Correlation of turbulent burning velocities of ethanol–air, measured in a fan-stirred bomb up to 1.2 MPa D. Bradley ⇑ , M. Lawes, M.S. Mansour School of Mechanical Engineering, University of Leeds, Leeds LS2 9JT, UK article info Article history: Received 11 January 2010 Received in revised form 13 April 2010 Accepted 3 August 2010 Keywords: Turbulent burning velocity Reaction progress variable Ethanol–air Explosion flames High pressure combustion Flame surfaces abstract The turbulent burning velocity is defined by the mass rate of burning and this also requires that the asso- ciated flame surface area should be defined. Previous measurements of the radial distribution of the mean reaction progress variable in turbulent explosion flames provide a basis for definitions of such surface areas for turbulent burning velocities. These inter-relationships. in general, are different from those for burner flames. Burning velocities are presented for a spherical flame surface, at which the mass of unburned gas inside it is equal to the mass of burned gas outside it. These can readily be transformed to burning velocities based on other surfaces. The measurements of the turbulent burning velocities presented are the mean from five different explosions, all under the same conditions. These cover a wide range of equivalence ratios, pressures and rms turbulent velocities for ethanol–air mixtures. Two techniques are employed, one based on mea- surements of high speed schlieren images, the other on pressure transducer measurements. There is good agreement between turbulent burning velocities measured by the two techniques. All the measurement are generalised in plots of burning velocity normalised by the effective unburned gas rms velocity as a function of the Karlovitz stretch factor for different strain rate Markstein numbers. For a given value of this stretch factor a decrease in Markstein number increases the normalised burning velocity. Compari- sons are made with the findings of other workers. Ó 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved. 1. Introduction The paper presents experimental values of the turbulent burn- ing velocity of gaseous ethanol–air mixtures measured during spherical explosions in a fan-stirred bomb, with rms turbulent velocities of up to 8 m/s. Equivalence ratios, /, range from 0.7 to 1.5, at pressures between 0.1 MPa and 1.2 MPa, at an initial tem- perature of 358 K. The values were obtained using high speed schlieren photography, pressure transducers and radial distribu- tions of mean reaction progress variable,  c, ranging from 0 to 1. Ethanol is an important bio-fuel and the present study is a com- panion to an earlier one on the measurement of its laminar burning velocities and Markstein numbers, up to 1.4 MPa [1]. The impor- tance of the study lies not only in revealing new data on the turbu- lent burning velocity of ethanol–air, but also in the opportunity to review the factors affecting the turbulent burning velocity, partic- ularly those that have made comparisons of u t determined by dif- ferent methods so difficult. Any definition of u t also requires that an associated surface area be characterised. Along with the density of the reactants, these yield the mass burning rate. It will be shown how surfaces defined by particular values of  c, are an aid to such characterisation. The derived generalised correlations for the tur- bulent burning velocity of ethanol–air have been found, through further studies, to apply also to other fuels and conditions, includ- ing higher pressures. Relevant turbulence parameters that must be measured are the rms turbulent velocity, u 0 and the turbulent integral length scale, L. Both should be uniform over the designated flame surface area. Cylindrical and slot nozzle burners, with upstream turbulence- generating grids and with u 0 measured upstream of the flame have been widely used for measuring u t . Disadvantages are that the boundary layer created at the burner wall can locally reduce the value of u 0 [2], which can also decay naturally downstream of the plane of its measurement. There can also be uncertainties in measurement at the tip of an elongated flame cone. The low-swirl burner creates a stabilising divergent, uniform, flow and generates reaction zones normal to the mean flow [3,4]. The centre line velocity at the leading edge of the flame brush can be measured by particle image velocimetry, PIV. There are several advantages in measuring u t during explosions in a fan-stirred bomb [5]. High values of u 0 can be achieved, with turbulence that is sufficiently uniform and isotropic in the central measurement region. The explosion flame geometry is reasonably 0010-2180/$ - see front matter Ó 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.combustflame.2010.08.001 ⇑ Corresponding author. Fax: +44 1132 424 611. E-mail address: d.bradley@leeds.ac.uk (D. Bradley). Combustion and Flame 158 (2011) 123–138 Contents lists available at ScienceDirect Combustion and Flame journal homepage: www.elsevier.com/locate/combustflame