Numerical and experimental studies on flow condensation in hydrophilic microtubes

Abstract

• A parametric study was performed to investigate the flow condensation in microtubes. • The dynamic contact angle was used to obtain wall temperature and shear stresses. • The effect of diameter, inlet vapor quality, and vapor mass flux was assessed. • A non-dimensional analysis was performed to define flow regime transition criteria. • A flow map for condensing flow in microtubes with 250 μm < D h ≤ 900 μm was obtained. Microchannels have increasingly been used to miniaturize heat transfer equipment, improve energy efficiency, and minimize heat transfer fluid inventory. A fundamental understanding of condensation in microscale will yield far-reaching benefits for the different areas of industry. In this study, microtubes with inner diameters of 250, 500, 600, and 900 µm were used to investigate the effect of microtube diameter, inlet quality, and mass flux on the liquid/vapor interface near the wall boundaries in condensing flow. After validation with the experimental results, a transient numerical model (based on the Volume of Fluid approach) was developed to investigate the hydrothermal properties of condensing such as bubble dynamics, flow map transitions, transient interface shear force, and temperature on flow condensation performance in terms of heat transfer coefficient and pressure drop. The liquid film thickness, slug velocity, and location of transition from annular flow to slug flow inside the microtube were characterized for different microtubes, and the resultant alteration in condensation flow heat transfer and pressure drop is discussed in detail. Using non-dimensional analysis, a flow map was constructed and compared with the available flow maps for flow condensation in microchannels. The obtained results indicated that the interfacial characteristics of condensing flow in microtubes with hydraulic diameters lower than 500 µm are majorly different from those with D > 500 µm.