Heat Transfer & Blade Life in a Siemens Gas Turbine

The Siemens gas turbine is an excellent example of a heavy-duty, robust, flexible turbine design that has been variously adapted for industrial, power generation, aeroderivative, and other uses. There are more than 7,000 Siemens gas turbine engines installed and operating in more than 60 countries worldwide. With Siemens as our model, let’s take a closer look at the effects of heat transfer on turbine blade longevity.

Sources of Heat Transfer

Heat transfers to turbine blades whenever there is a temperature differential between the heat in the chamber and the material of the blades. Three fundamental processes control heat transfer: conduction, convection, and thermal radiation. If your aim is to nail down the heat transfer coefficient, you’ll need to have the first law of thermodynamics on hand, in addition to Fourier’s law (for conduction), Newton’s law of cooling (for convection), and Stefan-Boltzmann’s law (for thermal radiation).

• Conduction: This is a form of energy transfer that occurs when hotter (higher energy) molecules collide with colder molecules. Energy transfers to the less energetic molecules.
• Convection: Convection is a form of heat transfer that occurs when a body of molecules in motion enters a body of molecules with a different temperature. When hot gas moves through a Siemens gas turbine, for example  (due to thrust from fan rotation), heat is transferred into the surrounding areas of the combustion chamber through an intervening “boundary” layer between the hotter and colder bodies.
• Thermal Radiation: Radiation is unique in that it does not require any contact between mediums for heat transfer to occur. Instead, the energy is transferred via electromagnetic waves, which are emitted from the hot material, liquid, or gas.

Effects of Heat Transfer on Blade Life

The rate of heat transfer (and total heat transferred) to the components of the engine has a profound effect on component longevity.  Perhaps the most pronounced risk is a buildup of thermal fatigue, which is a failure factor that results from high temperature differentials between start-up and shut-down temperatures. Rapid rises and falls in temperature between peak cycles exacerbates thermal fatigue, eventually leading to weakening and cracking in the blade (not unlike creep from load stresses at high temperatures). 

Siemens gas turbine engineers use a common rule of thumb to describe the risks of heat transfer from high turbine inlet temperatures (TIT): If the 1st stage nozzle metal surface temperature is 20°C above its operating limit, the life of the blade reduces by a factor of 3. In other words, a Siemens gas turbine blade rated for 30,000 operating hours would, in practice, have a blade life of only 10,000 hours in an engine that runs 20°C too hot. 

It’s critical to slow and minimize total heat transfer through both material design and efficient cooling processes, which can dramatically impact blade life.

Siemens Gas Turbine Cooling

Both internal and external cooling methods can be used to protect turbine blades from harmful levels of heat transfer. Because cooling air from the compressor negatively impacts aerodynamics and efficiency, it’s desirable to use as little cooling air as possible, but at maximum effect. Turbine blades 1 and 2 of Siemens Gas Turbines 700 and 800 (SGT-700, SGT-800) are cooled by air passing through internal channels, which cools the channel walls by convection. The Siemens gas turbine design includes features that increase the heat transfer coefficient and heat transfer surface area (generally by breaking the flow boundary layer and enhancing turbulence).  Done well, these measures can mitigate the damage of excessive heat transfer with minimal pressure loss.

Siemens Uses Extensive Validation & Testing

Siemens gas turbine engines include heavy-duty, industrial, and aeroderivative gas turbines that range up to a robust 593 MW. Machines this powerful are large and can be expensive to repair, so extensive up-front validation and testing to optimize heat transfer performance is a must. Siemens gas turbines are proven to provide outstanding efficiency and blade longevity in commercial operation, in large part due to their focus on temperature sensing in both gas flow and solid components.

Siemens knows that RF temperature sensors can be a critical ally in testing and tracking heat transfer and cooling performance in a gas turbine engine. The patented wireless RF sensors from Sensatek, for example, generate data to provide advanced temperature control with real-time blade health monitoring in the harsh environment of a Siemens gas turbine engine.

Test Heat Transfer With Sensatek RF Sensors

The durability of Sensatek on-blade temperature sensors has been tested and verified by Siemens through a wide range of rigorous stress tests. At the Siemens Casselberry Flame Rig, Sensatek demonstrated a bonded sensor’s persistence and stability across 600+ flame shock cycles at 1,000 C/min.  The sensors were also embedded and demonstrated with Siemens Energy on an SGT8000H Rotor and tested on a microturbine engine rotating wheel up to 127,000 rpm at 810°C. Long-term high-temperature tests and Arrhenius characterization have simulated 10,000-hour operation functionality.This performance allows for extensive, repeated heat transfer testing without reinstrumentation while providing robust temperature models for heat transfer and cooling optimization. Contact us at Sensatek to learn more about how RF sensors can help to predict thermal stress, accelerate design cycles, and ensure maintenance targets are met.