Thermal models

Thermal Modeling of Petroleum Generation Theory and Applications... 

The first step in the seleetion proeess is to determine the maximum i DS(on) for the desired MOSFETk. This is done by examining the thermal model (see Appendix A). The maximum i DS(on)is found by... 

Figure A-1 Development of the thermal model for power packages.

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. 

Figure A-4 Thermal model of a free-standing power package.

The thermal model for the case in Figure A-3 is shown in Figure A-4. The thermal equation becomes... 

Figure A-6 The thermal model for an axial-leaded diode.

The thermal model is that of Figure A-4, and the thermal equation is Equation A.2 rearranged to... 

This situation would fit the thermal model as seen in Figure A-7b. It does not matter in this ease not all the elements of the model are known sinee all the elements above the lead temperature node are known for this first step. The thermal expression for the temperature rise above the measured lead temperature is... 

The output of this task could, in addition to the list of properties, also include a more sophisticated building model, in which collected properties are built in infiltration model, thermal model, etc. 

Four methods for industrial air technology design are presented in this chapter computational fluid dynamics (CFD), thermal building-dynamics simulation, multizone airflow models, and integrated airflow and thermal modeling. In addition to the basic physics of the problem, the methods, purpose, recommended applications, limitations, cost and effort, and examples are pro vided. 

In an actual design, thermal modeling (Section 11.3) for diffetent seasoii-s will come fitst to. set tempetatute boundary conditions. Multizone aitflow simulation (Section 11.4) will follow to define ventilation needs in each zone. For large enclosed space.s, for natural ventilation, and for a variety of other special problems, CFD (Section 11.2) and integrated modeling (Section 11..S) are applied. 

Flowever, with CFD, configurations with mostly known or at least steady-state boundary conditions and surface temperatures are calculated. In cases where the dynamic behavior of the building masses and the changing driving forces for the natural ventilation are of importance, thermal modeling and combined thermal and ventilation modeling mu.st be applied (see Section 11..5). 

Many natural ventilation problems are related to the thermally driven air exchange in a building. Such cases must be most often treated using combined thermal and ventilation models or thermal models with an integrated natural ventilation model (see Section 11.5). For example, in COMIS, a simple, single-zone thermal model is included for transient single-sided ventilation calculations. 

Without a thermal model, energy studies are also very limited (see Section 11.5). 

This section deals mainly with the interaction of thermal models as outlined in Section J 1.3 and airflow models as described in Section 11.4 for the purpose of integrated modeling of thermally induced (stack-driven) natural ventilation, governed by the thermal behavior of the building. For the integrated analysis ol air velocity fields and radiative and thermal effects in the building using CFD codes, see also Section 11.2 and Ott and Schild.-... 

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). 

I FIGURE I (.49 In thermal models, the ventilation airflow rates are input parameters in airflow models, on the other hand, room air tmiperatures must be defined In the output. Since natural ventilation airflows and room air temperatures are interdependent, both parameters must be intcgratly considered in the solution process. This is possible only by an integration of the natural airflow model into the thermal model. 

However, in thermally driven naturally ventilated enclosures, airflow rates and room air temperatures are interdependent and can be determined only simultaneously by using an integrated airflow and thermal model. 

For the data exchange between the airflow and the thermal model, sevenil levels can be distinguished ... 

Integration. Here, at a certain time step in the simulation, the airflow model is repeatedly called within the iterative solution process in the thermal model, and the airflow results are considered within this iterative solution process. Thus, the resulting airflows and room air temperatures fully comply in terms of the underlying physics. 

Also, for this option, several levels can be observed concerning the integration of the airflow model. The lowest level of integration still requires separate input for the thermal and the airflow model respectively with a high degree of redundancy in the input parameters (e.g., zones must be input for both models), and the connectivity of the airflows and the zones in the thermal model must be established manually by the user. More sophisticated levels have reduced redundancy and automatic establishment of the link connectivity. 

A higher level of integration is achieved when the airflow model is fully integrated in the thermal model and the input for the airflow part is just an addition to the input required for the thermal model. 

The highest level of integration would be to establish one large set of equations and to apply one solution process to both thermal and airflow-related variables. Nevertheless, a very sparse matrix must be solved, and one cannot use the reliable and well-proven solvers of the present codes anymore. Therefore, a separate solution process for thermal and airflow parameters respectively remains the most promising approach. This seems to be appropriate also for the coupling of computational fluid dynamics (CFD) with a thermal model. ... 

II rime-dependent spatial airflow and temperature distributions in a room are requested, the coupling of CFD with a thermal model has to be considered. ... 

Input is required for both the thermal model and the airflow model. Input parameters are identical to the ones outlined for the thermal model in Section 11.3.3 and for the airflow model in Section 11.4.3. Depending on the data-exchange mode, additional information on the data links between thermal and airflow models must be given. 

The multizone airflow model COMIS is adapted as a TRNSYS type, to be used in combination with the TRNSYS Type 56 thermal multizone building model. Input is somewhat redundant. Separate input files are necessary for the thermal model and the ventilation model, but not for meteorological and link schedules. 

LESOCOOL is an easy-to-use computer program for the determination of passive cooling potential by nighttime ventilation. 21 [t is based on a simple combined airflow and thermal model, originally described in Van der Maas and Roulet. -... 

The relationships between air exchange rate and temperature difference were determined using COMB (Fig. 11.51) and then integrated as the ventilation model in the thermal model. The rhermai behavior is modeled with the TRNSYS multizone type, considering the hall and the room below the thick concrete test floor slab. For the hall, a room model with two air temperature nodes (one for the occupied zone and one for the rest of the hall) and geometrically detailed radiation exchange is used. 

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. 

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