These tutorials simulate the process of mixing air and methane gas in a dilution pipe whose geometry is shown below.
The geometry consists of two concentric pipes, with the inner pipe covering only one quarter of the total distance between inlet and outlet. An orifice plate protrudes from the outer wall half way along the pipe. The flow is three-dimensional and essentially symmetric, so only half of the problem geometry was considered in building the corresponding mesh, shown below.
Air at standard pressure and temperature (1 bar, 293 K), flows in through the outer annulus. Methane gas (molecular weight 16 kg/kmole) at a temperature of 323 K flows in through the inner pipe. The pipe walls (inner and outer) are assumed to be adiabatic and the orifice plate has negligible thermal resistance.
The simulation is divided into two parts, one dealing with a steady-state and the other with a transient analysis.