1、Chapter two Two-phase flow patterns and flow pattern maps （第二章两相流型和流型图）, gas-liquid two-phase flow has obvious interface（气液两相流动具有明显的流动界面） has various geometric configurations（流动具有各种不同的几何构型）,1、Flow pattern or flow regime（流型）,Introduction（简介）,2、Important physical parameters in determining the flow pat
2、tern:（流型）, surface tension（表面张力） Keep channels walls always wet Make small liquid drops and small gas bubbles spherical gravity（重力） Tends to pull the liquid to the bottom of the channel especially in non-vertical channel ,3、Flow patterns in vertical upward tubes gas and liquid phase both flowing upw
3、ards，adiabatic flow（垂直上升管内的流型 气液两相均向上流，绝热流）,When the quality is gradually increased from 0 to 1, the flow patterns can be classified: （1）Bubbly flow（泡状流） Liquid phase is the continuous phase Gas or vapor phase is the dispersed phase Gas or vapor bubbles are of approximately uniform size.,（2）Plug flo
4、w or Slug flow（弹状流）,Gas flows as large bullet-shaped bubbles（sometimes called Taylor bubble） Gas bubble and liquid slug flow across tube alternatively Some small gas bubbles distributed throughout the liquid phase.,（3）Churn flow（搅拌流，块状流）,Highly unstable flow，oscillatory flow the shape of gas bubbles
5、 is not regular Liquid near the tube wall continually pulses up and down,（4）Annular flow（环状流）,Gas flows in the center of the tube Liquid partially flows as an annular film on the walls of the tube Liquid partially flows as small droplets distributed in the gas flowing in the center of the tube,4、Flo
6、w patterns in horizontal tubes，adiabatic flow（水平管内两相流型，绝热流）,When the quality is gradually increased from 0 to 1, the flow patterns can be classified: （1）Bubbly flow（泡状流） Low quality and low velocity Liquid is the continuous phase Gas or vapor phase is the dispersed phase near the top of the tube,（2）
7、Plug flow（塞状流、间歇流）,Bigger quality and low velocity Individual small gas bubbles coalesce to produce long plugs Gas bubbles still tend to flow along the top of the tube,（3）Stratified flow（光滑分层流）,Gas and liquid phase have obvious interface The gas-liquid interface is smooth This flow pattern does not
8、usually occur Gas and liquid phase are both continuous phase Gas phase flows above and liquid phase flows below,（4）Wavy flow（波状分层流）,Gas and liquid phase have obvious interface The gas-liquid interface is wavy Gas velocity is bigger than the stratified flow Gas and liquid phase are both continuous ph
9、ase Gas phase flows above and liquid phase flows below,（5）Slug flow（弹状流）,the upper wall of the tube is alternatively flowing by the gas bubble and liquid phase the wave amplitude is so large that the wave touches the top of the tube Attention: The difference between slug flow and plug flow is: The u
10、pper wall of gas bubble has no liquid film in slug flow, but it has liquid film in plug flow.,（6）Annular flow（环状流）,Gas velocity is big enough Gas flows in the center of the tube Liquid partially flows as an annular film on the walls of the tube Liquid partially flows as small droplets distributed in
11、 the gas flowing in the center of the tube The liquid film at the bottom of the tube is much thicker than at the top,Comparisons of gas-liquid two-phase flow patterns in horizontal tube with whole gravity and small gravity（地球重力和微重力条件下水平管内气液两相流型的比较）, Many investigators also defined other flow pattern
12、s, and nearly a hundred different names have been used. These names of flow pattern mentioned before are main flow patterns and relatively recognized accepted to us. Further general details about flow patterns can be found in Collier and Thome(1994) and Hewitt(1982).,5、Flow patterns in vertical upwa
13、rd tubes gas and liquid phase both flowing upwards，heating up flow（垂直上升管内的流型 气液两相均向上流，加热流）,Heat addition Phase change More complicated than adiabatic flow The evolvement of flow patterns are greatly affected by the heat flux（热流密度）, From bottom to top, the flow patterns of steam-water two-phase flow
14、in the tube take turns as follows: Single phase flow Bubbly flow Slug flow Churn flow Annular flow, The flow patterns of steam-water two-phase flow in the tube when heat flux increases are as follows:,xx denotes ONB（onset of nuclear boiling） yy denotes onset of annular flow region zz denotes the poi
15、nt of CHF（Critical heat flux），it is a very dangerous point,Tip：the types of flow patterns increase in heating channel when heat flux increases.,6、Flow patterns in horizontal tubes，heating up flow（水平管内两相流型，加热流）,1 single liquid phase；2 bubbly flow；3 plug flow； 4 slug flow；5 wavy flow；6 annular flow,Th
