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Many types of heat exchanger failure are randomly faced on the petrochemical industry, oil refining, power generator, metallurgical industry etc. (Metal handbook, Fatigue failure, 1975). With the development of science and technology, the production capacity of the industries is becoming increasingly large, which has made the size of tubular heat exchanger larger. Some common causes of failures in heat exchangers are listed below:
Pipe and tubing imperfections
Improper operating conditions
Vibration can caused diminished efficiency fatigue failure and other form of damage including fretting wear and fluctuating between tube and support. This can leads to equipment breakdown, expensive repairs, process shutdown, and even entire tube replacement. According to Qian et al. (1988) and Qiwu et al. (1990) damage to heat exchanger by vibration is very serious and is summarized as follows:
Leakage at the joins of tubes and tubesheets caused by the maximum bending stress there.
Augmentation of pressure drop caused by vibration
When the frequency of exciting forces approaches the natural frequency of tube, resonant vibration will occurred and accompanied by several noise.
When vibrating, tubes suffer periodic alternating stresses and frequent bending, which can result fatigue failure of tubes and may be perforated by the baffles due to vibration.
When the vibration amplitude too large, impact failure occur between tube and tube bundle and shell wall.
Nuclear components include steam generator, heat exchanger, and condenser, piping system, reactor internal and nuclear fuels. A first tube failure occurred in 1971 and a second in 1977, some ten years after the initial start-up of the station. This shows that fretting-wear can be a slow but continuous damage mechanism. Some examples of tube failures in commercial nuclear components are reported in the review by Pettigrew et al. (1991).
Fig. 1.0: Heat exchanger tube failure due to fretting-wear at support locations.
Figure 1.1: Fatigue failure of titanium condenser tube
Figure 1.0 shows an example of tube-to-tube support fretting-wear. This failure took place in the inlet region of a process heat exchanger. Obviously, the tubes were inadequately supported for the high flow velocities in the inlet region. While Figure 1.1 shows a classical fatigue failure of a titanium condenser tube. This problem was largely due to abnormal operation of the condenser, resulting in higher than normal steam flow velocities.
Although most vibration problems have very costly consequences, they are usually solved by simple design modifications or changes in operating conditions. Thus, it is important to understand flow-induced vibration and damage mechanisms to prevent problems at the design stage and to assist station operators in predicting the life of nuclear components. Therefore, in order to make heat exchanger work securely for long times, it is important to study this failure caused by flow induced vibration.
In general, parallel and cross-flow exist in the tube bundles of heat exchanger components. In heat exchangers the tubes are often subjected to cross-flow particularly near inlets and outlets where flow velocities are generally high. Obviously the first step in a vibration analysis is to evaluate operating conditions and flow velocities. In cases where the flow paths are well defined the flow velocities are calculated simply from the flow areas. Often the flow is divided between several parallel flow paths. In heat exchanger the flow may go through the tube bundle, between the tube bundle and the shell and sometimes through a central tube-free lane. In such cases the flow calculations are based on equal pressure drop between regions common to all flow paths.
Flow-induced excitations of bodies, obstacles and structures in steady or unsteady flows, are at present both a relevant field of research as well as the subject of important studies of theoretical and experimental nature. In 1980 and afterwards, the development of new achievement and visualization techniques for describing flow field structures allowed the research to study by implementing physical experiments the direct assessment of the effects induced by the flow fields on the bodies (Blackburn & Henderson (1999), Lin et al.(1995), Sheridan et al.(1997)) and the dynamic responses of the obstacle. In these studies the body is thought as a boundary condition for the flow field.
The structure of an external flow around an immersed body which is the flow can be described and analyzed often depends on the geometry of the body. There are three main categories of bodies are usually considered such as two-dimensional objects (infinitely long and of constant cross-sectional size and shape), axisymmetrical bodies (formed by rotating their cross-sectional shape around the axis of symmetry), and three dimensional bodies that may or may not be symmetrical. The cylinder is one of the basic shapes most studied because of the simplicity of its form and because this form mimics a large number of practical applications. To be able to find the suitable measure that required controlling the fluid-dynamic response, the generation mechanism of this response should be clarified first. This mechanism is the flow pattern around the obstacle.
Flow-induced vibration in heat exchangers has been a major cause of concern in the nuclear industry for quite a few decades. Tube failures due to excessive vibration must be avoided in heat exchangers and nuclear steam generators, preferably at the design stage. Thus, a comprehensive flow-induced vibration analysis is required before fabrication of shell and tube heat exchangers. Some examples of tube failures in commercial steam generators are reported in the review by Pettigrew et al. (1991). These flow-induced vibrations are caused by time-dependent forces, which can be determined by measuring the pressure fluctuation at the tube surface. These tube failures are consistently associated with high velocity cross-flows and large amplitude vibrations. Vibration can cause diminished efficiency fatigue failure and other forms of damage, including fretting wear and impacting between tubes and supports. This can leads to equipment breakdown, expensive repairs, process shutdown, and even entire tube replacement.
