Investigation On Pool Boiling Heat Transfer Of Gradient Structures

Boiling heat transfer has high heat transfer efficiency.Compared with convective heat transfer,it can maintain low superheat of heat surface.At the same time,compared with other phase change heat transfer methods,boiling heat transfer is simple,economical and mature technology,so boiling heat transfer is widely used in various thermal engineering.Boiling heat transfer has the characteristics of fast transient,phase change,multistage,complex heat transfer process,and there are many limitations on the understanding of boiling heat transfer process and mechanism.Therefore,boiling heat transfer has always been of research in the field of phase change heat transfer.In the present study,gradient metal foam and square pillar arrays are used as research objects.The experimental results and numerical simulation are combined to study the boiling heat transfer performance and mechanism of gradient metal foam and square pillar arrays,including the effect of pore density,foam thickness and surfactant on the boiling heat transfer performance of the metal foam,as well as the influence of wettability and surface morphology on the heat transfer performance of the pillar arrays.In addition,a numerical model of the square pillar arrays is established,and the boiling heat transfer mechanism of the pool is revealed from the perspective of bubble dynamics.The main research contents and results are as follows.(1)Pore density,foam thickness and surfactant have a significant effect on the heat transfer performance of metal foam,and double-layer gradient metal foam can enhance heat transfer.The surfactant TX-100 and polyvinyl pyrrolidone(PVP)deteriorate the heat exchange as result of increasing the viscosity of the liquid,which is not conducive to the escape of bubbles.(2)After the square pillar arrays are plated with silver,the contact angle becomes larger and the heat transfer is deteriorated,and the gradient square pillar array further weakens the heat transfer due to the fact that bubbles are easily formed on the surface of the gradient array.After adding isopropanol or n-heptanol to deionized water,the number of vaporization cores and bubble volume both decrease,and the heat transfer is weakened at a low heat flux.However,at high heat flux,the bubbles can be effectively blocked to form a gas film,and the heat transfer is enhanced.(3)The numerical model of the square pillar array shows the influence of different surface morphology and different wettability on the formation and detachment of bubbles,which promoting the understanding of the mechanism of pool boiling heat transfer.