Todd Garrett Wetzel

Coupled Acoustic and Heat Transfer Modeling of a Synthetic Jet

Synthetic jets based on acoustic resonators have recently been applied to the active cooling of small, heat generating components such as microelectronics. The synthetic jets considered in this study are constructed using two piezoelectric bimorph disks separated by a donut shaped elastomeric wall with a small orifice in the wall. The piezo disks are energized to actuate out of phase at high frequency to alternately entrain and expel surrounding air. The resulting pulsating jet of air enhances local heat transfer by a factor of at least 3X, and exceeds 8x for small surfaces, compared to natural convection. These synthetic jets also create substantial noise at certain operating conditions that may be objectionable for some applications. This paper develops an analytical model that captures the coupled structural dynamic, acoustic and heat transfer physics, but is also simple enough to investigate the underlying physical behavior of the parameters that govern the jet’s performance and run many trade-o studies. Detailed comparison with experimental results is also discussed. Despite issues with the comparison of some parameters impacting jet performance, such as disk velocity and exit velocity, the predicted sound intensity and heat transfer coecient agree well with experimental test data.

Heat Transfer in a Complex Trailing Edge Passage for a High Pressure Turbine Blade: Part 1 — Experimental Measurements

A combined experimental and numerical study to investigate the heat transfer distribution in a complex blade trailing edge passage was conducted. The geometry consists of a two pass serpentine passage with taper toward the trailing edge, as well as from hub to tip. The upflow channel has an average aspect ratio of roughly 14:1, while the exit passage aspect ratio is about 5:1. The upflow channel is split in an interrupted way and is smooth on the trailing edge side of the split and turbulated on the other side. A turning vane is placed near the tip of the upflow channel. Reynolds numbers in the range of 31,000 to 61,000, based on inlet conditions, were simulated numerically. The simulation was performed using the Glenn-HT code, a full three-dimensional Navier-Stokes solver using the Wilcox k-omega turbulence model. A structured multi-block grid is used with approximately 4.5 million cells and average y+ values on the order of unity. Pressure and heat transfer distributions are presented with comparison to the experimental data. While there are some regions with discrepancies, in general the agreement is very good for both pressure and heat transfer.

Evolution of air-cooled turbine generator design

Recent market trends have seen air-cooled generators fulfilling application needs over an increasing range of power output ratings. While GE continues to provide and to further develop an extensive line of hydrogen-cooled generators, significant investment in the air-cooled product line has further enhanced the reliability and performance of the overall product structure. Concurrent with this demand for increased performance and range of product offerings, market demands for higher machine efficiency and cost effective design are increasingly driving the need for continuous product enhancement. To meet these demands, several new technologies and machine designs for the air-cooled product line have been developed. This paper describes these advances as applied to the air-cooled line of generator products.

Use of fiber optic-based distributed temperature measurement system for electrical machines

A fiber optic based distributed temperature measurement system was implemented in stator windings (straight copper bars) as well as in the end-windings (curved copper bars) of a motor. Usually, in electrical machines such as motors or generators, only a few conventional temperature sensors are used, whereas the distributed temperature system has the potential of providing very detailed temperature distribution by having hundreds of sensors in a single fiber. The sensors were made of Bragg gratings etched onto the fiber itself. For the present study, the spatial resolution of the sensors is 6 mm (nominally at 1/4” apart). The technique uses Optical Frequency Domain Reflectometry (OFDR) to process the back-reflected light signal indicative of the thermal filed. A prototype fiber optic system was implemented in a motor made by GE industrial systems. The sensing length (length of the stator) for the motor was 0.75 m containing approximately 150 sensors thus providing very detailed temperature data. Performance tests were conducted at different heat loads representing different electrical conditions. Continuous tests for the duration of 19 hours were conducted. The temperature of stator windings varied from ambient (~ 23°C) to approximately 85°C. As reference, Resistance Temperature Devices (RTDs) were installed in adjacent slots to the slot where fiber optic sensors were installed. A total of 8 sensors were installed but data were collected on only 3 fibers. Fiber sensor measurements were found to track the temperature trends very well. The fiber data agreed with RTD data within ± 3°C in the entire duration. The RMS value of difference between the fiber and RTD on one side was 0.3°C, and with the RTD on the other side was 0.5°C. The fiber measurements also showed how hotspots could be missed by using few RTDs, as is done in the industry. The fiber measurements also showed the temperature distribution in the endwindings, an area not normally monitored. The maximum temperature was an acceptable 110°C. The feasibility of this technique for measuring stator-winding temperatures is proved. Still some of the problems faced during the installation and experiments are (a) robustness of fiber and sheathing fiber and (b) fiber survivability during manufacturing process and repair.

MEMS Microvalve for Harsh Environment

There is a need for small valves which control flow at high temperature and pressure for a number of commercial and military applications. However, traditional solenoid actuated valves are typically expensive, heavy and subject to undesirable electrical and mechanical failure modes. Micromachining techniques, commonly used in the electronics industry, are finding more and more applications for Micro Electro Mechanical Systems (MEMS). Most of the previous work in the area of MEMS valves has been limited to low pressure/low temperature flows at ambient conditions. In this presentation, the development and testing of a two stage MEMS diaphragm valve that is capable of operating at high temperature and pressure gas flows is presented. Valve requirements, tolerances and thermal management are considered in the design. Valve fabrication processes, such as Reactive Ion Etching (RIE) and laser ablation, are discussed in detail. Issues related to the actuation of the microvalve are also discussed, including two approaches based on shape memory alloy and piezoelectric materials. Modeling and test results are presented throughout to identify successes and lessons learned.Copyright