Chemical reactions in turbulent flow

Keeler, R. N., E. E. Petersen, and J. M. Prausnitz (1965). Mixing and chemical reaction in turbulent flow reactors. AIChE Journal 11, 221-227. 

The methods of analysis involving numerical solutions appear sufficiently well advanced to permit a rapid expansion of the microscopic analysis of turbulent transport as soon as some of the basic experimental facts are obtained. The next advance of particular interest to the chemical engineer appears to be an understanding of the kinetics of chemical reactions in turbulent flow. The fluctuating temperatures and concentrations introduce perturbation in the normal approach to kinetics that may well yield interesting results in the field of combustion and perhaps in chemical processing. 

Fundamental regularities offast chemical reactions in turbulent flows... 

V. Z. Kompaniets, A. A. Ovsyannikov, A. S. Polak Chemical Reactions in Turbulent Flows of Gases and Plasma, Nauka, Moscow (1979) (in Russian). 

Fast Chemical Reactions in Turbulent Flows Theory and Practice... 

Magnussen, B.F. and Hjertager, B.W. (1981) On the structure of turbulence and a generalized eddy dissipation concept for chemical reaction in turbulent flow. 19th AIAA Aerospace Meeting, St. 

As the final topic in this chapter, we consider the rates of chemical reaction in turbulent flow. Such flow produces rapid mixing, so that the fluid appears homogeneous. Such mixing turns out to be only macroscopic. In other words, if we take 10 samples, each of 1 cm, we find that the average concentrations of the samples differ by only a few tenths of a percent. However, if we take ten samples of 10 cm, we find that their concentrations vary widely. For example, if we are mixing acid and base, we might find that some samples contain 10 mol/1 H, and other have 10 mol/1 H. ... 

Facilitating the insight into the complex inter-linked phenomena of chemical reactions and turbulent flow field behaviour as well as for the investigation of parametric effects, the simulation program AIOLOS is used as an effective tool for the investigation of the combustion processes in small and medium scale wood combustion systems. 

Osenbroch LKH (2004) Experimental and Computational Study of Mixing and Fast Chemical Reactions in Turbulent Liquid Flows. PhD Thesis, Aalborg University, Esbjerg... 

V.Z. Kompaniec, A.A. Ovsyannikov and A.S. Polak in Chemical Reactions in Turbulent Gas and Plasma Flows, Nauka, Moscow, Russia, 1979. [In Russian]... 

Now that we know how to estimate the size fx of the smallest scales of turbulence, it is simple to conclude by summing up the principles that govern mixing and chemical reactions in a flow with homogenous turbirlence. Table 11.1 shows, for a chemical reaction, for micromixing (molecular diffusion), and for macromixing (dispersion at different scales), the characteristic times and the scales with which these various processes are associated. For a stirred reactor, the phenomenon involves six parameters of different nature ... 

Hjertager LK, Hjertager BH, Solberg T (2002) CFD modelling of fast chemical reactions in turbulent liquid flows. Com and Chem Eng 26(4-5) 507-515... 

The last two sections in this chapter are concerned with reactions commonly described as fast or diffusion controlled. In Section 17.4, we discuss chemical reactions whose rates are controlled not by chemical kinetics but by Brownian motion of the reagents. These reactions are studied by suddenly changing the temperature or pressure and measuring the decay of the resulting perturbation. In Section 17.5, we investigate the speed of second-order reactions in turbulent flow. If these reactions are fast, their speed depends on mixing, not on chemistry. Thus their reaction rates are determined not by chemical kinetics but by the turbulent dispersion summarized in Chapter 4. These reactions can be described with mathematics like that for diffusion, and so are best treated here. 

Modelling plasma chemical systems is a complex task, because these system are far from thennodynamical equilibrium. A complete model includes the external electric circuit, the various physical volume and surface reactions, the space charges and the internal electric fields, the electron kinetics, the homogeneous chemical reactions in the plasma volume as well as the heterogeneous reactions at the walls or electrodes. These reactions are initiated primarily by the electrons. In most cases, plasma chemical reactors work with a flowing gas so that the flow conditions, laminar or turbulent, must be taken into account. As discussed before, the electron gas is not in thennodynamic equilibrium... 

