Study On Hydrodynamics And Heat Transfer Characteristics Of Droplet Impacting On Cylindrical Surface

The process of droplet impingement on the wall contains abundant physical mechanism,involving the intersection of many disciplines such as fluid dynamics,heat and mass transfer and interface science.Therefore,there are many influencing factors,and the influence of wall geometry on droplet impact dynamics and heat transfer characteristics is particularly important.At present,the vast majority of researches focus on the case of single or even multiple droplets impacting on the flat surface.However,due to the complexity of the engineering environment,the case of droplets impacting on the cylindrical surface is also very common,for example,the spray falling film on the horizontal tube,the spray cooling on the nuclear fuel rods,the quenching on the cylindrical steel and the cooling of cylindrical glass mold,etc,all involve the situation of droplets impacting on the cylindrical surface.Due to the anisotropy caused by the geometric structure of cylindrical surface,there is a significant difference of droplet hydrodynamic behaviors between the circumferential and axial directions after impacting,which affects the heat transfer characteristics,but there are few reports in this aspect.Therefore,in this paper,the hydrodynamic behaviors and heat transfer characteristics of droplets impacting on the cylindrical surface are systematically and deeply studied by means of theoretical analysis,numerical simulation and experiment.This paper started with the hydrodynamic behaviors of single droplet impacting on cylindrical surface.The contact time and the maximum spreading length of liquid film during the transient process(millisecond scale)of single droplet impact were studied.On the one hand,to explore the physical mechanism of the reduction of contact time after droplet impacting on the cylindrical surface,the water spring theory was extended to the case of droplet impacting on the cylindrical surface through force analysis.Combining the surface energy and Hooke’s law,the stiffness coefficient in the water spring theory was redefined to make the theory more universal in analyzing contact time.At the same time,the empirical correlation for predicting contact time was improved.Inspired by the influence of cylindrical anisotropy on contact time,a wettability controllable surface was designed to control the contact time during droplet impact.On the other hand,the influence of cylindrical anisotropy on the maximum spreading lengths of liquid film in the circumferential and axial directions was investigated through numerical simulation.Based on the numerical simulation and the principle of energy conservation,the prediction correlations of maximum spreading length of liquid film in the circumferential and axial directions and the theoretical relationship between them were established.The results showed that,the wall wetting characteristics and the droplet impact velocity have a great influence on the maximum spreading length of liquid film,but the diameter of the cylindrical surface does not.The prediction error of the relationship is within the acceptable range.On the basis of hydrodynamic research,the heat transfer characteristics of the initial transient process(millisecond scale)of successive droplets impact were studied.Through numerical simulation,the liquid film flow and heat transfer characteristics of a single droplet and successive droplets impacting on the heated cylindrical surface were compared.The influence of cylindrical anisotropy on local flow field in the liquid film was clarified.It was found that in the retraction stage of liquid film,the high-temperature liquid near the wall at the center first flows upward,and then flows along the gas-liquid interface to the three-phase line,which reveals the formation mechanism of the cross shaped high-temperature region at the center of gas-liquid interface.For the case of successive droplets impacting,the reason why the solid-liquid contact area continues to increase,but the total heat flow and convective heat transfer coefficient significantly decrease was proved through the quantitative analysis of the average temperature and thickness of the liquid film.In order to make up for the shortage of less successive droplets,shorter simulation time and limited process in numerical simulation,an experimental platform was built to explore the steady-state single-phase heat transfer mechanism of the whole process from the impact time to the liquid film dropping when successive droplets impact on the heated cylindrical surface.By using the measured local wall temperature as the boundary condition,the local heat flux and local convective heat transfer coefficient can be obtained by solving the heat conduction differential equation.Through the evolution law of local wall temperature,the circumference is divided into impingement zone,heat diffusion zone and tail detachment zone.The heat transfer performance gradually decreases from the impingement zone to the heat diffusion zone,but slightly increases in the tail detachment zone.The reason for the difference among the heat transfer performance in different zones is explained from the fluctuation of the gas-liquid interface and the disturbance in the flow field.When the impact frequency of successive droplets is large enough,the improvement of heat transfer performance is no longer obvious through further increasing the impact frequency.Increasing the droplet impact velocity mainly improves the heat transfer performance of the upper half of the heated cylindrical surface.When the droplet impact velocity exceeds a certain critical value,the droplet splashing will occur.The critical velocity of droplet splashing has a negative correlation with the impact frequency and droplet diameter of successive droplets.When the input heat flux increases,the heat transfer performance on all zones is improved.Finally,the corresponding heat transfer correlations were established for each zone.In order to further investigate the steady-state boiling heat transfer mechanism of successive droplets impacting on the heated cylindrical surface,an experimental platform with high heat flux was built for steady-state boiling heat transfer,and a programmable temperature acquisition system was designed to realize the purpose of rapid acquisition of temperature signals.The impact frequency of successive droplets has a great influence on the single-phase heat transfer stage and the early stage of nucleate boiling,but has a little influence on the late stage of nucleate boiling.The thermophysical mechanism was analyzed from two aspects,i.e.,sensible heat and latent heat of vaporization of liquid.The impact velocity of successive droplets has a great influence on the single-phase heat transfer stage and the early and late stages of nucleate boiling,and it was found that in the late stage of nucleate boiling,the change of the impact velocity of droplets mainly affects the latent heat of liquid vaporization,but has little influence on the sensible heat of liquid.According to the evolution of local wall temperature and the boiling behaviors of liquid film,the triggering mechanism of the cylindrical anisotropy on the critical heat flux was clarified.At last,in order to investigate the transient heat transfer mechanism of successive droplets impacting on a heated cylindrical surface with different initial wall temperatures,it was started from the experimental study of a single droplet impacting on the heated cylindrical surface,and then went deep into the study of successive droplets impact.The study of transient heat transfer characteristics of single droplet impact provided a basis for the analysis of successive droplets impact.First of all,in the experiment of a single droplet impact,the map of heat transfer characteristics was drawn by using the wall temperature and the Weber number.It was found that the Leidenfrost point decreases with increasing droplet impact velocity,which is contrary to the conclusion when droplet impacts on the heated flat surface.The formation mechanism of Leidenfrost state was analyzed by comparing the vapor pressure between solid and droplet with the dynamic pressure after droplet impact,and the heat transfer mechanism behind this phenomenon was revealed in combination with the influence of surface geometry.On this basis,the case of successive droplets impacting on cylindrical surfaces with different wall temperatures was conducted,by comparing the average wall temperature drop rate,it was found that the impact frequency of successive droplets has the most obvious influence on the nucleate boiling stage.When the initial wall temperature is high enough,the heat transfer state will change from film boiling to nuclear boiling along with the successive droplets impact,and the change of heat transfer state will be delayed with the increase of droplet impact velocity.On the one hand,this research enriches and develops the hydrodynamics as well as heat transfer theory of droplet impacting on solid wall.On the other hand,it also provides a scientific theoretical basis for the thermophysical design and application of the process of droplet impacting on the cylindrical surface in different fields.