Reducing mixing at the outlet bore of a cylinder

Summary In some technical applications mixing caused by contraction of cross section is to be avoided. An example is the outlet of a chromatographic column in preparative liquid chromatography (HPLC). First of all, it is desirable to reduce the volume of the cylinder within which the mixing occurs. This is done by placing a porous disk upstream of the contraction. Mixing within the contraction (or funnel) is avoided if a constant axial velocity component over every cross-section is achieved. Two such velocity fields have been found analytically. The funnel shapes have been taken to be the stream tubes of these fields. The mixing within these funnels, and indeed in other common funnels, is described by the breakthrough curves or residence-time distributions, which are numerically determined. An experiment has been set up to measure the breakthrough curves for different funnels. Received 6 September 1997; accepted for publication 26 January 1998