Manfred Piesche

Investigations on the flow and separation behaviour of hydrocyclones using computational fluid dynamics

Abstract Despite of the widespread use of hydrocyclones, their accurate design is often difficult because there exist many mathematical relationships to predict the separation efficiency and the pressure drop based on semi-empirical models which are restricted to particular cyclone geometry and operating conditions from which they are deduced. Some numerical approaches based on computational fluid dynamics (CFD) have been performed to describe the flow field. The present investigations deal with problems occurring when applying CFD methods to hydrocyclones and some basic information is given to yield reliable results concerning the velocity and pressure distribution and the grade efficiency curve as a fundamental basis in the design of these apparatuses.

Design of hollow cone pressure swirl nozzles to atomize Newtonian fluids

We present calculation models to predict the drop size depending on the hollow cone nozzles geometry and the volume flow for atomization of Newtonian fluids. The observations are valid for the field of the aerodynamic wave break-up of a swirling liquid sheet. The calculation methods are based on the knowledge of the flow at the nozzle outlet. Assisted by a flow simulation program it was possible to calculate the detailed and locally discrete flow field inside the nozzle taking into account the arising phase interface. The method allows to determine the film thickness, the velocities and the angle of the film which leaves the nozzle outlet. The calculation methods developed, however, allow to describe the formation of the sheet, the alteration of the sheet thickness in the direction of its spreading as well as to determine the Sauter mean diameter after the break-up of the sheet and the ligament under the influence of the gas phase which is in interaction with the liquid. Following the calculation results extensive experimental investigations varying the geometry and the transport properties of the fluids, in particular the viscosity, were undertaken. Comparing the numeric results with the experimental data and with citations in the literature the success of the investigations becomes evident. Thus a calculation basis for the design of hollow cone nozzles is available which allows to predict the Sauter mean diameter as a function of individual parameters of operation and material.

A model of the coagulation process with solid particles and flocs in a turbulent flow

Amathematical model was developed on the basis of population balances to predict the %oc size distribution in a coagulating suspension. The coagulation process is performed in a stirred tank reactor with a turbulent %ow -eld. In the population model the in%uence of the di/erent local energy charges inside the reactor is taken into account. Moreover two sorts of particles are distinguished, i.e. the originally present and completely dispersed primary particles and the %ocs. Contrary to the primary particles the %ocs can be disrupted due to pressure and shear forces as they are mechanically not very stable. This di/erent behaviour requires separate population balances for the two sorts of particles. The model parameters that are necessary are adapted to one single experiment. For the steady state the results represent di/erent %oc size distributions dependent on the solid concentration and the energy charge. Moreover it is shown that the assumption of an ideally mixed reactor that is often used cannot be maintained to be always true for the prediction of the resulting %oc size distribution. The calculation results achieved are validated by image processing measurements of coagulating quartz particles in an aqueous suspension. ? 2002 Elsevier Science Ltd. All rights reserved.

Numerical and Experimental Analysis of the Momentum and Heat Transfer in Exhaust Gas Sensors

Modern zirconia oxygen sensors are heated internally to achieve an optimal detection of the oxygen concentration in the exhaust gas and fast light off time. The temperature of the gas in the exhaust pipe varies in a wide range. The zirconia sensor is cooled by radiation and forced convection caused by cold exhaust gas. If the zirconia temperature falls, the oxygen detection capability of the sensor decreases. To minimize the cooling effects, protection tubes cover the zirconia sensor. However, this is in conflict with the aim to accelerate the dynamics of the lambda sensor. In this paper, the heat transfer at the surface of a heated planar zirconia sensor with two different double protection tubes of a Bosch oxygen sensor is examined in detail. The geometric configuration of the tubes forces different flow patterns in the inner protection tube around the zirconia sensor. The zirconia sensor is internally electrically heated by a platinum heater layer. At the sensor surface heat transfer caused by radiation takes place. In the inner protection tube radiation is absorbed, emitted and reflected at the surfaces. A fully 3d numerical model for the flow and the heat transfer is developed to predict the heat transfer and flow pattern and the temperature field of fluid and solids. The models for the internal heater and the radiant heat transfer including reflection are implemented in the commercial CFD code Fluent. The gas is modeled as a compressible, ideal gas with temperature dependent fluid properties. An experimental apparatus is designed to measure the temperature distribution in the reference channel of a heated zirconia oxygen sensor with protection tubes, that is mounted in an exhaust pipe. The comparison of the predicted values and the measured data show good agreement. With the numerical model it is possible to predict the temperature of the sensor element and the protection tubes as a function of the flow conditions, the heater power and the aging of the sensor. With the use of the non dimensional reduced temperature it is possible to reduce all measured temperature profiles to one single curve. Therefore the course of the temperature profile is dominated by the structure of the heater meander, the absolute value of the temperature by the boundary conditions of the flow and the heater.

