Effects Of Flow And Heat Transfer On Thermal Stress Of Water Wall Special-shaped Fin Region In Ultra-supercritical Boiler

Transverse cracks can be generated in the vertical water wall of supercritical boilers during actual operation,especially in the special-shaped fin region,which may lead to undesired leakage and even explosion.A hydrodynamic calculation model taking the water wall of a 660 MW ultra-supercritical boiler as the research object,along with a temperature and a thermal stress numerical model for the special-shaped fin region,was established in this work.The effects of flow and heat transfer on the thermal stress of the special-shaped fin region were calculated and analyzed.Using the existing standard methods for hydrodynamic calculation of power plant boilers,a hydrodynamic calculation model for the studied object was established.This model can calculate the pressure,flow,temperature,and enthalpy of all pipes from furnace distributor to ceiling outlet collecting header.Considering the uncertainty of the phase transition point and the non-uniformity of the furnace thermal load,integral phase transition point height calculation and sectional furnace thermodynamic calculation were performed.Calculations respectively for the 50%THA condition and the 30%VWO condition calculations were finished.The tube panel flow,phase transition point height,and metal temperature were then discussed by comparing these conditions.The results show that there is a change of 45 ℃ ～ 60 ℃ between the burner area and the adjacent tube panel at 38 m under these two conditions.The phase transition region and the transverse crack height are highly coincident.For these two operating conditions at 38 m and 51 m,the fluctuation of the phase transition point height even approaches 3 m during the peak-shaving stage.The maximum temperature change of the apex of the fire side is 60 ℃,which may cause fatigue thermal stress and transverse cracks.At present,there is no analysis on the thermal stress of the special-shaped fin region.Taking the complex structure of the special-shaped fin region into account,a 3D model was established.The standard k-ε model was selected in the temperature field model,while the unidirectional fluid-structure coupling was used to calculate the thermal stress.Based on calculation results,both the temperature and the thermal stress characteristics were analyzed.The influence of varying load operation and water wall geometry was also considered.The results show that both the axial tensile stress and equivalent stress of the tube wall beside special-shaped fin to the apex of the fire side are large,and the alternating load of the boiler is prone to alternating thermal stress,resulting in transverse cracks.The highest temperature of the whole model is located at the center of the water wall special-shape fin.Axial stress and equivalent stress are also large in this region.The largest equivalent stress is located at the connection point of the special-shape fin and the tube wall,implying material failure and cracks.The cracks may extend to the fireside tube of the special-shape fin region,which accelerate the generation of transverse cracks.The research results provide a theoretical basis for preventing transverse cracks and improving water wall system safety from the aspects of design and operation.