Balachandar Varatharajan

PULSED DETONATION ENGINE PROCESSES: EXPERIMENTS AND SIMULATIONS

Computational and experimental investigations of a pulsed detonation engine (PDE) operating in a cycle using ethylene/air mixtures are reported. Simulations are performed for two geometry configurations, namely, an ideal tube PDE with a smooth wall fueled with premixed C2H4/O2 and a benchmark tube PDE with internal geometry and a valveless air supply fueled with C2H4. Performance estimates of fuel-specific impulse (Ispf) of an ideal tube PDE, obtained using a two-step reduced mechanism for a C2H4/O2 mixture, are in good agreement with existing test measurements from the literature. Realistic simulations of all processes of the PDE cycle (fill, deflagration-to-detonation transition (DDT), detonation propagation, blowdown, and purge) of a benchmark tube PDE yielded important insights into continuous cycle operation. Experimental measurements include DDT visualizations and dynamic pressure measurements. Comparisons of experimental and computational visualizations show good agreement in cycle process timescales. However, run-up distance is underpredicted, indicating a need to improve the flame propagation mechanism. The predicted decrease in the fuel-specific impulse (Ispf) for the benchmark tube when compared to the I spf of an ideal tube may be attributed to nonuniformities in the mixture composition, the pressure drop resulting from internal geometry, and backflow in the benchmark tube due to a compression wave propagating into the upstream geometry.

Detailed and Reduced Mechanisms of Jet a Combustion at High Temperatures

For the Computational Fluid Dynamics (CFD) modeling of combustion and detonation of Jet A aviation fuel it is necessary to use the simplest kinetic mechanism that accurately describes the essential relevant phenomena. A surrogate that demonstrated good agreement with the parent fuel in the detonation process was chosen. A detailed kinetic mechanism was elaborated using a multilevel approach. A reduced mechanism was derived from the detailed mechanism for use in the CFD simulation of real detonation processes in combustors.

The Effect of Fuel Density on Mixing Profiles in a DACRS Type Premixer: Experiments and Simulation

A combined experimental and computational study was conducted to investigate the effect of fuel density variations on mixing from a double annular counter-rotating swirl (DACRS) nozzle operated at atmospheric pressure under non-reacting conditions using either helium (He) or a mixture of He and CO2 as fuel simulants. A small probe traversed through the flow collecting gas samples that were sent to gas analyzers measuring the concentration profiles. The resulting measurements are then used to validate the computational fluid dynamics (CFD) model. A commercial CFD code (CFX 10) with a Reynolds averaged Navier-Stokes (RANS) formulation was used to simulate the experiment. Multiple turbulence closures, such as standard and realizable k-e and SSG Reynolds stress model were evaluated. Additionally, several geometrical considerations, such as modeling a 72° sector versus a full 360°, were tested. While at high fuel-to-air momentum flux ratios (J) the fuel simulant concentration profiles were outward-peaked, and at low J the profiles were center-peaked. An analysis of the experimental results clearly indicate the momentum flux ratio is the most influential parameter controlling mixing in a DACRS nozzle. The simulations produced quantitative agreement with the experimental measurements using the realizable k-e turbulence closure and only modeling a 72° sector of the nozzle. The complexity of the studied problem required a considerable refinement of the grid to produce an accurate and grid independent solution. The validated model may now be used to explore the design space for optimization of a nozzle for utilization in a syngas application.© 2007 ASME

Quantitative estimation of the static and dynamic parameters of jet a-air combustion and detonation from the first principles calculations.

The Jet A surrogate, consisted from 72.7 wt % decane + 9.1 wt % hexane + 18.2 wt % benzene was selected. Detailed mechanism, consisted of 417 elementary reversible reactions and 71 components, for this Jet A surrogate combustion was elaborated. The capability of the 3-component Jet A surrogate to predict the ignition delay times for the Jet A fuel over wide temperature and pressure ranges and to predict the pressure, temperature, and velocity of detonation was demonstrated.

Structure of flame nidus in an opposed partially premixed flow with heat losses

The attachment and detachment of flames to cold surfaces is examined using mathematical and numerical solution methods for a simplified flame attachment/detachment problem. The flame structure considered here resembles flames near propellant surfaces, where the surface consists of alternating condensed-phase sectors of oxidizer and fuel species, which gasify and burn in the gas. The mathematical arguments originate from a constant-density flow analysis, focusing on the flow dynamics near the location on the diffusion flame (DF) arc near the cold surface where the DF ends. This location is defined as the flame nidus. Relationships are derived for the propagation speed and quenching distance of the flame (1) when it is strongly influenced by the wall and (2) when it is lifted from the wall. A criterion is produced to determine when a DF nidus is or is not attached to the surface. Numerical solutions demonstrate the validity and limitations of the simplified theoretical models by delineating the regions of applicability of theories based on a postulated premixed form of flame nidus propagation.