Evaporative coolers used in industrial or outdoor spaces have air flowing through the wetted porous material. At the air-water interface, water evaporates cooling the airflow over it. Usually, the air is driven by fans or blowers which also generate noise. The cooling pads used in evaporative cooling devices are sheets of porous material wetted with water. The most commonly used ones are of fibrous type with natural materials. This paper discusses noise reduction in an evaporative cooling application utilizing the same porous medium which acts as an interface for evaporative cooling. The noise reduction problem has various factors for consideration. Simplified plane wave incidence is assumed for this study from the porous material perspective of the cooling pads. Air is driven against flow resistance offered by the porous material. Both, microscopic properties and macro-arrangement of porous material affect the resulting pressure drop. Corrugated macro-geometries similar to those used in noise-reducing heat sinks developed recently are studied. Further, for evaporative cooling purposes, the acoustic performance of commonly used porous materials under wetted conditions needs to be studied.

In compact air convection cooling applications, it is desirable to satisfy noise reduction and heat dissipation within same functional space. Designing a solution for such application need to consider thermal, acoustic and flow aspects. The corrugated arrangement of porous material is utilized for the combined thermal-acoustic function, as it is known to offer a lesser mean-flow pressure drop than other geometries such as block shape, wedge shape. It consists of air channels separated by porous walls which conduct the heat away from base of the heat sink. The porous walls are considered to be consisting of a periodic foam material. The effect of higher-than-ambient temperatures on the acoustic performance of the corrugated arrangement is observed experimentally using a custom-built test setup. Two designed, additively manufactured corrugated porous heat sink samples with octet truss periodic foam are tested within the heat sink operating conditions observed typically in elec-tronics cooling applications (< 70 degrees C). A coupled thermal-acoustic model is presented to predict and study the performance of the corrugated porous heat sinks for the combined application. The relevant performance pa-rameters are pressure drop, overall thermal resistance and transmission loss (TL). Using this model, a simple parametric design study is enabled with variations in porosity and different macro-geometric parameters of the corrugated arrangement.

For applications of porous materials where mean pressure drop is a concern, packaging the material into a corrugated structure is better compared to other geometries such as block or wedge shapes. The goal of this study is to integrate noise reduction functionality within that material, which requires an understanding of sound propagation through corrugated porous structures, including flow effects. The corrugated porous structure involves porous partitions separating inlet and outlet fluid channels. The porous materials considered are periodic octet-truss and body-centered cubic unit cells, and sound propagation across these porous partitions is modeled using the Johnson-Champoux-Allard model. The predicted transmission loss (TL) is benchmarked using designed additively manufactured corrugated structures measured using a flow duct. The laminar flow regime is maintained across the porous structure to reduce flow-noise effects. It is shown that the TL for a given corrugated structure increases with a decrease in porosity, and the impact of flow becomes significant as the porosity decreases. The influence of flow on TL also depends on the unit cell configuration. Furthermore, the model provides insights into pressure and acoustic particle velocity distributions within the corrugated structure and reveals regions of the porous material that effectively participate in noise reduction.

In convective cooling heat sinks, fans are used to create the flow, which generates noise. Using a separate device for noise reduction would increase the system's overall size. Combining noise-reducing functionality with a heat sink is desired in many space-constrained applications. Fan noise typically contains both tonal and broadband frequency components. Metal foam heat sinks can be effectively used for broadband noise reduction, especially at mid and high frequencies, and heat dissipation in the same volume. Corrugated metal foam heat sinks or porous finned heat sinks show better heat transfer performance and better noise reduction at lower pressure drops than the block foam geometry. Helmholtz resonator is a classical acoustic device that can address the low-frequency tonal noise centered at the natural frequency of the Helmholtz resonator. This study involves the experimental investigation of corrugated periodic foam heat sink geometry for thermal and acoustic performance. Also, to address tonal low-frequency fan noise, the acoustic performance of the Helmholtz resonator combined with a corrugated periodic foam heat sink is experimentally studied.

In convective air-cooled heat sink applications with space constraints, corrugated geometries can be used as in-duct sound absorbing structures offering lower duct-flow resistance than other geometries such as block-shape, wedge-shape geometries. Sound wave propagation through this geometry is presented using a simple 1-D acoustic model. Using the model, acoustic performance of corrugated sample is evaluated in terms of its transmission loss in dB. Thermal resistance and pressure drop values are also reported and compared with acoustic performance as function of number of corrugations and length of corrugated sample. A rectangular corrugated geometry has alternate inlet and outlet channels separated by porous walls. Sound propagation across this arrangement is modelled by extending prior model from literature with similar geometries. Prior model by Allam and Åbom (2005) is highly symmetric about the channels and porous walls are modelled by simple steady flow resistance equation. In current work, appropriate considerations are taken into account for the configuration of corrugated geometries suitable to general heat sink applications and sound wave propagation through porous walls is predicted by using Johnson-Champoux-Allard (jca) model. The porous walls at ends of the geometry are modelled as in acoustically series-parallel network combinations. Further, effect of heat sink temperature on sound wave propagation is also explored using the model.

In many high-performance compact air-cooling technologies, it is desirable to adopt materials that provide combined heat dissipation with noise reduction (NR) in the same device. A device that adopts such material is termed here as a “noise-reducing heat sink” (NRHS). NRHS design would involve fundamental thermal-acoustic performance tradeoffs, thus necessitating investigation of their combined thermal-acoustic performance. In this article, the thermal-acoustic performance of NRHS is evaluated by determining their thermal-acoustic index (TAI) as a function of airflow rates (Mach number < 0.1). This index is determined by combining thermal resistance as the basis for thermal performance characterization and NR improvement to characterize the acoustic performance. Conventional parallel-plate heat sinks/slit-based geometries, stochastic metal foams, and a designed periodic foam structure are compared as NRHS candidates. The results show that, for the same material volume fraction, stochastic metal foam heat sinks have better combined thermal-acoustic performance compared with a conventional parallel-plate heat sink. A designed periodic foam NRHS is shown to have significantly improved TAI, thus leading the way for NRHS design in compact cooling technologies. For the best combined thermal and acoustic performance, laminar flow is identified as a suitable operational flow regime.

Reticulated foams are widely used in noise control applications. However, difficulties in controlling the foaming process makes it difficult to control the microstructure and consequently the absorption characteristics of stochastic foams. Present work is a step toward designing an appropriate microstructure of foam to obtain desired acoustics performance using the concept of periodic foams. Periodic foam structures are created from designed unit cells by uniform spatial repetition of the unit cell in the three-dimensional domain. Three different unit cell configurations are created, characterized and compared using the Johnson-Champoux and Allard (JCA) model. These three unit cell configurations are further used to create periodic foam structures and prototypes of those structures are fabricated using additive manufacturing techniques. The measured characteristic impedance, complex wave propagation constant, absorption coefficient, and transmission loss for the fabricated samples are in close agreement with the JCA model predictions. Designed periodic foam structures proposed in this study offer a novel way to tailor the absorption spectrum to specifications in noise control and other acoustics applications.