Measurements and visualization during condensation inside a 3 mm diameter channel

In the last years, convective condensation has been extensively studied inside large tubes (e.g. 8-12 mm diameter) and two main mechanisms have been identified: shear stress dominated condensation (annular flow) and gravity force dominated condensation (stratified flow). Recent investigations carried out in mini- and micro-channels evidence the less important role of gravity force when decreasing channel dimensions, with a reduction of the stratified zone, an enlargement of the intermittent region and a sensible effect of the channel shape. Investigations in the macro-to-micro transition region, that include both heat transfer, pressure drop and flow visualization, are very limited. From this background, a new test section for the study of convective condensation inside a 3.4 mm diameter circular channel has been designed and built. The test section allows quasi-local heat transfer coefficient measurements and flow visualization. Prior to perform two-phase tests, the experimental technique has been validated with single-phase heat transfer measurements. Condensation experiments have been run at 40°C saturation temperature and the effects of mass velocity and vapor quality are studied. Flow pattern has been investigated with a high speed camera. Finally, the new experimental data are compared with available correlations in the literature to assess differences and similarities of the phenomena with larger and smaller channels.