Introduction

The purpose of this discussion is to introduce readers to basic principles that underpin the applications of tracer techniques to fluid flow investigations.

The general concept of the tracer experiment is illustrated in Figure 8.1. It involves a three-stage process: (a) injecting the tracer T at A; (b) observing the response of the detector to the tracer at B; and (c) using the data to enhance knowledge of the ‘system’. As indicated in Section 8.2.1, the term ‘system’ may be interpreted in a very broad way. However, this discussion will be restricted to water flowing along a pipeline or through a rapidly stirred tank (Figure 8.6(b)).

Let it be assumed that the tracer T is injected into the bulk flow as a short pulse and traverses the cross section B in the time interval between t0 and (t0 + dt). A simple expression will be developed for the average transit time or residence time for water particles between the point of injection of the tracer A, and the sampling cross section at B. This expression will be used as the basis for:

• determining the mean residence time and residence time distribution of the tracer ‘particles’ between A and B;

• establishing the criterion for complete mixing of the tracer with the bulk flow of the water;

• measuring the flow rate of water at B assuming complete mixing has been achieved.

Residence time distributions of tracer particles between the points of injection and sampling

Suppose NT ‘particles’ of the tracer t are injected as an instantaneous pulse at A, are diluted and dispersed within the system (Figure 8.1) and monitored at B.