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Thermoptim FAQ in English
Last modified:
11/08/2007 07:37 PM
Here are some of the most frequently asked questions on Thermoptim
What is the THERMOPTIM documentation composed of?Answer: The THERMOPTIM documentation is composed of several complementary sections: · User guides, including a 3-volume reference manual and manuals on the interactive charts. · The first volume of the reference manual presents the software, the diagram editor and the optimization method. The second volume deals with the simulator (screens of the basic components and the tools available in the modeling environment). The third volume explains how to design external components to extend the features of THERMOPTIM. · Quick start guides enabling the user to become familiar with THERMOPTIM quickly (in less than 30 minutes), either by exploring an existing model or by building one. · Finally, a short guide called Quick Reference is available from the Help menu, with tabs presenting the main concepts. Where is the THERMOPTIM documentation located?Answer: The THERMOPTIM documentation (pdf format) is located in the folder Doc_En in the software installation directory. It is available from the Help Menu of the simulator. There is also a short documentation named "Quick Reference" also available from the Help menu. What is a process-point?Answer: A process point is an "exchange" process whose upstream and downstream points are the same. The points may become different as the project evolves. For example if you connect another process, this changes the upstream point. The process-point then becomes a simple "exchange" process. These processes can be used to define fluid inlets or outlets, or to connect nodes. They are also useful when creating large size projects. How do I change the temperature units?Answer: In THERMOPTIM, the temperatures can be express in °C or in K. The temperature units are selected in the global preferences editor (Help menu of the project screen), but they revert to the default setting when the application is restarted. What do the different energy types represent?Answer: Each process has an energy type that can be either "purchased", "useful" or "other" energy. Purchased energy generally represents all of the outside energy you had to supply to the cycle. Useful energy represent the net balance of the cycle, in other words, the sum of energies produced and consumed. The efficiency of the cycle is defined as the ratio of "useful" energy to "purchased" energy. In a steam power plant, the "purchased" energy is the energy supplied to the boiler, and the "useful" energy is the difference between the energy produced by the turbine and the energy consumed by the pump to pressurize the water. "other" energy is the heat released in the condenser. In a compression refrigeration cycle, "purchased" energy is the energy consumed by the compressor, "useful" energy is the refrigeration effect (heat extracted by the evaporator) and "other" energy is the heat to be evacuated in the desuperheater and the condenser. Why are the points of a process not deleted when the process is deleted?How do I delete them?
Answer: When a process is deleted, the points are conserved to avoid losing their settings and so they can be used with other processes. The option "not connected" in the points diagnostics table (Special Menu > Diagnostic Tools) will find all the points not connected to a process. After identifying the points, you can delete them from their screen. How do I check that the simulator components and the diagram editor components are really the same?Answer: The Diagram / Simulator interface (Special Menu > Diagram / Simulator Interface) displays the list of components existing in both environments. Across from each one, an "X" in the "diagram" box indicates that it is present in the diagram editor, and an "X" in the simulator box indicates that it is present in the simulator. You can easily check whether your model is described consistently in both environments. Note: Some diagram editor components do not have any equivalent in the simulator (the texts, the balances, and the utilities) and the process-points and the exchange processes are differentiated in the diagram editor but not in the simulator. Why can't I connect two nodes directly?Answer: This is due to structural reasons. A node is defined logically by an association of processes (1-n). One process corresponds to the main vein, and n processes correspond to the branches. The processes are connected to points, and the points are connected to processes. If you connect two nodes directly, it would not be possible to define the intermediate point or the flow rate. Why does the diagram editor not automatically propagate the names of the substances in the combustion chambers, the mixers, and the external components?Answer: This is because in these components, the composition of the substances can change. This is always true in the combustion chambers, and sometimes in the mixers and external components. Consequently, you must specify the name of the new substance. Why doesn't the calculation of an exchanger stabilize right away?Answer: The exchangers are always designed with the implicit assumption that the thermophysical properties of the fluid remain constant throughout the exchanger, whereas this assumption is not made during the calculation of the processes. The result is that when you recalculate a temperature based on the exchanger equations, slight differences may exist between the value of the exchange module and the value of the corresponding process. If you want extremely good accuracy, you must iterate several times. Generally two or three times is enough. Why are there sometimes two similar substances in the pure gases and the vapors?Is there any difference between them and if so, what is it?
Answer: They are the same substances, but they are represented with different models. For example, the vapor "water" gives the properties of the water around its liquid-vapor equilibrium, whereas the pure gas "H2O" is modeled as an ideal gas (which never condenses). The gas "H2O" can be a component of any compound gas, as a combustion product for example, whereas the vapor "water" cannot be mixed with other substances. The zero reference for enthalpies of ideal gases is 1 bar and 25 °C, whereas it is specific for each vapor. Can you give a quick review of the concept of exergy?Can you provide a definition of exergy efficiency?
