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Air Entrainment

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Fei Tang – 1st expert on this subject based on the ideXlab platform

  • fire induced temperature distribution beneath ceiling and Air Entrainment coefficient characteristics in a tunnel with point extraction system
    International Journal of Thermal Sciences, 2018
    Co-Authors: Fei Tang, Qing He, Lei Chen, Kaihua Lu

    Abstract:

    Abstract This study investigates the effect of a point extraction system on Air Entrainment characteristics of a one-dimensional smoke movement stage in a ventilation tunnel under various ceiling extraction velocities. Reduced scale model tunnel experiments are conducted. The vertical smoke layer temperature is recorded, and the variation characteristics of smoke layer thickness are calculated by an integral ratio method. Results show that the Air Entrainment coefficient increases with fire heat release rates for a given ceiling extraction velocity, but decreases with increasing ceiling extraction velocities for a given heat release rate. A dimensionless factor f ( V ) is introduced and the revised model of Air Entrainment coefficient under various ceiling extraction velocities is proposed. The Entrainment coefficient for smoke flows in tunnel fires is found to be a function of the Richardson number. All the experimental data are well correlated by the new proposed model for the Air Entrainment coefficient with a single-point extraction system. This study proposes new empirical Entrainment design formulae of tunnel fires under the effect of point ceiling extraction system.

  • longitudinal distributions of co concentration and temperature in buoyant tunnel fire smoke flow in a reduced pressure atmosphere with lower Air Entrainment at high altitude
    International Journal of Heat and Mass Transfer, 2014
    Co-Authors: Fei Tang, Longhua Hu, Lizhong Yang, Xiaochun Zhang

    Abstract:

    Abstract Smoke temperature and CO (carbon monoxide) concentration are two most important parameters concerning human safety in case of a tunnel fire. Their longitudinal distributions in the smoke flow along the tunnel are both closely related to fresh Air Entrainment from the surroundings; meanwhile heat loss process also contributes to the temperature decay but not to the CO concentration dilution at the same time. However, previous researches are all considering in default the condition with normal pressure, which is needed to be extended for condition at reduced pressure atmosphere such as at high altitude (for example, Tibet). This paper reports new findings for the distributions of these two parameters in a reduced pressure atmosphere with lower Air density and thus lower Air Entrainment. The longitudinal distributions of smoke flow temperature and CO concentration for a tunnel fire near sea level (1 atm) and at high altitude (0.64 atm) have been correspondingly computed and compared by Fire Dynamics Simulator (FDS). It is found that the longitudinal decay profiles of CO concentration are similar in these two pressures, as both the Air Entrainment mass flow rate during the smoke flow traveling (contributing to the dilution) and the Air Entrainment of the fire plume (dominating the initial mass flow rate of the smoke flow) are proportional to ambient pressure thus their ratio is independent of pressure. However, the longitudinal decay of the smoke flow temperature is faster with distance along the tunnel in the reduced pressure atmosphere, as the Air Entrainment of the fire plume (dominating the initial mass flow rate of the smoke flow) is lower in the reduced pressure atmosphere, meanwhile the heat loss term is independent of pressure giving their ratio (heat loss to initial mass flow rate) is larger in the reduce pressure. Therefore, the difference between normalized longitudinal profiles of CO concentration and smoke temperature in a tunnel fire is larger, as indicated by a higher λ coefficient value, in the reduced pressure atmosphere at higher altitude than that in the normal pressure atmosphere, although their values of λ for both these two atmospheric pressure can be well correlated by a reciprocal function with longitudinal Air flow speed.

