Coaxial Jets with Disparate Viscosity: Mixing and Laminarization Characteristics

Mixing of fluids in a coaxial jet is studied under four distinct viscosity ratios, m = 1, 10, 20 and 40, using highly resolved large-eddy simulations (LES), particle image velocimetry and planar laser-induced fluorescence. The accuracy of predictions is tested against data obtained by the simultaneous experimental measurements of velocity and concentration fields. For the highest and lowest viscosity ratios, standard RANS models with unclosed terms pertaining to viscosity variations are employed. We show that the standard Reynolds-averaged Navier-Stokes (RANS) approach with no explicit modelling for variable-viscosity terms is not applicable whereas dynamic LES models provide high-quality agreement with the measurements. To identify the underlying mixing physics and sources of discrepancy in RANS predictions, two distinct mixing modes are defined based on the viscosity ratio. Then, for each mode, the evolution of mixing structures, momentum budget analysis with emphasis on variable-viscosity terms, analysis of the turbulent activity and decay of turbulence are investigated using highly resolved LES data. The mixing dynamics is found to be quite distinct in each mixing mode. Variable viscosity manifests multiple effects that are working against each other. Viscosity gradients induce additional instabilities while increasing overall viscosity decreases the effective Reynolds number leading to laminarization of the turbulent jet, explaining the lack of dispersion and turbulent diffusion. Momentum budget analysis reveals that variable-viscosity terms are significant to be neglected. The scaling of the energy spectrum cascade suggests that in the TLL mode the unsteady laminar shedding is responsible for the eddies observed

Mechanistic analysis and reduced order modeling of forced film cooling flows

Abstract Unforced and forced film cooling jets are investigated in view to develop a reduced order model of the velocity and temperature fields. First, a vertical jet in cross-flow, a configuration well documented at high blowing ratios, is investigated at low blowing ratios using experimental visualizations and large eddy simulations. The unforced study reveals that dominant structures at low blowing ratio can be significantly different from the ones formed at high blowing ratio and describes their evolution and transition as the blowing ratio is changed. The forced jet investigations extend the results of past numerical studies in terms of starting vortex classification to partly modulated jets, and evidence a quantitative mismatch in the transition blowing ratio threshold between experiments and simulations. Film cooling performance estimations show that, at fixed mass flow rate, unforced jets perform better than the forced jets while at fixed pressure supply, forced jets can bring some improvements over their unforced counterpart. A survey of a more application relevant inclined jet is carried out using comparable methods. The unforced study shows the attached inclined and vertical jet vortical structures have strong similarities yet one of them is absent from the former configuration therefore leading to smoother regime transitions. The forced inclined jet study reveals some common starting vortex regimes with the ones of the vertical jet, but also exhibits unique sets of structures not observed before. Film cooling performance of inclined jets is also assessed and compared to relevant vertical jet results. Derivations of reduced order models of two-dimensional systems for both velocity and temperature fields using the Proper Orthogonal Decomposition (POD) - Galerkin method are used to establish the numerical methods and potential caveats of the method. Then, POD of unforced and forced inclined jet is carried out as a statistical analysis tool to evidence the energetically dominant flow structures. Finally, reduced order models of the unforced jet are obtained in the attached and transitional regimes and reduced order models for the forced jet are derived in both instantaneous and phase averaged fields. Most of the derived models show reasonable agreement with the projected empirical data once stabilized using appropriate linear damping methods. The flow analysis, models and methods presented herein constitute an essential step towards the development of close-loop controlled film cooling system

The Use of a Jet Column with Different Nozzles as a Reactor for Biodiesel Reaction with Crude Palm Oil as Feedstock

