Simulation of water flow in a conduit using radiotracer-axial dispersion model

Abstract

A simple radiotracer experiment has been carried out to simulate water flow in a conduit made of PVC plastic with a diameter of 2 in. (50.8 mm). The water was supplied from a water tank with a volume of 500 liters During the experiment flow rate of water in the conduit was maintained constant at a speed of 0.2 m.s−1 by adjusting valve at the farthest end of the conduit. Br-82 isotope solution in a concentration of 1 mCi.ml−1 was injected instantaneously into the conduit. Injected isotope follows the bulk flow of water in the conduit. A scintillation NaI(Tl) radiation detector which was placed on the conduit at a distance of 8 m from injection point captures the radiation energy of injected radiotracer when passing it and generate residence time distribution (RTD) curve which represent isotope particle spent in the conduit. To get meaningful information, a mathematical simulation so-called the axial dispersion model has been developed to evaluate the RTD curve with Peclet number, Pe, as a model parameter. The best fitting of the RTD curve model onto the experimental one was achieved when the Pe is equal to 13. The calculated fitting error was 0.003, very extremely low as expected. This result indicated that the axial dispersion model was able to quantify water flow and simulated Peclet number indicated that convection flow is more dominant compared to the radial flow.