### 8.2.1. Set Up

Most simulations are set up as transient. The choice of timestep is generally model dependent, but will usually fall into the range 0.5 s to 2 s. The Total Energy heat transfer model should be selected to fully model buoyancy.

Note: When modeling buoyancy, it is very important to correctly specify the buoyancy reference density when opening boundary conditions are used.

The RNG turbulence model is a good choice for combusting flows, with either no buoyancy terms in equations or buoyancy terms in both the equations (with C3=1). The SST model is also reasonable, but may not converge well for natural convection. The SSG model is accurate, but convergence may be very slow.

The most common fuels used are hydrocarbons such as methane, diesel and petroleum. Cellulosic materials, plastics and wood are also used. The simulation will dictate the type of materials to use as a fuel.

The Eddy Dissipation Model is widely used, combined with an Additional Variable for toxins. The flamelet model is more rigorous, and is a better choice when the fire is under ventilated.

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