Ronnie Andersson

Analysis of Thermoelectric Generator Performance by Use of Simulations and Experiments

A method that enables accurate determination of contact resistances in thermoelectric generators and which gives detailed insight into how these reduce module performance is presented in this paper. To understand the importance taking thermal and electrical contact resistances into account in analysis of thermoelectric generators, full-scale modules were studied. Contact resistances were determined by means of non-linear regression analysis on the basis of results from 3D finite element simulations and experiments in a setup in which heat flow, voltage, and current were measured. Statistical evaluation showed that the model and the identified contact resistances enabled excellent prediction of performance over the entire range of operating conditions. It was shown that if contact resistances were not included in the analysis the simulations significantly over-predicted both heat flow and electric power output, and it was concluded that contact resistance should always be included in module simulations. The method presented in this paper gives detailed insight into how thermoelectric modules perform in general, and also enables prediction of potential improvement in module performance by reduction of contact resistances.

Experimental and numerical investigations of a jet mixing in a multifunctional channel reactor: Passive and reactive systems

Mixing of two liquids in a new multifunctional channel reactor developed by AlfaLaval has been studied both experimentally and through computational fluid dynamics (CFD). As the channels are quite narrow the Reynolds numbers are low and the bulk of the channel is within the turbulent boundary layer. This makes accurate a priori predictions of the flowfield difficult and experimental validation necessary. Particle image velocimetry (PIV) was used to measure the flowfield, whereas planar laser-induced fluorescence was used for a scalar concentration field. CFD simulations were performed with the commercial software Fluent 5.5. Different turbulence models were tested and compared with PIV. The best predictions were obtained with a low Reynolds boundary layer k-e turbulence model. Mixing of a passive tracer, including mean concentration and concentration variance, was calculated with the turbulent mixer model of Baldyga. A reactive system with diazo coupling between 1-naphthols, 2-naphthols and diazotized sulphanilic acid was studied both experimentally and theoretically due to its sensitivity to mixing conditions. The interpolation model of Baldyga was used to predict the evolution of the species. Good agreement was found between simulations and experiments for both the flow field and the reactive system.

Computational Fluid Dynamics for Engineers: Frontmatter

Computational fluid dynamics (CFD) has become an indispensable tool for many engineers. This book gives an introduction to CFD simulations of turbulence, mixing, reaction, combustion and multiphase flows. The emphasis on understanding the physics of these flows helps the engineer to select appropriate models with which to obtain reliable simulations. Besides presenting the equations involved, the basics and limitations of the models are explained and discussed. The book, combined with tutorials, project and Power-Point lecture notes (all available for download), forms a complete course. The reader is given hands-on experience of drawing, meshing and simulation. The tutorials cover flow and reactions inside a porous catalyst, combustion in turbulent non-premixed flow and multiphase simulation of evaporating sprays. The project deals with the design of an industrial-scale selective catalytic reduction process and allows the reader to explore various design improvements and apply best practice guidelines in the CFD simulations.

Computational Fluid Dynamics for Engineers by Bengt Andersson

Computational fluid dynamics (CFD) has become an indispensable tool for many engineers. This book gives an introduction to CFD simulations of turbulence, mixing, reaction, combustion and multiphase flows. The emphasis on understanding the physics of these flows helps the engineer to select appropriate models with which to obtain reliable simulations. Besides presenting the equations involved, the basics and limitations of the models are explained and discussed. The book, combined with tutorials, project and Power-Point lecture notes (all available for download), forms a complete course. The reader is given hands-on experience of drawing, meshing and simulation. The tutorials cover flow and reactions inside a porous catalyst, combustion in turbulent non-premixed flow and multiphase simulation of evaporating sprays. The project deals with the design of an industrial-scale selective catalytic reduction process and allows the reader to explore various design improvements and apply best practice guidelines in the CFD simulations.

Mathematical modeling in chemical engineering

A solid introduction to mathematical modeling for a range of chemical engineering applications, covering model formulation, simplification and validation. It explains how to describe a physical/chemical reality in mathematical language and how to select the type and degree of sophistication for a model. Model reduction and approximation methods are presented, including dimensional analysis, time constant analysis and asymptotic methods. An overview of solution methods for typical classes of models is given. As final steps in model building, parameter estimation and model validation and assessment are discussed. The reader is given hands-on experience of formulating new models, reducing the models and validating the models. The authors assume the knowledge of basic chemical engineering, in particular transport phenomena, as well as basic mathematics, statistics and programming. The accompanying problems, tutorials, and projects include model formulation at different levels, analysis, parameter estimation and numerical solution.

