8. Predictive Formula of Wall Temperature Gradient

Systems of numerical solutions are obtained on wall temperature gradient of Al₂O₃-water nanofluid’s natural convection. On this basis, a correlation of the wall temperature gradient is developed. Such correlation has significant theoretical and practical value on heat transfer application, because it is based on the comprehensive consideration of physical properties and parameters, including nanofluid’s variable thermophysical properties, nanoparticle’s shape factor and fluid’s boundary temperatures. Then, it is found that comprehensive consideration of fluid’s variable thermophysical properties is importance for theoretical research of nanofluid’s convection heat transfer. The calculation examples demonstrate that Boussinesq approximation method will lead to an absurd calculation result of the related convection heat transfer research. Such absurd calculation result lies in that by using Boussinesq approximation method, as long as the flow average temperature is same, the calculation results, such as, the wall temperature gradient are same. However, such calculation results could be quite different according to the present calculation examples with consideration of fluid’s variable physical properties. Thus, proper consideration of fluid’s variable physical properties is necessary for exact theoretical research of convection heat transfer. The wall temperature gradient investigated in this Chapter is the key issue for research of convection heat transfer coefficient in actual fluids. This Chapter demonstrates that this issue can be resolved theoretically with comprehensive consideration of variable physical properties and parameters. In this way, the theoretical research result will have a significant theoretical and practical value on heat transfer application.