Experimental studies on density wave oscillation instability of two-phase flow in parallel ver-tical rifled tubes were conducted on two-phase high pressure test loops. The main characteristics of density wave oscillation instability were perceived. In the range of test parameters, the effects of heat flux, pressure, mass velocity, inlet sub-cooling, asymmetric heat flux on the density wave oscillation instability of two-phase were explored and analyzed. Based on the test result and using the homogeneous model, the threshold rela-tional expressions of density wave oscillation instability were obtained.

In the light of hydrodynamic characteristics of waterwall tubes of 600 MW supercritical circulating fluidized bed(CFB) boilers at low mass flow rate, experimental studies were carried out on instability of two-phase flow in parallel vertical rifled tubes.Hydrodynamic characteristic curves and two typical oscillation curves of pressure drop and density wave under different conditions were obtained.The effects of system pressure,mass flow rate,inlet degree of subcooling as well as upstream compressible volume on the instability of two-phase flow were analyzed within the range of experimental parameters.Meanwhile,the threshold relational expressions of instability were obtained using homogeneous model according to the test results,providing a basis for the design and safe operation of waterwall with parallel vertical rifled tubes of supercritical CFB boilers.

The experiments on two-phase flow instabilities in parallel vertical internally ribbed tubes were conducted to investigate the hydrodynamic flow characteristics under low mass flux conditions in a 600 MW super-critical W-shaped flame boiler.Two kinds of oscillations,pressure drop oscillation(PDO) and density wave oscillation(DWO),were observed.The influences of pressure,mass flux,inlet sub-cooling,compressible volume,exit throttling,and asymmetric heat flux on the two-phase instability were explored and analyzed.The results show that the PDO occurred at lower exit quality and in the negative slope portion of the Δp-G curves with the in-phase oscillating of both channel flow rates,and DWO occurred at higher exit quality and in the positive slope portion of the Δp-G curves with the out-phase oscillating of channel flow rates.Based on the testing results and the homogeneous model,the threshold expressions of instability were obtained.It is suggested that these findings may be used in the water wall design and its safe operation in super-critical boilers.

Air side heat transfer and fluid flow characteristics of four-row slotted fin surfaces with arc-type and X-type strip arrangement in pressurized air cooler were studied by visualization experiments.Experiments were carried out with PIV(particle image velocimetry)system and infrared thermo tracer to visualize the flow field and the temperature distribution of two slotted fin surfaces.The experiment results showed that the fluctuation intensity and pressure drop of the X-type slotted fin surface were higher than those of the arc-type slotted fin surface.However,the heat transfer performance and field synergy property of the X-type slotted fin surface were superior to those of the arc-type fin surface.A new definition of fluctuation intensity was proposed to indicate the magnitude of flow disturbance enforced by the configuration and arrangement of slotted fin surface.The higher the fluctuation intensity,the larger the velocity gradient and the vorticity magnitude.The visualization experiment results provide a reliable experimental support for later numerical simulation and optimization design for new fin structures.

Air side heat transfer and fluid flow characteristics of slotted fin surfaces with arc-type and X-type strip arrangement in a pressurizing air cooler were studied by visualization experiments.Experiments were conducted out by a particle image velocimetry(PIV) system and an infrared thermo tracer to visualize the flow field and the temperature distribution of two slotted fin surfaces.The experimental results show that the pressure drop of the arc-type slotted fin surface is lower than that of the X-type slotted fin surface and the former contributes more to improving the heat transfer performance at the back of the tube.However,the integrated heat transfer performance and field synergy character of the latter are superior to those of the former.According to the merits of the two slotted fin surfaces,a new type slotted fin surface(X-arc type) was designed.Heat transfer and fluid flow characteristics of three slotted fin surfaces were simulated and the results agree well with the experiments data.Numerical simulation results show that the field synergy character and heat transfer performance of the optimized fin surface are better than those of the other two slotted fin surfaces.Within Reynolds number range from 800 to 3 400,the integrated heat transfer performance of the new designed fin surface is about 7%-15% higher than that of arc-type fin surface and 3%-9% higher than that of X-type fin surface.