Experimental Investigation on Water Film Characteristics and Droplets Formation around Cascade Blade

Abstract

Power augmentation of gas turbines by fogging (ingestion of fine droplets from nozzles) have been gaining more interest among the gas turbine manufacturers mainly due to relatively being extremely simple and highly effective among the available augmentation techniques for gas turbines. The presence of water droplets ingestion due to fogging in gas turbine systems has shown significant improvement in the performances of gas turbine systems. Despite due to this positive effect, the presence of liquid phase in the gas turbine can cause damage to the components or may cause a change to the flow path of the working fluid, which can affect the flow physics of air around the compressor blades as well as can cause physical damages to the gas turbine system. In fact, the fundamental of the behaviour of the two-phase in gas turbines has not been completely understood, as our knowledge of the gas turbine systems is mainly limited from the single phase (i.e. air) point of view. Due to limited fundamental experimental study, the aim of this study is to get an insight about the fundamental phenomena involved due to the presence of liquid and how the characteristics of liquid phase are affected by different parameters. Geometrically simple blade profile is used in this study to investigate the formation of the liquid film, water accumulation at the trailing edge, ligament formation, droplets shedding angle and the droplets size distribution aft the trailing edge region of the aerofoil. In the present study, extensive shadowgraph images were taken by using a high-speed camera with a back-lighting technique and an in-house code was developed to measure the above characteristics. It was found that the water film formation on the blade surface was primarily governed by two parameters. Firstly, due to the external forces exerted by the air flow speed (i.e., depending on the Reynolds number of the air) and secondly, due to the liquid’s restoring forces due to surface tension (i.e., Weber number of liquid film thickness). Based on these, in the present study, the appearance of the water film was categorized as wavy-film, intermediate and mirror-like smooth wave pattern for the high, intermediate and low air velocity respectively. The instability of the water film was found to be due to the Kelvin-Helmholtz instability and was evaluated by Craik’s criteria for the instability of the thin liquid film. Moreover, a theoretical expression for the liquid film thickness was obtained based on the Couette flow with linear velocity profile. At the trailing edge, the experimental study revealed that the amount of water accumulation is a function of many parameters, such as the air momentum ratio, liquid’s flow rate, weber number and the thickness of the trailing edge. For thick trailing edge profile blades, a large amount of water gets accumulated due to the large surface area under the same aerodynamic and water load conditions to that of the thin trailing edge profile. From the visualization study, vortex shedding was found to be responsible for the stripping and breakup of ligaments at the trailing edge, and the shedding frequency was found to be relatively larger for thicker trailing edges and vice versa. The breakup of accumulated water occurs mainly due to the two modes. When the air momentum ratio is large, the surface waves formed due the aerodynamic forces plays an important role in the stripping of water droplets from the tip of the trailing edge. On the other hand, for low air momentum ratio, the surface tension forces get dominant which results in mainly the bag mode of the breakup of ligaments at the trailing edge. With the decrease in the air momentum ratio, the amount of accumulated liquid increases, which ultimately results in an increase in the length of the ligaments and ultimately resulted in the bag mode of the breakup of these ligaments. The length of ligament increases with an increase in liquid’s flow rate and decrease in the air velocity and vice versa. It had also been observed that the water gets accumulated at a region of flow separation and the amount of water accumulated increases along the span direction of the blade if the relative difference between the liquid film velocity and the surrounding air flow becomes very large. The effect of angle of attack on the characteristics of two-phase phenomena around a cascade blade was also investigated. From these experiments, it was concluded that the liquid film thickness as well as the size of droplets formed aft the T.E. of the blade increases due to the reduced velocity effects. The primary droplets size formed, due to the breakup of the ligament, aft the trailing edge region decreases with an increase in the air momentum and weber number (based on the trailing edge IV thickness). Theoretical models were proposed to predict the size of primary droplets formed aft the T.E. of the blade, the shedding frequency and the wavelength of the accumulated water at the T.E.. It was measured experimentally and proven theoretically that greater the thickness of the blade is, larger would be the primary droplets formed under the same weber number (based on the thickness of the trailing edge). Furthermore, the theoretical investigation had shown that the primary droplets produced are inversely proportional to the square of the air velocity and vice versa. As the chord-wise distance aft the trailing edge increases the gradient of droplet size change. For high air momentum ratio cases, this change in the gradient was usually small, mainly due to the vibrational mode of a breakup. However, for low air momentum cases, the gradient of droplet size changes was large mainly due to the bag mode of a breakup. Additionally, the primary droplets size decreases with an increase in the air momentum ratio and vice versa. The results further showed that the droplets distribution angle in the pitch-wise direction, increases with a decrease in the air momentum ratio, due to the bag mode of a breakup, whereas, it decreases for high air momentum ratio, due to the dominance of vibrational breakup. However, for the droplets distribution angle in the span-wise direction, it increases with an increase in air momentum ratio and vice versa. The increase in the span-wise angle was mainly due to the accumulation of water over a large are in the span-wise direction.