Dynamic Characteristics Of Components Of Elastic Tube Bundle Heat Exchanger

Energy shortage and environmental pollution has been increasingly serious, so it is urgent to find new energy resources, research on the theory and methods of improving efficiency of energy utilization and develop new types of heat transfer equipments that are efficient and energy-saving. Based on the National Key Basic Research and Development Plan "advanced theory and methods of energy-saving for the typical heat transfer equipments in high energy-consumption industries" (2007CB206900), combined with the practical engineering application and vibration control theory, dynamic characteristics of a new type of heat transfer equipment and components are studied systemically. A dynamics model of thin-wall cylindrical shell simply supported at both ends and excited by complex forces is established, driving point mobility matrix formulae are deduced, which give a theoretical basis of substructure mobility description for the vibration system whose basis is shell structure. Based on the combination of beam functions and the Galerkin method, the influence of various structural parameters on the inherent characteristics of shell is studied. Using component mode synthesis method and finite element method, dynamics characteristics of the elastic tube bundle itself and the fluid-solid interaction system that consists of the elastic tube bundle and the fluid inside and outside of it are investigated in detail. Major contents of research and creative achievements are listed as follows:Based on the classical shell theory, a dynamics model of cylindrical shell simply supported at both ends and excited by complex forces is built. Response expressions of the shell structure undergoing steady - state harmonic excitation are derived using modal superposition method. The analytical expressions of driving point force mobility, moment mobility and coupling mobility of the shell with force and moment acting on it together are deduced. A theoretical basis of substructure mobility description for the vibration isolation system whose basis is shell structure is established. Effects of geometric and physical parameters of the shell and the acting locations of excitation on the mobilities are discussed. The distribution and transmission characteristics of vibration energy in the shell are found, which provide a potential means for taking active or passive measures to reduce the vibration and noise level in structures. The results show that: when force or moment is imposed individually, the real part of mobility is always positive that means the vibration energy input into the structure; when force and moment are imposed together, the coupling mobility may have a negative real part that means the structure reacts against the external entertainment and the vibration energy outflows from the structure. As the excitation frequency-related flexibility of structures, mobility is closely related to the power flow inputting into the structure. By means of increasing thickness or reducing span of the shell, the energy transmission to the structure at low-frequencies can be decreased significantly as the results of mobility changes. The resonance peak value can be inhibited significantly by the structural damping, but the vibration energy will scatter from the resonance area to the non-resonance ones when the damping is increased, so appropriate choices must be made according to actual needs. Some sensitive frequencies could be avoided through the reasonable choice of excitation position, which is one of important aspects that worth considering in the vibration control of flexible structures.Based on Flugge thin-wall shell theory that has a broader application in practice, free vibration governing equation of curved shell is derived and solved by making use of both the beam function combination method and the weighted residual method. Taking advantage of the characteristics that axial distribution of mode shape of shells is similar to the mode shape function of beams with corresponding boundary conditions, mode shape function of shells is approximated by the combination of beam functions in axial direction and trigonometric functions in circumferential direction. Suitable displacement expressions are assumed according to boundary conditions, and then the vibration equation can be modified by these expressions. The corresponding characteristic equation can be solved by using the Galerkin method, and the dynamic characteristics of curved shell such as natural frequencies and mode shapes are got. The influences of geometric parameters, constraints and additional mass on the vibration characteristics of curved shell are analyzed in detail. The results show that: boundary conditions have a greater impact on natural frequencies of curved shell. A higher frequency can be got with stronger constraints if other conditions are the same. Different from cylindrical shells simply supported at both ends, the frequency corresponding to the rigid sliding mode is not zero for the curved shell with the same boundary condition. Compared with cylindrical shells, the mode shapes of curved shell are more complex, and the mode shapes of extrados are often different from that of intrados. Geometric parameters such as the ratio of section radius to bending radius, thickness, bending radius have a great impact on natural frequencies of curved