Abstract
This paper reviews the current state of knowledge on the initiation of a flame in a spray through the action of a spark or through local deposition of heat, and the subsequent flame development, in uniform and non-uniform dispersions of droplets and in the presence of turbulent flow. These processes are of importance in various applications such as gas turbine ignition (relight) and safety related to flammable liquid mists. The review focuses on the initial kernel development, the evolution of a spherical or edge flame, and the ignition of the spray flame when viewed at the whole combustor scale. The factors that determine success or failure of the ignition process at the various phases of the overall burner ignition are discussed through experiments and Direct Numerical Simulations, while modelling efforts are also assessed. The fuel volatility, droplet size, overall fuel-to-air ratio, and the degree of pre-evaporation are the important factors that distinguish spray ignition from gaseous flame ignition, and the extra fluctuations introduced by the random droplet locations, and how this may affect modelling and flame evolution, are highlighted. The flame propagation mechanism in laminar and turbulent sprays is one of the key aspects determining overall ignition success. Suggestions for future research are discussed.
Highlights
The initiation and subsequent complete establishment of combustion in a spray due to an externally-imposed means, such as an electrical or laserinduced spark, a plasma jet, or a heated surface, is here denoted as forced ignition and is reviewed from the perspective of the various temporal and spatial scales and the stochasticity of the underlying phenomena
This paper aims to review the present state of our knowledge on these topics, mostly from the point of view of the fundamentals behind spark ignition processes in spray systems and focusing on the effects of the spray and the turbulence on the initiation and evolution of the flame
We review some fundamental findings revealed by experiment, DNS, and modelling on forced ignition in turbulent non-premixed systems with gaseous fuels that are needed for understanding spray ignition, focusing on the stochastic behaviour and the range of temporal and spatial scales involved
Summary
The initiation and subsequent complete establishment of combustion in a spray due to an externally-imposed means, such as an electrical or laserinduced spark, a plasma jet, or a heated surface, is here denoted as forced ignition and is reviewed from the perspective of the various temporal and spatial scales and the stochasticity of the underlying phenomena. This knowledge is of fundamental interest, and of practical importance for a range of applications. This paper aims to review the present state of our knowledge on these topics, mostly from the point of view of the fundamentals behind spark ignition processes in spray systems and focusing on the effects of the spray and the turbulence on the initiation and evolution of the flame
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