Numerical studies on capability to focus solar radiation with mirrors of different curvatures

The article presents the methodology and results of a parametric analysis performed to investigate the effects of changes in the curvature of the solar concentrator mirror on its ability to focus the radiation. Working conditions of the concentrator, i. e. possible values of radiation intensity, were adopted according to irradiance typical for Poland, and, therefore, similar to the conditions in many European countries. The curvatures of examined mirrors were obtained by modification of a parabolic curve. The calculations were conducted for two cases: when the Sun radiation falls completely directly and when it's half diffuse. The 2-D simulations were conducted in ANSYS FLUENT 18.2 software. Discrete ordinates model was employed to simulate radiation phenomenon. Also, a sensitivity analysis was carried on discrete ordinates model parameters and density of the computational mesh. The results allow stating that some of the new curvatures provide only slightly worse focus than the classical parabolic shape, but also show greater insensitivity to the increasing share of diffusive part of radiation. The presented model is a quick and proven tool for testing new curvatures of solar concentrator mirrors.

A Comparative CFD Study on Simulating Flameless Oxy-Fuel Combustion in a Pilot-Scale Furnace

The current study presents a method to model the flameless oxy-fuel system, with a comparative approach, as well as validation of the predictions. The validation has been done by comparing the predicted results with previously published experimental results from a 200 kW pilot furnace. A suction pyrometer has been used to measure the local temperature and concentrations of CO, CO2, and O2at 24 different locations. A three-dimensional CFD model was developed and the validity of using different submodels describing turbulence and chemical reactions was evaluated. The standardk-εmodel was compared with the realizablek-εmodel for turbulence, while Probability Density Function (PDF) with either chemical equilibrium or the Steady Laminar Flamelet Model (SLFM) was evaluated for combustion. Radiation was described using a Discrete Ordinates Model (DOM) with weighted-sum-of-grey-gases model (WSGGM). The smallest deviation between predictions and experiments for temperature (1.2%) was found using the realizablek-εmodel and the SLFM. This improvement affects the prediction of gaseous species as well since the deviation between predictions and experiments for CO2volume percentages decreased from 6% to 1.5%. This provides a recommendation for model selections in further studies on flameless oxy-fuel combustion.

Analysis and Optimization of the Fan-Pad Evaporative Cooling System for Greenhouse Based on CFD

A CFD model was presented to simulate the distribution of air velocity and temperature in a greenhouse adopting the fan-pad cooling system in summer. The Boussinesq hypothesis was applied for the simulation of gravitation; the k-ε turbulent model and discrete ordinates model were selected to predict the distribution of air velocity and temperature inside greenhouse using the commercial software Fluent. The differences between simulated and measured air temperature varied from 0.9 to 4°C and the differences of air velocity were less than 0.15 m/s, which proved that the CFD method can estimate the distribution of air velocity and temperature in the greenhouse rationally and effectively. The validated CFD model was then used to evaluate the cooling effect and design the installment of fan and pad in terms of the crop size. The results implied that Case 3 and Case 5 should be chosen when the height of crop canopy varies from 2 m to 3 m. When it varies from 1 m to 2 m, all the cases can be effective except Case 1. When the canopy height is below 1 m, all the cases can be selected. This paper suggested that the CFD model can be used as an optimal tool for fan-pad evaporative cooling system in the greenhouse.

A Model for Radiative Cooling of a Semitransparent Molten Glass Jet

A mathematical model for the radiative cooling of a semitransparent molten glass jet with temperature-dependent viscosity has been developed and is implemented numerically. The axial velocity and jet diameter variations along the length of the jet, the axial bulk mean temperature distributions, and the centerline-to-surface glass temperature distributions are determined for different processing conditions. Comparisons are also made between the semitransparent predictions, which are based on a spectral discrete ordinates model, and predictions for an opaque medium.