerical Study on Opposed Flow Flame-Spread over Thin Parallel Fuel Sheets of Finite Widths in Microgravity

Lecture Notes in Mechanical Engineering

In this work a numerical study has been carried out to gain physical insight into the phenomena of opposed flow flame spread over an array of thin solid fuel sheets in a microgravity environment. The two-dimensional (2D) simulations show that the flame spread rates for the multiple-fuel configuration are higher than those for the flame spreading over a single fuel sheet. This is due to reduced radiation losses from the flame and increased heat feedback to the solid fuel. The flame spread rate exhibits a non-monotonic variation with decrease in the interspace distance between the fuel sheets. Higher radiation heat feedback primarily as gas/flame radiation was found to be responsible for the increase in the flame spread rate with the reduction of the interspace distance. It was noted that as the interspace distance between the fuel sheets was reduced below a certain value, no steady solution could be obtained. However, at very small interspace distances, steady state spread rates were obtained. Here, due to oxygen starvation the flame spread rate decreased and eventually at some interspace distance the flame extinguished. With fuel emittance (equal to absorptance) reduced to '0' the flame spread rate was nearly independent of the interspace distance, except at very small distances where the flame spread rate dropped due to oxygen starvation. A flame extinction plot with the extinction oxygen level was constructed for the multiple-fuel configuration at various interspace distances. The default fuel with an emittance of 0.92 was found to be more flammable in the multiple-fuel configuration than in a single fuel sheet configuration. For a fuel emittance equal to zero, the extinction oxygen limit decreases for both the single and the multiple fuel sheet configurations. However, the two flammability curves cross over at a certain fuel separation distance. The multiple-fuel configurations become less flammable compared to the single fuel sheet configuration below a certain separation distance. © 2013 © 2013 Taylor & Francis.

Combustion Theory and Modelling

Opposed flow flame spread over an array of thin solid fuel sheets in a microgravity environment

In this work a flame-spread model is formulated in three dimensions to simulate opposed flow flame spread over thin solid fuels. The flame-spread model is coupled to a three-dimensional gas radiation model. The experiments [1] on downward spread and zero gravity quiescent spread over finite width thin fuel are simulated by flame-spread models in both two and three dimensions to assess the role of radiation and effect of dimensionality on the prediction of the flame-spread phenomena. It is observed that while radiation plays only a minor role in normal gravity downward spread, in zero gravity quiescent spread surface radiation loss holds the key to correct prediction of low oxygen flame spread rate and quenching limit. The present three-dimensional simulations show that even in zero gravity gas radiation affects flame spread rate only moderately (as much as 20% at 100% oxygen) as the heat feedback effect exceeds the radiation loss effect only moderately. However, the two-dimensional model with the gas radiation model badly over-predicts the zero gravity flame spread rate due to under estimation of gas radiation loss to the ambient surrounding. The two-dimensional model was also found to be inadequate for predicting the zero gravity flame attributes, like the flame length and the flame width, correctly. The need for a three-dimensional model was found to be indispensable for consistently describing the zero gravity flame-spread experiments [1] (including flame spread rate and flame size) especially at high oxygen levels (>30%). On the other hand it was observed that for the normal gravity downward flame spread for oxygen levels up to 60%, the two-dimensional model was sufficient to predict flame spread rate and flame size reasonably well. Gas radiation is seen to increase the three-dimensional effect especially at elevated oxygen levels (>30% for zero gravity and >60% for normal gravity flames). © 2012 Copyright Taylor and Francis Group, LLC.

Journal	Data powered by TypesetLecture Notes in Mechanical Engineering
Publisher	Data powered by TypesetSpringer Heidelberg
ISSN	21954356
Open Access	No