Flow boiling in a 1.1mm tube with R134a: Experimental results and comparison with model

A detailed comparison of the three-zone evaporation model, proposed by Thome et al. (2004), with experimental heat transfer results of two stainless steel tubes of internal diameter 4.26 mm and 2.01 mm using R134a fluid was presented by Shiferaw et al. (2006). In the current paper the comparison is extended to flow boiling in a 1.1 mm tube using R134a as the working fluid. Other parameters were varied in the range: mass flux 100-600 kg/m2.s; heat flux 16-150 kW/m2 and pressure 6-12 bar. The experimental results demonstrate that the heat transfer coefficient increases with heat flux and system pressure, but does not change with vapour quality when the quality is less than about 50% for low heat and mass flux values. The effect of mass flux is observed to be insignificant. For vapour quality values greater than 50% and at high heat flux values, the heat transfer coefficient does not depend on heat flux and decreases with vapour quality. This could be caused by partial dryout. The three-zone evaporation model predicts the experimental results fairly well, especially at relatively low pressure. However, the partial dryout region is highly over-predicted by the model. The sensitivity of the performance of the model to the three optimized parameters (confined bubble frequency, initial film thickness and end film thickness) and some preliminary investigation relating the critical film thickness for dryout to measured tube roughness are also discussed

EAM Modulated DBR Laser Array for TWDM-PON Applications

4 Channel DBR laser arrays are fabricated for use in optical line terminals of TWDM-PON systems. These combine 1.4Q InGaAsP material in the DBR with EAMs using the identical active layer design. A tuning range ~10 nm and extinction ratio of >27 dB are measured

Fast tuneable InGaAsP DBR laser using quantum-confined stark-effect-induced refractive index change

We report a monolithically integrated InGaAsP DBR ridge waveguide laser that uses the quantum-confined Stark effect (QCSE) to achieve fast tuning response. The laser incorporates three sections: a forward-biased gain section, a reverse-biased phase section, and a reverse-biased DBR tuning section. The laser behavior is modeled using transmission matrix equations and tuning over similar to 8 nm is predicted. Devices were fabricated using post-growth shallow ion implantation to reduce the loss in the phase and DBR sections by quantum well intermixing. The lasing wavelength was measured while varying the reverse bias of the phase and DBR sections in the range 0 V to < - 2.5 V. Timing was noncontinuous over a similar to 7-nm-wavelength range, with a side-mode suppression ratio of similar to 20 dB. Coupled cavity effects due to the fabrication method used introduced discontinuities in tuning. The frequency modulation (FM) response was measured to be uniform within 2 dB over the frequency range 10 MHz to 10 GHz, indicating that tuning times of 100 ps are possible

Flat Dielectric Grating Reflectors with High Focusing Power

Sub-wavelength dielectric gratings (SWG) have emerged recently as a promising alternative to distributed-Bragg-reflection (DBR) dielectric stacks for broadband, high-reflectivity filtering applications. A SWG structure composed of a single dielectric layer with the appropriate patterning can sometimes perform as well as thirty or forty dielectric DBR layers, while providing new functionalities such as polarization control and near-field amplification. In this paper, we introduce a remarkable property of grating mirrors that cannot be realized by their DBR counterpart: we show that a non-periodic patterning of the grating surface can give full control over the phase front of reflected light while maintaining a high reflectivity. This new feature of dielectric gratings could have a substantial impact on a number of applications that depend on low-cost, compact optical components, from laser cavities to CD/DVD read/write heads.Comment: submitted to Nature Photonic

Cumulative dominance and heuristic performance in binary multi-attribute choice

Working paper 895, Department of Economics and Business, Universitat Pompeu FabraSeveral studies have reported high performance of simple decision heuristics in multi-attribute decision making. In this paper, we focus on situations where attributes are binary and analyze the performance of Deterministic-Elimination-By-Aspects (DEBA) and similar decision heuristics. We consider non-increasing weights and two probabilistic models for the attribute values: one where attribute values are independent Bernoulli randomvariables; the other one where they are binary random variables with inter-attribute positive correlations. Using these models, we show that good performance of DEBA is explained by the presence of cumulative as opposed to simple dominance. We therefore introduce the concepts of cumulative dominance compliance and fully cumulative dominance compliance and show that DEBA satisfies those properties. We derive a lower bound with which cumulative dominance compliant heuristics will choose a best alternative and show that, even with many attributes, this is not small. We also derive an upper bound for the expected loss of fully cumulative compliance heuristics and show that this is moderate even when the number of attributes is large. Both bounds are independent of the values of the weights.Postprint (author’s final draft

Experimental flow boiling study in a 0.52 mm diameter vertical tube using R134a

This article is also available online from the link below.Experimental results for R134a boiling on a vertical stainless steel tube of internal diameter 0.52 mm are presented in this paper. They form part of an ongoing study of flow boiling in a wide range of small diameter tubes. Other parameters were varied in the range: mass flux 300-700 kg/m2.s; heat flux 1.6-75 kW/m2 and pressure 6 -10 bar. The flow patterns visualised at the exit of the heated test section are first presented. The variation of the heat transfer coefficient with thermodynamic quality and its dependence on heat flux, mass flux and system pressure is then presented and discussed

Design, fabrication and characterization of a distributed Bragg reflector for reducing the étendue of a wavelength converting system

In this work, the design, fabrication and characterization are reported for a distributed Bragg reflector (DBR) filter with a specific wavelength and angular dependency, which aims to improve the light collection from a wavelength-converter-based light source into a smaller angle than the full angle Lambertian emission. The desired design is obtained by optimizing the transmission characteristics of a multi-layer structure. Titania (TiO2) and silica (SiO2) are used as high and low refractive index materials, respectively. The deposition is made by electron beam evaporation without substrate heating, followed by a post-annealing procedure. The optical properties of the evaporated layers are analyzed by ellipsometer and spectrometer measurements. The angular and wavelength dependency of the fabricated DBR is in good agreement with simulations for the designed structure. (C) 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreemen

High-Q distributed-Bragg-grating laser cavities

Applying Bragg gratings in Al2O3 channel waveguides, we demonstrate distributed Bragg reflectors with Q-factors of 1.02x10e6. An integrated Al2O3:Yb3+ waveguide laser with 67% slope efficiency and 47 mW output power is achieved with such cavities

California's Proposition 8: What Happened, and What Does the Future Hold?

Analyzes the characteristics of voters most likely to have supported or opposed Proposition 8 in 2008, including party affiliation, religiosity, and age. Reviews exit poll data suggesting strong African-American support for the measure