Presentation of the exercise
This exercise is primarily intended to illustrate how to calculate adiabatic compressions and expansions. It is centered around the study of a small capacity gas turbine, called micro-turbine. This micro-turbine is an existing machine, used for several applications, including cogeneration.
In session S46En we will give a cogeneration facility example based on this machine.
This exercise is divided into three parts:
- in the first part (session S21En) we assume that the machine is traversed by a single gas, air, initially modeled as a perfect gas, and in a second time as an ideal gas. The resulting model is obviously simplistic, but it is interesting for beginners, because it allows them to make the connection between analytical theoretical calculations and resolution in Thermoptim.
- in this part (session S22En), we consider the combustion reaction that takes place in industrial machines. The model is much more realistic. The comparison with the analytical model allows us to analyze its limits
- in the third part(session S23En), we analyze quantitatively the irreversibilities of the cycle by setting up its exergy balance, and deduce ways of improving the machine cycle (regeneration).
Before starting working with Thermoptim, we strongly recommend that you study session S07En_init, which introduces all the concepts which you will be using.
(Session realized on 06/16/11 by Renaud Gicquel)Thermoptim
Thermoptim
Thermoptim
Comparison with the ideal gas model
This session enabled you to build a realistic model of gas turbine, taking into account the combustion reaction.
You can now compare the results with the previous model (session S21En), where we assumed that the machine was traversed by air, without change in composition.
The following table summarizes the results:
- in terms of net mechanical power, the error of the perfect gas model (PG) is 11.5% and that of the ideal gas model (IG) 6.7% compared to the model with combustion (C)
- in terms of thermal power provided to the cycle, they are respectively 15.6% and 6, 9%
- errors on the efficiency remains low, however
- the errors on temperature differences in the compressor are much lower than in the turbine, where they reach high values (14% for the PG model
In conclusion, even if the model (IG) is not too wrong, as we dispose of a modeling environment as Thermoptim, it is preferable to build a model as accurate as possible: it is no more difficult and there is no error.
Thermoptim project and diagram files are provided in the attached archive.
Review the summary of session 21En to remember the importance of looking critically at the results provided by a software package, and possibly refer to the note entitled “Model construction and verification methodology” (see link below).