Kenneth W. Van Treuren

Measurements in a Turbine Cascade Flow Under Ultra Low Reynolds Number Conditions

With the new generation of gas turbine engines, low Reynolds number flows have become increasingly important. Designers must properly account for transition from laminar to turbulent flow and separation of the flow from the suction surface, which is strongly dependent upon transition. Of interest to industry are Reynolds numbers based upon suction surface length and flow exit velocity below 150,000 and as low as 25,000. In this paper, the extreme low end of this Reynolds number range is documented by way of pressure distributions, loss coefficients, and identification of separation zones. Reynolds numbers of 25,000 and 50,000 and with 1 percent and 8-9 percent turbulence intensity of the approach flow (free-stream turbulence intensity, FSTI) were investigated. At 25,000 Reynolds number and low FSTI, the suction surface displayed a strong and steady separation region. Raising the turbulence intensity resulted in a very unsteady separation region of nearly the same size on the suction surface. Vortex generators were added to the suction surface, but they appeared to do very little at this Reynolds number. At the higher Reynolds number of 50,000, the low-FSTI case was strongly separated on the downstream portion of the suction surface. The separation zone was eliminated when the turbulence level was increased to 8-9 percent. Vortex generators were added to the suction surface of the low-FSTI case. In this instance, the vortices were able to provide the mixing needed to re-establish flow attachment. This paper shows that massive separation at very low Reynolds numbers (25,000) is persistent, in spite of elevated FSTI and added vortices. However, at a higher Reynolds number, there is opportunity for flow reattachment either with elevated free-stream turbulence or with added vortices. This may be the first documentation of flow behavior at such low Reynolds numbers. Although it is undesirable to operate under these conditions, it is important to know what to expect and how performance may be improved if such conditions are unavoidable.

Using Gurney Flaps to Control Laminar Separation on Linear Cascade Blades

This paper describes an experimental investigation of the use of Gurney flaps to control laminar separation on turbine blades in a linear cascade. Measurements were made at Reynolds numbers (based upon inlet velocity and axial chord) of 28×10 3 , 65×10 3 and 167×10 3 . The freestream turbulence intensity for all three cases was 0.8%. Laminar separation was present on the suction surface of the Langston blade shape for the two lower Reynolds numbers. In an effort to control the laminar separation, Gurney flaps were added to the pressure surface close to the trailing edge. The measurements indicate that the flaps turn and accelerate the flow in the blade passage toward the suction surface of the neighboring blade thereby eliminating the separation bubble. Five different sizes of Gurney flaps, ranging from 0.6 to 2.7% of axial chord, were tested. The laser thermal tuft technique was used to determine the influence of the Gurney flaps on the location and size of the separation bubble. Additionally, measurements of wall static pressure, profile loss, and blade-exit flow angle were made. The blade pressure distribution indicates that the lift generated by the blade is increased. As was expected, the Gurney flap also produced a larger wake. In practice, Gurney flaps might possibly be implemented in a semi-passive manner. They could be deployed for low Reynolds number operation and then retracted at high Reynolds numbers when separation is not present. This work is important because it describes a successful means for eliminating the separation bubble while characterizing both the potential performance improvement and the penalties associated with this semi-passive flow control technique.

Local Heat Transfer Coefficient and Adiabatic Wall Temperature Measurement Beneath Arrays of Staggered and Inline Impinging Jets

Recent work, Van Treuren et al. (1993), has shown the transient method of measuring heat transfer under an array of impinging jets allows the determination of local values of adiabatic wall temperature and heat transfer coefficient over the complete surface of the target plate. Using this technique, an inline array of impinging jets has been tested over a range of average jet Reynolds numbers (10,000–40,000) and for three channel height to jet hole diameter ratios (1, 2, and 4). The array is confined on three sides and spent flow is allowed to exit in one direction. Local values are averaged and compared with previously published data in related geometries. The current data for a staggered array is compared to those from an inline array with the same hole diameter and pitch for an average jet Reynolds number of 10,000 and channel height to diameter ratio of one. A comparison is made between intensity and hue techniques for measuring stagnation point and local distributions of heat transfer. The influence of the temperature of the impingement plate through which the coolant gas flows on the target plate heat transfer has been quantified.Copyright

Experimental Evaluation of Open Propeller Aerodynamic Performance and Aero-acoustic Behavior

