Two-phase bubbly flow is a fundamental phenomenon which occurs in many industrial processes such as thermal power plant, chemical processes, nuclear reactors, and so on. It has been investigated continuously over several decades. However, due to its complexity, some characteristics of the phenomenon have not yet been understood. The velocity profile of liquid and bubble is an important parameter in the bubbly flow. Also, it is used to calculate other parameters such as void fraction and slip velocity. Hence, measurement of the velocity profile in bubbly flow is required.
The Ultrasonic Velocity Profile (UVP) method [1] is a nonintrusive measurement, which can measure velocity profile of the fluid even though is opaque. The method does not require the transparent test section. The UVP measurement uses a pulsed echography of an ultrasonic wave reflected from moving reflector which has a similarity of velocity with a fluid such as a particle dispersed in a fluid. It contains Doppler signal. The frequency of Doppler signal fD (x) directly relates to a velocity of moving particle. Therefore, the instantaneous velocity profile of the fluid along its measurement line can be obtained by observing Doppler frequency. Although conventional UVP can obtain velocity profile of liquid and bubble, the velocity of both phase cannot be classified separately.
This study proposes a measurement technique to obtain velocity profile of liquid and bubble in two-phase bubbly flow using UVP method. The previous study by Murakawa [2], two resonant frequencies was used to measure both liquid and bubble velocity separately. The system requires more devices However, present study employs only single resonant frequency. Therefore, a measuring equipment is minimized. To measure and separate velocity distribution of liquid and bubble with single resonant frequency, a technique of measurement and separating velocity of both phases are developed. In bubbly flow, Doppler signal is demodulated from echo signals reflected by particle (liquid) and bubble. Its frequency and amplitude inform the velocity and the identity of each reflector respectively. In some case, the position of particle and bubble occurs in same measurement channels. When this behavior occurs, multi-frequency and different amplitude in Doppler signal are generated. Therefore, advanced signal processing is required to analyze the effect of Doppler signal for decomposing Doppler frequency of particle and bubble. Then, the phase separated velocity of liquid and bubble can be obtained.
For confirming the applicability of modified measurement system, the developed UVP was used to experiment on vertical pipe flow apparatus. The measurement accuracy of developed UVP was guaranteed due to the result had good agreement with UVP Original and PIV method. Furthermore, the UVP was applicable to observe velocity distribution of both phases in a bubble column.
References
[1] Y. Takeda, Velocity profile measurement by ultrasonic Doppler shift method, Int. J. Heat Fluid Flow, 7, (1986), pp.313-318.
[2] H. Murakawa, H. Kikura, and M. Aritomi: Application of ultrasonic Doppler method for bubbly flow measurement using two ultrasonic frequencies, Experimental Thermal and Fluid Science, 29, (2005), 843-850.