Research On Flow And Heat Transfer Enhancement Of Supercritical Water In The Large-capacity Heat Exchanger

The supercritical water is used in large heat exchanger, e.g. station boiler and nuclear reactor, to increase the thermal parameters at the outlet. However the special thermal characteristics in the so called Large Specific Heat Region and the heat absorption capacity in the high enthalpy region restrict the usability of the supercritical water. Therefore the investigation on the heat transfer enhancement of supercritical water under high temperature condition is very important. In this paper, the heat transfer enhancement of supercritical water is studied by using the commercial CFD code Fluent, aiming at the large capacity heat exchangers, such as the station boiler and nuclear reactor. The effects of the flow-disturbing devices, internal ribs and grid spacer in specific, on the heat transfer characteristic are discussed in detail. The objective has focused on the heat transfer differences in the radial and circumferential directions of flow channels and revealing the corresponding mechanism.For the rifled tube, the simulation results show that the local heat transfer in the circumferential and radial directions of the tube are not uniform. The average heat transfer near the rib top surface is much better than that near the substrate surface. The bulk enthalpy variation has little effect on the basic structure of the secondary flow. The individual vortex formed on the rib top surface creates a relative low-pressure region and augments the heat and mass transfer near this region. The heat transfer heterogeneity is more pronounced in the great specific heat region due to the drastically changed physical properties. Because the substrate is the weak point of the heat transfer, the internal structure of the tube in this region must be optimized.For the sub-channel of supercritical reactor core, the simulation results show that the circumferential distribution of cladding temperature, which indicates that the highest and the lowest value appears at the narrow gap region the sub-channel center respectively, is not uniform due to flow choking effects. The local heat transfer is greatly enhanced inside the grid spacer strap due to the increased flow velocity. The standard grid spacer is negative for decreasing cladding temperature because it increases circumferential difference of cladding temperature in its downstream region. The local heat transfer is enhanced just downstream of the grid spacer with split-vanes. However, big circumferential temperature difference and a decreased heat transfer region are also caused by the swirling flow, indicating that the swirling flow has adversely affected the local heat transfer. The improved heat transfer performance downstream of the grid spacer with split vanes is more pronounced for the higher Reynolds number case. The decreased heat transfer just downstream of the grid spacer might be an unique feature of the tight rod bundle and should be considered seriously during the design process.