Systems Design, Fabrication, and Testing of a High-Speed Miniature Motor for Cryogenic Cooler

The long-term storage of liquid hydrogen for space missions is of considerable interest to NASA. To this end, the Reverse Turbo-Brayton Cryocooler (RTBC) is considerably lighter than conventional designs and a potentially viable and attractive solution for NASA's long-term Zero-Boil-off (ZBO) hydrogen storage system for future space missions. We present the systems design, fabrication, and performance evaluation of the Permanent Magnet Synchronous Motor (PMSM) powering a cryocooler capable of removing 20 W of heat at 18 K with a COP of 0.005 and driven by two 2-kW permanent magnet synchronous motors operating at 200 000 rpm and at room temperature and 77 K. Structural, thermal, and rotordynamic aspects of system design are considered

Use Of Inlet Guide Vanes For A Miniature Centrifugal Compressor

A novel design for a high-speed, miniature centrifugal compressor for a miniature RTBC (reverse turbo Brayton cycle) cryogenic cooling system is the focus of this paper. Due to the geometrical restriction imposed by the cryocooling system, the outer radius of the compressor is limited to 2.5 cm. Such a small compressor must rotate at a high speed in order to provide an acceptable pressure ratio. Miniature design precludes the use of inducers with large angles. In order to compensate for the absence of conventional inducers, the proposed design uses inlet guide vanes (IGV) that produce preswirl at the impeller inlet. IGV is followed by a radial impeller and an axial diffuser. The design speed for this compressor is 313,000 rpm for an overall static-to-total pressure ratio of 1.7 with helium as the working fluid for the compressor and the cryocooling system. The analysis undertaken in this paper is for an aerodynamically similar design with air as the working fluid. The rotational speed is 108,000 rpm and the overall static-to-total pressure ratio of 1.55. This paper concentrates on computational prediction of the performance of the compressor. The three-dimensional transient simulation is performed by using sliding mesh model (SMM). Blade tip leakage is not considered in the computation presented here. The unsteady solution predicts the interaction between IGV and the impeller, and between the impeller and the diffuser. The isentropic efficiency of impeller is found to be 81% at the design point. Based on the results obtained in this study, the inlet angle of diffuser vanes is modified to match the gas flow at the impeller exit, resulting in an increase of the isentropic efficiency of diffuser from 8.6% to 74.8%. It is also found that the performance of upstream components - IGV and impeller, are not affected by the performance of the diffuser. Copyright © 2005 by ASME

Study Of Thermal Energy Transport Between Hydrogen Gas Molecules And A Single-Wall Carbon Nanotube Using Molecular Dynamics Simulations

The focus of the current research is the investigation and characterization of the energy transport between a (10,10) single-wall carbon nanotube (SWCNT) and surrounding molecular hydrogen gas using molecular dynamics (MD) simulations. The MD simulations use Tersoff-Brenner hydrocarbon potential for C-C, C-H, and H-H bonding interactions and the conventional Lennard-Jones potential for soft-sphere gas-CNT collision dynamics of H-H and H-C non-bonding van der Waals interactions. A simulation cell with periodic boundary conditions is created for 1200 carbon atoms in an armchair nanotube configuration and three distinct gas densities corresponding to 252, 500, and 1000 H2 molecules in the same volume. The MD simulation runs are performed with time steps of 0.1 fs and the total simulation times of 40 ps. The simulations are initialized by setting the gas species and CNT at two different temperatures. Initial gas temperatures range from 2000K to 4000K, whereas the carbon nanotube is held at 300K. After the equilibrium temperatures of the CNT and the gas molecules are achieved, the external constraints to maintain the temperature are removed and the thermal energy transport between the two is studied. The kinetic energy exchange between the nanotube and the surrounding gas is monitored to study thermal energy transport over the duration of the simulation. A parameter is proposed, the coefficient of thermal energy transfer (CTET), to characterize the thermal transport properties of the modeled systems based on parameters governing the transport process, thus mimicking the conventional heat transfer coefficient. Values for CTET vary directly with gas density and range from 50 MW/m2K to 250MW/m2K, showing that gas density has a significant impact with higher density corresponding to higher collision rates and higher rates of energy transfer. In contrast, the gas temperature has a lower impact on CTET, with colder gas providing in certain cases a slightly lower value for the coefficient. In order to validate the MD simulations, the time-series data of molecular vibrations of the CNT is converted to a vibrational frequency spectrum through FFT. The characteristic frequencies obtained on the spectra of isolated SWCNT and H2 simulations are compared against the known natural frequencies of the CNT phonon modes and vibrational modes of H2 molecules. The comparison is quite favorable. Copyright © 2005 by ASME

Retrieval Of Multidimensional Heat Transfer Coefficient Distributions Using An Inverse Bem-Based Regularized Algorithm: Numerical And Experimental Results

