Turbine Basics - Turbine and Blade Cooling Technology

Axial flow turbine structure

A turbine is a rotary power machine that converts the enthalpy of a working fluid into mechanical energy. It is one of the main components of aircraft engines, gas turbines, and steam turbines. The energy conversion between turbines and compressors and airflow is opposite in procedure. The compressor consumes mechanical energy when it is running, and the airflow gains mechanical energy when it flows through the compressor, and the pressure and enthalpy increase. When the turbine is running, shaft work is output from the turbine shaft. Part of the shaft work is used to overcome the friction on the bearings and drive the accessories, and the rest is absorbed by the compressor.

Only axial flow turbines are discussed here. The turbine in a gas turbine engine is usually composed of multiple stages, but the stator (nozzle ring or guide) is located in front of the rotating impeller. The blade channel of the turbine element stage is convergent, and the high-temperature and high-pressure gas from the combustion chamber expands and accelerates in it, while the turbine outputs mechanical work.

Heat transfer characteristics of turbine blade outer surface

The convective heat transfer coefficient between the gas and the blade surface is calculated using the Newton cooling formula.

For the pressure surface and the suction surface, the convective heat transfer coefficient is the highest at the leading edge of the blade. As the laminar boundary layer gradually thickens, the convective heat transfer coefficient gradually decreases; at the transition point, the convective heat transfer coefficient suddenly increases; after the transition to the turbulent boundary layer, as the viscous bottom layer gradually thickens, the convective heat transfer coefficient gradually decreases. For the suction surface, the flow separation that may occur in the rear section will cause the convective heat transfer coefficient to increase slightly.

Shock Cooling

Impingement cooling is to use one or more cold air jets to impact the hot surface, forming a strong convection heat transfer in the impact area. The characteristic of impingement cooling is that there is a high heat transfer coefficient on the wall surface of the stagnation area where the cold air flow impacts, so this cooling method can be used to apply focused cooling to the surface.

The impingement cooling of the inner surface of the leading edge of the turbine blade is a limited space impingement cooling, and the jet (cold air flow) cannot mix freely with the surrounding air. The following introduces the impingement cooling of a single-hole plane target, which is the basis for studying the impact of impingement flow and heat transfer.

The flow of a single-hole vertical impact plane target is shown in the figure above. The plane target is large enough and has no rotation, and there is no other cross-flow fluid on the surface. When the distance between the nozzle and the target surface is not very close, a section of the jet outlet can be regarded as a free jet, namely the core section (Ⅰ) and the base section (Ⅱ) in the figure. When the jet approaches the target surface, the outer boundary line of the jet begins to change from a straight line to a curve, and the jet enters the turning zone (Ⅲ), also called the stagnation zone. In the stagnation zone, the jet completes the transition from a flow perpendicular to the target surface to a flow parallel to the target surface. After the jet completes a 90° turn, it enters the wall jet zone (IV) of the next section. In the wall jet zone, the fluid flows parallel to the target surface, and its outer boundary remains a straight line. Near the wall is an extremely thin laminar boundary layer. The jet carries a large amount of cold air, and the arrival speed is very high. The turbulence in the stagnation zone is also very large, so the heat transfer coefficient of the impact cooling is very high.

Convection Cooling

（1）Radial direct cooling channel inside the blade

The cooling air flows directly through the inner cavity of the guide vane in the radial direction, absorbing heat through convection heat transfer to reduce the temperature of the blade body. However, under the condition of a certain cooling air volume, the convection heat transfer coefficient of this method is low and the cooling effect is limited.

(2) Multiple cooling channels inside the blade (multi-cavity design)

The multi-cavity design not only increases the convective heat transfer coefficient between the cold air and the inner surface of the turbine blade, but also increases the total heat exchange area, increases the internal flow and heat exchange time, and has a high cold air utilization rate. The cooling effect can be improved by reasonably distributing the cold air flow. Of course, the multi-cavity design also has disadvantages. Due to the long cooling air circulation distance, small circulation area, and multiple turns of the airflow, the flow resistance will increase. This complex structure also increases the difficulty of process processing and makes the cost higher.

（3）Rib structure enhances convective heat transfer and spoiler column cooling

Each rib in the rib structure acts as a flow disturbance element, causing the fluid to detach from the boundary layer and form vortices with different strengths and sizes. These vortices change the flow structure of the fluid, and the heat transfer process is significantly enhanced through the increase in fluid turbulence in the near-wall area and the periodic mass exchange between the large vortices and the mainstream.

Spoiler column cooling is to have multiple rows of cylindrical ribs arranged in a certain way inside the inner cooling channel. These cylindrical ribs not only increase the heat exchange area, but also increase the mutual mixing of cold air in different areas due to the disturbance of the flow, which can Significantly increase the heat transfer effect.

Film Cooling

Air film cooling is to blow out cold air from the holes or gaps on the hot surface and form a layer of cold air film on the hot surface to block the heating of the solid wall by the hot gas. Since the cold air film blocks the contact between the main airflow and the working surface, it achieves the purpose of heat insulation and corrosion prevention, so some literature also calls this cooling method barrier cooling.

The nozzles of film cooling are usually round holes or rows of round holes, and sometimes they are made into two-dimensional slots. In actual cooling structures, there is usually a certain angle between the nozzle and the surface being cooled.

A large number of studies on cylindrical holes in the 1990s showed that the blowing ratio (the ratio of the dense flow of the jet to the mainstream) will significantly affect the adiabatic film cooling effect of a single row of cylindrical holes. After the cold air jet enters the mainstream high-temperature gas area, it will form a pair of forward and reverse rotating vortex pairs, also known as a kidney-shaped vortex pair. When the blowing air is relatively high, in addition to forward vortices, the outflow will also form counter-rotating vortices. This reverse vortex will entrap the high-temperature gas in the mainstream and bring it to the trailing edge of the blade passage, thereby reducing the film cooling effect.