Pool boiling of refrigerants R-134a and R-404A on porous and structured tubes part I. Visualization of bubble dynamics

This is the first part of a two-part experimental work that is intended to provide a further understanding of the physical phenomenon for refrigerants R-134a and R-404A boiling on structured and porous tubes. This paper provides detail visualization results for bubble formation characteristics on smooth and enhanced tubes. The experimental observation results support the existence of flooded and suction-evaporation modes on structured tubes. Under low heat-flux conditions, the boiling mechanism is in the flooded mode. Bubbles generated from the sub-tunnel surface of structured tubes are similar to those generated from a smooth tube surface. Most of the bubbles joined together inside the tunnel to form large bubbles, moved up to the upper part of the sub-tunnel, and departed from the surface through the so-called active pores. The other bubbles passing through the pores directly are the small bubbles. Under high heat-flux conditions, the boiling mechanism transforms to the suction-evaporation mode. A thin liquid film evaporated in the tunnel and bubbles ejected from the structured pores. There are no small bubbles passing through the pores in this situation. In a porous tube, because of a large variety of pore sizes, the bubble density is very high even though under low heat-flux conditions. However, the ratio of the bubble density to the number of pores for the porous tube is lower than that of structured tubes B and TX. This means that most of the pores on the porous tube are not active and require further improvement.