Gas turbine performance and cooling efficiency

With anything related to gas turbines, ‘efficiency’ is the name of the game. An efficient turbine is economic on fuel, offers a productive output and is generally more reliable and long-lasting. It is important to consider efficiency at every step in order to maximise output and reap the highest possible profit.

When we think of the process of producing electricity using a gas turbine, we think of the combustion of fuel: extremely high temperatures and pressures for maximum output. It may seem odd, then, to discuss how cooling efficiency is in fact essential to optimum gas turbine performance.

What part does cooling have to play in the process and how can we use this knowledge to improve efficiency?

Why do gas turbine compressor blades need cooling?

In order to achieve optimum thermal efficiency, it’s essential to increase the turbine inlet temperature. Gas will often enter the turbine at temperatures as high as 1500 C, which is above the melting point of the turbine’s superalloy surfaces.

Thermal stresses in gas turbine blades can lead to pits, cracks, and even the complete failure of the blade itself. Any imperfections or deformities of the blades (let alone them breaking completely) compromise the aerodynamics within the gas turbine impacting efficiency. And, as we know, efficiency is the name of the game.

So, we are faced with a problem; extremely high temperatures are required for efficient combustion, yet those temperatures compromise the very equipment of the gas turbine itself. The answer? An effective, efficient gas turbine blade cooling system, preventing damage to the blades due to that intense heat.

How to cool gas turbine compressor blades

There are different methods of blade cooling:

Internal convection cooling

Turbine blades are hollow and cooling air is able to pass into the blade and circulate, reducing the temperature of the blade and protecting it from damage.

Internal impingement cooling

Building upon the above method, for this alternative the blade is made of two ‘shells’. Cooling air enters the inner shell, which has holes in it. Air jets at the outer shell, cooling it from within.

Discrete film cooling

In partnership with one of the above methods, which are limited in their effectiveness, a thin layer of cooling air can be directed to flow over the outer side of the blade. This buffer absorbs some of the heat, further protecting the blade.

A particular problem area is the trailing edge. The very thin trailing edge of the blade simultaneously suffers higher temperatures due to the way the hot gases flow over and around the blade, and is difficult to cool as it is too thin for a cooling air channel. To combat this, the cooling air is often allowed to exit at this point, particularly cooling this area and creating a ‘protective film’ over the blade surface.

Full blade film cooling

As above, the whole blade is covered in a protective layer of cooling air

Full blade transpiration cooling

For this final method, the blades are made from a porous material which allows cooling air to continuously pass to the surface. This idea is still in the developmental stages as a suitable porous material able to withstand the heat and pressure within a gas turbine is yet to be developed.

Conclusion

There are so many variables to consider when it comes to gas turbines, yet it’s worth spending some time considering and developing your blade cooling efficiency. The performance and lifespan of the turbine itself depends on this apparently small detail.

Cooling efficiency directly impacts thermal efficiency which in turn dictates the performance and profitability of your turbine.

Get in touch

At Rochem, we have been specialists in gas turbine compressor cleaning products for over 40 years. The science behind creating an optimum environment within gas turbines fascinates us and we’d love to share our ideas with you to help at your site.

If you’re interested in maximising your gas turbine’s potential through cleaning chemicals and hardware, . Our team of knowledgeable experts is on hand to answer all your questions.

https://netl.doe.gov/sites/default/files/gas-turbine-handbook/4-2-2-2.pdf [Accessed 2/3/24]

Further references:

https://www.linkedin.com/pulse/gas-turbine-blade-cooling-its-importance-magesh-john#:~:text=Turbine%20blade%20temperatures%20are%20kept,the%20stator%20and%20rotor%20blades. [Accessed 29/2/24]

https://www.sciencedirect.com/science/article/pii/S2666202723000484#sec0006 [Accessed 29/2/24]