Numerical Simulation Of Boiling Bubble Motion In Nano-Fluid Pool Based On LBM Method

As an effective heat transfer method,nuclear boiling is widely used in industrial production,such as cooling of electronic components,steam generators,air conditioning refrigeration and power generation.Nanofluids as a heat transfer medium is an effective heat transfer enhancement method,and the mechanism of nanofluids enhancement of heat transfer is currently a hot research topic.At present,there are many experimental studies on boiling heat transfer of nanofluids,but there are still divergent conclusions about the effect of the addition of nanoparticles on boiling heat transfer.At present,there are few numerical simulations of boiling heat transfer of nanofluids.Traditional simulation methods are difficult to study the random motion of nanoparticles,the interaction between nanoparticles and fluids,heat transfer,and the small size effect of nanoparticles on boiling transfer The effect of heat.The lattice Boltzmann method(LBM method),as a mesoscopic scale simulation research method,can better understand the influence mechanism of physical property parameters on its boiling process.In this paper,the lattice Boltzmann method is used to simulate the natural convection filled nanofluids triangle cavity,the growth and deperture process of single bubble in nanofluids pool boiling,and the merger process of double bubbles in nuclear boiling process.The effects of gravity G,particle volume fraction δ and particle size a on the growth and deperture behavior of water-based alumina nanofluids boiling bubbles were studied.The effect of double bubble merging on boiling heat transfer was analyzed,as well as the effects of bubble separation distance,wall superheat,heater length,gravity and wettability on bubble growth.First,according to the nuclear boiling process,the mathematical model is established and simplified,the appropriate nanofluids model is selected,the boundary conditions of the model are set,and the relevant programs for simulation are written in C language.The analysis of the results obtained from the simulation mainly includes the following aspects:1.The natural convection of nanofluids in an isosceles right-angled triangular cavity was studied,and the effects of Rayleigh number(103≤Ra≤106),particle volume fraction(2%≤δ≤10%),heat source position(0.2≤D≤0.8),particle size(10nm ≤a≤100nm)on natural convection heat transfer mechanism were analyzed,The effects of two nanofluids(water-alumina)models on the simulation results were compared.2.Simulated the growth and deperture behavior of single bubbles during the boiling process of pure water and water-based alumina nanofluids,mainly studying the behavior of gravity G,particle volume fraction δ and particle size a on the bubble growth and deperture behaviors of water-based alumina nanofluids influences.3.Simulated the merging of double bubbles in the process of nuclear boiling,and introduced the merging process and temperature distribution of double bubbles,and calculated the wall heat transfer rate under different conditions.Mainly studied the effect of double bubble merging on wall heat transfer,and the effect of bubble separation distance,wall superheat,heater length,gravity and wettability on bubble growth,and whether bubble separation distance and wettability affect bubble merging influences.Through the comparative analysis of the results,the following conclusions are obtained:1.Using a single-phase nanofluids model,the simulation results show that the average Nusselt number ratio(Num,nf/Num,f)of the hot wall surface increases approximately linearly as the volume fraction increases.The results of the improved nanofluidic model show that the average Nusselt number ratio increases with increasing volume fraction,but the slope of the change in the average Nusselt number ratio gradually decreases.Comparing the two models,the heat transfer efficiency simulated by the improved model is higher than that of the single-phase model.This is because the improved model considers the force between particles and heat transfer,which is more in line with the actual situation.2.Compared with pure water,the addition of nanoparticles increases the surface tension of the nanofluids,which increases the diameter of bubbles detached from the nanofluids.The change trend of single bubble separation time with acceleration of gravity in nanofluids is similar to pure water.As gravity increases,the difference between bubble separation time of nanofluids and pure water decreases.Nanoparticles increase the thermal conductivity of the nanofluids and can enhance heat transfer.The bubble deperture time of the alumina nanofluids decreases as the volume fraction of the particles increases.The smaller the nanometer particle size,the more intense the Brownian motion of the particles.Therefore,as the particle size of the nanoparticles decreases,the bubble deperture time gradually decreases.3.The dynamic behavior of double-bubble merging is studied.The simulation results of the shape and equivalent diameter of the double-bubble merging process are in good agreement with the experimental results in the references.A typical nucleation-growth-merging-oscillation-deperture process was found.The heat flux change of the bubbles has two main peaks,which are respectively due to the bubble merging and bubble deperture.(The first peak is due to the evaporation of the microlayer liquid captured between the bubbles,and the second peak is due to the departure of the merged bubbles).The average heat flow during bubble merging is much higher than that of single bubbles.There is a critical distance for merging bubbles.If the separation distance is less than the critical distance,the average heat flow increases with the separation distance.If the separation distance is greater than the critical distance,the average heat flow decreases as the separation distance increases.The critical distance increases with increasing superheat,heater length and contact angle,and decreasing gravity.It is found that the bubble separation diameter is the main factor that determines the critical separation distance.The larger the bubble separation diameter,the larger the critical separation distance.The growth of double bubbles has a critical merging distance(if it is greater than it,the bubbles will not merge).When the contact is small(30-60°),the critical merge distance will not change.As the contact angle increases(60-80°),the critical merge distance gradually decreases.When the contact angle approaches 90°,the critical merge distance tends to be constant.When the contact angle continues to increase(90-110°),the critical merge distance increases exponentially.Whether the two bubbles merge depends on the distance between the bubbles and the shape of the bubbles.