High Quality Region Research Articles

Preferential incorporation of defects in monocrystalline diamond films

Cathodoluminescence (CL) spectroscopy and microscopy and Raman scattering measurements of homoepitaxial diamond films, grown at substrate temperatures of 900 and 1200°C by microwave plasma-assisted chemical vapor deposition, reveal the preferential incorporation of defects in films grown on the (111) and (320) faces of diamond substrates compared with those grown on the (001) faces. The CL features observed include the broad bands centered at around 440 nm (2.82 eV) and 550 nm (2.25 eV) and a narrow band at about 740 nm (1.675 eV). The diamond films grown on the (111) faces exhibit strong non-uniformities in the distribution of impurities ( e.g. Si) and defects. The distribution of the 550 nm and 740 nm bands can be correlated with the topographic features in these films. The diamond films grown on the (001) faces show no 550 nm and 740 nm bands, and high-quality regions exhibit uniform monochromatic (440 nm) CL images.

Enhancement of Heat Transfer to a Non-Azeotropic Refrigerant Mixture in a Horizontal, Spirally Grooved Evaporator Tube.

An experiment was performed to investigate the enhancement of local heat transfer to a boiling or evaporating non-azeotropic refrigerant mixture of HCFC22 and CFC114 in a horizontal, spirally grooved tube. There was no notable difference in the enhancement among the mixture and the pure refrigerants at a high flow rate. At a low flow rate, the heat transfer to the mixture in the high quality region was found to be enhanced over the smooth tube as effectively as to the pure refrigerants ; therefore, no features of the mixture were recognized. In the low quality region where the effective enhancement was also achieved for the pure refrigerants, however, the enhancement for the mixture was greatly degraded. This degradation was theoretically explained by taking account of the variation of composition in the meniscus liquid film formed in the grooves on the upper part of the tube perimeter.

Rainfall patterns in a major wheat-growing region of Australia

Rainfall is an important variable in the wheat production areas of Australia. This analysis examines, firstly, the pattern of rainfall over 2.3 million ha of a high-quality wheat-producing region, and secondly, develops regression equations for rainfall prediction over this region. Most of the variation in rainfall pattern across the region is accounted for by differences in October-to-March (summer) rainfall and in April-to-September (winter) rainfall. The summer rainfall differences account for over two thirds of the variation. Based on these two rainfall periods, a partitioning of the study area reveals five distinct regions. The second part of the analysis uses multiple regression to provide a set of equations for rainfall prediction at any location in the region, for a number of rainfall periods. These equations use altitude, longitude and latitude as predictors. Nearly all of the equations explain between 80% and 94% of the variation in rainfall. Differences between regions are accounted for in the analysis, making the equations widely applicable. The validity of the mean rainfall equations was tested on three further sites: the mean prediction error was 6.9%. This approach may be applicable where large land masses with similar geographical features occur.

A Correlation for Forced Convective Boiling Heat Transfer of Single-Component Refrigerants in a Horizontal Smooth Tube.

An experimental study is reported on convective boiling heat transfer for HFC134a, HCFC22, CFC114 and CFC12 inside a horizontal smooth tube. The test evaporator is of the double-tube type in which the heating water flows in the outer annulus, and the local heat transfer coefficients are measured for both counter and parallel flow. Based on the supposition of Chen that the total heat flux q is represented as the sum of forced convective contribution qcv and nucleate boiling contribution qnb, a general correlation is developed for the data in the annular flow regime. The mean deviation between the calculated and measured heat transfer coefficients is 12.2% for the present experimental data and 9.5% for available data collected from previous research. According to the proposed correlation, nucleate boiling is not suppressed even in the high-quality region in the case of counter flow, while two-phase forced convection has been recognized as predominant in the high-quality region with uniform heat flux conditions.

Heat Transfer of a Bolling Two-Phase Flow in Helical Coils. Effects of Coil Diameter and Inclined Angle.

The wall temperature distributions of a boiling two-phase flow of R113 in helically coiled tubes were measured, and the effects of a coil diameter and inclined angle on the heat transfer characteristics were investigated. There were large differences in the temperature distributions between the vertically placed coil and the horizontally placed coil at the lower quality region. When the coil was vertically placed, the ratio of the heat transfer coefficient of a boiling two-phase flow to that of a liquid single-phase flow was not affected by coil diameter. When the coil was horizontally placed or inclined by 45 degrees, the heat transfer coefficient at the upper part of the coil was larger than that at the lower part in the lower quality region but smaller in the higher quality region. The overall average heat transfer coefficient of a horizontally placed coil was larger than that of a vertically placed coil.

An experimental study of disturbance waves in boiling two-phase flow.

