A CFD Study of the Performance of Horizontal Dilution Tubes

Since about 20 years, horizontal dilution tubes are in use to study soot formation close to the main reaction zone, in order to characterize the properties of nascent soot nanoparticles and to obtain insight into the soot formation process. In this study, the performance of horizontal dilution tubes, both free standing and embedded, is investigated by RANS and LES. The flame gas enters the dilutionctube through a pinhole and, in experimental studies, it is claimed to mix quickly with the cold, inert gas flow in the dilution tube. Previously, the distortion of the flow and temperature profiles around the dilution tube were investigated. Here, the orifice flow as well as the dilution process inside the tube are studied. The volume flow through the orifice is shown to be proportional to the square root of the pressure drop. The discharge coefficient is the range 0.9  0.3 in the cold air (calibration) case and drops to 0.35 under hot (flame) conditions. The resulting dilution ratio is roughly a factor of 5 below typical literature data. The gas sample is found to remain in the wall boundary layer and, the mixing process is not complete at the end of the dilution tube. ... mehrTurbulence decays rapidly behind the tube inlet and, in the main body of the tube, the flow is in the laminar to turbulent transition regime. Turbulence increases significantly in the outlet section which has much smaller pipe cross sections. Despite its relatively low Reynolds number, the outlet flow to the particle sizer (or to the gas analyzer) is clearly turbulent and, interactions with the wall are probable. The results are in agreement with previouscfindings from laminar jets in crossflow. Guidelines for optimization of the sampling conditions are suggested.