Littérature scientifique sur le sujet « Ηmin and ηmax »

The present work aims at optimizing titanium dioxide morphology for dye-sensitized solar cells applications. Five different anatase phase mesoporous titanias were prepared and tested as photoanodes in dye-sensitized solar cells. The materials were prepared by using a template approach. Two materials were synthesized by using monodisperse silica nanospheres and the other three using two different organic templating agents (Pluronic P123 and Brij 58). A complete characterization of the obtained materials was performed by powder XRD, FEG-SEM, UV-Vis reflectance spectroscopy, BET surface area measurements, and TG-DTA. Several cells were assembled using N719 as dye and a nonvolatile electrolyte based on benzonitrile. The cells were tested by means ofJ-Vcurves under simulated solar radiation, IPCE, and dark current measurements. The highest efficiencies were achieved with titania prepared by using Pluronic P123 as template (ηmax=6.8%), while the lowest efficiencies were recorded with using titania samples prepared with the silica nanospheres template (ηmin=5.7%). The different performances of the samples are examined and discussed.

Abstract Self-consolidating concrete (SCC) is one of the trending low-yield stress material innovations of the new age building industry due to its ability to flow through heavily reinforced structural members without segregation and without losing its moderate viscosity. In this research paper, a V-funnel simulation has been developed for the flow time to establish its validity with allowable design conditions according to the European standard. The Bernoulli’s equation and the continuity flow state conditions were adopted for the V-funnel of 515 mm upper width, 75 mm thick, and vertical dimensions (depth) of 450 mm and 150 mm for the V-section and funnel duct (75 mm × 65 mm) respectively. Concrete shear stress with the funnel wall was considered as a factor of concrete unit weight (ρ) and frictional force under downward velocity with respect to variational height (dh). The result of the V-funnel simulation for the flow time shows that the kinematic viscosity (η) of the concrete is bound within the limits of 0 and 1/64. This implies that ηmin ≈ 0 (no friction with walls), and the minimum flow time (Tmin) becomes 8.7 sec while minimum shear stress is zero (τmin ≈ 0 N/m2). However, for ηmax ≈ 1/64 (maximum friction with walls of τmax τ ρ/64 N/m2 is attained) and Tmax becomes 18.9 sec. These values fall within the flow time after 10 seconds of mixing, ranging between 8 and 10 seconds, and the flow time after 5 minutes of mixing of 11 to 25 seconds, according to the European Guidelines for self-consolidating concrete. These results for the flow simulation of the concrete placement during construction, are also consistent with the United Nations sustainable development goals (SDGs) for technological innovation, infrastructure, and sustainable cities.

Abstract This paper addresses the laminar boundary layer flow of certain binary gas mixtures along a heated flat plate. To form the binary gas mixtures, light helium (He) is the primary gas and the heavier secondary gases are nitrogen (N2), oxygen (O2), xenon (Xe), carbon dioxide (CO2), methane (CH4), tetrafluoromethane (CF4), and sulfur hexafluoride (SF6). The central objective of this paper is to investigate the potential of this group of binary gas mixtures for heat transfer intensification. From fluid physics, two thermophysical properties, i.e., viscosity η and density ρ, influence the fluid flow, whereas four thermophysical properties, i.e., viscosity η, thermal conductivity λ, density ρ, and heat capacity at constant pressure Cp, affect the forced convective heat transfer. The heat transfer augmentation from the flat plate is pursued by stimulating the forced convection mode as a whole. In this regard, it became necessary to construct a specific correlation equation to handle binary gas mixtures owing Prandtl number Pr∊(0.1,1). Whenever there is heat transfer invigoration in forced flow, drag force accretion seems to be inevitable. A standard formula for estimating the drag force Fd exerted on the flat plate is available from the fluid dynamics literature. The descriptive equations for the heat transfer rate Qmix and drag force Fd,mix associated with the seven binary gas mixtures are channeled through the four thermophysical properties, i.e., density ρmix, viscosity ηmix, thermal conductivity λmix, and heat capacity at constant pressure Cp,mix, which depend on the molar gas composition w. Two case studies suffice to elucidate the modified convective heat and momentum transport that the binary gas mixtures bring forward. At a film temperature Tf=300 K and 1 atm, the He+SF6 mixture delivers the absolute maximum for the relative heat transfer Qmix,abs max/B=16.71 at an optimal molar gas composition wopt=0.96. When compared with the light primary He gas with a relative heat transfer rate QHe/B=12.04, the He+SF6 mixture generates a significant heat transfer enhancement of 39%. At a film temperature Tf=600 K and the same 1 atm, the relative heat transfer QHe/B for the light primary gas He comes down to 10.77. In reference to this, the He+SF6 mixture furnishes an absolute maximum heat transfer Qmix,abs max/B=18.11 at an optimal molar gas composition wopt=0.96, yielding a remarkable heat transfer enhancement of 68%. In the global context, the usage of exotic binary gas mixtures with light helium and selected heavier gases may be envisioned for special tasks in industries that demand high heat transfer rates.

