Study On The Failure Mechanisms Of The Functionally Graded Ceramic Coal Water Slurry Nozzles

The nozzle is one of the most critical parts in coal water slurry (CWS) burning system. In this paper, the researches were carried out on the internal flow field's pressure, velocity, concentration inside CWS nozzle. The temperature field, thermal stress field and worn surfaces of the functionally graded ceramic CWS nozzle were analyzed. The failure mechanism of the functionally graded ceramic CWS nozzle was investigated.With the theories of multiphase flow and computational fluid dynamics(CFD), the internal flow of ceramic CWS nozzle was numerically simulated. Results showed that pressure of the flow field decreased gradually along the nozzle's axis from entry to exit, the maximal pressure appeared at corner of the entry. The coal water slurry and the erosion particles moved in a accelerated motion, their radial velocities distribution abided by the parabola rule at the nozzle's exit side,and the maximum velocities appeared in the hole centre of nozzle. The concentration of the erosion particles at the corner of nozzle's entry was higher than those of other zones, and the maximal concentration of the erosion particles inside nozzle with right corner at entry was higher than that of nozzle with round corner.The thermal characteristics of the functionally graded ceramic CWS nozzle were analyzed with the finite element method (FEM). Results showed that radial temperature distribution at the exit surface of the nozzle took on a parabola shape, and the temperature inside nozzle was lower than that outside nozzle. The radial thermal stress at the nozzle's exit surface decreased firstly, then increased gradually from inside to outside. The maximal thermal gradient and thermal stress appeared in nozzle's exit. The maximal thermal stress of the functionally graded ceramic nozzle was lower than that of the homogeneous ceramic nozzle.Wearing experiment of the functional graded ceramic CWS nozzle was carried out. The eroded bore surfaces of the nozzle were examined by scanning electron microscope (SEM). The failure mechanism of the functionally graded ceramic CWS nozzle was discussed. Results showed that the impact of the rigid particles was the main reason that caused the failure of ceramic nozzle's entry, and the thermal shock damage of nozzle's exit was owing to greater thermal graded and thermal stress at the exit zone. Different parts of the nozzle showed different surface morphologies and different failure mechanisms. Many pits generated at the entry zone of the ceramic CWS nozzle, it appeared that the brittle fracture was the main failure mechanism. The smooth worn surface at nozzle's center zone revealed that the primary failure mechanism was polished by abrasive particles. Some cracks and grain pullout appeared at the nozzle's exit zone, it was suggested that the main failure mechanism was thermal cracking and collapsing. The compressive residual stress at the nozzle exit that was formed in fabricating process of the functional graded ceramic CWS nozzle could partially counteract the thermal stress generated in CWS burning process. The functionally graded ceramic CWS nozzle has superior thermal erosive wear resistance compared to the homologous stress-free ceramic nozzle.