Behaviors of disturbance waves in an evaporating tube are experimentally investigated in high quality region using conductance probe method under the following conditions : pressure 2.95 Mpa, mass velocity 300, 500 and 700kg/(m2 ·s), heat flux up to 1.2 MW/m2 and thermodynamic quality larger than 0.80. As the results of comparison with air-water adiabatic data, it is found that the disturbance waves in the boiling steam-warter system has the following remarkable features ; a) the wave does not take sharp spike like form and has a log tail, b) scattering of the propagation speed is larger than that of the air-water system, c) the space separation of two neighboring wave does not vary significantiy with the quality. It is also confirmed that the disturbance waves have a close connection with the wall temperature fluctuation at the dryout region.

Critical heat flux of forced convection boiling in an oscillating acceleration field—II. Contribution of flow oscillation

Critical heat flux (CHF) with forced convection generally decreases under an oscillating acceleration condition. Contribution of flow oscillation on the decrease in CHF was investigated experimentally and theoretically. The experiments were performed with a Freon-113 boiling loop. The results showed that, in the high exit quality region deteriotation of CHF could almost wholly be attributed to the variation of inlet flow rate induced by motion. The amplitude of flow oscillation for a given acceleration variation could be predicted from a transfer function derived with linearization technique. Prediction of the transient CHF on the basis of “local-conditions hypothesis” gave conservative values.

An investigation of critical heat flux and surface rewet in flow boiling systems

A high thermal capacity copper test section placed at the end of a heating tube is used for investigating the critical heat flux and heat transfer characteristics during rewetting process for Freon-113 upward flow at mass velocities of 357, 627, 1035 and 1465 kg m −2 s −1. The critical heat flux coincides well with the maximum heat flux obtained by the transient cooling test. The wall superheat at the onset of rewetting changes greatly with the inlet quality, and ranges from 120 to 140°C in the subcooled and low quality region and from 60 to 80°C in the high quality region.

The Czochralski growth of bismuth-germanium oxide single crystals

The conditions necessary for the growth of high quality Bi 12GeO 20 single crystals by the Czochralski method have been determined. If growth conditions are not optimal cloudy inclusions appear in the crystals. By use of X-ray microprobe analysis no platinum has been found in the cloudy area, but the ratio Bi 2O 3 : GeO 2 is 3 : 2, compared to 6 : 1 in high quality regions. Optical transmission, as well as the dependence of the linear expansion coefficient on temperature, have been measured.

An Evaluation of the Departure from Nucleate Boiling in Bundles of Reactor Fuel Rods

Flow boiling crisis can be categorized into at least two types: Departure from Nucleate Boiling (DNB) in the subcooled and low-quality region and dry-out in the high-quality region. To analyze the ...

A liquid metal Rankine topping cycle for a steam power plant.

This paper describes studies related to the use of a potassium Rankine-cycle power system as a topping cycle for a steam power plant. A cycle efficiency of 50% could be obtained for a 5000-kwe plant. Nickel and cobalt alloys, common to the well-established gas-turbine industry, could be used for the components. Turbines, bearings, seals, and a compact potassium boiler, which would also be directly applicable to the topping cycle, have been tested. Test results show that heat-transfer coefficients of the order of 50,000 Btu/hr-ft-°F can be achieved in the high-quality region. Also, stable boiling was readily obtained over a wide range of operating parameters.

Antenna analysis

The research on alternative refrigerants has been one of the urgent issues in the refrigeration and air conditioning industry for the sake of reducing ozone depletion and global warming effect. R161, as a refrigerant with good environmental acceptability (GWP = 12, ODP = 0), good thermo-physical properties, good heat transfer and flow characteristics, and highly similar thermo-physical properties to R22, has attracted broad attention as a substitute of R22 and R407C. In this paper, the boiling heat transfer and pressure drop characteristics of R161/POE mixture in a 7 mm O.D. horizontal micro-fin tube were investigated by experimental methods. The local heat transfer coefficients and pressure drops were measured at mass flux range of (100 ∼ 250) kg/(m2·s), heat flux range of (11.76 ∼ 52.94) kW/m2, bubble point temperature range of (−5 ∼ 8) °C, and oil concentration range of (0 ∼ 5)%. The results showed that: (1) at low vapor quality region, the local heat transfer coefficient increased with oil concentration, while at the high vapor quality region, it decreased rapidly with the increase of oil concentration; (2) the pressure drop increased with the increase of oil concentration, mass flux and heat flux; (3) the influence of oil on heat transfer coefficient and pressure drop decreased with the increase of mass flux. A new correlation to predict the local boiling heat transfer of R161/oil mixture inside small micro-fin tubes was developed on the basis of the local properties of refrigerant/oil mixture and structure of enhanced tube, and the predicted data agreed with the experimental data within the deviations of (−15 ∼ +20)%.