CFD simulation on effect of interfacial shear force on water vapor condensation in inclined flat tube

doi:10.11949/j.issn.0438-1157.20150976

Abstract

Abstract:

Considering the interfacial shear force, a mathematical model to the condensation of turbulent vapor flowing downward in an inclined flat tube is proposed and implemented in computational fluid dynamics (CFD). The predicted results from the CFD model are compared with the experimental results from the literature for the vapor condensation in a prototype tube (2600 mm length, 3 mm width and 50 mm altitude in 60° inclination to vertical). It is found that the condensate rate and mean condensation heat transfer coefficient (HTC) from CFD simulation agree very well with the experimental quantities. Using CFD model to calculate the interfacial shear stress by varying vapour velocity, the results demonstrate that the value of shear force depends on the vapor velocity at the tube inlet, and shear force decreases continuously with the vapor flow and condensation. Simulating the interfacial shear effect on the condensation, it shows that the interfacial shear increases the local condensation HTC, and meanwhile, reduces the local condensate rate. The simulation results also shows that the interfacial shear weakens the gravitational effect on the film accumulation and obviously decreases film thickness from 0 to 0.8 m in the tube axial length. However, from 1.0 m to the tube outlet, the gravitational force dominates over the shear force, and thus the shear effects can be completely neglected. It is also found that the condensate film is speeded up particularly from 0 to 0.2 m in axial length thanking for the interfacial shear.

Key words: turbulent flow, condensation, interfacial shear stress, inclined flat tube, numerical simulation

摘要：

针对倾斜扁平管内的湍流蒸汽,考虑交界面剪切力,建立蒸汽凝结的CFD模型。在一根实验样管上(长宽高尺寸:2600 mm×3 mm×50 mm,倾斜角:60°),对比管内平均凝结传热系数和蒸汽凝结率的CFD解与实验值,验证模型的有效性。计算不同蒸汽入口流速下的交界面剪切力值,发现剪切力的大小由蒸汽入口流速决定,其数值随蒸汽流动持续递减。对剪切力对蒸汽凝结影响进行CFD模拟,发现,剪切力增大管内局部凝结传热系数,减小蒸汽凝结质量;在0~0.8 m管段,剪切力明显破坏了重力对液膜的累积,削薄了液膜厚度,但从1.0 m到蒸汽出口,液膜厚度由重力控制,剪切力的影响可忽略不计;剪切力迫使液膜加速流动,其加速作用0~0.2 m管段表现突出。

关键词: 湍流, 凝结, 界面剪切力, 倾斜扁平管, 数值模拟

CLC Number:

TK124

DENG Hui, BAI Yan, LI Xinxin, ZHANG Dongming. CFD simulation on effect of interfacial shear force on water vapor condensation in inclined flat tube[J]. CIESC Journal, 2016, 67(4): 1215-1224.

邓慧, 白焰, 李欣欣, 张东明. 交界面剪切力对倾斜扁平管内蒸汽凝结影响的CFD模拟[J]. 化工学报, 2016, 67(4): 1215-1224.