Diffractive scattering processes have two main signatures: one or both incoming protons remain intact after the interaction and one or more rapidity gaps appear as forbidden regions in the rapidity distribution of scattering products. Rapidity gaps are associated to the exchange of pomerons between the interacting protons, the pomeron being described in QCD in terms of an exchange of gluons in a colorless configuration. In the context of diffractive physics, the study of rapidity gaps is then of particular interest. In the work reported in this thesis a study of rapidity gaps has been conducted in Double Pomeron Exchange events produced in pp collisions at √s = 8 TeV, using a dataset collected in July 2012 by the TOTEM experiment at the LHC during a common data taking with the CMS experiment. In DPE events both incoming protons remain intact in the collision and a system of particles is generated in the central zone, separated from the two protons by two rapidity gaps. Thanks to the combined TOTEM-CMS apparatus, which provides an exceptionally large pseudorapidity coverage, the tagging of protons with the TOTEM Roman Pot detectors and the reconstruction of the central system with the CMS apparatus has been performed. The TOTEM T2 telescope also provided the reconstruction of charged particles in the forward region, where no information from CMS is available. The aim of this work was the development of an analysis to study the rapidity gaps in DPE events, and compare them with a sample of DPE events obtained by a Pythia8MBR Monte Carlo simulation. Since such MC sample is based on a pure 2-gluon colorless exchange during the interaction, a deviation of rapidity gap probability could represent an indication of additional exchange not related to pomerons. In this study, an important role was covered by the charged particle tracks reconstructed in the TOTEM T2 telescopes and by final-state stable particles reconstructed and identified by means of a particular algorithm, known as “particle-flow" (PF), combining the information from the CMS subdetectors. They allowed to define in a wide |η| range the two rapidity gaps in DPE event candidates. The evaluation of the size of the rapidity gaps was possible through the direct leading proton measurement by the RP detectors. As first step, an optimization in the selection of PF neutral particles (neutral hadrons and photons) at √s = 8 TeV has been performed in order to suppress most of the detector noise in data, since standard cuts were previously obtained by the CMS Collaboration for data at √s = 7 TeV. The new cuts have been found in each CMS sub-region by using a Zero-Bias sample, collected during the same data taking period of the dataset used for DPE event selection (triggered by the TOTEM RPs). Then, the typical variables for the identification of DPE processes have been introduced: the fractional longitudinal momentum loss of each scattered proton (ξ1,2) reconstructed from the proton tracks; the two values of pseudorapidity ηmin and ηmax (related to the central diffractive system) which characterize the two rapidity gaps; the mass MX of the diffractive system obtained from the RP measurements, and the central mass Mcentral measured from the PF objects in the CMS region. In the next step of this study a selection of DPE event was performed in order to remove/reduce the main sources of background. The dominant background due to elastic events overlapped with pile-up processes has been removed by vetoing on the diagonal (TB/BT) configurations for the protons in the RPs. In order to reduce the pile-up effects in the selected parallel (TT/BB) configurations, we have selected only events with one proton per arm, simultaneously tagged by the two vertical RPs in the 220-station, and with no more than one CMS vertex. Then, by requiring a value of mass of the diffractive system greater than the value of the central mass, it was possible to reject events affected by residual noise and pile-up (NP) effects. After the selection of DPE events, a data driven correction method has been applied bin per bin to the probability distributions of ηmin and ηmax as an iterative procedure in order to correct for NP effects. This procedure has been based on studies on the Zero-Bias sample. In the following step, it was necessary to also consider the contribution of another source of background: the single diffractive process. Detailed studies performed on MC simulation showed that this contribution is dominant (up to about 19%) and is due when a proton produced by the breaking of one of two incoming protons arrives to RP detectors, simulating a leading proton. This contribution has been reduced by requiring activity in the CMS region. Based on MC studies, the iterative method correction has been updated in order to account for the residual SD background. Then, a comparison with MC expectations given by the Pythia8MBR generator was made. Here, a clear enhancement in events with reduced or absent reconstructed rapidity gaps in the very forward regions is observed in DATA, leading to a discrepancy in global normalization of the probability distributions in the central region, where a substantial agreement is found for the shapes. A better agreement between DATA and MC expectations is indeed found in the subsample where forward RGs are required (T2 veto on both sides). This result cannot a priori exclude that the violation of the expected rapidity gaps is due to some other process not characterized by the exchange of colorless objects, or that the MC generator we are considering is not properly modeling the DPE processes. However, further investigation should be performed in order to be sure that the observed behaviour is not due to some subtle residual background effect, or to some detector simulation related effect as well. For instance, it should be interesting to perform a separate study of events with zero and one CMS vertex, which are expected to be characterized by a different background bias.

