Flow characterization

LDV is the traditional method using tracer particles to measure velocity and one-point statistics of turbulent properties [2]. It is still a very useful technique and has the advantage that it can measure closer to walls compared to PIV. An inherent problem with LDV is that it does not measure at a specific point but rather at places 

Optical systems can be used in multiphase flows at a very low volume fraction of the dispersed phase. Through a refractory index matching of hquid-liquid or liquid-solid systems, it is also possible to measure at high void fractions. However, it is not possible to obtain complete refractory index matching since the molecules at the phase boundary have different optical properties than the molecules in the bulk. Consequently, it is possible to measure at a higher fraction of the dispersed phase with larger drops and particles because of the lower surface area per volume fluid. 

Ultrasonic Doppler velocimetry is a nonintrusive technique that has been developed into a very useful technique for opaque liquid flows [3]. This technique provides good measurement of velocity new high-frequency techniques give a space resolution on the millimeter level, and even the large turbulent scales can be resolved. 

The rate of dissipation of turbulent kinetic energy, s, is more difficult to measure. 

A very fine space resolution is required to measure the gradient of turbulent velocity fluctuations and calculate turbulent dissipation directly from the definition [5, 6]. 

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If we consider a flow, characterized by a typical velocity of V and a scale size L, the strength of the viscous forces on a unit mass of the fluid is i VL. The strength of the of the inertial forces in the flow is (i.e., the force... 

The catalyst concentration can be varied in a wide range for the above-mentioned parameter set, without changing the reaction kinetics [9]. Since gas/liquid micro reactors span a broad range of residence times, typically much shorter than for conventional apparatus, this allows a flexible adaptation of the test procedure to the needs of micro flow characterization. 

Table 4.4 presents wastewater flow characterization for the foundry industry by casting metals. Also presented in this table is the level of process water recycle, and the number of plants surveyed with central wastewater treatment facilities for all of the processes at that plant. The discharge flow represents all processes within the specific metal casting facilities. 

Wastewater Flow Characterization of the Metal Finishing Industry... 

For flow characterized on the shell-side by the clean condition ... 

Given the uncertainties associated with the calculations, especially those on the shell-side, a sensible design basis for the heat transfer area specification would be the shell-side flow characterized by the clean condition. Of course, the fouling coefficients for the shell-side and tube-side should be included to account for the surface fouling resistance. 

G. Q. Lu and C. Y. Wang. Electrochemical and flow characterization of a direct methanol fuel cell. Journal of Power Sources 134 (2004) 33 0. 

In the both the Hayes and Hickey zones mineralization occurs in three forms Type I mineralization consists of massive (> 90%) sulfide veins that range from 1 to 5 cm in width. These veins are for the most part restricted to the massive rhyolite of the Archibald Settlement Formation. However, where Type I veins intersect carapace-breccias or marginal zones of flows characterized by zones of contorted flow-layering and/or auto-breccia development, sulfides fill and cement the interstices such that rhyolite appears to be cemented by sulfide. 

Since the bubbling bed represents such severe deviations from ideal contacting, not just minor ones as with other single-fluid reactors (packed beds, tubes, etc.), it would be instructive to see how this problem of flow characterization has been attacked. A wide variety of approaches have been tried. We consider these in turn. 

Flow characterization methods, reactive intermediates, 46 156-164 Fluorescence, 19 68, 46 156 emission spectrum, Holobacfer, 36 420, 422 microscopy, autotrophic organisms, 36 118-119... 

Reaction rate, 46 101-102 Reactions, see specific types Reactive intermediates, 46 101-107, 164 flow characterization methods, 46 156-164 gas-phase studies, 46 107-121 lifetimes, 46 106... 

A number of investigators have modeled the Tsuji and Yamaoka data [104]. In these investigations the flame was modeled as a semi-infinite stagnation flow, with the outer potential flow characterized by the velocity-gradient parameter a (see Section 6.3.1). For the cylindrical geometry, this characterization is correct in the neighborhood of the center stagnation-flow streamline. 

Schnitzlein, M.G. and Weinstein, H. (1988). Flow Characterization in High-Velocity Fluidized Beds Using Pressure Fluctuations. Chem. Eng. Sci., 43,2605. 

