LN2 spray droplet size measurement via ensemble diffraction technique

Abstract

The size of subcooled liquified nitrogen droplets are measured with a 5 mW He-Ne laser as a function of pressure difference (delta P) across flat spray and full cone pressure atomizing nozzles. For delta P's of 3 to 30 psid, the spray sauter mean diameter (SMD) ranged between 250 to 50 microns. The pressure range tested is representative of those expected during cryogenic fluid transfer operations in space. The droplet sizes from the flat spray nozzles were greater than those from the full cone nozzle. A power function of the form, SMD varies as delta P(exp a), describes the spray SMD as a function of the delta P very well. The values of a were -0.36 for the flat spray and -0.87 for the full cone. The reduced dependence of the flat spray SMD on the delta P was probably because of: (1) the absence of a swirler that generates turbulence within the nozzle to enhance atomization, and (2) a possible increase in shearing stress resulting from the delayed atomization due to the absence of turbulence. The nitrogen quality, up to 1.5 percent is based on isenthalpic expansion, did not have a distinct and measurable effect on the spray SMD. Both bimodal and monomodal droplet size population distributions were measured. In the bimodal distribution, the frequency of the first mode was much greater than the frequency of the second mode. Also, the frequency of the second mode was low enough such that a monomodal approximation probably would give reasonable results.