Study On Flow And Heat Transfer Characteristics Of Vapor-liquid Two-phase Flow In Helical Coil Tube Once-through Steam Generator

Helical coil tube is widely used in the design of small volume heat exchangers in industry because of its compact structure,high heat transfer efficiency and good safety features.However,systematic analysis of heat transfer and flow characteristics of helical coil tube is of insufficiency in the published literatures.In particular,for the circumferential non-uniformity in helical coil tube,the current research is limited to the single-phase scenarios.Characteristics and correlations about onset of nucleate boiling(ONB),subcooled boiling heat transfer,and post dryout heat transfer had hardly mentioned and flow instability in parallel channels of helical coil tubes has rarely been reported as well.All these bring uncertainty to the design analysis and application of the helical coil once-through steam generator and affect its operation safety and life assessment.To address the insufficiency,the circumferential heat transfer characteristics of single-phase heat transfer,ONB,subcooled boiling,saturated boiling,dryout and post dryout heat transfer are studied sequentially in this paper.A dimensionless Lu number is proposed to reflect the relative strength of buoyancy and centrifugal force.Based on the Lu number,it is possible to predict circumferential non-uniformity of single-phase heat transfer,ONB and subcooled boiling in helical coil tube.The circumferential non-uniformity of saturated boiling in helical coil tubes is primarily affected by both nucleate boiling and convection boiling mechanisms.The nucleate boiling mechanism dominates longer in the low heat transfer circumferential region of the tube than convection boiling mec hanism,while in the high heat transfer region the condition is contrary.The order of circumferential dryout in helical tubes is independent of the wall temperature during saturated boiling,and there are chiefly three types of dryout in helical coil tube.When the flow rate is low,the dryout starts from the upper side of the tube and spreads along the outer side to the inner side;As the flow rate increases,the dryout occurs at upper and lower sides of the tube simultaneously and spreads to the middle of the tube.When the flow rate is high,the perimeter of the tube wall dries out simultaneously.The circumferential non-uniformity in post dried out region is affected by the order of drying out in circumferential direction,and the temperature is highest at the first dry out circumferential position.The effects of thermal-hydraulic and helical tube structure parameters on the heat transfer in helical coil tube are also studied in this paper.In the experimental range,increas ing the hydraulic diameter,reducing the coil diameter,increasing the system pressure together with mass flow rate and heat flux can increase the average heat transfer coefficient of the whole process in helical coil tube.In addition,it is found out that the two-phase frictional pressure drop increases first with the increase of quality and then decreases after reaching the peak value in the study of flow characteristics in helical coil tubes.An increase in system pressure and a decrease in mass flow rate can reduce the two-phase frictional pressure drop.The heat flux and helical tube structure have no significant effect on the two-phase frictional pressure drop.The flow instability of parallel helical coil tubes channels can be suppressed by reducing the power flow ratio and increas ing the inlet resistance.When the system pressure and inlet resistance are low,the stability of the system increases with the increase of inlet subcooling.As the system pressure and inlet resistance increase,the influence of inlet subcooling on system stability becomes complicated.The stability enhancement due to system pressure rise will decrease with the increase of inlet subcooling.The period of oscillations to transit time ratio of the helical parallel channel is in the range of 1.38?2.28,whic h is the same as period of density wave oscillations to transit time ratio in the straight parallel channel.Through the systematic research,this paper suggests perfected empirical correlations for different heat transfer and flow zones in helical coil tube and new empirical correlations are proposed for the ONB,post dryout heat transfer and two-phase frictional pressure drop.Furthermore,a one-dimens ional thermo-hydraulic code for helical coil once-through steam generator on the basis of RELAP5 and these correlations is developed.This code was used to calculate the inlet throttling coefficient and flow and power distributions between the tube bundles of the CAP50-S helical coil once-through steam generator which provides theoretical and practical guidance for the preliminary design of the CAP50-S helical coil steam generator.