HEAT-TRANSFER TO NONMISCIBLE LIQUID-LIQUID MIXTURES FLOWING IN A VERTICAL TUBE

Phase distribution and heat transfer measurements were carried out in an n-heptane-water mixture flowing upward in a vertical stainless stee l tube of 16-mm inner diameter. For the phase distribution measurement a very high frequency impedance probe was used with a tip diameter of 0.58 mm. The volumetric phase fraction of n-heptane was varied betwee n 0 and 1, the mixture velocities between 0.5 and 2.3 m/s, and the tem perature between 30 and 80 degrees C. By means of a high-speed data ac quisition board implemented in a personal computer, two methods for si gnal conditioning were tested: level thresholding and derivative thres holding. Derivative thresholding proved to be advantageous because it is insensitive to baseline drift and allows detection of droplets of t he discontinuous phase. Heat transfer to the liquid-liquid mixture dep ends on the phase distribution in the boundary layer at the wall. It w as found that the change from water-dominated heat transfer to n-hepta ne-dominated heat transfer occurs nearly abruptly at n-heptane volumet ric phase fractions between 0.6 and 0.7. An estimate of the heat trans fer coefficient as a function of the mean volumetric fraction epsilon by a linear interpolation between the heat transfer coefficients at ep silon(ns) = 0 and epsilon(ns) = 1 would therefore lead to significant errors. The few correlations available in the literature are also not suitable to predict heat transfer to water-n-heptane mixtures. Therefo re a preliminary prediction method is proposed based on the experiment al data for phase distribution near the heated wall.