Biodiesel may be produced by trans-esterification reaction of vegetable oil, which transforms triglycerides into alkyl esters as biodiesel and glycerol as a byproduct, in the presence of an alcohol reactant and a acid or base catalyst. The major obstacle of preventing biodiesel commercialisation is low mass transfer rates from methanol into oil phase to achieve high yield due to large difference in fluid viscosities, i.e. low viscosity methanol and high viscosity oil. Many techniques have been proposed to overcome this obstacle, most of which involve high mole ratio of methanol to triglycerides exceeding 6, but none of them utilised fluid mechanic techniques to fix up the obstacle. The present research adopts a finding in fluid mechanic field that notched and tabbed nozzles are capable of intensifying shear stress between 2 different flows, which consequently increases the contact areas of the flows considerably. For this purpose, in the present research, a jet column was utilised as a reactor where the mixture of reactants, i.e. crude palm oil (CPO) and methanol with catalyst NaOH were recirculated and injected downward vertically into the reactor column from a nozzle at the top of reactor. The type of nozzles and the mole ratio of methanol to CPO were varied (3.75:1; 4.5:1; 5.25:1 and 6:1) to investigate their effects on yield and conversion of the reaction conducted for 60 minutes at temperatures 53-58oC. Nozzles used were notched, tabbed and conventional circular nozzles for comparison. The highest conversion and yield of biodiesel were achieved at mole ratio 6:1 attaining respectively 87.2% and 96.8% using notched nozzle, 87.8% and 96.6% using tabbed nozzle and 71.2% and 75.1 % using circular nozzle for comparison. Therefore, using notched and tabbed nozzles can reduce the excess of methanol reactant thus saving its separation cost while producing high yield of biodiesel

Flow and heat transfer characteristics of turbulent swirling impinging jets [thesis]

Numerous industrial applications rely on impinging jets to impart convective heat and mass transfer in processes ranging from the cooling of electronic devices and gas turbine blades to drying of paper and food products. Conventionally, non-swirling impinging jets have been employed, but some studies have shown that inducing swirl allows better control of uniformity and improved convective fluxes. A better understanding of the underlying physical mechanisms that lead to such behaviour warrants deeper insights into the flow and heat transfer characteristics of impinging jets, both swirling and non-swirling. Whilst important to achieve, the flow field of an impinging jet is already quite complex even before the addition of swirl which, in free (not impinging) jets, induces vortex breakdown and other instability modes. The addition of swirl to impinging jets thus has the potential to affect the transient and steady-state convective behaviour, both of which are crucial in industrial applications. This study features experimental and numerical investigations of incompressible turbulent impinging air jets that utilize aerodynamically generated swirl. The research focuses on the velocity field, upstream near the nozzle exit plane as well as further downstream, and the way in which it affects heat transfer at the impingement plane, both under transient and steady-state conditions. Boundary conditions at the nozzle exit were measured using Constant Temperature Anemometry. The surface temperature distribution of a thin foil heater, which forms the impingement surface cooled by the ambient temperature jet, was measured using infrared thermography for a range of Reynolds numbers (Re=11,600-35,000), swirl numbers (S=01.05), and impingement distances (H/D=2-6). The effects of different inflow conditions for non-swirling and weakly swirling impinging jets were also simulated (numerically) using ANSYS Fluent (version 16.2). Particle Image Velocity was utilized to resolve the flow field, over low (S=0.30) and higher (S=0.74) swirl over a range of Reynolds numbers (Re=11,60035,000) and nozzle-to-plate distance (H/D=2 and 4). Whilst the use of non-intrusive infrared thermography has been widely reported in studies of the steady-state heat transfer behaviour of impinging jets, an image processing methodology to resolve the time-dependant (transient) convective heat transfer behaviour was lacking. In this context, a MATLAB based method was developed to quantify the role of various impinging jet parameters on the time to reach steady-state. The effect of spatial discretization, image resolution, and the threshold value of time-dependent Nusselt number, on the time to reach steady-state, was also analysed. The role of various operating (Re, S) and geometric conditions (H/D) on the temporal evolution of turbulent impinging jets was also resolved. By applying the innovative image processing methodology developed, results show that for non-swirling jets, transient heat transfer characteristics at some conditions (H/D=4) are distinct if compared to others (H/D=2 and 6) and that the heat transfer distribution over the impingement plate changes significantly over a small interval of time. For swirling jets, the peak Nusselt number shifts to the wall jet region as the intensity of the swirl increases. Two correlations (no-to-low swirl, moderate-to-high swirl) are proposed to predict the time needed to reach a steady-state for Re=35,000. Computational Fluid Dynamics was then used to resolve the role of various (upstream) nozzle exist conditions (velocity profiles) on the emerging heat transfer characteristics at the impingement plane. Results showed that under some conditions (S=0.31, uniform velocity profile) a small recirculation zone, stabilised on the impingement plane, affects the heat transfer compared to other tested velocity profiles. This study also gave valuable insights on the impact of using (simple) geometric inserts to generate for swirl into impinging jets, a method widely used for its simplicity. Results showed that this can fundamentally perturb the results unlike the use of aerodynamic swirl which relies on tangential air ports. For the experimentally measured flow field, vortex breakdown is observed for two of conditions (Re=11,600 and 24,600 at S=0.74) out of the six tested. Impingement affects the position, shape, and strength of the vortex breakdown. For Re=24,600, impingement significantly affects (shape and position) the recirculation bubble when compared to impingement at Re=11,600. Heat transfer characteristics at high swirl are compared with low swirling impinging jets. The vortex breakdown (at high swirl) affects the impingment heat transfer and showed comparatively uniform heat transfer distribution in contrast to low swirling impinging jets. Vortex breakdown significantly deteriorates stagnation zone heat transfer and the Nusselt number peak occurs in the wall jet region. Benefits derived from this study include identifying impingement conditions that allow quicker stabilisation of heat transfer (shorter transients) as well as an improved understanding for the role of impingement on the upstream and downstream velocity field and heat transfer characteristics

