Heat transfer enhancement of turbulent drag-reduced flow

• Main Author: Kok, Carvin Zhi Long
• Other Authors: Leong Kai Choong
• Format: Final Year Project
• Language: English
• Published: 2015
• Subjects: DRNTU::Engineering::Mechanical engineering
• Online Access: http://hdl.handle.net/10356/64019
• Institution: Nanyang Technological University

Energy conservation is becoming more important over time as the demand for energy supply is increasing. However, cheap energy supply is diminishing, further strengthening the importance of energy conservation. Hence, energy efficiencies of systems are becoming a main concern. One example is the introduction of District Heating and Cooling systems (DHC) in large cities. In such systems, centralised heating/cooling plants supplies heated or chilled fluids through series of pipes to buildings to meet their cooling/heating load. This eliminates the need for separate cooling/heating plants, thereby improving overall efficiency. Drag reducing agents (DRAs) were found reduce pressure loss in pipe flows which can be used in DHC. DRAs form thread-like structures which suppress turbulence in high flow rates, thereby reducing the drag losses. Surfactants were found to be of the best fit in DHC systems due to their regenerative nature after being destroyed temporarily in high shear stress sections such as pumps and fittings. However, DRAs also cause reductions in heat transfer that coupled with drag reduction benefits in which both heat transfer and drag reduction are important for DHC. In this project, experiments were being carried out in a two-dimensional recirculation channel with a water channel cross section area of (400 × 20 mm). Water was heated on either the upper or lower side of the channel, resulting in symmetric heating. Ethoquad 18/25 surfactant and NaSal counterion of 1:1 mass ratio were used. Heat transfer enhancers with a series of nine flat inserts at various angles of attack were studied. ii The experiments show that Ethoquad 18/25 achieved the highest drag reduction of 10 % at a concentration of 30 ppm at Reynolds number of 8000. The overall drag reductions, however, were low. It was also found that increasing the surfactant concentrations above 30 ppm decreased the drag reduction. Heat transfer reduction averaged at 10 % and was more often or not higher than that of drag reduction at the same flow condition. Drag induced by the heat transfer enhancement inserts largely depends on the frontal area blockage rather than the orientation. Results show that the drag increase stands at 40 % and 95 % at the insert angle of attack of 10 and 20 degrees, respectively. Mixing was found to be more uniform when the alternate inserts are inverted. This results in a much higher performance due to the more uniformly distributed heat transfer between both heaters. The overall performances of the inserts were higher when they were used in surfactant solutions, suggesting that there were noticeable effects against the micelle structures of surfactants.