A topic of increasing importance in the Unmanned Aerial System (UAS) community is the design and performance of open propellers used in hand launched, small UASs. The performance of these small propellers directly influences the operational capabilities of the UAS. As such, the design and testing of these propellers is necessary to accurately predict UAS performance. This experimental investigation examined the relationship between diameter, pitch, and number of blades to aerodynamic efficiency and aero-acoustic sound pressure levels. Thrust, torque, propeller rotational speed, and sound pressure level were measured for twelve aero-nautCAMcarbon (ACC) folding propeller configurations currently being used on an operational UAS with diameters ranging from 12 to 15 inches, pitches from 6 to 13 inches and increasing from two to three propeller blades. Each configuration was tested at 44 ft/s tunnel velocity, the typical cruising velocity of a small UAS, while the propeller rotational speed was varied to determine the rotational speed needed to produce 2.5 lbf of thrust, a typical cruise thrust required for a small UAS. As expected, the rotational speed required to achieve the desired thrust decreased approximately 7.7% per inch increase in the propeller diameter. At the same time, the noise signature decreased by approximately 0.8 dB per inch increase and overall efficiency rose by 2.9% per inch increase. Similar results were found for increasing both the number of propeller blades and also increasing the pitch of the propeller. Increasing from two to three blades decreased the rotational speed by 9.1% with a 2.1 dB drop in sound pressure level and an increase in overall efficiency of 3.2%. Increasing the pitch generally decreased rotational speed by 4.6% per inch of pitch increase and decreased noise level by 0.7dB per inch of pitch increase. Overall efficiency slightly increased by 0.6% for an inch increase in pitch. For a given diameter propeller there seems to be an optimum pitch for minimum sound pressure level. For design, this indicates there is an optimum angle of attack for the propeller, which translates to an optimum beta twist angle to achieve minimum sound pressure level. Noise generation was found to be a strong function of propeller rotational speed. Lower rotational speed generally produced less noise.

Effect of Passive and Active Air-Jet Turbulence on Turbine Blade Heat Transfer

The heat transfer distribution on a turbine blade shape has been measured in a linear cascade wind tunnel for turbulence levels between 0.5% and 15%. The measurements were conducted at a low Reynolds number (80,000). This is typical of low pressure turbine stages at high altitude where unpredicted losses, attributed to transition and separation, have been observed. The heat transfer distributions provide insight into the transition and separation behavior. Turbulence levels from 5% to 10% were generated with a passive biplane lattice grid, while turbulence levels from 10% to 15% were generated by an active air-jet grid. As turbulence levels increased, stagnation heat transfer increased and the location of the boundary layer transition advanced toward the leading edge on the suction side of the blade. The stagnation Nu/Re0.5 increased by 18.6% when going from a clean tunnel turbulence of 0.5% to 15% turbulence. The heat transfer was measured using a uniform heat flux liquid crystal technique. At turbulence intensities of 0.5% stream wise streaks of varying heat transfer were recorded on the concave pressure side of the turbine blade characteristic of Gortler vortices. At higher levels of turbulence these streaks disappeared. Turbulence decay rates, along with micro and macro length scales, are reported for both active and passive grid test conditions.Copyright

Designing Small Propellers for Optimum Efficiency and Low Noise Footprint

Abstract : A topic of increasing importance in the Unmanned Aerial System (UAS) community is the design and performance of open propellers used in hand launched, small UASs. The design and testing of these propellers is necessary to accurately predict UAS operation. This paper describes the design methodology used by Baylor University and the USAF Academy to design propeller blades for optimum efficiency and low noise. Propeller blade design theories are discussed as well as an overview of several of the existing design codes. Included is a discussion on geometric angle of attack, the induced angle of attack, and their impact on propeller design. The design program BEARCONTROL was developed which incorporates the programs QMIL and QPROI). Supplemental codes were also developed to work with Bearcontrol to design a propeller with a constant chord and variable twist. This resulted in the angle of attack for L/Dmax being used from the propeller hub to the tip. BEARCONTROL is a program written in MATLAB that gives a user the ability to quickly design a propeller, predict its performance, and then create a 30 model in SolidWorks. The MATLAB GUl ultimately results in a mostly automated process that is simple to use for individuals who are unfamiliar with command prompt programs and SolidWorks modeling. Also incorporated into BEARCONTROL is the program NREL AirFoil Noise (NAFNOISE) developed by the National Renewable Energy Laboratory (NREL). This program predicts the noise of any airfoil shape and provides a comparison for optimizing/minimizing predicted noise for the propeller being designed. Construction methods and materials also have a direct impact on cost, durability and operability when using a rapid prototype process to fabricate propellers. An overview of materials and construction methods used in this research are discussed. Incorporation of a hub with interchangeable blades is also presented as a more efficient testing method.

Comparison and Prediction of Local and Average Heat Transfer Coefficients Under an Array of Inline and Staggered Impinging Jets

Recent work, Van Treuren et al. (1993, 1994), has shown the transient method of measuring heat transfer under an array of impinging jets allows the determination of local values of adiabatic wall temperature and heat transfer coefficient over the complete surface of the target plate. Using this technique, an inline and staggered array of impinging jets was tested over a range of average jet Reynolds numbers (10,000–40,000) for three impingement plate to target plate separations (1, 2 and 4). The array was confined on three sides and spent flow was allowed to exit in one direction. Local and average values are presented. These values for the two array configurations are then compared with each other as well as with previously published data in related geometries. A new correlation technique is presented, based on the local data, which breaks the target surface into jet and crossflow areas of interest, with excellent results. The correlation uses the local jet Reynolds number and local jet-to-crossflow mass velocity ratio. This new technique compared favourably with published correlations. Also presented is the influence of the impingement plate on the target plate heat transfer in the form of an effectiveness parameter. This influence is accounted for in the correlation.Copyright

