Biporous Structures Research Articles

Pore-scale simulation of two-phase flow in biporous media

Enhancing both permeability and capillary pumping in porous structures has emerged as a key focus for researchers, leading to the development of biporous media. While experimental studies on these structures have been conducted recently, there is a lack of numerical simulations due to difficulties in describing the geometry. To address this gap, the present study explores pore-scale numerical simulation of two-phase capillary flow in biporous media. A new simplified biporous structure is proposed, featuring a staggered arrangement of clusters, with each cluster composed of closely packed solid particles. For comparison, a monoporous media case is contrasted and represented using a conventional staggered arrangement of solid particles. Both passive and active capillary flow modes are considered in the present study. The numerical results align well with previous experimental findings on biporous media, indicating that the proposed biporous geometry effectively models two-phase flow in complex structures at a reasonable computational cost. The results show that capillary effects in biporous media are up to two times more effective than in monoporous structures. Simultaneously, permeability is enhanced by a factor of four in biporous media under similar circumstances, with most of the mass flow rate (more than 95%) passing through the larger pores between clusters. This combined impact of increased capillary action and higher permeability leads to enhanced wicking performance in biporous structures. The outcomes can help to understand two-phase flow physics in the biporous structure and develop reliable models for the simulation of biporous media on a macroscopic scale. Numerical modeling and comprehension of capillary structures play a crucial role in designing optimized geometries to enhance their performance.

Two methods for thermal conduction analysis of bi-porous sintered structures

Loop heat pipes (LHPs) have consistently garnered attention for effectively managing thermal conditions in critical electronic components of spacecraft and satellites. Particularly intriguing are LHPs equipped with bi-porous capillary wicks, demonstrating commendable flow and thermal dissipation capabilities. Despite its significance for evaluating the phase-transition heat transfer in porous structures, the effective thermal conductivity (ETC) has not been understood mechanistically. This study aims to establish an efficient procedure for predicting ETC in bi-porous wicks through a comparative analysis of analytical and numerical methods. The analytical ETC expression was formulated using thermal resistance network and neck formation theory. Meanwhile, the numerical model utilized the Finite volume method (FVM) combined with a 3D porous structure created by Diffusion-Limited Aggregation (DLA) algorithm. The investigation revealed a congruence between two methods and the measured ETCs of interstitial-pore samples containing fine nickel powders. However, undesirable results were shown for coarse samples due to the irregular particle shapes. In samples with formation pores, the numerical model excels in characterizing the spatial distribution and scale of pores, albeit at the expense of time consumption and model complexity compared to the analytical one. Both methods prove valuable for ETC modeling of sintered bi-porous structures according to desired applications.

Trace Quantity Detection of H2PO4– by Fluorescent Metal–Organic Framework (F-MOF) and Bioimaging Study

Drinking water quality management and sustainable environment to water bodies are a major concern to Public Health Engineering departments. Inorganic Phosphate (Pi), one of the major inorganic pollutants, is addressed for the last two decades regarding trace quantity detection. Over the past few years, a large number of fluorescent metal-organic frameworks (F-MOFs) have been studied to explore the desirable method for selective water analysis. In this article, a Zn(II) 3D F-MOF, {[Zn2(PCDF-INH)(BDC)2].(H2O)}n (1) (H2BDC = 1,4-Benzenedicarboxylic acid, and PCDF–INH = N, Nʹ-diisonicotinoyl-2-hydroxy-5-methyl-isophthalaldehyde dihydrazone) is used for selective recognition of H2PO4− anion by ‘turn-off’ fluorescent sensing technique. Single-crystal X-ray analysis accounts that the asymmetric unit is constituted of two different Zn(II) centres by PCDF–INH and the bridging group, BDC2− ions are responsible for the generation of helical polymer and the biporous structures (14.807×13.096 A2 and 24.905×24.932 A2). The compound, 1, exhibits strong emission at 505 nm in CH3OH-H2O solution and is 'turn-off' upon the addition of H2PO4−. The selectivity and specificity for H2PO4− have been checked in the presence of fifteen different anions including different phosphates (PO43−, HPO42−, P2O74−). The entire sensing activity of 1 is examined by spectrofluorometric method, 1H–NMR titration, PXRD analysis, bio-imaging study, and the limit of detection is 3.903 μM. However, in vitro bio-sensing analysis confers that F-MOF (1) is sensible toward the H2PO4− ion at a trace level in the human lung fibroblast cells and human liver cancer cell line Hep G2.

