A Study On Subcooled Flow Boiling Incipience And Heat Transfer Under Flow Fluctuation

The flow rate can fluctuate in offshore nuclear power systems which are exposed to wind and waves,as well as in loops where flow instabilities occur,resulting in different thermal-hydraulic characteristics compared with that under steady flow.Among the thermal-hydraulic characteristics,onset of nucleate boiling(ONB)model determines whether the fluid is boiling,and boiling heat transfer is crucial to equipment performance and safety,both being key issues in subcooled flow boiling.Therefore,researches ranging from single-phase convection,to boiling incipience,to subcooled flow boiling under flow fluctuation were studied by theory,numerical simulation and experiment.Single-phase pulsatile flow heat transfer was studied,including the parameters' fluctuation,flow regime transition and heat transfer characteristics.By separation of variables and Green function method,temperature field of laminar flow considering wall thermal inertia was obtained.It was found that the wall thermal energy,Prandtl number,the fluctuation amplitude and period of the flow rate are the key parameters that affect the fluctuations of heat flux,temperature and Nusselt number.The influence of each parameter was analyzed.In the experiment,flow regime transitions were found affected by flow fluctuation.Flow transition leads to large heat transfer variation,resulting in a peak normalized temperature fluctuation.In addition,the average heat transfer coefficient is also affected by the flow regime transition.The critical nucleation radius and heat flux at ONB were obtained by the theory of nucleation from cavities.The critical nucleation radius and heat flux of boiling incipience controlled by superheat equation are functions of wall superheat,and the heat flux of boiling incipience controlled by cavity size is a function of cavity size,fluid subcooling and heat transfer coefficient.With convective heat transfer considered,the influence of pressure,liquid subcooling and heat transfer coefficient on ONB condition were analyzed.In addition,the limiting factor of boiling incipience will change with the increase of pressure,heat transfer coefficient and fluid subcooling.Based on self-consistent thermodynamic theory,the relation between heat flux,critical nucleation radius at ONB and parameters such as wall superheat,heat transfer coefficient and fluid subcooling were obtained.The result was compared with the result from the cavity nucleation theory.A thermodynamic model of nucleation from insoluble bubbles was established.The dominant factor and characteristic of change in Gibbs free energy were analyzed.The presence of insoluble bubbles,the contribution of the line stress and the effects of wall temperature,temperature gradient and surface tension on the nucleation energy barrier and the critical radius were obtained.Based on the cavity nucleation theory,analytical method of ONB under flow fluctuation was proposed by considering the impact that flow fluctuation introduces.The mechanism of the superheat variation at ONB was revealed.The effect of wall temperature fluctuation under different heat transfer coefficient and liquid subcooling was analyzed,and under certain conditions the above effect are relatively large.The experimental trend is consistent with the theory,but the values are slightly larger.In addition,flow fluctuation may cause flow regime transition,the consequential heat transfer affects wall superheat at ONB.Subcooled flow boiling visualization and heat transfer experiments were conducted.The variation of thermal-hydraulic condition caused by flow fluctuation will affect the maximum diameter and the bubble departure type,but the bubble growth and condensation still agrees with the correlations of Zuber or Akiyama.The surface topography of the test section causes high wall superheat at ONB,in the case of high liquid subcooling,if the condensed bubbles interact with the subsequent growing bubbles,the gas-liquid interface will be unstable,and then the compound relaxation instability and acoustic instability occur.Considering the effects of the relative low flow rate and the nucleation characteristics of the heated surface on heat transfer,the convective heat transfer and the pool boiling heat transfer were modified,and then Liu-Winterton correlation achieves more accurate prediction.Under intermittent boiling condition,the average heat transfer is influenced by flow fluctuation,and the prediction method of the average heat transfer was proposed.The calculated results are in good agreement with the experimental data.