Research

Subcritical to supercritical mixing

Density and density-gradient values are quantified for sub-to-supercritical jets based on a non-resonant PLIF method that allows accurate detection of the entire jet core. A novel method is applied for calculating the core lengths using the structures detected across the entire jet center plane. Jet spreading angles are evaluated and a model is used for the spreading angle dependence on chamber-to-injectant density ratio. A detailed comparison has been provided with existing theoretical models and experimental data for both core lengths and spreading angles. Single and binary species mixtures were used to study the injection and mixing characteristics of both subcritical and supercritical jet. Of particular interest, the injection of supercritical jets in subcritical environment has identified the re-assertion of surface tension and formation of drops downstream of the injection location.

Flameholding in supersonic reacting flows

Flameholding in supersonic flow depends on local conditions in the recirculation region, and on mass transfer into and out of this region. Large gradients in local gas composition and temperature exist in the recirculation region. Hence, stability parameter correlations developed for premixed flames cannot be used to determine blowout stability limits for non-premixed flames encountered in practical devices. In the present study, mixture samples were extracted at different locations in the recirculation region and the shear layer formed behind a rearward-facing step in supersonic flow, and analyzed by mass spectrometry to determine the species concentration distribution in the region. The point-wise mass spectrometer measurements were complemented by acetone planar laser-induced fluorescence (PLIF) measurements to get a planar distribution of fuel mole fraction in the recirculation region. Non-reacting flow tests and combustion experiments were performed by varying various fuel related parameters such as injection location, injection pressure and fuel type. Fuel injection upstream of the step was not effective in supplying enough fuel to the recirculation region and did not sustain the flame in combustion experiments. Fuel injection at the step base was effective in sustaining the flame. For base injection, the local fuel mole fraction in the recirculation region determined from experiments was an order of magnitude higher than the global fuel mole fraction based on total moles of air flowing through the test section and total fuel injected in the test section. This suggests substantial difference in flame stability curve for non-premixed conditions in the scramjet engine compared to premixed flow. For base injection, fuel remained in the recirculation region even at higher injection pressure. Due to slower diffusion rate, the heavier fuel had higher local mole fraction in the recirculation region compared to lighter fuel for a unit global fuel mole fraction injected in the test section. Hence fuel molecular weight will affect the non-premixed flame stability limits in scramjet engine; the heavier fuel will have better fuel-lean and worse fuel-rich stability limit compared to lighter fuel. This is in addition to the fact that a lighter fuel such as hydrogen has a much wider flame stability limit than a heavier fuel such as propane. The data obtained in the study can help develop a stability parameter for non-premixed flames and validate computational models.

Isolator/Combustion chamber interactions

Isolator and test section flow evolution in a supersonic reacting flow subject to transient operation has been studied experimentally at three conditions with isolator Mach number entrance of 1.6, 1.9 and 2.5. These conditions are representative of flight transient operation starting around Mach 4 and continuing to Mach 6 and Mach 7, which cover the range from ramjet start to transition to full scramjet operation. Along with flameholding analyses, discussed in the previous report, this study will provide direct insight into the key physiochemical mechanisms and design attributes that dictate supersonic combustor behavior; it can also be used to facilitate the design and optimization of an entire scramjet flowpath consisting of the inlet, isolator, combustor and nozzle. Heat release through combustion in the model scramjet was simulated using incrementally blocking the flow exit until upstream-interaction was induced in the combustion chamber and the isolator. The conditions included here exceed the thermal chocking limit. Imaging and wall pressure distributions indicate the stability of the supersonic flow and the transition from fully supersonic to dual, subsonic-supersonic, regime. An initial evaluation of the length of the shock train formed in the isolator under separated flow indicated that further improvement of the predicted tools is necessary.

Effects of Pylon-Aided Fuel Injection on Mixing in a Supersonic Flowfield

Analyses have shown that mixing can be enhanced using thin pylons that have only a negligible impact on pressure losses. In this study, helium and argon have been transversely injected into a Mach 1.6 airflow simulating a light and a heavy fuel injection behind a thin triangular pylon placed upstream, in the isolator. Penetration and mixing in the test section were observed at three cross-sections including the recirculation region and beyond with planar laser-induced fluorescence (PLIF). Results were compared to the no-pylon cases. In the near field, the presence of the pylon resulted in improving both penetration and spreading of the jet and, at the same time, in lowering the concentration gradients in the recirculation region, an indication of improved flameholding ability; however, in the far-field spreading is improved by other factors, notably by the large vortical structures induced by the presence of side walls.

Cavitation in thermosensitive fluids

Cavitation is studies in a closed-loop tunnel using a thermosensitive fluid that simulates cryogenics’ cavitation behavior. Unlike water, which has been studied exhaustively, cryogenic fluids undergo cavitation with significant thermal effect. Past attempts at analyzing this behavior in water have led to poor predictive capability due to the lack of data in the regime defined as thermosensitive cavitation. Fluids flowing near their thermodynamic critical point have a liquid-vapor density ratio that is orders of magnitude less than typical experimental fluids, so that the traditional equation-of-state and cavitation models do not apply. Thermal effects in cavitation have not been fully investigated due to experimental difficulties handling cryogenics. This work investigates the physical effects of thermosensitive cavitation in a model representative of a turbopump inducer in a modern rocket engine. Unsteady surface pressures and high speed imaging collected over the span of thermophysical regimes ranging from thermosensitive to isothermal cavitation offer both quantitative and qualitative insight into the physical process of thermal cavitation. Physical and thermodynamic effects are isolated to identify the source of cavity conditions, oscillations and growth/collapse behavior. Planar laser imaging offers an instantaneous look inside the vapor cavity and at the behavior of the boundary between the two-phase region and freestream liquid. Nondimensional parameters are explored, with cavitation numbers, Reynolds Numbers, coefficient of pressure and nondimensional temperature in a broad range. Results in the form of cavitation regime maps, Strouhal Number of cavity collapse, and cavity length offer a mechanistic analysis of the phenomenon. Linear stability analysis of the boundary is performed, as well as analysis of the thermal effects in the cavity and the oscillatory behavior of the cavity and reentrant jet.

High pressure combustion research

Combustion of hydrogen-air and hydrogen-oxygen mixtures under a wide range of equivalence ratios and pressures are studied in the high-pressure combustion chamber up to 60 atm. Conjugate, 3-D, wall-heat transfer and OH-PLIF at 281 nm are measured. A study of error sources evaluated the effect of 18 parameters on the uncertainty of the PLIF measurement with the local temperature having the larges effect, as high as 15%. Together, all the parameters may lead to as much as 22% uncertainty in the measurement of OH mole fraction.

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