The synchronization between the air pressure fluctuations and the depth of liquid penetration into the nozzle during bubble departures was investigated using joint recurrence quantification analysis. In the experiment, the bubbles were generated from a glass nozzle into distilled water. During the analysis, the recurrent rate coefficients were calculated for the depth of liquid penetration into the glass nozzle and pressure changes in the gas supply system. The study was conducted by two air volume flow rates, i.e. 0.023 l/min and 0.026 l/min. The air volume flow rates were selected so that the appearance and disappearance of period bubble departures were clearly visible. It has been shown that the synchronization of the pressure changes and the depth of liquid penetration appears when periodic changes in the depth of liquid penetration occur in a relatively long period of time. The process of changing the distance between the extremes of liquid penetration into the nozzle and pressure changes in the gas supply system was observed. It has been found that the decrease in the distance between these extremes is responsible for the appearance of periodic bubble departures. This behaviour has not been reported in previous papers. This process was modelled by numerical simulations.

In this paper, the boiling synchronization of two-phase flow patterns in two parallel minichannels of 1 mm in diameter with connected compressible volumes was analyzed. The analysis was performed using images recorded with a high-speed camera. The degree of synchronization between channels was evaluated by assessing the presence of liquid flow in the small part of the minichannels. It can be concluded that boiling is synchronized when small bubble flow or wavy annular flow occur in neighboring channels. The occurrence of slug flow in one channel causes the boiling in neighboring channels to become unsynchronized. The result of the image analysis shows that the correlation coefficient based on the evaluation of the presence of liquid flow in the small part of the minichannels over a long enough time period allows for the detection of boiling synchronization.

In the paper, a self-organizing map combined with the recurrence quantification analysis was used to identify flow boiling patterns in a circular horizontal minichannel with an inner diameter of 1 mm. The dynamics of the pressure drop during density-wave oscillations in a single pressure drop oscillations cycle were considered. It has been shown that the proposed algorithm allows us to distinguish five types of non-stationary two-phase flow patterns, such as bubble flow, confined bubble flow, wavy annular flow, liquid flow, and slug flow. The flow pattern identification was confirmed by images obtained using a high-speed camera. Taking into consideration the oscillations between identified two-phase flow patterns, the four boiling regimes during a single cycle of the long-period pressure drop oscillations are classified. The obtained results show that the proposed combination of recurrence quantification analysis (RQA) and a self-organizing map (SOM) in the paper can be used to analyze changes in flow patterns in non-stationary boiling. It seems that the use of more complex algorithms of neural networks and their learning process can lead to the automation of the process of identifying boiling regimes in minichannel heat exchangers.

Boiling in a minichannel occurring at a low mass flow rate is accompanied by non-stationary twophase flow. The analysis of pressure fluctuations during non-stationary boiling in minichannel shows that quasi-periodic changes in flow patterns can be observed in such fluctuations. We can define in such a way the sequences, which are called “oscillating boiling patterns”. In the present paper the model, which allows us to simulate the appearance of “oscillating boiling patterns” has been presented. In the proposed model the mass flow rate changes (because of evaporation and condensation) are modelled by compressible volumes representing various sizes bubbles. In the paper the good quality agreement between experimental data and simulation results has been achieved. Experimental data was collected during the boiling in open minichannels system with inner diameter of 1 mm.