Enhanced pool boiling heat transfer on freeze-casted surfaces

Distinct from monoporous structures with only one characteristic pore size, biporous structures are characterized by two distinguished pore sizes. In this paper, novel double-layered biporous coatings consisting of a biporous top layer and a monoporous base layer are fabricated by freeze casting from aqueous nickel slurry, where the particle size (D50) is 400 nm. The biporous layer of the freeze-casted coating is characterized by lamellar vertical large pores formed by templating ice crystals and small pores formed by particle accumulation. The sizes of the large pores are in the order of 10~100 μm depending on coating thicknesses and freezing temperatures, while the sizes of small pores are in the order of ~1 μm. Pool boiling heat transfer performances of freeze-casted double-layered biporous coatings are investigated. For comparison purposes, saturated pool boiling experiments under the same conditions are also carried out on a slurry-evaporation-deposited surface as well as on a smooth surface. Experimental results indicate that freeze-casted coatings enhance boiling heat transfer performance significantly by increasing nucleate site density, decreasing bubble departure diameter, increasing departure frequency, and enhancing surface wickability. Compared to a smooth surface, the inception temperature on a freeze-casted surface is significantly reduced from about 10 K to less than 5 K while the heat transfer coefficient (HTC) and the critical heat flux (CHF) are improved by 180% and 67%, respectively. It is found that there is an optimum coating thickness to maximum HTC due to a tradeoff between nucleation site density and vapor removal efficiency. In addition, liquid slurry frozen at low supercoolings has better boiling heat transfer performance since the pore size is bigger as a result of larger ice crystal size.

Development of nickel bi-porous wicks for miniature loop heat pipe applications

PurposeMiniature loop heat pipes (MLHPs) are highly efficient passive heat transfer devices, which have considerable advantages over conventional heat pipes. Currently, miniature LHPs with ammonia and water as working fluids have been developed and utilized in electronics cooling within temperature range of 50°C-70°C at any orientation in 1-g conditions.Design/methodology/approachThe authors studied the standard procedure for the development of bi-porous nickel wicks and their characterization. Three different shaped nickel powders were studied, and best fitting nickel powder for electronics cooling application was reported. The manufacturing of bi-porous wick structures was analyzed with parameters such as porosity, permeability, capillary pressure and effective thermal conductivity for efficient performance of MLHP.FindingsThe study investigated the sintering process for number of samples to identify effective sample for the particular application. It is found that carbonyl nickel powder (type 287) with particle size of 2.6-3.3 µm gives promising results. Permeability and porosity were found to be highest in this case.Originality/valueIt is found that carbonyl nickel powder type with particle size gives promising results. Permeability and porosity was found to be highest in this case.

Layer-by-layer assembled carbon nanotube-polyethyleneimine coatings inside copper-sintered heat pipes for enhanced thermal performance

Biporous structures at the nano–microscale are promising candidates for controlling phase change heat transfer, through their enhanced capillary wicking and fluid transportation. However, existing methods for fabricating biporous structures involve complex process which is not suitable for small-scale thermal devices such as heat pipes, owing to their confined and non-flat inner structures. Herein, we report the biporous structures inside copper-sintered heat pipes, enabled by layer-by-layer (LbL) assembled multi-walled carbon nanotube (MWCNT)-polyethyleneimine (PEI) coating for enhanced thermal performance. The repetitive filling and removing of the oppositely charged solutions with MWCNT-PEI and carboxylic-functionalized MWCNTs assembled the nanoporous MWCNT-PEI coatings (10, 20, and 40 bilayers) on the microporous copper-sintered inner surfaces. The fiber-like MWCNT networks structurally manipulated morphology and thickness of biporous structures, while the hydrophilic PEI shells chemically optimized wettability. A reduced thermal resistance (∼14.3%) was observed for MWCNT-PEI coating in 10 bilayers, due to the enhanced capillary wicking, interfacial contact areas, and bubble dynamics, whereas the 40 bilayers did not exhibit improved thermal performance owing to the redundant nanoporous layers causing reduced volume of microporous structures and increased thermal resistance. The LbL-assembled MWCNT-PEI coatings would act as functional layers to improve the performance of miniaturized and thin-film-based thermal devices.

