Basic Engineering Science – A Systems, Accounting, and Modeling Approach (Richards)

Classification by Working Fluid

If you examine the various devices mentioned earlier that operate as thermodynamic cycles, you discover that they all operate by changing the properties of a substance inside the closed system. This substance is called a working fluid. All of the examples presented earlier fall into one of two categories. In the cycles for the automobile engine or the jet engine, the working fluid remains a gas throughout the entire cycle. These are examples of cycles that operate with a single-phase working fluid. The cycles that form the basis for most refrigerators and for the fossil-fueled steam power plants change the phase of the working fluid from a liquid to a vapor and then back to a liquid in the cycle.

These are examples of cycles that operate with a two-phase working fluid. These distinctions will not be very significant to us this quarter; however, to the designer they have a great significance in determining the size, weight, cost, and performance of a specific cycle.

Classification by Physical Structure

Most devices that operate as or can be modeled as a thermodynamic cycle have a physical structure that fits into one of two categories-a closed, periodic cycle or a closed-loop, steady-state cycle. These are illustrated in Figure 7-23.

The closed, periodic cycle is modeled as a fixed quantity of matter contained inside of a simple piston cylinder device [see (a)]. This cycle is characterized by spatially uniform intensive properties that vary periodically with time. This is the classic cycle that has been studied by engineers for years. It is the model for the early steam engines and is still the model for the modern internal combustion engine where a gas is compressed and expanded within a piston engine.

What do you think?

The claim is made that work is more valuable than heat transfer. Can you think of any system and process that can only be done by a heat transfer of energy? [If you can, could the system be redefined and the heat transfer be replaced by a work transfer of energy? The net heat transfer (or transfer rate) of energy into the system equals the net work transfer (or transfer rate) of energy out of the system for thermodynamic cycle.

This result leads to the common interpretation of a thermodynamic cycle as an energy conversion device for converting heat transfer of energy into work transfers of energy.

Classification by Purpose

Experience has shown that work transfers of energy are more valuable than heat transfers of energy. This means that we can do more things with a work transfer of energy than we can with a heat transfer of energy. Because of this, we will choose to define the purpose of a device in terms of the work transfer of energy for the cycle.