Experimental Investigation On The Bubble Growth And Coalescence Characteristics In Rectangular Narrow Channel Based On Observation From Bottom

The structure of narrow channel is compact,and the heat transfer coefficient of the boiling two-phase flow in the channel is large,so the narrow channel is currently widely used in the core design of the advanced nuclear reactor.The bubble dynamic behavior accompanying by the boiling two-phase flow is extremely complicated.After the bubble grows to the size corresponding to the channel,the shape of bubble is deformed due to the restriction of the flow channel wall,which causes the bubble behavior in the narrow channel differs from that in the conventional channel.In this paper,the deionized water was used as the working medium,and the visual experimental research was carried out under different thermal conditions.A rectangular narrow channel with four visual sides was used as the experimental section.Combining with the experimental result,the bubble dynamic behavior was studied by image analysis.The main research work of this paper is as follows:Aiming at the image analysis of boiling bubble behavior,an improved image automatic analysis method was established by adding processing methods,like grayscale stretching,edge detection and closing operation.This image analysis method can effectively solve the problem that the bubble profile is not closed caused by the spot in the bubble image segmentation,and realize automatic bubble recognition and bubble parameter acquisition.The extracted bubble parameters include the number of bubbles,the length of the horizontal and vertical axes,the area,and the equivalent diameter.Combining with the image analysis method established in this paper,the bubble growth curve and bubble coalescence behavior in the narrow channel were obtained.By analyzing the bubble growth curve,it was found that the bubble growth rate is faster in the early stage and slower in the later stage,and they are in the inertial controlled growth stage and the thermal diffusion growth stage,respectively.Whether the bubble leaves from the nucleation point after it grows to a certain size or not depends on the force acting on the bubble.As the volume of the bubble increasing,the bubble is elongated under the action of buoyancy and gravity;the shape changes from a sphere to an ellipsoid,and the lengthwise ratio of the longitudinal and transverse axes is gradually greater than 1.In the bubble coalescence process,there are three stages: drainage,mergence and post-coalescence behavior.During the drainage process,the reduction of the inter-bubble liquid film to a critical thickness is an important factor causing bubble coalescence.In the mergence stage,the evolution of dry spots,thick liquid film and residual droplets were found at the bubble bottom.According to the factors inducing bubble coalescence,the bubble coalescence is divided into thermodynamic coalescence and hydraulic coalescence.The hydraulic factors include fluid fluctuation,bubble interface fluctuation and extrusion.Combining with the image analysis technology,a calculation method of the void fraction was proposed for the characteristics of bubble behavior in the narrow channel.In this calculation method,the influence of the channel wall restriction and the contact angle between the bubble and the channel wall were taken into account.Through a large number of the automatic image analysis,the void fraction of the boiling two-phase flow in the narrow channel under different thermal parameters was obtained.Meanwhile,the variation characteristics of bubble density and void fraction under different interface phenomena were analyzed.Different interface phenomena include multi-bubble growth,relatively stable bubble morphology and large bubble slipping.