Search

The primary heat transfer source in the conventional multi-cylinder drying of paper and board is conduction. Conduction is facilitated by high-tension contact with steam-heated cylinders, while convective drying, the main mass transfer source, operates as heated air flows over the article web in the pockets. This conduction process occurs at elevated temperatures and contact pressures to ensure effective contact between the wet paper web and the heated cylinders. The contact pressures and temperatures of the hot surface significantly influence the conductive drying characteristics. This research paper presents an experimental study that involves designing a simple lab-scale setup with in-situ and continuous sensing capabilities for various commercially available grades of paper and board. Embedded thermocouples measure the temperatures of the heated platen and the sheet, allowing the collection of flux data to determine heat transfer characteristics. The goal of this study is to ascertain the instantaneous contact coefficients for different basis weights as a function of moisture content. The acquired data will provide valuable insights and information toward process development, design, and simulation of the article drying process. [ABSTRACT FROM AUTHOR]

The gas flow through sheet-like porous materials such as paper can show marked lateral variations due to a heterogeneous, locally varying microstructure. Hence, reliable predictions of such lateral flux variations require an appropriate consideration of local variations in the microstructure. The flow through such sheet-like materials is commonly described with Darcy's law in which permeances are formulated in terms of microstructure properties, such as porosities, tortuosities, or hydraulic radii. This work proposes an extension of existing permeance models that directly considers the variation and the cross-dependence between local microstructure properties. The extended model is applied to local air fluxes through a paper sheet to exemplarily reveal the joint impact of local porosities and local tortuosities on the air flux. The key extension is to consider a joint distribution of porosity and tortuosity. The latter is constructed from the univariate property distributions using a copula approach and yields local tortuosities including their variation for any encountered local porosity. These values jointly enter any permeance model that qualitatively captures the dependence of the air flux on the porosity. To assess the merit of the model, variations in the air flux and in the pore space properties are independently determined from the same measured microstructure of paper. Air flux variations are provided by computational fluid dynamics simulations on multiple, nonoverlapping segments taken from the microstructure. A statistical analysis of the entire microstructure provides the distribution of local porosity, tortuosity, and thicknesses. Our model quantitatively explains that porosity-dependent variations in the tortuosity, in particular the ones associated with high-volume pathways, decisively determine air flux variations. Article Highlights: Computational fluid dynamics simulations determine local variation of air flow through an actual microstructure of paper. Construction and use of the copula approach to model joint distribution of local porosities and tortuosities is explained. Existing permeance models equipped with variations in porosity and porosity dependent tortuosity capture most variations in air flow. [ABSTRACT FROM AUTHOR]

• Density of a paper has a decisive impact on its air permeability. • Exposure of both historical and common papers to A. niger decreases permeability. • Nano-wollastonite is recommended to improve biological resistance of paper. The effect of nano-wollastonite and Aspergillus niger fungus on permeability of historical and common paper was studied here. Specimens were immersed in aqueous wollastonite nano-suspension (NW) at 10, 20, 30, and 40% concentration, and their permeability was finally compared with that obtained from control specimens. Permeability of each specimen was measured before and after the immersion in NW. To fix solid particles on NW-treated specimens, polyvinyl acetate (PVA) was mixed in all treatments. Results showed that wollastonite significantly limited the growth of A. niger. Permeability of historical specimens was significantly lower than that of the common paper, 29 in comparison to 190 cm3.s−1, respectively. This was due to the higher density in the historical paper, providing lower empty voids and spaces for fluid to pass through. Fungi culture resulted in significant decrease in both paper types. In fact, fungal hyphae penetrated into the specimens, eventually blocking the paths through which fluid could transfer. High significant R-square values were found between permeability values before and after the exposure to A. niger. It was concluded that permeability is closely correlated to the density of paper, and it has a significant effect on the potentiality of fungi hyphae to be penetrated into the texture of papers, affecting the weight loss values. Moreover, NW-suspension can be used to decrease permeability as well as susceptibility to fungal attack. [ABSTRACT FROM AUTHOR]

The extension of the Darcy law (the Forchheimer flow equation) relating second-order nonlinear pressure drop with flow velocity is studied during fast transient through air drying of sheets of porous biobased materials such as paper. A range of the paper materials with open structure consistent with tissue and towel products (basis weights 25 and 50 g/m2) made using different production processes are analyzed for the factor-specific influences with regard to changes in the fluid resistance from the removal of moisture from the material interstices. A characteristic dimension suitable for the drying process is applied from viscous and inertial momentum transport analysis. [ABSTRACT FROM AUTHOR]

An experimental method using a novel design to characterize the air flow and water removal during vacuum dewatering in paper manufacturing is discussed. The experimental setup involves the intermittent application of vacuum, similar to commercial systems, using a rotating disk with slot opening arrangement. The system is capable of commercially realistic residence times of the order of milliseconds. The intermittent application of vacuum simulates vacuum dewatering on commercial paper machines. The air flow rate is calculated from changes in pressure and temperature in the vacuum tank underneath the sample. The role and importance of air flow during vacuum dewatering is studied by accurately measuring the air flow, properly taking into account the leaks during vacuum dewatering. The method described here provides for the first time accurate air flow and water removal data during vacuum dewatering. Methods of analysis of the experimental data are also presented. This information can be used to better understand the water removal process as well the role and importance of air flow during vacuum dewatering. [ABSTRACT FROM AUTHOR]

Airflow sensor is a crucial component for monitoring environmental airflow conditions in many engineering fields, especially in the field of aerospace engineering. However, conventional airflow sensors have been suffering from issues such as complexity and bulk in structures, high cost in fabrication and maintenance, and low stability and durability. In this work, we developed a facile direct-writing method for fabricating a low-cost piezoresistive element aiming at high-performance airflow sensing, in which a commercial pen was utilized to drop solutions of single-walled carbon nanotubes onto tissue paper to form a piezoresistive sensing element. The encapsulated piezoresistive element was tested for electromechanical properties under two loading modes: one loading mode is the so-called pressure mode in which the piezoresistive element is pressed by a normal pressure, and another mode is the so-called bending mode in which the piezoresistive element is bended as a cantilever beam. Unlike many other developed airflow sensors among which the sensing elements are normally employed as cantilever beams for facing winds, we designed a fin structure to be incorporated with the piezoresistive element for airflow sensing; the main function of the fin is to face winds instead of the piezoresistive element, and subsequently transfer and enlarge the airflow pressure to the piezoresistive element for the normal pressure loading mode. With this design, the piezoresistive element can also be protected by avoiding experiencing large strains and direct contact with external airflows so that the stability and durability of the sensor can be maintained. Moreover, we experimentally found that the performance parameters of the airflow sensor could be effectively tuned by varying the size of the fin structure. When the fin sizes of the airflow sensors were 20 mm, 30 mm, and 40 mm, the detection limits and sensitivities of the fabricated airflow sensors were measured as 8.2 m/s, 6.2 m/s, 3.2 m/s, 0.0121 (m/s)−2, 0.01657 (m/s)−2, and 0.02264 (m/s)−2, respectively. Therefore, the design of the fin structure could pave an easy way for adjusting the sensor performance without changing the sensor itself toward different application scenarios. [ABSTRACT FROM AUTHOR]