John D. Coull

Winglets for Improved Aerothermal Performance of High Pressure Turbines

© 2014 by ASME. This paper investigates the design of winglet tips for unshrouded high pressure turbine rotors considering aerodynamic and thermal performance simultaneously. A novel parameterization method has been developed to alter the tip geometry of a rotor blade. A design survey of uncooled, flat-tipped winglets is performed using Reynolds-averaged Navier-Stokes (RANS) calculations for a single rotor at engine representative operating conditions. Compared to a plain tip, large efficiency gains can be realized by employing an overhang around the full perimeter of the blade, but the overall heat load rises significantly. By employing an overhang on only the early suction surface, significant efficiency improvements can be obtained without increasing the overall heat transfer to the blade. The flow physics are explored in detail to explain the results. For a plain tip, the leakage and passage vortices interact to create a three-dimensional impingement onto the blade suction surface, causing high heat transfer. The addition of an overhang on the early suction surface displaces the tip leakage vortex away from the blade, weakening the impingement effect and reducing the heat transfer on the blade. The winglets reduce the aerodynamic losses by unloading the tip section, reducing the leakage flow rate, turning the leakage flow in a more streamwise direction, and reducing the interaction between the leakage fluid and end wall flows. Generally, these effects are most effective close to the leading edge of the tip where the leakage flow is subsonic.

High Performance SOI-CMOS Wall Shear Stress Sensors

Here we present for the first time, a novel silicon on insulator (SOI) complementary metal oxide semiconductor (CMOS) MEMS thermal shear stress sensor for turbulent flow measurements based on aluminum hot-film as a sensing element. These devices have been fabricated using commercial 1 mum SOI-CMOS process followed by a deep reactive ion etch (DRIE) back-etch step, offering low cost and the option of circuit integration. The sensors have a good spatial resolution (size 130 mum times 130 mum) and a very efficient thermal isolation (due to their location on a 500 mum times 500 mum, low thermal conductivity silicon oxide membrane). Results show that these sensors have a high temperature coefficient of resistance (TCR) (0.319%/degC), a low power consumption (below 10 mW for 100degC temperature rise) and a high reproducibility within a wafer and from wafer to wafer. In constant temperature (CT) mode, the sensors exhibit an average sensitivity of 22 mV/Pa in a wall shear stress range of 0-1.5 Pa and an ultra-short time constant of only 17 mus, which corresponds to a high cut-off frequency of 39 kHz.

VELOCITY DISTRIBUTIONS FOR LOW PRESSURE TURBINES

A parametric set of velocity distributions has been investigated using a flat plate experiment. Three different diffusion factors and peak velocity locations were tested. These were designed to mimic the suction surfaces of Low Pressure (LP) turbine blades. Unsteady wakes, inherent in real turbomachinery flows, were generated using a moving bar mechanism. A turbulence grid generated a freestream turbulence level that is believed to be typical of LP turbines. Measurements were taken across a Reynolds number range of 50,000-220,000 at three reduced frequencies (0.314, 0.628, 0.942). Boundary layer traverses were performed at the nominal trailing edge using a Laser Doppler Anemometry system and hot-films were used to examine the boundary layer behaviour along the surface. For every velocity distribution tested, the boundary layer separated in the diffusing flow downstream of the peak velocity. The loss production is dominated by the mixing in the reattachment process, mixing in the turbulent boundary layer downstream of reattachment and the effects of the unsteady interaction between the wakes and the boundary layer. A sensitive balance governs the optimal location of peak velocity on the surface. Moving the velocity peak forwards on the blade was found to be increasingly beneficial when bubblegenerated losses are high, i.e. at low Reynolds number, at low reduced frequency and at high levels of diffusion. Copyright

High-Sensitivity Single Thermopile SOI CMOS MEMS Thermal Wall Shear Stress Sensor

In this paper, we present a novel silicon-on-insulator (SOI) complementary metal-oxide-semiconductor (CMOS) microelectromechanical-system thermal wall shear stress sensor based on a tungsten hot-wire and a single thermopile. Devices were fabricated using a commercial 1-

