Investigations On The Free Jet With PIV Experimentand Numerical Simulation

Jetting phenomenon could be found in many places in nuclear reactor engineering.A free jet could be considered as a simplification of a jet.The study on the instaneous velocity field is beneficial for the modelling of turbulent flow,which is also meaningful for the design of safety-related system and the analysis of the consequences of accidents relevant to jet phenomenon.In this thesis,the particle image velocimetry?PIV?technique with planar laser was incorporated to measure the instaneous velocity for a round free jet.The velocity field,the fluctuation of velocity and the Reynolds stress were derived from the measurement and analyzed in detail.The numerical simulation of the flow field of the round free jet was conducted by using different turbulence models or modelling methods.The comparison between the simulation results and experimental measurement was made and the models were evaluated.The free jet experimental facility was set up with high resolution PIV measurement system installed.Based on the methodology to assess the measurement error proposed by the Japan Visualization Association,the bounded relative error was then estimated as within2.391%.Polyamide resin was selected as tracing particles for PIV measurement.The sensitivity analysis on the concentration of tracing particles and number of images to calculate time-average quantities were performed.The acceptable values were then recommended.The free jet experiments were carried out under different inlet Reynolds number?Re?conditions.The decay behaviors of dimensionless velocity along the axis of round jet were measured by using PIV technique.The relationship between the developing section and the Reynolds at the nozzle was identified.Through analysis on the radial dimensionless velocity at different axial locations,the self-similarity phenomena could be found in full-developed section of the free jet when Re was over 2400.The results agreed well with the prediction from Gortler theory.And the entrainment of free jet agreed well with the prediction from Tollmien theory.By conducting statistics analysis of instaneous velocity,the fluctuation of local velocity with respect to time-averaged velocity in both axial and radial direction was quantified.The results showed that in developing section,the ratio of fluctuation over time-averaged velocity in either direction had two peaks located at r/b_1/2?±1;in full-developed section,the same ratio presented the only peak at the centerline.Non-dimensional Reynolds stress and gradient of time-averaged velocity showed double crest distribution and self-similarity could also be found.The turbulent viscosity coefficient was given by Reynolds stress and gradient of the experiment date.Finally,the numerical simulations of cases with Re=531.5 and 2444.9 respectively were conducted by using different turbulence models of standard k-?,RNG k-?and large eddy simulation method as well.The results were compared against experimental measurement.It was shown that standard k-?worked well for the relative high Re case and LES took the advantage for the relative low Re case.