7. Skin-Friction Coefficient

Local and average characteristic coefficients of skin friction of nanofluid’s natural convection are proposed with consideration of nanofluid’s variable physical properties. Meanwhile, a defined characteristic similarity velocity is defined. The formulae of the local and average characteristic coefficient of skin friction of nanofluid’s natural convection are obtained, with the defined characteristic similarity velocity whose value is set to be a unit. The system of the rigorous numerical solutions of the skin velocity gradient is provided for Al₂O₃-water nanofluid’s natural convection under consideration of variable physical properties. With the further analysis on the system of the rigorous numerical solutions of the skin velocity gradient, the effects of some physical variables and parameters are found on skin velocity gradient for Al₂O₃-water nanofluid’s natural convection. With increase of the fluid bulk temperature, the skin velocity gradient, and the skin friction coefficient will decrease. With increase of the wall temperature, the skin velocity gradient and the skin friction coefficient will increase. With increase of the Al₂O₃ nanoparticle’s volume fraction, the skin velocity gradient and the skin friction coefficient will increase as a linear function. However, the effect of the nanoparticle’s volume fraction from 0 to 0.1 is not obvious compared with the effect of the flow boundary temperatures.