Discovering an empirically new relation and obtain the flow pattern map for dimpled tubes in two-phase flow for refrigerant R600-a

In present work, heat transfer and flow pattern map of dimpled and horizontal tubes were studied during phases of boiling and condensation of mandatory convection of hydrocarbon R600-a. Empirical relationship will be expressed based on empirical data obtained by such experiments and fit the experimental results and Matlab software to show flow pattern map for such refrigerant with proper accuracy. The cycle consists of a pump, two preheaters, two testing parts, two condensers, flow meter and reversing valve. In this study, experimental evaporator is a copper tube with internal diameter of 8.7 mm and length of 1200 mm. the heat required for fluid evaporation is supplied by thermal element twisted around it. Such tube has been designed in forms of dimpled and straight models to evaluate effect of placement of dimples inside tubes. The test was conducted by refrigerant mass flow between 155 and 470 kg/m^2 s and vapor quality between 0.05 and 0.78. Moreover, flow patterns and their transitions for refrigerant R-600a during flow boiling inside a helically dimpled tube and a smooth tube were visually observed and analyzed. Annular, intermittent, and stratified-wavy flow were recognized for plain tube whereas there was no stratified-wavy flow in flow pattern visualization of dimpled tube. Investigation clearly shows that the dimples in evaporation significantly impact the two phase flow pattern. Inside the helically dimpled tube the intermittent/annular transitions occurs at lower vapor quality value than for the smooth tube.