The structure of two experimental heavy fuel oil flames is examined by using a mathematical model of spray combustion based on an Eulerian description of the gas phase and a Lagrangian description of the drops. The flame of drop size 0 2 6 0 Mm has been predicted with much better agreement than that of big drops ( 0 1 0 0 0 Mm). The calculations have shown that the initial size distribution of a spray has a major effect on the spray flame structure. The need for a reliable formulae describing (a) the physics of multicomponent evaporation/pyrolysis of the heavy oils (b) the combustion of coke cenospheres and (c) the blow-off criteria of individually burning drops, was also demonstrated. Introduction Spray combustion of residual fuels, heavy oils and alternative fuels has become of increasing importance in the last decade. Although such flames are considered mainly for large combustion chambers, recently interest in the use of such fuels in small applications has grown. The spray combustion mechanism of lighter distillate fuels, such as kerosene, is relatively better understood than the mechanism of spray combustion of heavy fuel oil. The recent experiments by Yule and Bolado /1 / demonstrated the fundamental role of the initial parameters (at the atomiser nozzle) on spray flame structure. This theoretical study is related to two of their flames marked elsewhere / l , 2/ as flames 2a and 2c. In both flames the total (air + drops) momentum and the fuel flow rates were kept constant whereas the initial size distribution of the drops were different (see Table 1). Although in the experiments a heavy fuel oil was used, both flames were computed by a model relevant to spray combustion of a distillate fuel. Despite the strong simplification of the physics of combustion of a heavy fuel oil, the application of the computer model to both flames gives a considerable insight into some of the phenomena occurring in these experiments. Moreover, it was considered that it would be valuable to know, how far one can go with application of a light fuel combustion model to spray modelling of a heavy fuel oil. Since both the physics of combustion of a heavy fuel oil and its mathematical description have not been fully established yet, such a simplification is often accepted without any assessment of the error it may carry. The detailed description of both the experimental rig and the internal mixing twin fluid air blast atomiser may be found in the literature /1, 2/. The following were measured within the flames: (a) The radial gas temperature distribution at four stations above the atomiser. (b) The wall temperature distribution along the combustion rig. (c) Total emissivity of the flames at four stations above the atomiser. (d) The spray thrust and initial size distribution of the drops. (e) The size distribution of drops at the station 0.1 m above the atomiser. The Theoretical Formulation The calculation method solves the Eulerian conservation equations for the gas-phase and Lagrangian equations of the drops. The Eulerian gas phase * Presently with International Flame Research Foundation, 1970 CA Ijmuiden, The Netherlands