Three-dimensional Numerical Simulation Of Rayleigh-Bénard-Marangoni Convection Of Cold Water In A Cylindrical Pool

Rayleigh-Bénard-Marangoni convection is a kind of natural convection which is driven by the buoyancy as well as thermocapillary force.Because of its wide applications in engineering,such as development of semiconductor,phase change cool-thermal energy storage system,pellicle spray and crystal growth.It has become a hotspot in academic circles.Up to now,the studies of Rayleigh-Bénard-Marangoni convection is mainly focused on the common fluid that the density of which can be regarded as a linear function of temperature.However,the investigation of Rayleigh-Bénard-Marangoni convection of fluid near its density maximum in a cylindrical pool heated from below and with a free surface at the top is lacked.In this thesis,aiming at the thermal convection phenomenon in the equipment of phase change cool-thermal energy storage system,by using the finite volume approach,a series of three dimensional numerical simulations of Rayleigh-Bénard-Marangoni convection of cold water near its density maximum were carried out.The influence of Rayleigh number(Ra),Biot number(Bi),density inversion parameter(?m)and aspect ratio(?)on the flow patterns is obtained.Furthermore,we have also discussed the effect of some control parameters on the heat transfer ability as well as gotten the evolution of flow structures in this paper.The study results not only have a important academic significance on developing the research of natural convection of working fluid with density maximum,but also can provide the theoretical basis for the optimal design of cold storage air conditioning in engineering.The results show that:(1)The strength of free surface tension is related to the Biot number.When the Biot number is smaller,the free surface temperature gradient is larger,which leads to a strong surface tension.Due to the fact that the surface tension can help to overcome the viscous resistance of fluid,the critial Rayleigh number for the onset of convection is very small and the flow structure is relatively simple at this time.With the increase of the Biot number,the strength of the surface tension is weakened gradually,which makes the flow structure become more complex and diverse.In this case,the density inversion parameter as well as Rayleigh number has been the main factor affecting the flow patterns.Meanwhile,with the increase of density inversion parameter,the critical Rayleigh number increases synchronously.(2)When free surface is adiabatic,the system is always in conductive state when the calculation is stable.At Bi?0,the flow patterns observed from the first bifurcation of the conductive state at different inversion parameters are identical.When Ra=1000,with the increase of the Biot number,the two-roll pattern and one-torus pattern are observed respectively.Furthermore,when the Biot number is large enough,the system decays to the conductive state.At Ra=5000,the single-roll pattern and two-roll pattern are come to light with the increase of the Biot number,moreover,only when ?m=0.1 and Bi>75,the sadman pattern is obtained.(3)When the aspect ratio of the cylinder is small,the flow structure is relatively simple and critical Rayleigh number is very large.With the increase of aspect ratio,the inhibition effect of side wall on flow gets weakened gradually,which makes the critical Rayleigh number become smaller and the increase of the number of cells in the system.(4)With the increase of Rayleigh number,the average Nusselt number will increase at the same time.With the increase of the Biot number,the average Nusselt number will increase at first and then decrease.At the critical Biot number that the flow pattern bifurcates from a relatively stable state to a violent state,there is a sudden increase of Nusselt number.For all flow patterns,at the same Rayleigh number or Biot number,when the density inversion parameter is smaller,the average Nusselt number is larger.