Research On Condensation Heat Transfer Performance For Vapor In Inclined Tubes

At present, the direct air-cooling technology is widely applied in China where it isrich in coal but short of water resource, and steam condensing in its core components,the condenser determines the heat exchange efficiency of the air cooling unit. However,literitures of studies on condensation heat transfer in air-cooled condenser(ACC) arevery rare up to now. In this thesis, experiment and numerical simulation on steamcondensation in two smooth inclined tubes has been investigated in order to providereference for future heat transfer research on steam condensation in more complex andlarger cross section inclined tubes.The steam condensation in tubes relates to a complex gas-liquid two-phase flow.Therefore, the primary objective of this thesis is to design and establish a newexprimental setup for investigating the flow pattern, the pressure drop and heat transfercharacteristics in the inclined tubes, and to generalize correlations and a couplingrelation model of heat transfer and flow resistance based on experimental data forsimulating the property of steam condensation in the inclined tubes.For the short test section, flow patterns are researched under different workingconditions by visualizing the flow from a transparent glass tube connected to the testone, and four flow patterns such as the annular flow, wave annular flow, stratified wavyflow and stratified flow are found. Based on test data and Kattan flow chart, a new flowpattern is developed in the range of test parameters for vapor flow in a smooth inclinedtube. In a certain range, the inclination angle does not have a significant effect on thepressure drop. By combination of the frictional pressure drop correlations, void fractioncorrelations, and the test data, an applicable correlation for the steam condensation ininclined tubes under this test condition is determined. The condensation heat transfercoefficient has few relationship with cooling water temperature. Steam flow speed, heattransfer temperature difference and inclination angle influence significantly on thecondensation heat transfer coefficient. In some degree, the heat transfer coefficientincreases with the increase of the steam flow speed but decreases with the increase ofthe temperature difference in heat transfer. Furthermore, there exists an optimum anglecorresponding to the best condensation heat transfer coefficient, and the optimum angleis30oin this range of experimental conditions.Based on the short experimental section, the long tube test section tests are carriedout to study the condensation heat transfer characteristics, and the distribution of cooling water temperature, steam temperature, wall temperature and dryness of steamalong the orientation of steam flow are acquired. Cooling water temperature, steamtemperature, wall temperature, and dryness of steam reduce along the direction of steamflow, and vapor quality has more significant effect on the heat transfer coefficient whichincreases as vapor quality increase. The heat transfer coefficient of a smooth inclinedtube in30ois1.06~2.98times than that of the horizontal tube. Based on comparison ofthe test results and the prediction results from several correlations, a new correlation ispresented, and most deviations of the prediction results from the new correlation toexperimental data are less than15%.Through analysis of the steam condensation heat transfer and flow characteristics,and segmenting the inclined tube, the heat transfer and flow resistance couplingdifferential equations for differential section of condensation are established on basis ofthe experimental data and the developed correlation, so that the numerical study onsteam condensation characteristics in inclined tubes is to be carried out. In steamcondensation process, the vapor quality is not transformed linearly, the integral averagedry degree in smooth inclined tube can’t be calculated according to the arithmeticaverage method, and steam pressure gradient decreases gradually along the tube, so thesteam pressure along the tube decreases more slowly. The effects of shear stress recedein the steam flow process and friction resistance is gradually decreased.The results of flow patterns, pressure drop, and heat transfer characteristics forsteam condensation in inclined tubes of this thesis could be the reference toperformance study, design and optimization of the direct air-cooled condenser in powerplants.