Investigation into Stability, Transition and Turbulence of Thermal Plumes

In this thesis, the stability, transition and turbulence of thermal plumes were investigated by numerical simulation. Experiments were also conducted, but only for the validation of the simulation code being used. The effect of variable transport properties on a large eddy simulation of a turbulent axisymmetric plume was examined, and it was shown that an in-house incompressible Navier-Stokes solver, which is based on a standard Smagorinsky LES model, with the effects of variable properties incorporated using a modified Sutherlands law, predicts the correct statistical behaviours of the turbulent plume. The near-field puffing instability in thermal planar plumes, which had received little attention in the literature, was investigated by direct numerical simulation. The associated lapping flow instability, forming bulge structures over a heated floor section, was studied using a channel flow model, which allows the lapping flow velocity to be varied. The parametric dependencies were found for the bulge formation and the oscillation frequencies in the lapping flow. Further, the Prandtl number dependent transitional behaviours in the near-field were investigated, and direct stability analysis was conducted to study the lapping flow and stem instabilities. Experiments using a shadowgraph technique and an in-house, two-dimensional, two-component particle image velocimetry, with water as the working fluid, provided validations for the near-field unsteady behaviours of thermal plumes. A ventilated filling box flow with a transitional planar plume was also investigated by direct numerical simulation. A mapping of transitional flow behaviours was obtained, and the parametric dependencies of turbulence statistics and mean flow characteristics were investigated. The three-dimensionality was shown to have only minor effects on the transitional ventilated filling box flows being considered

Investigation into Stability, Transition and Turbulence of Thermal Plumes

In this thesis, the stability, transition and turbulence of thermal plumes were investigated by numerical simulation. Experiments were also conducted, but only for the validation of the simulation code being used. The effect of variable transport properties on a large eddy simulation of a turbulent axisymmetric plume was examined, and it was shown that an in-house incompressible Navier-Stokes solver, which is based on a standard Smagorinsky LES model, with the effects of variable properties incorporated using a modified Sutherlands law, predicts the correct statistical behaviours of the turbulent plume. The near-field puffing instability in thermal planar plumes, which had received little attention in the literature, was investigated by direct numerical simulation. The associated lapping flow instability, forming bulge structures over a heated floor section, was studied using a channel flow model, which allows the lapping flow velocity to be varied. The parametric dependencies were found for the bulge formation and the oscillation frequencies in the lapping flow. Further, the Prandtl number dependent transitional behaviours in the near-field were investigated, and direct stability analysis was conducted to study the lapping flow and stem instabilities. Experiments using a shadowgraph technique and an in-house, two-dimensional, two-component particle image velocimetry, with water as the working fluid, provided validations for the near-field unsteady behaviours of thermal plumes. A ventilated filling box flow with a transitional planar plume was also investigated by direct numerical simulation. A mapping of transitional flow behaviours was obtained, and the parametric dependencies of turbulence statistics and mean flow characteristics were investigated. The three-dimensionality was shown to have only minor effects on the transitional ventilated filling box flows being considered

Experimental investigation of negatively buoyant sediment plumes resulting from Dredging operations

In a first step to investigate the behaviour of sediment plumes released from dredging vessels, an experimental facility has been built to release scaled fine sediment plumes in the presence of cross flow. High-frequency measurements of velocity components and sediment concentration are obtained using acoustic and optical backscatter instruments. The paths of the axis of the experimental buoyant plumes in cross-flow have been compared to integral laws by Fisher et al. (1979), showing relatively good agreement for plumes not influenced by the dredger’s hull. Plumes with low relative density difference and high crossflow to outflow velocity ratio deviate from the integral laws due to additional mixing induced by the hull boundary layer and wake

