Convective heat transfer enhancement in drag reducing channel flow

• Main Author: Muhammad Jemal Amsah.
• Other Authors: Leong Kai Choong
• Format: Final Year Project
• Language: English
• Published: 2009
• Subjects: DRNTU::Engineering::Mechanical engineering
• Online Access: http://hdl.handle.net/10356/16360
• Institution: Nanyang Technological University

Polymers and surfactants are commonly used to reduce the fluid flow drag in a channel flow. The discovery of the Tom’s effect [3], where polymers can be used as drag reducing additives, have opened up avenues for reducing undesired drag of fluid flow that occurs over long distance transportation. This discovery helps save pumping energy in pipelines and thus save cost. Nevertheless, the use of polymers as additives to reduce flow drag can be permanently impaired when subjected to high shear stress rates and extended period of turbulent flow. Surfactants, on the other hand, are more desirable due to their temporary mechanical degradation when exposed to high shear stress rates and prolonged period of turbulence. Despite all the virtues of the use of surfactants, they have a main disadvantage of reducing the heat transfer rate. This phenomenon is unfavourable in re-circulating systems especially in district heating and cooling systems. This project aims to investigate the impact of surfactants in reducing the drag of fluid in a water channel flow. The adverse effects of heat transfer reduction were explored and the effectiveness of using an array of micro-jets to enhance the heat transfer rate was examined. For this project, a two dimensional, closed loop re-circulating water channel of 4.5 m long was used. The experimental set-up includes a settling tank, two reservoir tanks, two pumps, an electromagnetic flow meter and a heating element. The heating element was installed along the water channel. Different concentrations of surfactants were used to study its influence on the skin friction drag and the heat transfer rates. The maximum drag reductions attained at surfactant concentrations of 30 ppm, 60 ppm and 90 ppm are approximately 11.5%, 14.1% and 44.7% respectively. For surfactant concentration of 90 ppm, the critical Reynolds number was discovered to be 17 000. It was noted that there was minimal heat transfer reduction at surfactant concentration of 30 ppm. The heat transfer reduction at Reynolds number 17 000 was recorded as 4.0%, 38.8% and 48.7% for surfactant concentration of 30 ppm, 60 ppm and 90 ppm respectively. In order to enhance the heat transfer rate in the drag reducing channel flow, an array of 4 X 4 micro-jets of diameter 150 μm was used. These micro-jets pumped water into the channel flow to create spiraling vortices that help disrupt the orientation of the orderly aligned micelle structures. At surfactant concentration of 60 ppm, a maximum of 106.8% of heat transfer enhancement was attained at Reynolds number 17 000. This process promotes the heat transfer enhancement by causing the disruption of the rod-like microstructure of the micelle layer formed by the surfactants, hence resulting in better heat transfer enhancement. The effects of different micro-jet velocities were also investigated in this project. Three different micro-jet velocities were used to observe its impact on the heat transfer enhancement of the system namely, 1.18 m/s, 2.94 m/s and 4.71 m/s. The maximum heat transfer enhancement was recorded as 32.3%, 79.7% and 100.6% respectively at various Reynolds number. The maximum overall heat transfer enhancement of 100.6% was achieved at Reynolds number of 17 000 and micro-jet velocity of 4.71 m/s.