Bartosz Olszański, ZA, MEiL, PW, Experimental Research of an Airfoil Cascades in Varying Air Humidity Conditions

The development of the increased efficiency low pressure turbine (LPT) is one of the most popular and emerging topics in current turbomachinery flow research. As the LPT provides the power to drive the fan, that diameter is gradually increasing, the more focus are put on the design of LPT. More sophisticated aerodynamics of a fan means that the load of LPT is also substantially increased so there is a strong need for minimization of secondary flow losses from one side, and the development of a new, high lift blades to reduce blade numbers for weight, manufacturing and operating cost savings from another to maintain the improved engine performance.

Nowadays, the design and optimization process of blade is carried out by using specialized, in-house codes and commercial CFD software. Routinely, the fluid medium in computations is assumed as a dry air. In fact, the air has a small fraction of water vapor which alter the properties of a perfect, dry gas. The latent heat in water vapor is released to surrounding non-condensed gas when the phase change occurs, increasing local temperature and pressure. This obviously non-adiabatic effect induces a nonlinear phenomenon, the so-called "condensation shock", that can be captured by flow visualization techniques.

The main purpose of current work is to estimate the air humidity influence on turbine blade cascade performance. The experimental study of two separate turbine cascades (custom-made cascade and VKI LS-59) consisting of 4 separate blades for various humidity was carried out using stationary, linear high-speed cascade wind tunnel. It is an open circuit, intermittent, in-draft (suction) type of wind tunnel designed to test turbine and compressor blade cascades as well in cold-gas conditions. Among the other important parameters taken into consideration were vacuum tank pressure as well as inlet angle of attack (AoA). The additional task were to identify the intake Mach number of test section in turbine configuration in a function of vacuum tank pressure and throttle valve opening width and qualitative evaluation of the flow field near the rounded trailing edge of turbine blade for the separation shock wave pattern identification.

The study proved that the favorable pressure gradient along blade is negatively influenced when humidity rise, due to the presence of condensation. The evolution of condensation shock position in function of humidity was observed. For a low humidity (below 20-25% for a standard experiment temperature 20 degrees Celsius), when the partial vapor pressure cannot exceed equilibrium vapor pressure in a lowest pressure point, the flow can be treated as a dry air case.

slides (5,67 MB, 20/06/2017 12:25)