The Effect of Flow-Induced Vibration on Heat and Mass Transfer Performance of Hollow Fiber Membranes in the Humidification/Dehumidification Process

Cross-flow hollow fiber membranes are commonly applied in humidification/dehumidification. Hollow fiber membranes vibrate and deform under the impinging force of incoming air and the gravity of liquid in the inner tube. In this study, fiber deformation was caused by the pulsating flow of air. With varied pulsating amplitudes and frequencies, single-fiber deformation was investigated numerically using the fluid–structure interaction technique and verified with experimental data testing with a laser vibrometer. Then, the effect of pulsating amplitude and frequency on heat and mass transfer performance of the hollow fiber membrane was analyzed. The maximum fiber deformation along the airflow direction was far larger than that perpendicular to the flow direction. Compared with the case where the fiber did not vibrate, increasing the pulsation amplitude could strengthen Nu by 14–87%. Flow-induced fiber vibration could raise the heat transfer enhancement index from 13.8% to 80%. The pulsating frequency could also enhance the heat transfer of hollow fiber membranes due to the continuously weakened thermal boundary layer. With the increase in pulsating amplitude or frequency, the Sh number or Em under vibrating conditions can reach about twice its value under non-vibrating conditions.

Long-Term Monitoring of a Tunnel in a Landslide Prone Area by Brillouin-Based Distributed Optical Fiber Sensors

This paper shows the results of the monitoring of the deformations of a tunnel, carried out using a distributed optical fiber strain sensor based on stimulated Brillouin scattering. The artificial tunnel of the national railway crosses the accumulation zone of an active landslide, the Varco d’Izzo earthflow, in the southern Italian Apennines. Severely damaged by the landslide movements, the tunnel was demolished and rebuilt in 1992 as a reinforced concrete box flanked by two deep sheet pile walls. In order to detect the onset of potentially dangerous strains of the tunnel structure and follow their time trend, the internal deformations of the tunnel are also monitored by a distributed fiber-optic strain sensor since 2016. The results of the monitoring activity show that the deformation profiles are characterized by strain peaks in correspondence of the structural joints. Furthermore, the elongation of the fiber strands crossing the joints is consistent with the data derived by other measurement systems. Experiments revealed an increase in the time rate of the fiber deformation in the first and last part of the monitoring period when the inclinometers of the area also recorded an acceleration in the landslide movements.

Analysis of impacts of thermal shocks on mechanical properties of E-glass fiber reinforced polyester composites

Glass fiber reinforced polyester composites are economic and high-performance composite materialsthat has gained a wide range of applications. Besides the developments in composites design, scientific studies addressing the consequences of thermal changes on the mechanical properties of fiber reinforced polymer composites(FRPCs) are scarce. Therefore, the main aim of the present work is to investigate the physical/mechanical properties of glass fiber reinforced polyester composites under thermal shocks. The effects of thermal cycle duration (2, 5 and 20 hours) on the porosity and mechanical properties (maximum stress, strain, elastic modulus and impact resistance) of polymeric composites reinforced by glass fiber, woven fabric and copper/silica nanoparticles (NPs) were investigated. The results exhibited that the porosity and mechanical properties changed obviously in long duration cycles, i.e., 20 hours. Major reduction trends were observed when the fabric reinforced samples were further reinforced by NPs. It was concluded that although NPs reduce porosity and pose filling effect in composite matrix, can also provide stress concentration locations. The composites reinforced by woven fabric and prepared by RTM method provide better mechanical properties. Moreover, after thermal shocks, the fibers within the composite structure formed curved shapes. Consequently, a reduction occurred at the elastic modulus of fibrous reinforced composites (fiber or fabric) after thermal cycles. Besides theelevated porositywas the predominant factor reducing elastic modulus, fiber deformation was also considered as a hidden factor which has never been discussed in previous research studies. A model of bicomponent structure was used to explain the effects of fiber deformation on elastic modulus of the FRPCs.

Differences and similarities between kraft and oxygen delignification of softwood fibers: effects on mechanical properties

Charged groups in pulp have been shown to enhance the tensile strength of the paper produced from the pulp. Oxygen delignification introduces charged groups and it is of interest to determine how the delignification should be distributed between the cooking and the oxygen stage with respect to mechanical properties. A number of unbleached kraft cooked and oxygen delignified pulps within a wide kappa number range were produced and refined, and the effects of the refining on the morphology and mechanical properties were studied. The WRV correlated with the fiber charge and at a given fiber charge, kraft cooked and oxygen delignified pulps had the same WRV development in refining, although they had significantly different kappa numbers. The tensile strength development during refining depends on the fiber rigidity which is affected by the lignin content, the fiber charge and the chemical and mechanical processes used. Refining increased the curl of the kraft cooked pulps and decreased the curl of oxygen delignified pulps, irrespective of kappa number. A greater increase in tensile strength was seen for the pulps with a higher fiber charge and WRV, probably because of the greater degree of fibrillation achieved in the beating process. Despite the greater fiber deformation in the oxygen delignified pulps, the strength can be increased by a larger amount of charged groups and a greater swelling of the fibers. Graphic abstract