C. I. Morris

Plif imaging of hypersonic reactive flow around blunt bodies

Qualitative OH planar laser-induced fluorescence (PLIF) imaging experiments of supersonic reactive flows over a 40° wedge and a 19-mm-diameter blunted cylinder are reported. Hydrogen-, ethylene-, and methane-based mixtures have been used in this work. Three different free stream conditions. M ∞ =6.7, 5.2 and 4.2, were used to study the combustion modes around the wedge and blunted cylinder. Separate schlieren imaging results were also obtained for several of the test cases, providing complementary shock wave visualization. In several cases for the wedge, shock-induced combustion was observed a finite distance behind the nonreacting oblique shock wave. For the most sensitive hydrogen mixture, an oblique detonation occurred in which the shock wave and combustion front were fully coupled. In the experiments with the blunted cylinder, two different regimes of combustion were observed. When the velocity of the flow was larger than the CJ velocity of the mixture, the mode was that of an adiabatic shock followed by a smooth combustion front. For cases of flow velocity similar to the CJ velocity or less, regular oscillations were observed on the image and in the stagnation pressure history. These observations are qualitatively consistent with previous work.

Experimental Investigation of H2 Transverse Jet Combustion in Hypervelocity Flows

This paper describes on-going research at Stanford on combustion and mixing enhancement concepts in high total enthalpy supersonic flows. The experiments, performed in an expansion tube facility, are designed to study the near-field mixing and autoignition process of an underexpanded transverse hydrogen jet injected into flight-Mach 13 total enthalpy flow conditions (M=4.7, T=1300K, V=3300m/s, P=0.75psi). Simultaneous OH-PLIF and schlieren imaging are performed to obtain information on the location of the shock waves, the jet penetration, and the region of combustion. Schlieren images show the shock structure around the jet and the periodically formed coherent structures in the jet/freestream interface. Overlaid OH-PLIF and schlieren images indicate the flameholding capability of a hydrogen jet in air crossflow for different jet-to-freestream momentum flux ratios. Experiments studying the interaction of on an oblique shock wave with the jet are also reported. The results demonstrate that the jet plume is bent towards the wall along with the OH radicals present along the jet/freestream interface.

PLIF imaging and thermometry of NO/N2 shock layer flows in an expansion tube

Planar laser-induced fluorescence (PLIF) imaging and thermometry of quasi-steady hypersonic flows over a wedge and circular cylinder in an expansion tube facility are reported. The facility performance as inferred from the PLIF measurements are compared with that determined from shock speeds, static pressure, pitot pressure and infrared absorption measurements. The collisional quenching corrections required in these high-temperature shock layer flows are explored and direct comparisons are made between theory and experiment for the quenching of nitric oxide fluorescence.

THE IMPACT OF A CONVERGING-DIVERGING NOZZLE ON PDE PERFORMANCE AND ITS ASSOCIATED FLOWFIELD

Performance impact of a converging-diverging nozzle on a pulsed detonation engine (PDE) is studied experimentally. Thrust and specific impulse are determined from integrated pressure measurements. Burned gas velocity is obtained from optical measurements of seeded cesium atom. Comparison of measured quantities with prediction from finite-rate chemistry computational fluid dynamics (CFD) models indicate heat transfer plays a significant role at late times during blowdown.

A Pulsed Detonation Tube with a Converging-Diverging Nozzle Operating at Different Pressure Ratios

A pulsed detonation tube (PDT) is operated at initial pressures equal to and greater than 1 atm in order to evaluate the effect of these pressures on the relative gains achieved by a converging-diverging nozzle, while the ambient pressure is maintained at 1 atm. Local heat flux is measured and is determined to play a significant role in overall energy release during blowdown, especially when a converging -diverging nozzle in implemented. With the addition of the nozzle, a decrease in Isp is observed at low initial pressures (near 1 atm) , whereas the nozzle becomes beneficial at higher initial pressures (above 2 atm) .

