Air entrainment by inclined circular plunging water jets with various impingement heights

This paper presents an experimental study on air entrainment by inclined circular plunging water jets issued from long nozzles with various jet impingement heights up to 6.1 m. Particular attention was paid to the jets having a large impingement height, and which disintegrated into small droplets before reaching the water surface. The dominant sizes and velocities of the droplets generated by jet breakup were 1–4 mm and 4–9 m/s. The results show that for plunging water jets having the same hydraulic head, the air entrainment rate increased as the jet impingement height at first grew, then dropped as the jet impingement height kept increasing. The early increase in the air entrainment rate was due to the growth of jet surface roughness, while the later decrease was caused by the water jet disintegration. The momentum of the plunging jet decreased dramatically due to the reduced mass and the significant energy dissipation through the atmosphere caused by jet disintegration. This led to a much smaller penetration depth and air entrainment rate compared to with a continuous jet.

Effect of Primary Variables on A Confined Plunging Liquid Jet Reactor

The effects of operating conditions including a novel downcomer geometry on the gas/air entrainment rate, Qa, were investigated for a local vertical confined plunging liquid jet reactor (CPLJR) as an alternative aeration process that is of interest to Kuwait and can be used in various applications, such as in wastewater treatment as an aerobic activated sludge process, fermentation, brine dispenser, and gas–liquid reactions. Operating conditions, such as various downcomer diameters (Dc = 45−145 mm), jet lengths (Lj = 200–500 mm), nozzle diameters (dn = 3.5–15 mm), and contraction angles (Ɵ =20–80°), were investigated. A newly designed downcomer with various mesh openings/pores (Dm = 0.25ʺ (6.35 mm)–1ʺ (25.4 mm)) was also investigated in the current study. The air entrainment results showed that these were the primary parameters for the measured air entrainment rate in confined systems. The highest gas entrainment rates were achieved when the ratio of the downcomer diameter (Dc) to the nozzle diameter (dn) was greater than approximately 5, as long as the liquid superficial velocity was sufficient to carry bubbles downward. Furthermore, a downcomer with mesh openings (Dm) less or equal to 0.5ʺ (12.7 mm) provided a higher entrainment rate than that of conventional downcomer (without a mesh).

Numerical modeling of spillway aerators in high-head dams

Due to high flow velocity, the spillway surfaces of high-head dams can expose to cavitational damage. The most effective and economical method of protection from this damage is aerated to flow using aerators. In this study, a spillway aerator of the roller-compacted concrete dam of 100 m height was analyzed using two-phase computational fluid dynamic model to overcome the cavitation damage on the spillway surface. The numerical analysis with prototype dimensions was performed for various flow conditions (5223, 3500, 1750 and 1000 m3/s of flow rate), and obtained results were compared with some experimental observation in the literature. Numerical and experimental results indicated that the cavitation occurs on the surface after a certain downstream point based on cavitation indices. The air entrainment rate and air concentrations supplied by means of the aerator were determined to avoid the cavitational damage. While the experimental results can contain considerable scale effect in terms of air entrainment rate owing to, e.g., viscous effects especially for small scales, the numerical models with prototype dimensions gave much more accurate results. In other words, it can be also mentioned that the actual aeration amount is much greater than that obtained from the model experiments. The results based on numerical analysis showed that the aerator device meet air demand to prevent the cavitation damage.

Estimation of the air entrainment characteristics of a transient high-pressure diesel spray

The air entrainment characteristics of a transient high-pressure diesel spray were investigated with respect to time and location for injection pressures ( Pinj = 76 or 137 MPa) and ambient density (ρa = 15.6 kg/m3) under the non-evaporating condition (303 K). A particle image velocimetry analysis was introduced and some parameters were defined to express air entrainment characteristics. The air entrainment rate increased greatly as the flow moved downstream owing to a larger contact surface area and a recirculating flow. Higher pressure led to a greater entrainment rate with higher effectiveness. The speed (spray tip and front ambient gas) and volume (spray and laterally entrained gas) relations suggested the possibility for the renewal against the lateral-dominant entrainment mechanism.

