Flame spread over the horizontal surface of polymethyl methacrylate (PMMA) has been studied numerically by a coupled model of heat and mass transfer describing the feedback between gas-phase flame and solid fuel. Mathematical formulation has been defined by non-stationary two-dimensional elliptic equations applied both for gas phase and solid fuel. The computational procedure is based on modification of the OpenFOAM open-source code. Results of predictions have been compared with the data of comprehensive experimental investigation of the thermal and chemical structure of PMMA flame. Good agreement has been obtained for the detailed gas-phase and the solid fuel temperature and species concentrations profiles, as well as for the macroscopic parameters: the flame spread rate, the total mass regression rate and the length of the pyrolysis zone. Based on the analysis of thermal degradation of methylmethacrylate in inert surrounding, the concept of reduced molar weight for gaseous products of PMMA pyrolysis has been proposed, which provided better agreement for fuel distribution in the gas phase. © 2018 Elsevier Ltd

Amit Kumar

Department of Aerospace Engineering

Computer simulation

Fuels

Gas fuel analysis

Gases

Heat transfer

MASS TRANSFER

Open source software

Open systems

Polymethyl methacrylates

Pyrolysis

Computational procedures

COUPLED HEAT TRANSFER

Experimental investigations

FLAME SPREAD

Flame structure

Heat and mass transfer

MACROSCOPIC PARAMETERS

Mathematical formulation

Flame research

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

Applied Thermal Engineering

Experimental and numerical investigations of upward and downward flame spread over flat polymethyl methacrylate (PMMA) slabs are presented here. Experiments have been carried out using PMMA slabs of different thickness in the range of 1.6 mm–5.4 mm. Downward and upward flame spread processes have been recorded under atmospheric pressure and normal gravity conditions. Careful repeatable high resolution measurements of temperature and species fields have also been carried out, to fill the scarcity of such data in literature. These data illustrate the structure and spread rates of flames established over PMMA slabs. A simple numerical model, used widely to simulate flame spread over condensed surfaces, called Fire Dynamics Simulator (FDS), has been employed to numerically simulate the experimental cases. Infinite rate chemistry and sublimation based interface model have been used. FDS is economical when compared to CFD tools such as FLUENT and OpenFOAM. It provides predictive results when compared to theoretical models. Results from FDS have been validated against numerical and experimental data from literature, by comparing quantities such as mass loss rate, flame spread velocity and flame structure. FDS is seen to capture essential transport processes of a spreading diffusion flame. Even though discrepancies have been observed between the numerical and experimental results near the fuel surface, the overall comparison of the trends has been quite reasonable. Numerical model is capable of predicting the unsteady and steady features of downward spread as well as transient rapid upward flame spread, as observed in the experimental results. Detailed structure and flow field have been presented and discussed. © 2017 Elsevier Ltd

Investigation of the structure and spread rate of flames over PMMA slabs

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.

On the role of radiation and dimensionality in predicting flow opposed flame spread over thin fuels

A steady-state flame spread model has been used to study the effect of side-edge burning on flame spread over thin solid fuel strips of finite width. Simulations have been carried out for fuel strips with both inhibited (by metallic strips) and uninhibited side edges. The effect inhibition on both normal- and microgravity flame spread along with several intermediate gravity levels has been investigated. Such a study is important for understanding the physiochemical processes controlling the flame spread in low gravity where human experience is limited. Although simulations have shown an overall increase in spread rate for uninhibited cases for both normal- and microgravity flames, some effects such as flame spread variation with external imposed velocity and flame extinction limits show different behavior for microgravity and normal gravity flames. The heat and mass transport processes in the flame have been discussed in detail to explain the observed trends. Copyright © Taylor & Francis Group, LLC.

Combustion Science and Technology

A computational study on opposed flow flame spread over thin solid fuels with side-edge burning

Combustion and Flame

An experimental study of horizontal flame spread over PMMA surface in still air

Numerical study of horizontal flame spread over PMMA surface in still air

Journal	Data powered by TypesetApplied Thermal Engineering
Publisher	Data powered by TypesetElsevier Ltd
ISSN	13594311
Open Access	No