Experimental Investigation of the Spray Autoignition of n Heptane in a Jet in Cross Flow Configuration and Comparison to Single Droplet Ignition Simulations

Abstract

Modern combustion machines, which inject liquid hydrocarbon fuels into compressed air at high temperatures, aim for a high compression ratio to achieve high process efficiency. This means, there is only a limited residence time of the fuel in the combustion chamber before autoignition. The residence time however is important in terms of vaporizing and turbulent mixing of the fuel with air. A well stirred mixture with a lean overall equivalence ratio (near the lean flammability limit) will reduce the combustion temperature and in turn reduce the production of nitric oxides (NO, N2O and NO2) through the Zeldovich mechanism. This thesis details the results obtained under the ESA MAP project CPS III (Combustion Properties of Partially Premixed Spray Systems), where (amongst other topics) n-heptane sprays are observed under machine conditions and these experimental results are compared to numerical results of a simulation for single droplet ignition. The experiments were conducted in the hot wind tunnel Bremen (HWK), a newly operational Ludwieg tube type wind tunnel, which provides high temperature and high pressure flows with moderate flow rates. The comparison of the experimental data and simulations shows, that for the simple case of an unobstructed jet in cross flow (JICF) configuration, the induction times and their scatter can be predicted fairly well by assuming for the simulation, many super-positioned monodisperse equidistant sprays of different droplet diameter and their respective volume fractions assuming a Rosin Rammler distribution.