Multidimensional turbulence spectra – identifying properties of turbulent structures

Development of models for several phenomena occurring in turbulent single and multiphase flows requires improved description and quantification of the turbulent structures. This is needed since often the phenomena are very fast or nonlinear. Previously the authors have presented experimental measurements that show that the breakup of bubbles and drops in turbulence is due to interaction with single turbulent vortices. Hence, it is not sufficient to use average turbulence properties when developing models for CFD simulation of engineering applications. In this paper the results from analysis of individual turbulent structures are presented. Results from analysis of the turbulent kinetic energy in turbulent structures, using Eulerian vortex identification methods, are presented. The amount of turbulent kinetic energy associated with a coherent vortex defined using different vortex identification methods is quantified. It is shown that the peak turbulent kinetic energy is located near the edge of the region identified as coherent, making the analysis challenging and development of models difficult. However, detailed analysis of a small number of coherent vortices from LES of turbulent pipe flow reveals new information about their life history. The growth (i.e. entrainment of the surrounding liquid), enstrophy, lifetime, and energy of a specific coherent vortex are tracked over time.

Time Resolution in Transient Kinetics

This study presents the mathematical background of deconvolution of concentration data in transient kinetic studies. In a case study with a flow reactor setup, it has been shown that the deconvolution algorithm results in a significant reduction in the time lag of an FTIR detector from approx. 23 s to approx. 3 s. This is an important achievement as otherwise the dynamic information of a reactive system (like the rate of adsorption or accumulation of surface species) would have been lost during that time interval. Using the regularizing theory of ill-posed, inverse problems, an algorithm for deconvolution of concentration measurements has been developed based on the discrepancy principle. Our software package, TranKin, can be easily adapted to various other laboratory reactor systems to enhance the time resolution of transient experiments.

CFD Modeling of Thermoelectric Generators in Automotive EGR-coolers

A large amount of the waste heat in the exhaust gases from diesel engines is removed in the exhaust gas recirculation (EGR) cooler. Introducing a thermoelectric generator (TEG) in an EGR cooler requires a completely new design of the heat exchanger. To accomplish that a model of the TEG-EGR system is required. In this work, a transient 3D CFD model for simulation of gas flow, heat transfer and power generation has been developed. This model allows critical design parameters in the TEG-EGR to be identified and design requirements for the systems to be specified. Besides the prediction of Seebeck, Peltier, Thomson and Joule effects, the simulations also give detailed insight to the temperature gradients in the gas-phase and inside the thermoelectric (TE) elements. The model is a very valuable tool to identify bottlenecks, improve design, select optimal TE materials and operating conditions. The results show that the greatest heat transfer resistance is located in the gas phase and it is critical to reduce this in order to achieve a large temperature difference over the thermoelectric elements without compromising on the maximum allowable pressure drop in the system. Further results from an investigation of the thermoelectric performance during a vehicle test cycle is presented.

An integrated system for energy efficient exhaust after-treatment for heavy-duty vehicles

This chapter presents a unique system approach applied in a joint academic–industrial research programme, E4 Mistra, to attain the goals of high energy efficiency and low emissions in an exhaust aftertreatment system for heavy-duty vehicles. The high energy efficiency is achieved by heat recuperation, onboard hydrogen production for NO x reduction, and by finding new solutions for making the aftertreatment system active at low exhaust temperatures. To reach low particulate emissions, a mechanical filter using a sintered metal powder is developed and coated with catalytic material to improve the soot oxidation efficiency. Low NO x emissions are achieved by an efficient NO x reduction catalyst.

Computational Fluid Dynamics for Engineers: Turbulent mixing and chemical reactions

The purpose of this chapter is to give an introduction to problems faced by engineers wanting to use CFD for detailed modelling of turbulent reactive flows. After reading this chapter you should be able to describe the physical process of turbulent mixing and know why this can have an effect on the outcome of chemical reactions, e.g. combustion. The problem arises when the grid and time resolution is not sufficient to resolve the concentration and the average concentration in the cells is a poor estimation of the actual concentration as shown in Figure 5.1. The local concentration changes fast, and we need models that can predict the space- and time-average reaction rate in each computational cell. The average concentration in a computational cell can be used to describe macromixing (large-scale mixing) in the reactor and is relatively straightforward to model. The concentration fluctuations, on the other hand, can be used to describe micromixing (small-scale mixing on the molecular level). To quantify micromixing, the variance of the concentration fluctuations is used. Chemical reactions can take place only at the smallest scales of the flow, after micromixing has occurred, because reactions occur only as molecules meet and interact. An expression for the instantaneous rate of chemical reactions is often known for homogeneous mixtures. However, the average rate of chemical reactions in a reactor subject to mixing will depend also on the rate of micromixing.