Study On Fluidelastic Instability Of Bundles Consisting Of Different Frequency Tubes

Baffle heat exchanger is widely used because of its simple structure and high efficiency of heat transfer.However,existence of support failure in the heat exchanger will lead to the reduction of the natural frequency of tubes in the tube array,forming the unequal frequency array,which is very easy to cause fluidelastic instability and cause heat exchanger to destroy.It is of great significance to study support failure and fluidelastic instability of unequal frequency tube arrays for the design and operation of baffle heat exchangers.In this paper,a numerical method is used to study natural frequency of the tube when the supporting failure occurs.The influence of the different position and number of support failures on the natural frequency and mode of the heat transfer tube is obtained.The results show that natural frequencies of tubes are related to the location and number of supporting failures.The single support failure can obviously reduce the 1st order natural frequency of tube with a maximum decrease of 62.1%,but it has no obvious influence on the higher order(2nd order)modes.When two support failure happened,tube’s mode is related to the distance between the two failure supports and the original constraint direction.Water tunnel and visual acquisition system based on high speed photography are used for acquiring vibration characteristics in the tube bundles of normal square array and rotated triangular array with pitch ratio 1.28 and 1.4.A concept of frequency dispersion rate is proposed to describe the frequency characteristics of the tube array,and the relationship between characteristics of the fluidelastic instability and frequency dispersion rate of the unequal frequency array is also obtained.Results show that critical velocity of the square tube array increases with the increase of frequency dispersion,and this phenomenon exists in the two pitch ratios.The critical velocity of bundle with 8mm tube increased 28% when tube bundles changed to unequal frequency tube bundles.But same phenomenon is not found in the angle triangle.The spectrum analysis shows that the vibration characteristics of identical frequency array and the unequal frequency array are basically the same.Numerical method is used to study the fluid-solid coupling law of normal square tube array.The difference between numerical calculation and experimental results is analyzed.Critical velocity of rotated square array and triangular arrangement of the unequal frequency array is also studied.Result shows that the error of calculated critical velocity and experimental result is about 13%.Spectrum analysis shows that the calculated spectrum of tube is basically the same as that of the experiment.Based on those results,this numerical simulation method is used to study the fluid elastic instability of the unequal frequency of triangular array and rotated square array with pitch ratio of 1.4.Results show that the critical velocity of the triangle arrangement increases with the increase of frequency dispersion.In rotated square array,critical velocity of the identical frequency array is smaller than that of unequal frequency array,while critical velocities of the unequal frequency array are the same.