77 research outputs found
Lattice and Energy Band Engineering in AlInGaN/Ga Heterostructures
We report on structural, optical, and electrical properties of AlxInyGa1âxâyNGaNheterostructures grown on sapphire and 6HâSiC substrates. Our results demonstrate that incorporation of In reduces the lattice mismatch, Îa, between AlInGaN and GaN, and that an In to Al ratio of close to 1:5 results in nearly strain-free heterostructures. The observed reduction in band gap,ÎEg, determined from photoluminescence measurements, is more than 1.5 times higher than estimated from the linear dependencies of Îa and ÎEg on the In molar fraction. The incorporation of In and resulting changes in the built-in strain in AlInGaN/GaN heterostructures strongly affect the transport properties of the two-dimensional electron gas at the heterointerface. The obtained results demonstrate the potential of strain energy band engineering for GaN-based electronic applications
Forced Convection Heat Transfer from a Finite-Height Cylinder
[EN] This paper presents a large eddy simulation of forced convection heat transfer in
the flow around a surface-mounted finite-height circular cylinder. The study was carried out
for a cylinder with height-to-diameter ratio of 2.5, a Reynolds number based on the cylinder
diameter of 44 000 and a Prandtl number of 1. Only the surface of the cylinder is heated
while the bottom wall and the inflow are kept at a lower fixed temperature. The approach
flow boundary layer had a thickness of about 10% of the cylinder height. Local and averaged
heat transfer coefficients are presented. The heat transfer coefficient is strongly affected
by the free-end of the cylinder. As a result of the flow over the top being downwashed
behind the cylinder, a vortex-shedding process does not occur in the upper part, leading
to a lower value of the local heat transfer coefficient in that region. In the lower region,
vortex-shedding takes place leading to higher values of the local heat transfer coefficient.
The circumferentially averaged heat transfer coefficient is 20 % higher near the ground than
near the top of the cylinder. The spreading and dilution of the mean temperature field in the
wake of the cylinder are also discussed.The simulation was carried out using the supercomputing facilities of the Steinbuch Centre for Computing (SCC) of the Karlsruhe Institute of Technology. MGV has been partially supported by grant TRA2012-37714 of the Spanish Ministry of Economy and Competitiveness.GarcĂa Villalba, M.; Palau-Salvador, G.; Rodi, W. (2014). Forced Convection Heat Transfer from a Finite-Height Cylinder. Flow, Turbulence and Combustion. 93(1):171-187. https://doi.org/10.1007/s10494-014-9543-7S171187931Ames, F., Dvorak, L.: Turbulent transport in pin fin arrays: experimental data and predictions. J. Turbomach. 128(1), 71â81 (2006)Armstrong, J., Winstanley, D.: A review of staggered array pin fin heat transfer for turbine cooling applications. J. Turbomach. 110, 94 (1988)Breuer, M., Rodi, W.: Large eddy simulation of complex turbulent flows of practical interest. In: Hirschel, E. (ed.) Flow Simulation with High Performance Computers II, Notes on Numerical Fluid Mechanics, Vol. 52, pp 258â274. Vieweg, Braunschweig (1996)Chen, S., Sanitjai, S., Ghosh, K., Goldstein, R.: Three-dimensional vortex flow near the endwall of a short cylinder in crossflow: uniform-diameter circular cylinder. Appl. Therm. Eng. 49, 73â78 (2012)Delibra, G., Hanjalic, K., Borello, D., Rispoli, F.: Vortex structures and heat transfer in a wall-bounded pin matrix: LES with a RANS wall-treatment. Int. J. Heat Fluid Flow 31(5), 740â753 (2010)Denev, J.A., Fröhlich, J., Bockhorn, H.: Large eddy simulation of a swirling transverse jet into a crossflow with investigation of scalar transport. Phys. Fluids 21, 015101 (2009)Donnert, G.D., Kappler, M., Rodi, W.: Measurement of tracer concentration in the flow around finite-height cylinders. J. Turbul. 8, 33 (2007)Frederich, O., Thiele, F.: Turbulent flow dynamics caused by a truncated cylinder. Int. J. Heat Fluid Flow 32(3), 546â557 (2011)Fröhlich, J., GarcĂa-Villalba, M., Rodi, W.: Scalar mixing and largeâscale coherent structures in a turbulent swirling jet. Flow Turbul. Combust. 