Definition of void fraction in horizontal adiabatic two-phase flow based on momentum pressure drop characterisation

• A novel low-intrusive technique for void fraction definition was developed. • New estimations of ammonia void fraction were obtained for 4, 6 and 8 mm tubes. • A new approach for the homogeneous frictional pressure losses was proposed. This paper presents a straightforward and low-intrusive experimental method for determining the void fraction of ammonia in horizontal tubes at high vapour quality. The method is based on the characterisation of momentum pressure drop and has been applied for ammonia flow in horizontal tubes with internal diameters of 4, 6, and 8 mm under operating conditions, including saturation temperatures ranging from 45 to 65 °C, mass velocities between 100 and 300 kg·m -2 ·s -1 , and vapour qualities from 0.2 to 0.8. The estimated void fractions are consistent with those obtained in the previous study using the quick-closing valve technique at the low vapour qualities below 0.3. The homogeneous flow model provides upper-bound estimates of the ammonia void fraction, whereas heterogeneous models, such as those proposed by Rouhani and Axelsson (1970) and Tibrica et al. (2017), offer lower-bound predictions in the high vapour quality region, underestimating the void fraction by 10–15 %. Assuming that the obtained two-phase flows are close to homogeneous, the frictional pressure losses measured in the smooth tube segment with internal diameters of 4, 6, and 8 mm were correlated with well-established homogeneous models. The comparison showed that they underpredict the actual mechanical energy losses by 45 % to 65 %. A new phenomenological approach was proposed, considering that the pressure losses of the nearly homogeneous two-phase flow in a smooth tube estimated from void fraction measurements are due to local pressure losses caused by velocity-field deformation near wave structures. The new model predicts the test data within ±25 % uncertainty range. However, the approach fails to match the heterogeneous frictional pressure drop model developed by Müller-Steinhagen and Heck (1986) when used with a modified, pattern-dependent empirical coefficient, which predicts the pressure drops with an accuracy of ±15 %.