A comparison between spray cooling and film flow cooling during the rewetting of a hot surface

The paper is dealing with a research carried out at the Institute of Thermal-Fluid Dynamics to investigate the rewetting of a hot surface. The rewetting of the hot surface by spray cooling has been analyzed in previous works. After the droplet impingement, the liquid film falls along the surface, and rewetting by falling film takes place. The experiment was characterized by a 1-dimensional liquid spray, i.e., drops having a uniform, constant diameter, impinging on the heated surface. The cooling rate of the hot surface has been detected as a function of wall temperature, drop diameter and velocity, and impact point of the spray. The working feature of the spray is based on the varicose rupture of the liquid jet: imposing a periodic (symmetrical) perturbation with appropriate amplitude and frequency on the jet surface, the flow is “constrained” to break soon after leaving the nozzle, eventually obtaining constant diameter drops, depending on the nozzle diameter and liquid velocity. In this paper, previous results with spray cooling are compared with experimental runs in which the spray injection is replaced with a falling film all along the test section. The rewetting velocity has been calculated from the response of the thermocouples placed on the heated wall and using a digital image system based on the video image registered during the runs.     

LES Modeling of the Impact of Heat Losses and Differential Diffusion on Turbulent Stratified Flame Propagation: Application to the TU Darmstadt Stratified Flame

Common combustion chambers often exhibit turbulent flames propagating in partially-premixed mixtures. This propagation is generally governed by aerodynamics, unsteady mixing and chemical processes and may also be affected by conductive heat losses when the reactive zone develops close to the burner lips. The Filtered TAbulated Chemistry for Large Eddy Simulation (F-TACLES) model has been recently developed to include tabulated chemistry in Large Eddy Simulation (LES) of adiabatic stratified flames in flamelet regimes. The present article proposes a modeling approach to account for both differential diffusion and non-adiabatic effects on flame consumption speed following the F-TACLES formalism. The adiabatic F-TACLES model is first detailed using a generalized formalism for diffusive fluxes allowing either to account for differential diffusion or not. The F-TACLES model is then extended to non-adiabatic situations. A correction factor based on the non-adiabatic consumption rate is introduced to recover a realistic filtered flame consumption speed. The objective is here to tackle flame stabilization mechanisms when heat losses affect the reaction zone. The proposed approach is validated through the simulation of the unconfined stratified turbulent jet flame TSF-A for which stabilization process is affected by heat losses. Five simulations are performed for both adiabatic and non-adiabatic flow conditions comparing unity Lewis number and complex diffusion assumptions. The adiabatic F-TACLES model predicts a flame anchored at the burner lip disagreeing with experimental data. The non-adiabatic simulation exhibits local extinction due to heat losses near the burner exit. The flame is then lifted improving the comparison with experiments. Results also show a significant impact of molecular diffusion model on both mean flame consumption rate and angle.     

Control of Flow and Heat Transfer in a Porous Enclosure due to an Adiabatic Thin fin on the Hot Wall

Natural convection flow in a differentially heated square enclosure filled with porous matrix with a solid adiabatic thin fin attached at the hot left wall is studied numerically. The Brinkman–Forchheimer-extended Darcy model is used to solve the momentum equations, in the porous medium. The numerical investigation is done through streamlines, isotherms, and heat transfer rates. A parametric study is carried out using the following parameters: Darcy number (Da) from 10⁻⁴ to 10⁻², dimensionless thin fin lengths (L _p) 0.3, 0.5, and 0.7, dimensionless positions (S _p) 0.25, 0.5, and 0.75 with Prandtl numbers (Pr) 0.7 and 100 for Ra = 10⁶. For Da = 10⁻³ and Pr = 0.7, it is observed that there is a counter clock-wise secondary flow formation around the tip of the fin for S _p = 0.5 for all lengths of L _p. Moreover when Da = 10⁻² the secondary circulation behavior has been observed for S _p = 0.25 and 0.75 and there is another circulation between the top wall and the fin that is separated from the primary circulation. However, these secondary circulations features are not observed for Pr = 100. It is also found that the average Nusselt number decreases as the length of the fin increases for all locations. However, the rate of decrease of average Nusselt number becomes slower as the location of fin moves from the bottom wall to the top wall. The overall heat transfer rate can be controlled with a suitable selection of the fin location and length.     

