Fabio Toshio Kanizawa

A new correlation for single and two-phase flow pressure drop in round tubes with twisted-tape inserts

Twisted-tape inserts are frequently used in heat exchangers as a passive and inexpensive heat transfer enhancement method. However, their use results not only heat transfer coefficient increments, but also pressure drop penalties. The present study analyses the literature on single and two-phase frictional pressure drop inside tubes with twisted-tape inserts focusing on the physical mechanism and the effects of the use of twisted-tape in comparison to plain tubes. Experimental data were gathered from the open literature and compared against the available correlations developed in order to predict two-phase frictional pressure drop in tubes containing twisted-tape inserts. It was found that none of the correlations was able to predict such a database accurately. A new correlation to estimate the friction factor for two-phase flows inside tubes with twisted-tape is also proposed. Contrarily to previous studies, the proposed correlation presents reasonable predictions under single and two-phase flow conditions and obeys the trends when the twisted-tape ratio tends to zero and infinite.

STATE-OF-THE-ART REVIEW ON FLOW PATTERNS, SUPERFICIAL VOID FRACTION AND FLOW-INDUCED VIBRATION DURING TWO-PHASE FLOWS ACROSS TUBE BUNDLES

This paper presents a comprehensive review on the open literature concerning two-phase flow across horizontal tube bundles. Initially, experimental studies available in the literature are described in an organized manner. This detailed description includes bundle configurations and the experimental techniques used for evaluating flow patterns, void fraction and flow-induced vibration. Most of studies were performed having air-water as working fluids. Several excitation mechanisms can be reproduced such as flow periodicity, fluid-elasticity and turbulence. Flow patterns are discussed and their predictive methods are compared. Void fraction predictive methods are also discussed and comparisons among them are presented. It was found that the predictive methods worked reasonably well when compared against the databases used during their development. Generally, the studies have obtained bundle average measurements which are not representative of the local conditions and, therefore, are not recommendable to be used as accurate designing tools. Most studies concerning fluid-induced vibration have addressed this problem through simplified bench structures, usually consisting of a single rigid tube over a flexible suspension.