Answer: Exergy: H - T0.S situates a quantity of energy with respect to a temperature level T0. To illustrate, 100 kW at 20°C is not the same as 100 kW at 800 °C. You can't do the same thing with it. For an open system, the first two laws of thermodynamics taken together lead to the expression of the difference in exergy between the input and the output. This is: + useful (recoverable) work, + the quantity of heat multiplied by (1 - T0/Tc), + the generation of entropy. The multiplication factor (1 - T0/Tc) is the Carnot factor (efficiency of a Carnot cycle functioning between T0 and Tc). The exergy balance leads to adding actual work and an ideal work that you could obtain from a given quantity of heat. For a machine with a dual heat source, the energy efficiency is W/Q, and the exergy efficiency is the ratio of actual work to ideal work. Exergy efficiency is therefore deduced from the energy efficiency using the Carnot factor. The choice of the reference temperature T0 is difficult. The exergy efficiency is an intrinsic characteristic of a machine if we choose the cold source carefully (example discharge temperature for a gas turbine). How do I choose between polytropic efficiency and isentropic efficiency?Answer: There are two ways to determine whether a compression or expansion process is isentropic or not. Either you use the isentropic efficiency, which is the ratio of actual work to theoretical work for an expansion process and the opposite for a compression process. Or you use a polytropic exponent k such that pvk = cste. You can find the value of k from the inlets / outlets of the pevek = psvsk conversion determined using the isentropic efficiency approach above. I am having problems modeling combustion in THERMOPTIMI get a message error: "there is no oxygen in the reagents" when I ask it to calculate, whereas my combustion model does have an inlet port and a perfect air intake port exiting the compressor.
Answer: For THERMOPTIM to be able to calculate a combustion, the oxidant or the fuel must contain oxygen. The substance "perfect air" is a very special pure gas, which was included only to enable users to perform calculations on perfect gases. It does not contain any oxygen. You have to chose the protected compound gas "air" or "atmospheric air", or create a moist air compound gas if you want to take into account the humidity of the air. Another common error when modeling combustion is to chose a vapor instead of the corresponding ideal gas as a fuel, for example the vapor "methane" instead of the pure ideal gas "CH4-methane". THERMOPTIM calculates the combustion by decoding the chemical formula of the different components of the ideal gases representing the oxidant and the fuel. What is meant by primitive or basic types?Answer: The primitive types of THERMOPTIM are its basic components, namely substances, processes, nodes, and heat exchangers. What is meant by an invalidated type or a calculable type?Answer: A primitive type can be calculable or not. It is calculable if all the information required to calculate it is known and valid. There are two types of information required: its own characteristics, and those coming from other primitive types. A primitive type can be validated or invalidated. It is validated if it was calculable and it has been calculated. It becomes invalidated when any of the information required for its recalculation is modified. A primitive type has prerequisites and/or subordinates. Its prerequisites are the other primitive types that contain part of the information it needs to be calculated, and must therefore be validated in order for it to become calculable. Its subordinates are the other primitive types of which it is a prerequisite. A type can become invalidated in two ways: either when one of its own characteristics is changed, or when at least one of its prerequisites is invalidated. It is calculable when all of its prerequisites are validated. Substances do not have any prerequisites, and certain types have no subordinates. A project tracks the evolution of invalidated types. When you click the Recalculate button in the project or recalculation window, all of the invalidated types that are calculable are calculated. Why is it, that during the convergence of the recalculation engine, there are always a certain number of calculable, yet invalidated values outputted?Answer: The recalculation engine computes a complete set of calculations and then stops. Every time the engine is restarted, it recomputes all the invalidated computer-calculable parameters, which results in invalidating some of the others. So, for this reason, it is quite intentional that Thermoptin does not iterate automatically. It may happen that the computer, after a recalculation, does not change the previous output values of the parameters but does change the number of computable but invalidated parameters.Answer: Indeed, this means that the calculations have been carried out without changing the energies of the system. What is the benefit in locking and stopping the recalculation engine, thus preventing the propagation of the new modification, and doing the calculation with old information?Answer: It is simply to renew the tables of invalidated types. What is the order of the recalculation?Answer: This is explained in the second volume of the Thermoptim handbook. What is a multi-functional component?Answer: At the beginning of Thermoptim, the only components available in the software were single-function. These components are so-called single-function because they are related either to a mechanical energy, either to a thermal energy. For instance, the following devices are called single-function: compressors, nozzles for adiabatic expansion (by far the most known and widespread), nozzles for isenthalpic expansion, heat-exchangers, and combustion chambers. Multi-functional components are more sophisticated. They can handle several thermal couplings at various temperature levels. In the previous versions of the software, it was not possible to describe such components. What is a thermocoupler?Answer: The thermocoupler system completes the heat exchanger system by allowing components other than exchange processes to connect to one or more exchange processes to represent a thermal coupling. This system is different from the exchanger system: two exchange processes cannot be connected by a thermocoupler. This system has a number of benefits, because it can be used to represent many thermocouplers that do not constitute a heat exchange in the traditional sense, like for example cooling the walls of the combustion chamber of a reciprocating engine, cooled compression, and above all supply or removal of heat from the multi-functional external components. |
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