Research On Inversion Of Heat Flux And Characteristics Of Boiling Heat Transfer On Cylindrical Surface During Quenching

When a high-temperature test section is submerged in a low-temperature cooling medium,the boiling phenomenon that occurs on the surface of high-temperature test section is called quenching boiling.The quenching boiling is widely used in the fields of metal heat treatment process and nuclear reactor core safety.Compared with heating boiling,the quenching boiling has its own characteristics and laws under the effect of initial conditions and boundary conditions.The acquisition of the surface temperature and heat flux of the quenching test section is the necessary conditions for obtaining the boiling curve.In order to prevent the installation of temperature sensors from destroying the boiling surface morphology and affecting the boiling process,this research proposed the inverse heat conduction problem to inversely obtain the surface temperature and heat flux during the quenching boiling process.The one was improving the optimization algorithm of the inverse heat conduction problem to improve the accuracy and efficiency of inversion and the other was summarizing the phenomenon,characteristics and laws of quenching boiling of the high-temperature cylindrical test section to reveal the heat and mass transfer mechanism of quenching boiling.The main research contents and results are as follows:Firstly,this paper optimized the mathematical model of inverse heat conduction problem,coupled the conjugate gradient method and the sequential function specification method and proposed the sequential conjugate gradient method.The accuracy and efficiency of proposed inversion mathematical model were verified by numerical experiments and quenching boiling experiments.Compared with the conjugate gradient method,the proposed method significantly improves the inversion calculation efficiency;compared with the sequential function specification method,it effectively improves the inversion calculation accuracy.Secondly,the experimental platform of cylinder quenching boiling in water pool was designed and constructed.Through visual research,the effects of coolant subcooling,surface roughness and surface materials on the evolution of vapor film were clarified during cylinder quenching boiling,and the apparent cognition of the quenching boiling phenomenon was obtained.By programming the MATLAB image processing program to process the visualized images,the change laws and fluctuation characteristics of the vapor film in the quenching process were obtained,and the change rules of the propagation velocity of the quenching front with the coolant subcooling was summarized.The experiment results shown that the duration of the film boiling and the duration of the quenching process decrease gradually with the increase of coolant subcooling and surface roughness,and the decrease of solid thermophysical property.The film thickness of the film boiling gradually decreases,the film fluctuation intensity decreases and the propagation velocity of the quenching front increases with the increase of coolant subcooling.Finally,based on the measured temperature inside the test section during quenching boiling,the surface temperature and heat flux were obtained by proposed inverse heat conduction problem.The influence of coolant subcooling,surface roughness and surface materials on the surface temperature and heat flux in quenching boiling were analyzed.The internal relationship between the evolution of vapor film and the surface temperature and heat flux was analyzed.The relationship between the minimum film boiling temperature and film boiling Nusselt number and coolant subcooling,surface roughness and solid material thermophysical properties were summarized.The prediction models of the minimum film boiling temperature and film boiling Nusselt number were established to scientifically describe the multiphase flow and heat and mass transfer mechanism of submerged quenching boiling.The experiment results shown that the intensity of quenching boiling heat transfer is enhanced,the cooling rate increases and the minimum film boiling temperature increases with the increase of surface oxidation,coolant subcooling and surface roughness and the decrease of thermophysical properties of solid materials.The Nusselt number of film boiling heat transfer increases with the increase of coolant subcooling,but it is almost not affected by the surface roughness.The above-mentioned research further improves the understanding of principle of the inverse heat conduction problem and the quenching boiling of high-temperature test section.The inverse heat conduction problem can provide an effective non-destructive indirect measurement method for parameters that are difficult to be directly measured due to special safety requirements or structural integrity requirements in engineering practice.In this study,the law of heat and mass transfer,multi-phase flow and heat and mass transfer mechanism in the quenching boiling process were studied and discussed.It is expected that the research results can provide a theoretical basis for the research of metal heat treatment process and large-break loss of coolant accident in pressurized water reactor.