| It is necessary to use wall heat flux partitioning model to calculate thermal hydraulic parameters in the thermal hydraulic design of nuclear reactor core.However,the existing wall heat flux partitioning models are based on conventional channels,and the bubble parameters used in the model are mostly proposed under pool boiling conditions.The bubble behaviors in a narrow rectangular channel is distinctly different from that in a convectional channel due to the effect of geometry confined.It is necessary to carry out in-depth experimental research on wall heat flux in the narrow rectangular channel and propose a wall heat flux partitioning model suitable for the narrow rectangular channel conditions,thereby,improve the calculation accuracy of the thermal hydraulic design and meet the requirements of refined design.In order to observe the bubble behaviors and analyze the wall heat flux partitioning characteristics in a narrow rectangular channel,the experiments were performed at pressures ranging from 0.1 MPa to 0.5 MPa,with inlet subcooling ranging from 20 to 60 K,mass flux ranging from 400 kg/(m~2·s)to 1600 kg/(m~2·s)and heat flux ranging from 100 kW/m~2 to 300kW/m~2.In the present work,we conducted visual investigation to study the bubble behaviors in subcooled flow boiling in a narrow rectangular channel by a high-speed digital camera.A sequence of automatic image processing technology was used to deal with the original bubble images to get relevant bubble parameters,including bubble diameter and bubble velocity,etc.We found that almost all the bubbles will slide along the heating wall,and two types of bubbles can be observed in our experiments:Lift-off bubbles and Sliding bubbles.Wall superheat and pressure have significant influence on the ratio of the number of two kinds of bubbles.The growth rates of these two types of bubbles conform to different power models,respectively.The bubble characteritics parameters are obtained by the bubble basic parameters by automated algorithms.As the wall superheat increases,the bubble nucleation site density,bubble diameter,bubble velocity and bubble density increase.System pressure,liquid subcooling and flow have no effect on bubble nucleation site density.The higher the liquid subcooling degree,the smaller the bubble departure diameter and lift-off diameter,and the product of the bubble departure frequency and the departure diameter is constant.In each operating condition,the bubble diameter and bubble velocity distributions agree well with normal distribution,and the bubble velocity will exceed the liquid velocity as the void fraction increases.Due to different behavior characteristics of the two types of bubbles,the wall heat flux partitioning in the narrow rectangular channel is divided into three parts:the Lift-off bubble heat flux,the Sliding bubble heat flux,and the single phase convection heat transfer heat flux outside the bubble-affected area.Considering the proportion of the two types of bubbles,the wall heat flux partitioning model suitable for narrow rectangular channels is obtained,and the prediction error is within±30%.Moreover,with the increase of the superheat degree of the heating wall,the heat flux of Lift-off bubbles and Sliding bubbles increases,and single phase convection heat transfer heat flux decreases but always accounts for a large proportion,while the Lift-off bubble heat flux always occupies a small proportion.Combining the methods of experimental research and theoretical analysis,the characteristics of bubble behavior in narrow rectangular channels are determined in this paper.A wall heat flux partitioning model suitable for narrow rectangular channels is established for the flow and heat transfer characteristics in the narrow rectangular channel.It provides a more accurate model for the calculation of two-phase flow in the core,and lays a foundation for the refined thermal hydraulic design in narrow rectangular channels. |
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