Understanding Turbine Flow on a Mollier Diagram

A Mollier diagram, also known as an enthalpy-entropy chart or h-s diagram, is a thermodynamic chart that plots total heat against entropy. It is widely used in engineering, especially in the design and analysis of turbines, compressors, and other thermodynamic systems.

Let's consider the flow through a turbine. A turbine is a device that extracts work from a fluid stream. This process typically involves the expansion of the fluid, causing its pressure and temperature to decrease. In an ideal turbine process, this expansion is considered isentropic.

What is an Isentropic Process?

An isentropic process is a thermodynamic process where the entropy of the fluid remains constant. In other words, it is a reversible adiabatic process (no heat transfer and no irreversibilities like friction or turbulence).

Representing Processes on a Mollier Diagram

The axes of a Mollier diagram are enthalpy (h) on the y-axis and entropy (s) on the x-axis. Key thermodynamic properties like pressure and temperature are shown as lines on the chart.

• Lines of constant enthalpy are horizontal.
• Lines of constant entropy are vertical.
• Lines of constant pressure and constant temperature (often coinciding in certain regions) are also shown as curves.

Ideal Turbine Expansion on a Mollier Diagram

In an ideal turbine, the expansion process is assumed to be isentropic. Since entropy remains constant during an isentropic process, the path followed by the fluid on an h-s (Mollier) diagram is a line of constant entropy. Lines of constant entropy on a Mollier diagram are vertical.

Therefore, the flow through a turbine, when considered ideally (isentropic expansion), is represented by a vertical line on a Mollier diagram, starting from a higher enthalpy and pressure state and ending at a lower enthalpy and pressure state at the same entropy value.

Real Turbine Expansion

In reality, turbine expansion is not perfectly isentropic due to irreversibilities. A real process involves an increase in entropy. On a Mollier diagram, this would be represented by a line starting at the initial state and moving downwards and to the right (increasing entropy) to the final state.

However, the question asks for the representation, and often in basic thermodynamic analysis or when discussing the ideal case for comparison, the isentropic process is the primary representation considered.

Analyzing the Options

Let's look at the given options:

• Curved line convex up: This does not typically represent an ideal turbine process.
• Curved line convex down: This could represent a real, irreversible expansion process with increasing entropy, but not the ideal isentropic flow.
• Horizontal line: A horizontal line represents constant enthalpy. This is not representative of a turbine process where work is extracted, leading to a decrease in enthalpy.
• Vertical line: A vertical line represents constant entropy, which corresponds to an ideal isentropic expansion process, typically used to represent the flow through a turbine in its ideal form on a Mollier diagram.

Based on the definition of an isentropic process and the construction of a Mollier diagram, the ideal flow through a turbine is represented by a vertical line.