Large Height Ratio Research Articles

Effect of aspect ratio of semitransparent window on interaction of the collimated beam with natural convection: part 1

The effect of the semitransparent window's aspect ratio, i.e., the height ratio (hr) and window width ratio (wr) and Planck numbers on the interaction of the collimated beam with natural convection has been investigated numerically. The cavity is convectively heated from the bottom, and a semitransparent window is created on the left wall, and a collimated beam is irradiated on the window at an azimuthal angle (ɸ) of 135°. The dynamics of two vortices inside the cavity change considerably by combination of window's aspect ratio and Planck number (Pl) of the medium. The thermal plume flickers depending on the situation of the dynamics of two vortices inside the cavity. The localised heating of the fluid happens mostly for the large height ratio of the semitransparent window. The conduction, radiation, and total Nusselt numbers are also greatly affected by the aspect ratio and Planck number of the medium.

Wind interference effects of high-rise building on low-rise building with flat roof

Wind pressure measurements were conducted in a wind tunnel to investigate interference effects of a high-rise building on a low-rise building with a flat roof. The influences of the building spacing ratio, building height ratio and wind direction are examined. The results show that positive pressures are observed on the roof of the low-rise building when it is located in front of the high-rise building with small spacing. Increasing the height of the high-rise building or decreasing the spacing induces an increase in positive pressures. The critical condition of the sign change of wind pressures on the roof is presented. Significant amplification of suction is observed when the low-rise building is right behind the high-rise building with small spacing and large height ratio. Increasing the height of the high-rise building greatly amplifies the suction on the whole roof of the low-rise building. The mean lift coefficient on the roof is about 2.5 times that on an isolated low-rise building roof when the spacing ratio is small and the height ratio is large. The maximum interference factors of the most unfavorable minimum and maximum coefficients in the roof area near the high-rise building are larger than 1.5 and 3.5, respectively.

Numerical study on overtopping performance of a multi-level breakwater for wave energy conversion

Overtopping wave energy converter (OWEC) is capable of converting the unstable wave energy into the relatively stable potential energy in its reservoir. The small wave height and the large tidal range in China will cause a lower efficiency of the traditional single-level overtopping device especially at the lower tidal level. The design of the multi-level reservoirs may be a solution to improve the overtopping performance, which can be integrated in the sloping-ramp breakwater. In this study, a multi-level breakwater for wave energy conversion, consisting of two reservoirs with sloping walls at different levels, is proposed. A numerical wave tank based on the VOF model is established in ANSYS-Fluent® 12.0 for the OWEC's performance investigation. The numerical model on predicting the overtopping process and time-averaged discharges is validated by corresponding experimental data. Effects of the opening width of the lower reservoir, sloping angles of two reservoirs and the gap height between the reservoirs on the dimensionless overtopping discharges are studied numerically. It is found that a smaller opening width, larger height ratio, and two sloping angles of 30° have positive effects to improve the overtopping performance of the device.

Heat transfer and flow structure in turbulent channel flow over protrusions

In this study, heat transfer characteristics and flow structures over protrusions in a turbulent channel flow were systematically investigated by DES numerically. Densely arranged protrusions with different depth ratios (h/D) are considered for Reynolds number Re2H (based on bulk velocity and full channel height) between 3000 and 6000 while Prandtl number Pr is fixed at 0.7. It is found that larger height ratio induces higher friction factor and heat transfer. However the performance factor, on the other hand, first increases then reaches its asymptotic limit and even decreases with increasing height ratio. It is also observed that the highest skin friction, form drag and localized Nusselt number are found at the upstream portion of protrusion. Additionally, the distributions of friction factors and Nusselt number exhibit symmetrical features for the low protrusion configuration and asymmetric characteristics for the high protrusion arrangement. The asymmetric distribution of localized Nusselt number at large height ratio (h/D⩾15%) is found to be closely linked to the asymmetric vortex shedding from the back ridge of protrusions.

Enhancement and Saturation Phenomena on Luminous Current and Power Efficiencies of Organic Light-Emitting Devices by Attaching Microlens Array Films

Compared to the OLED with planar substrate, the luminous current efficiency and luminous power efficiency of the device are shown 25% and 36% enhancement, respectively, by attaching the microlens array film (MAF) having a fill factor of 0.788 and a height ratio of 0.46. Both the luminous current efficiency and luminous power efficiency of the organic light-emitting device (OLED) increased monotonically with increasing the fill factor of MAFs, regardless of their arrangements. However, the curve-fitting equation of the luminous power efficiency has a negative second-order term, which shows a saturation phenomenon. Based on the experimentally verified configuration, simulation showed that the luminous current and power efficiencies also increase along with increasing height ratio and they were found to have more pronounced saturation phenomena. As for the spectral characteristics, the peak wavelength of the planar OLED spectra got blue-shift and the full-width-at-half-maximum (FWHM) of its spectra decreased with increasing the viewing angles due to the microcavity theories. After MAFs are attached to the OLED, the FWHM of the OLED spectra decreased linearly by increasing the fill factor of the MAFs. We also observe blue shifts at different viewing angles which is the evidence that the waveguiding modes are being extracted. The saturation phenomena of efficiency enhancement imply that high fill factor and large height ratio of MAFs will certainly benefit for efficiency enhancement, however, full factor cases are not always the most desirable. Optimal fill factor and height ratio, which may be less than unity, will gain the best efficiency enhancement, suffer from less color deviation, and make fabrication easier.