Droplet velocity and diameter distributions in flash boiling liquid nitrogen jets by means of phase Doppler diagnostics

Abstract

Due to current and future environmental and safety issues in space propulsion, typical propellants for upper stage or satellite rocket engines such as the toxic hydrazine are going to be replaced by green propellants like the combination of liquid oxygen and hydrogen or methane. The injection of that kind of cryogenic fluids into the vacuum atmosphere of space leads to a superheated state, which results in a sudden and eruptive atomization due to flash boiling. For a detailed experimental investigation of superheated cryogenic fluids, the new cryogenic test bench M3.3 with a temperature controlled injection system was built at DLR Lampoldshausen. After a first test campaign with high-speed shadowgraphy of flash boiling liquid nitrogen sprays, a laser-based Phase Doppler system was setup to determine the spatial distributions of droplet velocities and diameters in highly superheated sprays. The spatial distributions revealed a core region with high mean velocities close to the injector orifice. With increasing distance from the injector orifice, the sprays develop a more and more monodisperse pattern. These distributions also showed that atomization due to flash boiling generates finer sprays with growing degrees of superheat. In certain spray regions, two droplet populations varying in their direction of motion, velocity and diameter due to possible recirculation zones were observed. The experimental data of flash boiling liquid nitrogen generated within this study provide a comprehensive data base for the validation of numerical models and further numerical investigations.

Difficulties in Defining the Degree of Superheat in Flash Boiling Liquid Nitrogen Sprays

“…In a consecutive study, the droplet velocity and diameter distributions of highly superheated flash boiling LN2 sprays with constant injection conditions were measured by means of PDA and preliminary results like possible recirculation zones or the existence of a second droplet population were shown [13]. A subsequent in-depth data analysis of this measurement campaign showed, that the droplet velocity and diameter distributions of flash boiling LN2 sprays have a sufficient degree of similarity with data of storable fluids from literature [14]. In a former study [15], the characteristic morphology like break-up patterns and spray angles of flash boiling LN2 sprays was determined by means of high-speed shadowgraphy in dependence on the injection parameters and derived dimensionless quantities.…”

“…Since the dominating parameters for flash boiling are the injection temperature T inj and the back pressure p ch , it is important for an experimental investigation to make them adjustable, to keep them constant during the injection period and to make them reproducible. This is why the new test bench M3.3 with a temperature-controlled injection system was built at DLR Lampoldshausen for a detailed experimental investigation of cryogenic flash boiling processes [12,13,14,15,16].…”

“…The test fluid nitrogen gets liquefied during the chill-down process of the test bench. Further details about the test bench M3.3 and its possible range of injection conditions can be found in references [12,13,14,15].…”

“…In this study, we are using the four LN2 sprays #1, #6, #9 and #14 which were generated by a single sharp-edged injector with a diameter of D inj = 1 mm and a length-to-diameter ratio of L /D = 2.9. For further injector details we refer to former works [12,13,14,15,16]. The injection conditions in terms of the injection temperature T inj , the injection pressure p inj , the chamber pressure p ch , the mass flowṁ, the degrees of superheat R p and ∆T * as well as the dimensionless numbers like the gaseous and liquid Weber numbers W e g and W e l , respectively, the Jacob number Ja, the Reynolds number Re and the Ohnesorge number Oh of the sprays #1, #6, #9 and #14 are summarized in Table 1.…”

Selected Results of the Collaborative Research Center “Droplet Dynamics under Extreme Ambient Conditions” SFB-TRR 75

“…It is then applied to a jet of flashing cryogenic nitrogen case that is investigated experimentally in TP-B4. Some flow "anomalies" have been observed in shadowgraph movies where dark regions appear to be stationary in the flow or even floating slightly upstream (Rees et al [9]). The simulations show that a recirculation zone forms after the Mach disk due to the interaction of regions of high speed at the side of the jet and of low speed after the shock front.…”