窄缝环形管内汽液两相流型及流动沸腾传热特性研究/Researtch on Vapor-liquid Two-phase Flow Pattern and Con
During recent years, boiling in narrow spaces has become a research subject of great importance as this phenomenon occurs in many practical situations such as Integrated Nuclear Power Device (INPD), compact heat exchangers, motor cooling and electronic component cooling. The narrow gap annulus tube studied in this thesis is a new kind of element of steam generator installed in an INPD. For the reason of design accuracy and operating safety, both the hydrodynamic and heat transfer characteristics of upward flow in narrow gap annulus tubes are studied theoretically and experimentally.In this paper, experiments were carried out to investigate the characteristics of single-phase forced convection in upward narrow gap annulus tubes with the gap size of 1～2.5mm, and also the frictional resistance characteristics were studied. The experimental results showed that transition Reynolds number is in the range of 500～1000. The heat transfer coefficients in narrow gap are generally higher as compared with round tubes, and the enhancement increases with the gap size decreases. Maximum enhancement is obtained for narrow gap with size of 1mm. When both sides of the gap are heated, heat transfer coefficient of inner tube wall is much higher than that of outer tube wall. Equations both for calculating frictional pressure drop and heat transfer coefficients of single-phase flow in annulus gap are presented from experimental data.Convective boiling heat transfer characteristics and pressure drop in annulus gaps are studied experimentally as considering the effect of gap size, mass flow rate and heat flux et al.. Vapor-liquid two-phase flow pattern is obtained by optical experiment. Five flow regimes are defined and flow pattern map is developed to distinguish the different boiling region. The research showed that due to the limitation of narrow space the two-phase flow regime changes, and the boiling heat transfer is enhanced obviously. Boiling heat transfer coefficients are calculated respectively for saturated nucleate boiling region and forced flow heat transfer region. While calculating the heat transfer coefficients for saturated nucleate boiling region according to the asymptotic model, the concept of boiling angle is introduced to consider the effect of various factors on heat transfer. The results of the deduced expression of heat transfer coefficients are in good agreement with the experimental data.In theoretical analysis, the heat transfer mechanism was studied taking into account the effect of the bubble behavior and two-phase flow regime in narrow gap annulus tubes. We think the reason of heat transfer enhancement is due to the facts that bubbles continually remove the superheated liquid near the tube wall, and also the decrease of the thickness of liquid film reduce the heat transfer resistance. Finally on the base of the former explanation a theoretical model for heat transfer enhancement in narrow gap is developed, and the results predicted by this model well agree with the experimental results of heat transfer enhancement.