Void Fraction Definition and Correlations

There are two different methods to experimentally define void fraction. The first is based on analyzing the images of gas-liquid interfaces recorded in the observation window of the channel (Triplett et al. 1999b Serizawa et al. 2002 Kawahara et al. 2002). For example, in experiments by Kawahara et al. (2002) at low liquid flow rates, most of the recorded images showed either liquid flowing alone or a gas core 

The second method to calculate of void fraction is based on gas velocity measurement. A high-speed motion analyzer was employed by Zhao and Bi (2001b) to visualize bubble behavior in the test sections. 

Zhao and Bi (2001b) concluded that the drift-flux model with zero drift velocity and Co = 1.2 - 0.2 Pg/Pl agrees with the measured gas velocities for the three tested miniature channels. 

Homogeneous flow may be described using volumetric gas quality. 

Empirical void fraction correlations, relating to the Lockhart-Martinelli factor X and from there to volumetric quality x, have been provided by Lockhart and Mar-tinelli (1949), Baroczy (1963), Wallis (1969), and have been discussed by Butter-worth (1975) and Chen and Spedding (1983). Butterworth (1975) showed that Lockhart and Martinelli s correlation (1949) for void fraction, as well as several other void fraction correlations, can be represented in the following generic form  

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