Fig xA Temporal variations of the axial acoustic
Studies of the effect of ambient pressure or temperature on fuel spray combustion have received much attention in past years , , , , ,  and . However, most of the studies focused on the effects of high ambient air pressures (>0.1 MPa). For instance, Hiroyasu et al.  studied the effects of high ambient pressure (3.0 MPa) and jet velocity on the breakup of high-velocity water jets under conditions similar to those encountered in diesel engines. Similar tests using diesel-type nozzles with water as the test fluid were also conducted by Arai et al. . These studies reported that Verteporfin an increase in ambient pressure causes the breakup length to diminish and indicated that ambient pressures in the range of 0.1–3 MPa have a strong effect on breakup length and spray angle of the high speed jet. DeCorso  was among the first to investigate the effect of ambient air pressure on the spray characteristics of simplex swirl atomisers. That measured an increase in drop size when the ambient pressure was increased from 0.1 to 0.79 MPa and attributed genetic code increase in drop size to increased coalescence of the spray drops as the ambient pressure increased. Lee and Reitz  investigated the effects of gas density and velocity on the breakup mechanisms of liquid drops injected into a transverse high-velocity gas flow at four spray chamber pressures (1, 3.7, 6.4 and 9.2 atm). Over a pressure range of 0.1–0.5 MPa, a marked rise in the Sauter mean diameter (d32) was observed with an increase in ambient air pressure (Pa ) (d32∝Pa0.27). For a given nozzle, known as the pressure-swirl atomizers, many equations have been proposed for d32 via correlation of available experimental data where both ambient air and liquid properties were taken account ,  and .