Scaling Small-Scale Wind Turbines for Wind Tunnel Testing

Wind tunnel testing of wind turbines can provide valuable insights into wind turbine performance and provides a simple process to test and improve existing designs. However, the scale of most wind turbines is significantly larger than most existing wind tunnels, thus, the scaling required for testing in a typical wind tunnel presents multiple challenges. When wind turbines are scaled, often only geometric similarity and tip speed ratio matching are employed. Scaling in this manner can result in impractical rotational velocities. For wind tunnel tests that involve Reynolds numbers less than approximately 500,000, Reynolds number matching is necessary. When including Reynolds number matching in the scaling process, keeping rotational velocities realistic becomes even more challenging and preventing impractical freestream velocities becomes difficult. Turbine models of 0.5, 0.4, and 0.3 m diameter, resulting in wind tunnel blockages up to 52.8%, were tested in order to demonstrate scaling using Reynolds number matching and to validate blockage corrections found in the literature. Reynolds numbers over the blades ranged from 20,000 to 150,000 and the tip speed ratio ranged from 3 to 4 at the maximum power point for each wind speed tested.Copyright

An Examination of the Effect of Reynolds Number on Airfoil Performance

The standard of living throughout the world has increased dramatically over the last 30 years and is projected to continue to rise. This growth leads to an increased demand on conventional energy sources, such as fossil fuels. However, these are finite resources. Thus, there is an increasing demand for alternative energy sources, such as wind energy. Much of current wind turbine research focuses on large-scale (>1 MW), technologically-complex wind turbines installed in areas of high average wind speed (>20 mph). An alternative approach is to focus on small-scale (1–10kW), technologically-simple wind turbines built to produce power in low wind regions. While these turbines may not be as efficient as the large-scale systems, they require less industrial support and a less complicated electrical grid since the power can be generated at the consumer’s location. To pursue this approach, a design methodology for small-scale wind turbines must be developed and validated. This paper addresses one element of this methodology, airfoil performance prediction. In the traditional design process, an airfoil is selected and published lift and drag curves are used to optimize the blade twist and predict performance. These published curves are typically generated using either experimental testing or a numeric code, such as PROFIL (the Eppler Airfoil Design and Analysis Code) or XFOIL. However, the published curves often represent performance over a different range of Reynolds numbers than the actual design conditions. Wind turbines are typically designed from 2-D airfoil data, so having accurate airfoil data for the design conditions is critical. This is particularly crucial for small-scale, fixed-pitched wind turbines, which typically operate at low Reynolds numbers (<500,000) where airfoil performance can change significantly with Reynolds number. From a simple 2-D approach, the ideal operating condition for an airfoil to produce torque is the angle of attack at which lift is maximized and drag is minimized, so prediction of this angle will be compared using experimental and simulated data. Theoretical simulations in XFOIL of the E387 airfoil, designed for low Reynolds numbers, suggest that this optimum angle for design is Reynolds number dependent, predicting a difference of 2.25° over a Reynolds number range of 460,000 to 60,000. Published experimental data for the E387 airfoil demonstrate a difference of 2.0° over this same Reynolds number range. Data taken in the Baylor University Subsonic Wind Tunnel for the S823 airfoil shows a similar trend. This paper examines data for the E387 and S823 airfoils at low Reynolds numbers (75,000, 150,000, and 200,000 for the S823) and compares the experimental data with XFOIL predictions and published PROFIL predictions.© 2011 ASME

Experimental Analysis of a Counter-Rotating Wind Turbine

Increases in wind turbine efficiency have helped to provide cost-effective power to an ever-growing portion of the world. This paper explores the possibility of increasing power production using two counter-rotating sets of wind turbine blades. A review of design characteristics, such as number of blades, blade angle of twist, chord length, and generator efficiencies, resulted in the design of a counter-rotating wind turbine incorporating three different National Renewable Energy Laboratory (NREL) cross-sectional blade profiles along the span of the blades. A three-blade front system and two different three-blade rear systems were studied. The blade prototypes were modeled in SolidWorks® , produced using a Dimension SST 3D printer, and then tested using two Parallax™ four-pole stepper motors as generators in a model 406B ELD wind tunnel. Testing was performed between 15 mph and 40 mph in 5-mph increments. Preliminary results indicate that a counter-rotating assembly is promising for increasing energy extraction from a column of air. The counter-rotating system reached its optimum operating efficiency in wind tunnel testing at 25 mph using an exact reflection for the rear fan. At these test conditions 0.40% of the energy in the air column was converted into usable power. This outcome compares to a 0.21% power conversion when testing only the front-blade system. Additional testing will be completed using flow visualization in a ELD 502 water tunnel along with CFD analysis. The purpose of this testing is to discover air column behavior behind the upstream and downstream blade systems to optimize the design and increase total system efficiency. An appropriate scaling method must also be found. Currently, an energy model is being used to scale from the wind tunnel to the water tunnel. These tests would make it possible to design blade sets to create a maximum total efficiency at a specific wind speed. It would also be valuable to determine if counter-rotating systems could expand the range of possible turbine locations by lowering the required wind speed for significant power generation.Copyright