Yumei Ding

Experimental Study of Heat Transfer Enhancement and Friction Loss Induced by Inserted Rotor-assembled Strand (I) Water

Abstract The single-phase pressure drop and heat transfer in a rotor-assembled strand inserted tube were measured using water as the working fluid. Experiment using a smooth tube was carried out to calibrate the experimental system and the data reduction method. In the experiment, fixed mounts were used to eliminate the entrance effect. The experimental results of smooth tube show that employment of fixed mounts leads to a visible bias of friction factor at relative low Reynolds numbers, although it does not significantly affect the Nusselt numbers. The measured data of inserted tube reveal that rotor-assembled strand can significantly improve heat transfer with the Nusselt number increased by 101.6%-106.6% and the overall heat transfer coefficient increased by 58.1%-67.4% within the Reynolds number range of 20000 to 36000. Meanwhile, friction factor increases by 52.2%-84.2% within the same Reynolds number range. The correlations of Nusselt number and friction factor as function of the Reynolds number and Prandtl number were determined through multivariant linear normal regression.

Effect of electric field on gas-assisted melt differential electrospinning with hollow disc electrode

Abstract The concept of a gas-assisted melt differential electrospinning device with hollow disc electrode is presented. As the electric field force is the only drawing force stretching polymer melt jet to fibers, it is necessary to study the distribution and electric field intensity of the electric field created in the spinning region caused by the hollow disc electrode. A series of electric field simulations, including the distribution of the electric field and the relationship between electric field intensity and various parameters were carried out by the finite element method. In addition, experiments of melt electrospinning were conducted, mainly focusing on several electrical parameters affecting the fiber diameter. The results of simulations were compared with those of experiments, proving experimental phenomena and conjectures. The results of simulations and experiments were mutually corroborated and consistent with each other. All results provided significant support and basis for future exploration and development of melt electrospinning.

Interfacial Diffusion and Bonding in Multilayer Polymer Films: A Molecular Dynamics Simulation.

As a stacked form of ultrathin polymer films, multilayer nanostructures are of great interest in various applications. Coarse-grained molecular dynamics simulations were carried out to understand the behaviors of interfacial diffusion and bonding of multilayer polymer films. We found two obvious stages for the interfacial diffusion of polymers in the multilayer film, and it is 3 times faster in the first stage than in the second one due to the evolution of molecular conformations. The polymers near the interfaces have an in-plane mobility much higher than the out-of-plane one. The strength of interfacial bonding has been characterized by the fast tensile stress-strain curve along the normal direction. It shows multiple yielding points for the multilayer polymer films, which is distinct from the tensile behavior of the bulk. The ultimate tensile stress (UTS) and corresponding separating strain, surprisingly, do not necessarily increase with diffusion time. Because of the dramatic molecular rotation and extension during the first stage of interfacial diffusion, the interlayer interpenetration is nonuniformly distributed in the plane of the interface. Such a nonuniform distribution may be one of the reasons for the decrease of the UTS and separating strain.

Experimental Study of Heat Transfer Enhancement and Friction Loss Induced by Inserted Rotor-assembled Strand (II) Lubricant Oil 22 *

Abstract The paper described experimental investigation of heat transfer and single-phase pressure drop through tubes with different rotor-assembled strands inserted in the Reynolds number range of 800–9000 with lubricant as working fluid. In the experiment, fixed mounts were employed to eliminate the entrance effect. The experimental results showed that the employment of fixed mounts led to a visible bias of friction factor in the laminar regime while it could not affect the Nusselt numbers significantly. Experiment for the tube inserted with rotors-assembled strand showed remarkable improvement for heat transfer with the Nusselt number increased by 200%–225% in the laminar regime and 125%–160% in the transitional regime. Meanwhile, the friction factor increased inevitably by 200%–300% within the same range of Reynolds number. The comparison of different rotor-assembled strands inserted tubes and plain tube showed that the heat transfer benefited from the increase of the diameter of rotor-assembled strand with the same lead and the decrease of the lead of rotor-assembled strand, so does the friction factor. Based on experimental data and thorough multivariant linear normal regression method, the correlations of average Nusselt number and friction factor are established.

Design of polymer multilayer heterostructure broadband reflector for the near-infrared using genetic algorithm

Abstract. A broadband reflector for the near-infrared (NIR) was designed using a multilayer heterostructure consisting of several quarter-wave stacks (QWSs), which were composed of polymethyl methacrylate (PMMA) and polyethylene terephthalate (PET) films. Taking the solar power density as the target and using the Bragg wavelength of each QWS as the variable, the genetic algorithm was applied to look for the optimal multilayer heterostructure for broadband NIR reflection. As high as 99.46% total energy reflectivity in the short-wavelength NIR region (780 to 1100 nm) and 89.56% total energy transmissivity in the visible light region (380 to 780 nm) were realized by a multilayer heterostructure consisting of six quarter-wave PMMA/PET stacks, which can be easily fabricated based on the micronano multilayer coextrusion technology. The designed structure possesses good stability, and its total energy reflectivity is not sensitive to the incident angle of light. The proposed broadband NIR reflector can be applied to buildings as energy-saving films.

Heat transfer and pressure drop in tubes fitted with grooved lobe rotor-assembled strands

The heat transfer enhancement of grooved lobe rotors and smooth tube were investigated in the integrated heat transfer performance test bench of shell-and-tube heat exchanger. The results indicated that heat transfer rate of tube with the grooved lobe rotors increased about 80-120% compared with that of smooth tube, but the coefficients of resistance also increased about 65-75%. It would be necessary to modify the structure of grooved lobe rotors in the future in order to obtain high heat transfer coefficient and a relatively low friction factor.