Numerical Analysis Of Dynamic Characteristics Of Thermowell Based On Bidirectional Thermo-fluid-solid Coupling

Due to the vortex is periodically fallen off on both sides of thermowell,the thermowell in the pyrolysis gas pipeline used for protecting the temperature measuring equipment will cause the changes to the magnitude and direction of fluid forces imposed on the thermowell so as to induce the fatigue damages generated by the thermowell vibration.At the initial moment of operation,the thermowell wall surface and flow field environmental temperature are changed unevenly when the thermowell is shocked by the high-temperature gas,and the thermowell stress cycling caused by the temperature gradient will also trigger the thermowell to generate the fatigue damage.In terms of the damages frequently occurred in the application of thermowell,this paper is targeted to improve the safety of thermowell,through the numerical simulation of the dynamic characteristics of thermowell based on bidirectional thermo-fluid-solid coupling,study the intrinsic mechanism of thermowell damage.The main research work includes:First of all,under the conditions of temperature load and the combined action of pressure load and temperature load,it respectively establishes the models for the thermowell without interference equipment and the thermowell with interference equipment,as well as completes the numerical simulation for the bidirectional thermo-fluid-solid coupling.The conclusion for the circumstance without interference equipment is:The thermowell top has the most serious deformation under the action of temperature load with the maximal deformation is 0.167mm.The deformation is distributed in the gradient rule along the height direction,and the maximal deformations caused to the thermowell are respectively 0.986mm,1.12mm under the pressure load and the combined action of temperature load and pressure load.The deformation of thermowell appears the deviation at X and Y directions due to being interfered by the gas flow factor.The deformation is oriented with deviation at X direction,and the deformation is gradually reduced along with the height direction.The thermowell stress is continuously increased along with the height direction,the peak values for mechanical stress and total stress are centralized to distribute at the restrained end of root segment,which respectively are 118.63,128.71 MPa.The stress is firstly stabilized at about 28MPa along with the height without significant change,and then rapidly reaches to the peak value closet to the retrained end.The conclusion for the circumstance with interference equipment is:the maximal deformation for thermowell under the combined action of temperature load and pressure load is 0.478mm,as well as the thermowell and interference device will have the deviating deformation of different degrees at X and Y directions under the interference of gas flow.The thermowell is oriented with the deviation at X direction,and the interference device is oriented with the deviation at Y direction.The thermowell stress is centralized to distribute at the restrained end of root segment,and the peak value of stress is 35.8MPa.The stress of interference device is mainly distributed at the positions of four comers of restrained end,and the peak value of stress is 52.06MPa.If making a comparison to the numerical results of being without the interference device and with the interference device,it is able to identify that the interference device can effectively reduce 57.2%displacement deformation and 72.1%equivalent stress for the thermowell so as to ensure the structural safety of thermowell.Secondly,through performing the modal analysis and harmonic response analysis to the thermowell and interference device,it is able to identify that the different coupling methods and gas pressure will have the significant influences on the fixed frequency of thermowell.The variation of gas speed doesn't have the significant influences on the fixed frequency,but having the influences on the equivalent stress under the different fixed frequencies.The higher the fixed frequency of thermowell,and the more complex its motion form of vibration mode.Through the harmonic response analysis to the thermowell and interference device,it is able to identify that the displacement response at 1 and Y directions is the most intensive around the No.2nd,8th and 10th orders fixed frequencies.The stress response of thermowell at X,Y and Z directions is only the most intensive around the No.8th order fixed frequency.The displacement response of interference device at X and Z directions are the most intensive around No.2nd,4th,6th and 10th order fixed frequencies.Within the first ten-order fixed frequencies,the displacement response of interference device at Y direction shall have seven resonance response points.The stress response peak values of interference device at X,Y,Z directions are all occurred around No.2nd,4th,6th and 10th order fixed frequencies.Through performing the dynamic characteristic analysis to the thermowell and interference device,it further illustrates the relationship between the structural deformation form in coupling calculation and its own structural property so as to provide the theoretical support for the safety application of thermowell.