Porous Absorber Research Articles

Prediction and uncertainty quantification of the diffuse sound absorption of finite absorbers

This contribution presents a numerical approach to predict diffuse sound absorption. Conventionally, this is done by considering an infinite absorber coupled to an acoustic halfspace. However, the diffuse absorption coefficient for a finite absorber can be quite different from the corresponding infinite-size value due to what is referred to in literature as the size or edge effect. A finite size correction has been developed previously, but it is only applicable to homogeneous absorbers, neglects boundary conditions, and is based on a computationally costly numerical integration. This contribution presents an alternative approach which overcomes these drawbacks. In this approach, a deterministic model of the absorber, e.g., constructed with the finite element or modal transfer matrix method, is coupled to a diffuse model of the room through the diffuse field reciprocity relation. The theoretical uncertainty on this diffuse sound absorption that is inherent in the diffuse field assumption can also be quantified, or more precisely, the variance of the sound absorption that can be theoretically expected across a diverse ensemble of rooms of the same volume. It is demonstrated that the diffuse absorption coefficient corresponds to the average of the absorption coefficient across an ensemble of rooms, implying that the former is of practical use even at low frequencies. The methodology is experimentally validated for porous and membrane absorbers.

The oil absorption capacity of polypropylene porous absorbents fabricated through iPP/EVA in situ microfibrillar composites: effect of fibrillar morphology

The oil absorption capacity of polypropylene porous absorbents fabricated through iPP/EVA in situ microfibrillar composites: effect of fibrillar morphology

Multiscale porous with coiled-up channel for low-frequency broadband sound absorption

A compact structure comprising the gradient perforated porous material and the coiled-up channel (GPPC) is proposed. The sound absorption performance is investigated by a theoretical model based on the transfer matrix method (TMM) and simulation with the finite element method (FEM), and the simulations are experimentally validated. Broadband absorption of porous material can be shifted to a lower frequency and enhanced by coiled-up channel of GPPC, which are due to the resonances of the meso-pore with coiled-up channel and the coupling effect of multiscale porous material and coiled-up channel, and the energy trapping between the meso-pore and porous material matrix. The effects of the geometry parameters of the meso-pore and coiled-up channel on the absorption properties are also discussed. Moreover, the absorption enhancement of GPPC relative to homogeneous porous relies on a proper porous material matrix. Finally, the GPPCs with a width and thickness of 70 × 100 mm is designed by parallel coupling, and the absorption coefficient exceeds 0.7 at [240, 3000] Hz, which is promising in engineering noise control and provides some preliminary understanding of the combination of the gradient perforated porous absorber and the coiled resonator.

Three-dimensional porous aerogel-bead absorbent with high dispersibility of lanthanum active sites to boost phosphorus scavenging

Excessive phosphorus (P) emission from sewage to water bodies is still a global challenge that threatens environmental safety. Here, La-based aerogel beads (LCM) with a lanthanum (La) complex core were prepared via crosslinking to serve as a novel P scavenger. The chelation of the central La active sites with the alginate ligand to form complexes made the LCM beads stable in water. La active sites, as the core of LCM, existed high dispersibility in the surface and interior of the aerogel beads, which made it exhibit high utilization of La active sites and a high P/La molar ratio (P/La: 2.80), which was 2 ∼ 10 times that of other La-loaded adsorbents. Due to the high dispersion of La active sites, LCM achieved a high adsorption capacity of 624.57 mg P/g La. Besides, the LCM beads also achieved a high mass transfer efficiency due to the unique three-dimensional porous structures of the beads. Apart from the excellent P capture performance of the LCM aerogel beads, the physicochemical properties of LCM facilitated their separation from the treatment water and subsequent regeneration, making this a promising adsorbent for P uptake from wastewater.

Comparison of the sound absorption properties of acoustic absorbers made from used copy paper and corrugated board

Due to the increasing demand for eco-friendly, inexpensive sound absorbers, this study investigated composites made from recycled paper pulp and urea-formaldehyde adhesive. By varying the pulp contents, five samples from used copy paper and five samples from used corrugated board were fabricated. For the same type of porous absorber, one with a lower bulk density has a higher total porosity, resulting in a higher sound absorption coefficient (SAC) spectrum. Sound-absorbing performance of copy paper composites with bulk densities below 442.4 kg/m3 and corrugated board composites with densities of less than 474.8 kg/m3 can be alternatives to commercial polyurethane foam of the same thickness. The noise reduction coefficient (NRC), as well as the average SAC of all corrugated board composites at medium (αM) and high (αH) frequencies were greater than those of copy paper composites. However, the average SAC at low frequency (αL) was not explicitly different for copy paper and corrugated board composites. In conclusion, corrugated board composites are a better candidate as sound absorbers than copy paper composites with the same pulp content.

