| The vertically arranged sinter cooler (VASC) has been proposed as one of the promising approaches toward energy efficiency improvement for sintering procedure in iron and steel industry, as the design would be able to lower the air leakage, and hence to improve the heat recovery efficiency. The characteristic of gas-particle flow and heat transfer is one of the key issues corresponding to the feasibility of these devices. This work aims to have a better unstanding on the particle characteristics and gas flow in the VASCs. Experiments were performed on the pressure drop in the packed beds of sintered ore particles (SOPs), based on the obtained particle characteristics and thermophysical properties of SOPs.Industrial SOPs sampled from an iron and steel plant were characterized to obtain a comprehensive set of fundamental structural and thermophysical properties. The SOPs investigated were identified as self-fluxed sinters having a basicity of 1.175 after composition analysis, and two peaks with the ranges of 15-40μm and 160-250μm were observed on the pore size distribution. The specific heat capacity of the SOPs at elevated temperatures up to 200 ℃ was measured, with a power-law fit curve being established for extrapolation. The thermal conductivity of the SOP samples with various porosities between 0.12 and 0.24 was determined experimentally, while a Sierpinski-carpet-based fractal model was also utilized to predict the multi-sized porous structure and effective thermal conductivity as a function of porosity. The fractal model was validated by the experimental results, which can be applied over a wide range of porosity as 0.036-0.326, with an average relative error of 9.72%.Experiments on the pressure drop over packed beds for SOPs with bed-to-particle diameter ratio ranging from 7 to 35 and glass beads with the ratio between 6 and 22 were performed to study the wall effects on pressure drop. The results revealed that the voidage effect tends to be pronounced for relatively low ratio values, while the friction effect becomes more significant for relatively high ratio values when it is lower than 40. Comparing to the confining walls, the particle characteristics were shown to have a dominant effect on the flow regime in the voids of the SOP-filled beds, whereas an opposite comparison was observed for the smooth glass beads as a reference. The correlations for the wall corrected friction factor proposed in the literature (Exp. Heat Transfer 12 (1999) 309), with varying coefficients as the function of packed bed parameters, fit well to predict the pressure drop through packed beds for both SOPs and glass beads with considering the wall effects. Improved correlations were also proposed for SOPs and spherical particles with the predictive relative error less than 17%.Pressure drop for the packed beds with various mix ratios of SOPs amongst the sizes of 30-40 mm,40-50 mm and 50-60 mm has been studied experimentally in order to investigate the effects of the size distribution on the packed bed characteristics and gas flow resistance. The flow transits early as the mixture of the particles aggravates the inhomogeineities of the packed beds and results in a smaller voidage of the beds. The wall corrected friction factor for mixed SOPs decreases comparing with the monosized ones, with lower values of viscous and inertial coefficients. Also, the value of the coefficients shows a great scatter, and a different trend of viscous coefficient was observed comparing to the mono-sized beds.Experiments for packed beds of different particle shapes were performed, including glass beads, wooden cylinders, irregularly shaped white marble with smooth surfaces, irregularly shaped crushed rock with rough surfaces and irregularly shaped SOPs with various pores. With the particle shapes ranging from regular to irregular ones, the critical velocity for different flow pattens increases. And, wall effects seem unapparent on the flow pattens comparing with the shape variation. The equation in the form of fv=A+BRemn could give a satisfied prediction for the pressure drop in high Reynolds numbers, with a maximal relative error of 12%-15%. As revealed by the equation of Raichura, the proportion of the wall friction on the total viscous term increases to a constant value when the particle sphericity increases form 0.6 to 1.0. Wall friction has a greater effect on the packed beds with regular particles. When the sphercity decreases, the portion of the near wall "annular region" of higher voidage reduces, and so do for the wall effects on the inertial term. Thus, Raichura’s equation could not give an accurate description of the higher voidage "annular region" for the particle spheritity between 0.6 and 0.8. It is necessary to improve the applicability of the inertial constant values in Raichura’s equation. |
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