Experimental Investigations on the Mixing Behaviour of Impingement Mixers for Polyurethane Production

By means of experimental investigations on the mixing behaviour of impingement mixers it was possible to determine the influence of geometrical parameters, material properties and operating parameters on the mixing quality. Our investigations show that identical mixing behaviours can be observed with geometrically similar mixers and under dynamically similar flow conditions. The characterisation of the mixing quality as a function of the above parameters was done by a tomographic measuring technique.

Numerical Investigation on the Deformation of Droplets in High-Pressure Homogenizers

In the present paper a high pressure emulsification process in a homogenizing orifice is investigated. Contrary to former published papers no turbulence model is applied to calculate the turbulent and quasiturbulent effects, but a direct numerical simulation is carried out. Especially in narrow gaps turbulence models are not valid, hence important effects are damped out. In the wake of the orifice emerging vortex rings can be described. To gain further knowledge about the predominant deformation and break-up mechanisms the flow type in the high pressure homogenizer is evaluated. In a second step the deformation and break-up of a single droplet is investigated with a modified Volume-of-Fluid method.

Numerical simulation of the transport and deposition behaviour of particles on filter fibres using Euler‐Lagrange Method and coupling of CFD and DEM

The aim of this investigation is the development of a simulation method which accounts for the transport behaviour of particles in the vicinity of a single filter fibre and describes the particle deposition on the fibre element correctly. Two coupling schemes are used to connect the CFD software Fluent™ with the DEM software EDEM, the Euler‐Lagrange as well as the Euler‐Euler coupling. In this study both methods will be compared to each other and with results where the motion and reallocation of dendrite structures is neglected. Furthermore, the numerical results are compared to experimental data. The investigation is aimed for the consideration of spherical particles but the method can be expanded easily to calculate the transport behaviour of non spherical particles. The motion and reallocation of dendrite structures on a filter fibre which are built‐up from deposited particles is analysed. To determine the transport behaviour of particles in the fluid flow in respect of their finite size of the Euler‐E...

Numerical and Experimental Analysis of the 3D Flow-Pattern in Exhaust Gas Sensors

In new exhaust system specifications such as single cylinder balancing, closed coupled catalyst systems, sensor locations close to the engine, turbo applications, fast light off situations and diesel engine applications the dynamic behavior of the lambda sensor becomes more important. This demands a detailed knowledge and modeling of the relevant parameters. In former analysis of exhaust gas sensors the main focus has been the electrochemical processes in the sensor. The influence of flow structure and protection tubes had lower priority. In this paper we present the numerical and experimental analysis of cold air flowing in a pipe including mounted exhaust sensors. Two double-protection tubes from the Robert Bosch GmbH have been examined named (a) and (b). The predicted results have been compared with values measured with Laser Doppler Anemometry (LDA). The flow pattern in the protection tube type (a) depends on the geometric configuration of the sensor element and the tubes. Particles and droplets in the flow reach the surface of the heated zirconia sensor, which sometimes leads to damages of the sensor element. The protection tube type (b) causes a helical flow pattern with negligible influence of the geometric configuration. In addition the centrifugal force deposits particles and droplets at the wall of the inner protection tube. The predicted velocities show a good agreement with the measured values. Based on these results a one-dimensional, analytical model to predict the mass flow through the protection tube type (b) is developed. The analytical data show good agreement with the results of the three-dimensional CFD analysis. A theoretical limitation of the mass flux through the sensor as a function of geometric parameters is found.

Numerical Simulation of the Fluid Flow and the Separation Behavior in a Single Gap of a Disk Stack Centrifuge

Using physicomathematical modeling and numerical simulation, a disk stack centrifuge was examined. With these tools it was possible to derive criteria for the stability of the flow behavior. Subsequently, this knowledge led to regularities describing the separation efficiency which can be written in dimensionless laws. The theoretical and numerical results were proven by experiments.

Characterization of Exhaust Sensors Through Modeling of Multi- Component Gas Transport and Reaction

Transport and reaction of gas mixtures in porous media are common phenomena in many chemical engineering applications. One common method of modeling the transport processes is to notionally substitute a uniform bundle of tortuous capillaries for the irregular porous structure. Then, accurate equations of motion for the gas flow and diffusion inside these small-sized capillaries can be used. This advantage comes at the cost of two additional parameters that enter into the equations, the tortuosity factor and the equivalent capillary diameter. In this work, an existing model for transient transport of multi-component gas mixtures is expanded to comprise heterogeneous fluid domains and chemical reaction. It can be applied to fluid domains that partially or completely enclose porous regions. The potential of the present model is demonstrated by simulating the electrochemically induced and transport-limited signal formation inside an exhaust gas sensor.