Recent advances in understanding of flammability characteristics of hydrogen

Transverse combustion instabilities: Acoustic, fluid mechanic, and flame processes

This paper provides an overview of recent advances in the theoretical modeling and numerical simulation of cryogenic fluid injection and mixing in transcritical and supercritical environments. The basis of the analysis is a general theoretical and numerical framework that accommodates full conservation laws and real-fluid thermodynamic and transport phenomena. All of the thermophysical properties are determined directly from fundamental thermodynamics theories, along with the use of corresponding-state principles. Turbulence closure is achieved using a large-eddy-simulation technique, in which large-scale motions are calculated explicitly and the effects of unresolved small-scale turbulence are modeled either analytically or empirically. The analysis has been applied to study: (1) fluid jet dynamics, (2) swirl injection of liquid oxygen through a simplex swirl injector, and (3) shear co-axial injection and mixing of liquid oxygen and methane. Various effects, including density stratification, she...

Cryogenic fluid jets and mixing layers in transcritical and supercritical environments

Results from large eddy simulations (LES) and direct numerical simulations (DNS) of a two-dimensional, spatially developing, compressible planar free jet undergoing an idealized, exothermic, chemical reaction of the type F+rOx→(1+r)P are presented in order to assess several subgrid-scale (SGS) combustion models. Both a priori and a posteriori assessments are conducted. The SGS turbulence model used is the dynamic Smagorinsky model (DSM). Two classes of SGS combustion models are employed in this study. These include the conserved scalar approach and the direct closure approach. Specifically, the SGS combustion models involve several forms of direct filtered reaction rate closures, including a scale similarity filtered reaction rate model (SSFRRM), and a mixing controlled strained laminar flamelet model (SLFM) in the form of thermochemical state relationships, obtained from the DNS, and two assumed forms for the subgrid mixture fraction filtered density function (FDF). In general, LES results are in reasonable agreement with DNS results and highlight the performance of the various SGS combustion models. In particular, in the context of the present study, it is found that: (1) the SLFM cases overpredict product formation due to their inability to capture finite-rate chemistry effects; (2) due to the relatively low values of the SGS mixture fraction variance in the flow under study, the SLFM results are not sensitive to the form of the assumed FDF; and (3) in comparison to the other models investigated, the SSFRRM combustion model provides the best agreement with the DNS for product formation.

Large eddy simulation of a nonpremixed reacting jet: Application and assessment of subgrid-scale combustion models

A theory that explains the operating pressures where liquid injection processes transition from exhibiting classical two-phase spray atomization phenomena to single-phase diffusion-dominated mixing is presented. Imaging from a variety of experiments have long shown that under certain conditions, typically when the pressure of the working fluid exceeds the thermodynamic critical pressure of the liquid phase, the presence of discrete two-phase flow processes become diminished. Instead, the classical gas-liquid interface is replaced by diffusion-dominated mixing. When and how this transition occurs, however, is not well understood. Modern theory still lacks a physically based model to quantify this transition and the precise mechanisms that lead to it. In this paper, we derive a new model that explains how the transition occurs in multicomponent fluids and present a detailed analysis to quantify it. The model applies a detailed property evaluation scheme based on a modified 32-term Benedict-Webb-Rubin equation of state that accounts for the relevant real-fluid thermodynamic and transport properties of the multicomponent system. This framework is combined with Linear Gradient Theory, which describes the detailed molecular structure of the vapor-liquid interface region. Our analysis reveals that the two-phase interface breaks down not necessarily due to vanishing surface tension forces, but due to thickened interfaces at high subcritical temperatures coupled with an inherent reduction of the mean free molecular path. At a certain point, the combination of reduced surface tension, the thicker interface, and reduced mean free molecular path enter the continuum length scale regime. When this occurs, inter-molecular forces approach that of the multicomponent continuum where transport processes dominate across the interfacial region. This leads to a continuous phase transition from compressed liquid to supercritical mixture states. Based on this theory, a regime diagram for liquid injection is developed that quantifies the conditions under which classical sprays transition to dense-fluid jets. It is shown that the chamber pressure required to support diffusion-dominated mixing dynamics depends on the composition and temperature of the injected liquid and ambient gas. To illustrate the method and analysis, we use conditions typical of diesel engine injection. We also present a companion set of high-speed images to provide experimental validation of the presented theory. The basic theory is quite general and applies to a wide range of modern propulsion and power systems such as liquid rockets, gas turbines, and reciprocating engines. Interestingly, the regime diagram associated with diesel engine injection suggests that classical spray phenomena at typical injection conditions do not occur.

