Terminal Bubble Rising Velocity in One-Component Systems

Activity description
Experimental facility (Test Section)
Experimental facility for single bubble experiments
Applications
Publications
Photo gallery

Activity responsible: Dr. Francesco D'Annibale
Address: ENEA C.R. Casaccia, Institute of Thermal Fluid Dynamics, Via Anguillarese 301 (S.P. 092), 00060 S. M. di Galeria RM, Italy 
Phone: +39 06 3048 6467 Fax: +39 06 3048 3026 

Email: dannibale_f@casaccia.enea.it


Activity description

  • Lack of information on bubble rising velocity in one-component systems, where bubbles are generated by the evaporation of the liquid (diabatic conditions), all in saturated conditions, with a range of pressure and temperature from ambient conditions until the critical point.
  • To obtain experimental data (close to the critical pressure) which can be directly used in microgravity or reduced gravity experiments
  • The aim of the experiment is the measure of bubble rising velocity in a liquid column up to the critical pressure. Fluids used: R-114 and FC-72. The vapour bubble is generated at the bottom of the liquid column (under saturated conditions) using a tiny electric heater. Vapour generated is collected in a glass cylinder ending with a small hole, 0.057 mm to 0.234 mm (six different nozzles) and, after reaching the necessary local overpressure, injected as a bubbles train in the liquid column. For each fluid, the system pressure is increased from the saturation value at room temperature (about 20 °C) up to the critical value (3.26 MPa for R-114 and 1.82 MPa for FC-72). Bubble evolution is recorded with high speed cinematography from the injection to the reaching of the terminal velocity in the liquid column.
    Single bubble tests to ascertain the wake effect on the bubble rising velocity in this range of diameters have been carried out, showing a negligible effect of the wake effect for the tested bubble diameters.
     
     
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    Experimental facility

    The components of the experimental facility are:

    • a rectangular test section made up of an insulated vessel (brass) with heated walls (to heat the wall and the fluid at the test temperature, and to compensate heat losses during the test, though the test section is insulated); its inner volume filled with the test fluid is about 1 dm3, the maximum pressure is 5.0 MPa while the maximum temperature is 180 °C; glass windows on the four sides of the test section allow complete visualization of the process
    • a system for the generation of saturated vapour bubbles: it is made up of a Pyrex tube at the bottom of which a tiny electric heater is placed; the upper part of the Pyrex tube is closed with a device which hosts the nozzle for the injection of vapour bubbles into the liquid column; the nozzle, made of glass with smooth edges, can be changed (diameters from 0.07 to 0.3 mm are used in this research) obtaining different bubble diameters
    • a high-speed video camera (Speedcam 512 Weinberger, 1000 fps, at a resolution of 512x512 pixel), and a stroboflash light (Kodak, duration of the flash about 20 ms)
    • a tank, with heated walls, to store the fluid
    • instrumentation such as thermocouples and pressure transducers 
    • a data acquisition system (Labview software) and an image digital treatment system (Labview/Imaq software)

     

    ImpBRIV.gif (12 K)

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    The test fluid is supplied from the storage tank to the test section through a filter (porosity 15 mm) and a valve by heating the walls of the storage tank itself, which are wrapped with an electric heater. The valve closure allows the complete separation between the test section and the storage tank after the refill procedure of the test chamber. Care is taken to avoid air presence in the test section during the charge phase. After the refill, heat is supplied to the liquid in the test section through the wall heater. As the liquid in the test section is already under saturated conditions after the charge process, the heating allows to increase the temperature and the pressure inside the test chamber, until the test temperature (and pressure) is reached. The saturated liquid is further heated in the Pyrex tube with the additional electric heater placed at its bottom, thus causing the evaporation of the saturated liquid in the Pyrex cylinder. Vapour rises up in the Pyrex tube and enters the liquid column as a bubble train, the diameter of which is determined by the nozzle diameter and the system pressure (for the given fluid). Bubbles evolution rising into the liquid column is recorded with the high-speed video camera for few seconds (4 to 8 s) from the nozzle exit up to the liquid free surface. The recorded sequence is the data set of a single test. The procedure is repeated heating the fluid and increasing the temperature and the pressure until the thermodynamic critical conditions are approached.

    A selected sequence of frames (about 100 consecutive frames are selected, with a 'clean' trajectory, separated bubbles, etc. to avoid misinterpretation) are then analyzed with the image digital treatment system. The program is able to recognize and measure the single bubble, providing the trajectory, the velocity, the shape, the dimensions, the volume as well as other parameters of interest. The volume is derived from the cross area and the axes, considering the rotation ellipsoid. All bubble parameters are average values calculated over the about 100 frames or so. Such parameters are eventually correlated to data measured in the test chamber (fluid pressure and temperature), in order to make possible theoretical analysis and comparison with existing calculation methods. Click to see an example composed by: a typical image (left), the working picture of the image digital treatment system (center) and the output of the image digital treatment system (right)

    The experimental apparatus allows to get well established experimental conditions: the liquid is certainly under saturated conditions, as well as the vapour bubbles, as their production is obtained with a very small superheating and the vapour is  in contact with the saturated liquid for a relatively long time before leaving the nozzle.
     
