As turbomachinery technology continues to advance in efficiency as well as overall power, many engineers want an estimate on how long these manufactured machines will operate. Specifically, in high-temperature and high-flow turbomachinery applications, one of the main sources of performance degradation can be attributed to increases in surface roughness. Gas turbine and compressor blades in particular experience a substantial amount of surface degradation over their lifetime.
There are many mechanisms that contribute to surface degradation in airfoils and annulus surfaces. Foreign particles adhering to the material surface (or fouling) is generally caused by any increase in contaminants such as oils, salts, carbon, and dirt in the airflow. Corrosion occurs when there is a chemical reaction between the material surface and the environment that causes further imperfections on the machine surfaces. Additional mechanical factors such as erosion and abrasion will play a part in a machine’s surface degradation as well.
Some of these mechanisms, however, are more prevalent in higher temperature systems. Hot corrosion, for example, occurs at a temperature range of around 730 to 950°C due to the chemical interaction of molten salts on the component’s surface. In this case, high temperature gas turbines are most notably affected by this phenomenon. Even with protective cooling flow systems, most gas turbines start to lose efficiency early in their life cycles. For this reason, it is important to analyze the future performance of the machine before it is put into operation. Attributing the necessary roughness factors can lead to a more accurate description of the performance of a machine over time. Using the AxSTREAM software platform, these roughness factors can be implemented into a design to compare what a turbomachine might experience 5 or 10 years into its lifecycle as opposed to at its birth. Furthermore, a user could model the effect of particular surface coating and polishing techniques and analyze whether the increases in efficiency outweighs the investment costs.
By assigning different surface roughness grades in AxSTREAM, one can analyze a number of parameters of interest such as efficiency and power output. Using the streamline/meanline calculation module, (seen to the left), the user is able to seamlessly overlay a number of different performance factors and curves due to the prescribed changes in material roughness. With increases in surface roughness, laminar to turbulent transitions will occur much sooner and will result in substantial losses in the machine. Because of this, it is crucial to investigate what kind of investments and preventative measures need to be made to ensure proper treatment of a machine’s surface.
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