Shao-Wen Chen

Flow Characteristics and Void Fraction Prediction in Large Diameter Pipes

Two phase flows in large diameter pipes have immense importance in a wide variety of industrial applications. As a first approximation for the prediction of a two-phase flow and as a beginning for the development of more complex models, the drift-flux model is often used to characterize and predict flows for many geometries and flow conditions. In this chapter, the flow characteristics in flows in large diameter pipes are illustrated based on the experimental data. The flow regimes and their transition criteria are discussed. The existing drift-flux models are summarized, their strengths and weaknesses are noted and the data that can be used to evaluate these models are presented. Based on the flow regime transitions in large diameter pipes, all of the available drift-flux models are evaluated systematically in both low (bubbly) and high (cap and churn-turbulent) void fraction flows. The drift-flux type correlations of Hibiki and Ishii [14] and Kataoka and Ishii [24] are found to be able to give the best predictions for the existing low and high void fraction databases respectively and are recommended for void fraction predictions in flows in large diameter pipes.

Experimental Study on Interfacial Area Transport of Two-Phase Flow under Vibration Conditions

An experimental study on air-water two-phase flow under vibration condition has been conducted using double-sensor conductivity probe. The test section is an annular geometry with hydraulic diameter of 19.1u2009mm. The vibration frequency ranges from 0.47u2009Hz to 2.47u2009Hz. Local measurements of void fraction, interfacial area concentration (IAC), and Sauter mean diameter have been performed along one radius in the vibration direction. The result shows that local parameters fluctuate continuously around the base values in the vibration cycle. Additional bubble force due to inertia is used to explain lateral bubble motions. The fluctuation amplitudes of local void fraction and IAC increase significantly with vibration frequency. The radial distribution of local parameters at the maximum vibration displacement is specifically analyzed. In the void fraction and IAC profiles, the peak near the inner wall is weakened or even disappearing and a strong peak skewed to outer wall is gradually observed with the increase of vibration frequency. The nondimensional peak void fraction can reach a maximum of 49% and the mean relative variation of local void fraction can increase to more than 29% as the vibration frequency increases to 2.47u2009Hz. But the increase of vibration frequency does not bring significant change to bubble diameter.

Experimental study of gas–liquid two-phase flow through packed bed under natural circulation conditions

Dry-out phenomena in packed beds or porous media may cause a significant digression of cooling/reaction performance in heat transfer/chemical reactor systems. One of the phenomena responsible for the dry-out in packed beds is known as the counter-current flow limitation (CCFL). In order to investigate the CCFL phenomena induced by gas–liquid two-phase flow in packed beds inside a pool, a natural circulation packed bed test facility was designed and constructed. A total of 27 experimental conditions covering various packing media sizes (sphere diameters: 3.0, 6.4 and 9.5 mm), packed bed heights (15, 35 and 50 cm) and water level heights (1.0, 1.5 and 2.0 m) were tested to examine the CCFL criteria with adiabatic air–water two-phase flow under natural circulation conditions. Both CCFL and flow reversal phenomena were observed, and the experimental data including instantaneous and time-averaged void fraction, differential pressure and superficial gas–liquid velocities were collected. The CCFL criteria were determined when periodical oscillations of void fraction and differential pressure appear. In addition, the Wallis correlation for CCFL was utilized for data analysis, and the Wallis coefficient, C, was determined experimentally from the packed bed CCFL tests. Compared to the existing data-sets in literature, the higher C values obtained in the present experiment suggest a possibly higher dry-out heat flux for natural circulation debris systems, which may be due to the water supply from both top and bottom surfaces of the packed beds. Considering the effects of bed height and hydraulic diameter of the packing media, a newly developed model for the Wallis coefficient, C, under natural circulation CCFL is presented. The present model can predict the experimental data with an averaged absolute error of ±7.9%.