Experimental investigation of shear driven liquid films for film cooling applications in liquid rocket engines

Ryan P Miller, Purdue University

Abstract

Liquid film cooling is an important method for cooling the walls of a liquid rocket engine. Mass transfer via entrainment decreases the effectiveness of the film coolant and it is therefore important to estimate the amount of film coolant that establishes itself along the wall of a combustion chamber if the coolant flow rate is to be optimized. However, film entrainment research is limited in regards to film cooling applications in rockets. The correlations and theories that have been published are often limited in scope and have only been tested at momentum fluxes that are an order of magnitude less than those typically experienced in rockets. Experimental research has been conducted in a cold-flow test article at AFRL in order to investigate the effects of the gas stream momentum flux on the coolant flow rate that remains attached to the wall. Specifically, the objective of this study was to investigate the establishment of a shear driven liquid film introduced into a rectangular test section by a .38 mm X 25.4 mm slot, perpendicular to the stream-wise direction of the gas phase. A secondary objective of this thesis was to investigate the ability of several diagnostics to measure the interfacial shear stress, mean film thickness, and the film mass flow rate of a liquid water film shear-driven by nitrogen gas in cold flow conditions. Ultimately, a film removal slot was chosen to measure the film mass flow rate, differential pressure taps were chosen to indirectly deduce the interfacial shear stress by measuring a stream-wise pressure drop, and a laser focus displacement meter was chosen to measure the liquid film thickness. Lastly, a high-speed video camera was used to obtain qualitative visual data of the surface of the shear-driven liquid film. Experiments were conducted during June and November 2011 in order to accomplish the objectives of the thesis. The experimental apparatus consisted of a rectangular channel through which nitrogen gas flowed at momentum fluxes ranging from 2.5 to 110 kPa. A liquid water film was introduced into the test section at flow rates ranging from .0034 kg/s to .018 kg/s through a .38 X 25.4 mm slot that was perpendicular to the gas flow. In the experiments described herein, the film removal slot failed to completely remove the entire film, and, therefore, the results from the film removal slot were inaccurate. However, from the data that was obtained, it appears as though the liquid that becomes entrained into the gas phase increases with both increasing gas phase momentum flux and injected liquid flow rate. The amount of liquid entrained appears to vary from about 20 to 60 % of the injected liquid flow rate for the conditions studied. In addition, the correlations for entrainment proposed by several other authors did not agree well with the data that was obtained nor each other. The pressure drop data that was obtained was inaccurate due to problems with the flow, an electrical error, or leaks in pressure lines. Due to difficulties encountered with the differential pressure transducers, it is not recommended that the interfacial shear stress be estimated from pressure drop data. The thickness of the liquid film was shown to decrease with increasing gas phase momentum flux and increase with increasing liquid film flow rate. The film thicknesses measured ranged from 4 µm at the highest momentum fluxes and lowest liquid flow rates to 160 µm at the lowest gas phase momentum fluxes and highest liquid flow rates, with a 25 % relative uncertainty. The film thickness that were measured were of the same order of magnitude as estimated or measured for similar conditions by other researchers. A numerical model was used to determine an upper bound on the film thickness results, and the experimental results fell below and exhibited the same trends as the upper bound. It was also discovered that the turbulent surface of the liquid film and entrained liquid droplets at high momentum fluxes adversely affected the ability of the LFD to directly detect the surface of the liquid film. A new method for using the LFD to measure film thicknesses based on geometric optics is proposed. However, the results obtained by this new method still require further validation.

Degree

M.S.A.A.

Advisors

Anderson, Purdue University.

Subject Area

Aerospace engineering

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