Udo Fritsching

Measurement of phase interaction in dispersed gas/particle two-phase flow

Abstract For simultaneous measurement of size and velocity distributions of continuous and dispersed phases in a two-phase flow a method is proposed and applied to increase the sensitivity of a phase-Doppler-anemometry (PDA) system. Design considerations are presented to increase the detectable size range of the PDA system to approximately 1:200 (in diameter) for simultaneous detection of tracers and particles. For this the optical properties for light scattering of the particles are properly adjusted to the measurement problem by using homogeneously coloured particles and a special signal processing procedure, which is developed to guarantee reliable signal processing of the tracer signals even with poor signal-to-noise ratios (SNR). The application of the experimental configuration is described by simultaneous measurements of gas and particle velocity and velocity fluctuation profiles in a two-phase jet arrangement with a particle diameter range from 1 (tracers) up to 160 μm (particles). In this two-phase flow at high Stokes numbers ( St ≫1) different turbulence structure modification effects are identified. The height of influence of these effects depends on the local position in the jet. Near the nozzle exit high gas velocity gradients exist and therefore high turbulence production in the shear layer of the jet is observed. Here the turbulence structure in the jet mainly depends on lateral turbulence transport. Due to a changed turbulence structure with reduced intensity of large eddies, this lateral transport in the two-phase jet is decreased in comparison to the single-phase jet in this area. In the area at greater nozzle distances where the jet is nearly developed, the velocity gradient in the shear layer is lower and due to this lateral turbulence transport effects become less important. Here axial turbulence transport along the jet dominates and turbulence intensity reduction is higher.

A mathematical model for cooling and rapid solidification of molten metal droplets

Abstract During the spray forming process, a continuous molten metal stream is atomized by impinging high speed inert gas jets. In the generated spray cone, the resulting metal droplets are rapidly cooled by the huge temperature difference to the surrounding gas phase and thereby partly solidify. After a certain flight and residence time inside the spray cone, the droplets impinge on the substrate and form the product (deposit). The material properties of this product depend on several process parameters and especially on the thermal state of the deposited droplets at impingement. Smaller droplets cool very fast and may impinge onto the product in a completely solidified state as solid metal powder particles. Larger droplets contain a higher amount of thermal energy and impact during the state of phase change or even still completely liquid. Therefore, describing the thermal history of metal droplets during flight in the spray cone is of great importance. In this contribution, a mathematical model is introduced to describe the cooling and solidification of individual metal droplets in the spray cone during the droplet–gas interaction in flight. By introducing this model into a standard two phase flow simulation model for the spray cone description, it is possible to calculate the transient droplet temperature and solid fraction contents of individual particles depending on overall process parameters and flight path.

Jet break up of liquid metal in twin fluid atomisation

For melt disintegration in spray forming and metal powder production, the liquid metal typically is atomised by means of twin fluid atomisation e.g. with inert gases. The first stage of the atomisation process is covered by the initialisation and development of surface perturbations on the liquid jet surface that subsequently grow and finally lead to the break up of the melt jet. The primary disintegration process affects all further stages of the atomisation process and hence influences the resulting spray characteristics. In this contribution, the influence of different material properties on the primary disintegration process of a metal melt in a free fall atomiser configuration is investigated. Surface instabilities of liquid metal jets up to the primary break up are analysed experimentally as well as analytically. Results of a linear instability analysis are discussed and compared to evaluations of high-speed video images. In comparison of both results the periodicity and main mechanism of the initialisation process is analysed.

Fluid flow and heat transfer in gas jet quenching of a cylinder

Heat treatment by quenching of individual metallic parts with multiple impinging gas jets is an environmentally friendly alternative to conventional surface hardening and quenching in liquids. In the present investigation the gas flow field and simultaneous heat transfer process in gas quenching is studied by numerical simulation for surface treatment of a cylindrical sample geometry. Aim of the investigation is the evaluation of optimized flow conditions and nozzle arrangements to achieve: a maximum overall heat release (high integral heat transfer rates) to maximize the quenching efficiency; a local smooth distribution of the cooling process (spatially homogeneous heat transfer) for avoidance of spatial hardness variations. These aims are achieved by derivation of an optimized nozzle arrangement and appropriate operation conditions of the gas jet array with respect to the three dimensional sample geometry of a cylinder to be quenched.

In-situ particle temperature, velocity and size measurements in the spray forming process

The structure and material properties of spray formed products depend directly on the thermal state of particles before they impact the substrate or on the already deposited layer. Monitoring particle temperature, velocity and size can thus provide a unique tool for optimizing the material properties as well as controlling spraying conditions during deposition. In this paper, an optical sensing device based on the principle of high-speed pyrometry, developed for on-line monitoring of particle temperature, velocity and diameter of in-flight particles during thermal spraying conditions (e.g. plasma guns), is for the first time applied and examined in the spray forming process. Thermal radiation emitted by the particles is collected by a sensing head attached to the spray cone and transmitted through optical fibers to a detection cabinet located away from the dusty environment. Tests were carried out with different materials, spray pressures and measurement positions to exhibit the efficiency of the measurement system in the spray forming process.

