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A one-stage dilution tunnel has been developed to sample and dilute diesel exhaust. The tunnel has the capability of simulating many aspects of the atmospheric dilution process. The dilution rate and overall dilution ratio, temperature, relative humidity, and residence time in the tunnel, as well as residence time and temperature in the transfer line between the tunnel and exhaust sampling point may be varied. In this work we studied the influence of the exhaust transfer line, tunnel residence time, and dilution air temperature on the exhaust particle size distribution. The influences of fuel sulfur content on the size distribution and on the sensitivity of the size distribution to dilution and sampling conditions were also examined. We do not suggest an optimum dilution scheme, but do identify critical variables.

A single-stage dilution system has been designed to simulate the process of engine exhaust dilution in the atmosphere. An exhaust sample stream is introduced into a partial flow tunnel where it is diluted at a controlled rate. Temperature, relative humidity, dilution ratio and rate, and residence time are all adjustable. The system includes a turbulence generator to adjust the intensity of turbulence in the tunnel and a wake disk to control the initial mixing rate. Numerical methods were used to simulate flow fields, velocity fields, and mixing profiles for gases and particles. Mixing profiles for a gaseous tracer and particles of different sizes were also determined experimentally and compared with the model predictions. Critical parameters that influence mixing profiles and dilution rates predicted by modeling were demonstrated experimentally. Predicted and measured normalized mixing profiles were found to be in good agreement.