Experimental study of the velocity field in a vortex-flow heat exchanger with isothermal conditions

The velocity field in a vortex heat cell was investigated experimentally using laser Doppler velocimetry for a wide range of flow conditions. Experimental results point out the three dimensionality of the exchanger's flow, which is composed into a main vortex flow developing along the side walls. The strength of the flow increases up to a limiting value reached for a Reynolds number ranging between 15,000 and 30,000; a secondary flow, caused by interaction between centrifugal and inertial forces, extends perpendicularly to the main flow and remains Reynolds number dependent. It is composed of multiple counter-rotating structures occurring at the exchanger periphery with low inlet Reynolds numbers, thus reducing the rate of centripetal momentum transfer. With increasing inlet Reynolds number, the secondary flow extends across the whole exchanger radius, thus increasing the rate of mixing of the treated fluid. The appearance of so-called Taylor–Görtler vortices tends to reduce the z- and r-axis vorticity transfer.     

Flow of gas and water through flooded jointed and porous media under conditions of falling pressure

Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 3, pp. 168–171, May–June, 1990.     

Advective fluxes in multiphase porous media under nonisothermal conditions

We consider the case in which more than one fluid phase occupies the void space of a porous medium. The advective flux law is formulated for a fluid phase, under nonisothermal conditions and with the presence of solutes in the fluid phases. The derivation of the flux laws is based on an approximated version of the averaged balance equation for linear momentum. Taking into account momentum transfer through the interface between the fluid phases, leads to coupling between the flow in adjacent phases. Fluxes are also shown to depend on the surface tension at the interface between the adjacent fluid phases. Since the latter depends on temperature and solute concentration in the two phases, the advective flux is shown to depend on both temperature and solute concentration gradients in the two phases. A preliminary order of magnitude analysis gives conditions under which the coupling phenomena are not negligible. The approach is applied to the unsaturated zone, as a typical example of a multiphase porous medium.The main conclusion is that the well known Darcy law for single phase flow, may have to be modified for a multi fluid phase system, especially when temperature and solute concentration are not uniform.     

Shear-thinning fluids flow in fixed and fluidised beds

In the paper the results of experimental studies directed on the effect of liquids’ properties (aqueous solutions of polymers and surfactants) on resistance of the flow through porous and fluidised beds, are presented. It was shown that the determination of the values of minimal fluidisation velocity on the basis of an analysis of pressure drop related to the current two-phase system height gives the more accurate values than the method based on the initial bed height. Independently of the Newtonian or shear-thinning properties of the liquid flowing through motionless or fluidised bed, the relation of the friction factor on well-defined Reynolds number (related to real rheological parameters of a liquid studied) is analogous. It has been shown that the diagram proposed by Koziol et al. can be stated as the generalized one, not only for the determination of the solid particles motion in Newtonian fluids, but for the shear-thinning liquids too. In the last case it should be taken into account that the critical value of porosity cannot be taken equal to 0.4, but should be appropriate to the real porosity in the critical conditions for a given system solid particle–liquid. The generalization of both, the map of Bi and Grace related to the characteristic fluidisation ranges and the diagram of the classification of particles fluidised proposed by Goossens for gas-fluidisation, on any systems of solid particles–power law fluids, has been proposed.     

Convection in a porous medium with velocity slip and temperature jump boundary conditions

The onset of convection in a rarefield gas saturating a horizontal layer of a porous medium has been investigated using both Darcy and Brinkman models. It is assumed that due to rarefaction both velocity slip and temperature jump exist at the boundaries. The results show that (i) when the degree of rarefaction increases the critical Rayleigh number as well as the critical wave number for the onset of convection increases, (ii) stabilizing effect of temperature jump is more than that of velocity slip, (iii) Darcy model is seen to be the most stable one when compared to Brinkman model or the pure gaseous layer (i.e. in the absence of porous medium).     

