Preface

This book is intended to serve as a reference text for presenting a broad range of topics on fluid dynamics to advanced scientists and researchers. The chapters have been contributed by the prominent specialists in the field of fluid dynamics cover experimental and numerical fluid dynamics, aeroacoustics, multiphase flow analysis, convective instability, combustion, and turbulence modeling.

> **Dr. Hyoung Woo Oh** Department of Mechanical Engineering, Chungju National University, Chungju, Korea

**1** 

*Mexico* 

**An Experimental and Computational Study of** 

**the Fluid Dynamics of Dense Cooling Air-Mists** 

Jesús I. Minchaca M., A. Humberto Castillejos E.\* and F. Andrés Acosta G.

Spray cooling of a hot body takes place when a dispersion of fine droplets impinges upon its surface to remove a large amount of heat by evaporation and convection (Deb & Yao, 1989). In metallurgical processes such as continuous casting of steel (Camporredondo et al., 2004) the surface temperature, Tw, of the hot steel strand exceeds considerably the saturation temperature, Ts, of the cooling liquid (water), i.e., Tw-Ts ranges between ~600 to 1100°C. These harsh temperature conditions have traditionally called for the use of high water impact fluxes (w, L/m2s) to remove the heat arriving to the surface as a result of the solidification of the liquid or semi-liquid core of the strand. The boundary between dilute and dense sprays has been specified at w= 2 L/m2s (Deb & Yao, 1989, Sozbir et al., 2003). In modern continuous casting machines the w found are well above this value. Most of the impingement area of the spray or mist jets will have w 10 L/m2s, with regions where w can be as large as ~110 L/m2s. Heat treatment of alloys requiring the rapid removal of large

amounts of heat also makes use of dense sprays or mists (Totten & Bates, 1993).

Sprays and air-mists are dispersions of drops produced by single-fluid (e.g., water) and twin-fluid (e.g. water-air) nozzles, respectively. In sprays, the energy to fragment the water into drops is provided by the pressure drop generated across the narrow exit orifice, while in air-mists nozzles a high speed air-stream breaks the water-stream generating fine, fastmoving droplets (Lefebvre, 1989; Nasr et al., 2002). In air-mist nozzles with internal mixing and perpendicular inlets for the fluids, as those shown in Fig. 1, the water splatters against a deflector surface and the resulting splashes are further split by the shear forces exerted by the axial air-stream, which also accelerates the drops as they move along the mixing chamber toward the exit port. Thus, the liquid emerges in the form of drops with different sizes and velocities and with a non-uniform spatial distribution (Hernández et al., 2008). In addition to w, the size, dd, and velocity, u, of the drops in dense air-mists play a crucial role in the cooling of highly superheated surfaces (Bendig et al., 1995; Jenkins et al., 1991; Hernández et al., 2011). This behavior stresses the important relationship between the heat transfer process and the droplet impact or deformation and break-up behavior. Since, for a specified fluid those two parameters, dd and u, determine the local impingement Weber number (Wezs= duzs2dd/), which in general has been agreed to characterize the impact behavior (Wachters & Westerling, 1966; Araki & Moriyama, 1981; Issa & Yao, 2005). As the

**1. Introduction**

\*

Corresponding Author

*Centre for Research and Advanced Studies – CINVESTAV, Unidad Saltillo*
