IGNITION OF A DROPLET IN A CONVECTIVE ENVIRONMENT

Abstract

The study of single droplet combustion is a basic ground to understand the spray combustion. Ignition is one of the most important combustion phenomena. It is intimately connected with the initiation of combustion. Hence a true understanding of the physics of all the processes involved in ignition, ultimately leading to droplet combustion, is of great importance. The early induction period, thermal run-away and the subsequent combustion of the droplet is studied in a quiescent environment, in the light of the importance of droplet heating. Further to understand the effect of forced convection on the location and structure of the flame, the two-dimensional case of the droplet combustion in a low Reynolds number (Re = 0(1)) flow-field is also studied. In the case of quiescent environment, the coupled and unsteady liquid-gas phase problem of the droplet combustion in one dimension is solved numerically. The pressure is assumed to be uniform and constant. This assumption is utilized in a novel way to formulate and solve the continuity equation. The effects of some important initial parameters viz. the droplet temperature, ambient temperature and the ambient equivalence ratio are also considered. It is found that the ignition does not wait for the droplet to reach its wet-bulb temperature. It occurs as soon as it finds ii suitable conditions somewhere in the neighbourhood of the droplet. It is only after the occurrence of thermal run-away, that the chemical reactions take place in the region close to the droplet surface and during this short period, surface temperature rises appreciably. It is also found that even in the quiescent environment, the radial velocity produced by the evaporation of the droplet and by combustion leads to a local Reynolds number of order one, based on the local parameters and instantaneous droplet radius. Hence convection effects are as important as the diffusional effects. Realizing this fact about the importance of convection, the effect of free stream forced convection with Reynolds number of the same order (unity) superimposed on the radial convection is also studied. This makes the problem two-dimensional one and more complicated. It is found that the structure of the flame gets distorted because of this mild convection. However, the chemical reactions taking place in the inner zone near the droplet surface, following ignition, may lead towards establishing a more symmetrical surface temperature and heat flux distribution even for a convective case.