Air Entrainment Rates Research Articles

An Investigation on the Use of Venturi Weirs as an Aerator

Abstract 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

Abstract 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.

Some aspects of the mathematical modelling of fireballs

The development of large-scale fireballs is a typical result of explosions associated with accidental fuel releases in the oil and process industries. The two major mechanisms that lead to fireball formation are vapour cloud explosions and boiling liquid expanding vapour explosions (BLEVEs). An ability to model fireballs is extremely important from the point of view of safety and risk analysis. The present paper provides a state-of-the-art review of the computational techniques available for the mathematical modelling of fireballs. The emphasis is placed on computational fluid dynamics (CFD) modelling, based on Reynolds-averaged Navier—Stokes (RANS) equations for reactive flow. The results of calculations and estimations of fireball hazards are compared with available experimental data. New results are presented for problems that are usually given less attention in fireball studies. These concern estimations of air entrainment rates into fireballs and potential strategies for fireball suppression.

Monitoring air entrainment with breaking wave noise

It is now known that the dominant component of wind-driven oceanic noise comes from breaking waves. Bubbles ranging in size from tens of microns to centimeters are forced into the water column during the first second or so of whitecap formation. At the moment of creation, each bubble emits a pulse of sound at a center frequency inversely proportional to its radius. The ensemble of such events amounts to a burst of noise that continues throughout the active phase of bubble creation within the whitecap. As the noise spectrum is related to the bubble size distribution within the whitecap, it is natural to explore the possibility that underwater oceanic ambient noise could be used as tool to remotely monitor air entrainment rates across the ocean surface. One of the problems with developing such a tool is understanding the relationship between bubble formation processes occurring within whitecaps and the concurrent noise emission. Here we will report recent progress in our understanding of bubble formation mechanisms in whitecaps, their role in ambient noise generation, and the implications for monitoring air entrainment rates.

Effects of Liquid Property on Air Entrainment and Oxygen Transfer Rates of a Plunging Jet Reactor.

The effects of liquid property on air entrainment and oxygen transfer characteristics of a plunging jet reactor are investigated in air-corn syrup solution system with changing the viscosity up to 40.5 mPa.s. The effect of liquid property on air entrainment rate is divided into three regions. The transition points of the regions are represented with jet Reynolds number. The change in air entrainment rate with liquid property is discussed in relation to the change in jet surface disturbances. Empirical correlations are also presented to predict the air entrainment rate, gas holdup and oxygen transfer coefficient. The oxygen transfer performance of the reactor is superior to that of the conventional bubble columns and stirred tank reactors over the range of viscosity from about 1 to 40.5 mPa.s. It is concluded that the plunging jet reactor is one of the most promising bioreactor treating a viscous fermentation broth.

Detergents in drainage systems for buildings

Current soil–waste–vent (SWV) system design guides aim to produce ventilated drainage networks for buildings which protect attached water trap seals from unwanted air pressure effects. Such effects may cause trap seal failure that are caused by airflow induction during discharge. The ventilation design guides are based on steady-state experiments utilising cold clean water as a test media. However, most ‘grey’ and ‘black’ water sources are dosed with detergent, and are often warm, which significantly alters the behaviour when compared to clean water. Thus, current design standards provide only approximations of SWV system response. Results indicate that induced airflows in warm detergent-dosed water can be significantly higher than those measured in clean water systems, by a factor of more than 2. This paper quantifies the effect of detergents in SWV systems in terms of observed air entrainment rates and previously published research on the factors that contribute to air entrainment. Results of previous work are cast into dimensionless groupings suitable for inclusion into a mathematical simulation model based on a finite difference scheme. It utilises the method of characteristics as a solution technique to simulate drainage system operation via the equations that define unsteady partially filled or full bore pipe flows and the boundary conditions represented by water traps and other common system components.

Detergency in soil and vent pipe (SVP) systems

Current soil and vent pipe (SVP) system design guides aim to produce ventilated drainage networks, which protect attached water trap seals from unwanted air pressure effects that cause failure of trap seals. The ventilation design guides are based on steady-state experiments utilising cold clean water as a test medium. However, most 'grey' and 'black' water sources are dosed with detergent, and are often warm, which significantly alters the behaviour when compared to the use of clean water. Thus, current design standards provide only approximations of SVP system response. This paper quantifies the effect of detergents in SVP systems in terms of observed air entrainment rates and previously published research on the factors that contribute to air entrainment.