Electrospun Biodegradable Poly(L-lactic acid) Nanofiber Membranes as Highly Porous Oil Sorbent Nanomaterials.

Crude oil spills seriously harm the ocean environment and endanger the health of various animals and plants. In the present study, a totally biodegradable polymer, poly(L-lactic acid) (PLLA), was employed to fabricate highly porous oil absorbent nanofibrous materials by using a combination of electrospinning technique and subsequent acetone treatment. We systematically investigated how the electrospinning parameters affected formation of the porous structure of PLLA nanofibers and demonstrated that PLLA nanofibers with decreased and uniform diameter and improved porosity could be rapidly prepared by adjusting solution parameters and spinning parameters. We also demonstrated that the acetone treatment could obviously enhance the pore diameter and specific surface area of as-optimized electrospun PLLA nanofibers. The acetone treatment could also improve the hydrophobic property of as-treated PLLA nanofiber membranes. All these led to a significant increase in oil absorption performance. Through our research, it was found that the oil absorption of PLLA nanofiber membrane increased by more than double after being treated with acetone and the oil retention rate was also improved slightly.

Control of thermoacoustic instability of liquid-spray flame with porous sound absorber

Control of thermoacoustic instability of liquid-spray flame with porous sound absorber

Cellular-like sericin-derived carbon decorated reduced graphene oxide for tunable microwave absorption

Sodium hydroxide (NaOH) activation on biomass-derived carbon materials has been a developed strategy to construct the porous and lightweight microwave absorber. However, the resulting random porous structures still remain a formidable challenge to optimize the impedance matching and microwave absorption performance. Here, a sericin-guided NaOH activation and subsequent carbonization process were proposed to synthesize uniform cellular-like sericin-derived carbon decorated reduced graphene oxide (SC/rGO) nanohybrids. The NaOH was used as a bi-functional activator to regulate the self-assembly of sericin protein between graphene oxide lamellas, while the sericin-guided NaOH further in situ activates the nanohybrids to produce uniform cellular nanostructures by carbonization. The abundant natural doped nitrogen and oxygen atoms derived from the industry wastes sericin are in-situ maintained after carbonization, which act as polarization centers to promote the attenuation of microwave energy. Benefiting from these merits, the synthesized cellular-like SC/rGO nanohybrids exhibit excellent impedance matching performance, promoted multiple electromagnetic transmission, excellent relaxation polarization phenomenon and microwave absorption properties. The optimized microwave absorption of SC/rGO40-500 reached −53.68 dB at 15.30 GHz, and the effective absorption bandwidth achieved 4.4 GHz (13.35–17.75 GHz) at the thickness of 1.51 mm with a low surface density of 0.035 g/cm2. Our work demonstrates a novel porous structure design strategy on biomass-derived carbon materials as high-efficiency microwave absorbers.

Modeling the acoustic behavior of stepwise gradient porous structures

Spatial property gradients can significantly enhance the noise reduction potential of porous structures. However, predicting the acoustic behavior of structures with gradient microstructures continues to remain a challenge. For porous structures with controlled periodic microstructures, researchers have recently demonstrated the use of a multiscale asymptotic method to extract the acoustic transport properties necessary for use in semi-empirical predictive models. Here, we propose the adaptation of this method to enable the modeling of porous absorbers with stepwise property gradients. The unit cell of the chosen microstructure is modeled in COMSOL Multiphysics and the acoustic transport properties are extracted using the multiscale method. The extracted properties are then used to formulate the transfer matrix of each unit cell. The global sound absorption and transmission loss behavior of the stepwise gradient structures are further predicted by combining the appropriate local transfer matrices. Our results show that the integration of the unit cell and transfer matrix methods provides a robust way of predicting the acoustic behavior of stepwise gradient porous structures with various combinations of layer thicknesses and geometries. The method provides a computationally efficient method to model such structures and can further the development of porous structures with application-specific acoustical properties.

