GEOMETRICAL EFFECTS ON DISCHARGE COEFFICIENTS FOR PLAIN-ORIFICE ATOMIZERS

Discharge coefficients were measured for 40 different plain-orifice atomizers. Orifice length/ diameter ratios were varied from 2 to 5 while maintaining the nominal diameter constant at 0.254 mm. Test fluids employed were water and a universal calibration fluid, Viscor 1487C, and were sprayed into air at normal atmospheric pressure by using injection pressures ranging from 3 to 109 MPa. Special attention was paid to the influence of the condition of the nozzle inlet edge on discharge coefficient. Tests were carried out on nominally sharp-edged orifices, rough-edged orifices as produced by an electro discharge machining (EDM) process, and orifices with rounded inlets. Results indicated that the effects of orifice length/diameter ratio and Reynolds number on discharge coefficient are small in comparison with the condition of the orifice inlet edge. Sharp-edged inlets were found to be the most sensitive to small perturbations of this edge. Increasing the orifice inlet radius raises the discharge coefficient and also causes the discharge coefficient to become more dependent on Reynolds number.