Concentration measurements in dense isothermal gas clouds with different starting conditions

Experimental results are presented for instantaneous finite-volume dense-gas releases in a channel of 20 m length. The gases used were argon and argon-air mixtures with density ratios relative to air varying from 1.15 to 1.35. The aspect ratio (height to length) of the initial volume was varied from 0.42 to 1.68 with gas volumes from 1.89 to 3.78 m 3. Measurements with 50 fast-response gas sensors (aspirated hot-wires) gave concentration signals with good resolution in time and space. The data obtained allow the air entrainment rates (both local and global) to be determined and give detailed information on the influence of the initial aspect ratio. Comparisons with theoretical solutions from the shallow-layer approximation (code SHALADIS) and with a dynamic integral model (adapted from Van Ulden, 1986, J. Haz. Mat. 16, 411–425) are also included. In view of the limitations of the models, the agreement was good for both.

Air entrainment at drops

This paper describes the results of a laboratory study of air entrainment below drops for a range of Drop numbers from 0.0007 to 0.0046. Characteristics of the plunging flow that occurs when the tailwaler depth yt is in the range of yt* . to h were studied where yt* . is the value of yt when a hydraulic jump is formed at the section where Ihe deflected stream below the drop becomes parallel to the bed and h is the height of the drop. Air entrainment below the drop was studied for the jump with yt = yt* as well as for the plunging flows and approximate expressions have been developed for their air entrainment rates. These results can contribute to the estimation of the reaeration of polluted streams below drops.

Measurements and prediction of air entrainment rates of pool fires

Motivated by the various applications of entrainment rate correlations in fire research and the large uncertainty in the efficacy of existing correlations and experimental data, the first particle imaging velocimetry (PIV)-based measurements of fire-induced flow field around pool fires burning methanol, heptane, and toluene were obtained. Air entrainment rates for 15-cm and 30-cm pool fires burning the three different fuels were calculated based on the mean velocity field. The entrainment data for the six fires could be correlated well using the fire Froude number as the nondimensional parameter. An existing kinematic approach to the prediction of the fire-induced flow field was extended to the present fires. The driving processes for the entrainment flow, namely, the volumetric heat release and the baroclinic vorticity generation, were evaluated based on correlations of buoyant diffusion flame structure in the literature. The predicted entrainment velocities were substantially higher than the measurements but were in qualitative agreement with the data. On this basis the heat release rate and vorticity correlations used in the analysis were corrected by using a smaller radius for the 1/c point in the velocity profile. The modified predictions were in better agreement with the experimental data. Therefore, further evaluation of the kinematic approach with proper heat release rate and vorticity distributions is warranted.

Validation Of Zone Models: Entrainment Rates And Convective Heat Losses From Model Atria Tests

Detailed experiments of smoke layer development in Atria revealed the need for refining the relationships for 1) air entrainment rates into low Froude number ( large diameter) pool fires, and 2) the convective heat transfer coefficient to the walls. Experiments were conducted in a square cross section enclosure 2 m x 2 m x 5 m high having adjustable size openings near the bottom ( for the fires air supply) and at the ceiling ( for smoke exhaust) (see Fig. 1). Steady state conditions were established prior to the measurements for methane pool fires produced by a 30 cm diameter sand box burner at two heat release rates, 10 kW and 20 kW, and various combinations of inlet and outlet vent sizes. Temperature measurements by a vertical thermocouple rack were used to determine the height of the smoke layer. Exhaust flow rates were precisely measured by using the tracer gas method, which in the present experiments included two gases: C02 and O2 (simultaneous CO measurements showed no detectable carbon monoxide levels, i.e., the combustion was complete). The pressure difference distribution and then neutral plane were determined by differential pressure transducers along the enclosure height. The redundancy and accuracy of the measurements in this project were essential for the development and estab1ishment of a new entrainment relationship for the buoyant plume of low Froude number (large diameter) pool fires the normalized entrainment rate varies proportionally to Z/D as it was proposed in a previous publication. Moreover, the convective heat transfer coefficient can be determined by using a constant Stanton number, while currently used correlations in zone models failed to reproduce the experimental results

Physical modeling of water sprays—comparison against field operations

AbstractThis paper discusses the procedure developed for simulating water sprays at a reduced scale (i.e., physical modeling) and, more impotantly, compares the model spray characterstics to corresponding full scale spray characteristics. An ideal physically modeled spray would simulate actual spray momentum, water volume flow rate, air entrainment rate, discharge water pressure, spray angle, spray pattern and drop size distribution. Full scale measurements or estimates of air discharge velocity, momentum, entrainment rate and drop size distributions were obtained for two types of sprays at two discharge pressures, various spray angles and at a 500 gpm flow rate. Physical modeling was conducted to simulate the full scale nozzles at a 1:50 scale reduction using commercially available spray nozzles. Measurements of spray momentum, volume flow rate and air entrainment rate were obtined for a subset of spray angles used in the full scale testing. Particle size distribution was not measured since it was considered of secondary importance in the simulation. The results of the study showed that model and full scale momentum, volume flow rate and air entrainment rates compared well with corresponding full scale observations (i.e., within 10 percent for momentum, 20 percent for volume flow rate and 25 percent for air entrainment rate). The results indicate that the method used for simulating water sprays is valid and that the model sprays adequately simulate the mechanical action of full scale sprays on vapor clouds. The results provide evidence that physical modeling can be used to evaluate the effectiveness of water sprays as a mitigation technique.

