An Introduction to Turbomachinery 2: Gas Turbine Engines

Gas turbines are a broad branch of mechanical devices whose thermodynamic analysis differs based on the application of the mechanism. For this reason, it is crucial to understand the working of each process and stage of the cycle upon the given parameters. 

The Rankine Cycle Proposition

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Rankine cycle is the most basic form of an ideal thermodynamic process on which the entire system works. The above illustration depicts the relation between enthalpy and entropy, along with the various stations of the cycle. Using the graph, one can calculate the amount of heat exchange, the work done by the system, and the efficiency of the machine. 

Here's a general workflow:

  • Process 1-2 represents an isentropic heat into the system.
  • Process 2-3 is an isobaric heat addition, also known as the combustion process.
  • Process 3-4 is an isentropic heat subtraction.
  • Process 4-1 is the activity to return to ambient conditions.

The efficiency is the ratio of the amount of work done by the system to the amount of energy used by the system. 

The heat supplied into the system can be calculated by the subtraction of enthalpy from processes 3 to 4. This is essential as the system parameters will be based on the results of this process.  

The prime application of the Rankine Cycle is seen in steam turbine engines. 

The Simple Impulse Turbine

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The illustration depicts the working of a simple impulse turbine that converts high-pressure steam into useful mechanical energy. 

The steam will enter with a low velocity and a high pressure-head value. The nozzle helps to increase the subsonic velocity of the fluid into supersonic velocity. This fluid then reaches the rotor blades and transfers the gained energy.

The lost velocity is marked on the graph below the labeled diagram. The thick "C" sections on the illustration are the orientation of the blades on the axis. The left-hand side of the diagram is known as the inlet or the leading edge. The right-hand side is known as the outlet or the trailing edge. 

It is to be noted that the pressure and velocity changes occur on a gradual and stage-wise process, as there are stationary and non-stationary blades present in the turbine. 

A Birds-Eye-View of the Gas Turbines

Gas turbines deliver mechanical power in the form of thrust, with jet engines being the prime example. Industrial gas turbine engines are used to generate electricity and power. 

The gaseous fluid exiting the nozzle of the gas turbine engine is used to generate thrust. Since thrust is a force, the unit of measurement of thrust is considered as Newton (N). 

The major applications are in the field of aviation, power generation, in the oil and gas industry, and marine propulsion, to name a few. 

Open Cycle

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The above illustration is an open circuit gas turbine plant with the inlet and exhaust in ambient conditions. Here's a breakdown of the process:

  • The stream is first passed through the compressor, where the velocity reduces, and the pressure increases.
  • The stream then enters the combustion chamber, where it burns along with the fuel with known calorific value.
  • After combustion, the stream then increases in pressure after passing through the low and high-pressure turbine stages and then allowed to enter back to the ambient conditions.

Closed Cycle

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In the closed cycle gas turbine plant, the setup and process of the energy exchange are identical to the open cycle. The difference is the addition of heat exchangers that keep the process as an endless cycle. 

The heat exchangers work in opposites. The first heat exchanger heats the incoming stream, while the second heat exchanger cools down the incoming stream. The addition of the heat exchangers may increase the overall efficiency of the powerplant. 

The Brayton or Joule Cycle

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The more commonly known Brayton Cycle is the ideal representation of the parameters in the process of the gas turbine engines. The process is as follows:

  • Process 1-2 is compression of the stream isentropically.
  • Process 2-3 is heat addition in the stream (known as combustion) isobarically.
  • Process 3-4 is the expansion of the stream (known as the turbine section) isentropically.
  • Process 4-1 is the heat reduction in the stream isobarically. If this process is neglected, then the cycle follows an open cycle configuration.

The efficiency of the Brayton Cycle is known to be 100% as the stream obeys the ideal gas laws. The processes mentioned form the working principle of the Brayton Cycle, with the compressor, the burner, or the combustor, and the turbine making up the three most essential elements. 

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The above schematic is a typical gas turbine with the shaft behaving as an inlet, a multi-stage compressor, the combustion chamber to burn the air-fuel mixture, and a multi-stage turbine. 

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The above schematic is a blueprint of a simple power generator mechanism. The generated energy is converted into electricity by the transformer. The remaining is stored in the generator of the powerplant. 

The above examples try to follow the Brayton Cycle for maximum efficiency, but as is known, all machines concur some losses due to heat dissipation or other forms of energy. 

Softwares for Simulations

Here is a list of software to help guide and simulate processes of systems and mechanisms before they can be inculcated on a large scale. 

  1. ANSYS and its tools
  2. AxStream and its tools
  3. Concepts NREC
  4. OpenFoam
  5. CFTurbo
  7. StarCCM+

The mentioned are available online and perform specific simulations based on the application of the project. 

The Market Buzz

The following is a list of engineering backgrounds that make students eligible for turbomachinery as a career option: 

  • Aeronautical engineering
  • Space research
  • Automobiles
  • Energy and utilities
  • Pharmaceuticals
  • Refineries
  • Oil and gas fields

The list of job openings or roles as a turbomachinery background expert is as follows:

  • Ph.D. research
  • Design engineer
  • Safety engineer
  • Design manager
  • Project engineer
  • Maintenance engineer
  • Stress/Finite element analyst
  • Boiler thermal performance engineer

To Conclude

The applications of turbomachinery are innumerable. Hence, in such a wide industry, it becomes essential to be thorough with the basics of turbomachinery. Later on, you can move on to further studies based on the area of expertise required for the particular industry or research. 

For a hands-on and comprehensive course to equip you with the right knowledge and skills for a fulfilling career in the domain of turbomachinery, click here
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