Flow Boiling Heat Transfer And Flow Patterns In Three-Dimensional Micro-finned Helically Coiled Tube

Because of the high efficiency in heat transfer and compactness in volume, helically coiled tubes are used extensively in heat exchangers, nuclear reactors, solar collectors, and the food, drug and refrigeration industries. It is well known that the secondary flow in the cross section of the helically coiled tube, due to centrifugal force and torsion force, is a significant factor affecting the flow and heat transfer. In the open literature, the experimental fluids of flow boiling heat transfer in helically coiled tubes are often water and steam. And the focuses are mainly pertinent to the behaviors of critical heat flux (CHF) or dryout in helically coiled tubes. The heat transfer performance of refrigerants flow boiling in helically coiled tubes has seldom been involved. Moreover, microfinned surface, widely used and investigated in straight tubes, has not been applied to helically coiled tubes. The flow boiling two-phase flow patterns and their transitions in helically coiled tube, another interesting and challenging field in two-phase flow study, have also not been sufficiently studied, compared to that in straight tube.In this paper, the heat transfer augmentation technology of a new type of three dimensional (3-D) microfinned surface was applied to helically coiled tube for the first time. The flow boiling heat transfer and flow patterns have been experimentally studied, in such kind of 3-D microfin helically coiled tube, using refrigerant R134a, an environment- friendly refrigerant. The flow boiling flow patterns have been recorded and filmed through flow visualization method. Several different two-phase flow pattern maps, together with the transitions between the main flow patterns, were obtained based on the experimental data. The performance of heat transfer and pressure drop have also been measured and the mechanism of heat transfer augmentation of three-dimensional microfin helically coiled tube were analyzied. Finally, several correlations of heat transfer and pressure drop were correlated based on different flow patterns.(1) Two-phase flow patters of R134a flow boiling in smooth and 3-D microfin helically coiled tubes were observed and filmed for the first time. The flow patterns in both kinds of helically coiled tubes could be divided into bubbly flow, plug flow, stratified flow, stratified-wavy flow, intermittent flow and annular flow. Compared to the flow patterns in adiabatic two-phase flow, the formation mechanism of bubbly flow in flow boiling was quite different, i.e., a lot of vapor bubbles could also be found in continuous liquid phase even though in stratified or intermittent flow regime. Further more, the continual formation, growth, and departure of bubbles from the heated surface made the interface of vapor-liquid strongly wavy and the droplet splash out of the two-phase interface. This caused the two-phase interface usually quite irregular geometry. On the other hand, because of the secondary flow in helically coiled tubes, the stratified effect was weakened, so that annular flow could be formed more easily. The modified Mandhane flow map can essentially descript the flow regimes and their transitions in helically coiled tubes. Taitel-Dukler flow map showed that the transition curves between stratified-wavy and intermittent or annular flow were almost the same, i.e., . The transition curves between intermittent flow and annular flow were Xtt＝0.4 and Xtt=0.7 for smooth and microfin helically coiled tubes, respectively. In the G-x flow map, it was obvious that not all the flow patterns mentioned above appeared one by one under each mass velocity. Only stratified-wavy flow existed in both kinds of tubes while the mass velocity less than 100 kg/(m2s).And in some cases, stratified-wavy flow could directly transform to annular flow.(2) The experimental data showed that flow boiling pressure drop in 3-D microfin helically coiled tube increased with the increases of both mass flux and the vapor quality, and decreased with the increase of saturated pressure. The comparison...