Thomas Joseph Condra

Numerical Modeling of Thermoelectric Generators With Varing Material Properties in a Circuit Simulator

When a thermoelectric generator (TEG) and its external load circuitry are considered together as a system, the codesign and co-optimization of the electronics and the device are crucial in maximizing the system efficiency. In this paper, an accurate TEG model is proposed and implemented in a SPICE-compatible environment. This model of thermoelectric battery accounts for all temperature-dependent characteristics of the thermoelectric materials to include the nonlinear voltage, current, and electrothermal coupled effects. It is validated with simulation data from the recognized program ANSYS and experimental data from a real thermoelectric device, respectively. Within a common circuit simulator, the model can be easily connected to various electrical models of applied loads to predict and optimize the system performance.

Further study of the gas temperature deviation in large-scale tangentially coal-fired boilers☆

Gas temperature deviation in upper furnace is an important but a less reported issue in large-scale tangentially fired boilers, since they endanger largely boilers operation. Simulations are conducted in this paper to study the deviation. Perfect agreement between the simulation results and key boiler design values and available site operation records indicates that the calculations are reliable. Based on the simulations, effect of some factors, including residual airflow swirling at furnace exit, super-heaters panels, coal particle trajectories and their combustion histories, on temperature deviations are studied in details. The most important cause and how it affects the temperature deviation are located. Two new methods, a nose on front-wall and re-arranged super-heater panels, are put forward unprecedentedly to alleviate the deviations.

Irreversible transfer processes of thermoelectric generators

We discuss a novel tool based on heat flow diagrams for analyzing irreversible processes associated with thermoelectric devices and discuss some ambiguous descriptions and errors in related investigations. We consider thermoelectric generators as a paradigm of a heat engine cycle and determine the heat flow distribution by treating the one-dimensional heat transfer differential equation. Representative heat flow diagrams are used to study the influence of internal and external irreversible processes of heat conduction and Joule heat generation.

Transient Behavior Study of Thermoelectric Generators through an Electro-thermal Model Using SPICE

A thermoelectric generator (TEG) usually works in dynamic operating conditions due to the time change, in real applications, of the electric load and hot or cold temperatures. Thus understanding transient thermal and electrical behavior of the device, besides the steady-state behavior, is important in order to investigate the global device performance. The major objective of this work is to describe the transient behavior of TEG operating in high temperature environments through a SPICE model based on an electrothermal analogy. The SPICE model presented is derived from a one dimensional (1-D) heat transfer differential equation. An important feature of the model is its ability to calculate the temperature profile taking the real temperature dependence of the materials properties into account. This feature is essential in simulating TEG exposed to a large temperature difference. In combination with considering the finite heat transfer rate at the interface between TEG and thermal ambient, the model is able to reflect the thermo-electric coupled multi-field system effect of TEG. A test rig is developed for verifying the proposed model. Commercially available TEG is tested with respect to stabilizing time under sharply changed electric load. The preliminary results of experiments and modeling are analyzed. It is expected that the model presented can assist, not only in the optimal design of TEG itself, but also in the evaluation of the whole energy system

Multi-physics simulation of thermoelectric generators through numerically modeling

The governing equations taken from the assumption of local equilibrium and the heat transfer rate form of Onsager flux have been compared with those based on classical heat transfer formulation by a simplified one dimensional (1-D) thermoelectric generator (TEG) model. In this paper, the simulation of coupled multi-physics effects in a TEG is realized in a three dimensional (3-D) way, based on the heat transfer formulation, through finite-difference numerical method and PSPICE computational tool. The feature to take the real temperature dependence of the materials properties into account is included in the computation.

Solving differential-algebraic equation systems by means of index reduction methodology

Abstract With the overall goal of optimizing the design and operation of steam boilers, a model for optimizing the dynamic performance has been developed. The model has been developed as three sub-models that are integrated into an overall model for the complete boiler. Each of the sub-models consist of a number of differential equations and algebraic equations—a so called DAE system. Two of the DAE systems are of index 1 and they can be solved by means of standard DAE-solvers . For the actual application, the equation systems are integrated by means of MATLAB’s solver: ode23t, that solves moderately stiff ODEs and index 1 DAEs by means of the trapezoidal rule. The last sub-model that models the boilers steam drum consist of two differential and three algebraic equations. The index of this model is greater than 1, which means that ode23t cannot integrate this equation system. In this paper, it is shown how the equation system, by means of an index reduction methodology , can be reduced to a system of ordinary differential equations—ODEs.

