Cooling of a Heated Surface by Mist Flow

Abstract

Cooling requirements in modern industrial applications, such as the removal of heat from electronic equipments, often demand the simultaneous attainment of a high heat flux and a low and relatively uniform and steady temperature of the heated surface to be cooled. The conventional single-phase convection cooling obviously cannot be expected to function adequately, since the heat flux there is directly proportional to the temperature difference between the heated surface and the surrounding medium. To maintain a high heat flux, the temperature of the heated surface usually must be kept at a high level. An attractive alternative is cooling by a spray, which takes advantage of the significant latent heat of evaporation of the liquid. However, in conventional industrial spray coolings, such as in the case of the cooling tower of a power plant, the temperature of the heated surface usually remains relatively high and is nonuniform and unsteady containing numerous flashy hot spots. In order to optimize the performance of the spray cooling of a heated surface by a mist flow, a clear understanding is required of (1) the dynamic interaction between the droplets and the carrier fluid and (2) the thermal reception of the droplets at the heated surface. It is the dynamic interaction between the phases that is causing the droplets to deposit onto the heated surface. The thermal reception at the heated wall develops mass and heat transfer leading to the mode of cooling of the heated surface. In the present study, an experimental investigation was made of the combination of the dynamic depositional behavior of droplets in a water droplet-air mist flow with the use of a specially designed particle-sizing two-dimensional laser-Doppler anemometer. Also, the heat transfer characteristics at the heated surface were investigated in relation to droplet deposition on the heated surface for wide ranges of droplet size, droplet concentration, mist flow velocity, and heat flux. It was discovered that over a certain suitable range of combination of these parameters, a superbly effective cooling scheme could be established by the evaporation on the outside surface of an ultrathin liquid film. Such a film was formed on the heated surface by the continuous deposition of fine droplets from the mist flow. Under these conditions, the heat flux is primarily related to the evaporation of the ultrathin liquid film on the heated surface and thus depends less on the temperature difference between the heated surf ace and the ambient mist flow. The heated surface is quenched to a low, relatively uniform and steady temperature at a very high level of heat flux. Heat transfer enhancement as high as seven times has been found so far. This effective heat transfer scheme is here termed mist cooling.