Chloride diffusion in porous concrete under conditions of variable temperature

A diffusion model is presented in this study for describing diffusion of chloride ion in porous concrete under nonisothermal conditions. The process is to simulate chloride diffusion in concrete exposed to daily cyclic temperature change. The effects of cyclic and constant temperatures on the chloride diffusion behavior in concretes are investigated. The time for chloride concentration to reach the corrosion threshold of the reinforcing steel is determined for a given water-to-cement ratio and thickness of concrete layer. Results indicate that the temperature and water-to-cement ratio play a vital role in governing the chloride diffusion process.

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Chlorid-Diffusion in porösem Beton unter dem Einfluß von Temperaturänderungen

Zusammenfassung Es wird ein Diffusionsmodell vorgestellt, das die Diffusion von Chloridionen in porösem Beton unter nichtisothermen Bedingungen beschreiben soll, wobei sowohl die täglichen zyklischen, wie auch sprunghaften Temperaturänderungen zugrunde gelegt werden. Berechnet wird die Zeitspanne bis zum Erreichen der Korrosionsschwelle an der Stahlarmierung, und zwar als Funktion des Wasser-Zement-Verhältnisses und der Schichtdicke des Betons. Die Ergebnisse zeigen, daß der Chlorid-Diffusionsprozeß hauptsächlich vom Temperatur- und Wasser-Zement-Verhältnis regiert wird.

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Nomenclature b amplitude parameter in the cyclic temperature function - C chloride concentration - Cp specific heat capacity - D effective chloride diffusion coefficient - D _o preexponential factor - E activation energy - k thermal conductivity of concrete - L thickness of concrete slab - R gas constant - t time - T temperature - T _o initial temperature - w water-to-cement ratio - x spatial coordinate - thermal diffusivity,k/( Cp) - density of concrete     

Modeling of turbulent, isothermal and cryogenic cavitation under attached conditions

Cavitation is often triggered when the fluid pres- sure is lower than the vapor pressure at a local thermo- dynamic state. The present article reviews recent progress made toward developing modeling and computational strat- egies for cavitation predictions under both isothermal and cryogenic conditions, with an emphasis on the attached cav- ity. The review considers alternative cavitation models along Reynolds-averaged Navier-Stokes and very lager eddy simu- lation turbulence approaches to ensure that the computational tools can handle flows of engineering interests. Observing the substantial uncertainties associated with both modeling and experimental information, surrogate modeling strategies are reviewed to assess the implications and relative impor- tance of the various modeling and materials parameters. The exchange between static and dynamic pressures under the influence of the viscous effects can have a noticeable impact on the effective shape of a solid object, which can impact the cavitation structure. The thermal effect with respect to evaporation and condensation dynamics is examined to shed light on the fluid physics associated with cryogenic cav- itation. The surrogate modeling techniques are highlighted in the context of modeling sensitivity assessment. Keywords     

Flow over rectangular porous block in a fixed width channel: influence of porosity and aspect ratio

Flow over a rectangular porous block placed in a fixed width channel is considered and the influence of block aspect ratio on the heat transfer rate from the block is examined. A non-porous solid block is also accommodated to compare the effect of porosity on the flow field and heat transfer characteristics. Aspect ratio and the porosity of the block are varied in the simulations. A numerical scheme employing a control volume approach is considered when predicting the flow and temperature fields. The Reynolds number is selected to yield the mix convection situation in the flow field. It is found that the aspect ratio significantly influences Nu and Gr numbers, in which case increasing the aspect ratio enhances Nu while lowering Gr. Increasing porosity improves the heat transfer rates from the porous block, provided that at high aspect ratios, this situation ceases due to blockage effect of the body in the channel.     

A model for expansion ratio in liquid-solid fluidized beds

Liquid-solid fluidized beds are used in mineral processing industries to separate particles based on parti- cle size, density, and shape. Understanding the expanded fluidized bed is vital for accurately assessing its performance. Expansion characteristics of the fluidized bed were studied by performing several experi- ments with iron ore, chromite, quartz, and coal samples. Using water as liquid medium, experiments were conducted to study the effects of particle size, particle density, and superficial velocity on fluidized bed expansion. The experimental data were utilized to develop an empirical mathematical model based on dimensional analysis to estimate the expansion ratio of the fluidized bed in terms of particle character- istics, operating and design parameters. The predicted expansion ratio obtained from the mathematical model is in good agreement with the experimental data.     

