CALCULATION OF FLOW AND MIXING PROPERTIES IN A 2-DIMENSIONAL DILUTIONTUNNEL

A computational fluid dynamic analysis was performed to study flow cha racteristics and mixing performance in a full-flow dilution tunnel. Th e tunnel, 0.457 m diameter and 4.878 m (16 ft) long, consists of trace r (propane gas) and dilution air introduced as co-current flow, with t urbulent mixing enhanced by an orifice. The velocity and concentration profiles of a propane gas tracer were predicted at the tunnel samplin g zone for 0.18, 0.47, and 0.94 m(3) s(-1) volumetric flow rates. Due to limited access of the mixing zone, the characterisation of the flow field under different flow conditions were impractical. A fluid dynam ic code, FLUENT, which uses the primitive variable approach, solves th e full equations of motion, energy and species mass fraction, was empl oyed to simulate the velocity and species concentration fields. The fl ow characteristics and the mixing phenomena of the tunnel equipped wit h one hole, and three hole orifices were studied. For the same tunnel geometry, the mean residence-time of gas from the entrance to the samp ling zone location were predicted. The mean residence-time and mixing parameter (p) predictions were in agreement with the experimental meas urements. The results indicate that the gas mean residence-time in the tunnel was larger for a lower flow rate. Also, the predicted mixing p arameter was mainly dependent on the Peclet number, Pe (i.e. the ratio of convection to dispersion), and the orifice configuration. The resu lts of this study show how a computational fluid dynamic analysis of t he dilution tunnel could aid in optimising the tunnel size, and obtain its fluid flow and mixing characteristics, with less cost and effort.