A comparison between the craig-cox and the kacker-okapuu methods of turbine performance prediction

Summary Two complete comprehensive loss correlations for estimating the efficienty of axial-flow turbines are examinated in this paper: the well-known Craig-Cox method, and the recent development of the Ainley-Mathieson method, presented by Kacker and Okapuu, which reproposes the validity of this approach.Firstly, a comparison is done by evaluating the losses in a number representative cascades, having various solidities, aspect ratios, Reynolds and Mach numbers. It is shown that the two methods are in good agreement for subsonic cascades having high flow coefficients, while significant differences are found in high deflection blades, especially for the secondary losses.In the second part, it is investigated how the choice of correlation affects the project of a turbine stage. A design procedure, automatically carried out by a computer program, was applied to a number of cases: the differences between the two solutions of the same project problem obtained by using the two correlations are discusses. Parameters like specific speed, expansion ratio and size parameter are used to generalize the results.

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Sommario In questo articolo vengono considerati due metodi per la valutazione completa delle perdite fluidodinamiche in turbine assiali: il primo, ben noto, è quello proposto da Craig e Cox, mentre il secondo rappresenta il recente sviluppo del metodo di Ainley e Mathieson, operato da Kacker e Okapuu a conferma della validità dell'approccio originale.Un primo confronto è stato effettuato valutando le perdite in alcune schiere tipiche, con diversi numeri di Reynolds e di Mach. Viene mostrato che i due metodi sono in buon accordo per schiere subsoniche aventi elevati coefficienti di flusso, mentre si rilevano differenze significative in pale ad alta deflessione, specialmente per ciò che riguarda le perdite secondarie.Nella seconda parte si è voluto indagare come la scelta di una o dell'altra correlazione di perdite possa influenzare il progetto di stadi di turbine. Un certo numero di stadi tipici è stato ottimizzato mediante un codice automatico: vengono discusse le differenze tra le due soluzioni dello stesso problema progettuale risultate dall'uso di ambedue le correlazioni. I risultati sono presentati in funzione di parametri quali il numero di giri caratteristico, il rapporto di espansione e il coefficiente di «taglia», nell'intento di generalizzare i risultati.

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Bulk modulus of particulate composites: A comparison between a statistical and micro-mechanical approach

Two models are compared. One is based on the theory of elastic continua, and describes the interaction between filler and matrix in terms of an interfacial layer of varying volume fraction and elastic properties. The other derives from an equation of state for the constituents and the composite, based on molecular considerations. The filler-matrix interaction is then expressed in terms of segmental attractions and repulsions. We examine the dependence of the bulk modulusK _c ( _f ) on the volume fraction _f of filler and then show the correspondence between the two theories in terms of the infinite dilution limit of the ratio [K _c ( _f ) –K _m ]/(K _{m f} ) where the indexm refers to the matrix.Dedicated to Prof. Dr. F. R. Schwarzl on the occasion of his 60th birthdayOn leave from Rajdhani College, University of Delhi, India     

DNS of Mixing and Reaction of Two Species in a Turbulent Channel Flow: A Validation of the Conditional Moment Closure

We consider the chemical reaction in a turbulent flow for the case that the time scale of turbulence and the time scale of the reaction are comparable. This process is complicated by the fact that the reaction takes place intermittently at those locations where the species are adequately mixed. This is known as spatial segregation. Several turbulence models have been proposed to take the effect of spatial segregation into account. Examples are the probability density function (PDF) and the conditional moment closure (CMC) models. The main advantage of these models is that they are able to parameterize the effects of turbulent mixing on the chemical reaction rate. As a price several new unknown terms appear in these models for which closure hypothesis must be supplied. Examples are the conditional dissipation 〈 χ ∣ φ 〉, the conditional diffusion 〈 κ ∇² φ ∣ u, φ 〉 and the conditional velocity 〈 u ∣ φ 〉. In the present study we investigate these unknown terms that appear in the PDF and CMC model by means of a direct numerical simulation (DNS) of a fully developed turbulent flow in a channel geometry. We present the results of two simulations in which a scalar is released from a continuous line source. In the first we consider turbulent mixing without chemical reaction and in the second we add a binary reaction. The results of our simulations agree very well with experimental data for the quantities on which information is available. Several closure hypotheses that have been proposed in the literature, are considered and validated with help of our simulation results. This revised version was published online in July 2006 with corrections to the Cover Date.     

