Book Description

The last decade has seen a dramatic rise in the research on synthetic jet (SJ) impingement cooling and heat transfer. This is primarily due to the many advantages of a SJ that include its low cost, simple structure, light weight, ease of installation and ability to be miniaturised using MEMs technology, making it a very promising alternative to other technologies in use. Consequently, a large body of literature exists on the impingement of SJ normally on a heated flat surface. The demand for cooling technologies however is not limited to target objects having flat surfaces only. Real applications can present a whole range of geometrical situations, with curved surfaces, and tight spaces with bounding walls. In spite of this, there exists very little research and information beyond the SJ impinging on flat surfaces in unconfined environments. An obvious deviation from a flat surface is a surface with curvature. For the specific situation of a circular cylinder, very few studies, if any, have been carried out in the past for analysing the heat transfer characteristics of SJ impingement. This research work was therefore aimed at investigating the flow and heat transfer characteristics of a slot synthetic jet (SJ) impinging on a circular cylinder. Specifically, it focussed on the influence of the geometric arrangements and flow conditions on the flow dynamics of the slot SJ, flow characteristics associated with the SJ impingement on a circular cylinder and the resulting thermal behaviour of the SJ. A bench-top synthetic jet actuator driven by a magnetic shaker via a loudspeaker diaphragm was utilised for the study. The SJ was generated from a slot of dimensions w = 6.4 mm x h = 160 mm (aspect ratio h/w of 25), with a jet Reynolds number of 2,400-3,900 (based on the slot width). To help understand the characteristics of the impingement fluid dynamics and heat transfer, a detailed investigation of the SJ flow field (in the absence of the cylinder) was first carried out. In this regard, an important aspect of the research was aimed at generating detailed understanding of the SJ flow-field characteristics in a bounded region. In a number of generic situations, this work is of high importance as the SJ could potentially be deployed for cooling applications in constrained environments. To attain a constrained environment, two parallel sidewalls were mounted along the shorter side of the slot extending in the streamwise direction to constrain the flow along the slot span. Hot-wire anemometry was used to explore the flowfield characteristics of the SJ ensuing in both a free (i.e. without sidewalls) and the constrained environment. To establish the flow and thermal characteristics of the SJ impingement, two instrumented aluminium cylinders of diameter, D of 19 mm, having a curvature ratio D/w of 3 were fabricated. The first was equipped for unsteady surface pressure measurements, while the second with a uniform surface temperature for heat transfer analysis. The cylinders were traversed along the jet centreline over non-dimensional distances from the slot of H/w = 5-50, corresponding to the SJ jet near-field through the developing region to the fully developed region. In addition, smoke flow visualizations were conducted to gain insights into the flow dynamics associated with the SJ flow-field with and without sidewalls, and SJ impinging on the cylinder. The experimental investigation for the SJ with and without sidewalls revealed that the presence of the sidewalls strongly influences the SJ flow-field. For instance, jet spreading rate reduced by almost 31.5 % with a corresponding rise in the statistically two-dimensional region in the slot downstream with the inclusion of the sidewalls. In addition, the phenomenon of axisswitching was found to be absent in the SJ flow-field in the presence of the sidewalls. Other jet properties such as the turbulence intensity, skewness, and flatness factors further revealed the differences in the flow-field of the two configurations. Furthermore, the experimental results for the SJ impinging on a circular cylinder showed that the flow-field behaviour differs significantly from that of a cylinder in uniform flow and is largely affected by the jet-cylinder separation distance and the operating environment i.e., free or constrained. For instance, the plots of the pressure distribution and normalized standard deviation of the fluctuating pressure around the cylinder surface revealed higher flow fluctuations associated with SJ impingement. The flow visualization and the hot wire measurements further unveiled that there was no obvious vortex shedding that occurs in the cylinder wake; instead evidence for vortex dipoles rising from the cylinder surface was found, on either side of the cylinder. Under uniform cylinder surface temperature conditions, the thermal performance of SJ impingement was found to be governed by the Reynolds number, jet cylinder separation distance and the excitation frequency, as might be expected from the literature on SJ impingement on flat surfaces. The SJ was found to perform better in the constrained environment, attributed to relatively higher flow fluctuations developed by the complex interaction of the vortex with the sidewall boundary layer and the cylinder. Almost 12% higher average heat transfer was observed in the case of the constrained environment over the range of parameters employed in the current work. Moreover, a strong dependence of heat transfer on the jet cylinder separation distance was also found. In contrast to the SJ impinging on flat target surfaces where the maximum heat transfer was attained in the intermediate field, at H/w = 14 to 18, for SJ impingement on the circular cylinder however, this was consistently attained in the near field i.e., H/w = 5. Also, the thermal performance as a function of the flow Reynolds number was found comparable to the uniform flow case, when the Reynolds number was based on the approach flow velocity (i.e. local velocity based on the cylinder location) instead of the velocity at the slot exit. The present study revealed for the first time, the flow and heat transfer behaviour of the SJ impinging on a circular cylinder in free and constrained environments. The results of the study may serve as a guide for SJ based solutions for various heating or cooling applications.