Turbine Blade Cooling – An Integrated Approach

It is a well-known fact in the turbomachinery community that the highest temperature achievable at the inlet of the turbine is a critical performance parameter for the turbine. For any given pressure ratio and adiabatic efficiency, the turbine specific work is proportional to the inlet stagnation temperature. Typically, a 1% increase in the turbine inlet temperature can cause a 2-3% increase in the engine output.

Increase in net power output of a gas turbine over a one percentage point rise in turbine inlet temperature
Figure 1 Increase in net power output of a gas turbine over a one percentage point rise in turbine inlet temperature

The major limitation for the maximum achievable value of the turbine inlet temperature comes from the material used for the turbine. The maximum material temperature has to be kept in check for multiple reasons, from the physical integrity to the structural reliability, and resulting temperature needs to be less than the turbine blade material’s maximum temperature.

Keeping these limitations in mind, elaborate methods are used for cooling the turbine nozzle and rotor blades. If the turbine blades are properly cooled, the turbine inlet temperature can be further raised while keeping the turbine blade temperature at a relatively lower and acceptable level. Any integral approach to turbine blade cooling takes in to account the dynamics of both the cooling flow path as well as the turbine main flow, and produces the complete turbine performance results with the inclusion of cooling. All these calculations need to be done within an acceptable time frame and with a very good level of confidence.

The first requirement of such an analysis is to simulate the flow through a cooling flow pipe network which can produce the desired cooling in the turbine blades. AxSTREAM NET™  provides a solid and robust tool for these types of studies. In AxSTREAM NET™, such networks are modelled using 1D numerical simulation, where the fluid path and solid structure are represented as a set of 1D elements which can be connected to each other to form a thermal-fluid network. For each fluid path section, the program calculates the fluid flow parameters for inlet and outlet cross-sections, like velocity, pressure, density, temperature, mass flow rate, total parameters, etc. by simulating the convective heat transfer between fluid flow and solid structure. The resizing of the cooling flow channels can be performed in case the desired goals are not met. The effectiveness of the cooling or heating systems can be estimated, based on the values of heat flow rates, temperatures of fluid flow and solid bodies (in this case the blade temperature). Calculated parameters of the cooling gas flow and the internal surface temperature of the blade are the outputs of such analysis for the given turbine inlet temperature and cooling flow path geometry.

Rotor Cooling Modeling 115 MW Gas Turbine
Figure 2 Turbine blade cooling flow network in AxSTREAM NET

After cooling network calculation, the results can be incorporated into the turbine flow path and performance calculation. AxSTREAM® helps with design, analysis, and optimization of the turbine while incorporating the cooling flows into the calculation. The major inputs for such analysis include the non-cooled turbine performance and the data from cooling flow calculations. After the calculation, the complete performance is known for the cooled turbines. Some minor corrections and resizing in the cooling flow network might be required at this stage in case the calculated turbine performance or cooling effects are below the requirement. Such resizing and correction measures have to be applied to the cooling network analysis step, and the loop has to be executed again. It often takes several iterations before the final state is reached in terms of the cooling network design and sizing, as well as the final cooled turbine performance.

Cooling flow inputs for a turbine rotor blade in AxSTREAM
Figure 3 Cooling flow inputs for a turbine rotor blade in AxSTREAM®
Mass balance diagram and Mollier diagram for a cooled turbine calculated in AxSTREAM
Figure 4 Mass balance diagram and Mollier diagram for a cooled turbine calculated in AxSTREAM®

The time of such calculations can be significantly reduced if the two steps, described above, are integrated into an optimization loop. This allows smooth execution of the iterations between the two calculation stages, without manual intervention from the user at every level.  AxSTREAM ION™ provides all of the capabilities required for such integration and automation for the processes. In fact, one can go a step further and include the thermodynamic cycle design step (which can be performed in AxCYCLE™) into the loop, in order to analyze the thermodynamic cycle while incorporating the compressor bleeds and cooled turbine performance. Once the cooling network is calculated while simultaneously calculating the turbine performance, the user can confidently speak not only of the reliability of the designed cooling system, but also of the acceptability of cooled turbine performance.

To learn more about how SoftInWay can help you, contact us at Info@Softinway.com to schedule a demo!

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