| The entrainment and mixing of accelerating turbulent jets were experimentally investigated in aqueous media with a Schmidt number of about 600. The jet velocity at the nozzle exit was increased linearly, quadratically and exponentially with respect to time. A steady turbulent jet with a Reynolds number of 3,000 had been established before the acceleration started. During the acceleration, the nozzle velocity was increased by ten fold, resulting in acceleration rates up to 1.10 m/s$sp2$. Both the planar laser induced fluorescence (PLIF) flow visualization experiments and the quantitative LIF measurements revealed that the acceleration creates a 'front' region within the jet flow field. Within the range of present experimental parameters, the front region has a length of about half of the distance from the nozzle exit to the position of its leading edge. The concentration level is approximately constant along axis within the front and it is higher compared to the steady jet value at the same downstream location. The concentration level within the front depends on the acceleration rate, acceleration scheme, and the axial position of the front. The front travels with higher speed in contrast to the steady jet at the same downstream location.;The jet concentration field can be approximately classified into three regions. (1) downstream of the front, the steady jet flow is not yet affected by the acceleration. (2) the front region. (3) upstream of the front, the flow becomes quasisteady with 10%-20% higher concentration level compared to the steady jet at the same location a few diameters downstream from the nozzle.;A model based on the passive scalar conservation and quasi-steady trajectory of parcels within the jet was developed to predict the jet front penetration and the front concentration. The models works fairly well for both linearly and exponentially accelerating jets. |
