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Liquid Reactions Using the Unsteady Laminar Flamelet Model

Modeling Liquid Reactions in CIJR Using the Unsteady Laminar Flamelet Model

Introduction

The purpose of this tutorial is to demonstrate setup and solution of liquid chemical reactions using the unsteady laminar flamelet model in ANSYS FLUENT. Reactions in liquid typically display slow chemical kinetics and low species diffusivity (or high Schmidt number). Slow chemistry implies that fast chemistry models of ANSYS FLUENT, such as the non-premixed equilibrium model, steady laminar flamelet model, and the eddy-dissipation model cannot give accurate results. Low molecular diffusion especially for reactors at low Reynolds number, causes slow mixing and high species variance. Finiterate chemistry models, in particular the laminar finite-rate and eddy dissipation concept (EDC) model of ANSYS FLUENT do not rigorously account for the turbulence-chemistry interaction and may not predict conversion, selectivity, and scale-up accurately. The PDF transport model can handle both the finite-rate chemistry and the low diffusivity of the species, but the computational cost involved is very high since kinetic calculations are performed using particle based methods (Monte-carlo methods) which are computationally intensive. This tutorial demonstrates how to do the following:

  • Set up liquid chemical reactions in a confined impinging jet reactor.
  • Set up unsteady laminar flamelet model with liquid micro-mixing extension.
  • Import a CHEMKIN mechanism.
  • Calculate a solution for steady and unsteady flamelet models.
  • Examine results using graphics.

Problem Description

The unsteady laminar flamelet model is able to account for both the slow chemistry and the reduced mixing due to the low diffusivity. It also offers significant reduction in run time by reducing stiff kinetic calculations to one dimension. This tutorial solves for the following set of liquid kinetics occurring in bulk species in a confined impinging jet reactor.

The first reaction is very fast, and the second is very slow. When the liquid micro-mixing model is enabled, ANSYS FLUENT assumes that the activation energy for the reaction is small, so reaction proceeds immediately after mixing. A diagram of the CIJR with two reactants impinging at the center of the reactor is shown in figure below.

Result

Summary

This tutorial has demonstrated that the unsteady laminar flamelet model can simulate reactions in liquids with slow chemistry and low diffusivity.

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