A Study On The Calibration-free Measurement Technique For Wall Shear Stress Using Dual-layer Hot-film Sensors

Viscous drag is one of the important sources of resistance for vehicles,reducing viscous friction resistance is of great significance.The wall shear stress distribution determines the friction resistance,the measurement of wall shear stress has always been a difficult point in experimental fluid mechanics research.The existing technology is difficult to be applied for the measurement of the vehicle.Aiming at the limitations of existing measurement methods,this thesis proposed a calibration-free wall shear stress measurement method for double-layer "sandwich"hot film sensors.This method uses two metal nickel films with the same working temperature as the sensitive unit to form a double-layer hot film structure.When the two layers of hot film work at the same temperature,the lower hot film insulates the heat transfer of the upper thermal film to the wall,ensuring that all the Joule heat emitted by it is transferred to the fluid.This technology greatly improves the measurement stability and sensitivity of the hot film sensor.This thesis analyzed the convective heat transfer of a tiny isothermal film immersed in the turbulent boundary layer.Based on the assumption of the polynomial temperature distribution in the thermal boundary layer,this thesis investigated the heat transfer intensity corresponding to the 1st-4th polynomial temperature curve,and formed a theoretical model describing the relationship between the heat transfer intensity and the wall shear stress.In addition,this paper discovered the influence of direction changes of shear stress on heat transfer.According to these theories,the wall shear stress can be calculated directly by using the Joule heat of a thermal film sensor,and the wall shear stress vector can be calculated by using the Joule heat of a pair of two-layer thermal film sensors arranged perpendicular to each other.In this thesis,a series of experimental studies have been carried out,using air and water as media,in-depth research on the above theories and experimental methods in the boundary layer of long pipes and flat plates,confirming the correctness of the theory and the practicability of the method.In order to apply the calibration-free wall shear stress method to underwater submersibles,this paper uses vacuum evaporation sedimentation technology to attach a nano-thickness parylene(paraxylene polymer)insulating coating on the surface of the sensor to protect it from electrochemical corrosion underwater.In this paper,the internal heat transfer model of the coating is developed,and the calibration-free measurement method of the coating sensor is formed.The experiment proved the feasibility of this method.