| Countercurrent flow widely exists in practical applications of industrial systems.It has a very important effect on the safety and economics of the equipment.For a large breach in a nuclear power plant reactor,the flooding in the annular channel relate to the core melting.Most of the existing flooding prediction models are based on the Wallis correlation,which is mainly a simple linear fitting of the dimensionless liquid and gas superficial velocity.The interface friction between the two phases is an important factor in flooding,which has been paid little attention.Combining the experiment and numerical simulation,the physical characteristics and mechanism of flooding was studied in this paper.Finally,based on the analysis mentioned above,a new prediction model for flooding was established.In this paper,based on the countercurrent flow experiment in a tube with inner diameter 25 mm at ambient temperature and pressure,a parallel electrode probe was used to measure the film thickness and the variation characteristics of liquid film with gas flow rate were obtained.Flow pattern was photographed by the high-speed camera.Combining the variation characteristics of liquid film and visualization results,different flooding mechanisms respectively at low and high liquid flow rate can be obtained.One is the upward propagation of the interface wave.The other is the bridge of liquid film in the tube so that a turbulent flow has been formed.It is believed that there is a transition region containing two flooding mechanisms between the two liquid flow rate.In addition,the variation of the liquid film during the flooding was analyzed from the time domain and the frequency domain to obtain the liquid film fluctuation characteristics.It was obtained that the spectral characteristics of the flooding distributed in a wave shape,and the peaks basically appeared at the frequency of 0-10 Hz.Based on the analysis of the physical characteristics during flooding,a momentum equation for flooding was established to obtain a flooding prediction model that takes into account the interface and wall friction forces.The existing interface friction factor model were evaluated and modified the model based on the Moeck et al model.The present interface friction factor model was established in different range of liquid flow rate which was suitable for present experimental conditions.At the same time,the Bo number was introduced into the model to apply to different type of channel and sizes.The applicability of present flooding prediction model was well established.By comparing the calculated data with the flooding data in round tubes and rectangular channels with different sizes,the relative error is basically within 30%,within 95% confidence interval.Based on numerical calculation,the interface waves during flooding were analyzed.The Variation in the interface wave form and the film thickness were compared.It was found that the two trends were basically the same.At the same time,by observing the variation in the flow field during the growth of the interface waves,a pressure difference between the windward side and the leeward side of interface wave has been found,which created an upward force on the interface wave and accelerated the wave peak to be torn by the airflow or caused the interface wave to move upward to trigger flooding. |
