Anthony J. Dean

High temperature shock tube study of reactions of CH and C-atoms with N2

The reactions of CH and C-atoms with N2, which are believed to be initial steps in the prompt-NO mechanism, were studied at high temperature behind reflected shocks. CH was formed from the pyrolysis of highly dilute mixtures of CH4 or C2H6 ( C ( 3 P ) + N 2 → C N + N , (1) leading to k1=6.3×1013 exp(−23160 K/T) (±30%) cm3 mol−1 s−1 over the temperature range 2660 to 4660 K and pressure range 0.5 to 1 atm. In order to determine the rate coefficient of C H ( X 2 I I ) + N 2 → H C N + N , (2) a perturbation technique was employed. In this technique, the CH profile resulting from pyrolysis of CH4 or C2H6 dilute in argon was perturbed by the addition of N2. A detailed analysis of the CH profiles led to a rate coefficient, k2=4.4×1012 exp(−11060 K/T)(±50%) cm3 mol−1 s−1 over the temperature range 2500 to 3800 K and pressure range 0.6 to 1 atm. N-atom measurements provided an independent verification of k2 and the products of reaction 2.

Development of a laser absorption diagnostic for shock tube studies of CH

Abstract A sensitive and quantitative diagnostic for CH based on cw laser absorption has been developed for CH kinetics experiments in a shock tube. A number of transitions in the A2 δ(ν = 0) ← X2Π(ν = 0) band and C2Σ+(ν = 0)← X2Π(ν = 0) band which are promising for accurate CH detection were considered, leading to the selection of 431.1311 nm (vac.) as the wavelength of maximum absorption over a range of temperature from 1500 to 4000 K. This corresponds to the coincidental overlap of the Q1d(7) and Q2c(7) transitions. Reflected shock experiments involving the pyrolysis of highly dilute mixtures of ethane or methane in argon were used to verify details of the spectroscopic modelling and to investigate the sensitivity of the method for monitoring CH. The detection limit for single-pass absorption (14.3 cm) was found to be below 0.2 ppm at 2000 K for post-shock pressures of 1 atm. This new diagnostic is well suited for studies of elementary reactions involving CH.

Wave Attenuation and Interactions in a Pulsed Detonation Combustor-Turbine Hybrid System

A large-scale, multi-tube pulsed detonation combustor with eight tubes arranged in a can-annular configuration was integrated with a single-stage axial turbine nominally rated for 8 lbm/s, 25000 RPM and 1000 hp. This PDC-turbine system was operated using ethyleneair mixtures with each tube firing at 20 Hz. High frequency pressure transducers were installed throughout the flow path to investigate the wave interactions and attenuation across the turbine. The multi-tube PDC was operated at a range of conditions with three firing patterns: a single tube firing, all tubes simultaneous and all tubes sequential. Analysis of these data reveal wave interactions in the transition plenum between the PDC exit and turbine inlet plane can affect the optimum operation of the multi-tube PDC. In addition, there is over 20 dB attenuation of the peak pressure pulse and 10 dB attenuation of the broadband acoustic noise through the single-stage, axial flow turbine.

Wave Interactions in a Multi-tube Pulsed Detonation Combustor-Turbine Hybrid System

A large-scale, multi-tube pulsed detonation combustor (PDC) with eight tubes arranged in a can-annular configuration was integrated with a single-stage axial turbine nominally rated for 10 lbm/s, 25000 RPM and 1000 hp. This PDC-turbine system was operated using ethylene-air mixtures with each tube firing at 20 Hz. High frequency pressure transducers were installed throughout the flow path to investigate the wave interactions at a range of conditions with three firing patterns: a single tube firing, all tubes firing simultaneously and all tubes firing sequentially. Analysis of these data revealed that wave interactions in the transition plenum between the PDC exit and turbine inlet plane can affect the optimum operation of the multi-tube PDC.

Performance on a Pulse Detonation Engine Under Subsonic and Supersonic Flight Conditions

In a Pulse Detonation Engine (PDE), an exit nozzle enhances thrust generation, maintains operating pressure and also controls operating frequency. The performance of a PDE, under subsonic and supersonic flight conditions, was assessed using two parametric studies. The first of these parametric studies employs a 2D CFD model and quantifies the relative impact of four different but separate exit nozzle shapes namely a Converging and Diverging (CD) nozzle, a diverging nozzle, a straight nozzle and a converging nozzle where as a second parametric model uses a quasi-1D model for predicting the effect of a CD nozzle geometry parameters on the systems-level performance of a PDE. The 2D CFD performance model predictions for the subsonic flight (Mach number (Minf) = 0.8 and altitude (hinf) = 20 kft), using single pulse simulations of the detonation and the blowdown processes, show that the fuel-specific impulse Ispf, is higher for the case of a diverging nozzle when compared to all other nozzles. These results are in agreement with the results reported earlier [14,20] in the literature. The 2D CFD performance model predictions of a PDE under supersonic flight conditions (Minf = 3.0 and hinf = 30 kft), obtained using single pulse simulations of the detonation and the blowdown processes, show that the Ispf, total impulse and thrust generated are higher for the case of the diverging nozzle and the CD nozzle, when compared to the performance metrics of the straight nozzle or the diverging nozzle. A second parametric study employs the Q1D limit cycle model, and the exit CD nozzle contraction ratio (Rnc) is varied from 1.6 to 6.4. For the subsonic flight conditions, Ispf increases with increasing values of the exit nozzle contraction ratio and attains a constant value of 1600 for an exit nozzle contraction ratio of 3.0. For the supersonic flight conditions, Ispf increases with increasing values of the exit nozzle contraction ratio and attains a constant value of 1950 for Rnc > 5.0. The design choice of the optimum value for Rnc is predicted to be a compromise between optimizing the performance metric Ispf and generating the required thrust.

