Combustion characteristics of diesel sprays from equivalent nozzles with sharp and rounded inlet geometries

Abstract

The ignition delay, flame structure, temperature, and soot distribution in a diesel spray injected at 80 MPa in a high-temperature (830 K) and high-pressure (6 MPa) quiescent air was studied for two nozzles, one with 0% hydrogrinding (HG) and another with 20% HG. HG in diesel nozzles is the process of forcing an abrasive fluid through the nozzles with sharp inlets; the abrasive fluid wears the sharp inlet edge of the spray holes until a prescribed flow rate is achieved. The percentage of HG used in this article is a measure of an increase in the volume flow rate after the HG process in a low-pressure flow test. The difference is substantial. For convenience, at some instances 0% HG is referred to as the sharp inlet and 20% HG as the rounded inlet. Spray impingement studies were made to evaluate the time-resolved spray momentum, nozzle discharge coefficient, and turbulence kinetic energy to characterize the nozzle internal flow effects on spray combustion. Equivalent nozzles were selected such that the momentum rates of the spray from both nozzles, as determined by the spray impingement, were the same. This was obtained by increasing the orifice diameter of the nozzle with 0% HG to compensate for the higher friction losses and lower discharge coefficient of the nozzle. The differences in discharge coefficient indicate that the flows inside the nozzles have different turbulence and cavitation levels. Inspite of the strong differences in internal flow, the sprays, which had the same momentum rate, behaved identically. In particular, the spray dispersion, penetration, ignition delay, combustion temperatures, flame volumes, soot concentration, and liftoff distances were almost the same for both sprays. Also, the use of noncircular injection orifices was shown not to change the combustion and emission performance of a diesel engine when the momentum of the fuel jets is the same. The work thus shows that diesel spray combustion is fully controlled by the spray momentum and that for realistic injection and combustion conditions the internal nozzle flow structure does not matter as long as it does not change the momentum.