16、e determination or identification methods of Flow patterns（流型确定或流型识别的方法）, By naked eye (direct method)Adaptive to low velocity Subjective Transparent tubes required, By various types of instrument (indirect method) Gamma-ray densitometer Conductive probe Optical probe Pressure difference signals Cap
17、acitance probe High-speed photography and so on.,signal characteristics of void-fraction probe to determine the flow patterns,Transitions in vertical upward flow（垂直上升管内流型转变）, bubbly to plug flow（泡状流向弹状流转变） Mechanism： （1）Gas velocity increases (void fraction increases) （2）Small bubbles get closer and
18、 collisions occur （3）Collisions lead to coalescence of bubbles （4）Formation of plugs Main factors preventing bubbles coalescence includes: （1）Surface active agent (surfactant) （2）High liquid flow rate (liquid turbulence intensity), plug to churn flow（弹状流向搅拌流转变）,Mechanism： （1）Gas velocity increases (
19、void fraction increases) （2）Gas plug rises through the liquid and gas velocity in the plug is upwards （3）The thin film around the plug is downwards, flow becomes countercurrent （4）Plug flow break down to give the pulsating and highly unstable churn flow, churn to annular flow（搅拌流向环状流转变）,（1）Gas veloc
20、ity increases (void fraction increases) （2）Gas is able to support the liquid as a film on the tube walls，then annular flow occursTwo terms: Flow reversal（回流）when the gas velocity is reduced and the liquid firstly starts to flow down instead of up flooding（液泛） when the gas velocity increases and the
21、liquid firstly starts to flow up instead of down,Flow pattern maps（流型图）,A useful and convenient method to determine flow pattern A two-dimensional graph (has x axis and y axis) to separate the space into areas corresponding to the various flow patterns Simple flow pattern maps use the same axes for
22、different transitions Complex flow pattern maps use different axes for different transitions,1、introduction,2、flow pattern map in vertical channel（垂直流的流型图） Hewitt-Roberts flow pattern map,Scope: Air-water two-phase flow Round tube Diameter 31.2mm Pressure 0.140.59MPa x and y axis are both momentum f
23、lux,3、flow pattern map in horizontal channel（水平流的流型图） modified Baker flow pattern map,widely used The axes are and ，here and are modified physical property factors:,4、Mandhane flow pattern map,Based on 5935 experimental data The axes are gas and liquid superficial velocity,5、Taitel and Dukler flow p
24、attern map,For horizontal flow Have some theoretical basis Complex type of flow pattern map Four parameters defined，they are X，Fr，T and K, The transitions between stratified flow、wavy flow and intermittent flow（光滑分层流、波状分层流和间歇流之间的转变）,The relationship of Fr and X is adopted.Froude numberMartinelli par
25、ameter(dp/dz)l indicates the frictional pressure gradient if the liquid in the two-phase flow were flowing alone in the tube. (dp/dz)g indicates the frictional pressure gradient if the gas in the two-phase flow were flowing alone in the tube., The transitions between stratified flow and wavy flow（光滑
26、分层流向波状分层流转变）,The relationship of K and X is adopted.indicates liquid viscosity, The transitions between bubbly flow and intermittent flow（泡状流向间歇流转变）,The relationship of T and X is adopted.Taitel and Duklers flow pattern map for horizontal flow does well although some unreasonable assumptions have be
27、en used. A complex type of flow pattern map has also been proposed by Taitel and Dukler for vertical flow, but is has not been so well tested.,Other geometries（其他非圆通道内流型）,Rectangular channels（矩形通道） Flow patterns are very similar to those in round tubes Liquid tends to collect in the cornersVertical
28、downflow (in tubes)（垂直下降圆形通道） When gas and liquid phase are both flowing downwards annular flow occurs very frequency.,Obstructions on the wall（壁面有障碍物） There is a tendency for the liquid to be thrown into the center of the channelHelical twisted tape insert（螺旋状插入物） This tend to throw the liquid on t
29、o the walls of the tube produce good heat transfer (heat transfer advanced method),Helically coiled tube（螺旋盘旋管） This tends to throw liquid onto the walls of the tube at point A or point B or somewhere in between (on ACB) due to the effect of gravity.,Rod bundles （as in nuclear reactors）（棒束通道） similar to those found in round,
