STUDY OF FLOW INSTABILITIES AND ITS EFFECT ON HEAT TRANSFER IN THERMOSYPHON REBOILERS

Abstract

Static instabilities in reboiler tubes are manifested in terms of increasing wall temperatures resulting in boiling crisis. Onset of flow oscillations indicates limit of operating conditions in natural circulation boiling systems. Sustained flow oscillations are undesirable with respect to the control and performance of such systems. These instabilities hamper the smooth and continuous circulation of liquid and vapour through the reboiler. With large amplitude of these flow oscillations, dry out conditions occur at different points along the reboiler tube resulting in vapour locking 'which in turn reduces drastically the heat transfer coefficients there by increasing wall temperatures or temperature differences. Therefore, study of these instabilities are necessary for proper design of such systems. Two types of instabilities namely Static and Dynamic (macroscopic) are more predominant with respect to performance of systems having them. Static _refer_to its_base_as_original_ state_ i.e._steady state performance state while Dynamic instabilities refer to cyclic flow patterns, transitional aspects in flow patterns causing highly oscillatory behaviour. Study of both is important for efficient design, control and performance of systems. However, present work studies only static one that too its aspect in boiling crisis i.e. region of reboiler tube where wall temperature and thereby temperature difference increases suddenly resulting in boundary layer separation causing instability. In the present work, available experimental data [1] have been used for static instability studies.