M.F. Lightstone

Buoyancy-driven heat transfer enhancement in a two-dimensional enclosure utilizing nanofluids

Abstract Heat transfer enhancement in a two-dimensional enclosure utilizing nanofluids is investigated for various pertinent parameters. A model is developed to analyze heat transfer performance of nanofluids inside an enclosure taking into account the solid particle dispersion. The transport equations are solved numerically using the finite-volume approach along with the alternating direct implicit procedure. Comparisons with previously published work on the basis of special cases are performed and found to be in excellent agreement. The effect of suspended ultrafine metallic nanoparticles on the fluid flow and heat transfer processes within the enclosure is analyzed and effective thermal conductivity enhancement maps are developed for various controlling parameters. In addition, an analysis of variants based on the thermophysical properties of nanofluid is developed and presented. It is shown that the variances within different models have substantial effects on the results. Finally, a heat transfer correlation of the average Nusselt number for various Grashof numbers and volume fractions is presented.

A review of low-flow, stratified-tank solar water heating systems

Researchers have discovered that by using low collector flow rates (roughly one-seventh of those that have been generally used) and by taking measures to ensure the water in the storage tank remains stratified, the energy delivered by a forced-flow solar system can be increased substantially. In addition, the lower collector flow rates permit substantial savings in system cost, mainly through reduction in plumbing costs. This paper reviews the state of the art in this highly promising change in approach. Items discussed include physical reasons for the predicted performance improvement, savings in system cost, current knowledge in tank stratification, methods of analyzing and computer-simulating the systems, and finally, full-scale system experiments.

Modeling soot formation in turbulent kerosene/air jet diffusion flames

Abstract Soot volume fraction and number density in a turbulent diffusion flame burning kerosene/air were predicted using two approaches. The first used a conventional soot inception model based on the acetylene concentration and is referred to as the acetylene model. The second used a soot inception model based on the formation rate of three and two ring aromatics [1] and is referred to as the PAH inception model. The soot models account for inception, coagulation, surface growth, and oxidation processes. The Favre-averaged governing equations of mass, momentum, and energy in the turbulent field were solved in conjunction with the k − e turbulence model. A recently developed detailed reaction mechanism for kerosene/air [2] was coupled to the turbulent flow field by the stretched laminar flamelet approach. A radiation heat transfer model that considered the soot, water and CO2 levels is included. Models are validated by comparing the numerical results to the experimental results of Young et al. [3] for a turbulent jet-flame burning pre-vaporized aviation kerosene. Significant improvements in the prediction of soot volume fraction are obtained using the PAH inception model for soot inception compared to the conventional acetylene approach.

Mixed convection heat transfer in two-dimensional open-ended enclosures

Mixed convection heat transfer in open-ended enclosures has been studied numerically for three different flow angles of attack. Discretization of the governing equations is achieved using a finite element scheme based on the Galerkin method of weighted residuals. Comparisons with previously published work on special cases of the problem are performed and the results show excellent agreement. A wide range of pertinent parameters such as Grashof number, Reynolds number, and the aspect ratio are considered in the present study. The obtained results show that thermal insulation of the cavity can be achieved through the use of high horizontal velocity flow. Various results for the streamlines, isotherms and the heat transfer rates in terms of the average Nusselt number are presented and discussed for different parametric values. 2002 Elsevier Science Ltd. All rights reserved.

Optimization of soot modeling in turbulent nonpremixed ethylene/air jet flames

ABSTRACT Two-equation soot models, which solve conservation equations for soot number density and mass concentration, have been extensively used to study soot formation in laboratorial turbulent flame and practical gas-turbine combustors. This study investigates the effects of different inception, growth coagulation, and oxidation source terms in a two-equation semi-empirical soot model that has been implemented to model two turbulent ethylene/air jet flames. The gas-phase chemistry is modeled using the laminar flamelet approach. A new soot inception submodel is proposed that is based on the naphthalene formation rate calculated by the detailed chemical kinetics. The expected value of the formation rate is stored in the flamelet library. Model predictions were compared with the measurements of Young and Moss. The predictions of the soot volume fraction are very sensitive to the soot surface growth rate. The soot predictions agree well with measurements when the surface growth rate is assumed to be proportional to the square root of the surface area. The result also indicate that the naphthalene inception route exhibits better performance. Finally a new soot model with an optimal combination of rates was developed. The model predictions provided good agreement with the experimental temperature, mixture fraction, and soot volume fraction distributions along both the axial and radial directions. The optimal soot model was also successfully validated on another turbulent ethylene/air jet flame.