Experimental Study of the Influence of Different Weir Types on the Rate of Air Entrainment

Aeration is used in water treatment to alter the concentration of dissolved gases, to strip volatile organics, and to reduce tastes and odours. This can be obtained by creating turbulence in the water. One method of producing such turbulence is via the overflow jets downstream of weir structures. This paper investigates the effect of varying weir types on the air entrainment rate. Empirical correlations predicting the maximum penetration depth and air entrainment rate were developed for different weir types. It is demonstrated that the air entrainment rate of the broad-crested weirs is generally much better than for the sharp-crested weir and the labyrinth weirs. The air entrainment rate increased as the weir longitudinal slope of the broad-crested weirs and the weir sill slope of the labyrinth weirs became larger.

An Investigation on the Use of Venturi Weirs as an Aerator

The most classic example of a hydraulic structure where gas transfer occurs is a weir. Transfer of gases between the atmosphere and river water can occur in a free overfall jet from a weir. A free overfall jet from a weir plunging into downstream water causes entrainment of air bubbles if the free overfall jet velocity exceeds the critical value where aeration occurs. This paper investigates the free overfall jets from Venturi and rectangular notch weirs and their effect on air entrainment rate. A Venturi weir was placed at the upstream channel end in order to increase the flow velocity of the free overfall jet and, in turn, to increase air entrainment. It was demonstrated that the air entrainment rate of the Venturi weir is significantly better than the rectangular notch weir, and this advantage becomes more pronounced as the throat width of the Venturi weir is decreased. These results demonstrated that Venturi weirs can be used as highly effective aerators in streams, rivers, constructed channels, fish hatcheries, water treatment plants, etc. Moreover, a regression equation was obtained for the Venturi weirs, relating air entrainment rate to unit discharge, weir crest width, drop height and throat width of Venturi weir. There was good agreement between the measured air entrainment rates and the values computed from the predictive equation.

Role of Nozzles with Air Holes in Air Entrainment by a Water Jet

When a water jet impinges a pool of water at rest, air bubbles may be entrained and carried a distance below the pool's surface. This process is called plunging water jet entrainment and aeration. This paper describes an experimental study of the air entrainment rate of circular nozzles with and without air holes, and in particular, the effect of varying numbers and positions of air holes and distance between the location of the air holes and nozzle exit. A negative pressure occurred due to the air holes on the circular nozzles. This phenomenon affected air entrainment. The differences in air entrainment rate were related to changes in the jet shape. It was demonstrated that the air entrainment rates of the circular nozzles with air holes were significantly higher than those of circular nozzles without air holes.

The Effect of Nozzle Type on Air Entrainment by Plunging Water Jets

In this study, for the plunging water jet aeration system using various inclined nozzle types, bubble penetration depth, air entrainment rate, water jet expansion, effect of water jet circumference at impact point, oxygen transfer coefficient and oxygen transfer efficiency which changed depending on the water jet velocity, were researched in an air-water system. Numerous studies were conducted with circular nozzles. The present study describes new experiments performed with different nozzle types. Three types of nozzles were examined, i.e., those with circular, ellipse and rectangle duct with rounded ends. Experimental results showed that water jets produced with ellipse and rectangle duct with rounded ends nozzles have very different flow characteristics, entrainment patterns on free water jet surface, and submerged water jet region within the receiving tank. Higher air entrainment rate and oxygen transfer efficiency was observed in the rectangle duct with rounded ends nozzle due to water jet expansion. Bubble penetration depth, however, is lower for the rectangle duct with rounded ends nozzle than for the other nozzles. The ellipse nozzle provided the highest bubble penetration depth. These results showed that it is appropriate to use ellipse nozzle in aeration of deep pool and rectangle duct with rounded ends nozzle in the applications where high bubble concentration is desirable.