Saturday, September 22, 2012

Tutorial: Reacting Flow - Liquid Fuel Combustion

This tutorial models the evaporation and combustion of a liquid fuel, using the dispersed phase modeling capability to compute coupled gas flow and liquid spray physics. The mixture-fraction/PDF equilibrium chemistry model is used to predict the combustion of the vaporized fuel. In this tutorial you will learn how to:
  • Prepare a probability density function (PDF) file for a liquid fuel system
  • Define FLUENT inputs for PDF chemistry modeling
  • De fine a discrete second phase of evaporating liquid droplets
  • Calculate the flow field using the pressure based solver, including coupling between the discrete - liquid fuel droplets and continuous phase

The mixture-fraction/PDF modeling approach allows you to model non-premixed turbulent combustion by solving a transport equation for a single conserved scalar, the mixture fraction. Multiple chemical species including radicals and intermediate species, may be included in the problem definition and their concentrations may be derived from the predicted mixture fraction using the assumption of equilibrium chemistry. Property data for the species are accessed through a chemical database and turbulence-chemistry interaction is modeled using a betha-PDF.

Problem Description
The liquid fuel combustion system considered here is shown in the Figure. A liquid spray of pentane fuel enters a 2D duct in which air is flowing at 650 K and 1.0 m/s. The duct walls are held at a constant temperature of 1200 K. The model considered includes a duct length of 10H where H is the duct height (1.0 m). The Reynolds number based on inlet conditions is roughly 100,000 and the flow is turbulent. As pentane evaporates it enters the gas phase and reacts. The combustion is modeled using the mixture-fraction/PDF approach, with the equilibrium mixture of chemical species. The spray is assumed to consist of 100 micron diameter liquid droplets injected at 300 K over a filled spray half-angle of 30 degrees at the duct center-line. The mass flow rate of liquid fuel is 0.004 kg/s, corresponding to very fuel-lean conditions in the flow.


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