RANS based models of reactive

RANS-BASED MODELS OF REACTIVE FLOW PROCESSES... 

By far, the most widely employed models for reactive flow processes are based on Reynolds-averaged Navier Stokes (RANS) equations. As discussed earlier in Chapter 3, Reynolds averaging decomposes the instantaneous value of any variable into a mean and fluctuating component. In addition to the closure equations described in Chapter 3, for reactive processes, closure of the time-averaged scalar field equations requires models for (1) scalar flux, (2) scalar variance, (3) dissipation of scalar variance, and (4) reaction rate. Details of these equations are described in the following section. Broadly, any closure approach can be classified either as a phenomenological, non-PDF (probability density function) or as a PDF-based approach. These are also discussed in detail in the following section. 

For illustrating of the second case, the component system of a reactive distillation system for the production of butyl acrylate (see Zeng et al.") will be used here as an example. Butyl acrylate is produced by an esterification reaction of acrylic acid and butanol with a byproduct of water. The total system includes four components acrylic acid (AA), butanol (BuOH), butyl acrylate (BA), and water. The base-property method was chosen as NRTL-HOC because AA is known to have vapor association. There are a total of six binary pairs in the whole component system. In these six binary pairs, built-in model parameters can be found for the four pairs of AA-BuOH, AA-H2O, BuOH-BA, and BuOH-HaO. The model parameters of the remaining two pairs (AA-BA and BA-H2O) need to be estimated by using the group-contribution method of UNIFAC before any simulation ran can be done. 

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