Hydroisomerization of n-decane over micro/mesoporous Pt-containing bifunctional catalysts: Effects of the MCM-41 incorporation with Y zeolite

Hydroisomerization of n-decane (n-C10) over the bi-porous Pt-containing catalysts was investigated. Two micro/mesoporous materials were respectively derived from the composition and mechanical mixing of MCM-41 and Y zeolite. This study aimed to estimate the influences of the mesoporous MCM-41 presence on catalytic performances of bifunctional catalysts during the hydroisomerization of long-chain alkanes. Compared to the Pt/Y catalyst, bifunctional catalysts with bi-porous structures were more selective to isomerized products, particularly the mono-branched isomers, which was attributed to the improved metal–acid functional balance and reduced diffusion limitations caused by the incorporation of MCM-41 with Y zeolite. Furthermore, the Pt-loaded Y/MCM-41 composite, characterized with the overgrowth of MCM-41 on Y crystals, outperformed the Pt-loaded Y/MCM-41 mechanical mixture in the regard of limiting secondary reactions. Given this fact, the migration scheme of alkene intermediates over bifunctional catalysts with bi-porous structures was proposed, and meanwhile, the differences of molecular migrations over Pt-loaded Y/MCM-41 mechanical mixture and composite were discussed.

Planar vapor chamber with hybrid evaporator wicks for the thermal management of high-heat-flux and high-power optoelectronic devices

Heat spreaders based on compact vapor chambers offer one attractive approach to the thermal management of high-power electronics. We report our design and experimental characterization of advanced evaporator wicks and thin planar vapor chambers incorporating these wicks. The hybrid wicks combine distributed high-permeability liquid supply structures with thin (monolayer) evaporation layers to achieve both low thermal resistance and high limiting heat fluxes over large heating areas. We model and experimentally characterize the capillary and heat transfer performance of liquid spreading layers consisting of mono-layers of Cu particles and identify a range of optimal particle diameters maximizing their performance. The thin liquid spreading layers are integrated with three different types of liquid supply structures, namely, columnar arteries, converging lateral arteries, and bi-porous structures. The resulting hybrid wicks show comparable heat transfer performances with critical limiting heat fluxes >350W/cm2 over heating areas of 1cm2 and peak heat transfer coefficients >20W/cm2K. These results confirm the effectiveness of our hybrid wick designs and also that evaporation heat transfer is dominated by the liquid spreading layers. A prototype vapor chamber incorporating CTE-tailored envelopes and the hybrid wick is developed for potential applications in the thermal management of laser diode arrays. We demonstrate an evaporator resistance of approximately 0.075K/(W/cm2), while removing over 1500W from a 4cm2 heating area.

Development of a platinum resistance thermometer on the silicon substrate for phase change studies

Resistance temperature detectors are commonly used measurement sensors in heat transfer studies. In many resistance temperature detectors, the platinum resistance thermometer (PRT) is chemically stable, has a wide temperature measurement range and possesses high measurement accuracy. In phase change studies of carbon nanotubes, bi-porous structures for microelectronic thermal management, 100 nm thick PRTs are developed on silicon substrates with 10 nm titanium adhesive to achieve precise and interface-free temperature measurements. After an annealing at 375 °C, the PRT samples are calibrated at a temperature range from 20 to 180 °C. Measurement hysteresis of temperature appears in thermal cycles. Electrical resistance tends to become low during all heating periods, which establishes the maximum measurement deviation of 10 °C. Experimental results from two different thin-film PRTs indicate that accurate and repeatable temperature measurements can be achieved by either reducing heating speed or using data in the cooling period.

Structural analysis of permeable powder materials with random and matrix structures

We have theoretically analyzed permeable powder materials with bidisperse and biporous structures based on a conceptual approach using the concept of the matricity of porous systems. Approximate formulas are obtained for the porosity, specific surface area, equivalent pore radius, monophase and interphase contact area. Using the percolation model, we estimate the matricity of powder materials and demonstrate the feasibility of structural analysis of various powder systems using such an estimate.

Effectiveness of biporous catalysts for zero-order reactions

Supported porous catalyst pellets with bidispersed pore structures are formed by agglomeration of porous particles. The present note is an attempt to extend an analysis of zero-order reaction for monoporous catalysts (which has been presented by Weekmann and Gorring) to biporous structures.