Kelli Hendrickson – 2nd expert on this subject based on the ideXlab platform

  • wake behind a three dimensional dry transom stern part 1 flow structure and large scale Air Entrainment
    Journal of Fluid Mechanics, 2019
    Co-Authors: Kelli Hendrickson, Gabriel Weymouth, Xiangming Yu

    Abstract:

    We present high-resolution implicit large eddy simulation (iLES) of the turbulent Air-entraining flow in the wake of three-dimensional rectangular dry transom sterns with varying speeds and half-beam-to-draft ratios $B/D$
    . We employ two-phase (Air/water), time-dependent simulations utilizing conservative volume-of-fluid (cVOF) and boundary data immersion (BDIM) methods to obtain the flow structure and large-scale Air Entrainment in the wake. We confirm that the convergent-corner-wave region that forms immediately aft of the stern wake is ballistic, thus predictable only by the speed and (rectangular) geometry of the ship. We show that the flow structure in the Air–water mixed region contains a shear layer with a streamwise jet and secondary vortex structures due to the presence of the quasi-steady, three-dimensional breaking waves. We apply a Lagrangian cavity identification technique to quantify the Air Entrainment in the wake and show that the strongest Entrainment is where wave breaking occurs. We identify an inverse dependence of the maximum average void fraction and total volume entrained with $B/D$
    . We determine that the average surface Entrainment rate initially peaks at a location that scales with draft Froude number and that the normalized average Air cavity density spectrum has a consistent value providing there is active Air Entrainment. A small parametric study of the rectangular geometry and stern speed establishes and confirms the scaling of the interface characteristics with draft Froude number and geometry. In Part 2 (Hendrikson & Yue, J. Fluid Mech. , vol. 875, 2019, pp. 884–913) we examine the incompressible highly variable density turbulence characteristics and turbulence closure modelling.

  • Air Entrainment and multiphase turbulence in the bubbly wake of a transom stern
    International shipbuilding progress, 2013
    Co-Authors: Kelli Hendrickson, Gabriel Weymouth, Sankha Banerjee

    Abstract:

    Accurate prediction of the highly-mixed flow in the near field of a surface ship is a challenging and active research topic in Computational Ship Hydrodynamics. The disparity in the time and length scales and the scales of Entrainment dictates the use of bubble source and mixed-phase flow models in which the current state of the art models are ad hoc. This paper presents the Air Entrainment characteristics and multiphase turbulence modeling of the near-field flow of a canonical stern with the inclusion of simple geometry effects. Using state of the art Cartesian-grid numerical methods with the full field equations, high-resolution two-phase flow data sets of a canonical stern with three different half-beam to draft ratios are simulated down to the scales of bubble Entrainment. These data sets are used as the foundation for: (1) characterization of wake structure and near-wake Air Entrainment of the stern; (2) analysis of turbulent mass flux in the wake of the stern; and (3) a priori testing of multiphase turbulence models for turbulent mass flux. Results are obtained to show that these techniques enable analysis and physics-based parameterization of near-field Air Entrainment about surface ships for use in Computational Ship Hydrodynamics.

Suzanne M Kresta – 3rd expert on this subject based on the ideXlab platform

  • Air Entrainment in baffled stirred tanks
    Chemical Engineering Research & Design, 2007
    Co-Authors: S Bhattacharya, D Hebert, Suzanne M Kresta

    Abstract:

    Abstract The impeller speed at which Air is first entrained from the surface of a stirred tank ( N E ) is an operational limit. Where Air Entrainment is desirable, it is a lower limit, but where Air Entrainment is detrimental it is an upper limit. This study (1) determines parameters which affect N E and (2) develops a mechanistic model of Air Entrainment. Experiments were conducted to determine the effect of impeller submergence, impeller diameter, baffle geometry, and the physical properties of the fluid on N E for an up-pumping (PBTU), and a down-pumping pitched blade turbine (PBTD). Mean and RMS velocity profiles were measured for selected cases using laser Doppler velocimetry (LDV). Using this data, Air Entrainment in stirred tanks and at other free surfaces is compared and is found to depend on the balance between gravity, surface tension and surface turbulence. There must be sufficient turbulence at the surface to overcome surface tension and form bubbles. The entrained bubble size is determined by the mean flow below the surface, which acts to pull the bubbles into the tank. It is shown that impeller variables, such as the power number, impeller speed and diameter, cannot predict the point of Air Entrainment at the surface. The key predicting variable is the ratio of u , the RMS velocity at the surface, to the mean downward velocity U . At the point of Air Entrainment, this velocity ratio just balances the physical properties of the fluid.