Abstract
Jet velocity is an important parameter affecting the air entrainment rate of plunging liquid jet processes. While the vast majority of researchers have investigated the effect of jet velocity, only a few of them considered the effect of jet length in calculating the jet velocity at impingement point. This study investigates the difference (ΔV) between the jet velocity at the inception of the nozzle (Vj) and the impingement point (VL) for a range of operating conditions. Furthermore, bubble voidage inside the downcomer, another critical parameter in plunging jets, is estimated using three different voidage equations incorporated inside a momentum balance model to predict the two-phase elevation level (HR) inside the downcomer. Results showed that ΔV is significant (VL > Vj), especially at low jet flow rates and high jet lengths. Generally, the momentum balance model predicted the HR well, and its prediction improves with downcomer diameter. Given that, the model still needs to be refined for more accuracy for a wide range of operating conditions.
Highlights
Gas entrainment occurs due to the impingement of a falling liquid into a receiving water body like in waterfalls, rivers, and streams that result in a dispersion of air bubbles into the water body, which results in self-purification or reaeration of the water body [1]
In environmental scenarios like breaking waves, the aeration rate can increase 200 times since many air bubbles entrain into the wave, and there is a tremendous increase in the air–water interfacial area [2]
velocity calculation (VL) velocity is the velocity effect coupled with the gravitational forces due to jet length Lj, which adds to the momentum of the flowing jet, and is worth considering in the equations used to calculate air entrainment rates
Summary
Gas entrainment occurs due to the impingement of a falling liquid into a receiving water body like in waterfalls, rivers, and streams that result in a dispersion of air bubbles into the water body, which results in self-purification or reaeration of the water body [1]. The PLJR is an excellent gas-liquid contactor as it is energy efficient and can achieve sufficiently high mass transfer rates This is possible since it can entrain gas bubbles into a liquid phase at low costs (operating and capital costs) [3,4,18] The liquid phase ejects from a nozzle, thereby forming a jet, which falls through ambient headspace and plunges into the surface of a receiving pool of the same liquid. The role of this annulus is to collect the upward gas-liquid flow that exits from the bottom of the downcomer resulting in entraining pure freshwater from the surrounding through the bottom of the annulus in the upward direction This is mixed with the saturated liquid from the bottom of the confining tube for additional mass transfer [3]. This is achieved by incorporating three voidage equations (n1 , n2 , and n3 ) into a momentum mass balance to predict HR theoretically and compare it against local experimental data obtained in the College of Life Sciences laboratory at Kuwait University
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