Elliptical Hollow Fiber Research Articles

Laminar flow and heat transfer between a regularly distributed elliptical hollow fiber membrane tube bank

Effects of the various semiaxis ratios (ble/ale, bri/ari), longitudinal and transverse pitch-to-diameter ratios (SL/d, ST/d) on the longitudinal fluid flow and heat transfer between a regularly distributed elliptical hollow fiber membrane tube bank (EHFMTB) employed for air humidity control are investigated. In the tube bank, the liquid stream flows inside the fibers, while the air stream flows axially between the fibers in a counter flow arrangement. Two regularly populated arrangements of the tube bank, in-line and staggered, are considered. Two representative elements containing two fibers and the air stream flowing axially between the fibers are selected as the calculating domains. The governing equations for the fluid flow and the heat transfer between the EHFMTB are established and numerically solved via a boundary-fitted coordinate transformation method. The fully developed Poiseuille numbers (fRe) and local Nusselt numbers under a uniform temperature condition (NuT) under various semiaxis ratios, longitudinal and transverse pitch-to-diameter ratios are then obtained. It can be found that the fRe and NuT have their largest values when the ble/ale and bri/ari both are equal to 1.0 and 1.5 for the in-line and the staggered arrangements, respectively. The SL/d and ST/d have larger influences than the ble/ale and bri/ari on the fRe and NuT.

Twisted hollow fiber membranes for enhanced mass transfer

Computational fluid dynamic simulations are used to examine incompressible flow through twisted elliptical and helically-coiled cylindrical fibers for typical flow Reynolds numbers employed in membrane filtration processes (Re<200). The resulting flow fields are compared in terms of the distributions of vorticity and wall shear rate, the pressure drop, and the azimuthally-averaged normal velocity in the vicinity of the wall. The secondary flow in twisted elliptical fibers generates significant recirculation flow leading to near-wall velocities directed normal to the surface of the membrane. The azimuthally-averaged normal velocity near the wall of a twisted elliptical fiber is found to be two to four orders of magnitude larger than typical ultrafiltration velocities. For Re<75, the average wall shear rate and normal velocity near the wall are found to be significantly higher in twisted elliptical fibers compared to their helical counterparts. These findings highlight the potential for mass transfer and flux enhancement using twisted elliptical hollow fiber membrane systems.

Elliptical metallic hollow fiber inner-coated with non-uniform dielectric layer.

We report on the fabrication and characterization of an elliptical hollow fiber inner coated with a silver layer and a dielectric layer for polarization maintaining and low loss transmission of terahertz (THz) radiation. The primary purpose of adding the dielectric layer is to prevent the silver layer from oxidation. The thickness of the dielectric layer is non-uniform owing to the surface tension of the coating, which was initially applied as a liquid. Transmission loss and polarization maintenance are experimentally characterized. Effects of the dielectric layer on transmission properties are analyzed by comparing the fiber to Ag-only fiber. Results show that a dielectric layer with thickness less than λ/10 can effectively decreases the power distributed on the metal surface and thus can practically reduce loss resulting from roughness of the silver layer. Bending effects on transmission loss and polarization maintenance are also investigated.

Effects of the random distributions on the longitudinal transport phenomena between an elliptical hollow fiber membrane bundle

An elliptical hollow fiber membrane bundle (EHFMB), which is installed in a shell to form a counter flow tube-and-shell heat exchanger like hollow fiber membrane contactor, is commonly used for air humidity control. EHFMB is usually randomly packed in practical applications because of the randomness in the assembly process. The longitudinal transport phenomena between a randomly distributed elliptical hollow fiber membrane bundle (REHFMB) are studied. To disclose the effects of the random distributions on the transport phenomena, a unit cell containing 20 fibers based on Voronoi tessellation technology and the air flowing axially between the REHFMB is selected as the calculating domains. The governing equations governing the fluid flow and heat transfer between the REHFMB are established and numerically solved. The friction factor and Nusselt number in the unit cell under various fiber distributions, packing fractions (φ), rotation angles (β) and semiaxis ratios (b/a) are then obtained, analyzed, and experimentally validated. It can be found that the elliptical semiaxis ratios, the random distribution in positions, and rotation angles all have effects on the axial transport phenomena between the REHFMB. Further, the random distribution in positions is the dominant factor.

