Interactions Between Flame Topology and Turbulent Transport in High-Pressure Premixed Combustion

Flow, Turbulence and Combustion - Direct numerical simulations of a turbulent premixed stoichiometric methane-oxygen flame were conducted. The chosen combustion pressure was 20 bar, to...     

Cold-gas experiments to study the flow separation characteristics of a dual-bell nozzle during its transition modes

An experimental investigation has been carried out to study the effect of test environment on transition characteristics and the flow unsteadiness associated with the transition modes of a dual-bell nozzle. Cold-gas tests using gaseous nitrogen were carried out in (i) a horizontal test-rig with nozzle exhausting into atmospheric conditions and, (ii) a high altitude simulation chamber with nozzle operation under self-evacuation mode. Transient tests indicate that increasing δP ₀/δt (the rate of stagnation chamber pressure change) reduces the amplitude of pressure fluctuations of the separation shock at the wall inflection point. This is preferable from the viewpoint of lowering the possible risk of any structural failure during the transition mode. Sea-level tests show 15–17% decrease in the transition nozzle pressure ratio (NPR) during subsequent tests in a single run primarily due to frost formation in the nozzle extension up to the wall inflection location. Frost reduces the wall inflection angle and hence, the transition NPR. However, tests inside the altitude chamber show nearly constant NPR value during subsequent runs primarily due to decrease in back temperature with decrease in back pressure that prevents any frost formation.     

Surface flow studies of restricted shock separation in a thrust optimized parabolic nozzle

In this paper, surface flow features such as length of separation region and separation bubble are studied during restricted shock separation condition in a thrust optimized parabolic nozzle. It is found that peaks in the variation of these surface flow features are directly related to beginning of conditions for flow transition suggesting changes in exhaust flow features which are responsible for initiating flow transitions in a nozzle. Results are obtained from surface oil tests performed during the shut down phase for a range of nozzle pressure ratio during which restricted shock separation condition is prevalent.     

Cold gas dual-bell tests in high-altitude simulation chamber

An experimental investigation has been carried out to study dual-bell transition behavior in different set ups inside a high-altitude test facility. Cold gas tests were carried out under two different operating conditions namely (i) test nozzle operating in self-evacuation mode and, (ii) test nozzle operating with an additional ejector nozzle (pre-evacuated condition). Although forward transition nozzle pressure ratio does not show any change in its value irrespective of the type of test facility and test set up, the re-transition nozzle pressure ratio shows a significant increase (7–8%) in its value when tested in the high-altitude facility. The latter is caused due to plume blowback effect which dominates during shut down transients in such facilities. Driven by the high atmospheric pressure, the jet exhaust is pushed backwards into the altitude chamber causing the re-transition to occur earlier than that observed in sea-level tests. Further the reduced mass flow rates for nozzle operation in different test set ups in a high-altitude test facility also reduces the magnitude of side-load peaks during the dual-bell transitions.