Gas hydrodynamics

Frisch, U., D. d Humieres, B. Hasslascher, P. Lallemand, Y. Pommeau, and J. Rivet. 1987. Lattice gas hydrodynamics in two and three dimensions. Complex Syst. 1 649-707. 

K2 Ideal Gas Hydrodynamics Physical chemistry Thermics External 692... 

Commercialization of new CFB processes for the production of chemicals— specialty or commodity—has been hindered due to scale-up concerns and operational complexity. In particular, the effect of diameter, high solids mass flux, and high pressure on gas hydrodynamics are undocumented in the open literature. Innovative research aimed at the design of new solids feeding devices and gas-solids separation techniques may reduce operational complexities. However, studies on small scale equipment must be performed prudently and documented concisely to be useful for scale-up. Scale effects at the entrance region are considerable, and sufficient attention has not been devoted to this region. 

Original developments in this area stem from the work of Frisch et al. (1986) who employed the technique of lattice gas hydrodynamics in which the fluid is modelled as a cellular automaton and the flow represented by the motion of particles on a lattice. More numerically efficient variants of this method, such as the lattice Boltzmann approach (McNamara and Zanetti, 1988), were subsequently developed. 

Rivet, J. P, and Boon, J. P. 2001. Lattice Gas Hydrodynamics. Cambridge University Press, Cambridge. 

The equation for the gas phase is especially complicated because it should reflect toe influence not only of toe gas hydrodynamics, but also of toe liquid phase. 

If, however, the reservoir pressure drops below the bubble point, then gas will be liberated in the reservoir. This liberated gas may flow either towards the producing wells under the hydrodynamic force imposed by the lower pressure at the well, or it may migrate... 

If these assumptions are satisfied then the ideas developed earlier about the mean free path can be used to provide qualitative but useful estimates of the transport properties of a dilute gas. While many varied and complicated processes can take place in fluid systems, such as turbulent flow, pattern fonnation, and so on, the principles on which these flows are analysed are remarkably simple. The description of both simple and complicated flows m fluids is based on five hydrodynamic equations, die Navier-Stokes equations. These equations, in trim, are based upon the mechanical laws of conservation of particles, momentum and energy in a fluid, together with a set of phenomenological equations, such as Fourier s law of themial conduction and Newton s law of fluid friction. When these phenomenological laws are used in combination with the conservation equations, one obtains the Navier-Stokes equations. Our goal here is to derive the phenomenological laws from elementary mean free path considerations, and to obtain estimates of the associated transport coefficients. Flere we will consider themial conduction and viscous flow as examples. 

This corresponds to the physician s stethoscope case mentioned above, and has been realized [208] by bringing one leg of a resonatmg 33 kHz quartz tiinmg fork close to the surface of a sample, which is being rastered in the x-y plane. As the fork-leg nears the sample, the fork s resonant frequency and therefore its amplitude is changed by interaction with the surface. Since the behaviour of the system appears to be dependent on the gas pressure, it may be assumed that the coupling is due to hydrodynamic mteractions within the fork-air-sample gap. Since the fork tip-sample distance is approximately 200 pm -1.120), tire teclmique is sensitive to the near-field component of the scattered acoustic signal. 1 pm lateral and 10 mn vertical resolutions have been obtained by the SNAM. 

If a particularly parallel beam is required in the chamber into which it is flowing the beam may be skimmed in the region of hydrodynamic flow. A skimmer is a collimator which is specially constructed in order to avoid shockwaves travelling back into the gas and increasing 7). The gas that has been skimmed away may be pumped off in a separate vacuum chamber. Further collimation may be carried out in the region of molecular flow and a so-called supersonic beam results. When a skimmer is not used, a supersonic jet results this may or may not be collimated. 

Interparticle Forces. Interparticle forces are often neglected in the fluidization Hterature, although in many cases these forces are stronger than the hydrodynamic ones used in most correlations. The most common interparticle forces encountered in gas fluidized beds are van der Waals, electrostatic, and capillary. 

Hydrodynamic principles for gas bearings are similar to those involved with Hquid lubricants except that gas compressibility usually is a significant factor (8,69). With gas employed as a lubricant at high speeds, start—stop wear is minimized by selection of wear-resistant materials for the journal and bearing. This may involve hard coatings such as tungsten carbide or chromium oxide flame plate, or soHd lubricants, eg, PTFE and M0S2. 

A general flow map of different hydrodynamic conditions (Fig. 23) consists of regions of flooding, dispersion, and recirculation on a plot of N vs for a Rushton turbine. For a low viscosity aqueous/air system, the gas flow numbers for the three conditions are given hy FI = 30Fr[D/TY for flooding, = 0.2Fr° (F/r)° for complete dispersion, and =13FF D/TY for recirculation. 

Some concerns directly related to a tomizer operation include inadequate mixing of Hquid and gas, incomplete droplet evaporation, hydrodynamic instabiHty, formation of nonuniform sprays, uneven deposition of Hquid particles on soHd surfaces, and drifting of small droplets. Other possible problems include difficulty in achieving ignition, poor combustion efficiency, and incorrect rates of evaporation, chemical reaction, solidification, or deposition. Atomizers must also provide the desired spray angle and pattern, penetration, concentration, and particle size distribution. In certain appHcations, they must handle high viscosity or non-Newtonian fluids, or provide extremely fine sprays for rapid cooling. 

To evaluate the flow pattern efficiency, a knowledge of the actual hydrodynamic behavior of the process gas circulating in the centrifuge is necessary. Primarily because of the lack of such knowledge, the flow pattern efficiency has been evaluated for a number of different assumed isothermal centrifuge velocity profiles. 

The important point to note here is that the gas-phase mass-transfer coefficient fcc depends principally upon the transport properties of the fluid (Nsc) 3nd the hydrodynamics of the particular system involved (Nrc). It also is important to recognize that specific mass-transfer correlations can be derived only in conjunction with the investigator s particular assumptions concerning the numerical values of the effective interfacial area a of the packing. 

For the liquid-phase mass-transfer coefficient /cl, the effects of total system pressure can be ignored for all practical purposes. Thus, when using Kq and /cl for the design of gas absorbers or strippers, the primary pressure effects to consider will be those which affect the equilibrium curves and the values of m. If the pressure changes affect the hydrodynamics, then Icq, and a can all change significantly. 

Influence of Chemical Reactions on Uq and When a chemical reaction occurs, the transfer rate may be influenced by the chemical reac tion as well as by the purely physical processes of diffusion and convection within the two phases. Since this situation is common in gas absorption, gas absorption will be the focus of this discussion. One must consider the impacts of chemical equilibrium and reac tion kinetics on the absorption rate in addition to accounting for the effec ts of gas solubility, diffusivity, and system hydrodynamics. 

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