A thermopile based SOI CMOS MEMS wall shear stress sensor

\mu \text{m}

Minimizing the loss produced by a turbulent separation using vortex generator jets

SOI-CMOS process followed by a deep reactive ion etching back-etch step to release a silicon dioxide membrane, which mechanically supports and thermally isolates heating and sensing elements. The sensors show an electrothermal transduction efficiency of

High efficiency cavity winglets for high pressure turbines

50~\mu \text{W}

Investigation of Wake Induced Transition in Low-Pressure Turbines Using Large Eddy Simulation

/°C, and a very small zero flow offset. Calibration for wall shear stress measurement in air in the range of 0-0.48 Pa was performed using a suction type, 2-D flow wind tunnel. The sensors were found to be extremely sensitive, up to 4 V/Pa for low wall shear stress values. Furthermore, we demonstrate the superior signal-to-noise ratio (up to five times higher) of a single thermopile readout configuration compared with a double thermopile readout configuration (embedded for comparison purposes within the same device). Finally, we verify that the output of the sensor is proportional to the cube root of the wall shear stress and we propose an accurate semiempirical formula for its modeling.

Diode-based CMOS MEMS thermal flow sensors

In this paper we present for the first time, a novel silicon on insulator (SOI) complementary metal oxide semiconductor (CMOS) MEMS thermal wall shear stress sensor based on a tungsten hot-film and three thermopiles. These devices have been fabricated using a commercial 1 μm SOI-CMOS process followed by a deep reactive ion etch (DRIE) back-etch step to create silicon oxide membranes under the hot-film for effective thermal isolation. The sensors show an excellent repeatability of electro-thermal characteristics and can be used to measure wall shear stress in both constant current anemometric as well as calorimetric modes. The sensors have been calibrated for wall shear stress measurement of air in the range of 0-0.48 Pa using a suction type, 2-D flow wind tunnel. The calibration results show that the sensors have a higher sensitivity (up to four times) in calorimetric mode compared to anemometric mode for wall shear stress lower than 0.3 Pa.

Numerical investigations of different tip designs for shroudless turbine blades

Vortex generator jets have been applied to control a separating turbulent boundary layer on the suction surface of compressor blades in a linear cascade at high incidence. With the jets operating in a steady blowing mode, loss measurements were taken over a range of jet velocities. An increase in the jet blowing ratio yielded a reduction in the loss coefficient up to a blowing ratio of 70%. At this condition, a loss reduction of 61% was measured relative to the case with no control. Higher jet velocities yielded a slight increase in the loss coefficient. In order to explore the behavior of the boundary layer over this range of blowing ratios, four sets of experiments were performed: static pressure measurements, wall shear stress measurements, stereoscopic Particle Image Velocimetry (PIV) measurements and smoke flow visualization. The static pressure measurements showed that the point of separation moves downstream from 60% surface length with no control, to approximately 92% with a blowing ratio of 70%. At higher blowing ratios, the boundary layer remains attached up to the trailing edge. Shear stress measurements were taken on the suction surface using a streamwise array of novel, dual element MEMS hot-film sensors. The mean quasi-wall shear stress measured with the sensors indicated attached flow in the region of the jet holes and separated flow downstream at all blowing ratios including and below 50%. At a blowing ratio of 75%, the quasi-wall shear stress measurements suggest that the flow is attached, but close to separation. These results suggest that minimum loss is obtained at the blowing ratio required to just keep the boundary layer attached. This conclusion is approximate because the flow from the discrete jets is inherently three-dimensional, and so the separation location will vary across the span. The three-dimensionality of the flow produced by the jets was evident in the quasi-wall shear stress and PIV measurements, as well as the smoke flow visualization. A vortex skewed relative to the streamwise direction was identified in the PIV measurements. By correlating the location of this vortex with the shear stress measurements, this vortex was identified with a region of elevated shear stress. A second region of elevated shear stress was, however, identified between the vortices. Turbulent kinetic energy extracted from the PIV measurements allowed the identification of this region with secondary flow between the co-rotating vortices produced by adjacent jets.