A study of flow boiling phenomena using real time neutron radiography / Academic Article uri icon

  •  
  • Overview
  •  

abstract

  • The operation and safety of both fossil-fuel and nuclear power stations depend on adequate cooling of the thermal source involved. This is usually accomplished using liquid coolants that are forced through the high temperature regions by a pumping system; this fluid then transports the thermal energy to another section of the power station. However, fluids that undergo boiling during this process create vapor that can be detrimental, and influence safe operation of other system components. The behavior of this vapor, or void, as it is generated and transported through the system is critical in predicting the operational and safety performance. This study uses two advanced penetrating radiation techniques, Real Time Neutron Radiography (RTNR), and High Speed X-Ray Tomography (HS-XCT), to examine void generation and transport behavior in a flow boiling system. The geometries studied were tube side flow boiling in a cylindrical configuration, and a similar flow channel with an internal twisted tape swirl flow generator. The heat transfer performance and pressure drop characteristics were monitored in addition to void distribution measurements, so that the impact of void distribution could be determined. The RTNR and heat transfer pipe flow studies were conducted using boiling Refrigerant 134a at pressures from 500 to 700 kPa, inlet subcooling from 3 to 12°C and mass fluxes from 55 to 170kg/m 2 -s with heat fluxes up to 40 kW/m2 . RTNR and HS-XCT were used to measure the distribution and size of the vapor phases in the channel for cylindrical tube-side flow boiling and swirl-flow boiling geometries. The results clearly show that the averaged void is similar for both geometries, but that there is a significant difference in the void distribution, velocity and transport behavior from one configuration to the next. Specifically, the void distribution during flow boiling in a cylindrical-tube test section showed that the void fraction was largest near the tube center and decreased with increasing radial distance. For swirling flow, the void concentration was highest in the center of each subchannel formed by the twisted tape insert, producing two local void maxima at each axial position. Furthermore, the instantaneous RTNR results show that the effects of bubble agglomeration change from one geometry to the next. To further examine the application of RTNR for void distribution measurement, both vertical and horizontal orientations were examined. These experimental results show similar cross sectional averaged axial distributions of the void fraction but significant differences in the local void behavior. The HS-XCT experiments were conducted on swirl-flow boiling of Refrigerant 123 at similar conditions as the RTNR experiments. These tests were conducted to qualitatively compare and verify the void distribution and behavior obtained using RTNR techniques. The HS-XCT results verify that during smooth flow boiling in a vertical tube the void tends to concentrate in the center of the channel and decrease outward to the channel walls. For swirl flow, the void tends to concentrate near the center of each subchannel formed by the twisted tape. Furthermore, wall region void fraction for smooth-flow boiling was significantly higher than swirling flow conditions due to the significant centrifugal forces present in swirl-flow. These centrifugal forces may improve the heat transfer and dryout behavior during swirl-flow conditions. This work contributes to the development of two-phase flow diagnostics based on penetrating radiative techniques, i.e., RTNR and HS-XCT for void distribution measurement, and enhances the knowledge of flow boiling systems. The application of HS-XCT and RTNR for the study of flow boiling phenomena using smooth and swirl-flow geometries has clearly demonstrated that differences in local void distribution result in differences in heat transfer behavior.

publication date

  • September 2000