Simultaneous PLIF and schlieren imaging of hypersonic reactive flows around blunted cylinders

Simultaneous OH PLIF and schlieren imaging experiments of hypersonic reactive flow fields around spherical-nosed and flat-faced cylinders, 19 and 25 mm in diameter, have been performed. Methaneand ethylene-based fuel-oxidizer mixtures were used at two different freestream conditions. Three different combustion modes were observed as flow velocity was varied relative to the Chapman-Jouget speed, consistent with previous work. In the unsteady modes, the frequency of the disturbances varied between 50 and 55 kHz for the regular regime, and 16 to 25 kHz for the large-disturbance regime. The period of oscillations was observed to vary inversely with the induction time of the mixture at the postshock condition. (Author)

THE EFFECT OF WALL HEAT LOSS ON THE FLOW FIELDS IN A PULSE- DETONATION WAVE ENGINE

The present study addresses the influence of convective heat losses on the flow field and performance of pulse detonation engines (PDE). We investigate the simplest PDE configuration of a straight detonation tube open at one end. The computations are carried out for stoichiometric hydrogen-oxygen, propane-oxygen and ethylene-oxygen mixtures at standard conditions using the method of characteristics for non-isentropic flows using a polytropic gas assumption. The thermodynamic foundation of this gas assumption is investigated. It is found that the model provides a very good approximation for an equilibrium flow. Previous heat flux measurements obtained immediately behind hydrogen-air and hydrogen-oxygen detonations are used to calibrate the assumed convective heat flux model. The calculations agree very well with experimental pressure and velocity profiles at various locations in the tube. The integrated impulse is also found in very good agreement with existing experimental results. The present study clarifies the previous observations of specific impulse deficits with increasing tube length. It is shown that the non-dimensional tube length L / D, where D is tube diameter, governs the amount of losses, the rate of pressure decay at the thrust wall, and hence the performance of PDE’s. The present study reveals that approximately 20% specific impulse losses are expected in typical tube geometries of L / D = 50, the losses varying quasi-linearly with L / D. Computations for ethylene-oxygen using the same heat loss model but including the exact thermo-chemical properties of the detonation products enabled validation of the polytropic gas calculations. Moreover, these finite rate calculations permitted comparisons with experimental temperature measurements, where excellent agreement is found. NOMENCLATURE

Numerical simulations and planar laser-induced fluorescence imaging results of hypersonic reactive flows

This paper shows comparisons between computational e uid dynamics (CFD) calculations and planar laserinduced e uorescence and schlieren measurements of inert and reactive hypersonic e ows around two-dimensional and axisymmetric bodies. In particular, both hydrogen ‐oxygen and methane ‐oxygen chemical reactions are considered for the shock-induced combustion in hypersonic e ows. The hydrogen ‐oxidation mechanism consists of an existing mechanism of 8 reacting species and 19 elementary reactions. The reduced model of the methane ‐ oxidation mechanism is newly derived from the GRI-Mech 1.2 optimized detailed chemical reaction mechanism, and consists of 14 species and 19 chemical reaction steps. Both chemical reaction mechanisms are combined with a point-implicit Euler CFD code. The OH species density distributions of the present numerical calculations and imaging experiments for both mixtures are found to be in qualitative agreement.

Experimental investigation of ram accelerator flow fields and combustion kinetics

Ram accelerator-related research at Stanford in the areas of hypersonic reactive flows and high-pressure combustion kinetics is presented. Research on reactive flows includes investigation of the combustion modes observed in hypersonic reactive flows over blunt cylinders and 2D bodies. In the experiments reported herein, simultaneous OH PLIF and schlieren imaging experiments of hypersonic reactive flow fields around spherical-nosed and flat-faced cylinders, 19 and 25 mm in diameter, have been performed. Stagnation pressure histories were recorded using a pressure transducer embedded in the cylinders. Methane-, ethylene-, and hydrogen-based fuel-oxidizer mixtures were used at different free stream conditions. Three different combustion modes were observed as flow velocity was varied relative to the Chapman-Jouget speed, consistent with previous work. These experiments represent the first time pressure disturbances in the unsteady combustion modes have been directly measured. Gas-phase combustion kinetics research involves the use of a high-pressure shock tube facility for ignition delay time measurements and detailed kinetics modeling for ram accelerator mixtures and conditions. Ignition time measurements are presented, and a detailed kinetics mechanism developed to model CH4/O2 ram accelerator ignition is reviewed.

PLIF imaging of the supersonic reactive flows around projectiles in an expansion tube

We report qualitative planar laser-induced fluorescence (PLIF) imaging experiments of supersonic reactive flows over a wedge test-body in an expansion tube. Both hydrogen-based and ethylene-based reactive mixtures were investigated in this work. Ongoing facility characterization efforts investigated three different freestream conditions, Moo = 6.7, 5.2, and 4.2. These conditions were used in an initial investigation of combustion modes around a 40°half-angle wedge. In several cases, shock-induced combustion was observed a finite distance behind the non-reacting oblique shock wave. However, in the case of the most sensitive mixture, an oblique detonation wave was observed in which the shock wave and combustion front were fully coupled.