Jayanta S. Kapat

Design of a superhigh-speed cryogenic permanent magnet synchronous motor

This paper presents the design and simulation of a superhigh-speed permanent magnet synchronous motor (PMSM) that operates in the cryogenic temperature of 77 K. The designed PMSM is used to drive a two-stage cryocooler for zero boil-off and long duration storage of liquid hydrogen systems. The paper addresses electromagnetic and thermal finite-element analysis, selection of materials for cryogenic applications, stress analysis, rotor dynamic analysis, and some tradeoffs used in the design. A prototype PMSM was built to verify the design methodology.

Molecular Dynamics Simulation of Adsorbent Layer Effect on Tangential Momentum Accommodation Coefficient

The tangential momentum accommodation coefficient (TMAC) is used to improve the accuracy of fluid flow calculations in the slip flow regime where the continuum assumption of zero fluid velocity at the surface is inaccurate because fluid “slip” occurs. Molecular dynamics techniques are used to study impacts of individual gas atoms upon solid surfaces to understand how approach velocity, crystal geometry, interatomic forces, and adsorbed layers affect the scattering of gas atoms, and their tangential momentum. It is a logical step in development of techniques estimating total TMAC values for investigating flows in micro- and nano-channels or orbital spacecraft where slip flow occurs. TMAC can also help analysis in transitional or free molecular flow regimes. The impacts were modeled using Lennard-Jones potentials. Solid surfaces were modeled approximately three atoms wide by three atoms deep by 40 or more atoms long face centered cubic (100) crystals. The gas was modeled as individual free atoms. Gas approach angles were varied from 10 to 70deg from normal. Gas speed was either specified directly or using a ratio relationship with the Lennard-Jones energy potential (energy ratio). To adequately model the trajectories and maintain conservation of energy, very small time steps (approximately 0.0005 of the natural time unit) were used. For each impact the initial and final tangential momenta were determined and after many atoms, TMAC was calculated. The modeling was validated with available experimental data for He gas atoms at 1770m∕s impacting Cu at the given angles. The model agreed within 3% of experimental values and correctly predicted that TMAC changes with angle. Molecular Dynamics results estimate TMAC values from high of 1.2 to low of 0.25, generally estimating higher coefficients at the smaller angles. TMAC values above 1.0 indicate backscattering, which numerous experiments have observed. The ratio of final to initial momentum, when plotted for a gas atom sequence spaced across a lattice cycle typically follows a discontinuous curve, with continuous portions forward and backscattering and discontinuous portions indicating multiple bounces. Increasing the energy ratio above a value of 5 tends to decrease TMAC at all angles. Adsorbed layers atop a surface influence the TMAC in accordance with their energy ratio. Even a single adsorbed layer can have a substantial effect, changing TMAC +∕−20%. The results provide encouragement to continue model development and next evaluate gas flows with Maxwell temperature distributions involving numerous impact angles simultaneously.

Effect of Increasing Pitch-to-Diameter Ratio on the Film Cooling Effectiveness of Shaped and Cylindrical Holes Embedded in Trenches

The continuous push for higher gas turbine inlet temperatures and operating efficiencies has led to increasingly sophisticated film cooling schemes. One such setup—trench cooling—consists of having film cooling holes embedded inside a gap, commonly called a trench. The coolant hits the downstream trench wall which forces it to spread laterally, resulting in more even film coverage downstream. Recent literature has focused on the effect that trenching has on cylindrical cooling holes only. In addition, researchers have limited their findings to a narrow range of pitch-to-diameter ratios (P/D). The current trends in the turbine industry of increasing or maintaining film cooling effectiveness while reducing the amount of coolant used dictate that P/D be increased, meaning less holes per row. In this study, we address both cylindrical and fan-shaped holes embedded in trenches. Tests have been conducted on 8 test plates with one row of cooling holes each varying the pitch-to-diameter ratio from 4 to 12 (12 configurations in total), and the blowing ratios from 0.5 to 2.0. We investigate the effect that P/D has on film cooling effectiveness for both hole geometries and compare them to similarly pitched baseline plates—fan and cylindrical—not in trenches. It is a known fact that increasing the pitch between holes, while maintaining all other conditions constant, decreases the average film effectiveness, however trenching has been shown to significantly increase film coverage. In this study, it has been shown that film cooling effectiveness of a cylindrical configuration can be maintained by the addition of a trench while cutting the number of holes in half. We also explore the behavior of shaped trenched holes, of which little has been said and find that their performance is actually hurt by trenching.Copyright