Performance enhancement of a solar still using magnetic powder as an energy storage medium‐exergy and environmental analysis

AbstractThe use of different energy storage materials can have a high effect on the water productivity of solar desalination. This study evaluates the impact of magnetic powders on modified solar still (MPSS) performance and compares the results with conventional solar still (CSS). Black iron oxide magnetic powder was selected to increase solar radiation absorption. The black iron magnetic powder simultaneously acts as a thermal storage material and a porous absorber medium. The thermal energy stored in the magnetic powder improved the performance of the MPSS during peak solar irradiation hours, resulting in higher productivity. The results showed that the thermal performance of MPSS was higher than CSS. The MPSS exhibited a 39.8% higher evaporative and 14.5% higher convective heat transfer rate compared with CSS. Results showed that the cumulative water productivity of MPSS was 31.2% greater than CSS. Also, the energy and exergy efficiencies of MPSS were improved by 18.9% and 19.04%, respectively, compared with CSS. Moreover, the predicted payback period in MPSS and CSS were 3.2 and 4.3 months. Additionally, the CO2 reduction of MPSS was improved by approximately 45.53% compared with that of CSS. The results also showed that the exergoeconomic parameter of MPSS and CSS with energy base was equal to 33.1 and 24.56 kWh/$, respectively.

Perfect Broadband Sound Absorption on a Graphene-Decorated Porous System with Dual-3D Structures.

Noise is a threat to human life quality and an invisible killer that causes many chronic diseases. The vast majority of porous sound absorbers are single-phased, which limits their sound absorption potential. Herein, a graphene-decorated porous system (GDPS) prepared using the immersion-hydrothermal-freezing self-assembly method is reported as an efficient sound absorber based on its unique consecutive double three-dimensional (dual-3D) structure. Due to the increased tortuosity and other gain effects, the novel structure can achieve the perfect broadband absorption at a wide bandwidth in which the sound absorption coefficient exceeds 0.9 easily. Within the effective thicknesses, the widest perfect absorption bandwidth of 814-6400 Hz is realized. Moreover, a higher graphene concentration and the addition of a polymer are found to be able to decrease the absorption frequency to the lowest values of 1979 and 1544 Hz, respectively. The design of a unique dual-3D structure opens up new strategies and applications to use graphene aerogels in noise and vibration applications.

Role of bioconvection, porous medium, and activationenergy on the dynamic of Sisko nanofluid: the case of anenlarging cylinder

The current problem is attributed to various features of Sisko nanofluids flow over-stretching cylinder along bioconvection of motile microorganisms in the presence of activation energy and non-Fourier thermal diffusion. The appropriate methodology is adopted to transform the governing boundary layer equations of fluid flow into the dimensionless nonlinear ODEs. The transformed coupled nonlinear differential equations are resolved numerically with MATLAB software with Bvp4c (Lobatto-IIIa) solver. The effect of dissimilar parameters viz., the Sisko material parameter, porosity parameter, bioconvection Lewis number, thermophoresis parameter, and bioconvection parameter on the velocity, temperature, concentration, and micro-organisms distribution are presented in numeric and graphics modes. The porous drag force of the Darcian linear system is used to evaluate the porosity parameter. It is also noticed that the numerical values of the porosity parameter reduced the velocity while it is enhancing the temperature. This research of wall cooling and heating bears is indispensable applications in solar porous water absorber systems, chemical engineering, space technology, metallurgy, substantial processing, and so forth.

Surface Wettability of Cellulose Sponges on Effective Oil Uptake.

Designing absorbents having specific wettability toward both oil and water is the key for selective and effective oil absorption and removal. For this purpose, establishing explicit correlations between surface tension of oils and surface wettability of absorbent is crucial. In this study, we modified common low-cost cellulose sponges with various organosilanes to achieve a range of hydrophobicity/oleophilicity and then assessed their oil uptake selectivity and capability. Oil uptake was followed as mass uptake versus time and analyzed based on the spreading coefficient (S) of a liquid over a solid surface. The results showed that sponges needed to be hydrophobic, not necessarily superhydrophobic, to selectively absorb oil from an oil/water mixture. To achieve a fast uptake and a high uptake capacity, an S ≥ 0 was necessary, that is, when the sponges were completely wet by the oil. Increasing the porosity of cellulose sponge led to a slight increase in oil uptake capacity, and a greater increase resulted when bacterial cellulose sponges that consisted of smaller and more uniform voids/pores were used. S ≥ 0 could be used as a criterion for evaluating effective and rapid oil uptake for porous absorbents, especially for those containing heterogeneous pore structures, such as common cellulose sponges.

