Study On Multiphase Flow And Numerical Simulation In Composite Quench Chamber

The study focused on quench chamber which was one of the critical components in an entrained-flow coal gasifier. The atomization characteristics and gas temperature distribution in composite quench chamber were mainly studied. The effects of gas velocity, liquid flow and nozzle number on droplet diameter, droplet concentration, droplet entrainment and pressure distribution were investigated. Mathematical model of flow field and temperature field in composite quench chamber were established using numerical simulation. The feasibility of engineering application of composite quench chamber was analyzed. Main contents and results were summarized as follows:(1) According to industry data, the temperature distribution in dip tube of industrial-scale quench chamber had been predicted using numerical simulation. The simulation results indicated that the significant temperature drop occurred at the 0-2 m near dip tube inlet, then the decreasing rate became slower at 3-6m. Based on the research, quench chamber was optimized by reducing the length of dip pipe and using spray cooling in order to solve the problem of dip tube plug. A experimental device of cold mode had been built and the effects of gas and liquid flow rate on pressure distribution had been studied. Results showed that pressure distribution was uniform in quench chamber in dry tower due to the small effect of bed resistance. In wet tower, the atomized droplets and cross flow had a disturbance on the bed pressure, resulting in the increase of pressure drop nearly 50%.(2) The droplet entrainment at the gas outlet of the composite quench chamber was studied The effects of superficial gas velocity, nozzle flow velocity, cooling water flow and radial position on droplet entrainment were investigated. The experimental results showed that droplet entrainment was increased with the increase of superficial gas velocity and liquid flow velocity, while there was no significantly change in the radial direction of chamber. The increase of liquid flow velocity could reduce the distribution gradient of droplet size. The smaller droplet was easily entrained and droplet entrainment fraction was increased. However, smaller droplets were aggregated into larger droplets in the gas flow process to ensure lower droplet entrainment rate. The increasing nozzle number can make the droplet well-distributed and reduce droplet entrainment fraction. The droplet entrainment was decreased about 40% when the number of nozzles increased from two to four. Droplet entrainment fraction was affected by the superficial gas velocity and liquid velocity. By the experimental data, the empirical correlation between droplet entrainment fraction and experimental conditions was achieved as shown below.(3) The atomized droplet size was measured by Malvern laser particle size analyzer for the new composite quench chamber. The droplet size decreased with the increasing liquid velocity. However, the droplet size at gas outlet increased with the increasing gas flow velocity. The droplet size gradient became larger with more nozzles resulting from dramatic effect among droplets. Droplet size was larger close to the radial position r/R=0.3 of atomized nozzle and droplet size was smaller in the quench chamber center. Droplet dispersion at axial position in the quench chamber was mainly relied on the gas flow carrying. With increasing axial distance, liquid droplet size of gas carrying became smaller, and the carrying critical droplets size is about 80～150μm at gas outlet.(4) The isokinetic sampling method was used to study droplet concentration distribution under gas-liquid mixing process in composite quench chamber. Through comparative analysis of different spray nozzle combination and gas-liquid flow, droplets concentration distribution has been obtained. The increasing liquid velocity was found to enhance the cross flow inductive effects and promote the droplets to spread to chamber wall. The increasing gas velocity enlarged the energy of the vortex and promoted the droplet to spread down along the axis. The droplet mass flux of four nozzles was almost twice as much as that of two nozzles in the same space. The results showed that increasing nozzle number could make the droplet well-distributed and eliminate dead zone. When four nozzles opened, the dead zones almost disappeared. According to the experimental results, the empirical formula about spray droplet concentration at radial position in composite quench chamber was obtained as shown following.(5) A flow and heat transfer 3-D model was built for the analysis of flow and temperature field in quench chamber. Single nozzle had a narrow range of atomized flow, temperature distribution was not uniform in quench chamber, and therefore the cooling effect was unsatisfactory. The droplet size became smaller with the increasing liquid flow rate, which was benefited for heat and mass transfer. Under the same heat flow, gas temperature was increased with increasing liquid flow rate. There were a large number of liquid droplets in the upper space of quench chamber. The gas temperature was low and the cooling effect was good. The lowest temperature of synthesis gas was obtained at axial position h/H=0.72. Near the bottom of quench chamber, liquid phase fraction was increased, reducing the effect of spray cooling. The effect of gas-liquid cross flow improved the efficiency of the evaporation of liquid phase and strengthened the effect of heat and mass transfer.