80, 47â59 (2008)Fröhlich, J., Rodi, W.: LES of the flow around a cylinder of finite height. Int. J. Heat Fluid Flow 25, 537â548 (2004)GarcĂa-Villalba, M., Fröhlich, J.: LES of a free annular swirling jetâDependence of coherent structures on a pilot jet and the level of swirl. Int. J. Heat Fluid Flow 27(5), 911â923 (2006)GarcĂa-Villalba, M., Li, N., Rodi, W., Leschziner, M.A.: Large eddy simulation of separated flow over a three-dimensional axisymmetric hill. J. Fluid Mech. 627, 55â96 (2009)Germano, M., Piomelli, U., Moin, P., Cabot, W.: A dynamic subgrid-scale eddy viscosity model. Phys. Fluids 3, 1760â1765 (1991)Hinckel, J.N., Nagamatsu, H.T.: Heat transfer in the stagnation region of the junction of a circular cylinder perpendicular to a flat plate. Int. J. Heat Mass Tran. 29(7), 999â1005 (1986)Hinterberger, C.: Dreidimensionale und tiefengemittelte Large-eddy-simulation von flachwasserströmungen. University of Karlsruhe (2004). Ph.D. thesisHölscher, N., Niemann, H.J.: Some aspects about the flow around a surface-mounted circular cylinder in a turbulent shear flow. In: Proceedings of 6th Symp. Int. Turbulent Shear Flows, ToulouseKrajnovic, S.: Flow around a tall finite cylinder explored by large eddy simulation. J. Fluid Mech. 676, 294â317 (2011)Lilly, D.: A proposed modification of the Germano subgrid-scale closure method. Phys. Fluids 4, 633â635 (1992)Morgan, V.T.: The overall convective heat transfer from smooth circular cylinders. Adv. Heat Tran. 11, 199â264 (1975)NiÄeno, B., Dronkers, A., HanjaliÄ, K.: Turbulent heat transfer from a multi-layered wall-mounted cube matrix: a large eddy simulation. Int. J. Heat Fluid Flow 23(2), 173â185 (2002)Palau-Salvador, G., GarcĂa-Villalba, M., Rodi, W.: Scalar transport from point sources in the flow around a finite-height cylinder. Environ. Fluid Mech. 11, 611â625 (2011)Palau-Salvador, G., Stoesser, T., Fröhlich, J., Kappler, M., Rodi, W.: Large-eddy simulations and experiments of flow around finite-height cylinders. Flow Turbul. Combust. 84, 239â275 (2010)Pattenden, R., Turnock, S., Zhang, X.: Measurements of the flow over a low-aspect ratio cylinder mounted on a ground plate. Exp. Fluids 39, 10â21 (2005)Pierce, C.: Progress-variable approach for large-eddy simulation of turbulent combustion. Stanford University (2001). Ph.D. thesisPopovac, M., Hanjalic, K.: Vortices and heat flux around a wall-mounted cube cooled simultaneously by a jet and a crossflow. Int. J. Heat Mass Transfer 52, 4047â4062 (2009)Rhie, C., Chow, W.: Numerical study of the turbulent flow past an airfoil with trailing edge separation. AIAA J. 21(11), 1061â1068 (1983)Rostamy, N., Sumner, D., Bergstrom, D.J., Bugg, J.D.: Local flow field of a surface-mounted finite circular cylinder. J. Fluids Struct. 34, 105â122 (2012)Sanitjai, S., Goldstein, R.J.: Forced convection heat transfer from a circular cylinder in crossflow to air and liquids. Int. J. Heat Mass Tran 47, 4795â4805 (2004)Sanitjai, S., Goldstein, R.J.: Heat transfer from a circular cylinder to mixtures of water and ethylene glycol. Int. J. Heat Mass Tran. 47, 4785â4794 (2004)Sparrow, E.M., Stahl, T.J., Traub, P.: Heat transfer adjacent to the attached end of a cylinder in crossflow. Int. J. Heat Mass Tran. 27(2), 233â242 (1984)Stone, H.: Iterative solution of implicit approximations of multidimensional partial differential equations for finite difference Methods. SIAM J. Numer. Anal. 5, 530â558 (1968)Sumner, D.: Flow above the free end of a surface-mounted finite-height circular cylinder: a review. J. 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Development of methods for the preparation of radiopure <sup>82</sup>Se sources for the SuperNEMO neutrinoless double-beta decay experiment
