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1. Introduction
Since the drag reduction phenomenon is discovered by Toms in 1948 [[1]], polymers and surfactants as two drag-reducing additives were studied by many researchers [[2]-[4]]. Polymer drag-reducing additives have been used extensively in crude oil pipelines of companies such as Shell and Conoco to increase the throughput and to save pumping energy [[2]]. However, due to the permanent decrease of drag reduction performance of polymer after passing through high shear stress sections of pipelines, they are not effective in closed circuit systems such as district heating and cooling (DHC) recirculation system. In contrast to polymers, the mechanical degradation of surfactants is only temporary [[3]].
In addition to the drag reduction effect, another noticeable effect of surfactant drag-reducing additives is the heat transfer reduction that occurs between a solid boundary and the flowing fluid [[4]-[6]]. Weiet al. [[6]] showed that heat transfer reduction is always a little larger than drag reduction. This reduced heat transfer behavior of drag-reducing surfactant solutions is an obstacle to their application in DHC systems since heat exchange in heating or cooling is essential.
Some research focused on the effect of the Reynolds number [[7]] and the concentration [[8]] on the drag reduction and heat transfer induction of the surfactant solutions. But there are few details, systematic investigations, and deep analyses of the concentration, the Reynolds number, and the temperature effects on the heat transfer reduction of the surfactant solution. In this paper, the combined effects of concentration, Reynolds, and temperature on the heat transfer behavior of CTAC/NaSal surfactant solution were experimentally investigated and analyzed from the viewpoint of thermal energy and shear stress energy in detail.
2. Experimental Apparatus and Procedure
2.1. Test Facility
The water channel test facility for heat transfer experiments was a closed loop shown schematically in [Figure 1]. The system consisted of a reservoir tank (0.45 m3 ), a stainless steel centrifugal pump, a settling chamber equipped with a nozzle, a two-dimensional channel, a diffuser, an electromagnetic flowmeter with a resolution of ±0.01 m3 /min, the stainless steel tube, and some necessary components to connect the equipments. In order to get different fluid temperatures, a control system with a 4.4 kW heater was installed in the reservoir tank. The temperature of the...