Flow-induced vibration is the vibration of a structure due to the flow of a fluid around it. It is a fluid-structure interaction problem. When certain critical conditions are exceeded, instability is induced in the system causing the tubes to vibrate with large amplitudes. Fluid-Structure Interactions (FSI) occurs in many engineering fields, such as nuclear engineering, ocean engineering, vehicle engineering and wind engineering. In a world of cause and effect, it becomes natural to study the way different mediums interact. The study of the interaction between fluid and structure has become an important area of scientific research since the tragic failure of the Tacoma Narrows Bridge on November 7, 1940. Research in this field attempts to measure the vibration of a structure caused by a fluid flowing past or through it. The first mention of this phenomenon in the literatures was made by Roberts (1966) which was later verified by the experiments of Connors (1970). Fluid-structure interaction technique is presently being used in many applications as bio-mechanics, pipe flow and immersed structure. This technique will help define the relationship between pipe wall vibration and the physical characteristics of turbulent flow. Resent work on this study was shown in publication of Hirota (2002).
Vortex-induced vibration is an important problem in many fields of engineering and possible phenomenon in situations where a bluff body interacts with a fluid flow. There are many potential areas where this phenomenon could be observed such as in heat exchanger tube bundles, marine structures, bridges, power transmission line and is cause for concern in many other practical applications. There are many problems caused by vortex- induced vibration has led to a large number of experimental and computational studies on the subject, including several review articles, for example: Sarpkaya (1979), Griffin and Ramberg (1982), Bearman (1984), Parkinson (1989), and more recently Williamson and Govardhan (2004).
Researchers face challenges unique to their method of solving this FSI problem through analytical, numerical or experimental means. Furthermore, the fluid-structure interaction produces a large number of turbulent scales that interact with each other, increasing the difficulty in the turbulence model. The Large Eddy Simulation (LES) technique is situated between the RANS and DNS in the sense that large scales are calculated directly while the small ones are approximated. Since direct numerical simulations (DNS) are, nowadays, yet limited at very simple flows, because of the large requirement of computational power, both in terms of memory resources and in terms of calculation time, large eddy simulations (LES), with the high performances of the modern computers, seem to be the actual more reasonable approach in dealing with such complex flow (Rodi et al. 1997). For turbulent flow, large eddy simulation (LES) has become a very popular and reliable numerical method, with which the complex turbulent flow characteristics can be captured accurately. LES models the fluid flow by spatially filtering the governing flow equations and solving for a local-averaged velocity rather than a time-averaged velocity, which will produce the pressure variations desired.
The focus of the project is to determine correlation between vibration and temperature on heat exchanger.
1.3 Scope of study
The scopes of study are briefly as follows:
3D flow analysis using commercial software (FLUENT)
Analysis based on external flow will be conducted
Model will be constructed for rigid cylinder
Overview of the report
Chapter 1: Introduction
Brief the content and background of the project. The scope of study and objectives are also discussed in this chapter.
Chapter 2: Literature review
Discuss about the previous studies related to fluid structure interaction, flow induced vibration, vortex induced vibration and LES modeling.
Chapter 3: Methodology
Development of methodology.
Develop modeling and analysis
Chapter 4: Result and discussion
This chapter will discussing the result and discussion
Chapter 5: Conclusions and recommendations
This chapter presents the conclusions of the project
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Blackburn, H.M. & Henderson, R.D.,A study of two-dimensional flow past an oscillating cylinder, Journal of Fluid Mechanics, 385, pp. 255-286, 1999.
Connors, H.J.,Jr., 1970. "Fluidelastic Vibration of Tuv=be Arrays Excited by Cross-Flow," In Flow Induced Vibration in Heat Exchangers, ASME Winter Annual Meeting, New York.
Dong Qiwu, Liu Minshan, 1990. The Dynamic Characteristic Analysis of Large Turbular Heat Exchanger with Dynamic Finite Element Method. Proceedings of the International Conferences on Dynamic Vibration and control, p104-109.
Griffin, O.M., Ramberg, S.E., 1982. Some recent studies of vortex shedding with application to marine tubulars and risers. ASME Journal of Energy Resource Technology 104, 2-13.
Hirota, K., Nakamura, T.,Kasahara, J., Mureithi, N.W., Kusakabe, T., and Takamatsu, H., 2002 "Dynamics of an In-Line Tube Array subjected to Steam-Water Cross-Flow. Part III: Fluidelastic Instability tests and Comparison with Theory," Journal of Fluids and Structures, 16(2), pp.153-173.
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