TED-AJ03-184 SPRAY COOLING OF DOWNWARD FACING NICKEL BODY WITH WATER : DEPENDENCE ON VOLUME FLOW DENSITY OF WATER

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Spray cooling, mist cooling or fog cooling has been widely applied because that the cooling performance can easily be varied in a wide range if the liquid flow rate is changed. However as is well known, the heat transfer characteristics are affected by many factors such as liquid flow density, velocity and size of droplets, properties of cooled surface and so on. One of other affecting factors is the geometric configuration. The existing researches were made almost exclusively with upward facing or vertical solid surfaces. Heat transfer is divided into high temperature, transition and low temperature regions. In the past, the first one was intensively studied, whereas the heat transfer characteristics have not been made clear enough. As applications spread in a wider area, the low temperature region will become more important. The present authors conducted a series of experiments on unsteady heat transfer from a hot nickel cylinder, which faced downward and cooled by upward water spray. The dependence of heat transfer on water flow density was investigated in a wide range of the cooled-surface temperature with the droplet velocity (air velocity) kept at about 20m/s. The heat transfer was also investigated on water jet cooling and air flow cooling, and compared with that by spray cooling. The experimental apparatus was made according to the JIS standard for the facility for a steel-hardenability test called the Jominy test. When spray cooling and air flow cooling were done, a spray nozzle was used in place of the water jet nozzle. The test body was a nickel cylinder. Two thermocouples were buried into the test cylinder near the cooled surface with their junctions on the central axis. The test cylinder was heated up to 650℃, set in the apparatus and then cooled. The signals from the thermocouples were converted to digital data and stored in a digital recorder. By solving a one-directional inverse conduction problem with the data, the temperature and heat flux were obtained at the cooled surface of the test cylinder. Temperature dependences of the thermal properties of nickel were taken into account. The water flow density D was varied in a wide range between 6.4×(10)^<-3>m^3/m^2s and 0.05×(10)^<-3>m^3/m^2s. The sizes of droplets were about 25μm and 45μm in average diameter for water flow densities of 0.7×(10)^<-3>m^3/m^2s and 2.0×(10)^<-3>m^3/m^2s, respectively, and scattered widely. The droplet temperature was 25℃±5K. After about seven cooling runs a small part was cut off from the test body near the cooled surface. Thus the test cylinder was initially 100mm and finally about 70mm long. Its diameter was 25mm. The purpose of the cutting is measurement of the positions of the thermocouples. The experimental results are summarized as follows. (1) The heat transfer coefficient at the surface superheat of 400K is proportional to D^<0.4> in the whole range of D. (2) The maximum surface heat flux is proportional to D^<0.8> in a wide range of D, while it tends to become less dependent on D for large water flow densities. The maximum surface heat fluxes are much less than those for water jet cooling. (3) For the largest water flow densities, the surface heat fluxes exceed those for water jet cooling in the low temperature region. (4) Surface heat fluxes are not very dependent on the surface superheat in the low temperature region. This holds true for water jet cooling. Especially for smaller water flow densities of D<0.3×(10)^<-3>m^3/m^2s the surface heat flux is almost constant independent of the surface superheat. It is nearly equal to the heat flux derived on the assumption that every attached droplet will evaporate entirely before the next droplet comes there. (5) For the smallest water flow density of D=0.05×(10)^<-3>m^3/m^2s, heat transfer is as much as that for steady air flow cooling in the high temperature region. However it is much more in the transition and low temperature regions.

TED-AJ03-184 SPRAY COOLING OF DOWNWARD FACING NICKEL BODY WITH WATER : DEPENDENCE ON VOLUME FLOW DENSITY OF WATER

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