CONDENSATION HEAT-TRANSFER ENHANCEMENT USING STEAM-ETHANOL MIXTURES ON A HORIZONTAL BANK OF TUBES

Significant heat-transfer enhancement using steam-ethanol mixtures can be obtained as compared to that of pure steam. Due to addition of small concentrations of ethanol, the condensate mode resembles that of typical dropwise condensation on a hydrophobic surface. Surface tension instabilities, also known as Marangoni phenomenon, take place when small amounts of ethanol are added to pure steam. It has been investigated extensively for the case of single tubes. However, it is difficult to extrapolate single-tube data to banks of tubes due to the complex nature of three-dimensional flows and interac-tions between vapor and liquid. In this paper, data are presented for condensation of pure steam as well as for steam-ethanol mixtures on a horizontal bank of tubes at atmospheric pressure. Present pure steam data are validated by comparing with earlier data and theoretical models available in literature. Condensation of steam-ethanol mixture was investigated using five concentrations of ethanol by mass in the boiler when cold prior to startup, i.e., 0.025, 0.05, 0.1, 0.5, and 1% at vapor velocity of 3.2 and 3.7 m/s. On the top rows maximum heat-transfer enhancement of around 4 and 2.5 times was obtained using ethanol mass fraction of 0.05 and 1.0%, respectively.