Surface maps of heat transfer coefficients (h) are often determined by transient liquid crystal or other similar transient experimental techniques. This involves (1) conducting an experiment with an impulsively imposed convective boundary condition on an initially isothermal test object, (2) measuring the resulting time-dependent surface temperature distributions, and (3) solving the one-dimensional transient heat conduction equation for different points on the convective surface. There are many practical cases where this approach fails to adequately model the temperature field and, consequently, leads to erroneous h values. In this paper, we present an inverse boundary element method (BEM)-based approach for the retrieval of spatially varying h distributions from surface temperature measurements. In this method, an efficient numerical algorithm requiring only a surface mesh is used to solve the conduction problem. At each time level, a regularized functional is minimized to estimate the time-dependent heat flux and simultaneously minimize the effect of experimental measurement uncertainties in surface temperature values on the calculated heat flux. Newton\u27s cooling law is then invoked to compute h. Results are presented from several numerical simulations and from a laboratory experiment. The method is applicable to unsteady as well as to steady-state convective systems

RETRIEVAL OF MULTIDIMENSIONAL HEAT TRANSFER COEFFICIENT DISTRIBUTIONS USING AN INVERSE BEM-BASED REGULARIZED ALGORITHM: Numerical and experimental results

Surface maps of heat transfer coefficients (h) are often determined by transient liquid crystal or other similar transient experimental techniques. This involves (1) conducting an experiment with an impulsively imposed convective boundary condition on an initially isothermal test object, (2) measuring the resulting timedependent surface temperature distributions, and (3) solving the onedimensional transient heat conduction equation for different points on the convective surface. There are many practical cases where this approach fails to adequately model the temperature field and, consequently, leads to erroneous h values. In this paper, we present an inverse boundary element method(BEM)-based approach for the retrieval of spatially varying h distributions from surface temperature measurements. In this method, an efficient numerical algorithm requiring only a surface mesh is used to solve the conduction problem. At each time level, a regularized functional is minimized to estimate the time-dependent heat flux and simultaneously minimize the effect of experimental measurement uncertainties in surface temperatme values on the calculated heat flux. Newton\u27s cooling law is then invoked to compute h. Results are presented from several numerical simulations and from a laboratory experiment. The method is applicable to unsteady as well as to steady-state convective systems

Comparison Of Pressure Drop And Endwall Heat Transfer Measurements To Flow Visualization Testing Of Solid And Porous Pin Fin Arrays

This paper examines the local and averaged endwall heat transfer effects of a staggered array of porous aluminum pin fins with a channel blockage ratio (blocked channel area divided by open channel area) of 50%. Two sets of pins were used with pore densities of 0 (solid) and 10 pores per inch (PPI). The pressure drop through the channel was also determined for several flow rates using each set of pins. Local heat transfer coefficients on the endwall were measured using Thermochromatic Liquid Crystal (TLC) sheets recorded with a charge-coupled device (CCD) camera. Static and total pressure measurements were taken at the entrance and exit of the test section to determine the overall pressure drop through the channel and explain the heat transfer trends through the channel. The heat transfer and pressure data was then compared to flow visualization tests that were run using a fog generator. Results are presented for the two sets of pins with Reynolds numbers between 25000 and 130000. Local HTC (heat transfer coefficient) profiles as well as spanwise and streamwise averaged HTC plots are displayed for both pin arrays. The thermal performance was calculated for each pin set and Reynolds number. All experiments were carried out in a channel with an X/D of 1.72, a Y/D of 2.0, and a Z/D of 1.72. Copyright © 2010 by ASME

Investigation On The Effects Of Wake Rod To Film Cooling Hole Diameter Ratio In Unsteady Wake Studies

In recent decades, greater interest in the effect of rotational wakes on gas turbine film cooling applications has produced increasing numbers of studies on these unsteady phenomena. Wakes are primarily shed from upstream components such as transition duct walls, stator vanes, and rotors. Studies have shown that in areas of unsteady flow, the best performing parameters in conventional steady investigations may not be the best for unsteady applications. One common method of modeling the rotor-stator interaction in subsonic flows is the use of spoke wheel type wake generators using cylindrical rods to produce the velocity detriment and local increase in turbulence intensity. Among the results published to date, no mention on the potential effect of the wake rod to film cooling hole diameter ratio has been addressed. Disagreement among investigators concerning the trailing edge thickness and the effect of boundary layer growth has led to diameter ratios from 0.5 to 5.6 in open literature. This investigation measures the effect of the diameter ratio on the adiabatic film cooling effectiveness over three diameter ratios of 2.375, 4.75, and 9.5 in order to determine any dependence on this typically unidentified parameter. Blowing ratios of 0.25, 0.5, and 0.75 will be tested at wake Strouhal numbers of 0 and 0.3 in order to determine the effects for weak and strong injection rates. Measurements are taken in a low speed induced flow annular duct with jet Reynolds numbers of 2000 to 8000. Among the cases studied no discernable trend based on the diameter ratio is found, indicating that this effect is negligible in wake impact comparisons. The effect of wakes is consistent for both weak and strong injection with similar magnitude of wake impact for each diameter ratio tested. The existence or lack of dependence on diameter ratio allows for more purposeful comparisons among present and future investigations which use this method of producing unsteady wakes. © 2011 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved