Abstract
In the present work, the flow characteristics of free and impinging synthetic jets formed from a single cavity but with a different number of orifices are investigated using the particle image velocimetry technique. A synthetic jet is a zero-net mass-flux device that formed by periodic ejection and suction of fluid through a small opening. The flow characteristics are used to explain the heat transfer results in the literature. The number of satellite orifices, distance between orifices, and location of the surface from orifices are varied to observe the phenomenon of jet interaction and its influence on flow recirculation. The results show that interaction between adjacent jets causes the diversified flow characteristics of the multiorifice jet with respect to a single orifice jet. The multiorifice jet exhibits significantly higher entrainment and mixing compared to its equivalent diameter of a single orifice jet. Moreover, results from the impinging jet corroborate that the impingement of vortex pairs, their breakdown, and formation of a strong wall jet are responsible for a high heat transfer coefficient for the case of larger surface spacing (z/d ≥ 8; normalized distance between orifice and surface). However, for impingement at small surface spacing (z/d ≤ 2), change in recirculation behavior with different orifice configurations can lead to a variation in the heat transfer coefficient. For the large and medium surface spacing (2 < z/d < 8), the center orifice gets sufficient time to develop, which draws the satellite and impingement jets as a single orifice jet. In the case of large surface spacing, the wall jet vortices are not attracted during the suction stroke. While for medium surface spacing, wall jet vortices are closer to the orifice and easily entrained during the suction stroke and form a recirculating region. In the case of small surface spacing, the center jet does not get sufficient time, and hence both center and satellite jets impinge separately. It was found that the center jet having higher ejection velocity impinges earlier than the satellite jet and the wall jet vortex formed from the center orifice drives along a wall jet. Therefore, recirculation is absent for smaller surface spacing, and fresh fluid is sucked and impinges on the surface in each cycle. This study expands the current knowledge of multiorifice free and impinging jets and establishes a relationship between heat transfer and fluid flow, which eventually facilitates an efficient and effective heat transfer system for compact devices.
Highlights
Air cooling is preferred in many heat transfer applications due to its availability and low cost
The results suggest that for small surface spacing with an increase in pitch circle radius (PCR), the satellite jets do not mix with the center orifice, and the jets rather maintain their individual identities
The present experimental study focuses on the interaction of the multiorifice synthetic jet and describes its consequences on the heat transfer study reported in the literature
Summary
Air cooling is preferred in many heat transfer applications due to its availability and low cost. Air is flowed over different fin geometries by using a fan to achieve high heat transfer. This approach can make the system bulky, which is undesirable. A synthetic jet is compact and has huge potential for localized cooling applications. It can be created by the oscillation of a speaker diaphragm inside a cavity, which gives rise to periodic suction and blowing, causing the formation of vortices at the exit of the orifice. Due to its wide applications, the fluid flow and heat transfer characteristics of a single orifice synthetic jet have received considerable attention both experimentally and numerically.. The synthetic jet is characterized by the formation of a pair of counter-rotating vortices during the ejection cycle, which travels in the axial direction, resulting in mass entrainment and mixing. Due to its wide applications, the fluid flow and heat transfer characteristics of a single orifice synthetic jet have received considerable attention both experimentally and numerically. The studies based on a single orifice synthetic jet have focused on the effect of several operational [effect of surface spacing, Reynolds number (Re), and frequency of excitation (f)] and geometrical parameters (orifice shape/thickness and cavity scitation.org/journal/adv geometry) on the heat transfer rate. The acoustic aspect of the synthetic jet has been reported by Bhapkar et al., who found an increase in the noise level with an increase in the orifice diameter
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