Numerical studies on the transient behaviour of a fire plume and ceiling jet

Transient behaviour of a thermally induced fire plume and ceiling jet is predicted using the field modelling technique. In the field model, the k-ε model is used to simulate the turbulent air flow. The traditional wall-function approach is applied to describe the solid-wall boundary effect. The building concerned is composed of a floor and a ceiling, but is without walls. The height is varied from 3 m to 9 m. Transient behaviour of air temperature, velocities, smoke concentration, etc., are predicted. The air entrainment rates to the fire plume and the entrainment constants are also calculated and compared with empirical expressions.

Air entrainment by plunging jets

This paper describes a theoretical and experimental study of the mechanisms by which plunging jets entrain air. Following other workers, it is proposed that the mechanism of air entrainment undergoes a qualitative change as the jet impact velocity ( U i ) is increased. Separate models are presented which predict air entrainment rates ( q a ) in the low- and high-velocity regimes. Both the theory and experimental results suggest that, at low velocities, q a ∼ U i 3, while at high velocities, q a ∼ U i 3 2 . Quantitative estimates of q a on high-velocity jets are found to be in good agreement with experimental results. Several reasons are proposed, based on both theory and experiment, for the scale effects that are observed on small-scale air-entraining flows.

Air entrainment into buoyant jet flames and pool fires

A simple model for the fire dynamics of turbulent fires is proposed and then used to derive correlations for air entrainment into turbulent jet fires and pool fires. Five parameters are identified to control the fire dynamics: the pool diameter D, the total buoyancy flow B ∞, a buoyancy force per unit mass at the adiabatic stoichiometric temperature ( ΔT ad T ∞ )g , the ambient density ϱ ∞, and the stoichiometric mass air to fuel ratio S. These parameters lead by dimensional arguments to correlations for air entrainment rates, which are further refined by using a simplified physical model. Three distinct regimes for air entrainment are identified in turbulent pool fires: (a) close to pool surface where entrainment rates vary as Z 1 2 (where Z is the vertical distance from the source), (b) near the neck-in area where entrainment rates vary as Z 3 2 , and (c) farther downstream but below the flame tip where entrainment rates vary as Z 5 2 . The proposed correlations are used to plot experimental data from which proportionality coefficients are determined. It is shown, also, that there is a relationship between entrainment rates up to the flame tip and visible flame feights; such a relationship would allow one to estimate entrainment rates by measuring visible flame heights.

Entrainment measurements in turbulent buoyant jet flames and implications for modeling

Air entrainment rates were measured along turbulent buoyant jet flames having heat release rates up to 45 kW and nozzle diameters 12 mm and 23 mm. We used the Ricou-Spalding measurement technique which involves supplying just enough ambient air to an enclosure around the flame to produce zero pressure drop across an exit orifice plate. The experimental enclosure, the orifice plate and the pressure lines were water-cooled because of the large radiative output from the fires. The entrainment rates at various positions in the turbulent flame for the larger nozzle (23 mm dia) were independent of the total heat release rate (20 kW, 31 kW and 45 kW) and increased with the 5/2 power of the distance from the nozzle exit. Similar behavior was observed for the smaller nozzle but with a virtual origin some distance above the nozzle exit. Implications of the present results for mathematical modeling of turbulent burning are discussed in conjunction with the heat release rates along turbulent buoyant jet flames.

DDT residues in crop and soil resulting from application to cowpea Vigna unguiculata (L.) walp. in the sub-humid tropics

DDT emulsion was applied by knapsack sprayer to cowpea Vigna unguiculata (L.) at a rate of 10kg a.i. ha −1 in each of two growing seasons per year for a four-year period in Ibadan, Nigeria. Partition of spray deposits between plant and soil was assessed at 1:3 respectively by methods involving crop cover estimation and dye recovery. DDT residues in soil and crop were determined by GLC analysis at the end of each growing season and showed disappearance of at least 60% of the applied pesticide. Field measurements were supplemented by studies on air entrainment and volatilisation rates from crop and soil. Accumulation of DDT in soil and its redistribution by ploughing were monitored. Residue levels reached a maximum of 1·7 μg g −1 total DDT after three spraying seasons and declined thereafter, despite repeated applications. In plots treated with a total of 70 kg a.i. ha −1 over the four years, the final residue in soil represented only 2–3% of the amount applied.