NUMERICAL MODELING OF CHEMISTRY AND HEAT TRANSFER IN TURBULENT COMPRESSIBLE FLOWS

Abstract

The reacting flows with heat transfer find application in many practical combustion systems, such as burners, flames, fires (at low Mach numbers), rocket nozzles, and re-entry vehicles (at high Mach numbers). There has been a continuous effort to understand the physics of low- to high-enthalpy reacting flows. A thorough understanding of the underlying physics may aid in designing optimal, safe, and efficient combustion equipment. The interplay between turbulence, heat transfer, and chemical kinetics makes simulations challenging, expensive, and complex. In high-fidelity combustion simulations, shock-capturing schemes, radiative modeling, and turbulence-chemistry interactions (TCI) play crucial roles. Numerical modeling of Radiation heat transfer is highly challenging as it relies on the complex nature of non-gray spectral absorption properties and the underlying integro-differential equation. In this thesis, I simulated low-speed and high-speed reacting flows to better understand the impact of various physical processes on the performance parameters of practical systems. The turbulence-chemistry interactions (TCI) plays a vital role in reactive flows. The interplay between turbulent mixing and chemical reactions becomes substantially more intense with increasing Mach and Reynolds numbers. These interactions lead to random species mixing and enhanced heat transfer. In this regard, for a low-speed reactive flow, I analyzed the influence of TCI on Turbulent Non-Premixed Flames (TNF): Sandia-D and DLR-A. To simulate the effect of TCI, the Eddy Dissipation Concept (EDC) and Partially Stirred Reactor (PaSR) methods were used in conjunction with the RANS approach, i.e., the k-epsilon model. For high-speed reactive flow, EDC with reduced chemistry using the in-situ adaptive tabulation (ISAT) method is applied to capture the TCI in a High Area Ratio (HAR) rocket nozzle. Two reaction mechanisms (hydrogen-oxygen and methane-oxygen) are considered to simulate the flow through the nozzle. The RANS and LES turbulence models are employed to resolve the turbulence scale in the HAR rocket nozzle. The simulation of high-speed flows with shock and other waves requires special attention to the treatment of convection terms in the governing equations. Shock waves cause discontinuities in the flowfield, with their strength strongly dependent on the Mach number. The shock and contact waves may lead to spurious oscillations and dissipation in numerical solutions. Evaluating numerical fluxes across a grid cell interface is vital in obtaining an accurate and efficient numerical solution. Some studies have compared the flux schemes for supersonic flows without considering chemical kinetics and radiation. The analysis presented in this thesis is one of the few to investigate the accuracy and capability of various flux schemes for hypersonic reacting flows with heat transfer. The flux schemes are compared for two cases: over a re-entry vehicle and inside a HAR rocket nozzle. The equilibrium composition serves as the starting point in many problems of combustion and heat transfer. In the case of the HAR rocket nozzle, the inlet boundary conditions for the mass fraction of species are specified based on their equilibrium values. We developed two novel stochastic algorithms using the Particle Swarm Optimization (PSO) and Monte Carlo (MC) techniques to estimate the equilibrium composition of the products. The results obtained from the PSO method are compared with the MC and the Chemical Equilibrium with Applications (CEA) codes. The results indicate that the PSO algorithm yields better results than the MC at a much lower computational cost. In high-temperature applications—as studied in this work—Radiative heat transfer is more significant than other modes of heat transfer. The radiative heat transfer can travel long distances and can affect the entire flow field through gas emission, absorption, and scattering. The heat transported through radiation can heat or cool the boundary layer flow and involve heat transfer through convection. In the case of re-entry vehicles, the shock layer's radiative cooling minimizes the shock stand-off distance and the convective heat load on the spacecraft's wall. The gases at high temperatures strongly emit radiation through atomic lines and molecular bands. The radiative properties of non-gray gases were modeled with theWSGG and k-distribution methods, such as the global full spectrum k-distribution (FSK) method. The Radiative Transfer Equation (RTE) is solved with the Discrete Ordinate Method (DOM) and the Spherical Harmonics Method (P1) to calculate the wall heat flux and radiative heat source term. The radiative heat transfer was found to be significant in the case of re-entry Martian spacecraft as well as in the HAR nozzle; however, for the case of low-speed methanol swirling flame, the effect was not significant.Reacting flows, turbulence-chemistry interactions, k-distribution, shocks, flux schemes.