Measuring hydrodynamic dispersion coefficients in unsaturated packed beds: Comparison of PEPT with conventional tracer tests

Abstract

Hydrodynamic dispersion has a major impact on mass transport within packed bed and porous media systems. In this paper, dispersion coefficients of neutrally buoyant tracer particles located with positron emission particle tracking (PEPT) are compared to results obtained using a conventional salt tracer experiment. It is demonstrated that the axial dispersion coefficients obtained from PEPT are very similar to those obtained using the salt tracer. The PEPT method has the advantage that the details of the flow behaviour can be observed, thus allowing analysis of the mechanisms at work to be carried out. In addition, the radial dispersion coefficient was obtained with PEPT, which is hard to obtain using conventional salt tracer tests. The main drawback with the PEPT method is that very low flow rates could not be studied as these result in very low saturations, which causes the tracer particle to become stuck. In this work the tracer particles used are 400μm in diameter, though as the tracer fabrication technology improves, the size of tracer available will continue to decrease, allowing a wider range of conditions and particle beds to be studied.