• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 800
  • 226
  • 172
  • 167
  • 151
  • 24
  • 21
  • 16
  • 14
  • 12
  • 10
  • 8
  • 6
  • 4
  • 3
  • Tagged with
  • 2193
  • 2193
  • 568
  • 422
  • 340
  • 326
  • 266
  • 249
  • 233
  • 232
  • 213
  • 210
  • 208
  • 166
  • 163
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
651

Statistical Error in Particle Simulations of Fluid Flow and Heat Transfer

Hadjiconstantinou, Nicolas G., Garcia, Alejandro L., Bazant, Martin Z., He, Gang 01 1900 (has links)
We present predictions for the statistical error due to finite sampling in the presence of thermal fluctuations in molecular simulation algorithms. Specifically, we present predictions for the error dependence on hydrodynamic parameters and the number of samples taken. Expressions for the common hydrodynamic variables of interest such as flow velocity, temperature, density, pressure, shear stress and heat flux are derived using equilibrium statistical mechanics. Both volume-averaged and surface-averaged quantities are considered. Comparisons between theory and computations using direct simulation Monte Carlo for dilute gases, and molecular dynamics for dense fluids, show that the use of equilibrium theory provides accurate results. / Singapore-MIT Alliance (SMA)
652

Coke yield and transport processes in agglomerates of bitumen and solids

Ali, Mohamed Ali Hassan 11 1900 (has links)
Agglomerate formation is a common phenomenon that can cause operating problems in the fluid coking reactor. When agglomerates form they provide longer diffusion paths of the reaction products through the liquid layers and liquid bridges within the agglomerate, which leads to higher mass transfer resistance, trapping of the reaction products and increasing the undesired coke formation reactions. Surviving agglomerates in the reactor can also cause fouling of the reactor interior and defluidization of the bed. The ultimate coke yield was determined for agglomerates of Athabasca vacuum residue and solid particles by heating on Curie-point alloy strips in an induction furnace at 503 oC and 530 oC and in a fluidized bed reactor at 500 oC until all toluene-soluble material was converted. Coke yields from agglomerates were compared to the results from reacting thin films of vacuum residue. The average coke yield from the agglomerates was 23%, while the coke yield from thin films of 20 m thickness was 11%, which supports the role of mass transfer in coke formation reactions. The ultimate coke yield was insensitive to vacuum residue concentration, agglomerate size, reaction temperature and agglomerate disintegration. The temperature profile within agglomerates was measured by implanting a thermocouple at the agglomerate center, and a heat transfer model was used to describe the temperature variation with time. The effective thermal diffusivity of the agglomerates was 0.20 x 10-6 m2/s. Control experiments on reactions in thin liquid films confirmed that heating rates in the range of 14.8 to 148 K/s had no effect on the ultimate yield of coke
653

The Biglobal Instability of the Bidirectional Vortex

Batterson, Joshua Will 01 August 2011 (has links)
State of the art research in hydrodynamic stability analysis has moved from classic one-dimensional methods such as the local nonparallel approach and the parabolized stability equations to two-dimensional, biglobal, methods. The paradigm shift toward two dimensional techniques with the ability to accommodate fully three-dimensional base flows is a necessary step toward modeling complex, multidimensional flowfields in modern propulsive applications. Here, we employ a two-dimensional spatial waveform with sinusoidal temporal dependence to reduce the three-dimensional linearized Navier-Stokes equations to their biglobal form. Addressing hydrodynamic stability in this way circumvents the restrictive parallel-flow assumption and admits boundary conditions in the streamwise direction. Furthermore, the following work employs a full momentum formulation, rather than the reduced streamfunction form, accounting for a nonzero tangential mean flow velocity. This approach adds significant complexity in both formulation and implementation but renders a more general methodology applicable to a broader spectrum of mean flows. Specifically, we consider the stability of three models for bidirectional vortex flow. While a complete parametric study ensues, the stabilizing effect of the swirl velocity is evident as the injection parameter, kappa, is closely examined.
654

MODELING AND ANALYSIS OF TURBOJET COMPRESSOR INLET TEMPERATURE MEASUREMENT SYSTEM PERFORMANCE

Binkley, Brian A 01 May 2011 (has links)
Accurate measurement of turbine engine compressor inlet total temperature is paramount for controlling engine speed and pressure ratio. Various methods exist for measuring compressor inlet total temperature on turbojet engines with hydromechanical control. One method involves the use of an ejector-diffuser system (eductor) to pull air from the engine inlet in order to measure the incoming total temperature. Analysis of historical test data has revealed that the inlet temperature measurement can be biased at certain flight conditions causing engine mis-scheduling and off-nominal engine operation. This bias is characterized primarily by adverse heat transfer effects and secondly by poor flow quality in the eductor tubing. Alternate eductor system configurations have been proposed to mitigate temperature bias. A one-dimensional engineering model of the eductor system was developed to facilitate the analysis of baseline and alternate eductor configurations. The model is calibrated with results from Computational Fluid Dynamics and validated with ground test data. The validated model is used to quantify the performance of several eductor configurations throughout the range of expected operating conditions and to quantify the amount of compressor inlet temperature measurement bias mitigation each configuration provides.
655

