Laser-Doppler measurements of the turbulent mixing of two rectangular water jets impinging on a stationary pool

Turbulent mixing of parallel jet flows has broad engineering applications. For example, in Gen. IV conceptual nuclear reactors, high-temperature flows mix in the lower plenum before entering the secondary cooling system. The mixing condition needs to be accurately estimated and fully understood. In addition, massive computational works involved in the design process necessitate high-fidelity experimental data sets for benchmarking simulation results. The purpose of this study is to use laser Doppler anemometry, a non-intrusive measuring technique, to evaluate the mixing characteristics of two submerged parallel jets issuing from two rectangular channels. The jets with a small spacing ratio of 3.1 were at room temperature. Flow characteristics including distributions of mean velocities, turbulence intensities, and Reynolds stresses were studied for the cases with equal and non-equal discharge velocities. The merging point (MP) was found to be between y/a = 1.72 and y/a = 3.45. The combining point (CP) was at y/a = 15.52. The Reynolds shear stress, a measure of the intensity of momentum transfer, reached its maximum after the merging point. An uncertainty analysis indicated that the average standard deviations of the streamwise mean velocity U and turbulence intensity U_rms at all locations in the five days' measurements were 1.5% and 1.6%, respectively. Spectral analyses including fast Fourier transform, power spectral density estimation and continuous wavelet transform revealed the scale and the evolution in time of varied-size eddies in the mixing region of the flow. Repeating flow structures were observed in different time segments. The experimental data obtained from the LDA measurements of the averaged quantities and transient are not only valid for benchmarking steady-state numerical simulations using turbulence models to solve RANS equations but they also enlarge the database of the experimental data for twin jets.