TWO-PHASE FLOW INSTABILITIES AND FLOW MAL-DISTRIBUTION IN PARALLEL CHANNELS

Abstract

Boiling flow instabilities are present in several heat exchanging systems, covering different applications and scales. Two–phase flow unstable phenomena can take place in nuclear reactors, cryogenic and chemical processes and high density power electronic devices. These types of undesired behaviors are counterproductive since they have a seriously detrimental effect on the efficiency of the involved processes. In particular, pressure drop oscillations are low-frequency oscillations (periods much larger than the residence time of a particle in the system) in pressure and mass flow rate. In addition to performance degradation and problems of system control, flow rate oscillations may induce oscillations in wall temperature which can lead to thermal fatigue and breakage of the equipment.

In the present dissertation two-phase flow boiling in single- and parallelhorizontal tube arrangements has been analyzed, both numerically and experimentally. Special focus has been put on the study of pressure drop oscillations (PDO) in such systems. A critical review on the state of the art of PDO has been performed, showing what has been done in the subject and which are still the open issues.

For the experimental part, an experimental facility for boiling R134a in horizontal channels has been designed and constructed during this PhD project. From here, new experimental data characterizing boiling heat transfer, frictional pressure drop and PDO have been obtained. The effects of inlet subcooling, pressure and heat flux on the pressure characteristic (N-shape) curve have been identified. While higher subcooling and lower inlet pressure made the N-shape negative slope steeper, an increase in heating rate did not show considerable effects. Nevertheless, the heat flux axial distribution along the pipe showed a strong impact on this N-shape plot. During PDO in parallel channels, a new limit cycle has been observed. In all of the unstable cases tested for two parallel horizontal channels, always one of the channels oscillated in the superheated vapor outlet region, leading to very high wall temperatures at the outlet of the channel.

Pressure drop oscillations and the characteristic curve of a horizontal boiling channel have also been numerically studied. A criterion for determining when the thermal capacity of the heated pipe should be considered in the modeling of PDO has been developed, based on a non-dimensional number. The analysis of the characteristic curve and the classical PDO model for a single-channel have been extended to a parallel-channel system. The different possible limit cycles present in a two horizontal parallel channel configuration have been identified and explained. Even when the two channels are identical and under the same conditions, both channels cannot follow the same limit cycle. While one channel follows the typical PDO cycle, the other channel oscillates in the superheated vapor outlet region. Effects like flow excursions and the existence of stable maldistributed solutions in coupled parallel channels have also been analyzed. The numerical results have been validated by the experimental results obtained in the facility.