Spatio-Temporal Linear Stability Analysis of Stratified Planar Wakes: Velocity and Density Asymmetry Effects

This paper explores the hydrodynamic stability of bluff body wakes with non-uniform mean density, asymmetric mean density, and velocity profiles. This work is motivated by experiments [S. Tuttle et al., “Lean blow off behavior of asymmetrically-fueled bluff body-stabilized flames,” Combust. Flame 160, 1677 (2013)], which investigated reacting wakes with equivalence ratio stratification and, hence, asymmetry in the base flow density profiles. They showed that highly stratified cases exhibited strong, narrowband oscillations, suggestive of global hydrodynamic instability. In this paper, we present a local hydrodynamic stability analysis for non-uniform density wakes that includes base flow asymmetry. The results show that increasing the degree of base density asymmetry generally has a destabilizing effect and that increasing base velocity asymmetry tends to be stabilizing. Furthermore, we show that increasing base density asymmetry slightly decreases the absolute frequency and that increasing the base velocity asymmetry slightly increases the absolute frequency. In addition, we show that increasing the degree of base density asymmetry distorts the most absolutely unstable hydrodynamic mode from its nominally sinuous structure. This distorted mode exhibits higher amplitude pressure and velocity oscillations near the interface with the smaller density jump than near the one with the bigger density jump. This would then be anticipated to lead to strongly non-symmetric amplitudes of flame flapping, with much stronger flame flapping on the side with lower density ratio. These predictions are shown to be consistent with experimental data. These comparisons support the analytical predictions that increased base density asymmetry are destabilizing and that hydrodynamic velocity fluctuation amplitudes should be greatest at the flame with the lowest density jump

Modeling Of Liquid Ceramic Precursor Droplets In A High Velocity Oxy-Fuel Flame Jet

Production of coatings by high velocity oxy-fuel (HVOF) flame jet processing of liquid precursor droplets can be an attractive alternative method to plasma processing. This article concerns modeling of the thermophysical processes in liquid ceramic precursor droplets injected into an HVOF flame jet. The model consists of several sub-models that include aerodynamic droplet break-up, heat and mass transfer within individual droplets exposed to the HVOF environment and precipitation of ceramic precursors. A parametric study is presented for the initial droplet size, concentration of the dissolved salts and the external temperature and velocity field of the HVOF jet to explore processing conditions and injection parameters that lead to different precipitate morphologies. It is found that the high velocity of the jet induces shear break-up into several μm diameter droplets. This leads to better entrainment and rapid heat-up in the HVOF jet. Upon processing, small droplets (\u3c5 μm) are predicted to undergo volumetric precipitation and form solid particles prior to impact at the deposit location. Droplets larger than 5 μm are predicted to form hollow or precursor containing shells similar to those processed in a DC arc plasma. However, it is found that the lower temperature of the HVOF jet compared to plasma results in slower vaporization and solute mass diffusion time inside the droplet, leading to comparatively thicker shells. These shell-type morphologies may further experience internal pressurization, resulting in possibly shattering and secondary atomization of the trapped liquid. The consequences of these different particle states on the coating microstructure are also discussed in this article. © 2008 Acta Materialia Inc

Modeling of plasma assisted formation of precipitates in zirconium containing liquid precursor droplets

This paper focuses on the modeling of heat and mass transfer in precursor containing droplets injected into a plasma jet and the estimation of precipitate formation in these droplets from the solute. A hybrid model is employed where the plasma temperature and velocity fields are obtained from previous experimental results and the heat and mass transfer around droplets are modeled. The precipitate formation zones from the zirconium acetate solution in these droplets are estimated based on the solute concentration field within the droplet. A simple homogeneous nucleation hypothesis is employed in predicting the regions of droplets where zirconia might precipitate. The effects of droplet size, injection velocity and angle, plasma conditions as well as the solute mass diffusivity are considered. Micrographs from single pass coating experiments give credible evidence of the presence of similar types of particle morphologies in agreement with this modeling study

Review Of Modeling Of Liquid Precursor Droplets And Particles Injected Into Plasmas And High-Velocity Oxy-Fuel (Hvof) Flame Jets For Thermal Spray Deposition Applications

This article presents a review of the current state-of-the-art in modeling of liquid chemical precursor droplets and particles injected into high-temperature jets in the form of DC-arc plasmas and high-velocity oxy-fuel flames to form coatings. Conventional thermal spray processes have typically utilized powders that are melted and deposited as a coating on hardware surfaces. However, production of coatings utilizing liquid precursors has emerged in the last decade as a viable alternative to powder deposition. Use of liquid precursors has advantages over powder in terms of their relative ease of feeding and tailoring of chemical compositions. In this article, we review the modeling approaches to injection of liquid precursors and particles into plasmas and high-velocity oxy-fuel flames. Modeling approaches for the high-temperature DC-arc plasma and oxy-fuel flame jets are first reviewed. This is followed by the liquid spray and droplet level models of the liquid precursors injected into these high-temperature jets. The various knowledge gaps in detailed modeling are identified and possible research directions are suggested in certain areas. © 2009 ASM International