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A stimulus-response technique was applied to a co-current flow with mixing orifice type full-flow dilution tunnel to determine its fluid residence time distribution and mixing characteristics. The technique involved the injection of a tracer at the inlet and the observation of the corresponding response at the sampling zone of the tunnel. Both propane gas and corn oil aerosol of approximately 0.5 {dollar}\\mu{dollar}m in diameter were used as the non-reacting tracers for the newly developed gas injection detection system (GIDS) and aerosol injection detection system (AIDS). Data were gathered from the tunnel operated under steady-state conditions and at three different volumetric flow rates which were 400, 1000, and 2000 SCFM. The moment method of analysis was applied on the response concentration-time data of tracer at both the tunnel inlet and sampling zone to determine the mean residence-time and mixing parameter. A second model, the one-parameter gamma distribution model (GDM) is employed to the response concentration-time at the tunnel sampling zone to represent the mixing characteristics. The mixing parameter, p, which relates to the convective and dispersive mixing condition of fluid at the tunnel sampling zone, is directly proportional to the Peclet number, Pe was determined. The lower p values can be attributed to the lower convective transport ability caused by the fully-developed velocity profile at the sampling zone. The higher values of p at a particular tunnel flow rate indicate that the velocity profile is not fully developed, and this was due to the higher convective transport ability. Model data shows that at 1000 scfm tunnel flow rate, the p value is the lowest, which indicates reasonably good mixing. Also, the p values for the hot propane tracer injections were lower in comparison to the cold propane tracer injections. The analysis shows that higher temperature indicates better mixing and lower mean residence-time. The GDM model agrees well with the experimental output showing that the model offers one elucidation for the measured data. A computational fluid dynamic analysis was performed to study flow characteristics and mixing performance of the full-flow dilution tunnel. The tunnel is 18 inches (0.457 m) in diameter and 16 feet (4.878 m) long, with co-current flow, connected via intermediate pipe to heavy duty tested engine exhaust manifold. The velocity and concentration profiles of a gas tracer (propane) were predicted at the sampling zone located at the end of the tunnel for 400, 1000, and 2000 SCFM (0.189, 0.472, and 0.944 m{dollar}\\sp3{dollar}/s) volumetric flow rates. A computer program, FLUENT, which solves the full equations of motion, energy, and species mass fraction was employed to simulate the velocity and concentration fields. The flow characteristics and the mixing phenomena of the tunnel equipped with a one hole and three hole orifices were studied. For the same tunnel geometry, the mean residence-time of gas from the entrance to the sampling zone location were predicted. The mean residence-time and mixing parameter (p) predictions were in good agreement with the experimental measurements. This indicates that the gas mean residence-time in the tunnel was larger for lower flow rate. Also, the predicted mixing parameter was mainly dependent on the Peclet number (Pe) and the orifice configuration.