Turbine components are placed right after the combustor and are therefore, subject to the highest temperatures in an engine. The turbine blades are directly in the line of fire (so to speak) of these incredibly high temperatures. Higher temperatures yield higher cycle efficiencies, meaning that the limit on efficiency for a cycle is determined by turbine materials. The current state of the art materials can only give so much heat resistance capacity, which makes blade cooling essential. In this post we’ll be taking a look at the various cooling methods that exist for turbine blades, and the tools to design them.
How important is cooling to the efficiency of gas turbine engines?
In a word, very. Let’s look at an example to better explain. Our fictitious engine without cooling has an overall pressure ratio of 40 where the maximum allowable turbine entry temperature (TET) is at 1498 K, yielding a thermal efficiency of 33%. When compared to a turbine with cooling, TET can be increased to 1850 K, yielding a thermal efficiency of 38%. This is an 8% increase in efficiency via the addition of cooling. In order to achieve good thermal efficiency in our cycles, turbine components must be cooled!
Another factor to consider is the non-uniform temperature profile at the exit of the combustor. The HP turbine has a radial distribution of temperature leading to hot spots called “hot streaks” from the dilution air injected around the flame in the combustor. This high-pressure (HP) turbine design becomes a matter of determining the peak temperature at the core of these hot streaks. Engineers place the leading edge of the inlet guide vanes (IGV) between these hot spots to provide a small margin for cooling before reaching the rotor blades.
Types of Blade Cooling
Blade Passive Protection
Blade passive protection methods consist of providing a thermal barrier coating on the turbine blade, particularly on the leading edge of the blade which is the area under the highest thermal stress. As an example, a 0.15mm layer ceramic coating sprayed onto the nozzle guide vanes (NGV) could be used, this coating has a low thermal conductivity of 1.3 W/mK. The objective of such a coating is to reduce heat flux through the blade and therefore reduce the blade’s wall temperature. In most cases a temperature drop of 100-200 °C can be achieved. This is usually not enough, especially for the HP turbine blades which handle exhaust gases directly from the high temperature combustor. Additional blade cooling measures are required in most cases, namely air-cooling methods.
Air Cooling Methods
Air cooling methods are used throughout the whole turbine arrangement. Turbine components are cooled using the air bled from the compressor. Stator blades and the outer wall of the turbine flow passage use cooling air routed between the combustor and the outer engine case, whereas turbine rotor blades, disks and the inner walls of the turbine flow passage use air that travels through the inner passage way. We’ll be taking a closer look at air cooling types used specifically on blades.
When talking about air cooling turbine blades, we can usually distinguish between two main types: internal cooling and film and transpiration cooling. The first type, internal cooling, maintains the blade metal temperature below the outside gas temperature by heat transfer to internal cooling air. This can include convection cooling and impingement cooling, where cooling air runs through channels within the blade. In the second type of cooling, film and transpiration cooling, the aim is to reduce heat transfer to the blades surface. This is done through film cooling, full-coverage film cooling and transpiration cooling. These methods can also be combined into a single blade for example by designing a blade with convection-, impingement- and film cooling.
Let’s take a closer look at film cooling…
Film cooling is the main strategy in use today since it provides the best compromise between improved efficiency from cooling and decreased cycle efficiency from multiple factors including manufacturing costs and complexity and reduced blade lifetime and strength. Cool air is bled from the compressor, channeled to the internal ducts of the turbine blades, and discharged through small holes in the blade walls forming cooling jets. This provides a thin layer of cool air along the blade’s external surface which acts as an insulating blanket. The ability of the cooling film to cool the blade is evaluated using a parameter called “cooling effectiveness”. With a maximum value of 1, cooling effectiveness measures how close the blades surface temperature is to the coolant flow temperature. Cooling effectiveness is mostly affected by the coolant flow parameters as well as the injection geometry. Advanced research is underway to improve cooling effectiveness and to better predict and extend blade life.
What about the effect of cooling on turbine efficiency?
Although cooling of turbine blades is necessary to ensure the longevity and high efficiency of the turbine, there exist three possible mechanisms which reduce the efficiency as a result of cooling. Firstly, cooling air increases aerodynamic drag of the blades. Secondly, the cooling air has a lower stagnation pressure when mixed into the downstream flow therefore the cooling air suffers a pressure loss in the cooling passages. Thirdly, entropy is increased by heat transfer from the hot primary gas flow to the cooling flow. While the positive effects of blade cooling tend to outweigh the negative effects in most applications, it is important to take into consideration these effects when designing the turbine.
What tools are at our disposal to help with blade cooling design?
During the design phase of the turbine blades it is very important to optimize the cooling flow, both for efficiency and reliability of the turbine. However, performing a full 3D analysis of the cooling passages and external cooling flow can be very computationally expensive. A 1D flow and heat network drastically simplifies the design task and ensures that optimal placement of cooling network and holes is achieved. Luckily for engineers, tools such as AxSTREAM NET drastically simplify the creation of 1D flow and heat transfer networks. AxSTREAM NET is a 1D system modelling solver based on the finite volume method which uses a thermal fluid network approach to simulate secondary flows and heat transfer at steady and unsteady conditions. This allows engineers to significantly reduce the iteration time required to optimize aerodynamic cooling losses and machine performance and to achieve optimal configuration of blade cooling, among other applications in the shortest amount of time possible.
The cooling flow pipe network in AxSTREAM NET is modeled using 1D numerical simulation, where the fluid path and flow structure are created by connecting built-in 1D elements to form thermo-fluid networks. By simulating the convective heat transfer between fluid flow and solid structure, the software calculates the fluid flow parameters for the inlet and outlet cross sections. These parameters include velocity, pressure, temperature, density, mass flow rate, etc. The outputs of this analysis are the calculated parameters of the cooling gas flow and the internal surface temperature of the blade. This allows checking the effectiveness of the cooling system and being able to see if the design goals are met. Then the cooling network calculations can be easily transferred to AxSTREAM® for further design, analysis and optimization of the turbine which allows for the incorporation of the cooling effects into the design of the turbine from the very beginning.
Turbine blade cooling is yet another complex task in designing highly efficient and reliable turbines. However, thanks to software tools such as AxSTREAM NET, the task is drastically simplified while still gaining further insight into design details, allowing the turbine to be pushed to its fullest potential. Turbine cooling methods are always evolving and this post only brushed the surface of the subject. Hopefully it has ignited your interest to delve deeper into the world of blade cooling, good luck!
- Al Taie, Dr. Arkan & Hussain, Najlaa. (2011). TURBINE BLADE COOLING BY AIR USING DIFFERENT METHODS. The Iraqi Journal For Mechanical And Material Engineering, Vol.11, No.2, 2011.