Determining and modeling of the extent of a treatment process in a reactor require knowledge of the flow characteristics. Residence time distribution (RTD), hydrodynamic dispersion coefficients, and velocity distribution are three important terms. Velocity distribution can be measured by a laser doppler anemometer (LDA) accurately, but RTD and hydrodynamic dispersion coefficients must be analyzed by mathematical modeling and dye testing. In this paper, the mathematical modeling and numerical simulation of a upflow reactor are developed. Rhodamine B and a fluorometer are used in laboratory testing. By applying the Euler FTCS (forward‐time‐center‐space) finite‐difference method, the results of dye tests and convection‐dispersion models are compared. The relationship between hydro‐dynamic dispersion coefficients and flow rates is discussed.