Example 6.14 Squeezing Flow between Two Parallel Disks This flow characterizes compression molding it is used in certain hydrodynamic lubricating systems and in rheological testing of asphalt, rubber, and other very viscous liquids.14 We solve the flow problem for a Power Law model fluid as suggested by Scott (48) and presented by Leider and Bird (49). We assume a quasi-steady-state slow flow15 and invoke the lubrication approximation. We use a cylindrical coordinate system placed at the center and midway between the plates as shown in Fig. E6.14a. 

Glenny, R. and Robertson, H., Fractal properties of pulmonary blood flow Characterization of spatial heterogeneity, Journal of Applied Physioloqy, Vol. 69, No. 2, 1990, pp. 532-545. 

Consider an unbounded, incompressible, Newtonian fluid undergoing a homogeneous shear flow characterized by the position-independent velocity gradient dyadic G, which can be decomposed into symmetric and antisymmetric contributions S and A, respectively, as... 

The flows may have vectorial or scalar characters. Vectorial flows are directed in space, such as mass, heat, and electric current. Scalar flows have no direction in space, such as those of chemical reactions. The other more complex flow is the viscous flow characterized by tensor properties. At equilibrium state, the thermodynamic forces become zero and hence the flows vanish... 

If we consider a membrane having the same solute concentration on both sides, we have All 0 However, a hydrostatic pressure difference AP exists between the two sides, and we have a flow Jv that is a linear function of AP. The term Lp is called the mechanical filtration coefficient, which represents the velocity of the fluid per unit pressure difference between the two sides of the membrane. The cross-phenomenological coefficient Ldp is called the ultrafiltration coefficient, which is related to the coupled diffusion induced by a mechanical pressure of the solute with respect to the solvent. Osmotic pressure difference produces a diffusion flow characterized by the permeability coefficient, which indicates the movement of the solute with respect to the solvent due to the inequality of concentrations on both sides of the membrane. 

LP is the hydraulic conductivity coefficient and can have units of m s-1 Pa-1. It describes the mechanical filtration capacity of a membrane or other barrier namely, when An is zero, LP relates the total volume flux density, Jv, to the hydrostatic pressure difference, AP. When AP is zero, Equation 3.37 indicates that a difference in osmotic pressure leads to a diffusional flow characterized by the coefficient Lo Membranes also generally exhibit a property called ultrafiltration, whereby they offer different resistances to the passage of the solute and water.14 For instance, in the absence of an osmotic pressure difference (An = 0), Equation 3.37 indicates a diffusional flux density equal to LopkP. Based on Equation 3.35, vs is then... 

It is important to note that, except for the heat transfer problems, which have not been considered here, the model contains, in a particular form, all the transport phenomena relationships given at the start of this chapter. From the mathematical viewpoint, we have an assembly of differential and partly differential equations, which show the complexity of this example. However, this relative mathematical complexity can be matched with the simplicity of the descriptive model. Indeed, it will be convenient to simplify general mathematical models in order to comply with the descriptive model. Two variants can be selected to simplify the flow characterization in the membrane filtration unit. 

The modelling example of the previous section shows that to simplify the general mathematical model of the studied process, the real flow in the filter unit has been considered in terms of its own simplified model. Indeed, it is difficult to understand why we have used a flow model, when in fact, for the flow characterization, we already have the Navier-Stokes equations and their expression for the computational fluid dynamics. To answer this question some precisions about the general aspects of the computational fluids dynamics have to be given. 

In the scientific literature, we can find a large quantity of experimental results where the flow characterization inside a porous medium has shown that the value of the dispersion coefficient is not constant. Indeed, for the majority of porous structures the diffusion is frequently a function of the time or of the concentration of the diffusing species. As far as simple stochastic models cannot cover these situations, more complex models have been built to characterize these dependences. One of the first models that gives a response to this problem is recognized as the modd of motion with states having multiple vdodties. 

Figure 3.41. Structure of carbon paper (left) and carbon cloth (right) used for gas diffusion layers in PEM fuel cells. A coating of 20% (by weight) fluorinated ethylene propylene has been applied. (From C. Lim and C-Y. Wang (2004). Effects of hydro-phobic polymer content in GDL on power performance of a PEM fuel cell. Electro chimica Acta 49, 4149-4156 G. Lu and C-Y. Wang (2004). Electrochemical and flow characterization of a direct methanol fuel cell.. Power Sources 134, 33-40. Used with permission from Elsevier.)...

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