Qinhong Zheng

A parametric study on the thermal performance of a solar air collector with a v-groove absorber

In this paper, a parametric study on the thermal performance of a solar air collector with a v-groove absorber has been investigated. In this single-cover collector, the air flowing in the channel formed by the v-groove absorber and the bottom plate—which is flat and insulated—is along the groove, aiming at enhanced heat transfer rate between the air and the absorber by increasing the heat transfer surface area, which is crucial to the improvement of thermal performance of a solar air collector. To quantify the achievable improvements with the v-groove absorber, a flat-plate solar air collector where both the absorbing plate and the bottom plate are flat, is also considered. The thermal performance of these two types of solar air collectors is analyzed and compared under various configurations and operating conditions. The results show that the v-groove collector has considerably superior thermal performance to the flat-plate collector. It is also found that to achieve better thermal performance, it is essential to; use a small size of the v-groove absorber for the v-groove absorber collector and to maintain a small gap between the absorber and the bottom plate for the flat-plate collector; to use selected coatings that have a very high absorptivity of solar radiation but a very small emissivity of thermal radiation on the absorber and glass cover; to maintain an air mass flow rate above 0.1kg/m2s; and to operate the collectors with the inlet fluid temperature close to that of the ambient fluid.

Approximate formula of the lowest-order TM mode of a pentagonal waveguide family

Based on two inequalities, the upper and lower bounds of the cutoff wavenumber of the lowest-order TM mode of a pentagonal waveguide family are derived. By means of taking the geometric mean value, an approximate formula to calculate the cutoff wavenumber of the lowest-order TM mode of a pentagonal waveguide family is developed. It is the function of the cross section geometric parameters of the pentagonal waveguide. Comparisons between the results obtained by the approximate formula and the available numerical published data fully validate accuracy of the approximate formula.

Analysis of inhomogeneously filled cavities by vector finite element method

Accurate analysis of inhomogeneously filled cavities by vector finite element method (VFEM) is presented in this paper. A generalized theory is presented and a tetrahedral subdivision element is exerted for the analysis of inhomogeneously loaded cavities. As a validation, the resonant frequencies of the rectangular and cylindrical cavities filled with dielectric of different types are computed and a comparison with analytical results is made. The examples show excellent flexibility and efficiency of the VFEM method in analysing inhomogeneously filled cavities.

An Efficient 2-D FDTD Method for Analysis of Parallel-Plate Dielectric Resonators

An efficient two-dimensional (2-D) finite-difference time-domain (FDTD) method is presented for the analysis of parallel-plate dielectric resonator. Three-dimensional (3-D) electromagnetic problems, with the description of z-dependence by kz, can be solved by compact 2-D FDTD method. Moreover, the perfect matched layer (PML), corresponding to the proposed FDTD algorithm, is also presented for the simulation of open system. Three representative examples-cylindrical, ring, and asymmetric parallel-plate resonators-are analyzed to verify the proposed compact 2-D FDTD method.

FD-TD analysis of bandwidth characteristics in the asymmetrical ridge rectangular waveguide

The finite-difference time-domain (FDTD) method is applied to analyzing the influence of the ridge position on the cutoff wavelength and bandwidth of the dominant mode in the ridge waveguide. The cutoff wavelength and bandwidth of the dominant mode of the asymmetrical single- and double-ridge rectangular waveguide have been analyzed. Numerical results demonstrate that FD-TD method is precise and efficient, and the cutoff wavelength and bandwidth can be adjusted by changing the ridge dimensions as well as by adjusting its position.

Analysis of loaded cavity by FDTD technique combining with sum algorithm and Padé approximation

An accurate and stable method, the combination of finite-difference time-domain technique, fast Fourier transform, Padé approximation, and sum algorithm, is presented for the calculation of the quality factors of cavity. Comparing with the fast Fourier transformation/Padé technique, without missing any interesting mode, this novel method not only eliminates the sensitivity of Padé approximation but also improves the accuracy of numerical results. As a validation, two cavities are analyzed.

Full-wave analysis of fin-line by finite difference time domain method

An efficient technique using the fast Fourier transform, sum algorithm in combination with the finite-difference time-domain (FDTD) method is proposed for the full-wave analysis of fin-line. In modelling the fin-line, substrate dielectric is treated as a complex permittivity dielectric. Cut-off wavelength, effective dielectric constant, propagation constant and attenuation constant are presented. And a comparison is made to validate the accuracy and efficiency of this method.

Natural convection boundary-layer flow in a Trombe wall channel

A scaling analysis has been carried out to obtain the scaling laws which relates the parameters of characteristic flow behavior of the natural convection boundary-layer flow of air in a Trombe wall channel with the operation parameters and the structure parameters. These scaling laws have been validated and quantified by a series of direct numerical simulation.