Effect of Turbulence

To analy2e premixed turbulent flames theoretically, two processes should be considered (/) the effects of combustion on the turbulence, and (2) the effects of turbulence on the average chemical reaction rates. In a turbulent flame, the peak time-averaged reaction rate can be orders of magnitude smaller than the corresponding rates in a laminar flame. The reason for this is the existence of turbulence-induced fluctuations in composition, temperature, density, and heat release rate within the flame, which are caused by large eddy stmctures and wrinkled laminar flame fronts. 

Detention efficiency. Conversion from the ideal basin sized by detention-time procedures to an actual clarifier requires the inclusion of an efficiency factor to account for the effects of turbulence and nonuniform flow. Efficiencies vaiy greatly, being dependent not only on the relative dimensions of the clarifier and the means of feeding but also on the characteristics of the particles. The cui ve shown in Fig. 18-83 can be used to scale up laboratoiy data in sizing circular clarifiers. The static detention time determined from a test to produce a specific effluent sohds concentration is divided by the efficiency (expressed as a fraction) to determine the nominal detention time, which represents the volume of the clarifier above the settled pulp interface divided by the overflow rate. Different diameter-depth combinations are considered by using the corresponding efficiency factor. In most cases, area may be determined by factors other than the bulksettling rate, such as practical tank-depth limitations. 

The accelerating effect of turbulence in alkaline attack on aluminum alloys is well illustrated in this case history. 

The effect of turbulent exchange l>etween the contaminated air in the process equipment enclosure and the room air (Fig. 7.11 lb) is described by Elter-nian. According to Elterman, the air velocity in the process equipment enclosure opening assuring contaminant concentration Q at the distance / from the opening can be calculated from... 

The design of low hoods is much simpler, since the adverse effects of turbulent mixing and cross-drafts are much less important than for high hoods. Low hoods are much more likely to capture a high percentage of the heated air and contaminants than high hoods, so they should be used whenever possible. 

Low Receptor Hoods Low receptor hoods are much easier to design, since entrainment of air into the plume and the effects of turbulent cross-drafts are not significant problems. In this case, the diameter of the plume at the hood face, dp, is assumed to equal the diameter of the source, d,.. Accord-... 

The effects of turbulence must be taken into account when sizing a relief area. For example, the explosion violence of turbulent methane-air mixture is comparable to that of zero turbulence of hydrogen-air mi.xtures. From the investigations [54], the nomograms from Figure 7-63 can be applied for turbulent gas mixtures under the following conditions [54] ... 

Yang, S.l. and Shy, S.S., Global quenching of premixed CH4/air flames Effects of turbulent straining, equivalence ratio, and radiative heat loss, Proc. Combust. Inst., 29,1841,2002. 

Barlow, R. S. and Frank, J. H., Effects of turbulence on species mass fractions in methane/air jet flames, Proc. Combust. Inst., 2J, 1087, 1998. 

Fig. 15. Effects of turbulent shear stress level and exposure time on cell viability measured by trypan blue staining. Cells were sheared in a concentric cylinder viscometer [1]...

The cluster reactor is attached to the pulsed cluster source s condensation channel, as shown in Figure 6. (16) To it is attached a high-pressure nozzle from which a helium/hydrocarbon mixture is pulsed into the reactor at a time selected with respect to the production and arrival of the clusters. The effect of turbulent mixing with the reactant pulse perturbs the beam, but clusters and reaction products which survive the travel from the source to the photoionization regime ( 600y sec) and the photoionization process are easily detected. 

Above a Reynolds number of around 2000, the condensate film becomes turbulent. The effect of turbulence in the condensate film was investigated by Colburn (1934) and Colburn s results are generally used for condenser design, Figure 12.43. Equation 12.51 is also shown on Figure 12.43. The Prandtl number for the condensate film is given by ... 

The combined effect of turbulent convection from liquid with high subcooling and radiation for film boiling on a flat surface was analyzed by Hamill and Baumeister (1967), resulting in the expression... 

A more realistic approach to quantify the pressure field is to consider the effect of turbulence [6]. For a pipe flow, the turbulent pressure fluctuations are due to velocity perturbations as a result of the formation of eddies. The instantaneous turbulent velocity can be calculated by assuming a sinusoidal velocity variation in... 

Moholkar VS, Pandit AB (1997) Bubble behavior in hydrodynamic cavitation Effect of turbulence. AIChE J 43 1641-1648... 

Leith and Licht (1972) incorporated the effect of turbulent reentrainment of the solids in a solution of Eq. (12-42) to derive the following expression for the grade efficiency ... 

Chemical engineers, however, have to find practical ways for dealing with turbulent flows in flow devices of complex geometry. It is their job to exploit practical tools and find practical solutions, as spatial variations in turbulence properties usually are highly relevant to the operations carried out in their process equipment. Very often, the effects of turbulent fluctuations and their spatial variations on these operations are even crucial. The classical toolbox of chemical engineers falls short in dealing with these fluctuations and its effects. Computational Fluid Dynamics (CFD) techniques offer a promising alternative approach. 

The velocities and other solution variables are now represented by Reynolds-averaged values, and the effects of turbulence are represented by the Reynolds stresses, (—pu pTl) that are modeled by the Boussinesq hypothesis ... 

These highly oversimplified explanations ignore the effects of turbulent flow, and the formation of vortices. 

In some practical processes, a high relative velocity may not exist and effects of turbulence on droplet breakup may become dominant. In such situations Kolmogorov, 280 and Hinze[27°l hypothesized that the turbulent fluctuations are responsible for droplet breakup, and the dynamic pressure forces of the turbulent motion determine the maximum stable droplet size. Using Clay s data, 2811 and assuming isotropic turbulence, an expression was derived for the critical Weber number 270 ... 

Cremer, M. A., P. A. McMurtry, and A. R. Kerstein (1994). Effects of turbulence length-scale distribution on scalar mixing in homogeneous turbulent flow. Physics of Fluids 6, 2143-2153. 

Komori, S., J. C. R. Hunt, K. Kanzaki, and Y. Murakami (1991b). The effects of turbulent mixing on the correlation between two species and on concentration fluctuations in non-premixed reacting flows. Journal of Fluid Mechanics 228, 629-659. 

Tsai, K. and R. O. Fox (1993). PDF modeling of free-radical polymerization in an axisymmetric reactor. EES Report 254, Kansas State University, Manhattan, Kansas. (1994a). Modeling the effect of turbulent mixing on a series-parallel reaction in a tubular reactor. ICRES Report 9403, Kansas State University, Manhattan, Kansas. 

The effect of turbulence in the fluid stream has been studied by Richardson and Meikle(25) who suspended a particle on a thread at the centre of a vertical pipe up which water was passed under conditions of turbulent flow. The upper end of the thread was attached to a lever fixed on a coil free to rotate in the field of an electromagnet. By passing a current through the coil it was possible to bring the level back to a null position. After calibration, the current required could be related to the force acting on the sphere. 

To examine the effect of turbulence on flames, and hence the mass consumption rate of the fuel mixture, it is best to first recall the tacit assumption that in laminar flames the flow conditions alter neither the chemical mechanism nor the associated chemical energy release rate. Now one must acknowledge that, in many flow configurations, there can be an interaction between the character of the flow and the reaction chemistry. When a flow becomes turbulent, there are fluctuating components of velocity, temperature, density, pressure, and concentration. The degree to which such components affect the chemical reactions, heat release rate, and flame structure in a combustion system depends upon the relative characteristic times associated with each of these individual parameters. In a general sense, if the characteristic time (r0) of the chemical reaction is much shorter than a characteristic time (rm) associated with the fluid-mechanical fluctuations, the chemistry is essentially unaffected by the flow field. But if the contra condition (rc > rm) is true, the fluid mechanics could influence the chemical reaction rate, energy release rates, and flame structure. 

There are many different aspects to the field of turbulent reacting flows. Consider, for example, the effect of turbulence on the rate of an exothermic reaction typical of those occurring in a turbulent flow reactor. Here, the fluctuating temperatures and concentrations could affect the chemical reaction and heat release rates. Then, there is the situation in which combustion products are rapidly mixed with reactants in a time much shorter than the chemical reaction time. (This latter example is the so-called stirred reactor, which will be discussed in more detail in the next section.) In both of these examples, no flame structure is considered to exist. 

The effect of turbulent mixing has been shown to follow the same behavior as molecular diffusion as previously shown in Eq. 3.6 (Fick s first law), where the diffusive flux, T (jiff(mol m s ), of a solute, C (mol/m ), is given by ... 

Reaction rates of nonconservative chemicals in marine sediments can be estimated from porewater concentration profiles using a mathematical model similar to the onedimensional advection-diffusion model for the water column presented in Section 4.3.4. As with the water column, horizontal concentration gradients are assumed to be negligible as compared to the vertical gradients. In contrast to the water column, solute transport in the pore waters is controlled by molecular diffusion and advection, with the effects of turbulent mixing being negligible. 

Improvements in deterministic (photochemical/diffusion) methods are based largely on accounting for more physicochemical effects in the structure of the model. Specific research subjects for improved models include photochemical aerosol formation and the effects of turbulence on chemical reaction rates. The challenge to the researcher is to incorporate the study of these subjects without needlessly complicating already complex models. How accurate a mathematical simulation is required What, roughly, will be the effect of omitting some particular chemical or physical component What is the sensitivity of model outputs to inaccuracies in the inputs ... 

Figure 27. Probstein s apparatus for investigating the effect of turbulence promoters in laminar-flow UF (10)...

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