On the transition between two-phase and single-phase interface dynamics in multicomponent fluids at supercritical pressures

The evolution of a cryogenic fluid jet initially at a subcritical temperature and injected into a supercritical environment, in which both the pressure and temperature exceed the thermodynamic critical state, has been investigated numerically. The model accommodates full conservation laws and real-fluid thermodynamics and transport phenomena. All of the thermophysical properties are determined directly from fundamental thermodynamics theories, along with the use of the corresponding state principles. Turbulence closure is achieved using a large-eddy-simulation technique. As a specific example, the dynamics of a nitrogen fluid jet is studied systematically over a broad range of ambient pressure. Owing to the differences of fluid states and flow conditions between the jet and surroundings, a string of strong density-gradient regimes is generated around the jet surface and exerts a stabilizing effect on the flow development. The surface layer acts like a solid wall that transfers the turbulent kinetic energy...

A numerical study of cryogenic fluid injection and mixing under supercritical conditions

http://www.yang.gatech.edu/publications/Journal/C&F%20(2008,%20Ribert).pdf

Counterflow diffusion flames of general fluids: Oxygen/hydrogen mixtures

The mixing and combustion of liquid oxygen (LOX) and gaseous methane of a shear coaxial injector operating under supercritical pressures have been numerically investigated. The near-field flow and flame dynamics are examined in depth, with emphasis placed on the flame-stabilization mechanisms. The model accommodates the full conservation laws and real-fluid thermodynamics and transport phenomena over the entire range of fluid states of concern. The injector flowfield is characterized by the evolution of the three mixing layers originating from the trailing edges of the two concentric tubes of the injector. As a consequence of the strong inertia of the oxygen stream and light density of methane, a diffusion-dominated flame is anchored in the wake of the LOX post and propagates downstream along the boundary of the oxygen stream. The large-scale vortices shedding from the outer rim of the LOX postengulf methane into the wake recirculation region to react with gasified oxygen. The frequencies of vortex shedding match closely those of the flow over a rear-facing step, mainly due to the large density disparity between LOX and gaseous methane. The effects of the momentum-flux ratio of the two streams are also examined. A higher-momentum methane stream enhances mixing and shortens the potential cores of both the LOX and methane jets.

Near-field flow and flame dynamics of LOX/methane shear-coaxial injector under supercritical conditions

A comprehensive numerical analysis has been conducted to explore the development of liquid-oxygen (LOX) flow in pressure swirl injectors operating at supercritical pressures. The model is based on full-conservation laws and accommodates real-fluid thermodynamics and transport phenomena over the entire range of fluid states of concern. Three different flow regimes with distinct characteristics, the developing, stationary, and accelerating regimes, are identified within the injector. Results are compared to predictions from classical hydrodynamics theories to acquire direct insight into the flow physics involved. In addition, various flow dynamics are investigated by means of the spectral and proper-orthogonal-decomposition techniques. The interactions between the hydrodynamic instabilities in the LOX film and acoustic oscillations in the gaseous core are clearly observed and studied. The influences of flow conditions (mass flowrate, swirl strength of the injected fluid, and ambient pressure) and injector g...

Nan Zong

Papers

A numerical study of cryogenic fluid injection and mixing under supercritical conditions

Cryogenic fluid jets and mixing layers in transcritical and supercritical environments

Counterflow diffusion flames of general fluids: Oxygen/hydrogen mixtures

Near-field flow and flame dynamics of LOX/methane shear-coaxial injector under supercritical conditions

Cryogenic fluid dynamics of pressure swirl injectors at supercritical conditions