多通道烘缸水平矩形管内冷凝换热特性及流型

Heat Transfer Characteristics of Condensation and Flow Regime in Horizontal Rectangle Channel of Multi-Channel Cylinder Dryer

为研究多通道烘缸内蒸汽的冷凝换热特性,提高烘缸干燥效率,降低造纸工业能耗,采用可视化方法对多通道烘缸中的单边换热水平矩形管内蒸汽冷凝现象进行了观测实验。实验观察到了环状流、波状流、弹状流以及塞状流4种流型。实验结果表明:当质量流率较小时,小干度下流型主要为塞状流和弹状流,随着干度的增大,流型转变为波状流;当质量流率较大时,即使小干度下流型仍主要为波状流和环状流,冷凝过程无塞状流和弹状流出现。经比较,Tandon流型图可较好地预测实验观察到的流型。此外,当质量流率较小(G<35kg·m-2·s-1)时,换热系数受质量流率和干度影响较小;随着质量流率增大,换热系数受质量流率和干度的影响变大,且随着质量流率和干度的增大而增大。最后,将实验值与传统经验关联式计算值进行了对比,发现Cavallini的关联式能较好地预测该实验结果。

The phenomenon of steam condensation within a horizontal rectangle channel with oneside cooling in multi-channel cylinder dryer is experimentally studied to investigate the characteristics of steam condensing in the multi-channel cylinder dryer,to improve the efficiency and to reduce the energy consumption of paper industry.Experiments are conducted via the visualization method under the quality of 0.2-0.8and mass flux of 20-50kg·m^-2·s^-1.Four flow patterns,including annular,wave,slug and plug flows,are observed.The slug flow,plug flow and wave flow are observed at low mass flux,the wave flow and annular flow are observed at high mass flux with a low steam quality,while the slug and plug flows are not found during the entire condensation process.It is found that the observed flow patterns agree well with Tandon＇s flow pattern map.The effects of quality and mass flux on condensing heat transfer are analyzed.The experimental results show that the effect of low mass flux（G〈35kg·m^-2·s^-1on condensing heat transfer coefficient can be ignored,and the condensing heat transfer coefficient increases obviously with increasing quality and high mass flux.Four kinds of correlations are employed to predict the coefficients of condensing heat transfer,and the resultreveals that Cavallini＇s correlation agrees with the experimental results relatively well.