| Based on the conveying system with high concentration, the venturi performance in the conveying system, effect of operating conditions and structure parameters on the gas-coal mixture through venturi, the pressure drop models of venturi and the phenomenon of adhesion and accumulation have been studied on the basis of the application of venturi in the dense phase pneumatic conveying of pulverized coal system. The main contents are summarized as follows:1. The roles of the venturi in the pneumatic conveying of pulverized coal with the high density are studied. The results from the three systems show that the raise of pipelines’ pressure and the restriction of conveying could be realized by the venturi, compared to general system. The less variation ratio of the conveying capacity with conveying pressure drop is attained as well. Yet, the energy consumption increases. In the pilot system, the conveying capacity increases from 0.2 to 1.2 times, the particle velocity increases from 0.5 to 1 times and the particle concentration increases from 0.13 to 0.17 times when the diameter ratio increases from 0.29,0.36 to 0.43.2. The flow characteristics and pressure drop of the gas-coal mixture through venturi are detailed. The results show that a sharp decrease in static pressure and volumetric loading ratio observed inside venturi. The volumetric loading ratio is even high in diffuser section. When keeping the average throat gas velocity same, the inlet gas velocity of gas-coal mixture flows is lower than that of single-phase gas flow, while the outlet gas velocity is higher. The pressure drop of the gas-coal mixture increases with the increase of superficial gas velocity, volumetric loading ratio and gas density. The pressure drop of each section and venturi decrease with back pressure when keep the same inlet pressure and velocity. The location of recovery of static pressure moved to throat section. The pressure loss transited to pressure recovery in diffuser section. The location of pressure recovery moved to the venturi. The short time of the flow into the developed flow is occurred. The ratio of outlet particle velocity decreases with the increasing outlet pressure. A continuous increase in the downstream pressure and great fluctuation in the near outlet pressure is found. The increase trend in the latter is only occurred when the ratio of outlet pressure is greater than 0.7. No significant relationship between the upstream pressure and outlet pressure is observed.3. The effect of structure parameter on the mixture flow through venture is investigated.The similar trends but varied degrees in the non-dimensionless parameter distribution are observed. The pressure drop of venturi and the corresponding section increase with the coal concentration. The most remarkable influence parameter was diameter ratio, which had the greatest influence on pressure drop. The increase in pressure drop of venturi and significant increase in that of diffuser is observed. The pressure drop of venturi increases 3.6 and 0.8 times, and that of diffuser increases 3.1 and 10.3 times, with a decrease of diameter ratio from 0.7,0.55 to 0.4. In the ranges of structural parameters, the little influence of convergence angle, throat length and diffuser angle on the total pressure drop are occurred. The pressure drop of venturi increases 7% and 15% when the convergence angles varied from 5 degree,9 degree to 2.5 degree. That increases 2.5% and 4% when the diffuser angles varied from 8 degree,13 degree to 2.5 degree. Compared to 23 d, the total pressure respectively increase 6% and 11% in 43d and 80d, even though the throat length increases 0.9 and 2.5 times. The greatest convergence pressure drop for the convergence angle series occurred in the 2.5 degree. The least diffuser pressure drop for the diffuser angle series occurred in the 8 degree. The greatest throat pressure drop for the throat length series occurred in the 80d. Under the condition of high concentration, there was little difference of convergence pressure drop between 5 degree and 9 degree, which resulted from the little decrease of pressure drop with the decrease of convergence length, as the remarkable collision and friction between particle and particle besides particle and wall occurred in the convergence section. The pressure drop of diffuser in the 13 degree is greater than 8 degree and approaching to 2.5 degree, which results from the short momentum exchange time between particles and gas, as the greater number of deceleration particles occurred in the diffuser section.4. Based on the large amount of experimental data, the pressure drop models from the system of typical gas-liquid through venturi, dense phase gas-solid through general pipe and dilute gas-solid metering are evaluated for the high-concentration gas-solid mixtures through venturi. The much simpler expressions but greater deviations of three empirical formulas from dilute gas-liquid system are found. Considering the acceleration of particles along the total length and frictional pressure drop of two-phase, the physics meaning of Boll model is most significant. Yet, the results from Boll model is not all accorded with the system. Meanwhile, the results is numerical solution, and its form is relatively more complex. By introducing the method of additional pressure drop and solid momentum flux, the deviation is within ±30%. While the physics meanings of two models are not clear. By adopting the relations between pressure drop and solid mass flowrate based on the modified St and considering the high pressure and concentration, the deviation is between 30% and-10%, even though the deriving process and final expression is relatively complex.5. The adhesion and accumulation phenomenon occurred in the venturi was revealed by the analysis of the accumulations. The weight and sizes of the accumulations increased with the increasing conveying time. The decrease of conveying capacity and increase of pressure drop proportion of total venturi and convergence section were observed within the same system pressure drop. Compared to BS coal, the relatively more content of mineral and major mineral element besides the clay minerals mainly composed by the kaolin in the accumulation was observed. The inner layer of the accumulations was best flat. The least mineral element while the most carbon element in the outer layer was obtained. |
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