AN EXPERIMENTAL STUDY ON NATURAL CONVECTION HEAT TRANSFER OF NON-NEWTONIAN FLUIDS FROM HORIZONTAL WIRES (RHEOLOGY, POLYMER SOLUTION, POWER-LAW, PURELY VISCOUS, VISCOELASTIC).

摘要

Natural convection heat transfer from horizontal wires to purely viscous and viscoelastic non-Newtonian fluids was studied. A new set of dimensionless numbers for power-law fluids was developed as a result of non-dimensionalizing the basic equations of change. These numbers are found to be simpler than the power-law dimensionless numbers currently in use and they are used here to correlate the experimental results for purely viscous fluids. The dimensionless equations of change also reveal that the heat transfer from horizontal wires to power-law fluids should be a function of the power-law index, Rayleigh and Prandtl numbers. In the case of high Prandtl number fluids, such as the present case, the heat transfer can be correlated as a function of only the Rayleigh number and power-law index.;For viscoelastic fluids it was found experimentally that the effects of elasticity on the heat transfer were negligible over a range of conventional Rayleigh numbers from 10('-4) to 10('+2). The use of the zero-shear-rate viscosity in the available Newtonian correlation, such as in Fand and Brucker's work yield predictions which are in good agreement with the experimental values.;The experimental results for the purely viscous fluids, which cover a range of generalized Rayleigh numbers from 10('-3) to 10('+2) show a decrease in the Nusselt number accompanying a decrease in the power-law index for a fixed Rayleigh number. This is in conflict with available analytical and experimental studies at high Rayleigh numbers where the Nusselt number increases with decreasing values of the power-law index. However, in the range of Rayleigh number less than unity, the available analytic study also suggests that the Nusselt number decreases with decreasing values of power-law index for a fixed Rayleigh number. This agrees with the physical observation that the purely viscous fluids of extremely low value of power-law index has a gel-like appearance and the heat transfer in these fluids approaches the conduction limit. Based on the present experimental data and the analytic results which are derived from the boundary layer assumptions, a new correlation of heat transfer from a horizontal cylinder to power-law fluids is proposed for the lower range of Rayleigh number.