Investigation Of Flow Instability Triggering Conditions Of A Subcooled Flow Boiling Natural Circulation System

The natural circulation refers to a common continuous flow without external drivenforce in a closed system. The buoyancy force, resulted by the bubbles generated duringthe boiling of fluid, drives the fluid flow by overcoming friction and gravity pressuredrop. In this process, due to system flow rate coupling each thermal parameters, systemflow shows the strong instability, which has an important effect on the safety andstability operation of nuclear reactor, boiler hydraulic system and other thermal devicesystems. hence, it is very important for the prediction and analysis of influent factors offlow instability. With water as the working medium, this paper has investigated thefactors which affect on the triggering points of flow instability in a natural circulationsystem at subcooled boiling conditions. According to the theoretical analysis of theexperimental system, an semi-empirical correlation and a dimensionless correlationhave been proposed which can predict the triggering conditions of density waveoscillation, and instability boundary has been calculated and analyzed.To analyse the relationship between the void fraction in the natural circulationtwo-phase length and various parameters, this paper studies the nucleation law ofbubbles in heating section in first. The influence of system parameters on nucleation sitedensity and bubble frequency is obtained.Using the experimental results of the nucleation law, this paper analyses themechanism of natural circulation instability. Study reveals that increasing inletsubcoolling and system pressure can improve the system stability. Increasing heat fluxmakes the subcooling boundary area of density wave oscillation bigger. And anon-dimension instability boundary map is obtained which shows the syntheticalinfluence of various parameters on the instability.To study the system-phase length on impact of instability boundary, this papercalculates the location of ONB points at the stability boundary. Result indicates thatboth of heat flux and system pressures have a high impact on the occurrence of ONBpoint at stability boundary. Consider the influence of two-phase length in non-heatedsection on instability, this paper evaluates the two-phase lengths at stability boundaryunder different subcooling and system pressure, which relationship with the subcoolingis a para-curve going upwards. This paper also finds that whether increasing systempressure at low subcooling region or reducing system pressure at high subcooling region may make the two-phase length larger.According to the dimensionless analysis of the two-phase flow differentialequations, an semi-empirical correlation has been proposed which can predict thetriggering conditions of density wave oscillation, the relative error of which is less than±10%. The instability boundary map are also obtained according to solving thecorrelation and system energy conservation equation.Finally, the paper has carried on the theoretical research on the onset conditions ofthe natural circulation flow instability, and obtain a dimensionless correlation whichcan predict the boundary of flow instability. The calculated two-phase length arecompared with the experimental calculation results, the relative error of which is lessthan±30%. This paper also calculated and analyzed the influence of various thermalparameters on the instability boundary. The calculation results are well consistent withthe prediction results of semi-empirical correlation and the experiment analysis results.