A Study on De-Freeze-Off of Layered Air-Water in a Circular Tube

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An experimental investigation has been performed to determine the efficient method to prevent the freeze-off which should be observed in a horizontal cooled circular tube. The experiments were carried out under a variety of conditions of airflow rate, water flow rate, cooled air temperature, inlet water temperature, and initial water level. Extensive photographical and visual observations of the developing ice layer along the tube wall were conducted along with the experimental determination of the characteristics of the freezing heat transfer. The effect of the flow separation induced by an orifice situated at the entrance section of an uniformly cooled circular tube was extensively determined.A. Free Convection Heat Transfer of Air-Water Layers(1) Initially cooled water in the tube flows downward along the tube wall. Next a secondary eddy is formed due to density inversion in the water, and it flows upward along the tube wall, while the initial downward flowing eddy becomes weaker.(2 … More ) The water at the air-water interface is supercooled by the density inversion, and this supercooled water gradually spreads to the tube bottom by free convection.(3) The local heat flux increases with the cooling rate, but the rate of increase is quite small during the experiment.B. Characteristics of Freezing Heat Transfer of Layered Air-Water Flow(1) The freeze-off region and steady-state region can be apparently classified by the equation, THETA_c = 1.92 x 10^<-1>Re_W0.35.(2) In the freeze-off region, there are three kinds of ice forms. Furthermore, in the steady-state region, there are two kinds of ice forms and no ice region.(3) Ice layer along the upper tube wall grows because of the splashing of the ripples at the air-water interface, which may be caused mainly by the developed ice layer and the ice transition (ice test formed).C. Effect of Flow Separation on the Freezing Heat Transfer and Freeze-Off(1) The flow separation increases the time from the start of experimental run to the onset of freeze-off condition. The time of freeze-off takes the maximum at d/D=1/2.(2) The ice-deposit formed in the separated region just downstream of an orifice is concave when d/D is less than 3/4.(3) The ice-deposit thickness along the length of the tube decreases and the station of the rapid expansion of flow passage tends to move upstream as the water-flow velocity increases.(4) The time of freeze-off decreases with a decrease in the tube diameter. Less