The natural convection heat transfer of a 60% sucrose solution in a vertical converging-diverging tube(CD) with regularly-spaced twisted tapes(RSTT) has been investigated numerically and experimentally. The effects of...The natural convection heat transfer of a 60% sucrose solution in a vertical converging-diverging tube(CD) with regularly-spaced twisted tapes(RSTT) has been investigated numerically and experimentally. The effects of wall temperature and number of RSTT on the Nusselt number were studied in detail. The distributions of velocity and temperature in the 60% sucrose solution were studied and the simulated results of CD with RSTT were compared with those of the smooth tube. The influence of Rayleigh number and RSTT on the Nusselt number was conducted experimentally. The results indicate that the Nusselt number of the 60% sucrose solution obviously increased with the number of RSTT but increased inconspicuously with 2 and more twisted tapes. The simulation shows that the distance for achieving an optimal heat transfer performance is 46 times the diameter of the tube. The mechanism of the natural convection heat transfer enhancement of the 60% sucrose solution in relationship with the CD and the RSTT was analyzed, and the change of average tangential velocity with the axial distance was presented to demonstrate that the enhancement of heat transfer was realized mainly because of the increase in tangential velocity.展开更多
The phenomena of magneto-hydrodynamic natural convection in a two-dimensional semicircular top enclosure with triangular obstacle in the rectangular cavity were studied numerically. The governing differential equation...The phenomena of magneto-hydrodynamic natural convection in a two-dimensional semicircular top enclosure with triangular obstacle in the rectangular cavity were studied numerically. The governing differential equations are solved by using the most important method which is finite element method (weighted-residual method). The top wall is placed at cold T<sub>c</sub> and bottom wall is heated T<sub>h</sub>. Here the sidewalls of the cavity assumed adiabatic. Also all the wall are occupied to be no-slip condition. A heated triangular obstacle is located at the center of the cavity. The study accomplished for Prandtl number Pr = 0.71;the Rayleigh number Ra = 10<sup>3</sup>, 10<sup>5</sup>, 5 × 10<sup>5</sup>, 10<sup>6</sup> and for Hartmann number Ha = 0, 20, 50, 100. The results represent the streamlines, isotherms, velocity and temperature fields as well as local Nusselt number.展开更多
The transition from an axisymmetric stationary flow to three-dimensional time-dependent flows is carefully studied in a vertical cylinder partially heated from the side, with the aspect ratio A = 2 and Prandtl number ...The transition from an axisymmetric stationary flow to three-dimensional time-dependent flows is carefully studied in a vertical cylinder partially heated from the side, with the aspect ratio A = 2 and Prandtl number Pτ=0.021. The flow develops from the steady toroidal pattern beyond the first instability threshold, breaks the axisymmetric state at a Rayleigh number near 2000, and transits to standing or travelling azimuthal waves. A new result is observed that a slightly unstable flow pattern of standing waves exists and will transit to stable travelling waves after a long time evolution. The onset of oscillations is associated with a supercritical Hopf bifurcation in a system with O（2） symmetry.展开更多
基金the Major Science and Technology Projects of Guangdong Province (No. 2011A080804012)
文摘The natural convection heat transfer of a 60% sucrose solution in a vertical converging-diverging tube(CD) with regularly-spaced twisted tapes(RSTT) has been investigated numerically and experimentally. The effects of wall temperature and number of RSTT on the Nusselt number were studied in detail. The distributions of velocity and temperature in the 60% sucrose solution were studied and the simulated results of CD with RSTT were compared with those of the smooth tube. The influence of Rayleigh number and RSTT on the Nusselt number was conducted experimentally. The results indicate that the Nusselt number of the 60% sucrose solution obviously increased with the number of RSTT but increased inconspicuously with 2 and more twisted tapes. The simulation shows that the distance for achieving an optimal heat transfer performance is 46 times the diameter of the tube. The mechanism of the natural convection heat transfer enhancement of the 60% sucrose solution in relationship with the CD and the RSTT was analyzed, and the change of average tangential velocity with the axial distance was presented to demonstrate that the enhancement of heat transfer was realized mainly because of the increase in tangential velocity.
文摘The phenomena of magneto-hydrodynamic natural convection in a two-dimensional semicircular top enclosure with triangular obstacle in the rectangular cavity were studied numerically. The governing differential equations are solved by using the most important method which is finite element method (weighted-residual method). The top wall is placed at cold T<sub>c</sub> and bottom wall is heated T<sub>h</sub>. Here the sidewalls of the cavity assumed adiabatic. Also all the wall are occupied to be no-slip condition. A heated triangular obstacle is located at the center of the cavity. The study accomplished for Prandtl number Pr = 0.71;the Rayleigh number Ra = 10<sup>3</sup>, 10<sup>5</sup>, 5 × 10<sup>5</sup>, 10<sup>6</sup> and for Hartmann number Ha = 0, 20, 50, 100. The results represent the streamlines, isotherms, velocity and temperature fields as well as local Nusselt number.
文摘The transition from an axisymmetric stationary flow to three-dimensional time-dependent flows is carefully studied in a vertical cylinder partially heated from the side, with the aspect ratio A = 2 and Prandtl number Pτ=0.021. The flow develops from the steady toroidal pattern beyond the first instability threshold, breaks the axisymmetric state at a Rayleigh number near 2000, and transits to standing or travelling azimuthal waves. A new result is observed that a slightly unstable flow pattern of standing waves exists and will transit to stable travelling waves after a long time evolution. The onset of oscillations is associated with a supercritical Hopf bifurcation in a system with O（2） symmetry.