Measuring and modeling the effect of density and pile height on sound absorption of double base Persian rug

The purpose of this study is to investigate the role of density and pile height on sound absorption coefficient in Double Base Persian (DBP) rug and possibility of prediction the acoustic behavior of DBP rug using the mathematical model. For this aim, in the first step, three double base rug samples were produced at different base densities (2, 4, and 6 warp yarn/cm) and the sound absorption coefficient of samples was measured with Impedance tube in two thicknesses (15 & 13 mm) to study the pile height effect. Moreover, the sound absorption of the double base zone was also measured by shaving off the pile from the double base rug samples. Three rug samples at different base densities were produced with very thin warp and weft yarns to avoid the base effect in this sample. Besides, the macroscopic empirical model (Johnson–Champoux–Allard (JCA)) was implemented on obtained data. The results showed that the sound absorption of the double base rug samples increases with increasing the pile height and base density. The role of the base zone in the sound absorption of the rug is bolder than the pile zone. What leads to improve the rug sound absorption by increasing density is increasing the sound absorption of base zone and the pile density changes do not play a major role in increasing the rug sound absorption. In addition, by assuming DBP rugs as a two-layer porous (pile + base zone) absorber, JCA model shows a good consistency with experimental data.

Ternary assembled MOF-derived composite: Anisotropic epitaxial growth and microwave absorption

In recent years, metal-organic framework (MOF)-derived absorbers have attracted much attention in the field of microwave absorption. It is interesting and difficult to construct multiple MOF-on-MOF heterostructures . Herein, we combine two unique anisotropic epitaxial growth strategies to build binary assemblies DUT-52@MIL-88B (DM) and DUT-52@MIL-88C (DMC), and then the ternary assembly DUT-52@MIL-88B@MIL-88C (DMM). The formation mechanisms of the assembled structure are clarified. Besides, they are converted into magnetic porous carbon-based absorbers (DM-700 and DMM-700) through a facile carbonization process . The microwave absorption mechanisms are elucidated. The results indicate that the exquisite multi-element composite structure endows the absorber with suitable impedance matching characteristics, complementary loss mechanisms and excellent electromagnetic loss capabilities. It is worth noting that DM-700 exhibits a remarkable minimum reflection loss (RL min ) of −67.5 dB under a matching thickness of 3.6 mm, corresponding to an effective absorption bandwidth (EAB, RL < −10 dB) of 2.0 GHz and the filler loading of 55%. The optimized absorber DMM-700 successfully achieves the optimization of the above-mentioned microwave absorbing properties. While maintaining the desirable RL min (−56.4 dB), the matching thickness is reduced (2.4 mm), and at the same time, EAB is doubled (4.0 GHz). This study might provide a new idea for the construction of sophisticated absorbers with multi-MOF-on-MOF structures. • The DUT-52@MIL-88B@MIL-88C composite is first synthesized. • The tip-to-end anisotropic MOF-on-MOF growth possess is proposed. • The microwave absorption mechanisms are elucidated. • The RL min exceeds −56.4 dB for the thickness of 2.4 mm.

Analysis of the sound field above finite absorbers in the wave-number domain.

This study examines the edge diffraction effect when a sound wave impinges and reflects off finite porous absorbers, flush-mounted in an infinite hard baffle. A theoretical analysis of the diffraction is given by taking a two-dimensional spatial Fourier transform of a plane wave impinging on a finite absorber. Numerical experiments are also presented to simulate the sound field above infinite and finite locally reactive absorbers and the measurement with an array of pressure sensors. In such cases, a regularized solution is used to separate the incident and reflected plane wave components, in the wave-number domain, including both propagating and evanescent waves. The properties of the wave-number spectrum are associated either with the specular reflection or with the diffracted components, caused by the interaction of the sound wave with the finite absorber. From the regularized solution, it is possible to reconstruct the surface impedance and the absorption coefficient of the sample. The influence of Gaussian noise on such measurements is also investigated. The use of propagating and evanescent waves on the sound field model led to an estimation of the absorption coefficient that depends just slightly on the size of the sample, which is a desired feature for in situ measurement methods.

Highly Efficient I2 Sorption, CO2 Capture, and Catalytic Conversion by Introducing Nitrogen Donor Sites in a Microporous Co(II)-Based Metal-Organic Framework.

Recently, the development of porous absorbents for efficient CO2 and I2 capture has attracted considerable attention because of severe global climate change and environmental issues with the nuclear energy. Hence, a unique porous metal-organic framework (MOF), {[Co(L)]·DMF·2H2O}n (1, DMF = N,N-dimethylformamide) with uncoordinated N atoms was rationally constructed via using a heterofunctional 4,6-bis(4'-carboxyphenyl)pyrimidine (H2L) linker. Interestingly, 1 exhibits exceptional properties for I2 sorption, CO2 capture, and catalytic conversion. Particularly, I2 can be efficiently removed in both vapor and solution forms, and the adsorption amount can reach 676.25 and 345.28 mg g-1, respectively. Furthermore, complex 1 displays high adsorption capacity for CO2 (53.78 cm3 g-1, 273 K). Consequently, 1 is expected to be a promising and practical material for environmental purification due to its excellent adsorption properties.

