Shuang-Ying Wu

Global simulation of a silicon Czochralski furnace in an axial magnetic field

Abstract Control of melt flow in crystal growth process by application of the magnetic field is a practical technique for silicon single crystals. In order to understand the influence of axial magnetic field on the silicon melt flow and oxygen transport in a silicon Czochralski (Cz) furnace, a set of global numerical simulations was conducted using the finite-element method for the magnetic field strength from 0 to 0.3 T, the crystal rotation rates from 0 to 30 rpm and the crucible counter-rotation rates from 0 to −15 rpm. It was assumed that the flow was axisymmetric laminar in both the melt and the gas, the melt was incompressible and a constant temperature was imposed on the outer wall of the Cz furnace. The results indicate significantly different flow patterns, thermal and oxygen concentration fields in the melt pool when a uniform axial magnetic field is applied.

Direct numerical simulation of Rayleigh-Bénard convection in a cylindrical container of aspect ratio 1 for moderate Prandtl number fluid

A series of three-dimensional numerical simulations were conducted for the Rayleigh-Benard convection in a cylindrical container with the aspect ratio Γ = 1. The Prandtl number of the fluid is 7 and the Rayleigh number varies from 103 to 105. Results show that the aspect ratio of the cylinder has an important influence on the multiplicity of the steady flow state. The sidewall of the cylinder at Γ = 1 restricts the increase of the number of rolls in the fluid layer. Therefore, only two-roll and single-roll flow patterns are observed at the whole simulation range of the Rayleigh number. During the transition of the Rayleigh-Benard convection to the unsteady flow, it is found that the unsteady flow pattern and the bifurcation sequence of the oscillation flow are very sensitive to the initial condition.

A Review on the Methodology for Calculating Heat and Exergy Losses of a Conventional Solar PV/T System

The heat loss and exergy loss from solar PV/T hybrid system can significantly reduce the first and second law efficiencies and consequently the cost effectiveness of the system. It is important to assess correctly these two losses and subsequently improve the performance of PV/T system. This article aims to present a comprehensive review and systematic summarization of methodology for calculating heat and exergy losses of conventional PV/T system. Included are the descriptions of heat and exergy losses calculating approaches in terms of heat transfer process analysis and exergy analysis theory, and the generalizations of correlations developed for heat loss and exergy loss prediction, particularly as applied to typical PV/T collectors. Based on the overview of research conducted to date, it is evident that there is still further works that need to be carried out with respect to the calculations of heat and exergy losses. This review work will be beneficial to the design, simulation, analysis, and performance assessment of the solar PV/T hybrid system.

An Investigation on the Exergo-Economic Performance of an Evaporator in Orc Recovering Low-Grade Waste Heat

Based on the perspective of exergy recovery (profit), an exergo-economic performance evaluation model of evaporator in ORC recovering low-grade waste heat is established. Selecting dry fluid R600a as a working fluid, an exergo-economic criterion was defined to estimate the effects of operating parameters and tube length on the performance of evaporator when the boiling temperature of evaporator is fixed. The optimal flow rate of organic working fluid Gt (flow rate of exhaust flue gas Gs), the flow rate ratio of exhaust flue gas to organic working fluid y, and tube length l have been discovered to maximize the annual net profit value NPV. Furthermore, there exists the critical values of Gt (Gs), y, and l which make NPV = 0. The results also show that, with the increase of the flow rate ratio of exhaust flue gas to organic working fluid, the maximum annual net profit value first increases sharply, and then decreases slowly, while the optimal tube length of evaporator reduces monotonously. Furthermore, the evaluation method from purely thermodynamic viewpoint is imperfect, and the economic considerations change the results obtained with only the maximization of recovered exergy.

Three-dimensional flow driven by iso- and counter-rotation of a shallow pool and a disk on the free surface

In order to understand the fundamental characteristics of the forced flow driven by iso- and counter-rotation of a shallow pool and a disk on the free surface, we conducted a series of unsteady three-dimensional numerical simulations in a shallow pool. The ratio of the disk to pool radius is Rs=0.3 and the aspect ratio of pool is H=0.06. The results indicated that the forced flow driven by disk and pool rotation is axisymmetric and steady at the small rotation Reynolds number. However, when Reynolds number exceeds a critical value, the flow will undergo a transition to three-dimensional oscillatory flow, which is characterized by the velocity fluctuation waves traveling in the azimuthal direction. The propagating direction and the velocity of the waves depend on the rotation rates and directions of the disk and pool. Besides, the critical conditions for the onset of the oscillatory flow were determined. The details of the flow fields were discussed and the mechanism of the flow pattern transition was also...

Asymptotic solution of thermocapillary convection in a thin annular pool of silicon melt

The free surface of the silicon melt in a thin annular pool is subjected to a radial temperature gradient. Since the surface tension depends on the temperature, it will create a thermocapillary force on the free surface and, in turn, yield to thermocapillary convection in the bulk of the liquid by the viscous traction. This paper presents an investigation on the steady two-dimensional thermocapillary convection in a differentially heated annular pool of the silicon melt using the asymptotical way. The pool is heated from the outer cylindrical wall and cooled at the inner wall. Bottom and top surfaces are adiabatic. The asymptotic solution is obtained in the core region in the limit as the aspect ratio, which is defined as the ratio of the pool height to the gap width, goes to zero. The numerical experiments are also carried out to compare to the asymptotic solution of the steady two-dimensional thermocapillary convection. The results indicate that the expressions of velocity and temperature fields in the ...

