Michael Benson

Effects of Wall Roughness on Particle Velocities in a Turbulent Channel Flow

Experimental measurements using a laser Doppler anemometer (LDA) system have been performed on 150μm dense glass particles in a fully developed downward channel flow in air. Tests were conducted in smooth, rough development, and fully rough wall conditions with a channel Reynolds number of 13,800, corresponding to a centerline gas phase velocity of 10.5m∕s with a dilute loading of particles of 15% by mass fraction. Velocities were measured and statistics compared to see the nature of the effects of the wall roughness in a rebuilt channel facility originally used for important works including Kulick, Fessler, and Eaton, (1994, “Particle Response and Turbulence Modification in Fully-Developed Channel flow ,” J. Fluid Mech., 277, pp. 109–134) and Paris (2001, “Turbulence Attenuation in a Particle-Laden Channel Flow ,” Ph.D. thesis, Stanford University, Stanford, CA). Wall roughness has a substantial impact on gas phase mean velocities across most of the channel width, except very near the wall. The turbulence intensity of the gas phase is enhanced across the entire channel in the presence of fully rough walls. The rough walls have an even greater impact on the particle phase. Streamwise particle velocities are reduced up to 40%, and become quite uniform across the channel. Particle fluctuating velocities are nearly doubled near the channel centerplane. Profiles appear uniform, due in large part to strong transverse mixing induced by particle-wall collisions. Much of the data of Kulick and Paris is shown here to be strongly influenced by wall conditions with poorly defined roughness in the development region, followed by rapid flow recovery in a relatively smooth test section.

Three-Dimensional Mass Fraction Distribution of a Spray Measured by X-Ray Computed Tomography

In order to design a spraying system with the desired characteristics, the atomization process has to be understood in detail, including the primary break-up of the liquid core. Accurate prediction of primary break-up is a major barrier to computer-based analysis of spray combustion. The development of models is hindered by the lack of validation data in a region where the fluid is dense, and optical access is therefore limited.The present experimental study is aimed at probing the spray structure by means of X-ray computed tomography (CT). A full-cone atomizer (0.79 mm orifice diameter) spraying in air at ambient pressure is investigated as a proof of concept. A mixture of water and iodine is used as the working fluid, providing elevated X-ray absorption and therefore improved signal-to-noise ratio. Several hundreds of X-ray projections are acquired as the spraying atomizer is rotated in front of the detector. Standard software for medical imaging is used to reconstruct the three-dimensional time-averaged distribution of liquid mass fraction in the full field of view, from the intact liquid core to the dilute spray region. A spatial resolution of 0.6 mm is obtained along the spraying direction, while the resolution is 0.3 mm in the other two directions. Significant asymmetries in the structure of the spray are revealed.Copyright

On the Analysis of the Aerodynamic Heating Problem

A complete analytical solution to the problem of aerodynamic heating is lacking in heat transfer textbooks, which are used for undergraduate and graduate education. There are many issues that are very important from a convective heat transfer point of view. In practice, poor analyses lead to poor design, thus faulty manufacturing. Since, over the years analysis has given way to numerical studies, the instructors do not take the necessary time to go through analytical details. Thus the students just use the results without any awareness of how to get them and the inherent limitations of the analytical solution. The only intent of this paper, therefore, is to present the detailed analytical study of the aerodynamic heating problem.

On the Similarity Solution for Condensation Heat Transfer

Analytical solutions for laminar film condensation on a vertical plate are integral to many heat transfer applications, and have therefore been presented in numerous refereed articles and in most heat transfer textbooks. Commonly made assumptions achieve the well known similarity solution for the Nusselt number, heat transfer coefficient, and film thickness. Yet in all of these studies, several critical assumptions are made without justifying their use. Consequently, for a given problem one cannot determine whether these restrictive assumptions are actually satisfied, and thus, how these conditions can be checked for validity of the results. This study provides a detailed solution that clarifies these points.