The launching efficiency (η) from semiconductor lasers into plane-ended (PE) multimode fibers is low(~10 20%). We polished PE into rooftop (RT) ends, having a RT angle of 65° to the fiber axis, and observed a substantial increase in η; 27% (PE) to 51 % (RT). This was achieved (ηmax) after optimizing the apex position of RT fiber with respect to the stripe; the two have to be parallel. When they were perpendicular, ηmin = 29% was obtained. The effect of longitudinal, lateral, and angular misalignments was compared for the PE and RT ends; the latter were always more critical. We further investigated, for the first time, the effect on the intermodal pulse dispersion and obtained the following results: (i) output FWHM after 1 m having PE or RT end was 0.57 ns, (ii) FWHM after 1 km with PE input was 0.70 ns, giving pulse dispersion (t m ) of 0.41 ns, (iii) FWHM after 1 km with RT input and r/max was 0.61 ns giving t m = 0.22 ns, (iv) FWHM after 1 km with RT end and ηmin was 0.65 ns giving t m = 0.31 ns. The material dispersion was negligible. Hence comparing (ii) and (iii) above, t m was reduced by a factor of 2. This was explained by alteration of the modal power distribution in fiber core and confirmed by output far-field measurements.

A unique way for maximizing turbulent free convection from heated vertical plates to cold gases is studied in this paper. The central idea is to examine the attributes that binary gas mixtures having helium as the principal gas and xenon, nitrogen, oxygen, carbon dioxide, methane, tetrafluoromethane and sulfur hexafluoride as secondary gases may bring forward. From fluid physics, it is known that the thermo-physical properties affecting free convection with binary gas mixtures are viscosity ηmix, thermal conductivity λmix, density ρmix, and heat capacity at constant pressure. The quartet ηmix, λmix, ρmix, and Cp,mix is represented by triple-valued functions of the film temperature the pressure P, and the molar gas composition w. The viscosity is obtained from the Kinetic Theory of Gases conjoined with the Chapman-Enskog solution of the Boltzmann Transport Equation. The thermal conductivity is computed from the Kinetic Theory of Gases. The density is determined with a truncated virial equation of state. The heat capacity at constant pressure is calculated from Statistical Thermodynamics merged with the standard mixing rule. Using the similarity variable method, the descriptive Navier-Stokes and energy equations for turbulent Grashof numbers Grx > 109 are transformed into a system of two nonlinear ordinary differential equations, which is solved by the shooting method and the efficient fourth-order Runge-Kutta-Fehlberg algorithm. The numerical temperature fields T(x, y) for the five binary gas mixtures He-Xe, He-N2, He-O2, He-CO2, He-CH4, He-CF4 and He-SF6 are channeled through the allied mean convection coefficient hmix/B varying with the molar gas composition w in proper w-domain [0, 1]. For the seven binary gas mixtures utilized, the allied mean convection coefficient hmix/B versus the molar gas composition w is graphed in congruous diagrams. At a low film temperature Tf = 300 K, the global maximum allied mean convection coefficient hmix,max/B = 85 is furnished by the He-SF6 gas mixture at an optimal molar gas composition wopt = 0.93. The global maximum allied mean convection coefficient hmix,max/B = 57 is supplied by pure methane gas SF6 (w = 1) at a high film temperature Tf = 1000 K instead of the He-SF6 gas mixture.