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.

Analysis of evaporating mist flow for enhanced convective heat transfer

“…In the present work, the transport process is predominantly diffusive, and the second terms on the right hand side of Eqs. (11) and (12) have only a small contribution. The mass, momentum and energy source/sink terms appearing in Eqs.…”

“…For instance, liquid spray cooling has received significant research attention as a high-heat-flux thermal management technology [2][3][4][5][6][7]. Mist impingement cooling is a variant of spray cooling wherein a two-phase mixture of finely dispersed liquid droplets in air is sprayed onto a hot surface [8][9][10][11][12][13][14]. It should be noted that the high heat-dissipation capacity of spray cooling-based technologies is primarily a result of direct contact of the cooling fluid with the hot surface and subsequent evaporation of the liquid at the surface.…”

Heat transfer of air-water dispersed flow in a vertical pipe

“…They found that the heat transfer rate was high in the region covered with the liquid film because of enhancement by evaporation heat transfer. Lee and others [2] studied air-water dispersed flow in a 41 × 41 mm 2 duct with a heating surface, by conducting LDA measurements for size distributions of water droplets and reported that a high cooling effect was achieved in the region where a very thin liquid film formed. On the other hand, Mastanaiah and GaniB [3] introduced an air-water dispersed flow into a heating circular pipe 12.95 mm in diameter.…”

A theoretical study of evaporative heat transfer in high‐velocity two‐phase flow of air–water in a small vertical tube

“…In addition, a few studies focused on the heat transfer for the annular or mist flow of air-water in a vertical passage without boiling phenomena. Lee [6] investigated the heat transfer characteristics of the mist flow of air-water in a rectangular channel (41 mm × 41 mm) and found that a very thin liquid film was attached consistently to the heated wall, and the liquid film greatly enhanced the heat transfer. Kitagawa [7] studied the heat transfer characteristics of the mist flow of air-water in a vertical round tube with an inner diameter of 11 mm.…”