Air-intake systems play a vital role in air quality improvement in a multitude of applications. Various engineering components such as gas turbines and compressors, diesel engines, and environmental filters utilize these systems to eliminate impurities and particulate matter suspended in air. A well-designed air intake system provides cool, clean air for combustion with a uniform 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 imperative to a gas turbine power plant. The intake system prevents the occurrence of a drop in the gross power output; high-pressure drop leads to decline in the power output. Optimizing the design of the air intake system is an increasingly challenging process due to the layout complexity and range of features that can be included in the intake system. Oftentimes, it is used in a combination of insect or trash screens, weather protection and filtration systems, silencers, anti-icing systems, ventilation systems, inlet heating or cooling systems for power augmentation. Poor designs could result in an uneconomical use of these components as well as a decline in engine performance owing to excessive pressure losses or distortion in the flow entering the gas turbine.
For instance, a 1” AWG (1 inch of water gauge) pressure reduction is equal to a gain of 0.355% power output. For a turbine with a power output of 160MW, this pressure loss is equal to 0.568MW. As a result, over a 4 month period of use at € 0.10/ kWh, the revenue will increase by €160000. High flow distortion, velocity, pressure or temperature, can induce compressor surge and high macro-mechanical stresses in compressor blades and vanes. In extreme cases, this may also cause blade or vane failures.
Moreover, 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 electrical installation, ventilation systems, firefighting system as well as other accessories as per requirement.
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.
Gas turbine power plant
CFD for Design Optimization
Computational Fluid Dynamics (CFD) analysis enables the user to understand and optimize the flow behaviour through the intake system (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. The friction slows it down resulting in pressure drop. The greater the friction, the greater the pressure drop. Think of it as 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.
Another leading cause for 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 the change in direction of the airflow, especially, during flows through a 90° bend. The type of fitting used for this type of bend can affect the resultant turbulence to a great extent.
With the help of the SimScale Platform, the appearance of flow separation in the bends and stagnant/dead zones which cause a 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 the reasons for and reduce the pressure drop in an air intake system. The system consists of a weather hood at the inlet through which the air enters. After passing through the weather hood, it enters the thin grills of pre-filter section followed by the main-filter section, which is modelled as a porous medium. The cleansed air from the filter enters the transition which leads to the silencer panels.
Schematic of Air Intake System
Post the panels, the air flows through 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 makes it more evident that the air is being drawn through the system.
To begin with, 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.
Different Designs for the air intake system
For all the geometries considered in the current project, we shall use Hex-dominant parametric (only CFD) meshing with 32 computing cores and multiple mesh refinements.
Below is an image of a prepared mesh with all the mesh refinements:
Mesh of Design 1
A steady-state analysis with incompressible, turbulent flow is performed using a k-omega SST turbulence model. Apart from the inlet and the outlet, all the volumes of the meshed geometry are assigned as walls.
The inlet is set to be a pressure inlet and the outlet to a velocity outlet with an assigned velocity of 6 m/s in the +x direction for all the three scenarios. After apposite numerics settings, the simulations are run taking approximately 2 hours to achieve convergence.
Results and Conclusions
The results presented below display 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. This pressure drop is the lowest for the optimized design with blades and rounded corners at the bends. Thus, we can conclude that rounded corners together with blades results in maximum reduction in pressure drop.
Pressure Contours (magnified scale)
From the velocity contours, it is evident that the recirculations are considerably reduced in the optimized design (design 3) as compared to the other two.
Pressure (Area Average) vs. Number of Iterations for all designs
Pressure drop for the three designs
Velocity streamlines across air intake system
 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