Fluid flow and heat transfer across an elliptical hollow fiber membrane tube bank with randomly distributed features

An elliptical hollow fiber membrane tube bank (EHFMTB) has better performances while being employed for air humidification. The EHFMTB is populated in a plastic shell to form a shell-and-tube heat exchanger like membrane contactor. The tube bank is always randomly populated in practical applications because of convenience and randomness in the manufacturing process. The fluid flow and heat transfer across a randomly distributed elliptical hollow fiber membrane tube bank (REHFMTB) are investigated. To disclose the influences of the fiber arrangements on the performances, three unit cells containing 20 fibers with different randomly distributions are selected as the calculating domains. A renormalization group k–ε (RNG KE) turbulence model with enhanced wall treatment is used for solving the equations governing the momentum and heat transports. The friction factor and Nusselt number across the REHFMTB under various fiber distributions, Reynolds numbers (Re), packing fractions (φ) and elliptical semiaxis ratios (b/a) are numerically obtained and experimentally validated. It is found that the comprehensive heat transfer performance is deteriorated for the fluid flow across the REHFMTB.

Heat and mass transfer enhancement in a cross-flow elliptical hollow fiber membrane contactor used for liquid desiccant air dehumidification

To improve the performances of a hollow fiber membrane contactor (HFMC) employed for liquid desiccant air dehumidification, an elliptical hollow fiber membrane contactor (EHFMC) is proposed. The contactor is assembled by a collection of elliptical hollow fibers populated in a shell. The liquid desiccant and the processing air streams flow inside and across the elliptical fibers, respectively. They are in a cross-flow configuration. The momentum and the conjugate heat and mass transports in the EHFMC are investigated based on Happel's free surface model. In this approach, a single elliptical fiber, an air stream across the fiber, and a liquid desiccant stream inside the fiber are selected as the calculating element. The air stream in the element is encompassed by a hypothetical outer free surface with an elliptical shape. The equations governing the momentum, heat and mass transports in the air and the solution streams are established and solved together with the conjugate heat and mass transfer boundary conditions. The fundamental data of the friction factor, Nusselt and Sherwood numbers in the element are then numerically calculated, experimentally validated, and analyzed. These basic data are compared with those obtained in the HFMC.

Fluid flow and heat transfer across an elliptical hollow fiber membrane tube bank for air humidification

In order to improve the performances of a hollow fiber membrane tube bank (HFMTB) used for air humidification, an elliptical hollow fiber membrane tube bank (EHFMTB) is proposed and employed. Fluid flow and heat transfer across the EHFMTB are investigated. Due to the higher heat and mass transport effectivenesses of a regularly populated tube bank as compared to a randomly spaced one, the tube bank with in-line and staggered arrangements are considered. Ten rows of the fiber tubes and the air stream across the tubes are selected as the computational domains. The equations governing the momentum transfer and the heat transfer are established via a renormalization group k–ɛ (RNG KE) turbulence model with enhanced wall treatment. The friction factor and Nusselt number across the EHFMTB under various fiber arrangements (in-line or staggered), pitch to diameter ratio (S/d), semiaxis ratio (b/a), and Reynolds numbers (Re) are numerically obtained, experimentally validated, and analyzed. It is found that compared to the HFMTB, the heat transfer of the air stream flowing across the EHFMTB along the minor semiaxis direction is enhanced. Further, the heat transfer enhancement is more obvious with a decrease in the semiaxis ratios (b/a).

