Numerical Simulation Of Multiphase Flow Instability In A Heat-absorbing Tube Under Different Temperature Boundary Conditions

Multiphase flow instability widely exists in nature and industrial production and has important applications in aerospace,material preparation,fluid transportation,and other fields.Due to the complexity of multiphase flow instability and its important influence on the flow and heat transfer of industrial production equipment,the related research has important application value for practical engineering applications.Front Tracking Method(FTM)has the unique advantages of high-precision interface tracking and efficient calculation of surface tension.In this paper,a two-phase flow model was established by FTM and thermal flow coupling in different temperature boundary conditions was used to numerically study the two-phase flow instability in heat collector tubes.Firstly,the evolution process of two-dimensional immiscible and incompressible two-phase flow Kelvin-Helmholtz(K-H)instability on inclined wall was simulated under the influence of no temperature field.It was found some basic rules that the evolution of interface on inclined wall.The results showed that the inclined wall had a positive effect on the upward and inward development of interface.The thickness of velocity gradient layer had a significant inhibition on the interface development.The influence of different gravity term of Richardson number Rig on K-H instability was counteracted by the interaction between the horizontal component of the upper fluid volume force and the vertical component.The influence of surface tension term Riσ on the development of interface was mainly the shape of the interface,which had little influence on the billow height of the interface.These conclusions were expected to provide further help for the exploration of the interaction between the heat-fluid coupling effect and interface fluctuations under different temperature boundary conditions.Secondly,the interaction between the interfacial fluctuation and heat transfer on an inclined wall with constant temperature boundary was investigated.The effects of different wall inclination angles(φ),Marangoni number(Ma),and Rayleigh number(Ra)on interfacial instability were investigated.The results showed that as the wall inclination increases,the interface became unstable and the heat transfer at the interface decreases.It was also found that the heat transfer was more stable at inclinationφ=45° in the four groups of φ=0°,30°,45°,60° comparison.The heat flux density between fluids decreased with the increase of the Ma number,which had little influence on the interface fluctuation,but mostly affected the interface morphology by changing the surface tension gradient at the interface,mainly at the end of the convolution interface.It was shown that the larger the Ma number,the inward development of the interface became more obvious and the inward development of the interface became more obvious with the increase of the inclination angle.The Ra number had an obvious inhibitory effect on the interface instability,which had an alternating change in the heat transfer at the interface.It was manifested in that the heat transfer between fluids when the wall was inclined in the early stage of development was greater than that when the wall was horizontal,but in the heat transfer in the later stage of development was less than that of the horizontal wall.A direct numerical simulation of the two-phase flow instability under the temperature boundary condition periodically fluctuating with time was carried out,and the influence of different wall inclination φ,fluctuation amplitude AT,angular frequency ωT,phase angle θT,and Ra numbers on the evolution of interface fluctuation and heat transfer were studied.The results showed that as the inclination angle increases,the maximum temperature that the upper fluid increases,and the minimum temperature decreases.When the wall was horizontal,the heat flux qy,y=0.5 in the ydirection on the horizontal line of y=0.5 reached a higher and lower extreme value.With the increase of the wall inclination,the mixing area gradually shifts to the latter half of x=0.5,and at φ=45° the radial heat transfer was the most stable.As the amplitude AT increases,the average temperature of the upper fluid was higher,but it had little effect on the time required to reach the maximum value.As the AT increased,the peak value of qy,y=0.5 was also higher.With the increase of ωT,the period of temperature fluctuation was shorter,and the highest and lowest value of the average temperature difference ΔTm of the upper fluid caused by the inclination of the wall was lower.It was found that the heat flow difference in the y-direction Δqy,5π/6 andΔqy,7π/6 was approximately symmetrically distributed.When θT=π/2 andθT=-π/2.ΔTm,φ=30° are symmetrically distributed.The influence of different θT on the temperature field was mainly caused by the superimposed effect of the influence of the disturbance in the middle of the basin on the temperature conduction and the temperature fluctuation of the upper wall caused by the development of interface instability.The acceleration effect of the velocity gradient on both sides of the interface and the downward suppression effect of the volume force on the cold and hot convolution fluid were counterbalanced,affecting the interface morphology of the mixed region.When Ra=39.24×104,a longer heat conduction stage step one appears.The absolute value of the maximum value of the peak H and trough D interface fluctuation Max |B(t)| decreased with the increase of Ra number,and the interface fluctuation of H smaller than D.The interface fluctuations of H and D were linearly distributed.