Numerical investigation of alternative process conditions for influencing the thermal history of spray deposited billets

Abstract In spray forming, during the spray deposition process and the subsequent cooling period a time dependent temperature field develops within the product and the solidification of the remaining liquid fraction takes place. In this paper, the time dependent thermal conditions and solidification behaviour in spray formed billets are investigated. A transient numerical simulation is carried out and compared with experimental results. A description of the relevant model and the developed program will be given and an investigation of the influence of different process parameters on the time dependent temperature field and the solidification history within billets are shown. The numerical model is based on a single-phase formulation of the energy equation. A non-orthogonal coordinate system is used for grid generation within the time dependent growing shape of the billet. Temperature measurements are carried out within the substrate and also in the lower part of the billet in the spray forming process. The material discussed throughout this contribution is CuSn6 (2.1020).

COUPLED ATOMIZATION AND SPRAY MODELLING IN THE SPRAY FORMING PROCESS USING OPENFOAM

Abstract The paper presents a numerical model capable of simulating the atomization, break-up and in-flight spray phenomena in the spray forming process. The model is developed and implemented in the freeware code openFOAM. The focus is on studying the coupling effect of the melt break-up phenomena with the local gas and droplets flow fields. The work is based on an Eulerian-Lagrangian description, which is implemented in a full 3D representation. The gas is described by the incompressible RANS equations, whereas the movement of the droplets is modeled by a tracking approach, together with a full thermal model for droplet cooling and solidification. The model is tested and validated against results from literature and experiments. Subsequently, the model is used to simulate the complex flow fields in the spray forming process and the results are discussed. The presented model of the spray forming process is able to predict the droplet size distribution of the spray from the process conditions, by introducing submodels for the melt fragmentation and successive secondary break-up processes as part of the spray model. Furthermore, the competition of drop break-up and solidification is derived by describing the thermal state of the particles in the spray. Therefore, the model includes a full thermal solver for the droplets, which also takes the rapid solidification of different drop sizes into account.

Dynamic forces on agglomerated particles caused by high-intensity ultrasound.

In this paper the acoustic forces on particles and agglomerates caused by high-intensity ultrasound in gaseous atmosphere are derived by means of computational fluid dynamics (CFD). Sound induced forces cause an oscillating stress scenario where the primary particles of an agglomerate are alternatingly pressed together and torn apart with the frequency of the applied wave. A comparison of the calculated acoustic forces with respect to the inter particle adhesion forces from Van-der-Waals and liquid bridge interactions reveals that the separation forces may reach the same order of magnitude for 80 μm sized SiO2-particles. Hence, with finite probability acoustically agitated gases may de-agglomerate/disperse solid agglomerate structures. This effect is confirmed by dispersion experiments in an acoustic particle levitation setup.

Modelling and simulation of flow boiling heat transfer

Purpose – The purpose of this paper is to describe the development and application of a numerical model for analysis of flow boiling phenomena and heat transfer.Design/methodology/approach – For flow boiling processes, the fluid and vapour flow regimes in connection with the conjugate heat and mass transfer problem for specimen quenching through the entire boiling curve is modelled. Vaporisation and recondensation, the vapour fraction distribution and vapour movement with respect to the liquid are considered in the calculation of the two‐phase flow and heat transfer process. The derived flow boiling model is based on a mixture model and bubble crowding model approach for two‐phase flow. In addition to the conventional mixture model formulation, here special model implementations have been incorporated that describe: the vapour formation at the superheated solid‐liquid interface, the recondensation process of vapour at the subcooled vapour‐liquid interface, the mass transfer rate in the different boiling p...

Numerical Investigation of Binary Droplet Collisions in All Relevant Collision Regimes

A numerical investigation of binary droplet collisions in a gaseous phase has been conducted. A volume of fluid (VOF) based interface capturing method, which is characterized by introducing an extra artificial compression term into the volume-fraction transport equation, is employed to capture the liquid-gas interface. The full Navier-Stokes equations coupled with the volume-fraction transport equation are discretized on a fixed, uniform grid using the finite volume method. The solution of the volume-fraction transport equation is bounded by an explicit universal multidimensional limiter. The simulations cover five major regimes of binary droplet collisions: (I) coalescence with minor deformation, (II) bouncing, (III) coalescence with major deformation, (IV) reflexive separation and (V) stretching separation. A ghost cell method is introduced in order to simulate bouncing and retarded coalescence in head-on collisions. Especially, for retarded coalescence collisions, the volumefraction boundary condition ...