An empirical study on evaporation heat transfer characteristics and flow pattern visualization in tubes with coiled wire inserts

In the present work, heat transfer and flow pattern visualization during forced convection evaporation of hydrocarbon R-600a (Isobutane) inside a horizontal tube with coiled wire inserts has been experimentally studied. The system consists of a pump, two preheaters, the test section, two condensers, flow meter and bypass. The test evaporator was an electrically heated copper tube of 1000 mm length, 8.1 mm inside diameter and a wall thickness of 0.71 mm. Coiled wires with three different wire diameters of 0.5, 1.0 and 1.5 mm and three different coil pitches of 10, 20 and 30 mm were made and used in full length of test evaporator. The test runs were carried out at refrigerant mass fluxes from 109.2 to 505 kg/m2 s and heat fluxes between 18.6 and 26.1 kW/m2. The use of coiled wires was found to increase the flow boiling heat transfer coefficients up to maximum value of 124% above the plain tube values with wire coiled having a twist ratio (PDin) of 1.243 and a thickness ratio (eDin) of 0.183. Four different flow patterns were observed and classified as bubbly, stratified, intermittent and annular flow. This study reveals the developments of the flow patterns. Also it was observed that the presence of coiled wire inserts induces disturbance into gas and liquid flows and so that the shape and motion of bubbles and slug are different from those observed in a plain tube. The correlation suggested by Shah [20] gave the best prediction to the flow boiling heat transfer coefficient in the plain tube within the studied ranges.