The benefits of using a bell mouth shape for the outlet of a top hat style separator were discussed extensively in this thread: http://www.jpthien.com/smf/index.php?topic=550.0
One of the charts from that thread presents a variety of duct end treatments that improve airflow over a bare duct end. Improvements are presented in terms of percent velocity pressure, which is difficult for most people to understand. The bell mouth showed the lowest losses of the six types listed.
In the course of measuring the performance of a 2 HP blower, I realized that I could measure the impact of three end treatments from the chart: the unflanged pipe, the flanged pipe and the unflanged cone - all using materials at hand.
One of the charts from that thread presents a variety of duct end treatments that improve airflow over a bare duct end. Improvements are presented in terms of percent velocity pressure, which is difficult for most people to understand. The bell mouth showed the lowest losses of the six types listed.
In the course of measuring the performance of a 2 HP blower, I realized that I could measure the impact of three end treatments from the chart: the unflanged pipe, the flanged pipe and the unflanged cone - all using materials at hand.