Flow structure

Counter-current flow. Structured packings. Gauze-type with triangular flow channels, Bravo, Rocha, and Fair correlation... 

Convection is influenced by the fluid flow adjacent to the solid surface. To appreciate the mechanics of this mode of heat transfer, the nature of the fluid flow in relation to the particular flow process must be known. Consideration of the flow structure created by the passage of a turbulent fluid over a smooth solid surface shows (see Fig. 4.24)... 

Gnatyuk, V. V., T. A. Gnatyuk, and L. P. Yarin. 1977. Studies of the axis-symmetric blocked flow structure. In Proceedings of the USSR Acad. Sci., Mechanics of Liquids and Gases, no. 2, pp. 16-23. Nauka, Moscow. 

J. Holuigue, O. Bertrand, E. Arquis. Solutal convection in crystal growth effect of interface curvature on flow structuration in a three-dimensional cylindrical configuration. J Cryst Growth 180 591, 1997. 

As more fuel is converted into combustion products per unit of volume and time, expansion flow becomes stronger. Higher flow velocities go hand in hand with more intense turbulence. This process feeds on itself that is, a positive feedback coupling comes into action. In the turbulent stage of flame propagation, a gas explosion may be described as a process of combustion-driven expansion flow with the turbulent expansion-flow structure acting as an uncontrolled positive feedback (Figure 3.2). 

Partial confinement hampers expansion and allows the introduction of a combustion enhancing flow structure. 

The major mechanism of a vapor cloud explosion, the feedback in the interaction of combustion, flow, and turbulence, can be readily found in this mathematical model. The combustion rate, which is primarily determined by the turbulence properties, is a source term in the conservation equation for the fuel-mass fraction. The attendant energy release results in a distribution of internal energy which is described by the equation for conservation of energy. This internal energy distribution is translated into a pressure field which drives the flow field through momentum equations. The flow field acts as source term in the turbulence model, which results in a turbulent-flow structure. Finally, the turbulence properties, together with the composition, determine the rate of combustion. This completes the circle, the feedback in the process of turbulent, premixed combustion in gas explosions. The set of equations has been solved with various numerical methods e.g., SIMPLE (Patankar 1980) SOLA-ICE (Cloutman et al. 1976). 

The flow structures of lean limit methane and propane flames are compared in Figures 3.1.2 and 3.1.3. The structure depends on the Lewis number for the deficient reactant. A stretched lean limit methane flame (Lepreferential diffusion, giving it a higher burning intensity. Hence, the flame extinction limit is extended. On the other hand, for a stretched lean limit propane flame (Le>l), the same effect reduces the burning intensity, which can... 

On comparing the two flames, it is evident that the flow structure of the lean limit methane flame fundamentally differs from that of the limit propane one. In the flame coordinate system, the velocity field shows a stagnation zone in the central region of the methane flame bubble, just behind the flame front. In this region, the combustion products move upward with the flame and are not replaced by the new ones produced in the reaction zone. For methane, at the lean limit an accumulation of particle image velocimetry (PIV) seeding particles can be seen within the stagnation core, in... 

Flow Structure in the Vessel during Rotation Transients.130... 

Although the detailed flow-structure in the vessel is unknown and can be predicted by numerical means, only the basic features of the flame-flow interaction can now be depicted. The interaction is shown schematically in Figure 6.4.7. Each of the Ekman layers formed at the sidewalls by the angular velocity perturbation of the flow induces two recirculation cells—one in front of the flame and one behind it, separated by the flame. The recirculation cell in front of fhe flame is of less importance, as the flow velocities there do not affect the flame. 

The CFB catalytic cracking reactor plays an important role in the petroleum industry because of its better gas-solids contact and narrow residence time distribution, but its non-uniform radial flow structure and the extensive backmixing of gas and solids lead to a lower conversion rate and poorer selectivity to desired intermediate products [14]. 

For the sake of developing commercial reactors with high performance for direct synthesis of DME process, a novel circulating slurry bed reactor was developed. The reactor consists of a riser, down-comer, gas-liquid separator, gas distributor and specially designed internals for mass transfer and heat removal intensification [3], Due to density difference between the riser and down-comer, the slurry phase is eirculated in the reactor. A fairly good flow structure can be obtained and the heat and mass transfer can be intensified even at a relatively low superficial gas velocity. 

Another major drawback stems from the disperse nature of the system itself involving a size distribution of the bubbles in the continuous liquid, which can be broad. The interface is not as defined as for two-phase continuous reactors, as described in Section 5.1.1. However, in the case of making foams, regular micro flow structures, such as hexagon flow, were described [22]. 

This section focuses on steady and unsteady hydrodynamic modes that emerge as the rotational speed of the inner cylinder (expressed by Ta) and pressure-driven axial flow rate (scaled by Re) are varied, while the outer cylinder is kept fixed. These modes constitute primary, secondary and higher order bifurcations, which break the symmetry of the base helical Couette-Poiseuille (CP) flow and represent drastic changes in flow structure. Figure 4.4.2 presents a map of observed hydrodynamic modes in the (Ta, Re) space, and marks the domain where all of the hydrodynamic modes that interest us appear. We will return to this figure shortly. 

A perturbed helical flow structure of the advection field is indicated by the data. 

Reactors which generate vortex flows (VFs) are common in both planktonic cellular and biofilm reactor applications due to the mixing provided by the VF. The generation of Taylor vortices in Couette cells has been studied by MRM to characterize the dynamics of hydrodynamic instabilities [56], The presence of the coherent flow structures renders the mass transfer coefficient approaches of limited utility, as in the biofilm capillary reactor, due to the inability to incorporate microscale details of the advection field into the mass transfer coefficient model. 

Janssen, E., 1967, Two Phase Flow Structure in a Nine-Rod Channel, Steam-Water at 1000 psia, Final Summary Rep. GEAP-5480, General Electric, San Jose, CA. (5)... 

Kosterin, S. I., 1949, Study of Influence of Tube Diameter and Position upon Hydraulic Resistance and Flow Structure of Gas-Liquid Mixtures, Izvestiza Akademii Nauk SSSR, Otdelema Tekhni-cheskikh No. 12, 1824, Translation 3085, Henry Brutcher Tech. Translation, Altadena, CA. (3) Kottowski, H., and G. Grass, 1970, Influence on Superheating by Suppression of Nucleation Cavities and Effect of Surface Microstructure on Nucleation Sites, Proc. Symp. LIM Heat Transfer and Fluid Dynamics, p. 108, ASME, New York. (2)... 

Hartge, E.-U., Li, Y., and Werther, J., Flow Structures in Fast Fluidized Beds, Fluidization V, Proc. 5th Eng. Foundation Conf. on Fluidization, Elsinore Denmark, p. 345 (1985)... 

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