A radiochemical method for producing 82Se sources with an ultra-low level of contamination of natural radionuclides (40K, decay products of 232Th and 238U) has been developed based on cation-exchange chromatographic purification with reverse removal of impurities. It includes chromatographic separation (purification), reduction, conditioning (which includes decantation, centrifugation, washing, grinding, and drying), and 82Se foil production. The conditioning stage, during which highly dispersed elemental selenium is obtained by the reduction of purified selenious acid (H2SeO3) with sulfur dioxide (SO2) represents the crucial step in the preparation of radiopure 82Se samples. The natural selenium (600 g) was first produced in this procedure in order to refine the method. The technique developed was then used to produce 2.5 kg of radiopure enriched selenium (82Se). The produced 82Se samples were wrapped in polyethylene (12 ÎŒm thick) and radionuclides present in the sample were analyzed with the BiPo-3 detector. The radiopurity of the plastic materials (chromatographic column material and polypropylene chemical vessels), which were used at all stages, was determined by instrumental neutron activation analysis. The radiopurity of the 82Se foils was checked by measurements with the BiPo-3 spectrometer, which confirmed the high purity of the final product. The measured contamination level for 208Tl was 8-54 ÎŒBq/kg, and for 214Bi the detection limit of 600 ÎŒBq/kg has been reached.</p
Pressure drop of laminar gas flows in a microchannel containing various pillar matrices
Measurement of the 2 nu beta beta decay half-life and search for the 0 nu beta beta decay of Cd-116 with the NEMO-3 detector
The NEMO-3 experiment measured the half-life of the
2
Μ
ÎČ
ÎČ
decay and searched for the
0
Μ
ÎČ
ÎČ
decay of
116
Cd
. Using 410 g of
116
Cd
installed in the detector with an exposure of 5.26 y, (
4968
±
74
) events corresponding to the
2
Μ
ÎČ
ÎČ
decay of
116
Cd
to the ground state of
116
Sn
have been observed with a signal to background ratio of about 12. The half-life of the
2
Μ
ÎČ
ÎČ
decay has been measured to be
T
2
Μ
1
/
2
=
[
2.74
±
0.04
(
stat
)
±
0.18
(
syst
)
]
Ă
1
0
19
â
â
y
. No events have been observed above the expected background while searching for
0
Μ
ÎČ
ÎČ
decay. The corresponding limit on the half-life is determined to be
T
0
Μ
1
/
2
â„
1.0
Ă
1
0
23
â
â
y
at the 90% C.L. which corresponds to an upper limit on the effective Majorana neutrino mass of
âš
m
Μ
â©
â€
1.4
â
2.5
â
â
eV
depending on the nuclear matrix elements considered. Limits on other mechanisms generating
0
Μ
ÎČ
ÎČ
decay such as the exchange of R-parity violating supersymmetric particles, right-handed currents and majoron emission are also obtained
Search for Neutrinoless Quadruple-beta Decay of Nd-150 with the NEMO-3 Detector
We report the results of a first experimental search for lepton number violation by four units
in the neutrinoless quadruple-ÎČ decay of 150Nd using a total exposure of 0.19 kg·y recorded with
the NEMO-3 detector at the Modane Underground Laboratory (LSM). We find no evidence of this
decay and set lower limits on the half-life in the range T1/2 > (1.1â3.2) Ă 1021 y at the 90% CL,
depending on the model used for the kinematic distributions of the emitted electrons
Measurement of the beta beta Decay Half-Life of Te-130 with the NEMO-3 Detector
We report results from the NEMO-3 experiment based on an exposure of 1275 days with 661 g of Te-130 in the form of enriched and natural tellurium foils. The beta beta decay rate of Te-130 is found to be greater than zero with a significance of 7.7 standard deviations and the half-life is measured to be T-1/2(2v)=[7.0 +/- 0.9(stat) +/- 1: 1(syst)] x 10(20) yr. This represents the most precise measurement of this half- life yet published and the first real-time observation of this decay
Calorimeter development for the SuperNEMO double beta decay experiment
SuperNEMO is a double-ÎČ decay experiment, which will employ the successful trackerâcalorimeter technique used in the recently completed NEMO-3 experiment. SuperNEMO will implement 100 kg of double-ÎČ decay isotope, reaching a sensitivity to the neutrinoless double-ÎČ decay (0ÎœÎČÎČ) half-life of the order of 1026 yr, corresponding to a Majorana neutrino mass of 50â100 meV. One of the main goals and challenges of the SuperNEMO detector development programme has been to reach a calorimeter energy resolution, ÎEâE, around 3%âE(MeV) Ï, or 7%âE(MeV) FWHM (full width at half maximum), using a calorimeter composed of large volume plastic scintillator blocks coupled to photomultiplier tubes. We describe the R&D programme and the final design of the SuperNEMO calorimeter that has met this challenging goal
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