     

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    Test section, test conditions & devices

    VESSEL:  volume V = 1 dm3 ; Pmax = 5 MPa ; Tmax = 180 °C
    Nozzle diameters:  0.057, 0.066, 0.075, 0.13, 0.16, 0.24 mm
    Fluids: R114 (Tc = 145.75°C, Pc = 3.36 MPa), FC-72 (Tc = 178.5°C, Pc = 1.84 MPa), Water
    Image processing: Speedcam 512 Weinberger, 1000 fps, (512x512 pixel), and a stroboflash light (Kodak, t = 20 ms); image digital treatment system (Labview+Imaq); 4-8 s recorded; 100 frames analyzed
     

    TEST MATRIX
     
    R-114 FC-72 Water
    Tc [°C] 145.75 178.5 374.15
    Pc [MPa] 3.36 1.84 22.12
    N.tests 76 140 21
    N.bubbles 2459 4059 163
    Prid 0.06-0.92 0.06-0.96 0.005-0.021
    Ti [°C] 23-142 53-174 102-150
    uy [mm/s] 39-214 (22-232) 37-186 (28-205) 145-317 (143-339)
    fc 0.48-0.86 (0.25-0.98) 0.47-0.92 (0.29-0.98) 0.48-0.95 (0.41-0.96)
    deq [mm] 0.14-1.38 (0.074-1.75) 0.12-1.42 (0.085-1.69) 1.07-4.00 (1.01-4.8)

     
    where:

    T= critical temperature
    P= critical pressure
    Prid  = reduced pressure ( P/Pc )
    uy  = vertical velocity
    deq = equivalent diameter (sphere with same volume of the ellipsoidal bubble)

    fc = elongation factor (ratio between minor and major axis) 

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    Single and controlled frequency bubble tests

    The bubble generation device: with electric pulse trains having different frequency it is possible to generate bubbles with the same frequency

    electric pulses t = 1 ms
    V = 10 and 30 Volt
    f from 1 to 150 Hz

    thin film type electric resistance (top view): 120 W and its size is 1.2 x 1.0 mm
     

    Abstract
    Experiment of bubble rising velocity in a liquid column using R-114 and FC-72 and water. Vapour bubbles are generated at the bottom of the saturated liquid column by a tiny electric heater and their evolution is recorded with high speed cinematography from the generation point until they reach the terminal velocity in the liquid column.
    Single bubble and bubble train (with controlled frequency) tests are carried out to ascertain the wake effect on the bubble rising velocity in the range of diameters from 0.1 to 0.8 mm, showing a negligible effect of the wake effect for the tested bubble diameters.
    Prediction of bubble terminal velocity with available correlations have been performed.
     
     

    Experimental facility for single bubble experiments

    The experimental facility is the same with, instead of the Pyrex tube, the system for the generation of saturated vapour bubbles.
     


     

    This latter is made up with a small electric resistance of the thin film type, which is a component of a strain-gauge typically used for pressure transducers manufacturing, flush-mounted in horizontal position inside the saturated liquid with the heating element upwards. 
     


     

    The electric resistance of the heating element is 120 W and its size is 1.2 x 1.0 mm. If we apply the resistance pulse (voltage) trains we get bubbles with different frequencies, depending on the pulse frequency. 
    The pulses are supplied by a waveform generator (Hewlett-Packard HP33120A), and a power amplifier (MB-Electronics, 2250MB). 
    The lasting of the electric pulse is 1 ms, the width ranges from 10 to 30 V, and the frequency (which affects the distance between two adjacent bubbles) from 1 to 150 Hz.

    As the liquid in the test section is under saturated conditions before the test, the additional heating coming from the electrical resistance causes the evaporation of the liquid on the surface of the heater.
    Bubble motion in the liquid column is recorded with the high-speed video camera as described in the previous section.

    Frequency controlled tests, carried out using both FC-72 and R-114, provide bubbles characterized by a frequency ranging between 1 and 150 bubble/s, while the corresponding distance between adjacent bubbles ranges between 3 and 300 d. Bubble diameter in these tests ranges roughly between 0.2 and 0.8 mm. Click to see an example of bubble trains
     
     

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    Applications

    The research if finalized to give informations on therising velocity in one-component systems, where bubbles are generated by the evaporation of the liquid in saturated conditions, with a range of pressure and temperature from ambient conditions until the critical point. Increasing the pressure there is a reduction of the driving force (due to the density difference between vapour and liquid), until the critical point, where the densities are equal.
    Theese experimental data are necessary to develop and validate models for the bubble motion in conditions like microgravity or reduced gravity, where the variation of the driving force is due to the reduction of the gravity acceleration. For that validation the experimental data obtained in the past for the chemical and nuclear industry, with big density difference, must be integrated with data in more extreme conditions, like ones obtained here.

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    Publications

    G.P. CELATA, M. CUMO, F. D'ANNIBALE and A. TOMIYAMA, Bubble Rising Velocity in Saturated Liquid up to the Critical Pressure
    2nd Japanese-European Two-Phase Flow Group Meeting, Tsukuba, 24-28 September 2000
    G.P. CELATA, M. CUMO, F. D'ANNIBALE and A. TOMIYAMA, Terminal Bubble Rising Velocity in One-Component Systems
    European Two-Phase Flow Group Meeting, Paper F3, Aveiro, 18-20 June 2001
    G.P. CELATA, M. CUMO, F. D'ANNIBALE and A. TOMIYAMA, On the Wake Effect in Bubble Rising Velocity for One-Component System
    12th International Heat Transfer Conference, pp. 485-490, Grenoble, 18-23 August 2002
    A. TOMIYAMA, G.P. CELATA, S. HOSOKAWA and S. YOSHIDA, Terminal Velocity of Single Bubbles in Surface Tension Force Dominant Regime
    International Journal of Multiphase Flow, Vol. 28, pp. 1497-1519, 2002

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    Photogallery
     
    Pictures of the single bubble experiments: