The stability boundary of tube arrays subjected to liquid-gas two-phase flow is usually estimated by a criterion similar to that for single-phase flow based on Connors’ equation. However, it has been revealed that two-phase flow instability is much more complex in our previous paper, where there exists two types of instability, intermittent instability and absolute instability.

Our theory is based on the assumption that the two phases liquid-phase and gas-phase can be separated on average. In this meaning the model is called a “separated flow model.” For each fluid phase, the energy balance between the excitation energy by fluid flow and the dissipation energy by tube vibration itself is estimated. By equating these two quantities (on average) the instability boundary is determined.

The flow distribution along the tube axis is not usually uniform. In our previous work this distribution effect was not considered. In this paper, our theory based on the energy balance in each separated flow component is expanded to include the flow-distribution effect along the tube axis.

Finally, a new criterion is for use at the practical design stage is formulated. An example showing comparison between the new criterion and the “Connors’ type formulation” is also presented.

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