Добірка наукової літератури з теми "Flux tubes; finite temperature; Monte Carlo methods"

Three dimensional numerical investigation of heat transfer enhancement in a non-uniformly heated parabolic trough solar collector using dimpled absorber under turbulent flow was incorporated in the current paper. The governing equations were solved using the finite volume methods (CFD) with certain assumptions and appropriate boundary conditions. The Monte Carlo ray trace technique was applied to obtain the heat flux distribution around the absorber tube. The numerical results were validating with the empirical correlations existing in the literature and good agreement was obtained. The present results demonstrate that the inclusion of inserts provide a good performance in heat transfer, also the receiver temperature gradient are shown to reduce with the use of geometrical modification, the absorber geometry have a remarkable effect on the HTF velocity distribution.

The Monte Carlo ray tracing method is applied and coupled with finite volume numerical methods to study effect of rotation on outlet temperature and heat gain of LS-2 parabolic trough concentrator (PTC). Based on effect of sunshape, curve of mirror and use of MCRT, heat flux distribution around of inner wall of evacuated tube is calculated. After calculation of heat flux, the geometry of LS-2 Luz collector is created and finite volume method is applied to simulate. The obtained results are compared with Dudley et al test results for irrotational cases to validate these numerical solving models. Consider that, for rotational models ,the solving method separately with K.S. Ball's results. In this work, according to the structure of mentioned collector, we use plug as a flow restriction. In the rotational case studies, the inner wall rotates with different angular speeds. We compare results of rotational collector with irrotational. Also for these two main states, the location of plug changed then outlet temperature and heat gain of collector are studied. The results show that rotation have positive role on heat transfer processing and the rotational plug in bottom half of tube have better effectual than upper half of tube. Also the contribution of rotation is calculated in the all of case studies. Working fluid of these study is one of the oil derivatives namely Syltherm-800. The power of wind can be used to rotate tube of collector.

The problem of predicting deposition rates and film thickness variation is relevant to many high-vacuum physical vapor deposition (PVD) processes. Analytical methods for modeling the molecular flow fail when the geometry is more complicated than simple tubular or planar sources. Monte Carlo methods, which have traditionally been used for modeling PVD processes in more complicated geometries, being probabilistic in nature, entail long computation times, and thus render geometry optimization for deposition uniformity a difficult task. Free molecular flow is governed by the same line-of-sight considerations as thermal radiation. Though the existence of an analogy between the two was recognized by Knudsen (1909, Ann. Phys., 4(28), pp. 75–130) during his early experiments, it has not been exploited toward mainstream analysis of deposition processes. With the availability of commercial finite element software having advanced geometry modelers and built-in cavity radiation solvers, the analysis of diffuse thermal radiation problems has become considerably simplified. Hence, it is proposed to use the geometry modeling and radiation analysis capabilities of commercial finite element software toward analyzing and optimizing high-vacuum deposition processes by applying the radiation-molecular flow analogy. In this paper, we lay down this analogy and use the commercial finite element software ABAQUS for predicting radiation flux profiles from planar as well as tube sources. These profiles are compared to corresponding deposition profiles presented in thin-film literature. In order to test the ability of the analogy in predicting absolute values of molecular flow rates, ABAQUS was also employed for calculating the radiative flux through a long tube. The predictions are compared to Knudsen’s analytical formula for free molecular flow through long tubes. Finally, in order to see the efficacy of using the analogy in modeling the film thickness variation in a complex source-substrate configuration, an experiment was conducted where chromium films were deposited on an asymmetric arrangement of glass slides in a high-vacuum PVD chamber. The thickness of the deposited films was measured and the source-substrate configuration was simulated in ABAQUS. The variation of radiation fluxes from the simulation was compared to variation of the measured film thicknesses across the slides. The close agreement between the predictions and experimental data establishes the feasibility of using commercial finite element software for analyzing high vacuum deposition processes.

We introduce a coupled multiscale, multiphysics method (CM3) for solving for the behaviour of rarefied gas flows. The approach is to solve the kinetic equation for rarefied gases (the Boltzmann equation) over a very short interval of time in order to obtain accurate estimates of the components of the stress tensor and heat-flux vector. These estimates are used to close the conservation laws for mass, momentum and energy, which are subsequently used to advance continuum-level flow variables forward in time. After a finite time interval, the Boltzmann equation is solved again for the new continuum field, and the cycle is repeated. The target applications for this type of method are transition-regime gas flows for which standard continuum models (e.g. Navier–Stokes equations) cannot be used, but solution of Boltzmann's equation is prohibitively expensive. The use of molecular-level data to close the conservation laws significantly extends the range of applicability of the continuum conservation laws. In this study, the CM3 is used to perform two proof-of-principle calculations: a low-speed Rayleigh flow and a thermal Fourier flow. Velocity, temperature, shear-stress and heat-flux profiles compare well with direct-simulation Monte Carlo solutions for various Knudsen numbers ranging from the near-continuum regime to the transition regime. We discuss algorithmic problems and the solutions necessary to implement the CM3, building upon the conceptual framework of the heterogeneous multiscale methods.