Longitudinal fluid flow and heat transfer between an elliptical hollow fiber membrane tube bank used for air humidification

The longitudinal fluid flow and heat transfer between an elliptical hollow fiber membrane tube bank (EHFMTB) used for air humidification are investigated. In the tube bank, the water stream flows inside the fibers, while the air stream flows axially between the fibers. The air and the water streams, in a counter flow arrangement, are exchanging heat and moisture effectively through the membranes. Two regularly populated arrangements of the tube bank, in-line and staggered, are considered. Two representative unit cells consisting of two fibers and the air stream flowing longitudinally between the fibers are selected as the calculation domains. The governing equations for the fluid flow and the heat transfer between the EHFMTB are established and numerically solved via a boundary-fitted coordinate transformation method. The friction factor and Nusselt number in the unit cells are then obtained, analyzed, and experimentally validated. They are compared with the data obtained for a hollow fiber membrane tube bank (HFMTB). The results are useful for structural optimization and performance evaluations of the EHFMTB employed for air humidification.

Heat and mass transfer deteriorations in an elliptical hollow fiber membrane tube bank for liquid desiccant air dehumidification

A hollow fiber membrane tube bank (HFMTB) is commonly used for liquid desiccant air dehumidification to prevent small liquid desiccant droplets from escaping to the processing air. Due to the extrusions during the fabrication process, cross-sectional shapes of the circular hollow fibers are easily changed to elliptical ones. To know the effects of the shape changes of the fibers, the conjugate heat and mass transfer in an elliptical hollow fiber membrane tube bank (EHFMTB) employed for liquid desiccant air dehumidification is investigated. The liquid desiccant flows inside the fibers, while the air flows in the shell side in a counter flow arrangement. In the tube bank, a typical unit cell consisting of a single fiber, the desiccant flowing inside the fiber and the air stream flowing outside the fiber, is considered. The air stream outside the fiber has an outer free surface. The equations governing the fluid flow and heat mass transfer in the two streams are solved together with the conjugate heat and mass transfer boundary conditions. The local and average Nusselt and Sherwood numbers in the unit cell are then obtained and experimentally validated. It is found that heat and mass transfer are substantially deteriorated in the EHFMTB.

Elliptical Hollow Fiber With Inner Silver Coating for Linearly Polarized Terahertz Transmission

Silver-coated elliptical hollow polycarbonate fibers for transmission of linearly polarized terahertz (THz) radiation are fabricated and characterized. A polarization ratio of 90% and a transmission loss of 0.79 dB/m at 0.65 THz are obtained for a 290-mm long fiber with major and minor inner radii of 2.42 and 1.18 mm. Modal birefringence in the order of 10-3-10-2 is predicted in the frequency region of 0.3-1.5 THz. We also investigate the transmission characteristics of deformed fibers and found that the fiber with a semicircle cross section could rotate the direction of a linear polarization.

Characterization of a novel dual-core elliptical hollow optical fiber with wavelength decreasing differential group delay

The modal distribution of a novel elliptical hollow optical fiber is experimentally and numerically characterized. The fiber has a central elliptical air hole surrounded by a germanosilicate lanceolate ring core. Experiments reveal that the fiber behaves like a dual core waveguide and it is found that the differential group delay of each core decreases with wavelength with a PMD coefficient slope of ~10(-2) ps/m/THz. Experimental results are also compared with numerical modeling based on scanning electron microscopy images.

Versatile control of geometric birefringence in elliptical hollow optical fiber

A novel optical fiber fabrication technique was developed by converting the symmetry of the silica substrate into the germanosilicate ring core to efficiently introduce geometric birefringence in an elliptical hollow optical fiber. Due to high ellipticity in the hollow ring core, the fiber provides an extremely high group birefringence of 2.35 x 10(-3) at 1550 nm. Single-mode single-polarization guidance was also experimentally confirmed, with a bandwidth of approximately 35 nm. The generic adiabatic mode conversion capability in the taper also provided a stable fusion splice to conventional single-mode fiber with low loss and high tensile strength.

High birefringence in elliptical hollow optical fiber

We propose a novel design of highly birefringent optical fiber composed of a central elliptical air hole, a circumferential elliptical ring core, and a circular cladding. The proposed waveguide structure is predicted to produce a linear birefringence higher by an order of magnitude than the solid elliptical core fiber. The large index contrast between the central air and germanosilica elliptical ring core is mainly attributed to the high birefringence and its characteristics are theoretically analyzed in terms of its waveguide parameters.