Interpretation of internal tracer experiments and local sojourn time distributions

Abstract Tracer experiments are a valuable tool in the study of transport phenomena. Most of the theoretical work about tracers has been concerned either with systems having clearly defined inlet and outlet or with recirculating flows. In this article the methods are generalized to more complex situations and an attempt is made to provide a uniform theoretical treatment for local tracer experiments performed at several sites inside the flow systems. Local purging rate and mixing rate are defined and their measurements and applications are discussed. Several sojourn time distribution are discussed, especially that of local remaining life which might be useful in the study of local processes in large flow systems. The results should be of interest in a wide variety of areas in which tracer experiments are used, such as chemical reactor design, physiology, hydrology and the study of dispersion processes in the atmosphere and in oceans.