Based on finite volume method, three-dimensional models are used to evaluate the effect of flow restriction device (plug) on outlet temperature and efficiency of LS-2 parabolic trough collector. In order to study the effect of plug, various positions of plug with different diameters are used as case-studies. In other hand, solar heat flux distribution on the outer wall of the receiver tube is calculated by Monte Carlo ray tracing method (MCRT). The MCRT method is applied and coupled with three-dimensional numerical methods, computational fluid dynamics (CFD), in order to study the effect of plug diameters and their positions on outlet temperature and efficiency of collector. The result of MCRT method shows nonuniform heat flux hits on outer wall of receiver tube. So, after simulation of absorber tube of LS-2 PTC, the nonuniform heat flux is applied in the computational code. In order to validate the numerical methods, the working fluid and physical simulated model and operating conditions are considered as Syltherm-800 and LS-2 parabolic trough collector which had been tested via Dudley et al. at Sandia National Research Laboratory (SNRL). After the validation of numerical method, several case-studies with variable plug diameter and plug positions are simulated. Other working fluids are also tested for modeling of mentioned case-studies too. Results show that if the amount of nondimensional displacement from center becomes +0.5, then outlet temperature will be gentler. It is independent of plug diameter and working fluid. Finally, the efficiency of each of cases is evaluated. Consider that, the evacuated receiver tube is utilized in simulation of parabolic trough concentrator (PTC) and therefore, the convective losses have been negligible.

The distribution of the gluon action density in mesonic systems is investigated at finite temperature. The simulations are performed in pure SU(3) Yang-Mills gauge theory for two temperatures below the deconfinement phase. The action-density isosurfaces display a prolate-spheroid-like shape. The curved width profile of the flux tube is found to be consistent with the prediction of the free bosonic string model at large distances. In the intermediate source separation distance, where the free string picture poorly describes the flux tube width profile, we find the topological characteristics of the flux tube converge and compare favourably with the predictions of the free bosonic string upon reducing the vacuum action towards the classical instanton vacuum. As a byproduct of these calculations, we find the broadening of the QCD flux tube to be independent of the UV filtering at large distances. Our results exhibit a linearly divergent pattern in agreement with the string picture predictions. We investigate the overlap of the ground state meson potential with sets of mesonic-trial wave functions. We construct trial states with non-uniform smearing profiles in the Wilson loop operator at T = 0. The non-uniformly UV-regulated flux-tube operators are found to optimize the overlap with the ground state. The gluon flux distribution of a static three quark system has been revealed at temperatures near the end of the QCD plateau, T/Tc ≈ 0.8, and another just before the deconfinement point, T/Tc ≈ 0.9. The flux distributions at short distance separations between the quarks display an action-density profile consistent with a rounded filled Δ shape iso-surface. However the Δ shape action iso-surface distributions are found to persist even at large inter-quark separations. The action density distribution in the quark plane exhibits a nonuniform pattern for all quark separations considered. We systematically measure and compare the main aspects of the profile of the flux distribution at the two considered temperature scales for three sets of isosceles triangle quark configurations. The radii, amplitudes and rate of change of the width of the flux distribution are found to reverse their behavior as the temperature increases from the end of the QCD plateau towards the deconfinement point. Remarkably, we find the mean square width of the flux distribution shrinks and localizes for quark separations larger than 1.0 fm at T/Tc ≈ 0.8 which results in an identifiable Y-shaped radius profile. Near the deconfinement point, the action-density delocalizes and the width broadens linearly with the quark separation at large quark separations. We present a method to include the thermal effects into the junction width of the baryonic string model. The profile of the baryonic gluonic distribution is compared with the width of the string picture’s junction fluctuations. The comparison reveals that the best fits to the junction fluctuations of the baryonic string are near the Fermat point of the triangle made up by the quarks. This result supports the underlying picture of Y-shaped string-like flux tubes connected at a junction.
Thesis (Ph.D.) -- University of Adelaide, School of Chemistry and Physics, 2011

The goal of this work was to measure the temporally varying heat flux and surface temperature of a pipe calorimeter in a pool fire, and assess its uncertainty. Three large-scale fire tests were conducted at the Sandia National Laboratories outdoor fire test facility. In each test a cylindrical calorimeter was suspended above a water pool with JP8 fuel floating on top. The calorimeter was a 2.4 m diameter, 4.6 m long, and 2.5 cm wall thickness pipe with end-caps suspended 1 m above the 7.2 m diameter pool. 58 thermocouples were attached to the calorimeter interior surface and backed with 8 cm of insulation. The Sandia One-Dimensional Direct and Inverse Thermal (SODDIT) code was used to determine the calorimeter external surface heat flux and temperature from the measured interior surface temperature versus time. To determine the uncertainty of the SODDIT results, a simulation of the calorimeter in a fire similar to the experiments was performed using the Container Analysis Fire Environment (CAFE) computer code. In this code, a Computational Fluid Dynamics (CFD) fire model applies a temporally and spatially varying heat flux to the exterior surface of a Finite Element (FE) calorimeter model. Flux is similar but not identical to the flux in the experiment. The FE model calculates the internal calorimeter surface temperature, which is used by SODDIT to calculate heat flux which was compared to the applied values. The absorbed heat flux and surface temperature at one calorimeter location was calculated by SODDIT and then compared to the CAFE applied heat flux and surface temperature. From this comparison a base case uncertainty due to inherent inverse calculation errors and frequency smoothing methods is presented. Uncertainties in temperature measurements, calorimeter material properties and wall thickness were applied to the SODDIT calculation and iterated using the Monte Carlo method to determine the overall heat flux and surface temperature uncertainty. The total absorbed heat flux uncertainty at the one studied location is ±4.8 kW/m2 at 95% confidence. The outer surface temperature uncertainty for all data at the one studied location is ±6.6°C at 95% confidence. For all 58 measurement locations, the overall combined total absorbed heat flux uncertainty is ±13.8 kW/m2 at 95% confidence, surface temperature uncertainty is ±7.6°C. These uncertainties are valid only when the calorimeter temperature is not within the Curie temperature range of 999 to 1037K.