Le refroidissement par pulvérisation

Cooling by atomization

¹ Société Bertin et Co. B.P. 3, F. 78370 Plaisir
² Université de Louvain 2, Place du Levant B-1348 Louvain-La-Neuve (Belgique)

Abstract

1. Introduction The disintegration of a liquid mass into several small droplets is generally called atomization. This process strongly increases the interfacial area between the liquid and the medium into which it penetrates. It can thus intensify the physical or chemical transfers occurring at the interface. This advantage is used in many industrial applications, among others for controlled cooling of a hot wall. Typical examples are the cooling of steel strips in hot rolling mills and of slabs in continuous casting. With water atomization, it is possible to evacuate from the wall a large amount of calorific power : from 10⁵ to 10⁷ W/m² (fig. 1)*. One can distinguish two main types of atomization nozzles : pressure nozzles and pneumatic nozzles. In the first type a single liquid (water) is forced to pass through a small orifice. The pressure difference upstream and downstream from the orifice causes disintegration of the liquid. In the second type two fluids are used : water and air. Besides the pressure difference, a turbulence of the air is produced which causes a dispersion of the water in to tiny droplets. 2. Correlations of heat transfer obtained with pressure nozzles Over the last twelve years various experimental results of heat exchange between a hot metal wall and pressure atomization have been published. Figure 2 shows some of these. Various correlations are reviewed in the full text. From the analysis of these measurements, it can be concluded that : a) the water flow-rate density, ml, is the most important parameter, b) the heat flux density qp varies as m_l^{0,5 ...0,7} so that the advantage of increasing the flow-rate density of water is limited. Such global correlation are useful in order to design cooling units. However, they do not permit to explain the physical phenomena. For a more fundamental approach one needs insight into the microscopic characteristics of atomization : diameters and velocities of atomized drops as well as spatial concentration. The determination of these characteristics can only be achieved with the help of rather sophisticated measuring techniques such as photomicrography or laser velocimetry. It is then possible to obtain for each atomization a distribution of drop diameters and of drop velocities. 3. Theoretical model of heat transfer Most atomized drops impinging on a hot wall break up into a large number of smaller droplets. This disintegration occurs when the Weber number of the drop is larger than 80. High speed films have been shot in order to follow all the stages of this disintegration. Shortly after impact a continuous liquid film surrounds a central zone which is dome-shaped. This dome disappears after a time given by the ratio of drop diameter and velocity. The remaining film becomes unstable and breaks up into smaller droplets. The time evolution of dome diameter and of film diameter have been correlated. The theoretical model we have set up is based on this behaviour and is divided into two steps : first the evaluation of the calorific energy extracted by a single drop of known diameter and velocity, then, the calculation of the effect of all the drops of the atomization. It takes into account three modes of heat transfer : a) by direct contact between the wall and the liquid under a central zone of the dome of each drop b) by conduction through a vapour film under the periphery of the liquid film c) by heat radiation between wall and environment. The expression of the theoretical heat flux density explicitly contains the diameters of the atomized drops, their velocities and the numbers of drops impinging per unit time on a unit surface area of wall. From the model it appears that the drops which extract the greater amount of heat are neither the smallest nor the largest ones but those with a diameter ranging from 200 to 400 microns. 4. Increased heat transfer for an air/atomized water mixture 4.1. The problem Research by Moureau has shown the atomized water flow density to be the essential parameter to consider for cooling of very hot surfaces. In this type of system, the quantity of atomized water is small, i.e. cooling effectiveness is low. A few experiments have shown use of a two-phase mixture of air and atomized water to result in an appreciable increase in heat transfer (Figs. 9 and 10). Use of compressed air or low-pressure air produces very similar results. We have therefore made a systematic study of heat exchange taking place between a mixture of air and atomized water and a very hot surface. 4.2. Test results The tests were carried out with circular and phase cooling nozzles. The main findings were as follows : - The water flow-rate density and distance from the nozzle to the cooled surface are the important parameters to consider at the point of impact of the two-phase mixture (Fig. 11) ; air velocity and mean droplet diameter have little effect (relevant relationships are stated in the report) ; for a given flow-rate density, twice as much heat is transferred as with atomized water alone. - Maximum heat transfer takes place in the droplet impact area ; outside this area, heat transfer is considerably reduced and does not depend on the water flow-rate ; amounts of heat exchanged, however, are substantially in excess of those associated with air alone (Fig. 12). Where several nozzles are arranged side-by-side, heat exchange in the droplet impact area is closely dependent on nozzle spacing and the distance from the nozzles to the cooled surface (Fig. 13). 4.3. Heat-transfer mechanism The heat-exchange mechanism is the same for both two-phase air and atomized-water mixtures and atomized water alone. Heat is transferred during impingement of the droplets, which, therefore, should then be evacuated as quickly as possible. Thus, by its mechanical droplet-entrainment action, the air helps to increase heat-transfer. Droplet paths should not coincide with the air flow streamlines. The air jet discharging from the nozzle, therefore, should remain aerodynamically undisturbed. 4.4. Cooling system dimensions To ensure satisfactory thermal performance the width of the air-jet impact area on the cooled surface should be less than half the cooling-nozzle spacing. The geometrical dimensions of the cooling system are interdependent by simple relationships (see report). 5. Conclusions Atomized-water cooling of very hot surfaces is a simple, highly-flexible solution. Uniform heat flux is achievable over large areas by use of atomized water alone. Higher heat flux is achievable with air and atomized-water mixtures, but at the cost of uniformity. In either process, heat transfer takes place on impact of the droplets against the cooled surface. (*) See figures in the French text.