Visual Study On Bubble Behaviors Under Flow Subcooled Boiling In Rectangular Channel

The emergence and change of vapor bubbles have big effects on boiling heat transfer efficiency and system safety. To study the bubble characteristics, a visual experiment is performed to photograph and analyze the bubble behaviors in rectangular channel under flow boiling by using a high speed camera.The flow boiling experiment is carried out when the test section keeps vertical and inclined station, and does the rolling motion.The parameters, such as mass flux, system pressure, inlet subcooling and heat flux, are changed to study their influence on the bubble behaviors. According to the bubble imaging features and different research aspects, both manual and programma methods are used to process the image data, which can give the necessary bubbles information, such as the bubbles number, location and area and so on.The concept of bubble characteristic diameter is put forward by analyzing plenty of nucleation bubbles, and it is found that there is a linear relationship between the bubble life time and the bubble characteristic diameter, which is a new discovery. The bubbles with shorter life time and smaller diameter occupy a large fraction than those with longer life time and bigger diameter. The sliding bubble can accelerate the mixture of subcooled and saturated liquid, so when the sliding bubble crosses the nucleation site, the nucleation process is suppressed by the sliding bubble. The bubble coalescence process can be divided into the approaching, coalescence, adjustment and steady stage. During the shape change of coalescence bubble between circle and ellipse, the new bubble velocity will increase then decrease until steady, and the length of the long axis will shorten under exponential law.Besides, a little amount of vapor may be separated from the top bubble during coalescence and then can grow to become a new bubble.When increasing the heat flux, the bubble projection area will also increase while the bubble number may increase or decrease. It is because that when the bubble number increase to center value,the coalescence effect will decrease the number. Both the inclined station and rolling motion have big effects on the bubble behaviors through changing the forces acted on bubble, and the effects will result in different changes. In general, when the inclined angle is positive, the vapor generated will decrease and so will the projection area, vice versa. The bubbles number and projection area will change periodic along with the rolling motion. The rolling motion cannot be simply regarded as the addition of inclined station because the rolling motion is unstable while the incline is steady and the mass flux and wall temperature will also fluctuate during the rolling motion due to the experiment system itself.