In general, comprehensive, multidimensional modeling of turbulent combustion is recognized as being difficult because of the problems associated with solving the differential equations and the complexities involved in describing the interactions between chemical reactions and turbulence. A number of computational models are available commercially that can do such work. These include FLUENT, FLOW-3D, and PCGC-2. 

In turbulent flow, axial mixing is usually described in terms of turbulent diffusion or dispersion coefficients, from which cumulative residence time distribution functions can be computed. Davies (Turbulence Phenomena, Academic, New York, 1972, p. 93), gives Di = l.OlvRe for the longitudinal dispersion coefficient. Levenspiel (Chemical Reaction Engineering, 2d ed., Wiley, New York, 1972, pp. 253-278) discusses the relations among various residence time distribution functions, and the relation between dispersion coefficient and residence time distribution. 

The value of the heat transfer coefficient of the gas is dependent on the rate of flow of the gas, and on whether the gas is in streamline or turbulent flow. This factor depends on the flow rate of the gas and on physical properties of the gas, namely the density and viscosity. In the application of models of chemical reactors in which gas-solid reactions are carried out, it is useful to define a dimensionless number criterion which can be used to determine the state of flow of the gas no matter what the physical dimensions of the reactor and its solid content. Such a criterion which is used is the Reynolds number of the gas. For example, the characteristic length in the definition of this number when a gas is flowing along a tube is the diameter of the tube. The value of the Reynolds number when the gas is in streamline, or linear flow, is less than about 2000, and above this number the gas is in turbulent flow. For the flow... 

In general, liquid-phase reactions (Sc > 1) and fast chemistry are beyond the range of DNS. The treatment of inhomogeneous flows (e.g., a chemical reactor) adds further restrictions. Thus, although DNS is a valuable tool for studying fundamentals,4 it is not a useful tool for chemical-reactor modeling. Nonetheless, much can be learned about scalar transport in turbulent flows from DNS. For example, valuable information about the effect of molecular diffusion on the joint scalar PDF can be easily extracted from a DNS simulation and used to validate the micromixing closures needed in other scalar transport models. 

Chemical reaction in a turbulent flow field with uniform velocity gradient. The... 

Ibrahim, S. S., R. W. Bilger, and N. R. Mudford, Turbulence Effects on Chemical Reactions in Smog Chamber Flows, Atmos. Environ., 21, 2609-2621 (1987). 

The problem of absorption accompanied by a chemical reaction in a liquid film flowing along a vertical wall was often treated by using the framework of the penetration theory [28 - 32]. This theory also constitutes the starting point of the renewal models of turbulence [31,33,34]. 

Consider the turbulent flow of a liquid through a tube whose wall is dissolving in the liquid and that the diffusion of the solute is accompanied by a first-order chemical reaction. The turbulent diflfusivity is employed to represent the turbulent transport. Since the dissolving species is consumed by reaction, only the region near the wall (whose thickness is small compared to... 

The intense heat dissipated by viscous flow near the walls of a tubular reactor leads to an increase in local temperature and acceleration of the chemical reaction, which also promotes an increase in temperature the local situation then propagates to the axis of the tubular reactor. This effect, which was discovered theoretically, may occur in practice in the flow of a highly viscous liquid with relatively weak dependence of viscosity on degree of conversion. However, it is questionable whether this approach could be applied to the flow of ethylene in a tubular reactor as was proposed in the original publication.199 In turbulent flow of a monomer, the near-wall zone is not physically distinct in a hydrodynamic sense, while for a laminar flow the growth of viscosity leads to a directly opposite tendency - a slowing-down of the flow near the walls. In addition, the nature of the viscosity-versus-conversion dependence rj(P) also influences the results of theoretical calculations. For example, although this factor was included in the calculations in Ref.,200 it did not affect the flow patterns because of the rather weak q(P) dependence for the system that was analyzed. 

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