Air intake systems play a vital role in air quality improvement in various engineering components such as gas turbines and compressors, diesel engines, and environmental applications. A well designed air intake system provides cool, clean air for combustion with an even and minimized pressure drop. This improves the combustion efficiency and also reduces air pollution. To optimize the air intake systems, understanding of the flow and the pressure drop through the system is essential.
An air intake system is an important part of a power plant gas turbine industry as the high pressure drop leads to turbine gross power output drop. Optimizing the design of the air intake system is an increasingly challenging process as both the layout complexity and range of features that can be included in the intake system expands. These include a combination of insect or trash screens, weather protection and filtration systems, silencers, anti-icing systems, ventilation system off takes and inlet heating or cooling systems for power augmentation. Poor designs can result in inefficient use of these components as well as losses in engine performance due to excessive pressure losses or distortion in the flow entering the gas turbine.
For example, a 1” wg (1 inch of water gauge) pressure reduction is equal to 0.355% power output gain and for a turbine with output power of 160MW, this pressure loss is equal to 0.568MW, so that over a period of 4 months of use at € 0.10/ kWh, revenue will increase by €160000. High flow distortion, velocity, pressure or temperature, can induce compressor surge and high acro-mechanical stresses in compressor blades and vanes. In extreme cases this may also result in blade or vane failures.
An acoustic enclosure system reduces the structure-borne noise or air-borne noise and aids in turbine cooling, fire protection, weather protection, etc. These systems can be integrated with the electrical installation, the ventilation system, the firefighting system as well as other accessories as per the requirements.
An exhaust system discharges exhaust gases away from a gas turbine into the atmosphere. It demands the proper material selections and a dedicated design which maintains low pressure level and satisfies the required noise attenuation.
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CFD for Design Optimization
Computational Fluid Dynamics (CFD) analysis aids in understanding and optimizing the flow behavior through the complete intake system including its air filter and ducting. In the initial design phase, CFD analysis of the base model can help in suggesting geometrical changes like guide vane placement in inlet plenum of the filter, enhanced filter utilization area, optimized sizing of filter mesh, removal of contraction in clean pipe, etc to improve the flow characteristics.
There are two primary reasons for pressure drop in ducts :
When air moves through a duct, it rubs against the inner surfaces of the duct and loses energy. Thus, it slows down resulting in pressure drop. The more it rubs, more the pressure drops. It’s like walking down a busy sidewalk with your shoulder rubbing against the walls. The amount of friction depends on the roughness of the material the duct is made of, how it was installed and how dirty it is.
The other primary cause of pressure drop is turbulence. Turbulence is characterized by chaotic changes in pressure and flow velocity. It is a kind of friction of the air rubbing against itself. The main cause of turbulence within ducts is turning of the air. When air flows through a 90° bend, the type of fitting used to do so can make a big difference.
With the help of CFD analysis, the appearance of flow separation in the bends, stagnant and dead zones which cause decrease in the total pressure of the gas entering the system can be visualized. The strong curves in the bends are responsible for the development of secondary flows comprising counter-rotating vortices which significantly degrade the performance of the system.
The purpose of this project is to investigate and reduce pressure drop in an air intake system. The system consists of a weather hood at the inlet through which the air penetrates through. After the weather hood are the thin grills of pre-filter section followed by the main-filter section modeled as porous media. The cleansed air from the filter enters the transition leading into the silencer panels. The panels exit is further connected to the bend and the flow finally exits through two outlet openings on which fixed value Velocity Outlet boundary condition is applied. Specifying the air flow rate (25.1 m3/s) on the outlet simulates more closely the fact that the air is being drawn through the system.
Three designs are analysed:
- Conventional design with sharp-corners at bend.
- Optimized design with blades (guide vanes) at bend.
- Optimized design with blades and rounded-corners at bend.
Figure 3: Different Designs for the air intake system
A steady state analysis with incompressible, turbulent flow is performed.
Results and Conclusions
The results presented below show that the maximum pressure drop occurs in the conventional design. It can be seen from the pressure contours that the variation in the pressure drop between the 3 designs occurs between the bend and outlet section and is the lowest for the optimized design with blades and rounded corners at bend. Thus, we can conclude that rounded corners together with blades results in maximum reduction in pressure drop.
Figure 3: Pressure Contour
Figure 4: Pressure Contour
From the velocity contours it can be seen that the recirculations are considerably reduced in the optimized design with blades and rounded corners at the bend as compared to the other two designs.
Figure 5: Velocity Contour
Figure 6: Pressure (Area Average) vs. Number of Iterations for all designs
Figure 7: Pressure drop for all designs
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 Losses in Air Intake Components of Industrial Gas Turbines, http://www.fst.tu-darmstadt.de/media/fachgebiet_fst/dokumente/forschung_1/verffentlichungen_1/losses_in_air_intake_components_of_industrial_gas_turbines_stoffel.pdf
 Power Plant Layout Planning –Gas Turbine Inlet Air Quality Considerations, https://powergen.gepower.com/content/dam/gepower-pgdp/global/en_US/documents/technical/ger/ger-4253-power-plant-layout-planning-gt-inlet-air-quality-considerations.pdf
 Maximizing the Efficiency ofGas Turbines and Compressors, https://www.freudenberg-filter.com/fileadmin/templates/downloads/Gasturbinen_BR_02-TM-323-August-2015-EN_low.pdf