Dynamics of vortical structures in a low-blowing-ratio pulsed transverse jet

Large Eddy Simulation is used to study the interaction of a 35° inclined jet into a crossflow. Steady state cases, with a BR ranging from 0.150 to 1.2, are firstly examined to understand the dynamics of the flow field. Iso-surface of Laplacian of the pressure, vorticity contour and velocity fields highlight the presence of four main vortical structures: shear layer vortices, horse-shoes vortices, wake vortices and CRVP. Qualitative comparisons are performed between simulations and experiments. The dynamics of the flow field is next characterized by pulsing the jet. The studied pulsed cases have same low BR and duty cycle respectively fixed at 0.150 and 50%. The presence of a vortex ring which evolves into a leading hairpin vortex is observed at the pulse. Good qualitative agreement is obtained between the numerical and experimental results. Film cooling effectiveness, temperature contour and jet trajectory are extracted for both steady and pulsed cases. Overall, steady state cases provide better results in term of film cooling performance. POD is performed on steady and pulsed cases to obtain the dominant modes of the flow

Characterisation of turbulence in an open channel flow and in a fountain with tomographic PIV.

This work aims to improve the understanding of the fundamental characteristics of environmental flows by interpreting the turbulence in a 3D measurement domain. This thesis primarily describes the Tomographic PIV technique and the results of three experimental investigations of environmental flows. Two experiments were conducted in an open channel flow, divided into four sequential, identical pools, by a combination of regular grids. The first set of TPIV measurements were in the water column, while the second set of measurements were made along the channel bottom. The instantaneous structures in the flow were visualised and the turbulent kinetic energy k, energy dissipation ε and vorticity ω were analysed; their decay along the streamwise direction was revealed. Ejections (Q2) and sweeps (Q4) were identified along the channel bottom. A major contribution that resulted from the investigation pertains to the vibration correction of the cameras. TPIV measurements were taken of a regime of turbulent, forced fountain flows. The fountains were created by injecting a salt-water solution through a circular opening into the bottom of a reservoir of a water-ethanol solution, with their refractive indices carefully matched. The evolution of the fountain in its initial stages was captured and described in a series of chronological measurement volumes. Measurements of the fully developed fountains captured the large scale structures and their characteristics were analysed by considering the topology of the invariants of the velocity gradient tensor. The TPIV system was designed and built in-house at the University of Sydney. The experimental investigations described in this work revealed some interesting features of the environmental flows. The applicability and versatility of TPIV for these flows were demonstrated. The measurements allowed for the quantification and visualisation of the turbulence in the flows and hence shed light on the physics behind them

Characterisation of turbulence in an open channel flow and in a fountain with tomographic PIV.

This work aims to improve the understanding of the fundamental characteristics of environmental flows by interpreting the turbulence in a 3D measurement domain. This thesis primarily describes the Tomographic PIV technique and the results of three experimental investigations of environmental flows. Two experiments were conducted in an open channel flow, divided into four sequential, identical pools, by a combination of regular grids. The first set of TPIV measurements were in the water column, while the second set of measurements were made along the channel bottom. The instantaneous structures in the flow were visualised and the turbulent kinetic energy k, energy dissipation ε and vorticity ω were analysed; their decay along the streamwise direction was revealed. Ejections (Q2) and sweeps (Q4) were identified along the channel bottom. A major contribution that resulted from the investigation pertains to the vibration correction of the cameras. TPIV measurements were taken of a regime of turbulent, forced fountain flows. The fountains were created by injecting a salt-water solution through a circular opening into the bottom of a reservoir of a water-ethanol solution, with their refractive indices carefully matched. The evolution of the fountain in its initial stages was captured and described in a series of chronological measurement volumes. Measurements of the fully developed fountains captured the large scale structures and their characteristics were analysed by considering the topology of the invariants of the velocity gradient tensor. The TPIV system was designed and built in-house at the University of Sydney. The experimental investigations described in this work revealed some interesting features of the environmental flows. The applicability and versatility of TPIV for these flows were demonstrated. The measurements allowed for the quantification and visualisation of the turbulence in the flows and hence shed light on the physics behind them