References 20

[1]	郭民臣, 纪执琴, 安广然, 等. 干-湿冷却系统对空冷机组热经济性影响的分析[J]. 化工学报, 2015, 66(1): 433-440. DOI: 10.11949/j.issn.0438-1157.20140990. GUO M C, JI Z Q, AN G R, et al. Thermal economy analysis of power unit with wet-dry hybrid cooling system[J]. CIESC Journal, 2015, 66(1): 433-440. DOI: 10.11949/j.issn.0438-1157.20140990.
[2]	明廷臻, 党艳辉, 刘伟, 等. 椭圆管矩形翅片空冷器流体流动与传热特性数值分析[J]. 化工学报, 2009, 60(6): 1380-1384. DOI: 10.3321/j.issn:0438-1157.2009.06.007. MING T Z, DANG Y H, LIU W, et al. Numerical analysis of fluid flow and heat transfer characteristics on elliptical tube with rectangular fins of air cooler[J]. CIESC Journal, 2009, 60(6): 1380-1384. DOI: 10.3321/j.issn:0438-1157.2009.06.007.
[3]	NUSSELT W. Die Oberflachenkondesation des Wasserdamffes the surface condensation of water[J]. Zetrschr. Ver. Deutch. Ing., 1916, 60: 541-546.
[4]	CHEN S J, REED J G, TIEN C L. Reflux condensation in a two-phase closed thermosyphon[J]. Int. J. Heat Mass Tran., 1984, 27(9): 1587-1594. DOI: 10.1016/0017-9310(84)90271-0.
[5]	FIEDLER S, AURACHER H. Experimental and theoretical investigation of reflux condensation in an inclined small diameter tube[J]. Int. J. Heat Mass Tran., 2004, 47(19): 4031-4043. DOI: 10.1016/j.ijheatmasstransfer.2004.06.005.
[6]	ROHSENOW W M, HARTNETT J P, CHO Y I. Handbook of Heat Transfer[M]. 3rd ed. New York, USA: McGraw-Hill, 1998: 14.7-14.8.
[7]	LEE K Y, KIM M H. Effect of an interfacial shear stress on steam condensation in the presence of a noncondensable gas in a vertical tube[J]. Int. J. Heat Mass Tran., 2008, 51(21): 5333-5343. DOI: 10.1016/j.ijheatmasstransfer.2008.03.017.
[8]	张俊霞, 王立, 李运刚,等. 界面剪切力对蒸汽垂直下流膜状凝结传热的影响分析[J]. 化工学报, 2011, 62(10): 2733-2739. ZHANG J X, WANG L, LI Y G, et al. Analysis of interfacial shear stress effects on vapor downward condensation heat transfer[J]. CIESC Journal, 2011, 62(10): 2733-2739.
[9]	HIRT C W, NICHOLS B D. Volume of fluid (VOF) method for the dynamics of free boundaries[J]. J. Comput. Phys., 1981, 39(81): 201-225. DOI: 10.1016/0021-9991(81)90145-5.
[10]	KAYS W M, CRAWFORD M E. Convective Heat and Mass Transfer,[M]. 3rd ed. New York, USA: McGraw-Hill, 1993.
[11]	MUNOZ-COBO J L, HERRANZ L, SANCHO J, et al. Turbulent vapor condensation with noncondensable gases in vertical tubes[J]. Int. J. Heat Mass Tran., 1996, 39(96): 3249-3260. DOI: 10.1016/0017-9310(96)00012-9.
[12]	KIM D E, YANG K H, HWANG K W, et al. Pure steam condensation model with laminar film in a vertical tube[J]. Int. J. Multiphase Flow, 2011, 37(8): 941-946. DOI: 10.1016/j.ijmultiphaseflow.2011.04.006.
[13]	MUNSON B R, YOUNG D F, OKIISHI T H. Fundamentals of Fluid Mechanics[M]. 4th ed. New York, USA: John Wiley & Sons, 2002.
[14]	OH S, Revankar S T. Analysis of the complete condensation in a vertical tube passive condenser[J]. Int. Commun. Heat Mass, 2005, 32(2): 716-727. DOI: 10.1016/j.icheatmasstransfer.2004.10.013.
[15]	ZHOU W, HENDERSON G, REVANKAR S T. Condensation in a vertical tube bundle passive condenser (Ⅰ): Through flow condensation[J]. Int. J. Heat Mass Tran., 2010, 53(5): 1146-1155. DOI: 10.1016/j.ijheatmasstransfer.2009.10.039.
[16]	杜小泽, 杨立军, 金衍胜, 等.火电站直接空冷凝汽器传热系数实验关联式[J].中国电机工程学报, 2008, 28(14): 32-37. DU X Z, YANG L J, JIN Y S, et al.Development of experimental correlation for heat transfer coefficient of direct air-cooled condenser in power plant[J]. Proceedings of the CSEE, 2008, 28(14): 32-37.
[17]	BATCHELOR G K.An Introduction to Fluid Dynamics[M]. Cambridge, UK: Cambridge University Press, 1967.
[18]	MCADAMS W H. Heat Transmission[M]. New York, USA: McGraw-Hill, 1954.
[19]	WEISSTEIN. Euler angles, Wolfram MathWorld[EB/OL]. Champaign, USA: Wolfram Research, Inc.,[2015-05-29]. http://mathworld. wolfram.com/EulerAngle.html
[20]	HARUN B, STEPHEN D, Edward K L. Numerical modeling of finned heat exchangers[J]. Appl. Therm. Eng., 2013, 61(2): 278-288. DOI: 10.1016/j.applthermaleng.2013.08.002.