Fitting a Three Dimensional PEM Fuel Cell Model to Measurements by Tuning the Porosity and Conductivity of the Catalyst Layer

A three-dimensional, computational fluid dynamics (CFD) model of a PEM fuel cell is presented. The model consists of straight channels, porous gas diffusion layers, porous catalyst layers and a membrane. In this computational domain, most of the transport phenomena which govern the performance of the PEM fuel cell are dealt with in detail. The model solves the convective and diffusive transport of the gaseous phase in the fuel cell and allows prediction of the concentration of the species present. A special feature of the model is a method that allows detailed modelling and prediction of electrode kinetics. The transport of electrons in the gas diffusion layer and catalyst layer is accounted for, as well as the transport of protons in the membrane and catalyst layer. This provides the possibility of predicting the three-dimensional distribution of the activation overpotential in the catalyst layer. The current density dependency on the gas concentration and activation overpotential can thereby be addressed. The proposed model makes it possible to predict the effect of geometrical and material properties on the fuel cell’s performance. It is shown how the ionic conductivity and porosity of the catalyst layer affects the distribution of current density and further how this affects the polarization curve. The porosity and conductivity of the catalyst layer are some of the most difficult parameters to measure, estimate and especially control. Yet the proposed model shows how these two parameters can have significant influence on the performance of the fuel cell. The two parameters are shown to be key elements in adjusting the three-dimensional model to fit measured polarization curves. Results from the proposed model are compared to single cell measurements on a test MEA from IRD Fuel Cells.© 2004 ASME

Parametric CFD Analysis to Study the Influence of Fin Geometry on the Performance of a Fin and Tube Heat Exchanger

Heat transfer and pressure loss characteristics of a fin and tube heat exchanger are numerically investigated based on parametric fin geometry. The cross-flow type heat exchanger with circular tubes and rectangular fin profile is selected as a reference design. The fin geometry is varied using a design aspect ratio as a variable parameter in a range of 0.1-1.0 to predict the impact on overall performance of the heat exchanger. In this paper, geometric profiles with a constant thickness of fin base are studied. Threedimensional, steady state CFD model is developed using commercially available Multiphysics software COMSOL v5.2. The numerical results are obtained for Reynolds number in a range from 5000 to 13000 and verified with the experimentally developed correlations. Dimensionless performance parameters such as Nusselt number, Euler number, efficiency index, and area-goodness factor are determined. The best performed geometric fin profile based on the higher heat transfer and lower pressure loss is predicted. The study provides insights into the impact of fin geometry on the heat transfer performance which help escalate the understanding of heat exchanger designing and manufacturing at a minimum cost.

Validation of guidelines for cfd modelling of a single tube row and in-line tube bundles

This paper questions and improves commonly used guidelines for modelling a tube bundle in cross-flow at ReD = 3.4 · 104 and ReD = 1.1 · 105. Especially, when the locations of flow separation are of high interest. A major conclusion of this paper is that near-wall modelling (y+ 5 should be avoided in relation to flow separation behind tubes in cross-flow. CFD modelling of a tube bundle may be simplified with the use of symmetric or periodic boundary conditions to account for the full geometry. The present work reveals periodicity in vorticity formation between a double cylinder row, though the wake region behind a single cylinder row is neither characterised as in-phase nor reversed phase. Likewise, periodic boundary conditions may result in a modelling with large wake deflections for a full tube bundle. Furthermore, since there is no unequivocal answer to which turbulence model to apply for tubes in cross-flow, the RNG k-e, Realizable k-e, SST k-ω, and RSM turbulence models are tested and compared.Copyright

Lagrangian prediction of particulate two-pase flows at high particle loadings

The Lagrange method for numerical prediction of particulate two-phase flows is discussed and reviewed. The numerical models used in calculating the particle trajectories, wall collisions and turbulence based movement, are presented. A venturi test case with high particle loading is treated and numerical and experimental results are compared. A circulating fluidised bed pre-separator is analysed and flow-particle coupled results at particle loadings up to 10 kg particles per kg air are presented. Convergence difficulties are discussed and a source smoothing method is introduced. Numerical and experimental results are compared and the reasons for the differences treated.