Mixing Of A Round Jet Into A Counterflow And Multiple Jets Into Crossflow

Ajet into counterflow and multiple jets into crossflow could enhance the dilution of flow, but they have been scarely investigated owing to their complex flowfield. A free or offset round jet into counterflow and multiple jets into crossflow were inverstgated in this paper. The mean and turbulent velocity of a free or offset round jet into counterflow were measured using particle image velocimetry (PIV). When a free round jet into a counterflow, the velocity of the jet decays beyond the potential core and until zero at stagnation. The distance, from the jet exit to this stagnation point, is called the penetration length lp. The penetration length linearly increases as the jet-to-current velocity ratio increase. Because of the entraintion between jet and counterflow, the wide of jet linearly increases with the distance increasing and its evolution is faster than that of in quiescent ambient condition. After reaching the stagnation point, the jet flow reverses and approximately assumes the velocity of the counterflow. The boundary between the forward and backward flows is defined as the "stagnation streamline". The half of stagnation streamline in radial direction increases to a maximum value (0.14lp) at the end of zone of established flow, and rapidly decays to zero after the peak. The variations relate to the jet-to-current velocity ratios and. The axial velocity decays more rapidly as the jet-to-current velocity ratio decreases and all of them decay more rapidly than that for the jet in quiescent ambient conditions. The mean axial velocity decays and has a continuous broadening in radial direction as axial distance increases. The profile of excess velocity is in accord with Gauss distribution and exhibits self-similarity in the zone of flow extablishment. Different with axial mean velocity, axial turbulent intensity rapidly increases after jet exit to a maximum value and then decreases generally. Near the stagnation point, the axial turbulent intensity reach a second local maximum value. The radial turbulent intensity rise sharply to a maximum value and decrease generally, but it do not have a second local maximum value near the stagnation point as the axial turbulent intensity. The axial and radial turbulent intensity increases exponentially with the axial distance (x/lp). The ratio of axial to radial turbulent intensity is0.5inside penetration length. The axial and radial turbulent intensity does not exhitbit self-similarity in the radial direction. The swekness, kurtosis and intermittency factor indicate a great intermittency exists in the vicinity of dividing streamline.When a offset round jet into a counterflow, the penetration length linearly increase with the jet-to-current velocity ratio increasing and the proportionality coefficient increase as the jet-to-current velocity ratio decreases. The evolutions of half width of the excess velocity and the zero velocity are the same as that of free round jet into counterflow. The axial mean velocity decay more slowly than that of a free jet into a counterflow. The retard degree of axial mean velocity increase as the offset distance decreases. However, the decay of the axial mean velocity on the axis is same as that of a free jet into counterflow when the offset distance is more than or equal to15D (D is the diameter of the jet). Axial mean velocity profile have continuous broadening and its maximum value decreases downstream. Some of them do not accord with Gauss distribution and not exhibit self-similarity. The axial and radial turbulent intensity on the axis increase sharply after the jet exit and decreases generally after a maximum value until equal to that of the counterflow beyond penetration length. Both of them do not exhibit self-similarity in radial direction. In the penetration length, the ratio of axial to radial turbulent intensity is less than0.5when then offset distance less than10D. However, when the offset distance greater than10D, the ratio of axial to radial turbulent intensity is equal to0.5. The swekness, kurtosis and intermittency factor exhibit that a great intermittency occur s in tine vicinity of dividing streamline and in the region near wall.A free round jet into a counterflow under different jet-to-current velocity ratios was also investigated using large eddy simulation. The results agree well with experimental measurements from laser-Doppler anemometry and laser-induced fluorescence that include velocity and mean concentrations along the centerline and radial direction. Vortex rings appear in the region near the jet exit and large-scale vortex structures still occur near the stagnation point. The flow becomes more chaotic and three-dimensional with the presence of these structures. In particular, their presence near the stagnation point results in large velocity fluctuations that enhance the mixing process and dilution. These fluctuations are described by probability deasity functions that deviate from Gaussian distribution. The three-dimensional streamlines indicate that the jet not only oscillates in three directions but also rotates about the jet axis and around the vortex. The second and third moments of the velocity or scalar fluctuations identify that the mixing processes are greater in the region before the stagnation point.In the last section, the characteristics of single and multiple tandem jets(n=2,3,4) in crossflow have been investigated using the realizable k-ε model. The results of this model agree well with experimental measurements using PIV (Particle Image Velocimetry) or LIF (Laser Induced Fluorescence). We analyzed the calculated results and obtained detailed properties of velocity and concentration of the multiple jets in the pre-merging and post-merging regions. When the velocity ratio is identical, the bending degree of the leading jet is greater than that of the rear jets. The last jet penetrates deeper as the jet number increases, and the shielding effect of the front jet declines with jet spacing increase. Interaction of the jet and crossflow induces formation and development of a counter-rotating vortex pair (CVP). CVP makes the distribution of concentration appear kidney-shaped (except in the merging region), and maximum concentration is at the center of the counter-rotating vortex. Concentration at the CVP center is1.03-1.4times that of the local jet trajectory. Post-merging velocity and concentration characteristics differ from those of the single jet because of the shielding effect and mixing of all jets. This paper presents a unified formula of trajectory, concentration half-width and trajectory dilution, by introducing a reduction factor.