Analysis of data to develop models for spray combustion

Abstract

[Abstract]: The design of modern petrol engines has placed an emphasis on lean combustion in order to increase efficiency, reduce operating noise and reduce pollutants. However, the closer the engine operates to the lean combustion limit the higher the possibility of engine misfires occurring. Misfires occur when there is not sufficient droplet density around the spark to allow the evaporation of the droplets and hence the release of fuel to the system. The advent of injection systems has enabled engineers to control the majority of the parameters of spray combustion, such as droplet size, droplet density and spray pattern. Therefore, the ability to model spray combustion would have wide ranging implications on the automotive industry. One model which lends itself to the modelling of spray combustion is the Conditional Moment Closure (CMC) model. However, the behaviour of the terms of the CMC model, namely the conditional scalar dissipation, conditional source term and the mixture fraction probability density function (pdf), is not understood well for this case. A number of Direct Numerical Simulations (DNS) have been performed on different cases of combustion where fuel droplets are present in cold air and a spark is used to evaporate the droplets and initiate a flame kernel. Data was collected and models were developed for the three key terms of the CMC model. Validation of the first order CMC model was performed by attempting to recreate the behaviour of the DNS data. The performance of the first order CMC model was found to be poor due to the inability of the model to account for fluctuations about the conditional mean of the quantities. Another conditioning variable, or second-order conditional modelling, may be needed in order for the CMC model to adequately capture the behaviour of spark assisted, spray combustion.