Nan Zong

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

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...

CRYOGENIC FLUID JETS AND MIXING LAYERS IN TRANSCRITICAL AND SUPERCRITICAL ENVIRONMENTS

ABSTRACT 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, shear-layer instability, volume dilatation, and property variations, dictating the evolution of cryogenic jets and mixing layers, are identified and analyzed in depth. The jet dynamics are found to be largely determined by the local thermodynamic state through its influence on the thermophysical properties of the fluid. The impact of injector configuration and operating conditions on the swirl injector behavior are also highlighted. These results not only shed light on the subject problems, but also provide a quantitative basis for identifying the design parameters and flow variables that exert the strongest influence on the underlying processes.

Cryogenic fluid dynamics of pressure swirl injectors at 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...

An efficient preconditioning scheme for real-fluid mixtures using primitive pressure-temperature variables

An improved preconditioning scheme incorporating a unified treatment of general fluid thermodynamics is developed for treating fluid flows over the entire regime of fluid thermodynamic states at all speeds. All of the thermodynamic and numerical properties (such as eigenvalues and Jacobian matrices) are derived directly from fundamental thermodynamics theories, rendering a self-consistent and robust algorithm. Further efficiency is obtained by employing temperature instead of enthalpy as the primary dependent variable in the preconditioned energy equation. No iterative solution of a real-fluid equation of state is required. This approach, combined with the use of explicit treatments of temporal and spatial derivatives, results in a scheme for which load balance is much easier to achieve in a distributed computing environment. A numerical stability analysis is performed to assess the effectiveness of the scheme at various fluid thermodynamic states. Sample calculations are also carried out. These include injection and mixing of cryogenic fluids and flame dynamics of coaxial jets of liquid oxygen and methane under supercritical conditions. The robustness and efficiency of the present work are demonstrated over a wide range of thermodynamic and flow conditions.

A Numerical Study of High-Pressure Oxygen/Methane Mixing and Combustion of a Shear Coaxial Injector

The mixing and combustion of cryogenic oxygen and methane in a shear coaxial injector operating under supercritical pressures are investigated numerically. The formulation accommodates full conservation laws and real-fluid thermodynamics and transport phenomena. The near -field fluid injection and mixing dynamics are characterized by the evolution of two mixing layers. The vigorous fluctuations generated from the inner mixing layer, which consists a string of large scale vortices emerging from the LOX post tip, provide a forcing on the outer shear layer. The formations of large structures do not follow the fundamental KelvinHelmholtz instability mechanism, but in a manner analogous to that produced at a backward-facing step. The effect of the momentum-flux ratio on the flow evolution is demonstrated. As the velocity of the methane stream increases, turbulent mixing is enhanced, and both the inner and outer potential cores are reduced. A diffusion dominated combustion occurs in the presence of large gradient of fluid pr operties. The work also identifies the flame anchoring mechanism.

Cryogenic Fluid Dynamic Response of Swirl Injector to External Forcing at Supercritical Conditions

The present study explores the effects of externally imposed excitations on the cryogenic fluid dynamics of pressure swirl injectors at supercritical conditions. The external forcing is imposed through periodic oscillations of the mass flow rate at the injector tangential inlet over a broad band of frequencies. The flow field evolution, film thickness, and the spreading angle at the nozzle exit, are investigated and compared with the results obtained from flow without external forcing. The results indicate that low frequency forcing results in coherent flow structures at the frequency of the external forcing, whereas very high-frequency forcing does not affect the flow field significantly due to large viscous dissipation of the high-frequency fluctuations. Due to the conservation of mass and momentum, external forcing does not lead to much variation in the time-averaged film thickness and spreading angle, however, their instantaneous quantities become more regular and periodic when external forcing is enforced. The effect of the forcing magnitude was also investigated. When the forcing magnitude is very large, the film thickness and spreading angle at the injector exit deviate from the low forcing magnitude cases.

Supercritical LOX/Methane Flame Stabilization and Dynamics of a Shear Coaxial Injector

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. Turbulence closure is achieved using a large-eddy-simulation technique. The injector flowfield is characterized by the evolution of 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 jet. The overall flow development is largely determined by the lighter methane stream. The large-scale vortices shedding from the outer rim of the LOX post engulf methane into the wake recirculation region to react with gasified oxygen. The frequencies of vortex shedding match closely those of a 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.

Supercritical Combustion of Liquid Oxygen (LOX) and Methane Stabilized by a Splitter Plate

A sys tematic numerical analysis has been conducted to explore supercritical mixing and combustion dynamics of liquid oxygen and methane separated by a splitter plate. The unified theoretical/numerical frameworks for the treatment of general fluid thermodynamics have been extended to accommodate turbulence/flame interactions. Different turbulent combustion models have been implemented. The applicability of those models is carefully assessed by comparing the chemical time to the characteristic turbulence time scales. The results indicate that the flamelet assumption is valid and the combustion process is mixing dominant throughout the entire flowfield under the present simulation conditions. The direct -losure approach over-predicts the reaction rate and gives rise to a thickened flame. The flow dynamics and combustion process in the vicinity of the splitter plate are quantified. Simulations of bot h the flamelet model and direct-closure approach confirm that the flame is stabilized by the wake recirculation zone with hot product right behind the splitter plate.

A Flamelet Approach for Modeling of (LOX)/Methane Flames at Supercritical Pressures

A comprehensive theoretical/numerical framework has been established to treat the combustion of liquid oxygen (LOX) and methane under supercritical conditions. The model accounts for detailed LOX/methane reaction mechanisms, and accommodates the effect of scalar dissipation on finite-rate chemistry. Turbulence closure is achieved by a large-eddy simulation technique. Several different turbulent combustion models are implemented and assessed by comparing the chemical and turbulence time scales at conditions typical of liquid-propellant engine rocket operation. Results indicate that the flamelet assumption is appropriate. The direct-closure approach may over-predict the reaction rate. The supercritical mixing and combustion LOX and methane downstream of a splitter are analyzed systemically, and the effects of real-fluid thermodynamics on the cryogenic-fluid flame evolution are quantified.