In many applications, non-uniform flows are undesirable and have a negative system impact. Non-uniform flows can arise in many ways such as in geometry changes and turns in piping/ducting, as well as with lip separation in certain aircraft engine inlets at high angle of attack. These non-uniformities can come with a variety of secondary flow patterns and thus highly three-dimensional flow. In a cylindrical coordinate system, these secondary (or in-plane) velocities have tangential and radial components. The tangential velocity is typically referred to as swirl and is the component of secondary flow that has the most system impact. These systems include industrial compressors, aircraft engines, and flow metering devices.
In industrial compressors and aircraft engines, swirl translates into off-design incidence angles on the blades. The off-design incidence can lead to blade stall, losses in performance, and loss in system operability. In flow metering, swirl can significantly reduce measurement accuracy, and is regulated in industries such as the oil and gas industry.
In the straightening of low-speed flows with approximately constant density and axial velocity, the velocity magnitude decreases and pressure increases along streamlines. This creates an unfavorable streamwise pressure gradient that inhibits the fluids ability to remain attached to the convex suction surface of the turning vane. This suction side separation causes the flow to under turn and exit at an angle not equal to the vane's trailing edge angle. The angle measured between the vane's trailing edge and the actual exiting flow angle is referred to as the deviation. Research on specific airfoil shaped turning vanes set up in linear cascade arrangements provides experimental data detailing the deviation measured at ranges of inlet flow conditions and vane spacing. These experimental data sets indicate that deviation angles were measured to be 10 degrees or higher in large vane spacing, high design flow turning arrangements. It is evident that in order to effectively straighten flow with turning vanes, the deviation must be predicted and accounted for in the design stage.
In this work, the design system of a new method of swirl mitigation is detailed and experimentally validated in a low-speed wind tunnel. The design system builds upon the fundamentals of the swirl-producing StreamVane design methods and is thus designated as the Inverse StreamVane. The complex arrangement of turning vanes in the StreamVane and Inverse StreamVane alike creates a spanwise variation of vane spacing. Calculated by a proximity approximation method, this local vane spacing, along with the local inlet flow conditions, become inputs to a derived function that predicts the local flow deviation. A root-finding method is utilized at each incremental vane section of each turning vane to converge on the design cambers that set the predicted local deviation angles equal to the local trailing edge angles.
Experimental and computational results validate the design method employed with the reduction of an experimentally measured 30 degree peak paired swirl profile to a 3 degree peak, 1.01 degree rms, swirl profile. Flow angularity and loss measurements detailed at 1/2 duct diameter downstream of the 1/6 duct diameter axial length of the device introduce the Inverse StreamVane as a very effective and efficient method of swirl mitigation. / MS / Fluid flows with components in directions other than just the dominant axial direction can create complexities and undesirable effects on a variety of systems. In applications such as aircraft engines and HVAC compression systems, these secondary velocity components can lead to lower efficiencies and loss in operable range. In applications such as flow rate measurement, these secondary velocity components can lead to losses in accuracy and increased measurement uncertainty. The underline effect of such losses in these systems is undesirable increase in system costs.
For these reasons, a variety of devices and efforts have been made in attempts to reduce secondary velocity components. Many methods of reducing secondary velocity components, or straightening the flow, involve the introduction of obstructions in the fluid’s path. These obstructions can create energy losses, thus reducing the amount of energy the fluid can impart, or increasing the amount of energy input required to the fluid in order to achieve the same system performance.
In the presented work, the design and wind tunnel validation of a flow straightening device is detailed. The design method utilizes curved airfoil shaped turning vanes strategically placed to reduce obstruction and increase straightening effectiveness. The device is referred to as the Inverse StreamVane, as it stems from the design principles of the secondary velocity producing StreamVane TM device introduced in 2013.
With the design method presented, flow straightening devices tailored to specified flow profiles can be rapidly generated. As is shown by experimental measurements in a wind tunnel, large secondary velocity components are removed by 90% or greater in very short axial distances downstream of the Inverse StreamVane. Losses at the flow regime tested are also shown to be very small downstream of the Inverse StreamVane. The work presented suggests that the design system and device has the potential to benefit a variety of industries and applications.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/97888 |
| Date | 30 October 2018 |
| Creators | Dawson, Dylan Paul |
| Contributors | Mechanical Engineering, O'Brien, Walter F. Jr., Lowe, K. Todd, Wicks, Alfred L. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | ETD, application/pdf, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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