Experiment And Calculation Of Boiling Heat Transfer Characteristics Of Aluminum Alloy Rod At High Temperature Value Research

As a reliable clean energy,nuclear energy has been widely used and developed in many countries and regions.However,because accidents in nuclear reactors may lead to serious consequences,the safety of nuclear reactors has always been a concern since the beginning of the development of nuclear energy.The fuel element is the core and key component of the nuclear reactors,and it is also the first safety barrier containing the release of fission products,most of the energy released by the nuclear fission chain reaction is first converted into internal energy in the fuel pack element,and then transferred to the cooling system.Therefore,the performance of fuel elements,especially their thermal and hydraulic performance,has always been one of the important contents of nuclear reactor design and development.The loss of water accident is one of the most important design basis accidents in nuclear power plants,when a water loss accident occurs,in order to prevent the exposed fuel elements of the core from overheating and burning,it is necessary to inject emergency cooling water into the core to quickly flood the core.During the re-flooding of the core,the surface of the fuel element cladding will boil,and its boiling heat transfer characteristics determine the maximum temperature and the degree of damage that the fuel element cladding can reach.This paper uses a combination of experiment and numerical simulation to study the boiling heat transfer characteristics of the high-temperature surface of the zirconium alloy rods.The specific research results are as follows:（1）Using the self-designed experimental device,carry out the zirconium alloy rod pool boiling experiment,and obtain the surface temperature curve of the zirconium alloy rod in the range of 650℃-1000℃and the cooling water at 80C,90℃and 100℃.The results show that when the initial temperature of the zirconium alloy rod increases from 750°C to 820°C,the change trend of the surface temperature curve is the same,and the duration of the film boiling stage is about 25 s;as the cooling water temperature increases,the temperature curve shifts to the right.The duration of film boiling is significantly increased;as the degree of surface oxidation increases,the temperature curve shifts to the left,and the film boiling stage is shortened.（2）The transition boiling and critical heat flux（CHF）are studied through the above experiments.As the initial temperature of the zirconium alloy rod increases,the CHF gradually increases.When the initial temperature of the zirconium alloy rod increases from 750°C to 820°C,the CHF increases from 536 k W/m² to 583 k W/m²;as the cooling water temperature increases,CHF gradually decreases.The corresponding CHF of 650℃zirconium alloy rod at 80℃,90℃and 100℃water temperature is 327k W/m²,310 k Wm² and 298 k W/m² respectively;as the thickness of the oxide film increases,the boiling curve moves downward.CHF is slightly reduced,and the boiling heat transfer on the surface of the zirconium alloy rod after oxidation is weakened compared with that before oxidation.（3）Through the above experiments,the relationship between the minimum film boiling temperature(T_min)and the temperature of the cooling water and the degree of surface oxidation was studied.T_min increases with the decrease of cooling water temperature and the increase of oxide film thickness.For zirconium alloy rods at 650°C,when the cooling water temperature decreases from 100°C to 80°C,T_min increases from490.1°C to 526.2°C.As the degree of surface oxidation deepens,T_min increases from480.5°C to 497.3°C.The experimental results are compared with the predicted value of the minimum film boiling temperature established in other literatures with the cooling water temperature.The experimental results and predicted values show a certain difference.Only when the cooling water temperature is 80℃,the predicted value is relatively close to the experimental value,and when the cooling water temperature is 90℃and 100C,there is a big difference.This may be because the T_min temperature is affected by other factors（for example,surface conditions,vapor phase film collapse mode,etc.）.（4）Numerical calculation model of the zirconium alloy rod and its surrounding coolant channels,and simulate the flow boiling heat transfer on its surface.The results show that when subcooled flow nucleate boiling occurs on the surface of the aluminum alloy rod,the gas phase is unevenly distributed along the axial direction.The volume fraction of the gas phase at the outlet increases with the increase of the inlet temperature and heat flow density.The closer the inlet temperature and heat flux density increase to the inlet end,that is to say,the earlier the boiling phenomenon occurs and the formation of bubbles.（5）The relationship between the thickness of the gas film,the velocity of the quenching front and the cooling water temperature and other factors are experimentally studied.In the pool boiling experiment with lower water temperature,the gas film on the surface of the zirconium alloy rod is thin,which indicates the enhancement of heat transfer.The thickness of the gas film becomes thinner as the temperature of the cooling water decreases,and the rapid cooling is accelerated due to the increase in the surface roughness of the zirconium alloy rod after oxidation,and the duration of the entire gas film is greatly shortened.The moving speed of the quenching front is a key factor affecting the cooling rate.The experimental results show that the moving speed of the quenching front decreases with the increase of the initial temperature of the zirconium alloy rod,and increases with the decrease of the cooling water temperature.（6）The numerical simulation captures the evolution process of the gas film on the surface of the zirconium alloy rod,as well as the formation and release of the top vapor mass,which is similar to the evolution process of the gas film during the experiment.The simulation results can clearly observe the hydraulic power around the gas-liquid interface waves,and the gas film on the surface of the zirconium alloy rod will not exist stably,it moves upward under the action of the force,and then leaves the surface of the zirconium alloy rod.