The fluid flow between noncontact mechanical seal faces is a composite flow consisting of pressure flow and shear flow.The flow state is influenced by various factors such as internal and external pressure difference,seal clearance,and rotation speed,etc.Combing related literature about the critical Reynolds number of the pressure flow,the shear flow,and the composite flow,it is found that most literature believe that the critical Reynolds number of the pressure flow is 2 000,and the critical Reynolds number of the shear flow is also close to 2 000.A method for calculating the Reynolds number of the combination of Poiseuille flow and Couette flow by using the composite velocity was presented,that is,taking the composite velocity as the characteristic velocity to calculate the Reynolds number,and called it as composite speed Reynolds number.The composite speed Reynolds number that is equal to 2 000 is used as a criterion of critical Reynolds number to judge whether the combined flow of the fluid film is in a laminar flow or in turbulent flow.For composite flows in which the pressure flow and the shear flow is perpendicular to each other,the composite speed Reynolds number determined by the composite velocity method is equivalent to the compound Reynolds number compounded by the pressure flow Reynolds number and the shear flow Reynolds number respectively.Taking Gabriel’s classic paper data as an example,the maximum composite Reynolds number calculated by the composite velocity method is 100,which is less than the critical Reynolds number of composite flow and is in a laminar state.