Exchange relaxometry of flow at small Péclet numbers in a glass bead pack

[Display omitted] ► Low-field NMR relaxation exchange experiments on a glass bead pack exposed to flow. ► Mathematical model including diffusive and advective transport. ► Experimental and theoretical assessment of multi-site exchange. ► Flow signature observed in T2–T2 exchange maps. ► Simulations reproduce qualitatively the effects of diffusive transport and flow. In this paper we consider low Péclet number flow in bead packs. A series of relaxation exchange experiments has been conducted and evaluated by ILT analysis. In the resulting correlation maps, we observed a collapse of the signal and a translation towards smaller relaxation times with increasing flow rates, as well as a signal tilt with respect to the diagonal. In the discussion of the phenomena we present a mathematical theory for relaxation exchange experiments that considers both diffusive and advective transport. We perform simulations based on this theory and discuss them with respect to the conducted experiments.