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Description
Jet impingement is a widely used forced convection technique characterized by a stagnation region and a radially developing wall jet, which can markedly thin the thermal boundary layer near the stagnation region and produce locally high heat transfer rates [1]. It is applied in systems with high heat fluxes and tight thermal constraints, including gas turbines, electronics [2], and grinding processes [3].
Conventional steady jets are typically generated by nozzles or open pipes supplied with compressed air, delivering relatively collimated flows whose thermal performance depends mainly on Reynolds number and nozzle-to-plate distance. In many industrial settings, the energetic cost of compressed air and the demand for more uniform, high-flow-rate jets motivate alternatives. Coandă-based air amplifiers use a small primary compressed-air flow to entrain ambient air, increasing total mass flow while potentially improving jet uniformity. Their performance depends on internal geometry and operating pressure. In the present work, air amplifiers are experimentally investigated to (i) determine local and average convective heat transfer coefficients over a heated thin foil, employed as a distributed heat-flux and temperature sensor in conjunction with Infrared Thermography (IRT), and (ii) compare their cooling performance with a conventional steady jet under matched reference conditions.
The devices under study (Meech A15005 and A15008) are annular anodized aluminium air amplifiers in which a primary flow is accelerated through an adjustable slit, generating a low-pressure region that entrains ambient air. The slit setting controls the primary mass flow rate and the entrainment ratio. The amplified jet impinges normally on a thin constantan foil, electrically heated by Joule effect. The nozzle-to-foil distance is adjusted through a micrometric positioning system. Surface temperature fields are measured by IRT, and convective heat transfer coefficients are obtained from an energy balance accounting for radiative and conductive losses.
Results are presented as time-averaged Nusselt maps. A high-Nusselt region appears in the central impingement zone and decreases toward the periphery. At small H/D the field is more localized and slightly non-axisymmetric, whereas larger H/D produces a smoother and more axisymmetric distribution.
Results are interpreted in terms of pneumatic efficiency by relating average heat transfer rates to primary compressed-air consumption to identify favorable operating envelopes, defined by supply pressure and nozzle-to-foil distance, where air amplification can offer a better balance between cooling effectiveness and energy expenditure compared with conventional jet impingement.
References
H. Martin, Heat and mass transfer between impinging gas jets and solid surfaces, Advances in Heat Transfer, vol. 13, pp. 1–60, 1977.
D. Babic, D. B. Murray, A. A. Torrance, Mist jet cooling of grinding processes, Int. J. Mach. Tools Manufact., vol. 45, pp. 1171–1177, 2005.
Y. Cheng, A. A. O. Tay, X. Hong, An experimental study of liquid jet impingement cooling of electronic components with and without boiling, Proceedings of the International Symposium on Electronic Materials and Packaging, IEEE, pp. 369–375, 2001.
