Browsing by Author "Schoenfeld, Paul David"
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- ItemPressure drop during reflux condensation of steam in an inclined elliptical tube(Stellenbosch : Stellenbosch University, 1998-09) Schoenfeld, Paul David; Kroger, D. G.; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.ENGLISH ABSTRACT: Zapke and Kroger developed a model for the pressure drop across flattened tubes employed in reflux condensers. This model is based on adiabatic two-phase countercurrent experiments and incorporates the theory on fluid friction during condensation by Groenewald. Zapke also developed a correlation with which the flooding velocity during adiabatic countercurrent flow may be predicted. The applicability of both the pressure drop model and the flooding correlation to reflux condensation had prior to this investigation not been verified by experiment. In this investigation the header-to-header pressure drop, the occurrence of flooding and the heat transfer performance of an inclined air-cooled elliptical tube in which reflux condensation of steam occurs were studied. The qualitative effect of the presence of a backflow region on the performance of the tube was also investigated. The 7 m long tube has a height of 97 mm (major axis) and a width of 16 mm (minor axis) and a sharpedged (90°) inlet. The tube is inclined at 60° to the horizontal. Steam temperatures are in the range of 45 °C to 65 °C. The pressure drop measurements obtained in this investigation showed that the header-to-header pressure drop in the tube is predominantly dependent upon the superficial vapor Froude number at the tube entrance. The pressure drop is also accurately predicted by the Zapke-Kroger pressure drop model in the range 0.1 ≤ Fr Hsv ≤ 0.4. At a certain steam flow rate a sudden sharp increase in the pressure drop occurs. This phenomenon is known as flooding. It was found that flooding during reflux condensation is governed by the superficial densimetric vapor Froude number with the duct height taken as the characteristic dimension. The measured vapor velocities at flooding are also in close agreement with those predicted by the Zapke flooding correlation. Flooding was found to have a limiting effect on the heat transfer rate of the elliptical tube in which all the steam condenses in the reflux mode. Finally, the formation of a steam backflow region in the tube resulted in the accumulation of noncondensable gases and the formation of a dead or cold zone. This led to a decrease in the heat transfer rate of the tube.