Forced convection of gaseous slip-flow in porous micro-channels under Local Thermal Non-Equilibrium conditions

Steady laminar forced convection gaseous slip-flow through parallel-plates micro-channel filled with porous medium under Local Thermal Non-Equilibrium (LTNE) condition is studied numerically. We consider incompressible Newtonian gas flow, which is hydrodynamically fully developed while thermally is developing. The Darcy–Brinkman–Forchheimer model embedded in the Navier–Stokes equations is used to model the flow within the porous domain. The present study reports the effect of several operating parameters on velocity slip and temperature jump at the wall. Mainly, the current study demonstrates the effects of: Knudsen number (Kn), Darcy number (Da), Forchheimer number (Γ), Peclet number (Pe), Biot number (Bi), and effective thermal conductivity ratio (K _R) on velocity slip and temperature jump at the wall. Results are given in terms of skin friction (C _f Re ^*) and Nusselt number (Nu). It is found that the skin friction: (1) increases as Darcy number increases; (2) decreases as Forchheimer number or Knudsen number increases. Heat transfer is found to (1) decreases as the Knudsen number, Forchheimer number, or K _R increases; (2) increases as the Peclet number, Darcy number, or Biot number increases.     

Gas residence time distribution in multistage fluidised beds

The residence time distribution of the gas in multistage fluidised beds is studied, using Helium as tracer, for different conditions of the solids concentration, the number of stages and the height-to-diameter ratio of the stage. The results are compared with that of batch fluidisation with and without baffles and it is found that continuous fluidisation gives better flow characteristics of the gas and possesses a wider range of operation than batch fluidisation.     

Flow regimes and flow transitions in liquid fluidized beds

Using two-dimensional liquid fluidized beds of glass particles in water, we have been able to identify at least four discrete fiow regimes. The points of transition between these regimes are sharp and non-hysteretic. The regimes include (in the order of increasing gm/gm_mf), wavy fiow, wavy fiow with transverse structure, fine-scale turbulent flow, and bubbling states. Characterization of each of these regimes is given in terms of the rime and length scales of the motion, as measured by light transmission, optical scanning, and digital time-series analysis. Features of the mechanics of these states are discussed. Observation of the bubbling state for particles of moderate density (ρ_s = 3990 kg/m³) in liquid beds is new, and is shown to be related fo anomalous expansion data reported by earlier investigators.     

Flow field and thermal characteristics in a model of a tangentially fired furnace under different conditions of burner tripping

Tangentially fired furnaces are vortex-combustion units and are widely used in steam generators of industrial plants. The present study provides a numerical investigation of the problem of turbulent reacting flows in a model furnace of a tangentially fired boiler. The importance of this problem is mainly due to its relation to large boiler furnaces used in thermal power plants. In the present work, calculation of the flow field, temperature and species concentration-contour maps in a tangentially-fired model furnace are provided. The safety of these furnaces requires that the burner be tripped (its fuel is cut off) if the flame is extinguished. Therefore, the present work provides an investigation of the influence of number of tripped burners on the characteristics of the flow and thermal fields. The details of the flow, thermal and combustion fields are obtained from the solution of the conservation equations of mass, momentum and energy and transport equations for scalar variables in addition to the equations of the turbulence model. Available experimental measurements were used for validating the calculation procedure. The results show that the vortex created due to pressure gradient at the furnace center only influenced by tripping at least two burners. However, the temperature distributions are significantly distorted by tripping any of the burners. Regions of very high temperature close to the furnace walls appear as a result of tripping the fuel in one or two of the burners. Calculated heat flux along the furnace walls are presented.