Adiabatic Turbine: What, How, Several Types, Examples


This article discusses about adiabatic turbine. The term adiabatic is often used in the field of thermodynamics. It refers to a system or boundary which does not allow heat transfer to take place.

A turbine is an essential device used in power plants which assists in generation of electricity. It contains elements that rotate. This rotation is transferred to generator which generates the electricity. We will study more about adiabatic turbines in this article.

What is an adiabatic system?

The system from which transfer of heat is restricted is called as adiabatic system. That is the heat cannot be transferred to surroundings and the heat cannot enter the system from surroundings.

The net heat transfer is zero. This is because of the walls of the system are adiabatic. Heat is not allowed to be transferred from the system. The entire content of heat remains inside the system only which becomes the internal energy of the system.

What is a turbine?

A mechanical device that is used to convert hydraulic energy to mechanical energy is called as turbine. The hydraulic energy comes in the form of moving water/steam which hits the propellers of turbine.

The movement of fluid that is the kinetic energy of fluid is transferred to the propellers of turbine. Because of this reason the shaft of turbine start rotating. This way the conversion of energy takes place from hydraulic to mechanical. Further sections will tell us more about turbine in detail. 

Working of turbine

After discussing in the above section we now know that the turbine converts kinetic energy of fluid to mechanical energy.

In power plants, steam is used as the working fluid and in dams water is used as the working fluid. Working fluid here refers to the fluid which is responsible for rotation of shaft. The shaft rotates after working fluid strikes the surface of propeller. The rotating shaft is connected to generator which generates electricity.

What is adiabatic turbine?

Now we know the meanings of both adiabatic system and turbine. Let us see what is an adiabatic turbine.

An adiabatic turbine is a turbine whose walls are adiabatic. These walls don’t allow heat transfer through them. This way no amount of heat can escape the turbine. Hence, the net transfer from the turbine is zero. No heat is added to the turbine and no heat escapes the turbine when the turbine is adiabatic.

Adiabatic turbine working

We know what an adiabatic turbine is. We can see that there is no physical difference between a normal turbine and an adiabatic turbine, only difference between them is that adiabatic turbine does not allow heat transfer to take place.

Similarly, the working of an adiabatic turbine is also same as that of normal turbine. In almost all cases, the walls of the turbine are kept adiabatic. Hence the working of an adiabatic turbine is same as that of a normal turbine that we have discussed in above sections.

Adiabatic gas turbine

As we have studied about the turbines used dams, they use water as the working fluid that rotates the propeller shaft in turbines.

When the working fluid used is a gas then the turbine is called as gas turbine. Gas turbines are mostly used in aeroplane power plants and thermal power plants used for generating electricity. The materials used in both the turbines are different because the type of working fluid used is different.

Adiabatic steam turbine

Adiabatic steam turbine is a turbine in which steam is used as the working fluid. Steam as we all know is the gaseous form of water.

The steam contains high kinetic energy which is transferred to the propeller of turbine. The turbine rotor starts moving when the high energy steam strikes the surface of propeller. Pressure drop will take place inside the turbine, while entering the steam is at high pressure. This pressure drops and the steam becomes low pressure steam after passing through turbine. 

Adiabatic reversible turbine

The term reversible refers to a turbine in which all the processes are ideal. The losses de to friction or energy leaks are not present in reversible processes.

When we talk about adiabatic reversible turbine, it means the expansion process taking place inside the turbine is nearly ideal. Usually a multiple staged turbine is considered as a reversible turbine. When the walls are adiabatic, this turbine becomes adiabatic reversible turbine.

Adiabatic turbine equation

The equation used in adiabatic turbine is given below-

Work done by turbine is given by-

Wt= h2-h1

Where,

h2 is the enthalpy at point 2

h1 is the enthalpy at point 1

Adiabatic turbine efficiency

The efficiency is defined as the output gained from a system or device after pumping in some input.

The efficiency of adiabatic turbine is given as-

Efficiency= Output/Input = Wt/Qa

Where,

Wt is the work done by turbine

Qa is the amount of heat added in the system

Is a turbine reversible ?

We have already discussed in above section that a turbine is considered reversible when the expansion process in the turbine is ideal.

In reality, no device can be considered ideal, but a turbine can be made nearly ideal by increasing the number of stages of expansion process. This type of turbine is called as a multi staged turbine. The process of using multiple turbines is called as reheating.

Is a steam turbine adiabatic?

When the transfer of heat is restricted completely, a steam turbine can be said to be adiabatic.

The steam changes its phase to liquid after passing through the condenser. In a turbine only the pressure drop in steam takes place. The steam coming in with high pressure gets converted to a low energy steam at exit.

adiabatic turbine
Image: Steam turbine with its case opened

Image credits: Siemens Pressebild, Dampfturbine Montage01CC BY-SA 3.0

Why are turbines adiabatic?

Turbines are adiabatic as the entire energy is taken to drive the turbine shaft.

No energy is lost in the form of heat or no energy enters the turbine in the form of heat. The only energy that participates in the energy transfer process is the kinetic energy of the fluid and kinetic energy of the turbine rotor.

Adiabatic turbine entropy

Before discussing about adiabatic turbine’s entropy, we shall know the meaning of entropy. Entropy’s widely accepted definition says that it is a measure of randomness in the system.

In actual turbines the entropy value increases. Whereas for reversible adiabatic processes the value of entropy generated is zero. This is because entropy is a state function and for a reversible process the end points are same and as the process is adiabatic, due to no heat transfer the change in entropy is zero.

Abhishek

I, Abhishek Khambhata, have pursued Btech in Mechanical Engineering. Throughout four years of my engineering, I have designed and flown unmanned aerial vehicles. My forte is fluid mechanics and thermal engineering. My fourth year project was based on performance enhancement of unmanned aerial vehicles using solar technology. I would like to connect with like minded people.

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