N. Sozbir 1 Y. W. Chang S. C. Yao e-mail: scyao@cmu.edu Department of Mechanical Engineering, Carnegie Mellon University, Pittsburgh, PA 15213 Heat Transfer of Impacting Water Mist on High Temperature Metal Surfaces Experimental studies were conducted to reveal the heat transfer mechanism of impacting water mist on high temperature metal surfaces. Local heat transfer coefficients were measured in the film-boiling regime at various air velocities and liquid mass fluxes. The test conditions of water mist cover the variations of air velocity from 0 to 50.3 m/s, liquid mass flux from 0 to 7.67 kg/m 2 s, and surface temperature of stainless steel between 525°C and 500°C. Radial heat transfer distributions were measured at different liquid mass fluxes. The tests revealed that the radial variation of heat transfer coefficients of water mist has a similar trend to the air jet cooling. At the stagnation point, heat transfer coefficient increases with both the air velocity and the liquid mass flux. The convective air heat transfer is consistent with the published correlation in the literature. The heat trans- fer contribution due to the presence of water increases almost linearly with the liquid mass flux. The total heat transfer coefficient can be established as two separable effects, which is the summation of the heat transfer coefficient of air and of liquid mass flux, respectively. This study shows that with a small amount of water added in the impacting air jet, the heat transfer is dramatically increased. The Leidenfrost temperature under water mist cooling was also measured. The Leidenfrost temperature increased with both the air velocity and the liquid mass flux. @DOI: 10.1115/1.1527913# Keywords: Boiling, Cooling, Droplet, Heat Transfer, Sprays Introduction Traditionally water sprays have been used extensively in met- allurgical industries for the cooling of high temperature metals at film boiling. Sprays are made of large drops, in the order of 100 mm or larger, and operated at high mass flux to give high heat transfer rates. However, the heat transfer distribution frequently is not uniform and water utilization efficiency is low. Recently, more and more industrial processes use water mist for cooling. Water mist contains small droplets, in the order of or less than 100 mm, and carries co-flowing air which are entrained or are used for atomization. The mass flux of water mist is low and therefore, the heat transfer rate is not as high as sprays; however, the water mist provides relatively uniform heat transfer and has high water usage efficiency. Furthermore, both water and air flow rates can be adjusted to provide a wide range of heat transfer variations. Due to these advantages, water mists are used prefer- ably for cooling of thin metal sheets and for tempering of glass at high temperature film boiling conditions. Water mists have also been proposed for the cooling of electronics at nucleate boiling conditions. At the film boiling regime, the impacting droplets of water mist will contact with surface for a very short period of time; however, the resulting heat transfer is significant. The contact heat transfer mechanisms include the convection in the layer of vapor under- neath the droplets and transient homogeneous nucleation at the point of droplet-surface contact. In additional to the droplet con- tact heat transfer, there is radiation from the surface and convec- tion to the flowing air on the surfaces. Due to the droplet-surface contacts, material properties and roughness also influence the heat transfer. When water mist cooling is applied to surfaces at low tempera- tures, nucleate boiling occurs. Many researchers have studied mist heat transfer at this condition. Typically, the recent ones are Gra- ham and Ramadhyani @1#, Lee et al. @2# and Yang et al. @3#. How- ever, studies of mist impingement on surfaces at high temperature film boiling regime are limited. Pedersen @4# studied water drop- lets impinging upon a heated surface at 1800°F. His data of 200 to 400 mm droplets showed that the approaching velocity is the ma- jor parameter affecting droplet heat transfer and that surface tem- perature has minor effect on heat transfer. Choi and Yao @5# in- vestigated an impacting spray experimentally. An impulse-jet liquid spray system and a solid particle spray system were com- pared. The effects of air convective were revealed. Nishio and Kim @6# reported the heat transfer of dilute spray impinging on hot surfaces. The report focused on the effects of the rebound motion and sensible heat of droplets on heat transfer in the high tempera- ture region. A simple model was developed to predict the heat flux distribution of a dilute spray impinging on a hot surface. They assumed that the droplet number flow rate of the spray is small that the heat transfer of each droplet is independent. Ideally, for the impaction cooling of water mist at high surface temperature, the heat transfer contribution of air and water could be separable and independent. An attempt was made by Deb and Yao @7#, who modeled the spray cooling by considering droplet impingement heat transfer and air convective heat transfer sepa- rately. The overall heat transfer is considered as the summation of these two contributions in addition to radiation cooling. However, no experimental verification was reported in terms of parametric study of these factors to validate this idea directly. In the present study, experiments were conducted on stainless steel disks at the high temperatures to verify these separable effects. Local heat transfer coefficients were measured in film boiling regime. The air flow and water flow are controlled independently and the opera- tional conditions included pure air and water mist at different liquid mass fluxes. As a result, the air and water effects are re- vealed independently and parametrically. The Leidenfrost tem- perature was obtained at various air velocities and liquid mass fluxes. Sozbir and Yao @8# investigated a water mist cooling for glass tempering experimentally. Very high velocity air jet im- pingement was applied during the cooling process of glass tem- pering. The heat transfer of multiple water mist jets on glass was 1 Visiting Assistant Professor from Sakarya University, Turkey Contributed by the Heat Transfer Division for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received by the Heat Transfer Division October 17, 2002; revision received September 3, 2002. Associate Editor: C. T. Avedisian. 70 Õ Vol. 125, FEBRUARY 2003 Copyright © 2003 by ASME Transactions of the ASME