Investigation On Measurement Methodologies Of Gas And Gas-solid Two Phase Average Velocities In A Short Duct

Pulverized coal (PC) fired boiler is the main unit for electric power generation in China. Combustion optimization is very important to improve the performance of PC boiler, which depends strongly on the measurement accuracy and reliability of combustion system. However, the flow rate measurement of combustion system is very difficult due to the limitation of the combustion equipment. The straight pipe is normally short for measuring flow rate in the air duct. This requires a real-time and accurate measurement method of the average flow rate of gas as well as a gas-solid two-phase flow in a pipeline with a lower pressure drop.The up-to-date structure, function and principle of the update flow meters used in the combustion system in PC boiler were investigated in this thesis. On accordance with the structure of the pipeline in the combustion system in PC boiler, the necessary condition to real-time and accurately measure the flow rate was suggested. Based on the comparison and analysis of these meters with different structures and principles, a novel average flow meter was suggested in gas flow or gas-solid two-phase flow. Its principle is local different pressure of fractional unit with parallel wings with distributed pressure measurement holes for gas flow or parallel back pipes with different lengths, where the three dimensions flow caused by the short straight pipe was straighten out by screen grid, and the local pressure are introduced to a common vessel. The average pressure was suggested to predict the flow rate, which is reasonable mathematically. The pressure resistant condition for the average pressure measuring as well as the screen grid size was deduced. The flow rectification and the reliability of the suggested flow meter were confirmed by the numerical simulations. And it was found that it is very insensitivity for installation deviation for the measurement, which indicates it could be used in the industrials. With the enlarged the different pressure by the primary unit, the flow meter was developed for gas and gas-solid two-phase flow. In the meter, the screen grid is distributed fully in the meter section. To further understanding the performance of the meter and improve continuously its structure, a cold test facility was designed and set up based on the scaling law and similarity conditions. And the experimental investigations were conducted.According to the cold test, it was found that the straight line wing is better with larger different pressure and lower pressure drop. And straight line wing with expanding angle of11°was selected for the meter. The experimental results showed that the pressure coefficient of the gas flow meter including the screen grid and the wings is very low ranging from0.08to0.13. With the decreasing of sectional area ratio m, the output of different pressure increases. The flow rate coefficient of the meter tends to a constant independent from Reynolds number when Reynolds number is higher than1×104. When the sectional area ratio m=0.4and0.5, the measurement error is extraordinary lower with more stable different pressure. The screen grid for rectification stabilizes the different pressure output with lower measurement error compared that without screen grid. The pressure measuring holes number and its distribution affects the measurement results. When the straight pipe is very short even0, the measurement error is less than2%with screen grid and reasonable wing with pressure measuring holes number and its distribution.The experimental results of gas-solid two-phase flow showed that the pressure coefficient of the gas flow meter of gas-solid two-phase flow is also very low ranging from0.1to0.3when sectional area ratio m decreases from0.18to0.04. The flow rate coefficient of the meter tends to a constant independent from Reynolds number when Reynolds number is higher than1×105. When the sectional area ratio m=0.4and0.5, the measurement error is extraordinary lower with more stable different pressure. When the straight pipe is2.5times diameter, the measurement error is less than2%. The additional error is less than1%caused by the angle less than6°departure. And the effect of the solid density in the gas-solid two-phase flow on the measurement can be ignored because it is very small.The flow meter suggested in this thesis was applied in the boilers for real-time measuring. The practices proved its accuracy and reliability with error of less than3%. The flow meter provides a reliable equipment for automatic optimization of combustion, leading to the boiler thermal efficiency increases1%with30%NOx reduction.