Heat and Mass Transfer on the Mixed Convection of Non-Newtonian Fluids Over a Vertical Wedge with Soret/Dufour Effects and Internal Heat Generation: Variable Wall Temperature/Concentration

Transport in Porous Media - The presented work compares the mechanical behavior from standard unconfined compressive strength and indirect tensile strength tests of natural sandstone and artificial...     

A reciprocity relation between the transmitting and the receiving properties of an antenna

Summary A reciprocity relation between the transmitting and the receiving properties of an antenna is derived. In the transmitting situation a certain part of the antenna, called source domain, is capable of carrying external currents, both of the electric and the magnetic type. In the receiving situation a plane electromagnetic wave is incident upon the antenna system.Whereas the customary way of deriving reciprocity relations in antenna theory starts with considering two separate antennas, the present method assumes a single antenna only.     

A first assessment of the NEPTUNE_CFD code: Instabilities in a stratified flow comparison between the VOF method and a two-field approach

This paper presents some results concerning a first benchmark for the new European research code for thermal hydraulics computations: NEPTUNE_CFD. This benchmark relies on the Thorpe experiment to model the occurrence of instabilities in a stratified two-phase flow. The first part of this work is to create a numerical trial case with the VOF approach. The results, in terms of time of onset of the instability, critical wave-number or wave phase speed, are rather good compared to linear inviscid theory and experimental data. Additional numerical tests showed the effect of the surface tension and density ratio on the growing dynamics of the instability and the structure of the waves. In the second part, a code to code (VOF/multi-field) comparison is performed for a case with zero surface tension. The results showed some discrepancies in terms of wave amplitudes, growing rates and a time shifting in the global dynamics. Afterward, two surface tension formulations are proposed in the multi-field approach. Both formulations provided similar results. The time for onset of the instability, the most amplified wave-number and its amplitude were in rather good agreement with the linear analysis and VOF results. However, the time-shifted dynamics was still observed.     

Heat exchange between a selectively emitting liquid and a laminar gas flow in the presence of an external source of radiation

An investigation is conducted in the solution of a number of practical problems of the radiative and combined heat exchange in nonuniform systems having widely different physical properties. The processes of thermal interaction between the ocean and the atmosphere have been treated in the paper [1], the effect of thermal radiation on the melting and solidification of semitransparent crystals has been investigated in [2], the flow of a selectively emitting gas around the lateral surface of an object evaporating under the action of radiative heating has been discussed in [3], and heat transfer from a jet to the molten mass of glass in a glassmaking furnace tank has been investigated in [4]. The radiative and combined heat exchange between a selectively emitting liquid and a transparent heat-conducting laminar gas flow in the case of a specified external thermal radiation field is discussed in this paper. The energy conservation equations are set up taking into account the heat transfer by radiation, convection, and molecular thermal conduction. A differential approximation is used in calculating the values of the radiation fluxes. A system of fundamental computational equations is solved by the method of finite differences and iterations and by the Runge-Kutta method. The results of the calculations are presented in the form of graphs.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 3, pp. 116–122, May–June, 1976.     