A shock tube study of reactions of NCO with O and NO using NCO laser absorption

The absorption coefficient of NCO at 440.4790 nm (vac) was determined using a cw ring dye laser and shock-heated HNCO/N2O/Ar mixtures at temperatures and pressures ranging from 2180 K to 3070 K and 0.9 to 1.1 atmospheres, respectively. The source of NCO in these experiments was the reaction O+HNCO→OH+NCO (2a) where the O-atoms were produced by the thermal dissociation of N2O. This determination of the NCO absorption coefficient allows quantitative measurements in shock tube studies of elementary reactions involving the NCO radical. This new NCO diagnostic was used to study the reactions of NCO with O and NO in shock-heated HNCO/N2O/Ar and HNCO/NO/Ar mixtures, respectively. The second-order rate coefficient of the reaction NCO+O→products (3) was determined to be: k3=4.7×1013 cm3 mole−1 s−1 (f=0.74, F=1.30) T=2180–3070 K, where f and F define the lower and upper uncertainty limits, respectively. The second-order rate coefficient of the reaction NCO+NO→products (8) was determined over the temperature range of 2380 K to 2660 K. These results were combined with the lower-temperature results for k8 from three previous studies, resulting in the expression: k8=1.4×1018T−1.73exp(−384/T, K) cm3 mole−1 s−1 T=294–2660 K.

C‐atom ARAS diagnostic for shock tube kinetics studies

Carbon atom detection using atomic resonance absorption spectroscopy (ARAS) has been developed for shock‐tube kinetics studies. C‐atoms (3P) are detected at 156.1 nm using a fast time response (<4 μs) ARAS system. Calibration is based on pyrolysis of methane highly diluted in argon over the temperature range 3000 K to 4800 K. This diagnostic has been applied to a high temperature study of the reaction C+N2→CN+N.

IGNITION AND DETONATION INITIATION OF MOVING HYDROGEN-AIR MIXTURES AT ELEVATED TEMPERATURE AND PRESSURE

Experimental and computational studies of detonation initiation, using Deflagration-to-Detonation Tran sition (DDT) processes, were performed to investigate the effect of varying inlet parameters at elevated temp eratures and pressures on the run-up to detonation in hydrogen-air mixtures for a repeating detonation rig. It is found that the gasdynamic and chemical processes which effect the initial flame acceleration are the rate-li miting processes in determining the time scale of run-up t o detonation. A parametric study was performed in which the independent parameters of temperature, T (290 ‐ 615 K), pressure, P (1.0 ‐ 4.0 atm) and inlet fill velocity, Vb (10 ‐ 40 m/s) were systematically varied, and thei r effect on location, L DDT , and time of detonation, t DDT , initiation was quantified. A Pareto of effects in this parametric study shows variation in fill veloc ity and rig pressure have the largest effect on t DDT , while it shows variation in fill velocity and rig pressure followed by initial gas temperature have the largest effect on L DDT . Dimensionalized best-fit correlations were obtai ned from the test measurements for t DDT and L DDT as functions of P, T, and V. A nondimensional be st-fit correlations was obtained from the test measurements for t DDT as a function of key nondimensionalized independent variables, namely density expansion ratio α, a nondimensional length scale l/l F (where l is the integral length scale and l F is the laminar flame thickness) and a nondimensional velocity scale M (flow Mach number in the reactants) which govern the flow and chemical processes that occur during the DDT process.

3.39 µm Laser Absorption Sensor for Ethylene and Propane Measurements in a Pulse Detonation Engine

A fiber-coupled mid-infrared (mid-IR) laser absorption sensor was designed to monitor ethylene and propane concentration in a multiple-cycle pulse detonation engine (PDE). The engine was operated at repetition rates ranging from 5 to 20 Hz with near-stoichiometric fuel/air mixtures. A 3.39 µm helium neon laser was fiber-coupled to the engine, enabling continuous, time-resolved measurements of fuel concentration. Design criteria are discussed to overcome sensor noise from optical emission, spark ignition, and the harsh mechanical vibrations of the fired engine. The sensor provides quantitative measurement of the stoichiometry to compare to PDE simulations and allows optimization of fuel valve timing and fill pressure for improved engine performance. Pulse-to-pulse interactions during fired engine tests can perturb the fuel loading compared to unfired tests, illustrating the need for in situ fuel monitoring for PDE development and testing.

Validation of a Short Kinetic Mechanism for Jet A-Air Detonations

Progress on comprehensive validation of the short chemical kinetic mechanism for the JetA/air mixtures under conditions, typical of Pulsed Detonation Engines (PDE) operating conditions, is reported. The reduced (13 reactions, 15 species) kinetic mechanism (under development in Kurchatov Institute for the high fidelity simulations of the PDEs) is subjected to a specially designed, multi-step validation procedure. The results of the zerodimensional thermodynamic- and kinetic-based simulations are briefly described. This report discusses the procedures and the results of the one-dimensional reactive CFD validation of the reduced kinetic mechanism. For the 1-dimensional validation of the mechanism (as well as any other kinetic mechanism for any fuel), implemented in the computational fluid dynamics software (CFD software), the test data of detonation initiation behind reflected shock waves are proposed as proper detonation initiation validation measurements. The predictions show acceptable agreement with the available experimental measurements on detonation initiation in Jet-A/air mixtures. Additional investigation of the mechanisms of detonation formation behind reflected shock wave in 1-dimensional simulations allows to explain several experimental observations, which could not be explained before.