Abstract
The convective heat exchange mechanism in two-phase regime, can allow high exchange coefficients, as long as particular conditions are respected, in terms of vapor quality. In evaporator tubes this problem is particularly important as a deviation from the optimal flow conditions can drastically drop the efficiency of the heat exchange. This work describes an innovative methodology for identifying the different two-phase flow regimes in evaporator pipes, through the processing of the vibrational data acquired on the pipe itself. An experimental apparatus with transparent pipes has been built up to recognize flow patterns by a fast image acquisition camera. Images have been associated to vibration data acquired on pipes by accelerometer. The analysis of frequency spectrum, led to first interesting results thus it is possible to distinguish between “no bubbles” regimes and different “bubbles” regimes. A wide test campaign has been realized on vertical and horizontal pipes, simulating steam bubbles with nitrogen bubbles, reproducing the main flow patterns. The paper demonstrates the possibility to design smart and non-intrusive sensor to be applied on evaporated tubes for the recognizing of anomalous thermal flow condition and sets the stage for future engineering work.
Highlights
Two-phase flows, in particular those where liquid and vapor phases coexist, are present in a multitude of plant applications, both industrial and civil
As regards the heat exchange, the two-phase flow conditions are characterized by a wide range of exchange coefficients depending on the preponderance of one phase over the other
These techniques are associated with high-speed photography [7] in transparent experimental pipes, or with the introduction of optical fibers inside the pipe [8].The alteration of the speed of sound as a function of the vacuum fraction can be related to the specific two-phase flow pattern by means of pressure transducers [9].Even thermometry and infrared analysis can give interesting results in the detection of anomalous conditions of two-phase flow [10,11].Last but not least is the technique based on the analysis of the vibrations induced by the two-phase flow on the pipe [12]
Summary
Two-phase flows, in particular those where liquid and vapor phases coexist, are present in a multitude of plant applications, both industrial and civil. Some of these techniques are based on the attenuation of gamma rays as a function of the vacuum degree of the liquid vapor mixture [2,3]. The analysis was developed starting from a campaign of experimental surveys in different conditions of the vacuum fraction, associated with a series of fast camera shots on a transparent section of pipe This technique may be more useful than other more mature and already in use, thanks to the fact that it is non-invasive and can identify different twophase flow patterns in real time. All this can make it possible to intervene on the system promptly, before even worse conditions and irreversible damage occur
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