Measurement and Simulation of the Turbulent Dispersion of a Radioactive Tracer in a Two-Phase Flow System

Studies on the measurement of turbulent dispersion of a radiotracer in an experimental setup with natural convection liquid-gaseous flow were carried out at the Forschungszentrum Rossendorf. Liquid-gaseous bubbly flow was generated in a narrow tank by injection of pressurized air into water or by catalytic disintegration of H₂O₂. A small amount of a positron emitting radiotracer liquid ( ¹⁸F in an aqueous NaF solution) was injected instantaneously at a certain point of the tank. The Positron Emission Tomography (PET) technique was used to observe the spreading of the tracer liquid in the bubbly flow. A double head gamma detector array was used for measuring the positron annihilation rate which is proportional to the tracer concentration. The dispersion coefficient D for the tracer liquid was calculated from the experimental data assuming an isotropic spreading of the concentration profile after separation of the linear displacement of the maximum concentration point. Calculations of the two-phase flow with spreading tracer were carried out for selected experiments using the computational fluid dynamics code CFX-4. The Euler-Euler continuum approach including a homogeneous low Reynolds number K,? -model was applied for the two-phase flow. Turbulent Prandtl numbers for gas and tracer dispersion were varied. In case of higher gas injection rates into water (superficial gas velocities j_gas of 5 - 15 mm/s), a reasonable agreement was achieved between calculated and measured values of the tracer transport velocity and the dispersion coefficient. A nearly linear correlation between j_gas and D was found in agreement with other authors. However, it was necessary to take into account the significant deviation of the tracer distribution from the ideal diffusion equation solution in some cases, in particular if the tracer has been spread over upwards and downwards regions of the natural circulation velocity field. Further investigations are necessary to improve the agreement between measured and calculated values of the dispersion coefficient and tracer transport velocity in the case of H₂O₂ disintegration and low gas superficial velocities.