Heat Transfer Enhancement Using Micro Porous Structured Surfaces

The parabolic trough solar collector as a popular technique is widely used in solar concentrating technologies (SCTs). The solar absorber tube is the key position of the trough solar thermal power system. The internal modification of the absorber tube is one of the most interesting techniques for increasing the collector’s performance. At present, most of the methods to enhance heat transfer efficiency focus on designing alternative parabolic trough collectors (PTC) absorbers and improving the internal structure of absorption tubes. Due to the limitation of temperature range, most absorption tubes use oil as heat absorbing liquid, and very few heat absorbing tubes directly use water as working fluid. This is because water is limited by critical heat flux in high temperature environment, resulting in low heat transfer performance. In this work, we designed a new porous absorber tube with the function of allowing liquid resupply and vapor overflow from different paths, which can effectively improve the critical heat flux of the absorber tube when using distilled water as working fluid. In order to obtain better heat transfer performance of the absorber and verify the feasibility of vapor–liquid separation mechanism, a simplified model of the absorber was carried out in pool boiling. In this work, we fabricated an arterial porous structure with the function of regulating vapor–liquid flow path based on vacuum sintering technique, and the effect of different heating methods on boiling heat transfer performance are analyzed. The maximum heat flux of 450 W/cm2 was achieved without any dry-out at the superheat of 42 °C, and the unique evaporation/boiling curve was obtained.

Prediction of sound absorption performance of fibers coming from cigarette butts using a phenomenological model

This work deals with experimental and theoretical research about cellulose acetate fibers coming from cigarette butts. This waste, considered one of the most hazardous, numerous, and polluting waste all over the world, represents a high environmental risk. The possibility of using these fibers in sound-absorbing panels is evaluated. The acoustical properties are measured according to ISO 10534-2, and predicted using a phenomenological model over the frequency range from 100 Hz to 6400 Hz. This work includes the study of important non-acoustical properties for a better understanding of the porous structure. The Johnson-Champoux-Allard model provides good accuracy. The experimental results of the sound absorption spectrums show mean errors ranging from 1.1% to 9.0%. Moreover, it is possible to obtain non-acoustic properties of cellulose acetate fibers, which are difficult to obtain experimentally, using the Johnson-Champoux-Allard model and an inverse technique. The input parameters of this model are successfully obtained with low errors concerning the measurements (0.6–5.2% for porosity or 2.6–10.5% for flow resistivity). The results of the sound absorption spectrum at normal incidence of the cellulose acetate show very accurate predictions. The experimental results show values of sound-absorption coefficients close to 1.0 and sound absorption average ranging from 0.42 to 0.8. Therefore, these waste fibers are an alternative to the traditional porous absorbers, mainly composed of synthetic fibers or foams and petroleum-based resins.

Optical analysis of a semi-transparent packed bed of spheres for next-generation volumetric solar receivers

Volumetric solar absorbers are attractive for high-temperature concentrating solar applications because they—theoretically—have lower re-radiation losses than surface absorbers since concentrated radiation penetrates their volume. In practice, however, the incident radiation is predominantly absorbed in the top/frontal surfaces of porous and distributed absorber materials. Another challenge with high-temperature absorbers, generally, is the risk of creating hot spots resulting from a non-uniform temperature distribution. To overcome these limitations, this study reports on a semi-transparent absorber design, enabling the peak temperature to occur within the internal volume. Eight different ‘constant-transmittance’ and ‘variable-transmittance’ combinations of transparent/semi-transparent/opaque spheres were packed into a cylindrical cavity and exposed to a solar simulator for a layer-by-layer transmission test. A 3-D ray-tracing model was also developed to further elucidate the relative optical absorption of the cavity walls and spheres. An optimal variable-transmittance design enabled ∼40% of the rays to penetrate to >70% of the absorber's depth, and ∼56% of the total rays were absorbed within the inner volume. Therefore, the optimal design yields uniform absorption, potentially leading to a uniform temperature distribution and a minimisation of thermal emission losses. This study provides the first experimental proof that a semi-transparent packed bed can significantly outperform an opaque design.