Surfactant Drag Reduction and Heat Transfer Enhancement.

摘要

Some cationic surfactants with suitable counterions can self-assemble into threadlike micelles (TLMs) in aqueous solution. The presence of TLMs imparts to the aqueous system interesting characteristics such as non-Newtonian behaviors and drag reduction (DR) capability. Such characteristics of TLMs systems can be controlled by tuning a number of factors such as temperature, counterion type, surfactant and counterion concentrations, pH, etc.;In this work, a pH-responsive surfactant-counterion TLMs system was developed. It is composed of 4 mM amphiphilic surfactant oleyl bis(2-hydroxyethyl)methylammonium chloride (commercial name is Ethoquad O/12 PG, EO12) and 8 mM trans -o-coumaric acid (tOCA) as the counterion. The rheological response of this TLMs system to pH is unique in that it has viscoelasticity in both low and high pH levels. Cryogenic transmission electron microscopy (cryo-TEM) images confirmed the presence of TLMs at pH 3.5 and pH 9.8. This system also had DR capability at low and high pH. Real applications may require that viscoelasticity can be reversibly switched on and off many times. Even after 5 cycles of pH changes the reversible changes in shear viscosity (η) and also first normal stress difference (N1) were still effective without significant decay. With its unique rheological behaviors, this TLMs system is potentially useful in either acidic or basic environment.;One promising application of surfactant DR solutions is in district heating or cooling systems (DHCS) to save pumping energy. However, surfactant DR solutions also have reduced heat transfer capability. Therefore, it is of practical importance to enhance the heat transfer capability of the drag reducing solution in heat exchangers while maintaining the DR capability in the rest of the DHCS.;Various mechanical devices have been employed to temporarily enhance the heat transfer capability of drag reducing surfactant solutions by disturbing the flow. However, in-flow mechanical devices result in additional pressure drop across the heat exchanger and so are not practical. In this work, a novel high-efficiency vortex (HEV) static mixer was designed and employed to locally enhance the heat transfer coefficient of a surfactant DR solution. Significant enhancement of heat transfer coefficients was observed with only modest pressure drop. The HEV static mixer had a performance number comparable to that of water. The enhanced heat transfer with moderate pressure drop by the HEV static mixer resulted from organized streamwise vortices naturally generated by the inclined tabs of the HEV static mixer.;To avoid the additional pressure loss by in-flow mechanical devices, this work also studied the use of external light irradiation to temporarily enhance the heat transfer capability. The ideal fluid should be drag reducing in its normal state. At the entrance of a heat exchanger, the fluid is irradiated by light, loses its drag reducing ability and has enhanced heat transfer capability in the heat exchanger. At the exit of the heat exchanger, the fluid is irradiated by a different light frequency which restores its DR capability. Thus this method combines the benefits of reduced pumping energy costs and good heat transfer.;A light-responsive surfactant DR solution was developed. After UV irradiation, its DR capability was reduced and its heat transfer capability was enhanced. The TLMs were also broken resulting in reduced η and N1. But the effect of UV irradiation on this solution is not reversible. As a result, the DR capability can not be restored, which prevents this solution from being used in DHCS. However, studies of this light-responsive surfactant DR solution led to an improved DR solution that had reversible responses to light irradiations. This surfactant DR solution is a promising candidate for use in district heating and cooling systems, where its drag reduction and high heat transfer can be switched on and off repeatedly by external light irradiation.