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Register a new userTransient Behavior near Liquid-Gas Interface at Supercritical Pressure
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Added by
Jordi Poblador Ibanez
Publication date
2018
fields
Physics
and research's language is
English
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Numerical heat and mass transfer analysis of a configuration where a cool liquid hydrocarbon is suddenly introduced to a hotter gas at supercritical pressure shows that a well-defined phase equilibrium can be established before substantial growth of typical hydrodynamic instabilities. The equilibrium values at the interface quickly reach near-steady values. Sufficiently thick diffusion layers form quickly around the liquid-gas interface (e.g., 3-10 microns for the liquid phase and 10-30 microns for the gas phase in 10-100 microseconds), where density variations become increasingly important with pressure as mixing of species is enhanced. While the hydrocarbon vaporizes and the gas condenses for all analyzed pressures, the net mass flux across the interface reverses as pressure is increased, showing that a clear vaporization-driven problem at low pressures may present condensation at higher pressures. This is achieved while heat still conducts from gas to liquid. Analysis of fundamental thermodynamic laws on a fixed-mass element containing the diffusion layers proves the thermodynamic viability of the obtained results.
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A two-phase, low-Mach-number flow solver is proposed for variable-density liquid and gas with phase change. The interface is captured using a split Volume-of-Fluid method, which solves the advection of the reference phase, generalized for the case where the liquid velocity is not divergence-free and both phases exchange mass. A sharp interface is identified by using PLIC. Mass conservation is achieved in the limit of incompressible liquid, but not with the liquid compressibility and mass exchange. This is a relevant modeling choice for two-phase mixtures at near-critical and supercritical pressure conditions for the liquid but away from the mixture critical temperature. Under this thermodynamic environment, the dissolution of lighter gas species into the liquid phase is enhanced and vaporization or condensation can occur simultaneously at different interface locations. The numerical challenge of solving two-phase, supercritical-pressure flows is greater than simpler two-phase solvers because: a) local phase equilibrium is imposed at each interface cell to determine temperature, composition, or surface tension coefficient; b) a real-fluid thermodynamic model is used to obtain fluid properties; and c) necessary phase-wise values for certain variables are obtained via extrapolation techniques. To alleviate the increased numerical cost, the pressure Poisson equation (PPE) used to solve the low-Mach-number flow is split into a constant-coefficient implicit part and a variable-coefficient explicit part. Thus, a Fast Fourier Transform method can be used for the PPE. Various verification tests are performed to show the accuracy and viability of the present approach. The growth of surface instabilities in a binary system composed of liquid n-decane and gaseous oxygen at supercritical pressures for n-decane is analyzed. Other features of supercritical liquid injection are also shown.
The injection of liquid fuel at supercritical pressures is a relevant topic in combustion, but usually overlooked. In the past, the wrong assumption whereby the liquid experiments a fast transition to a supercritical state was made, thus neglecting any role of two-phase interface dynamics in the early stages of the atomization process. However, recent studies have shown that local thermodynamic phase equilibrium and mixing between the involved species allow the coexistence of both phases in this pressure range. In this work, a Volume-of-Fluid method adapted to variable-density real fluids is used to solve the low-Mach-number governing equations coupled with a thermodynamic model based on the Soave-Redlich-Kwong equation of state. The mixing process, interface thermodynamics and early deformation of a cool liquid jet composed of n-decane surrounded by a hotter gas composed of oxygen at 150 bar are analyzed. Although heat conducts from the hotter gas into the liquid, net condensation can provide the proper local energy balance at high pressures. Then, vaporization and condensation may happen simultaneously at different interface locations. As pressure increases, liquid and gas mixtures become more alike in the vicinity of the interface. Thus, a combination of low surface tension force and gas-like liquid viscosities causes an early growth of surface instabilities. Early results indicate some similarity with high-Weber-number incompressible flows. The role of vortex dynamics on the interface deformation is analyzed by using the dynamical vortex identification method.
The high frequency dynamics of fluid oxygen have been investigated by Inelastic X-ray Scattering. In spite of the markedly supercritical conditions ($Tapprox 2 T_c$, $P>10^2 P_c$), the sound velocity exceeds the hydrodynamic value of about 20%, a feature which is the fingerprint of liquid-like dynamics. The comparison of the present results with literature data obtained in several fluids allow us to identify the extrapolation of the liquid vapor-coexistence line in the ($P/P_c$, $T/T_c$) plane as the relevant edge between liquid- and gas-like dynamics. More interestingly, this extrapolation is very close to the non metal-metal transition in hot dense fluids, at pressure and temperature values as obtained by shock wave experiments. This result points to the existence of a connection between structural modifications and transport properties in dense fluids.
Numerical analysis of a shear layer between a cool liquid n-decane hydrocarbon and a hot oxygen gas at supercritical pressures shows that a well-defined phase equilibrium can be established. Variable properties are considered with the product of density and viscosity in the gas phase showing a nearly constant result within the laminar flow region with no instabilities. Sufficiently thick diffusion layers form around the liquid-gas interface to support the case of continuum theory and phase equilibrium. While molecules are exchanged for both species at all pressures, net mass flux across the interface shifts as pressure is increased. Net vaporization occurs for low pressures while net condensation occurs at higher pressures. For a mixture of n-decane and oxygen, the transition occurs around 50 bar. The equilibrium values at the interface quickly reach their downstream asymptotes. For all cases, profiles of diffusing-advecting quantities collapse to a similar solution (i.e., function of one independent variable). Validity of the boundary layer approximation and similarity are shown in both phases for Reynolds numbers greater than 239 at 150 bar. Results for other pressures are also taken at high Reynolds numbers. Thereby, the validity of the boundary layer approximation and similarity are expected. However, at very high pressures, the similar one-dimensional profiles vary for different problem constraints.
Effects of a gurney flap were numerically investigated on the supercritical NASA airfoil by solving the two-dimensional Reynolds-averaged Navier-Stokes equations for a range of transonic Mach numbers and angles of attack, using turbulence compressible KW SST model. The height of the gurney flap was selected to be 1.65 percent chord length. A high-resolution mesh was applied to accurately predict the flow field specifically in the vicinity of the airfoil. Below the drag divergence Mach number, the gurney flap has a remarkable influence on the aerodynamic coefficients especially at -1 and 0 degrees angle of attack resulting in 50 percent increase in L over D ratio. At high Mach numbers and angles of attack, Gurney flap loses its effects and the clean airfoil has better aerodynamic performance since it significantly boosts both the pressure and shear drag. It was observed that the gurney flap mitigates the transonic lambda shock on both surfaces of the airfoil. Moreover, it alters the Kutta condition by changing the separation point location at the trailing edge which provides the airfoil more bound circulation and lift force.
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