Plume entrainment effects in solar domestic hot water systems employing variable-flow-rate control strategies

Abstract Solar domestic hot water heating systems perform more efficiently if their storage tanks are perfectly thermally stratified. In real tanks, which do not perfectly stratify, the most important mechanism destroying stratification is plume entrainment. Plume entrainment occurs when cooler water is inserted into the tank top which contains hotter water. The resultant falling plume of cool water causes mixing. This paper uses computer simulation to evaluate and compare two strategies by which plume entrainment is minimized by controlling the collector flow rate. One strategy (callea “SCOT”) maintains a constant collector outlet temperature, and the other (called “FCTR”) strategy maintains a constant temperature rise Δ T set from inlet to outlet of the collector. The results of the study show that the SCOT strategy always produces a system that performs more poorly than the corresponding system with a fixed flow rate. The FCTR strategy, on the other hand, consistently out-performs the fixed flow strategy, but only by a few percent. When the FCTR strategy is used, the optimum Δ T set to use is 20°C for the SDHW system simulated.

The Effect of Jet Mixing on the Combustion Efficiency of a Hot Fuel-Rich Cross-Flow

Abstract Many combustion systems inject air into a hot fuel-rich cross-flow to minimize carbon monoxide emissions. The aim of this study is to improve the understanding of the relationship between mixing, chemical kinetics, and combustion efficiency for air jets in a hot reacting cross-flow. Carbon monoxide and hydrogen concentration measurements have been made for nine different round jet configurations issuing air into a hot reacting cross-flow. The unmixedness was measured for the 18 round jets module. The study indicates that the maximum combustion efficiency depends primarily on the overall equivalence ratio. An equivalence ratio of approximately 0.8 leads to the best combustion efficiency for all of the round jet modules. The predominance of the equivalence ratio highlights the importance of premixing prior to combustion. At constant equivalence ratio, low momentum flux ratios yield higher emissions. The combustion efficiency improves as the number of jets increases. The same flow conditions optimized both H2 and CO combustion efficiencies.

Numerical simulation of the charging of liquid storage tanks; Comparison with experiment

The degree of thermal stratification maintained in hot water storage tanks has a significant impact on the performance of a solar energy system. This paper presents an axisymmetric finite volume model analysis of the charging of a tank with hot water, and compares the predictions with experimental results from the literature. The results show the capabilities and deficiencies of such a modeling technique for this type of problem. The importance of inlet fluid turbulence to tank destratification is demonstrated and inclusion of a simple turbulence model is found sufficient to yield good agreement with measurement. The model predictions also provide insight into when a simple one-dimensional plug flow model will be adequate. In addition, the model is used to evaluate the effect during charging of heat conduction in the tank wall on the temperature field in the fluid. Recommendations are made regarding future work on the development of detailed numerical codes of simulating the charging of liquid storage tanks.

A Stochastic Model of Particle Dispersion in a Turbulent Gaseous Environment

The prediction of particle dispersion by interactions with a turbulent gaseous fluid is an important, yet difficult, problem. This paper presents a new model to predict the motion of particles in a turbulent flow. This model, which solves for the probability density function (pdf) for particle velocity, treats the impact of the turbulence on the velocity pdf as a diffusion process. Particle concentrations are, in turn, found from the velocity distributions. Comparisons between the model predictions and both analytical and experimental results are presented. Results are reported for flows of homogeneous, isotropic turbulence; for grid-generated turbulence; and for round jets. This study includes a large range of particle diameters and densities. Good agreement is found between the predictions and measurements.

A Numerical Investigation of Turbulent Interchange Mixing of Axial Coolant Flow in Rod Bundle Geometries

This article is concerned with modeling turbulent interchange mixing within rod bundle arrays. Using a three-dimensional numerical solution, the characteristics and appropriateness of an isotropic κ–∊ turbulence model were investigated by comparing code predictions with experimental results. A novel methodology for solving for the fluid temperature field was also developed. Application of this methodology resulted in significant savings in computational effort for the problem of interest. Analysis of the results indicated that the isotropic model accurately predicted the radial component of turbulent eddy viscosity, but underpredicted the azimuthal component. As a result, radial diffusion processes (such as pressure drop) were predicted accurately, but azimuthal diffusion processes (such as turbulent interchange mixing) were underpredicted. Higher-order turbulence models such as algebraic stress models are therefore required if the details of the turbulent mixing are to be resolved.