Design and fabrication of meso-scale variable capacitance motor for miniature heat pumps

Miniaturization of the heat pump in the meso-scale power domain requires the development of the compressor integrated actuation units in the millimeter range. In this respect, a variable capacitance motor (VCM) is considered for compressor actuation. This work describes the design of a three-phase VCM. A microfabrication technique has been applied for the implementation of the motor using silicon technology. Some static electrical measurements have been conducted for the fabricated motor.

Design of a Dual Latent Heat Sink for Pulsed Electronic Systems

A conceptual design of a dual latent heat sink basically intended for low thermal duty cycle electronic heat sink applications is presented. In addition to the concept, end-application-dependent criteria to select an optimized design for this dual latent heat sink are presented. A thermal resistance model has been developed to analyze and optimize the design, which would also serve as a fast design tool for experiments. The model showed that it is possible to have a dual latent heat sink design capable of handling 7 MJ of thermal load at a heat flux of 500 W/cm 2 (over an area of 100 cm 2 ) with a volume of 0.072 m 3 and a weight of about 57.5 kg. It was also found that, with such high heat flux absorption capability, the proposed conceptual design can have a vapor-to-condenser temperature difference of less than 10°C with a volume storage density of 97 MJ/m 3 and a mass storage density of 0.122 MJ/kg.

Heat Transfer in Channels in Parallel-Mode Rotation at High Rotation Numbers

This study attempts to understand one of the most fundamental and challenging problems in fluid flow and heat transfer for rotating machines. The study focuses on electric generators for high energy density applications, which employ rotating cooling channels so that materials do not fail under high temperature and high stress environment. Prediction of fluid flow and heat transfer inside internal cooling channels that rotate at high rotation number and high wall heat flux is the main focus of this study. Rotation, buoyancy, and boundary conditions affect the flow inside these channels. A fully computational approach is employed in this study. Reynolds stress turbulence model with enhanced near-wall treatment is validated against available experimental data (which are primarily at low rotation and buoyancy numbers). The model was then used for cases with high rotation number (as much as 0.35) and high wall heat flux. Particular attention is given to how turbulence intensity, Reynolds stresses, and transport are affected by Coriolis and buoyancy/centrifugal forces caused by high levels of rotation number and wall heat flux. Variations of flow total pressure along the rotating channel are also predicted. The results obtained are explained in view of physical interpretation of Coriolis and centrifugal forces.

Processing and Characterization of Sc2O3-CeO2-ZrO2 Electrolyte Based Intermediate Temperature Solid Oxide Fuel Cells

ScCeZrO2 ceramic powders produced by two different manufacturers have been used for the manufacturing of ScCeZrO2 electrolyte based Solid Oxide Fuel Cells. Thin porous anode and cathode layers have been deposited on 150 µm thick dense electrolyte layers. Preliminary electrochemical testing of the produced cells has been performed.

Effect of Film Cooling on Low Temperature Hot Corrosion in a Coal Fired Gas Turbine

In recent times, the use of coal gas in gas turbines has gained a lot of interest, as coal is quite abundant as a primary source of energy. However, use of coal gas produces a few detrimental effects that need closer attention. This paper concentrates on one such effect, namely hot corrosion, where trace amounts of sulfur can cause corrosion (or sulfidation) of hot and exposed surfaces, thereby reducing the life of the material. In low temperature hot corrosion, which is the focus of this paper, transport of SO2 from the hot gas stream is the primary process that leads to a chain of events, ultimately causing hot corrosion. The corrosion rate depends on SO2 mass flux to the wall as well as wall surface temperature, both of which are affected in the presence of any film cooling. An analytical model is developed to describe the associated transport phenomena of both heat and mass in the presence of film cooling The model predicts how corrosion rates may be affected under operating conditions. It is found that although use of film cooling typically leads to lower corrosion rate, there are combinations of operating parameters under which corrosion rate can actually increase in the presence of film cooling.Copyright

