The Viscosity Of Dimethyl Silicone Oil And Shear Cavitation

In the gap of hydraulic components, like the valve plate of hydraulic pump and motor, the phenomenon of cavitation can always be seen because of the strong shear flow. Many problems like material wear, cavitation erosion and noise are caused by cavitation. Using the Couette rheometer, we researched the mechanism of shear flow cavitation. The paper states the main work of us which includes three aspects as following:(1) Improve the function of the new type Couette rheometer. In order to get the safe pressure range of the rheometer, we made a strength check. The method of liquid viscosity measurement at different ambient pressures using the Couette rheometer was analyzed and discussed after the problem of how to get the torque of the working section from the torque of the motor had been solved.(2) Do some research about the relationship between the viscosity of 500cSt dimethyl silicone oil and the amount of absorbed air. The viscosity of 500cSt dimethyl silicone oil with different amount of air was tested taking advantage of the new type Couette rheometer which can measure liquid viscosity at different ambient pressures. The experiment was done at the shear rate of 2196/s and 3660/s (the speed of the motor 150r/min and 250r/min) separately. According to the result of the experiment, we proposed the mechanism of viscosity decrease with the increase of the amount of air. The Erying viscosity model was modified from the aspect of the free activation using the Gibbs excess free energy. Meanwhile, the relationship between Gibbs free energy of activation and temperature was obtained. Finally, the method of how to get the molecular interaction energy of silicone oil and air from the viscosity of silicone oil was discussed which makes a good foundation for the further research of shear flow cavitation.(3) Research the mechanism of strong shear flow cavitation. Study how ambient pressure, shear rate and saturation affect the happen of cavitation. It is indicated that the PNSCC can not be used to predict the happen of cavitation in our experiment. When the Supersaturated silicone oil is not sheared, the air dissolved in it will separate out to reach a new state of equilibrium, and it is phase separation at static state. Shear cavitation is also the phenomenon of phase separation, but it is different with the situation of static state because of the shearing and it can be called phase separation at dynamic state. The phase equilibrium of nonideal liquid is related to the molecular interaction energy. Our experiment shows that when the silicone oil is sheared, the molecular interaction energy of silicone oil and air will get weak which causes the decrease of the silicone oil "dynamic solubility" to air, and the phase separation (the phenomenon of cavitation) will happen. The process stated above may be the mechanism of the shear flow cavitation.