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Impact of Water on Fire and Smoke Dynamics
Published in Bart Merci, Tarek Beji, Fluid Mechanics Aspects of Fire and Smoke Dynamics in Enclosures, 2023
An interesting recent study, presenting CFD results and experimental data on this phenomenon of air entrainment for water mist type nozzles, is Vaari et al. [21]. Whereas model improvement is still possible, the trends are well captured. The spray is divided into two regimes: a ‘momentum’ regime, close to the nozzle (the first 40 cm in Vaari et al. [21]), and a ‘gravitation’ regime, where the droplets evolve towards their equilibrium drop velocity (see Section 7.1.1). In the momentum regime, the spray widens and the droplets, injected with high velocity, break up into smaller droplets, due to the high Weber number (see Section 7.2.3). The air currents, induced by the water flow from the nozzles, drag the smaller droplets into the spray core, thereby increasing the drop density inside that region. Indeed, smaller droplets have a lower Stokes number, Eq. (7.13), and thus follow the air currents more easily than the larger droplets. As an order of magnitude, induced velocities of around 10 m/s are reported in Vaari et al. [21], with droplet inlet velocities in the order of 110 m/s.
Heat Transport Limits
Published in Calvin C. Silverstein, Design and Technology of Heat Pipes for Cooling and Heat Exchange, 2020
We now consider a circular flow duct, with vapor flowing through the center of the duct and liquid flowing along the duct periphery. The Weber number We is defined as the ratio of the vapor inertia force acting over the vapor space cross section (i.e., the change in the momentum flow rate as the vapor velocity increases from 0 to Vv) to the liquid surface tension force acting over the circumference of the vapor space. Thus, We=ρvVv2(π/4)Dv2σπDv=ρvVv2Dv4σ
Utilization of Biofuels in Compression-Ignition Engines
Published in K.A. Subramanian, Biofueled Reciprocating Internal Combustion Engines, 2017
The Weber number is a measure of the relation between the kinetic energy of the droplet and its surface energy. A very large Weber number means that the influence of the surface tension can be neglected in the droplet impingement process. The Weber number can be used for defining the probability of wall impingement (Wachters and Westerling, 1966). Whether the wall liquid wets the surface depends on the wall temperature, wall material, roughness, and other factors (Wachters and Westerling, 1966). The wall impingement model developed by Naber and Reitz (1988) (based on sticking, reflecting, and sliding) is used for multidimensional engine simulation. Wall impingement may result in a rich localized mixture and hence higher unburned hydrocarbon emission (Miers et al., 2005). The above discussion is based on droplet impingement on a hot surface plate. Subhash et al. reported that the probability of wall impingement with biodiesel is higher due to an increase in injection pressure (Lahane and Subramanian, 2012).
Numerical investigation of droplet-droplet collisions in a water and milk spray with coupled heat and mass transfer
Published in Drying Technology, 2020
Giulia Finotello, Johan T. Padding, Kay A. Buist, Annelien Schijve, Alfred Jongsma, Fredrik Innings, J. A. M. Kuipers
Once a collision pair is detected, the outcome of the binary collision needs to be predicted. Phenomenological models for the collision outcome are usually expressed in terms of Weber number, Ohnesorge number, impact parameter and size ratio: where ρd is the droplet density, ds and dl are the diameters of the smallest and largest droplet in the pair, respectively. σ is the surface tension and μd is the droplet fluid viscosity. The Weber number is the ratio between inertia forces and surface tension. To account for viscosity the Ohnesorge number is used, which represents the ratio of viscous forces and the combined effect of inertial forces and surface tension. The impact parameter B is defined, before the moment of impact, as the distance b between the two droplet centers in the plane perpendicular to the relative velocity vector, normalized by the average droplet diameter. When B is equal to 0 we are dealing with a head-on collision and when it is 1 a grazing collision.
Natural sources and experimental generation of bioaerosols: Challenges and perspectives
Published in Aerosol Science and Technology, 2020
Malin Alsved, Lydia Bourouiba, Caroline Duchaine, Jakob Löndahl, Linsey C. Marr, Simon T. Parker, Aaron J. Prussin, Richard J. Thomas
Many natural sources of bioaerosol arise from wet environments, as described in the previous section, and these are replicated in the laboratory by fragmentation of liquids. Fragmentation is the breakup of bulk fluid into droplets which occurs when forces imposed on the system overcome surface tension forces that tend to minimize creation of new surface area. The Weber number (We) is the non-dimensional number that quantifies competition between kinetic energy and surface energy, defined as We = (ρv2L)/σ, linking fluid density (ρ), speed (v), length-scale (L), and surface tension (σ). When We is high, creation of new surface in the form of fragmentation of a bulk fluid into droplets is possible (Lefebvre and McDonnell 2017; Bourouiba and Bush 2013). Fragmentation is induced by (i) impacts, transforming a bulk fluid into a sheet, then ligaments, and then droplets via a series of surface-tension dominated interfacial instabilities and processes (Wang and Bourouiba 2017, 2018b; Eggers and Villermaux 2008); (ii) shearing, from an airflow or one fluid moving faster over the interface of another, leading to classical hydrodynamic instabilities (i.e., Kelvin-Helmholtz), resulting in ligament, and then droplet formation (Eggers and Villermaux 2008); (iii) or bubble bursting, leading to the creation of secondary droplets, for example, from film rupture and destabilization into ligaments, and then droplets (Walls, Bird, and Bourouiba 2014; Poulain and Bourouiba 2018).
Numerical study on dynamic characteristics of double droplets impacting a super-hydrophobic tube with different impact velocities
Published in International Journal of Computational Fluid Dynamics, 2019
Han Chen, Xiaohua Liu, Kaimin Wang, Hongsheng Liu, Shengqiang Shen
The change of dimensionless spreading diameter at different Weber number was presented in Figure 15. In the figure, these black solid pointed present the impact moment of trailing droplet. From the above description, the impact model was in-phase when Weber number was not greater than 42.8, and out-of-phase when Weber number was not less than 56.9. In the case of the out-of-phase impact, the contact time of the liquid film with the super-hydrophobic tube was generally greater than that of the in-phase impact. When the Weber number was 6.9, the dimensionless spreading diameter moved smoothly, for the extrusion between the trailing droplet and the leading film was carried out in the air. In addition, other curves experienced multiple fluctuations over time due to the influence of trailing droplet impact on the movement of the coalescent liquid film, such as the breakage of the finger film, the development of the liquid crown. As the Weber number increased, the maximum dimensionless spreading diameter increased first and then decreased. This was because when the Weber number was 61.7 and 109.7, the liquid film had large kinetic energy, and the edge of the liquid film exceeded the three-phase contact line, and the degree of contact with the tube surface was rather reduced.