Experimental Study on the Initial Position Distribution of Taylor Bubbles in Cryogenic Upward Inclined Tubes*

ZHANG Hua (張華), WANG Shuhua (王淑華), LIU Yiping (劉夷平) and WANG Jing (王經(jīng))

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Experimental Study on the Initial Position Distribution of Taylor Bubbles in Cryogenic Upward Inclined Tubes*

ZHANG Hua (張華)**, WANG Shuhua (王淑華), LIU Yiping (劉夷平) and WANG Jing (王經(jīng))

Institute of Engineering Thermo-physics, Shanghai Jiao Tong University, Shanghai 200240, China

An experimental study was carried out to understand the phenomena of the boiling flow of liquid nitrogen in inclined tubes with closed bottom by using the high speed digital camera. The tubes in the experiment are 0.018 m and 0.014 m in inner diameter and 1.0 m in length. The range of the inclination angles is 0-45° from the vertical. The statistical method is employed to analyze the experimental data. The experiment was focused on the effect of the inclination angle on the initial position distribution of Taylor bubbles. The formation criterion of Taylor bubbles was confirmed by analyzing the images of Taylor bubbles. The experimental results show that the initial position of Taylor bubble increased first, and then decreased with the increasing inclination angle, with the maximum at 30°. The standard deviation of the initial position of Taylor bubble in tubes was different with different inner diameters. The lognormal shape was fitted to the measured the initial position distributions of Taylor bubbles in the cryogenic tubes.

cryogenic, nitrogen, boiling, initial position distribution, Taylor bubble, inclined tube

1 INTRODUCTION

Gas-liquid slug flow is highly complex with an inherent unsteady behavior. It is characterized by long bullet-shaped bubbles separated by liquid slugs that may be aerated by small dispersed bubbles. In cryogenic engineering, superheating always exists in conveyor and storage system of cryogenic fluids. So the cryogenic two-phase flow is unavoidable. The propagation and storage of cryogenic fluids bring many problems, such as stratification, geysering and rollover [1]. And these problems can cause high transient pressures and vapor flow rates, in some cases large enough to damage equipment. The diagnosis of flow patterns in vertical and inclined conveying pipe is an important issue in many research fields of cryogenic two-phase flow. Most of researches are focused on the Taylor bubble length, void fraction, liquid slug lengths and the Taylor bubble translation velocity, and often the ambient fluids such as air-kerosene and air-water are used. Most of those researches are carried out mainly for horizontal or slightly inclined slug flow and for vertical flow in developed slug flow[2-9].

The slug length, void fraction and Taylor bubble length are studied extensively. The mean liquid slug length in inclined tubes (including vertical and horizontal) has been studied by several researchers including Gu and Guo [2], Wang. [3], Barnea & Taitel [4], Mao & Dukler [5, 6], Xia. [7] and van Hout. [8]. They found that the mean liquid slug length decreases with increasing inclination angle.

The mean Taylor bubble length in inclined tubes (including vertical and horizontal) has been studied by Mao & Dukler [5] and van Hout. [8, 9]. And the mean Taylor bubble length has a minimum at about 30o and extends to much larger values for the small tube for the inclined tubes [8].

Bubble motion in inclined tubes has been studied by several authors. White and Beardmore noted the influence of the angle of inclination on bubble rise velocity [10]. Zukoski studied the influence ofas well as the effects of viscosity and surface tension on the rise velocity [11]. Bubble motion in inclined tubes (including vertical and horizontal) has also been studied by several other researchers including Maneri and Zuber [12], Bendiksen [13], Weber. [14], Cou?t and Strumolo [15], Alves. [16], and van Hout[8, 17]. All of these authors found that the bubble velocity first increases and then decreases as the angle of inclination increases. The normalized velocity would decrease to a more or less constant value at the exits of the vertical or inclined pipes.

Visualization study of cryogenic vapor-liquid slug flow is seldom studied in inclined tubes with closed bottom. Compared with normal atmospheric temperature liquid, cryogenic liquid has high compressibility, low density difference between vapor and liquid and low latent heat of vaporization. There are large differences on bubble motion in cryogenic two-phase flow and normal atmospheric temperature two-phase flow. Few investigations are performed to understand Taylor bubble initial position distribution in cryogenic liquid in upward inclined pipes.

The purpose of the present study is to investigate experimentally the initial position distributions of Taylor bubbles in inclined tubes with closed bottom. The liquid nitrogen is used as working medium.

2 EXPERIMENTAL

2.1 Experimental apparatus and image processing system

The experimental apparatus consists of a liquid nitrogen Dewar, a test jacketed Pyrex glass tube and a vacuum pump. The experimental tubes are 1.0 m long with inner diameters 0.014 m and 0.018 m. The test section can be inclined at 0-45o inclination angles from the vertical. More detailed information about the facility and the measurement method can be found in Ref. [18]. The vacuum in the jacket is kept at6×10－2Pa to serve as the thermal insulation to decrease the convection heat transfer.

