In this study, the flow rate distribution characteristics and the changes in the discharge angle of a multi-perforated tube with the number of rectangular orifices on both sides of a rectangle tube were investigated by using the change of BR (blockage ratio) and the distance between the orifices with the relatively high Reynolds numbers. In addition, the influence of orifice thickness was examined, and eventually it was implemented to find the optimum design parameters which affect the flow rate distribution between the orifices.

Besides, the experimental study for the characteristic of flow rate distribution of the orifices on a circular multi-perforated tube was performed, and the results were compared with the analytical study. To control the flow rate distribution of the orifices in the longitudinal direction of the multi-perforated tube, the influence of orifice area ratio was examined. In addition, the orifice flow field was visualized with the help of PIV (particle image velocimetry) system, and the real flow area, which influences the flow rate distribution was investigated. The optimum design parameters, which controls the characteristic of the flow rate distribution between the orifices on the multi-perforated tube, were also used to suggest a better method to improve the efficiency of the multi-perforated tube system.

With a catalyst-inserted SCR muffler, an analytical study to describe about the shape of the multi-perforated tube with its location in the chamber between the entrance of the muffler and catalyst affects on the inner flow was also conducted. A method is suggested to improve NOx reduction rate and to prevent ammonia slip, by visualizing the inner flow field in the chamber and was evaluated the uniformity index of the flow rate distribution between the front and the rear of the catalyst.

In order to prove these optimized flow characteristics, an engine test was carried out using the test equipments which consist of an engine dynamometer system, an emission spectrometer to measure a density of exhaust gas and an MDLT (micro dilution tunnel) to collect the particles.

This paper consists of series of experiments which includes, (1) the rectangular multi-perforate tube test as a fundamental research, (2) the circular tube test as an extended research of the rectangular multi-perforate tube, (3) the analytical investigation for the catalyst-inserted SCR muffler, and (4) the engine test to prove the optimum design parameters on the area ratio change of multi-perforated tube, can be summarized as follows.

▶ The rectangular multi-perforated tube test
1) The increasing in BR (blockage ratio) of the multi-perforated tube affects the uniform flow rate distribution between orifices. The linear gradient of flow rate distribution in the longitudinal direction of the multi-perforated tube is as to BR=0.893∼0.979

2) The discharge angle become more perpendicular to the longitudinal direction of multi-perforated tube as it gets closer to the end of orifices, exhibiting big differences at the entrance if BR is small.

3) The increasing in the thickness of multi-perforated tubes are improving the uniform flow rate distribution between orifices become as contraction coefficient increase.

4) The flow coefficient distribution of orifices at the entrance of the orifices in the multi-perforated tubes increases in the longitudinal direction of the multi-perforated tubes, exhibiting values ranging from 0.66 to 0.68 as to BR=0.893∼0.979.

▶ The analytical investigation for the circular multi-perforated tube
5) The effective flow area that is defined as the actual flowing area shows the tendency of linear increase in the longitudinal direction of the multi-perforated tube.

6) The normal velocity contours shows the linear increase in the longitudinal direction of multi-perforated tubes, and the horizontal velocity demonstrates the maximum distribution in the middle of multi-perforated tubes and the minimum distribution at the entrance and the exit of multi-perforated tubes.

7) The flow rate distribution at the exit of orifices can be controlled by changing the area ratio of the orifices of the multi-perforated tubes. The flow rate distributions tend to uniformalize for the orifice array which has large area ratio and the area decreasing in the longitudinal direction.

▶ The analytical investigation for the catalyst-inserted SCR muffler
8) The flow characteristics inside the chamber is largely affected by the geometry of the multi-perforated tube. Accordingly, it was confirmed that the bulk swirling flow field may be controlled by changing the orifice area ratio of multi-perforated tubes in the longitudinal direction.

9) The flow characteristics formed at the front of the catalyst is controlled by adjusting the bulk swirling flow field inside the chamber, and consequently it was clarified that the uniformity index of the flow field at the front of catalyst could be adjusted.

10) In the case that the orifice area of the multi-perforated tube is increasing along the longitudinal direction, the flow rate increases along the longitudinal direction as well, which results in the formation of strong bulk swirling flow field inside the chamber. Consequently, there were noticeable higher uniformity index at the front end of the catalysts and better flow rate distributions between two catalysts. However, these results tends to increase the pressures inside of chamber relatively.

11) Finally, it was clear that the optimization of the flow characteristics in the multi-perforated tube and inside the chamber improved the uniformity index of flow field at the front of catalyst, accordingly as the higher uniformity index of the flow field at the front of the catalyst was also contributed to the NOx reduction by maximizing the utilization of catalysts.

▶ The engine test with a catalyst-inserted SCR muffler
12) According to the engine test results, the condition of ?40, which is that the orifice area increases from the inlet to the end along the longitudinal direction of the multi-perforated tube set inside the SCR-muffler, showed the lower NOx emission, compared to other two engine test conditions of 0 and +40.

13) The above results clearly illustrated that the changing of the orifice area ratios along the longitudinal direction of the multi-perforated tubes had an impacted on the NOx reduction characteristics in a catalyst-inserted SCR muffler system. The condition of ?40 that the larger area orifices were set up with the increase of the longitudinal distance of the multi-perforated tube showed good agreement with the optimum design parameters representing the good uniform index and flow rate distributions in the experimental and the analytical investigations for the circular multi-perforated tube.