Abstract

Water droplet erosion (WDE) has always been a serious issue in the power generation industry. WDE is the continuous loss of material from a solid surface due to the accumulated impacts by water droplets. In gas turbines, ultra-fine droplets are purposely sprayed into the incoming air of the compressor in order to decrease the ambient temperature and increase the efficiency of the turbine. Unfortunately, this process involving water droplets, severely damages the leading edge of the compressor blades. Erosion, such as WDE, is not constant with time. It has been classified into early stages and advanced stages. Most studies on erosion have primarily focused on the early stages of erosion. Very few researchers focused on water droplets and solid interactions at the early stages of erosion, which is important in understanding WDE damage. Knowing where and how the droplets impact the target can contribute to understanding the WDE phenomenon further and propose a way to reduce WDE or even find a potential solution for WDE altogether. The purpose of our work is to explore several approaches in understanding this interaction.
One part of this work will study the effects of airflow on the trajectories of water droplets with the use of ink coated samples and computational fluid dynamics (CFD) simulation. A secondary part will investigate and compare how different solid target samples react to the impact of water droplets. The selection of samples is as follows: flat (as-received unaltered material), grooved, and metallic foam. The grooved and metallic foam samples are employed in this work in order to stabilize a water film and dissipate the energy caused by multiple droplets impacts. The erosion tests were conducted in a rotating disc rig in accordance to the ASTM-G73 standard. The size of the droplets was 460 μm and the impact speeds were set to 150 m/s, 200 m/s, and 250 m/s. Ti-6Al-4V alloy was chosen as a sample material because it is typically used in turbine compressor blades.
During the WDE tests, it was observed that the airflow had a significant influence to drift the water streak away from the rotation center. Moreover, it can affect the trajectories and the impact position of the droplets. The airflow causes the upper half of the sample to erode faster than the bottom half. This was obvious upon observing the eroded samples after WDE tests. Furthermore, the formation and distribution of secondary droplets after the initial impact could also be influenced by the airflow within the rig. This is attributable to the lower mass of the secondary smaller droplets that are easily affected by the airflow.
Metal samples having grooves of either 0.5 mm or 1 mm depth were employed in this work along with other samples having blocked grooves to prevent water flowing downward. Incubation period and maximum erosion rate are the two main erosion resistance parameters of interest in this research. The results showed that the 0.5 mm groove had a longer incubation period than the reference sample. It also fared better than the 1 mm and bottom blocked grooved samples. This was studied by CFD simulation and is concluded as the influence of water film formation in the groove by dampening the impacts of subsequent water droplets. The 0.5 mm groove had a more suitable airflow environment to stabilize the water film in the groove than the 1 mm groove. However, at the maximum erosion rate stage, the reference sample and both sizes of grooves showed similar erosion behavior. This indicates that the maximum erosion rate stage is not affected significantly by introducing grooves and the formation of water films in them.
Porous and polyurethane-blocked samples were used as both test and reference samples respectively in the current work. Their purpose is to stabilize the water film, dissipate the impact energy, and study the interactions between water droplets and porous structure further. The
results show that the porous structure can dissipate more impact energy than the solid substrate and mitigate the WDE damage to some extent.