The determination and matching analysis of pinch point temperature difference in evaporator and condenser of organic rankine cycle for mixed working fluid

ABSTRACT Exergo-economic analysis of the pinch point temperature difference (PPTD) in both evaporator and condenser of sub-critical organic Rankine cycle system (ORCs) are performed based on the first and second laws of thermodynamics. Taking mixture R13I1/R601a as a working fluid and the annual total cost per net output power Z as exergo-economic performance evaluation criterion, the effects of PPTD in evaporator ΔTe, and the PPTD ratio of condenser to evaporator y, on the exergo-economic performance of ORCs are analyzed. Moreover, how some other parameters influence the optimal PPTD in evaporator ΔTe,opt and the optimal PPTD ratio of condenser to evaporator yopt are also discussed. It has been found that the exergo-economic performance of ORCs is remarkably influenced by ΔTe and y, and there exists ΔTe,opt and yopt. In addition, ΔTe,opt and yopt are affected by heat transfer coefficient ratio of condenser to evaporator ß, the temperature of working fluid at dew point in condenser T1a, and composition of R13I1/R601a: larger ß and T1a lead to lower ΔTe,opt and yopt; by contraries, larger mass fraction of R13I1 makes ΔTe,opt and yopt increase, and yopt increases linearly. The effects of the temperature of working fluid at bubble point in evaporator T3a, mass flow rate of exhaust flue gas mg, and inlet temperature of exhaust flue gas Tgi on ΔTe,opt and yopt are very slight. For comparison, three additional working fluids, namely R601a, R245fa, and 0.32R245fa/0.68R601a, are also taken into account.

The net power output of ideal supercritical organic Rankine cycle with different flow arrangement evaporators

The theoretical models of net power output for ideal supercritical ORC (organic Rankine cycle) with the evaporator of counter flow, parallel flow, and cross flow are, respectively, proposed. The effects of the ratio of heat capacity rates of heat source and working fluid, the number of heat transfer unit, and the ratio of the cycle high and low temperatures on the net power output of ideal supercritical ORC are addressed. The numerical simulation results of ideal supercritical ORC elucidate that the larger rate difference between the heat capacity of working fluids and heat source will help to improve the net power output. The net power output will be kept constant when the number of heat transfer unit reaches a certain value. In addition, supercritical ORC with counter flow evaporator exhibits the largest net power output while one with parallel flow evaporator does the lowest.

Performance comparison investigation on solar photovoltaic-thermoelectric generation and solar photovoltaic-thermoelectric cooling hybrid systems under different conditions

ABSTRACT The performance of solar photovoltaic-thermoelectric generation hybrid system (PV-TGS) and solar photovoltaic-thermoelectric cooling hybrid system (PV-TCS) under different conditions were theoretically analysed and compared. To test the practicality of these two hybrid systems, the performance of stand-alone PV system was also studied. The results show that PV-TGS and PV-TCS in most cases will result in the system with a better performance than stand-alone PV system. The advantage of PV-TGS is emphasised in total output power and conversion efficiency which is even poorer in PV-TCS than that in stand-alone PV system at the ambient wind speed uw being below 3 m/s. However, PV-TCS has obvious advantage on lowering the temperature of PV cell. There is an obvious increase in tendency on the performance of PV-TGS and PV-TCS when the cooling capacity of two hybrid systems varies from around 0.06 to 0.3 W/K. And it is also proved that not just a-Si in PV-TGS can produce a better performance than the stand-alone PV system alone at most cases.

Wind Effect on Combined Convection and Surface Radiation Heat Losses of a Fully Open Cylindrical Cavity With Insulation

Wind effect, of both the wind incidence angle and the wind speed, on convection and surface radiation heat losses of a fully open cylindrical cavity with constant bottom wall temperature was numerically investigated. The impacts of cavity tilt angle and wall temperature were also considered. Temperature contours, velocity contours, and vectors inside and around the cavity were presented. The variations of average convection and radiation heat loss Nusselt numbers Nuc and Nur and percentages of heat losses with related parameters (wind speed, wind incidence angle, tilt angle, and bottom wall temperature) were also shown. In the end, correlations about Nuc and Nur for practical applications were proposed. Results show that compared with no-wind condition, Nuc under a wind condition is almost always higher except for head-on wind with velocity of 1.5 m/s, while Nur is always lower. Nuc varies slightly, while Nur increases rapidly as the bottom wall temperature increases. With the existence of wind, the effect of tilt angle on heat transfer becomes more complex. A critical wind direction close to 30° is detected, which maximizes Nuc and percentage of convective heat loss. The results also demonstrate that wind speed, wind incidence angle, and tilt angle should be considered simultaneously when analyzing heat transfer inside the cavity under a wind condition.