Investigations of Melt Spreading and Coolability in a LWR Severe accident

Konovalikhin, Maxim January 2001 (has links)
No description available.
656

Natural Convection Heat Transfer in Two-Fluid Stratified Pools with Internal Heat Sources

Gubaidullin, Askar January 2001 (has links)
No description available.
657

On Heat and Paper : From Hot Pressing to Impulse Technology

Lucisano, Marco Francesco Carlo January 2002 (has links)
Impulse technology is a process in which water is removedfrom a wet paper web by the combined action of mechanicalpressure and intense heat. This results in increased dewateringrates, increased smoothness on the roll side of the sheet, andincreased density. Although the potential benefits of impulsepressing have been debated over the past thirty years, itsindustrial acceptance has been prevented by web delamination,which is defined as a reduction in the z-directional strengthof paper. This thesis deals with the mechanism of heat transfer withphase change during impulse pressing of wet paper. The resultsof four complementary experimental studies suggest that littleor no steam is formed in an impulse nip prior to the point ofmaximum applied load. As the nip is unloaded and the hydraulicpressure decreases, hot liquid water flashes to steam. Weadvance the argument that the force expressed upon flashing canbe used to displace liquid water, in a mechanism similar tothat originally proposed by Wahren. Additionally, modelexperiments performed in a novel experimental facility suggestthat the strength of flashing-assisted displacement dewateringcan be maximized by controlling the direction of steam venting.If this solution could be exploited in a commercially viableimpulse press, delamination would cease to be an issue ofconcern. The thesis includes a study of the web structure ofdelaminated paper. Here, we characterized delaminated paper bythe changes in transverse permeability and cross-sectionalsolidity profiles measured as a function of pressingtemperature. We found no evidence that wet pressing and impulsepressing induced stratification in non-delaminated sheets andconcluded that the parabolic solidity profiles observed weredue to capillary forces present during drying. Further, thepermeability of mechanically compressed never-dried samples wasfound to be essentially constant for pressing temperatureslower than the atmospheric boiling point of water and toincrease significantly at higher pressing temperatures. Wepropose this to be a result of damage to the cell wall materialdue to flashing of hot liquid water in the fiber walls andlumina. Finally, we present a method and an apparatus formeasurement of the thermal properties of water-saturated paperwebs at temperatures and pressures of interest for commercialhigh-intensity processes. After validation, the method wassuccessfully applied to measure the thermal conductivity,thermal diffusivity and volumetric heat capacity ofwater-saturated blotter paper as functions of temperature andsolids content. Here, we found that the thermal conductivityincreased with solids content in the range from 30%\ to 55%,which is in conflict with the commonly stated assumptions of adecreasing trend. We propose that this discrepancy could be dueto the thermal conductivity of air-free fibers wetted byunpressable water only, being significantly different from thatof dry cellulose.
658

Modeling And Control Of High Temperature Oven For Low Temperature Co-fired Ceramic (ltcc) Device Manufacturing

Yucel, Ayse Tugce 01 October 2012 (has links) (PDF)
In the electronics the quality, reliability, operational speed, device density and cost of circuits are fundamentally determined by carriers. If it is necessary to use better material than plastic carrier, it has to be made of ceramics or glass-ceramics. This study dealt with the ceramic based carrier production system. The types of the raw ceramics fired at low temperature (below 1000&deg / C) are called Low Temperature Co-Fired Ceramics (LTCC). In this study, a comprehensive thermal model is described for the high temperature oven which belongs to a Low Temperature Co-fired Ceramic (LTCC) substance production line. The model includes detailed energy balances with conduction, convection and radiation heat transfer mechanisms, view factor derivations for the radiative terms, thermocouple balances, heating filaments and cooling mechanisms for the system. Research was conducted mainly on process development and production conditions along with the system modeling of oven. Temperature control was made in high temperature co-firing oven. Radiation View Factors for substrate and thermocouples are determined. View factors between substrate and top-bottom-sides of the oven are calculated, and then inserted into the energy balances. The same arrangement was made for 3 thermocouples at the bottom of the oven. Combination of both expressions gave the final model. Modeling studies were held with energy balance simulations on MATLAB. Data analysis and DOE study were held with JMP Software.
659

Temperature Distribution In Power Transformers

Karadag, Rukiye 01 June 2012 (has links) (PDF)
As in all other electrical equipments it is essential to estimate the temperature distribution in transformer components in the design stage and during the operation since temperatures above thermal limits of these components might seriously damage them. Thermal models are used to predict this vital information prior to actual operations. In this study, a three dimensional model based on the Finite Element Method (FEM) is proposed to estimate the temperature distribution in the three phase, SF6 gas insulated-cooled power transformer. This model can predict the temperature distribution at the specific discredited locations in the transformer successfully.
660

Temperature Distribution In Power Transformers

Karadag, Rukiye 01 June 2012 (has links) (PDF)
As in all other electrical equipments it is essential to estimate the temperature distribution in transformer components in the design stage and during the operation since temperatures above thermal limits of these components might seriously damage them. Thermal models are used to predict this vital information prior to actual operations. In this study, a three dimensional model based on the Finite Element Method (FEM) is proposed to estimate the temperature distribution in the three phase, SF6 gas insulated-cooled power transformer. This model can predict the temperature distribution at the specific discredited locations in the transformer successfully.

Page generated in 0.0605 seconds