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Axial Turbines
Published in Ahmed F. El-Sayed, Aircraft Propulsion and Gas Turbine Engines, 2017
Turbines may be defined as turbomachines that extract energy from the fluid and convert it into mechanical/electrical energy. A classification of turbines is shown in Figure 14.1. Turbines may be classified based on whether the surrounding fluid is extended or enclosed. An example of an extended fluid turbine is a wind turbine, which may be a horizontal axis wind turbine or a vertical axis wind turbine. Enclosed turbines may be classified based on whether the working fluid is either incompressible or compressible. Hydraulic turbines (mostly water turbines) deal with incompressible fluids. Compressible turbines may deal with either steam or gas. Gas turbines may operate as subsonic or supersonic turbines. Turbines may also be classified based on whether the gas flow direction within its passage is axial, radial, or mixed. In axial turbines, the flow moves parallel to the axis of rotation. In radial turbines, the gas moves perpendicular to the axis of rotation. In mixed-flow turbines, the gases have a combined radial and axial motion. Another classification is related to the role of the turbine rotor in extracting power from the gas flowing through its passages; thus, there may be either impulse or reaction turbines. In reaction turbines, both stator and rotor share power extraction from the gas, while in impulse types, stator only do the job.
Concentrated Solar Energy-Driven Multi-Generation Systems Based on the Organic Rankine Cycle Technology
Published in Subhas K Sikdar, Frank Princiotta, Advances in Carbon Management Technologies, 2020
Nishith B Desai, Fredrik Haglind
In ORC power systems, the expander is the most important component as it has the most effect on the techno-economic performance of the system. Expanders for the ORC power system can be grouped into two types: (i) turbo expanders (axial and radial turbines), and (ii) volumetric expanders (scroll expanders, screw expanders, reciprocation piston expanders, and rotary vane expanders). Turbines with an organic working fluid can reach a very high isentropic efficiency with only one or two stages. In systems with high flow rates and low pressure ratios, axial turbines (100 kWe to a few MWe) are the most widely used. In contrast, radial-inflow turbines are suitable for the systems with low flow rates and high pressure ratios. However, with decreasing power output and, hence, turbine size, the rotational speed increases proportionally. Therefore, for the low power range (mainly using radial-inflow turbines, < 100 kWe), it is necessary to design an adequate bearing system and to employ a high-speed generator and power electronics. Radial outflow turbine design allows a high volume flow ratio with the constant peripheral speed along the blade span (Zanellato et al., 2018). Radial-outflow turbines can be used for small to medium-scale applications with an advantage of reduced rotational speed, allowing direct coupling to a generator (Maksiuta et al., 2017). In systems with a capacity less than 50 kWe, the turbines cannot be used due to high rotational speed and high cost (Imran et al., 2016). Reciprocating piston expanders (Wronski et al., 2019) and screw expanders (Bao and Zhao, 2013) can be used for small capacity plants. Scroll expanders and rotary vane expanders can be used in small or micro-scale ORC power systems (Bao and Zhao, 2013).
A comprehensive review on organic Rankine cycle systems used as waste heat recovery technologies for marine applications
Published in Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 2022
Olgun Konur, C. Ozgur Colpan, Omur Y. Saatcioglu
The working principle of turbo-expanders is based on the rotation of turbine blades, while the high-pressure working fluid in the gaseous-phase expands and transfers its kinetic energy to the turbine blades through its way on the expander. Turbo-expanders can be categorized as axial and radial turbines following the different flow directions of the working fluid at the turbine inlet in relation to the output shaft. Both turbine types have the advantage of a compact structure with lightweight construction. However, in addition to the efficiency drops in unsteady operations, wet expansion is another major challenge on turbo-expanders because liquid formations may easily damage the turbine blades (Pantano and Capata 2017; Talluri et al. 2020). Axial turbine blades are not appropriate for operations with low mass flow rates and high-pressure ratios because the leakages from the small tip clearance between the turbine casing and the blades increase and cause a significant drop in the expander efficiency (Alshammari, Usman, and Pesyridis 2018). Therefore, axial turbine arrangements are well suited to large-scale applications of over 500 kW with a low-pressure ratio and high mass flow rate. Unlike axial turbines, radial turbines can effectively handle the required enthalpy drop in a single-stage expansion process at operations with lower mass flow rates and high-pressure ratios. They are also advantageous in high-density working fluids because of the robust turbine blade design that can support high loads on the blades. Radial turbines are predominantly preferred in medium-scale applications ranging from 30 to 500 kW power output capacity. The economic viability and high-efficiency values achieved in high power outputs start reducing below this range, since micro-scale (<10 kW) turbo-expander arrangements have not been well developed yet (Alshammari, Usman, and Pesyridis 2018; Talluri et al. 2020).