shell, but extents of the influence are not consistent for different constraint conditions and modal order. Concentrated mass can significantly decrease the natural frequencies of curved shell, but the effect is closely related to the location of the mass.According to the basic idea of "dividing the whole into components, synthesizing components into the whole" in the sub-structure method and the structural characteristics of the elastic tube bundle, six sub-structures including 3D curved beam and rectangular body (rigid or flexible) are constructed. Modal coordinates model of each substructure is built through coordinates transformation. The free vibration equation expressed by modal coordinates for the elastic tube bundle can be deduced by synthesizing the modal coordinates model of each component according to the coordination conditions at interface, and then natural frequencies and corresponding mode shapes are calculated by using numerical methods. The effects of the numbers of nominal modes and constrained modes on the results are investigated. The influence of different design parameters on the changes of natural frequencies is also probed. Some conclusions are got: mode shapes of the elastic tube bundle are complex and the in-plane vibration coexists with the out-of-plane one. Excited by broadband forces from internal and external fluid, the elastic tube bundle will vibrate incessantly and slightly, and thus the heat transfer performance and anti-sediment capability is enhanced due to the vibration. As long as the number of elements used to discretize the structure is large enough to ensure frequencies convergence, natural frequencies of the whole structure will depend on the sum of nominal and constrained modal orders. When both nominal modes and constrained modes are the fifth modal order, the natural frequencies are almost the same as those from the overall model. Among various design parameters that determining the inherent properties of the elastic tube bundle, the effect of pipe section radius is greater than that of thickness, and the influence of connecting body location is depend on different modal orders. On the basis of modal analysis, a frequency response analysis is also made. The contribution of each modal displacement to the overall response is inspected, which provide theory basis for the optimal design of the elastic tube bundle.Taking compressibility of fluid into account, the fluid-solid interaction system that consists of the elastic tube bundle and the fluid inside and outside is discretized by using a displacement - pressure format (that is, take displacement and pressure as the basic unknown variable of solid and fluid respectively), and thus the finite element model of coupling system is created. The dynamic properties of the elastic tube bundle, fluid inside and outside of tube and the fluid - solid coupling system are analyzed respectively, and special attentions are put on the influence of interaction between the structure and fluid on their dynamic characteristics. According to the structural characteristics of the elastic tube bundle and constraint conditions, a forced vibration model of the elastic tube bundle excited by impulse flow is constructed, and the dynamic response under the steady-state excitation is analyzed in detail. Some results are listed as follows: natural frequencies of the structure will decline at different extents after taking the fluid into account, and the contributions from fluid with different density are also different. Structural design must consider the effect of fluid with larger density in order to avoid resonances. In the coupling system, the fluid pressure changes will cause structural vibration, and the structural vibration will also change the distribution of fluid pressure and form new kinds of vibration. The interaction between fluid and structure will change the original modal frequencies and mode shapes; the vibration of one system at its modal frequencies will force the other system to produce corresponding vibration modes. Under the steady-state excitation of impulse flow, forced vibration will occur for the elastic tube bundle, but its vibration is always in a sub-resonance region. The amplitude of vibration is large enough to ensure heat transfer enhancement, but the resonance that may damage the structure is also avoided. Thus, both economic requirements and reliability of the heat exchanger are taken into account, and the heat exchanger can operate in efficient and safe means for a long time.In order to provide experimental data support for the theoretical analysis, experimental modal analysis of the elastic tube bundle is carried by utilizing modern test equipments. A supporting structure is specifically designed, measurement locations are carefully selected and reasonable system parameters are set to get an ideal test result. After testing and dynamic parameter identification, modal parameters of the elastic tube bundle such as natural frequencies, modal shapes and damping ratio are got. The results of finite element analysis have a good coherence with the experimental results, which verified the reliability of finite element model in the mesh control, attribute setting, boundary conditions equivalent and calculating method selection. It also provides the experimental evidence for further study of the vibration, noise and design optimization of the elastic tube bundle.