An Introduction To Turbomachinery Aerodynamics
Key Takeaways
Turbomachines are devices in which the aerodynamic action of steadily rotating blade rows transfers the work to or from a fluid.
Turbomachines are predominantly used for large volumetric flow rates, whereas positive displacement machines are used for small volumetric flow rates.
High-speed turbomachines work adiabatically, whereas positive displacement machines showcase isothermal operation.
The science of turbomachinery aerodynamics can be applied in a variety of ways
The science of turbomachinery aerodynamics can be applied to electricity generation, aircraft propulsion, fans, pumps, vacuums, washing machines, air conditioners, and ventilation systems. Turbomachines are the prime movers in power generation and aircraft propulsion. We will explore turbomachines and their associated aerodynamics further in this article.
Turbomachinery
Turbomachines are devices in which the aerodynamic action of steadily rotating blade rows transfers the work to or from a fluid. The energy transfer between the flowing fluid and rotating element brings a change in the momentum and pressure of the fluid. Turbines and compressors are common examples of turbomachinery. In turbines, the energy is transferred from the fluid to the rotor. However, it is just the reverse in compressors – with the energy transfer occurring from the rotor to the fluids.
Principal Parts of Turbomachinery
The principal parts of turbomachinery are:
Rotating element - The rotating element carries the vanes rotating in the fluid stream.
Stationary element - The guide vanes or passages are stationary elements and control the flow direction and energy transfer process.
Input or output shaft - The rotating element is usually fixed on the shaft.
A housing - Assembled turbomachinery components are encapsulated in the housing.
The Difference Between Turbomachinery and Positive Displacement Machines
Even though both machines release absorbed mechanical work into fluids, they are different. As mentioned above, turbomachinery utilizes rotational motion. However, positive displacement machines utilize piston movements. The reciprocating engine is an example of a positive displacement machine. One or more reciprocating pistons are used in such engines to convert pressure into a rotating motion.
In terms of speed, both turbomachines and positive displacement machines vary. The dynamic action of the rotor present in turbomachinery changes the energy level of the machine and runs at higher speeds. However, positive displacement machines are low-speed machines. The flow rates of positive displacement fluids are low. Compared to positive displacement machines, the flow rate of turbomachines is higher. Turbomachines are predominantly used for large volumetric flow rates, whereas positive displacement machines are used for small volumetric flow rates. The volume efficiency of turbomachines is 100% compared to positive displacement machines.
Types of Turbomachinery
Turbomachines can be classified into the following types, depending on the energy transfer.
Power-generating turbomachines - The energy transfer is from the fluid to the impeller fixed on the shaft. One example is turbines.
Power-absorbing turbomachines - Energy in the form of angular momentum is fed to the fluid from the impeller fixed on the shaft. Examples include pumps, compressors, and blowers.
The most common fluids flowing in turbomachinery are:
Air
Steam
Water
Petrol, diesel
Hot gases
The Direction of the Flow
The direction of the fluid flow through the vanes, blades, or rotating impeller is considered with respect to the axis of shaft rotation in this classification. The major types of turbomachinery based on the direction of flow are:
Axial flow – axial compressors or axial turbines
Radial flow – centrifugal pump or compressors
Tangential flow – Delton water turbines
Mixed flow – mixed flow pump or Francis turbine
Position of Rotating Shaft
Turbomachinery can also be classified based on the position of the rotating shaft:
Vertical shaft - Kaplan water turbines
Horizontal shaft - Stream turbines
Inclined shaft - Modern bulb micro hydel turbines
Fluid Condition in Turbomachines
There are a variety of fluid conditions found in turbomachines that impact their classification:
Reaction type (variable pressure). For example, Francies reaction turbines.
Impulse type (constant pressure). For example, Peloton water turbines.
Turbomachinery Aerodynamics and Machine Operation
High-speed turbomachines work adiabatically, whereas positive displacement machines showcase isothermal operation. In turbomachinery, there are both thermodynamic and aerodynamic actions between the rotating element and the flowing fluid. It is through thermodynamic and aerodynamic motions that energy transfer is realized in turbomachines with both pressure and momentum changes.
The aerodynamic action is important, as it makes the energy transfer, pressure, and momentum change a reality in turbomachinery. The aerodynamics involved in turbomachinery belong to the subcategory known as internal aerodynamics. In internal aerodynamics, the fluid flows through a confined passage or space, and it defines the type of liquid flow through the turbomachinery. There are other aerodynamic classifications associated with turbomachinery based on the flow regime such as subsonic flow, transonic flow, and supersonic flow.
Turbines and compressors are significant when discussing turbomachinery. The aerodynamics associated with both devices differ. The aerodynamic action in compressors is responsible for the increase in pressure of the fluid, whereas aerodynamics imparts decreasing pressure in turbines.
Designing Turbomachinery
Depending upon turbomachinery aerodynamics, the design details of the machine change. For a compressor, the design of stages and blades is made so that it can handle the adverse diffusing pressure gradients. In compressors, the design supports the increase in static pressure levels toward the flow direction. In contrast, turbines are designed for accelerating flow fields with decreasing static pressure.
It is important for an aerodynamic designer to have clarity on the aerodynamics associated with the turbomachinery they are designing. Cadence can assist designers in the design of turbomachines with CFD simulations. Cadence Fidelity CFD software offers industry-defining solver technology for designing turbomachinery.
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