The role of porous media in homogenization of diesel fuel spray combustion

Combustion homogenization of a fuel spray utilizing porous media (PM) has only recently been proposed and can be regarded as an untouched area of research. In this PhD program, the fundamental role of porous ceramic media on homogenization process of high pressure diesel fuel spray will be evaluated in diesel engine-like condition. In the study of high pressure diesel fuel spray combustion in presence of porous media, two main questions can be posed; what is the effect of porous media on fuel and air mixing process? How can porous media homogenize the diesel fuel spray combustion process? To answer these questions, cold (non-evaporating spray) and hot (combusting spray) experiments were conducted in a constant-volume chamber. Through the application of ultra-high speed imaging (UHSI) technique for cold flow experiment and developed image processing techniques, new insight into the transient nature of the fuel spray in different phases of spray interaction with a porous medium was gained. Besides developing a code for investigating the macroscopic characteristic parameters of the spray, a code was also developed to process the images for evaluating the probability density function (PDF) of the light intensity. Through PDF analysis, a discernible improvement of multijet dispersion and fuel atomization at higher chamber pressures in the last phase of spray interaction with the porous medium was achieved, irrespective of the injection pressure. This was interpreted as having a higher chance of uniform fuel distribution and a well homogenized mixture preparation for the combustion process, the first requirement for combustion homogenization, in both conventional as well as new diesel engine combustion concepts such as low temperature combustion (LTC). The combustion experiments were then conducted to shed light on the combustion characteristics of diesel spray in the presence of a porous medium with specific emphasis on combustion visualization and heat release analysis of the burning spray. Combustion imaging results showed the validity of the previous hypothesis gained from PDF analysis of cold flow results by showing the rapid development of homogenous combustion in the last phase of fuel interaction with the PM. Similar to the results of LTC combustion with retarded injection timing, a longer ignition delay, with a flatter and wider heat release rate pattern, compared to conventional diesel combustion, was observed in this study indicating the potential of PM assisted combustion in simultaneous reductions in soot and NOx emissions. Although more extensive experimentation will be required to determine performance over a wider range of conditions, these results suggest that porous media could be used to promote a distributed fuel air mixture resulting in slower, but more homogeneous heat release within the cylinder of an engine. The thesis provides detailed information for the experimental methodology developed in the Laboratory for Turbulence Research in Aerospace and Combustion (LTRAC) at Monash University