2025 ASEE Annual Conference & Exposition

A major emphasis of engineering thermodynamics courses is open-system component analysis and the application of Brayton, Rankine, and refrigeration cycles that model gas turbines, steam power plants, and refrigeration plants. Students are challenged by these problems since they must simultaneously keep three skills in mind at the same time: the component being analyzed (e.g. nozzle), the engineering model being applied to that component (e.g. reversible adiabatic expansion), and the procedure for analyzing working fluid properties (e.g. air table procedures or ideal gas isentropic process relationships).

We present a new pedagogical approach to open-system component, gas turbine, and steam plant analysis centered around the pressure-enthalpy (p-h) diagram. This approach was inspired by the extension of p-h diagram analysis from refrigeration cycles to steam cycles presented in “Engineering Thermodynamics a Graphical Approach” by Israel Urieli. To our knowledge, the further extension of the p-h diagram approach to gas turbine analysis is new in engineering pedagogy. We find that this approach is more digestible to students and allows for a deeper understanding of the limitations imposed by the first and second laws of thermodynamics.

Our approach begins with an analysis of nozzles, diffusers, turbines, compressors, and valves/constrictions on a p-h diagram for air. The change in enthalpy, corresponding to the horizontal separation on the p-h diagram, is equal to the associated work, heat, or change in kinetic energy once typical simplifying assumptions are applied. The visual association of horizontal separation with energy transfer allows students to more easily understand the processes occurring in these devices. For example, it is easily observed that an isentropic turbine produces the largest physically possible horizontal separation on the p-h diagram and therefore the largest possible work. Isentropic efficiency is easily visualized as a ratio of lengths on the p-h diagram.

On a typical problem, the students draw a component level diagram with state points, they translate that diagram onto a p-h diagram and then read off the fluid properties they need for any cycle calculations. This draws a strong connection between the physical system and the evaluation of the working fluid properties and reduces the cognitive switching penalty associated with a traditional approach. The p-h diagram approach also allows for a quicker path to understanding how operational and/or design changes influence engine performance. For example, it is visually clear that increasing the pressure ratio of a turboshaft increases the thermal efficiency of the engine. Another advantage of using the p-h diagram approach is that there is a common thread for gas turbines, steam plants, and refrigerators that makes each feel like a small extension of the previous cycle rather than a completely new approach.

As a quantitative measure of effectiveness, we compare final exam performance data for 330 students using a classical approach to open system components and cycle analysis to 370 students solving the same problems using the new p-h diagram approach.