A Detailed Uncertainty Analysis of Adiabatic Film Cooling Effectiveness Measurements Using Pressure Sensitive Paint

Pressure sensitive paint (PSP) can be a powerful tool in measuring the adiabatic film cooling effectiveness. There are two distinct sources of error for this measurement technique; the ability to experimentally obtain the data and the validity of the heat and mass transfer analogy for the problem being studied. This paper will assess the experimental aspect of this PSP measurement specifically for film cooling applications.Experiments are conducted in an effort to quantifiably bound expected errors associated with temperature non-uniformities in testing and photo-degradation effects. Results show that if careful experimental procedures are put in place, both of these effects can be maintained to have less than 0.022 impact on effectiveness.Through accurate semi-in-situ calibration down to 4% atmospheric pressure, the near-hole distribution of effectiveness is measured with high accuracy. PSP calibrations are performed for multiple coupons, over multiple days. In addition, to reach a partial pressure of 0 the calibration vessel was purged of all air by flowing CO2.The primary contribution of this paper lies in the uncertainty analysis performed on the PSP measurement technique. A thorough uncertainty analysis is conducted and described, in order to completely understand the presented measurements and any shortcomings of the PSP technique. This quantification results in larger, albeit more realistic, values of uncertainty, and helps provide a better understanding of film cooling effectiveness measurements taken using the PSP technique. The presented uncertainty analysis takes into account all random error sources associated with sampling and calibration, from intensities to effectiveness.Adiabatic film cooling effectiveness measurements are obtained for a single row of film cooling holes inclined at 20 degrees, with CO2 used as coolant. Data is obtained for six blowing ratios. Maps of uncertainty corresponding to each effectiveness profile are available for each test case. These maps show that the uncertainty varies spatially over the test surface, high effectiveness corresponds to low uncertainty. The noise floors can be as high as 0.04 at effectiveness levels of 0. Day-to-day repeatability is presented for each blowing ratio and shows that laterally averaged effectiveness data is repeatable within 0.02 effectiveness.Copyright

Sensitivity Analysis of Domain Considerations for Numerical Simulations of Film Cooling

The majority of computational fluid dynamics studies for turbine film cooling have employed the Reynolds-Averaged Navier-Stokes equations with various turbulence modeling techniques to achieve closure, most notably the various two equation (k-e or k-ω) models. For computational simulation of film cooling, modeling the entire testing domain with a row of multiple holes while sustaining a sufficiently fine mesh would demand a large number of grid cells and a hefty computational expense. A significant reduction in the computational domain can be and has been achieved without much harm to the overall accuracy of the film cooling prediction. The current study aimed to investigate the necessary domain parameters for reducing the grid cell count without significantly affecting the accuracy of the solution. The Box-Behnken design for response surface methodology was employed to determine the relative influence of each parameter on the cooling effectiveness prediction. The experimental design matrix was executed for multiple blowing ratios (0.5, 1.0, 2.0) to include the effects of the blowing ratio on the computational domain. The work was carried out using a three-dimensional computational fluid dynamics finite volume method with the RANS equations and k-e turbulence model. A cylindrical film cooling hole with a pitch-to-diameter ratio of 3.0, a length-to-diameter ratio of 7.5, and an inclination angle of 35° was studied. The results are compared against existing data in the literature as well as in-house experimental data. The data from each case is compared in terms of spatially-averaged effectiveness. The modeled entrance length was found to be the most important parameter, with the mainflow height a distant second. The size of the modeled plenum was not found to exert any significant influence on the effectiveness results. Explanations are offered for notable trends in the data and conclusions are drawn concerning the grid optimization process.© 2010 ASME