Thermal and flow models

The combined flow and thermal models can be a powerful tool for addressing various SOFC design issues. For example, during fast startup or fast cool-down, which may be needed in automotive applications, thermal stresses that develop within the fuel-cell stack must not exceed acceptable levels. It is therefore necessary to model in detail the gas flows as well as heat and mass transfer throughout the fuel-cell stack to analyse the transient temperature distribution. The latter, in turn, may be used to predict the thermal stresses. 

Combining the above rate equations and mass balances with the flow and thermal model equations presented in Section 11.2, detailed information about the variation of gas composition, fuel or oxidant utilisation, etc., in the flow channels may be generated. 

The coupled continuum-level electrochemical, flow, and thermal models are usually discretised in a finite element mesh [23,24]. When the necessary material properties, geometrical parameters, operating parameters, and boundary conditions are supplied, cell- and stack-level performance can be analysed. The combined models can determine the cell/stack voltage, the total current output, temperature distribution, species concentration, etc. 

The heat source terms discussed above are used in the energy balance equations of the flow and thermal models for both cell and stack simulations. To manage the computation cost, most simulations avoid the full CFD treatment of the flow. However, full CFD treatments provide more accurate and detailed information on the flow in an SOFC system. With increased computing power, more and more full CFD simulations are expected to appear. 

FIRAC is a computer code designed to estimate radioactive and chemical source-terms as.sociaied with a fire and predict fire-induced flows and thermal and material transport within facilities, especially transport through a ventilation system. It includes a fire compartment module based on the FIRIN computer code, which calculates fuel mass loss rates and energy generation rates within the fire compartment. A second fire module, FIRAC2, based on the CFAST computer code, is in the code to model fire growth and smoke transport in multicompartment stmetures. 

Steam generator primary side (one steam generator model with bypass valve flow and thermal characteristics). 

An alternative method was introduced using a hybrid model. In the hybrid model, a FLUENT sub-model was used to model glass flow and thermal conditions with plimger motion from the entrance of the glass tank to the orifice outlet. The transient temperature and velocity at the FLUENT/POLYFLOW interface are then mapped to a POLYFLOW sub-model as time-dependent boimdary conditions. The glass gob free surface deformation process was modeled in the POLYFLOW sub-model. 

Injection molding processing involves both a molten flow phase and a solidification phase. The major challenges of constitutive modeling of the liquid-solid transition involve two related topics. First, one needs to eonsider how the flow and thermal history influence the structure of the fluid. Seeondly, one needs to understand how the ehanges in the internal strueture stiffen the material. There are some investigations on the topics for semicrystaUine materials and a variety of approaches or models have been proposed by different authors. Most of these results are reviewed by Tanner and Qi (2005) and Pantani et al. (2005). However,... 

The coupling of flow and thermal effects with a temperature-dependent viscosity can lead to situations in which more than one solution to the model equations is possible, suggesting that the process can exist in more than one state for a given set of flow conditions. This phenomenon of multiplicity is often a surprise when first encountered, but it is a common characteristic of nonlinear systems. In combustion, for example, it is known that there is a finite range of flow conditions for which the system can exist in either of two states a low-temperature, low-conversion state, and... 

Specific reactor characteristics depend on the particular use of the reactor as a laboratory, pilot plant, or industrial unit. AH reactors have in common selected characteristics of four basic reactor types the weH-stirred batch reactor, the semibatch reactor, the continuous-flow stirred-tank reactor, and the tubular reactor (Fig. 1). A reactor may be represented by or modeled after one or a combination of these. SuitabHity of a model depends on the extent to which the impacts of the reactions, and thermal and transport processes, are predicted for conditions outside of the database used in developing the model (1-4). 

Every thermal model has as its ground, the ambient air temperature, unless the heat removing medium is water or a refrigerant, in whieh ease the ambient temperature of that medium is used. This must be the ease, sinee the power pro-dueing deviee ean be no eooler than the eoolest media around it and sinee heat flows from the warmer to the eooler body. 

In Gaussian plume computations the change in wind velocity with height is a function both of the terrain and of the time of day. We model the air flow as turbulent flow, with turbulence represented by eddy motion. The effect of eddy motion is important in diluting concentrations of pollutants. If a parcel of air is displaced from one level to another, it can carry momentum and thermal energy with it. It also carries whatever has been placed in it from pollution sources. Eddies exist in different sizes in the atmosphere, and these turbulent eddies are most effective in dispersing the plume. 

This section does not contain any fundamentals or mathematics bur tries to describe the basic energy flows and the methods used in thermal building-dynamics simulation codes to model these. Also, the methods are described without stating the underlying algorithms and equations, for which the reader is referred to the literature and references. A short outline of how these models affect the application possibilities and limits is given at the end of this section and also in Section 11.3.7. 

In thermal models, the ventilation airflow rates normally arc input parameters, to be defined by the user or to be calculated by the program on the basis of a nominal air exchange or flow rate) and some control parameters (demand-controlled ventilation, variable air volume flow ventilation systems), in airflow models, on the other hand, room air temperatures must be defined in the input (see Fig. 11.49). 

For the determination of downdraft risk in the winter case, three-dimensional and transient CFD computauons were performed using the TASC flow code. Boundary conditions were defined from the results of the thermal modeling. 

CFD is appropriate in cases where the detailed flow field is of interest in a configuration with mostly known or at least steady-state boundary conditions (surface temperatures). Combined thermal and ventilation modeling is more suited to cases where the dynamic behavior of the building masses and the changing driving forces for the natural ventilation are of importance. 

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