Study On The Evolution Mechanism Of Bubble Generation And Strengthrning Technology Based On Nucleation Boiling Heat Transfer

Compared with natural convection,nucleate boiling has the advantages of high heat transfer efficiency and safe and controllable operating conditions,and is widely used in various thermal engineering applications.Most of the existing studies focus on surface modification to achieve enhanced boiling heat transfer,but there is a lack of research on the kinetic properties of bubbles during nucleate boiling and the mechanism of enhanced boiling heat transfer by surface modification.Based on this,this paper builds an experimental platform for visualising pool boiling,which enables the study of bubble kinetic parameters and flow field behaviour on the heating surface;a VOF（Volume of Fraction）calculation model in two-phase flow is constructed to study the bubble kinetics and heat transfer characteristics on a smooth copper surface.The kinetic properties of the bubbles on the smooth copper surface were investigated by a combination of experiments and simulations.Firstly,the morphology,temperature and flow field changes during the detachment of a single bubble on the heating surface were analysed using numerical simulations,which provided the basis for the subsequent study of multiple bubbles.The results show that the bubble detachment process is influenced by the return flow of the surrounding supercooled liquid,and it is found that an increase in wall superheat accelerates the bubble detachment.The clustering of adjacent bubbles on the heating surface increases the detachment diameter and reduces the detachment time,which facilitates boiling heat transfer,but the clustering of large numbers of bubbles also affects convective heat transfer during bubble ascent.A self-assembled graphene oxide（GO）nano-coated surface was successfully prepared based on a visual cell boiling experimental platform,and the boiling heat transfer performance of the smooth copper surface and GO nano-coated surface was investigated experimentally using deionised water as the experimental medium.The results show that the GO nano-coated surface has a critical heat flow density（CHF）and heat transfer coefficient（HTC）that are 77.61% and 81.24% higher,respectively,compared to the smooth copper plane.The kinetic behaviour of bubbles on different heating surfaces was quantified by bubble visualisation.The results show that the high heat transfer performance of GO nano-coated surfaces is mainly due to more nucleation sites,faster bubble detachment frequency and smaller detachment diameter of bubbles on GO nano surfaces at the same wall superheat.These factors can effectively retard the formation of vapour films on the heated surface,resulting in a higher CHF value for the GO nano-coated surface.The effect of the flow field behaviour on the boiling heat transfer on the GO nano-coated surface was also analysed using PIV（Particle Image Velocimetry）.It was found that the bubbles on the GO nano-coated surface have small interactions with each other and the bubbles rise rapidly,which facilitates the convective heat transfer process.In this paper,we investigate the enhanced boiling heat transfer on smooth copper and GO nano-coated surfaces respectively,and obtain the bubble kinetic behaviour and heat transfer characteristics of single and multiple bubbles on smooth copper surfaces,explaining the mechanism of enhanced boiling heat transfer on GO nano-coated surfaces.