On the Teaching of Condensation Heat Transfer

The topic of condensation heat transfer is usually included in a chapter on Boiling and Condensation in most Heat Transfer textbooks. The assumptions made are those of laminar liquid film with constant thermo-physical properties, uniform vapor temperature equal to the saturation temperature of the vapor, negligible shear at the liquid-vapor interface, and negligible momentum and energy transfer by advection in the condensate film. The results presented are normally for the film thickness, the local convective heat transfer coefficient, and the Nusselt number. However, no means are presented to the student to determine if all of these simplifying assumptions are actually satisfied for a given problem. This investigation clarifies these points to improve teaching of the material and understanding by the student at the undergraduate and graduate level.Copyright

Assessment of Large Eddy Simulation Predictive Capability for Compound Angle Round Film Holes

Film cooling holes with a compound angle are commonly used on high pressure turbine components in lieu of axial holes to improve effectiveness or as a result of manufacturing constraints. Whereas large eddy simulation (LES) of axial holes is becoming more common place, assessment of LES predictive ability for compound angle hole has been limited. For this study, the selected compound angle round (CAR) hole configuration has a 30 degree injection angle, a 45 degree compound angle, and a density ratio of 1.5. The geometry, flow conditions, and experimental adiabatic effectiveness validation data are from McClintic et al. [28]. The low free stream Mach number of the experiment puts the flow in the incompressible regime. Two LES solvers are evaluated, Fluent and FDL3Di, on structured meshes with a range of blowing ratios simulated for plenum, inline coolant crossflow, and counter coolant crossflow feed holes. When a steady inlet profile is used for the main flow, LES agreement with the data is poor. The inclusion of a resolved turbulent boundary layer significantly improves the predictive quality for both solvers; consequently, resolved inflow turbulence is a required aspect for CAR hole LES. The remaining differences between the simulations and IR data are partly attributed to the steady coolant inlet profiles used for the counter and inline cross feeds.Copyright

A VIRTUAL GAS TURBINE LABORATORY FOR AN UNDERGRADUATE THERMODYNAMICS COURSE

Topics on gas turbine machinery have been successfully integrated into the thermodynamics course at the United States Military Academy (USMA). Because graduating cadets will encounter gas turbines throughout their service in the U.S. Army, it is important for all engineering students, not just mechanical engineering majors, to learn about gas turbines, their operation, and their applications. This is accomplished by four methods, one of which is an experimental analysis of an operational auxiliary power unit (APU) from an Army helicopter. Due to recent building issues, this gas turbine laboratory was improvised and offered as a fully digital virtual laboratory exercise. Since all undergraduate programs do not have the luxury of having a gas turbine laboratory, our experiences with the virtual laboratory are offered as an effective option. By digitally reproducing the laboratory setup, introduction, instrumentation, data collection and analysis, the virtual experience captures the essence of the laboratory. After viewing the web-based laboratory digital media files, students use one of two data sets, recorded from the data display panel in the real laboratory, in order to complete the laboratory report. While the tremendous advantage of actually seeing, testing, and analyzing the real engine cannot be denied, a well-planned and executed virtual laboratory adequately achieves learning objectives and provides students a unique opportunity to apply gas turbine fundamentals. An assessment of the virtual laboratory and results of student feedback are provided.Copyright

3D Velocity and Scalar Field Measurements of an Airfoil Trailing Edge With Slot Film Cooling: The Effect of an Internal Structure in the Slot

Measurements of the 3D velocity and concentration fields were obtained using magnetic resonance imaging for a pressure side cutback film cooling experiment. The cutback geometry consisted of rectangular slots separated by straight lands; inside each of the slots was an airfoil-shaped blockage. The results from this trailing edge configuration, the “island airfoil,” are compared to the results obtained with the “generic airfoil,” a geometry with narrower slots, wider, tapered lands, and no blockages. The objective was to determine how the narrower lands and internal blockages affected the average film cooling effectiveness and the spanwise uniformity. Velocimetry data revealed that strong horseshoe vortices formed around the blockages in the slots, which resulted in greater coolant non-uniformity on the airfoil breakout surface and in the wake. The thinner lands of the island airfoil allowed the coolant to cover a larger fraction of the trailing edge span, giving a much higher spanwise-averaged surface effectiveness, especially near the slot exit where the generic airfoil lands are widest.

