Vibrations of loosely supported tube bundles subjected to Streamwise Fluidelastic Forces

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Heat exchangers are important components in many industries, including power generation, petrochemical, and pulp and paper. These devices contain large numbers of tubes arranged in arrays. The purpose of these devices is to exchange heat between internal and external cooling and heating fluids. A great deal of energy is transported in the form of high pressure and heat. If a fraction of this energy is transformed into vibrations, a form of mechanical energy, the tube integrity will be compromised. Fluidelastic instability (FEI) is the most devastating excitation mechanism, characterized by a critical flow velocity beyond which a very large tube response occurs. These large vibrations will cause devastating failures, and need to be avoided. These failures manifest as fatigue, tube cracking, and fretting wear at the supports. Fretting wear occurs as the result of tubes impacting against their supports. In the past it was thought that FEI prevails only in the transverse (lift) direction. Design guidelines therefore focused on designing against instability in the transverse direction. All nuclear steam generators were made using these guidelines. Streamwise (drag) instability was never considered to be of importance in operating steam generators. This belief has been shattered after recent devastating failures in some replacement nuclear steam generators such as those at SONGS (San Onofre Nuclear Generation Station). These steam generators were designed to last for 40 years and instead, lasted for only 11 months. Investigating these failures has confirmed that streamwise (drag) instability was responsible for the damage caused by tube-to-tube fretting wear and failures. The magnitude of these failures sent shockwaves through the academic and the industrial community. In one plant, the price in terms of equipment damage and lost production costs was close to one billion dollars. In the wake of these incidents, both the research community and industry have made great efforts to understand this phenomenon. A few recent studies have confirmed beyond any doubt that streamwise FEI can occur. This mechanism is not fully understood, and there are no analytical or numerical tools available to predict it. There is therefore a pressing need to develop a model that can predict the stability threshold of this mechanism. The proposed research will develop a comprehensive time domain model that captures both transverse and streamwise FEI excitations. The developed model will then be used to study the effect of tube-to-support clearance, sliding support friction, and tube preload against the supports. This model will be used to study these effects through numerical simulations. These simulations will then be used to assess the structural integrity of in-service heat exchangers, and provide guidelines for the design of future heat exchanger units.