A comparison between crystal and isotropic strain gradient plasticity theories with accent on the role of the plastic spin

In the small deformation range, we consider crystal and isotropic “higher-order” theories of strain gradient plasticity, in which two different types of size effects are accounted for: (i) that dissipative, entering the model through the definition of an effective measure of plastic deformation peculiar of the isotropic hardening function and (ii) that energetic, included by defining the defect energy (i.e., a function of Nye's dislocation density tensor added to the free energy; see, e.g., [Gurtin, M.E., 2002. A gradient theory of single-crystal viscoplasticity that accounts for geometrically necessary dislocations. J. Mech. Phys. Solids 50, 5–32]). In order to compare the two modellings, we recast both of them into a unified deformation theory framework and apply them to a simple boundary value problem for which we can exploit the Γ-convergence results of [Bardella, L., Giacomini, A., 2008. Influence of material parameters and crystallography on the size effects describable by means of strain gradient plasticity. J. Mech. Phys. Solids 56 (9), 2906–2934], in which the crystal model is made isotropic by imposing that any direction be a possible slip system. We show that the isotropic modelling can satisfactorily approximate the behaviour described by the isotropic limit obtained from the crystal modelling if the former constitutively involves the plastic spin, as in the theory put forward in Section 12 of [Gurtin, M.E., 2004. A gradient theory of small-deformation isotropic plasticity that accounts for the Burgers vector and for dissipation due to plastic spin. J. Mech. Phys. Solids 52, 2545–2568]. The analysis suggests a criterium for choosing the material parameter governing the plastic spin dependence into the relevant Gurtin model.     

A Stochastic Method for Modelling the Geometry of a Single Fracture: Spatially Controlled Distributions of Aperture,Roughness and Anisotropy

Transport in Porous Media - We describe a simple but effective stochastic method to model the void structure of a single fracture in a form of voxel representation. A fracture void is delineated by...     

A comparison between a collocation and weak implementation of the rigid‐body motion constraint on a particle surface

In this work, we implemented and compared two different methods to impose the rigid‐body motion constraint on a solid particle moving inside a fluid. We consider a fictitious domain method to easily manage the particle motion. As the solid as well as the fluid inertia are neglected, the particle can be discretized through its boundary only. The rigid‐body motion is imposed via Lagrange multipliers on the boundary. In the first method, such constraints are imposed in discrete points on the boundary (collocation), whereas in the second the constraint is imposed in a weak way on elements dividing the particle surface. Two test problems, that is, a spherical and an ellipsoidal particle in a sheared Newtonian fluid, are chosen to compare the methods. In both cases, the analysis is carried out in 2D as well as in 3D. The results show that for the collocation method an optimal number of collocation points exist leading to the smallest error. However, small variations in the optimal value can generate large deviations. In the weak implementation, the error is only mildly affected by the number of elements used to discretize the particle boundary and by the Lagrange multiplier's interpolation space. A further analysis is carried out to study the effect of an approximated integration of weak constraints. A comparison between the two methods showed that the same accuracy can be achieved by using less constraints if the weak discretization is used. Finally, the rigid‐body motion imposed via weak constraints leads to better conditioned linear systems. Copyright © 2009 John Wiley & Sons, Ltd.     

Numerical simulation of mushrooms during freezing using the FEM and an enthalpy: Kirchhoff formulation

The shelf life of mushrooms is very limited since they are susceptible to physical and microbial attack; therefore they are usually blanched and immediately frozen for commercial purposes. The aim of this work was to develop a numerical model using the finite element technique to predict freezing times of mushrooms considering the actual shape of the product. The original heat transfer equation was reformulated using a combined enthalpy-Kirchhoff formulation, therefore an own computational program using Matlab 6.5 (MathWorks, Natick, Massachusetts) was developed, considering the difficulties encountered when simulating this non-linear problem in commercial softwares. Digital images were used to generate the irregular contour and the domain discretization. The numerical predictions agreed with the experimental time–temperature curves during freezing of mushrooms (maximum absolute error <3.2°C) obtaining accurate results and minimum computer processing times. The codes were then applied to determine required processing times for different operating conditions (external fluid temperatures and surface heat transfer coefficients).     

A possible case in the theoretical description of the relation between the stresses and deformations of an elastoplastic-creep material

In constructing the differential equation for stresses and deformations and their rates, it is proposed to use the nonlinear stress-deformation equations in the form of a quadratic parabola, when the loadings are instantaneous or very slow. Relaxation curves and curves of the creep and rate of creep of an elastoplastic material with nonlinear creep are obtained.