Upgrading the Undergraduate Gas Turbine Lab

A study of gas turbine engines is an important component of an integrated thermodynamics and fluid mechanics two-course sequence at the United States Military Academy (USMA). Owing to the ubiquity of gas turbines in military use, graduating cadets will encounter a variety of these engines throughout their military careers. Especially for this unique population, it is important for engineering students to be familiar with the operation and applications of gas turbines. Experimental analysis of a functional auxiliary power unit (APU) from an Army utility helicopter has been a key component of this block of instruction for several decades. As with all laboratory equipment, the APU has experienced intermittent maintenance issues, which occasionally render it unusable for the gas turbine laboratory in the course. Because of this, a very basic virtual laboratory was implemented which integrated video of the physical laboratory with key parameters and behind-the-screen data collection for use in engine analysis.A revitalized version of both the physical and virtual gas turbine laboratory experiences offered in the thermal-fluids course will include substantial improvements over the existing setup. The physical laboratory, which is centered on a refurbished APU from a medium-sized commercial aircraft, will continue to incorporate measurements of temperature and pressure throughout the combustion process, as well as fuel flow rate. In an improvement over the original laboratory setup, an orifice plate will be used to measure the flow rate of bleed air exiting the turbine, which had not previously been open during engine testing. Additionally, the air flow through the anti-surge valve was not metered in the original version of the physical laboratory. However, the anti-surge air flow can account for nearly 25% of the total air flow, and performance calculations in the physical laboratory will now account for this loss. The turbine output shaft will run a water-brake dynamometer. All instrumentation will be converted to digital signals and projected on a large screen outside the test area through a LabVIEW front panel. The virtual laboratory will include the same metering options as the operational APU. In addition, the virtual laboratory will include the option to alter engine operating parameters, such as inlet temperature and pressure or exhaust temperatures, and students may conduct broad parameter sweeps across ranges of possible inputs or desired outputs. These improvements will enable students to gain a deeper understanding of gas turbine operation and capabilities in practical applications. The improved laboratory will be implemented in Spring, 2014.© 2014 ASME

A Comparison of Shadowgraphy and X-Ray Computed Tomography in Liquid Spray Analysis

This work examines and compares two proven techniques for assessing key characteristics of liquid sprays for combustion applications: shadowgraphy and time-averaged X-ray computed tomography (CT). Atomization has key applications in combustion as it can improve fuel efficiency, increase heat release, and decrease pollutant emissions. To improve the design of fuel injection nozzles, the ability to conduct accurate analyses of sprays is crucial. Key characteristics of the liquid spray, such as mean particle diameter, spray-cone angle, mass distribution, and penetration length give insight into the effectiveness of a nozzle. Shadowgraphy is a relatively inexpensive method that produces a two-dimensional, instantaneous image of the spray particles or spray called a shadowgram. Shadowgrams can be used for analyzing mean particle size, spray-cone angle, and location of breakup regions. X-ray CT measures the time-averaged X-ray absorption of two-dimensional projection images of spray to produce a three-dimensional reconstruction of the spray. X-ray CT can provide valuable insight into the symmetry and mass distribution of a spray; however, X-ray absorption diminishes rapidly with increased distance from nozzles, thereby limiting analysis to the regions near the nozzle. A detailed comparison of the overall effectiveness and insights yielded by the two methods illustrates the unique uses, benefits, and shortcomings of each method. The results confirm that X-ray CT scanning proves more effective in the dense, near-nozzle spray region. Shadowgraphy effectively complements the X-ray CT analysis through particle analysis. It also allows for relatively simple spray cone analysis, though it cannot provide quantitative mass distribution analysis.Copyright