Heat transfer enhancement of turbulent drag-reduced flow

• Main Author: Lim, Mei Qi
• Other Authors: Leong Kai Choong
• Format: Final Year Project
• Language: English
• Published: 2014
• Subjects: DRNTU::Engineering::Mechanical engineering
• Online Access: http://hdl.handle.net/10356/60477
• Institution: Nanyang Technological University

In this century, energy conservation has demanded our attention as it is utmost important to ensure human survival not only for the present generation, but also for the future generations. Hence, there is a need to reduce energy consumption whenever necessary. In the case of district heating and cooling systems, turbulence drag due to the fluid friction on the wall has been a major factor in the wastage of precious energy generated. Thus, two chemical additives, namely polymers and surfactants, have been the focus of studies in reducing the drag present within such systems. Small amounts of surfactant additives can reduce the drag significantly over a range of turbulent flow rates, which results in lower pumping power required. Unlike polymer dragreducing additives which degrade irreversibly, surfactant additives are thermodynamically stable and are able to self-assemble quickly after degradation caused by high shear stresses. The characteristics of surfactant additives make it suitable for a re-circulatory system. However, the discovery of Tom’s effect also means that drag reduction is usually accompanied with a reduction in heat transfer as well. The heat transfer ability of the surfactants is decreased with the increase of the thermal resistance between the bulk fluid and wall. Various heat transfer enhancement methods, which can be classified into passive and active methods, have been used to compensate for this heat transfer reduction. There are a substantial number of investigations on wing/winglet-type vortex generators and the potential for bluff body vortex generators is promising. In this project, a bluff body in the form of cube-shaped protrusions are incorporated on the winglet-type vortex generator to make-up one of the vortex generators (VG 1). The other is the use of cylinders to make up the shape of a winglet-type vortex generator (VG 2). The effect of protrusions and geometries on heat transfer enhancement and drag reduction are the main area of interests. Experiments were conducted using a two-dimensional, closed loop re-circulating water channel. The flow rate of the water in the system was adjusted via the pump control panel and flow meter. The heating surface is made of high conductivity copper with dimensions of 0.3 m by 0.3 m, incorporated with heater mats and thermocouples to measure temperatures across the copper plate via data acquisition. The pressure drop across the heating assembly, where the vortex generators will be positioned at, was measured using a pressure transducer. The experimental results show that the vortex generator which provides the best heat transfer enhancement is VG 1. This vortex generator has an average heat transfer enhancement of 36%, with an average drag penalty of 18% for the Re between 8000 and 20000. The results from the flow visualisations also agree reasonably well with the experimental results, showing that VG 1 with its cube-like protrusions has better turbulence mixing and vortex shedding as compared to VG 2 made with cylinders. With the addition of the drag reducing additives to the vortex generator, the average heat transfer reductions are 7.22%, 11.37% and 15.02% for 30 ppm, 60 ppm and 90 ppm, respectively. The average drag reductions are 52.20%, 53.83% and 55.20% for 30 ppm, 60 ppm and 90 ppm, respectively. The Nusselt numbers are still higher than the experimental results obtained without the use of vortex generator even though there is presence of surfactant in the re-circulatory system.