Numerical Simulation Of Natural Convection And Enhanced Heat Transfer By Vibration Outside Microtubules

With the appearance of miniaturized devices,the solution of heat dissipation and cooling problem of high-density miniature devices is the guarantee of equipment performance and working life.The temperature distribution of traditional heat exchanger is not uniform and the flow has a dead angle.It is imperative to explore a new micro scale heat transfer mode.Therefore,a microtube heat exchanger for the exchange of hot and cold fluids is proposed in this paper on the basis of the predecessors,that is,the hot and cold tubes are placed horizontally in a closed circular cavity filled with medium water for heat exchange.Firstly,the influences of the distance,arrangement and Rayleigh number of the hot and cold tubes in the circular cavity on the natural convection intensity outside the tube group were investigated.The results show that the natural convection heat transfer effect of tube group is different under different tube spacing.When Rayleigh number Ra is between 2700-5400,the optimal tube spacing S is between 2d-2.5d.Under different arrangement modes,when Ra is between 5400-270000 and less than 2700,oblique row has the most prominent heat transfer effect.When Ra is between 2700 and 5400,the horizontal row has the best heat transfer effect.When Ra is greater than 270000,the average Nussel number Nu_aveof the column is the largest.In addition,given the spacing and arrangement of tubes,the Nu_aveof tube group increases with the increase of Ra.Except for the tube group,the simulation of the opposite cavity single tube shows that when Ra is greater than 2×10⁵,the Nu_aveof the heat pipe decreases with the increase of the inclination angle of the square cavity.Secondly,the influence of vibration on heat transfer is studied by moving grid technique.The simulation results show that the heat transfer performance of the heat pipe increases gradually with the increase of amplitude,but the improvement is limited.Between 0-10 Hz,Nu_avealmost stays the same;between 10-50 Hz,Nu_aveincreases with the increase of frequency,and the increase rate is less than 10%.When the frequency exceeds 50 Hz,the Nu_avelift rate increases dramatically.In addition,when the same vibration is applied,the Nu_avewith Ra of 2700 is 57.14%higher than that of Ra of 270,and the increase is 10.74%higher than that without vibration.When the vibration Reynolds number is expressed,the increase of Nu_aveis small when the Rev is less than 5,and the Nu_avenumber increases sharply when the Rev is more than 5.Finally,the simulation results were compared with Lemlich’s single tube vibration enhancement correlation curve,and it was found that the curve trend was consistent.However,in this paper,when the ratio between the forced convection term and the natural convection term induced by vibration was less than 8,the enhancement multiple was almost 1;when the ratio was greater than 8,the enhancement multiple increased sharply,and the maximum enhancement multiple was 3.16.