Atomization of impinging liquid jets in a supersonic crossflow

Abstract

THIS work describes a method to improve the performance of transverse liquid jet injectors in a high-speed gas flow. The measures of performance are the extent of the cross-stream penetration of the bulk of the liquid and the fineness of the droplets in the spray plume. It has been found that simple round injector ports often provide inadequate penetration and atomization for port sizes and injectant flow rates in the range of practical values—approximately 1 mm and q = pjV2j /p^ Vl, «5-15. A simple, passive type of injector is preferred for reasons of cost and reliability. We have studied the benefits to be gained by employing two smaller, inclined injectors so that the jets impinge just above the boundary surface rather than using a single perpendicular injector. Arrangements where the plane of the jets was aligned with and across the gas flow were both tested. Contents Approach The tests were conducted in the VPI 23 x 23 cm wind tunnel at MO, = 3.0with/?0 = 4.2atmand ro~300K. Water was used as the injectant for safety and cost reasons. The methods of observation were: 1) spark photomicrographs (10 ~8 s) to show instantaneous jet structure, 2) streak photographs (10 ~3 s) to give the time-averaged outer boundary of the plume for penetration measurements, and 3) diffractive scattering of a laser beam to give mean droplet size following the method of Refs. land 2. The injector was mounted in a 15.4x24.5 cm flat plate supported from the floor of the test section. After some preliminary testing, an injector with two 0.65 mm diam ports, inclined at 30 deg to the vertical and spaced to intersect 4.0 mm above the plate surface, was selected for intensive study. An injector with a single perpendicular port of 0.91 mm diam with the same total flow area as the impinging jet injector was used as a reference case. Results Typical spark pictures are shown in Fig. 1. These photographs show that when the injector is aligned with the flow, the plume formed after the impingement follows a vertical path up to a certain point, where it then disintegrates into clumps. This point corresponds to the location where the flow just behind the curved bow shock first becomes supersonic. For the case where the injector was oriented transverse to the freestream, the plume was not nearly as vertical as in the aligned case. The most important piece of data obtained from the streak pictures is the penetration of the liquid into the freestream. It was originally thought that in the process of angled injection and jet impingement, vertical momentum would be lost and penetration might be less than that of a circular injector. However, this was found not to be the case. Penetration measurements were taken at a station 20 mm downstream of the injector over a range of q for both the aligned injector and transverse arrangements. The nondimensional penetration h/dj was then plotted against the dynamic pressure ratio q and compared to the present results for a circular injector of equal total area and a data correlation for circular injectors from Ref. 3. This plot is shown as Fig. 2. It can be seen that the transversely oriented injector displays penetration almost identical to the circular injector; however, the aligned injector shows a significant increase in penetration, especially at low <?• The results of the droplet size measurements are of special interest, since a clear benefit will be derived from finer atomization of the spray. The optical arrangement used in this investigation yielded an average droplet size in an area the size of the laser beam (.071 cm2). The droplet size distribution was found at various stations in the plume. Measurements were made at x=x/dj locations of 10, 15, 25, 50, 90 from the injector. The y locations were chosen according to the height of the plume at each station. The results are presented graphically in Figs. 3 and 4 for q = 4 and 12, where mean droplet diameters are plotted as a function of position along the plume centerline. The x and y/h axes locate the space coordinates along the plume, and mean droplet diameters are plotted along the normal axis. The results for the single