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Session 2478

Computer Models Using Spreadsheets to Study Heat Engine Thermodynamics

Kenneth L. Tuttle U.S. Naval Academy

ABSTRACT Marine Power Systems is the second term of a two term course in thermodynamics at the U.S. Naval Academy. This is an applied thermodynamics course and is taught by the Marine Engineering faculty. One of the primary objectives of this thermodynamics course is to teach the thermodynamics of heat engines. Marine Power Systems takes the study of Heat Engine Cycles beyond the first step, introduction of how to work the Heat Engine Cycles. The Midshipmen at the Naval Academy write computer models of the heat engine cycles to study the thermodynamics of heat engines. The best example of the thermodynamic cycle modeled is the Brayton Cycle. The Marine Engineering students use a spread sheet program on their personal computers to model the Air Standard Brayton Cycle and run experiments by varying the independent variables.

INTRODUCTION Traditionally, engineering students learn most or all of the thermodynamic cycles that are in common use to model heat engines. They may learn to work around the Air Standard Cycle for Otto, Diesel and Brayton by assuming air behaves as an ideal gas with constant specific heats. They may also treat the working fluid as a real gas and use Gas Tables or in the case of the Rankine Cycle, Steam Tables. However, it is possible they may never exercise the models to run experiments because that requires working the cycles repeatedly. Learning to work all of the thermodynamic cycles fills up much of the course and working any of the cycles repeatedly is both laborious and very time consuming.

In order to learn what causes the cycle efficiency to increase and what does not, it is necessary to use the thermodynamic model to run experiments. The best way to study the thermodynamic cycles is to use the computer to solve the equations and find the net work and efficiency of the Air Standard Cycle. Assuming the working fluid is air and that it behaves as an ideal gas, allows the prediction of temperature in an isentropic process such as compression or expansion. Using constant specific heats it is possible to calculate the work and the cycle efficiency. Once the model is written, it is very easy to solve the cycle repeatedly using the computer while varying any independent variable of interest. Most of the work is in plotting the results of the experiments. The students learn the results of varying the parameters and at the same time develop a variety of computer skills.

The Marine Engineering Majors study applied thermodynamics in a second term course titled Marine Power Systems. They learn to work the ideal Otto, Diesel, Brayton and Rankine Cycles, even though they learned them to some degree during the first term of thermodynamics. In

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Tuttle, K. L. (1998, June), Computer Models Using Spreadsheets To Study Heat Engine Thermodynamics Paper presented at 1998 Annual Conference, Seattle, Washington. 10.18260/1-2--6975