Swanand V. Sardeshmukh

Prediction of Combustion Instability with Detailed Chemical Kinetics

Abstract : Combustion instability in an unstable single element rocket chamber using methane as the fuel is computationally studied. Effects of the kinetics mechanism are examined by comparing the results using a single step global mechanism and a detailed mechanism with 32 species and 177 reactions. Significant differences between the two predictions are identified including the amplitude of the unsteady pressure oscillations, and more importantly, the underlying mechanisms responsible for driving the combustion instabilities.

Implementation of an Enhanced Flux Formulation for Unsteady Navier-Stokes Solutions

Preconditioned time-marching CFD methods have become established as an accurate and efficient framework for all Mach numbers [1]. However, unsteady solution efficiency and accuracy suffer when the combination of low Mach numbers and high Strouhal numbers is encountered, especially in the context of high-fidelity turbulent and/or acoustics problems [1]. To counter this difficulty, an improved discrete formulation that is valid for all steady and unsteady regimes was proposed in an earlier article [2] for structured central-differenced algorithms. In this article, we further extend the modified approach to unstructured finite-volume formulations with implicit relaxation and test it for accurately resolving turbulence dynamics using a second-order discretization framework.

Computational Modeling of Supercritical and Transcritical Flows

We examine the use of a single-fluid model with the Peng Robinson equation of state to model supercritical and transcritical flows with combustion. Both non-reacting and reacting flows are considered to understand the modeling challenges. Several modifications of the equations of state and mixture rules are tested and shown to have varying strengths and weaknesses. No method works uniformly well for both non reacting and reacting flows, although the use of the Peng-Robinson model with Amagat’s mixture rule and modified compressibility factors appears to be robust and reasonably accurate for supercritical and transcritical combustion.

Kinetics Modeling of Hypergolic Propellants

Abstract : Multi-phase ignition of hypergolic propellants Mono-methyl Hydrazine (MMH) and Red Fuming Nitric Acid (RFNA) is studied numerically to understand fundamental processes such as gas phase ignition, vaporization and liquid phase chemistry for characterizing ignition. Such understanding will be critical for future design efforts targeting rapidly repeatable cyclic ignition of these propellants. Three test cases are considered: gas and liquid phase autoignition, an opposed jet diffusion flame and a liquid opposed jet diffusion flame. For the first case, three reduced chemical kinetics mechanisms are used to study autoignition behavior in the gas phase and with premixed liquids. In the second case, a laminar diffusion flame of the gas phase reactants under varying strain rates is studied. The third case investigates the interface behavior for liquid-liquid contact and its effect on gas phase ignition.

The use of OH* and CH* as heat release markers in combustion dynamics

The study examines chemiluminescence measurements and computations as a method of validation. Because chemiluminescent radicals are present in small concentrations and associated timescales are also small, a common assumption is that these are quasi-steady-state species and their transport can be ignored. This paper examines the above assumption using available data and simulation results, specifically aiming at a single element rocket combustor under strong pressure fluctuations. Variations in the kinetics rate constants are considered for assessing sensitivity of the results. Two additional aspects are explored: the ability of the excited species to represent the chemical heat release and the optical thickness of the medium. For the conditions of the study, the quasi-steady-state assumption in the case of OH* is found to be marginally insufficient while in the case of CH*, it is found to be acceptable. Modeled heat release is qualitatively better captured by CH* than OH* because of higher difference in the peak to asymptotic concentrations and lower ground state concentration.

Numerical Simulation of Combustion of Unlike Impinging Jets Near a Wall

Abstract : The combustion of gas-gas hypergolic propellants with MMH (Monomethylhydrazine) as fuel and RFNA (Red fuming nitric acid) as oxidizer are studied numerically for unlike impinging jets near an inclined wall using a detailed chemical reaction mechanism. The current study focuses on quantifying the effect of the inclined wall on the ignition characteristics: namely, contact time/location and ignition delay/location. Furthermore, the effect of wall is assessed with respect to mixing and flame spreading. The baseline three-dimensional simulation results compare two domains, with and without the inclined wall, under the same inlet flow conditions. These results show that the space between the wall surface and injector tips acts as a mixing zone with intensified vorticity and heat release rate. Two-dimensional results for various injection velocities are also presented and are compared with the three-dimensional results.