Experimental investigation on local heat transfer analysis of heated surface embedded with metal foam using circular orifice air-jet impingement☆

Jet impingement heat transfer is an effective localized cooling technique used in various engineering applications, including hot steel plates, gas turbine cascades, paper mills, metals, glass manufacturing, and electronic component cooling. Its simple design, low pressure loss, homogeneous distribution, short routes, and high efficacy make it widely used in industrial operations, including aircraft cooling, electronic component cooling, and textile manufacturing [[1], [2], [3]]. Researchers found that the thermal efficiency of impinging jets is influenced by factors such as Reynolds number, impinging distance, nozzle shape, jet confinement, turbulence intensity, and test specimen orientation [[2], [3], [4], [5], [6], [7], [8]]. Research on jet impingement tests on flat surfaces has mainly focused on circular orifices and nozzles, but limited studies have explored the use of metal foam as an additional method to enhance heat transfer.

Jet impingement cooling is a popular technique for achieving higher localized heat fluxes in thermal systems due to its flexible design, lower pressure drop, and shorter flow path. A fast-moving fluid, like air or liquid, impinges on a heated surface to effectively remove heat, creating a thinner boundary layer and faster heat dissipation from the surface. Different cooling fluids like air, water, air–water mixture, aqueous surfactant solutions, and nanofluids are used for industrial purposes. Heat transfer rate in impinging jets is influenced by parameters like jet Reynolds number, nozzle geometry, impinging distance, confinement, turbulence intensity, and test specimen orientation [4,6,7,9]. Numerous studies have examined the heat transfer and fluid flow characteristics of jet impingement, including tests conducted by Martin [1], Viskanta [3], Jambunathan et al. [7], and Livingood and Hrycak [10], as well as the fluid flow and heat transfer characteristics of unconfined axially symmetrical air jets.

Lytle and Webb [4] used an infrared camera to study heat transfer from a smooth plate to a round nozzle at a shorter influencing range. Lee et al. [11] found that heat transfer in stagnation regions improved with increased nozzle diameter. Other researchers studied the effect of nozzle geometry on jet impingement performance using experimental and numerical techniques. Kim and Park [12] found that non-circular turbulent jets had more dominant secondary flow than axisymmetric jets. Katti and Prabhu [6] examined factors like Reynolds number and non-dimensional impinging distance on circular jet impingement efficiency. Previous research mainly focused on experimental experiments, but most studies considered axisymmetric geometry and impingement on a smooth plate.

Researchers have utilized various techniques to improve heat transmission from flat surfaces during jet impingement, including swirl generators, longitudinal swirling strips, turbulence boosters, jet intermittency, mechanical tabs, and surface roughness [[13], [14], [15], [16]]. Air convective cooling is widely used for electronic components due to its simple design, with open-cell metal foams (OCMFs) being considered as a potential candidate for improved cooling performance. Metal foams, with desirable attributes like high surface area density, lightweight, rigidity, high porosity, customized thermal conductivity, and lower manufacturing costs, are being explored for thermal management applications [[17], [18], [19], [20], [21]]. Metal foam, characterized by porosity, PPI, size, and design, is preferred due to its higher surface area and enhanced mixing and turbulence, resulting in higher heat transfer [[22], [23], [24]]. Researchers have studied the integration of open-cell metal foams (OCMF) with plates during jet impingement cooling to improve heat transfer, analyzing parameters like foam thickness, porosity, pore density, and thermal conductivity. Calmidi and Mahajan [25] proposed an analytical model for estimating effective thermal conductivity of high-porosity fibrous metal. Bhattacharya et al. [26] studied the thermal performance of finned porous metal substrates for electronic device thermal management. They found that heat transfer performance in metal foam heat sinks is 1.5–2 times higher than conventional heat sinks. Jeng et al. [27] analyzed convective heat transfer and pressure drop characteristics of aluminium porous blocks, finding that the average Nusselt number increases with foam thickness and pore size. Paek et al. [28] study on porous material conductivity found that effective thermal conductivity increases with foam density and decreases with pore size. Shih et al. [29] suggested using an airflow-limiting mask and metal foam to direct cooling air towards a heated surface, improving cooling efficiency. They studied the effects of porosity, pore density, and restricted fluid flow on heat sink thermal performance. Wang et al. [30] found finned copper foam better than conventional sinks, and Yogi et al. [31] found lower heat transfer performance in aluminum OCMF with 40 PPI porous media.

Research suggests that open-cell metal foam can enhance heat dissipation by increasing contact space, improving interaction, and reducing pressure drop [[32], [33], [34]]. This is primarily due to its numerous advantages, including interconnected cavities providing ample contact area, enhanced amalgamation, and increased dispersion. Earlier studies involving air-jet impingement cooling using metal foam primarily analysed average thermal characteristics using circular, elliptical and other non axi-symmetric jets. The present study analysed both average and local heat transfer characteristics to effectively quantify the enhancement in the cooling rate in comparison to the heated surface without foam. Foam is an effective passive flow control method, offering substantial performance enhancements at an affordable cost in various applications [35,36]. Depending on the porosity and pore density, foam can either retard or accelerate the flow [37]. However, in the present study, foam distributes the incoming air jet into several micro-jets when they pass through porous material. These microjets then collide with the hot plate, hence increasing the coefficient of heat transfer.

Attached to the heating surface, the OCMF acts as a fin and increases the surface area per unit volume. Conduction transports heat from the plate to the foam's upper surface. The turbulence created due to the micro air-jets developed by the metal foam increases fluid mixing, distrupts, and breaks the thermal boundary layer. Circular jet impingement cooling effectively reduces the thermal boundary layer through increased turbulence and enhanced mixing, resulting in improved cooling efficiency. Similar mechanisms of heat transfer through metal foam have been reported by the earlier researchers [32,33,38,39].

Research on air-jet impingement using open-cell metal foam is limited to low Reynolds numbers and lacks information on foam area optimization's impact on heat transfer characteristics. Studies on the optimization of cooling properties of foam-embedded surfaces with circular orifices are limited. The literature has not explored the local and average thermal behaviour of circular orifices at higher Reynolds numbers for different foam sections under air-jet impingement. The present study examines the thermal behaviour of different foam sections on a heated plate with an axisymmetric orifice under air jet impingement.

The present analysis aims to estimate the thermal characteristics of a foamed surface and compare its performance with a plain surface without foam during jet impingement. The parameters under consideration include the foam thickness, coolant flow rate (Reynolds number Re), nozzle-to-plate distance (z/d), and spatial location away from the stagnation point. The experiments also examine the localized Nusselt number variation in the longitudinal (x/d) and transverse (y/d) directions. The infrared thermography technique is employed to monitor temperature, which is then used to evaluate thermal performance parameters. The experimental investigation aims to achieve the following objectives:

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  Examine the influence of a circular orifice on the thermal attributes of a copper foam embedded flat surface using air-jet impingement.
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  Study the local, average and stagnation heat transfer characteristics of metal foamed surface across the longitudinal and transverse directions.
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  Investigate the effects of varying impinging distance and Reynolds number on the thermal characteristics of a metal foamed surface.
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  Optimization of the foam area involved in the cooling of foam-embedded heated surface.