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AbstractReview: 50 refs.

Abstract Flow and heat transfer characteristics under rolling motion are extremely important to thermohydraulic analysis of offshore nuclear reactors. An experimental study was conducted in a heated rectangular channel to investigate flow and heat transfer in laminar–turbulent transitional flow regime under rolling motion. The results showed that the average friction factor and Nusselt number are higher than that of the corresponding steady flow as the flow rate fluctuates in transitional flow regime. Larger relative flow rate fluctuation was observed under larger rolling amplitude or higher rolling frequency. In the same manner, larger increases of average friction factor and Nusselt number were achieved under larger rolling amplitude or higher rolling frequency. The increases were mainly caused by the flow rate fluctuation through periodic breakdown of laminar flow and development of turbulence in laminar–turbulent transitional flow regime. First, turbulence, which enhances the rate of momentum and energy exchange, occurs near the crest of flow rate wave even the flow is still in laminar flow regime according to the average Reynolds number. Second, as a result of rapid increases of the friction and heat transfer with Reynolds number in transitional flow regime, the increases of the friction and the heat transfer near the crest of flow rate wave are larger than the decreases of them near the trough of flow rate wave, which also contributes to increases of average friction and heat transfer. Additionally, the effect of critical Reynolds number shift under unsteady flow and heating condition on flow and heat transfer was discussed.

Abstract During the start-up or operation of pool type lead cooled fast reactor, it is probable that loss of flow transient appears from forced circulation or mass flow rate less than that during forced circulation, due to any flow blockage, station blackout or pump malfunction. In this type of scenario temperature distribution and movement of thermal stratification layer will be different from that during loss of flow transient from forced circulation. Computational fluid dynamics (CFD) transient analysis of three-dimensional model of CLEAR-S was performed in this research with the use of commercial code FLUENT to analyze the influence of initial coolant mass flow rate on flow and coolant temperature profile in different constituents of engineering validation facility (CLEAR-S) during loss of flow transient. Firstly, steady state was simulated at mass flow rate equal to 100%, 50% and 25% of the rated mass flow rate respectively. Than loss of flow transient was simulated after started from the steady state of 100%, 50% and 25% of the rated mass flow rate as t = 0sec respectively to understand the movement and temperature distribution of thermal stratification within the pools of CLEAR-S. Results showed that after loss of flow transient from 100%, 50% and 25% mass flow rate, LBE in the hot pool was distributed into five to six layers of different temperatures and temperature of LBE on both sides of stratification layer increased with the decrease of initial mass flow rate. As time progressed hot pool stratification layer proceeded upward and width of top most layer of LBE with high temperature reduced which ultimately vanished after giving birth to a new layer at hot pool bottom. Stratification initiated from DHR outlet in the cold pool after loss of flow transient from 100% mass flow rate. Thermal stratification was observed to be started at heat exchanger (HX) outlet during loss of flow transient initiated form 50% and 25% of mass flow rate. Temperature of LBE on both sides of stratification layer increased with the decrease of initial cold pool mass flow rate of CLEAR-S. With the proceeding of time Stratification layer raised upward and top most layer vanished.

Abstract Some new structures of thin-film solar converters (SC) based on heterojunctions (HJ) with intermediate semiconductor layers are suggested. Thin protective layers and a quasi-electric field incorporated into the space charge region (SCR) prevent cross-diffusion of HJ components and increase efficiency of charge carrier separation. They also decrease the diode dark current and provide high stability of the converter parameters. Thin (∼ 0.1 μ M) (CdSe) x (ZnTe) 1− x or Zn x Cd 1− x Se layers were used as graded band-gap layers. They were places between a photosensitive II-VI-compound (CdTe, CdSe, CdSe x Te 1− x ) base layer and the transparent Cu 1.8 S layer. The above structures were prepared by vacuum closed space sublimation. The properties of these compounds were studied by electron microscopy and X-ray photoelectron spectroscopy (XPS) with ion etching. The photoelectron properties of structures such as Cu 1.8 S/(CdSe) x )(ZnTe) 1− x /CdSe are presented in detail. The manufacturing technology for the integrated solar batteries based on CdTe, CdSe, and CdSe x Te 1− x compounds was developed. The solar cell parameters under low illumination intensities are comparable to those of solar batteries based on c-Si and a-Si. The competitiveness of the polycrystalline thin-film SC is due to ease and low cost of fabrication (as compared with c-Si and a-Si) and also to the extended photosensitivity range (as compared to a-Si).

We present a methodology for applying silicon pho-nonic crystals (Si PnCs) to photovoltaics (PV). One-dimensional PnCs made from ultrathin Si, indium tin oxide, and graphene are designed, as well as two-dimensional Si-only PnCs. The general elastic wave equations are employed to solve the frequency band structures. The obtained bandgap can effectively suppress the carrier relaxation. The potential of Si PnCs for PV applications is theoretically assessed within our model. The calculated upper limit of the thermodynamic efficiency is approximately 58%, which is well beyond the Shockley–Queisser limit of either a Si solar cell or an all-Si tandem solar cell. Very importantly, our calculations show that it is not necessary to fully suppress the carrier relaxation to achieve ultrahigh efficiency. This work offers a strategy to develop ultrahigh-efficiency single-junction Si solar cells using the Si PnCs with ultrathin absorbers at extremely material cost.

A three-dimensional numerical model of the PEM water electrolyzer was constructed to account for the combined effect of the non-uniform depth of the flow channel and the non-uniform porosity of the anode gas diffusion layer on the mass transfer characteristics.

Abstract We have investigated the effect of boron (B) and phosphorus (P) compensation on the performance of Czochralski (CZ) silicon solar cells. Both majority and minority carrier mobilities are significantly reduced by the dopant compensation. Correspondingly, the minority carrier diffusion length becomes smaller. The compensated silicon solar cells show weaker spectral responses and therefore lower short-circuit current. However, the open-circuit voltage can be influenced by the net doping concentration in our silicon wafers. A higher open-circuit voltage could be obtained from the compensated solar cells due to its larger net doping concentration. As a result, the compensated solar cells could have the same efficiency as the conventional ones. It suggests that the dopant compensation in silicon with a doping level of ≤1017 cm−3 is not a serious issue for the improvement of solar cell efficiency. The results are of significance for the upgraded metallurgical grade silicon (UMG-Si) application in the fabrication of high efficiency and low-cost solar cells.

Abstract The turbulent flow and interchannel mixing in rod bundles is a concerned issue in the thermal-hydraulic design of fuel assemblies. In this paper, a large eddy simulation was performed to study the turbulent flow of water in a 2 × 2 rod bundle. The diameter of each rod and the pitch-to-diameter ratio were set to 20 mm and 1.25, respectively. The grid resolutions were carefully studied by examining the two-point correlation of velocity components along the streamwise direction, and by comparing the power spectral density with Kolmogorov’s −5/3 law. The numerical results showed that the root-mean-square (RMS) velocity is highly anisotropic in the gap area. The secondary flow on the cross-section of the rod bundle was nonuniform, and the intensity is strong close to the wall where small eddies were generated. The distributions of Reynolds normal and shear stresses and the budget of primary normal stress were presented.