Heat Transfer to Viscous Solutions

The focus of this study is on heat transfer to solutions with viscosities greater than those commonly encountered. An experimental facility was built to measure convective heat transfer coefficients for fluids with viscosities ranging up to 1,000 cP. Results for both single-phase (liquid) and two-phase (liquid and vapor) heat transfer are presented, for aqueous solutions of polyethylene glycol. Initial indications are that single-phase coefficients are reasonably well predicted by standard correlations, but two-phase coefficients are notably different than those correlated for non-viscous fluids. Introduction In two important process industries, namely food processing and polymer processing, fluids of high viscosities are often encountered [1]. Design of heat exchangers for such applications are often hampered by lack of published data or correlations, since the standard engineering models were predominantly based on data for common fluids (e.g. water and refrigerants) with viscosities of 1-10 cP and Prandtl numbers of 1-100 [2]. While one may expect that single-phase correlations may be extrapolated over wide ranges with fair confidence, there is no such assurance for two-phase correlations. The very nonlinear behavior of two-phase flows, especially where there is evaporative phase change, argues against extrapolation of existing correlations and models over orders of magnitude in viscosities and Prandtl numbers [3]. The objective of this experimental investigation was to measure convective heat transfer coefficients for viscous solutions, for both single-phase and two-phase flows. Experiment Figure 1 is a diagram of the experimental facility, showing the flow loop and details of the instrumented test section. Subcooled liquid solution flows from a surge tank and is further sub-cooled before entering a gear pump. The liquid exiting the pump is preheated to specified temperature before introduction to the test section. The solution then proceeds through the test section where it is heated in either single-phase convection or evaporated in two-phase convection. A double-pipe heat exchanger and a water-cooled condenser removes the added thermal energy, returning the test fluid to its initial enthalpy for recycle to the test section. In order to minimize dissolved gases in the test fluid, degassing is accomplished by pulling a vacuum throught a knockback condenser located above the main condenser. The pressure and temperature of the process fluid are measured directly before and after the fluid enters the test section. The solution temperatures are measured using 1/16” diameter K-type thermocouples. Pressure is measured using Validyne dp-15 pressure transducers. Flow is measured before the test section using a Flocat positive displacement flow meter. K-Type wall thermocouples are evenly spaced 3 inches apart along the 1⁄2” diameter, 6 feet long test section. Electrical bus bars attach a power supply to each end of the test section providing resistive heating to the test section. The outlet temperature and pressure are measured directly after the test section. A sight glass at the outlet allows the observation of flow patterns. Data are collected, analyzed, and organized in real-time using an AMUX-64T multiplexer and a 16 channel, 16 bit DAQ card using Labview.