Marangoni condensation of steam-ethanol mixture on wire-wrapped tube: effect of wire pitch on heat transfer augmentation

This study investigates the Marangoni condensation of steam-ethanol mixtures on wire-wrapped tubes at atmospheric pressure, with a constant vapor velocity of 0.48 m/s. Experiments were conducted using various mass ethanol concentrations (0.0125 %, 0.05 %, 0.1 %, and 0.3 %) with a 0.5 mm copper wire diameter, and wire pitches of (1.6 mm, 2 mm, 2.4 mm, and 2.9 mm). Special precautions were taken to eliminate air from the vapor phase and minimize experimental errors. Visual observations revealed distinct condensation modes, transitioning from film-wise condensation for the case of pure steam into pseudo-dropwise condensation with the addition of ethanol. The presence of ethanol improved heat transfer by reducing the film thickness, forming small droplets between the wire windings. Additionally, the wire wrapping increased the surface area, sliced film thickness, and minimized retention, thereby expanding the active surface area and significantly enhancing heat transfer. Both factors were thoroughly investigated to understand their combined effects. The results demonstrate significant improvements in heat transfer performance compared to pure steam, with notable increases in heat flux and heat transfer coefficients. The most significant enhancement ratio, defined as the ratio of observed heat transfer values to those predicted by Nusselt's theory (1916), was 9.9, occurring at an ethanol concentration of 0.1 % and a pitch of 2.9 mm.