2.2 Experimental condition

In the experiment, leak heat causes the boiling flow of liquid nitrogen. Due to the vacuum jacket, the convective heat transfer could be neglected. There is only a connection at the top of the upper tank, so the conducted heat transfer could be also neglected. Radiant heat transfer is the main mode of leak heat in the experiment. The heat flux of radiant heat transfer is calculated with

whereis Stefan-Boltzmann’s constant,2is the outer wall surface temperature of the tube,1is the inner wall surface temperature of the tube,0is emissivity of the Pyrex glass.

In the experiment, heat flux leaked into the test tube is estimated to be about 300 W·m－2for two tubes, which then generates about 130000 and 165000 bubbles (initial size below 0.6 mm) per second respectively as estimated from image analysis.

During the experiment, the range of inclination angle is 0-45o from the vertical. The test tube in the range of 5to 15from the bottom was measured with a high speed digital camera (REDLAKE Motion-Pro?X3, 1280 ×1024 pixels resolution, 1000 frames·s－1).

3 RESULTS AND DISCUSSION

3.1 Detection of initial Taylor bubbles

It is necessary to set a criterion for the occurrence of an incipient Taylor bubble in a horizontal or inclined tube. In this work, if a bubble is characterized with bullet-shaped nose, its body is columned, its bottom outline is clear, there is a dispersion of smaller bubbles after the bubble, the diameter of bubble is greater than the tube radius, and it is longer than the tube inner diameter, the bubble is identified as a Taylor bubble. Fig. 1 shows the initial Taylor bubbles in the 0.018 m tube at various inclination angles.

Figure 1 Taylor bubble images of the initial positions at various inclination angles(Example for0.018?m)

3.2 The initial position distribution of Taylor bubbles

The histograms showing the initial position distribution of Taylor bubbles in tubes with inner diameters 0.018 m and 0.014 m at different inclination angles are respectively given in Figs. 2 and 3. The mean and the most mode column section move rightward first, and then leftward with increasingwhereistheinclination angle of the tube.

Figure 2 shows that with the increase of, the initial position distribution of Taylor bubbles agrees well with lognormal distribution. Fig. 3 shows that in the tube with the diameter 0.014 m, the initial position distribution of Taylor bubbles is in line with lognormal distribution at all inclination angles.

Figure 2 The initial position distribution of Taylor bubbles at various inclination angels in the tube with inner diameter 0.018 m

Figure 3 The initial position distribution of Taylor bubbles at various inclination angels in the tube with inner diameter 0.014 m

3.3 Mean initial position of Taylor bubbles and standard deviation

Figure 4 also shows that with the increase of, the standard deviation has the trend that it increases first, and then decreases in the tube with inner diameter 0.018 m; and the mean standard deviation is almost the same when the inclined pipe with inner diameter 0.014 m changes from 0° to 45°.

3.4 Variation of initial position of Taylor bubbles with inclination

The cause of variation of the initial position of Taylor bubbles is explored by analyzing the images shot in the experiment with pipe of 0.018 m diameter as an example.

When the tube is vertical, the small bubbles at the bottom of the tube are gradually gathering at the center of the tube through ascending along the tube. At this condition, the disturbance in tube is weak, so that the initial Taylor bubbles are formed at a lower position, as shown in Fig. 5.

Figure 5 The images of bubbles at0° (Example for0.018 m)

When the tube began to incline (the inclination angles were 10°, 20° and 30° from the vertical), the disturbance in pipe is gradually strong, vortices occurred in the pipe. The small bubbles are affected by the vortices, so that the initial Taylor bubbles are formed at a higher position, and with the inclination angle increasing, the position would be higher and higher. And the vortices could be found in the area marked in Figs. 6 and 7.

Figure 6 The images of bubbles at10° (Example for0.018 m)

Figure 7 The images of bubbles at30° (Example for0.018 m)

When the inclination angle was 45°, the small bubbles rise rapidly to the pipe upper wall after formation. For so many small bubbles gather at the tube wall, the bubbles movement along the pipe is hindered, and the small bubbles coalesce rapidly. So the initial position of Taylor bubble is lower than those in the conditions with inclination angles 10°, 20°and 30°. In the areas in the loop in Fig. 8, the small bubbles mass near the upper wall of the pipe.

Figure 8 The images of bubbles at45° (Example for0.018 m)

Figures 5-8 are the consecutive images shot in the experiment when the inclination angles are 0°, 10°, 30° and 45° respectively (time interval between images is 1 ms).

3.5 Lognormal distribution of the Taylor bubble initial position

From Figs. 2 and 3, the initial position distributions of Taylor bubbles are right-skewed. The lognormal shape is fitted to the measured distributions and is depicted in Figs. 2 and 3 as a solid line. The probability density function of the lognormal distribution is

Table 1 Parameters λ and ξ of lognormal fit

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2008-08-27,

2008-12-14.

the National Natural Science Foundation of China (50476015) and National High-Tech Research and Development Program of China (2006AA09Z333).

** To whom correspondence should be addressed. E-mail: crazywawa@sjtu.edu.cn