Researches On Flow And Heat Transfer Of Supercritical Carbon Dioxide Natural Circulation In Closed Loop

Supercritical fluids are considered as the most attractive working fluids due to their outstanding thermodynamic properties.The advanced power cycles with supercritical fluids as the working medium have attracted more and more attention in the field of thermal engineering.As a passive heat transfer mode,natural circulation has the advantages of simple structure,high reliability,safety and stability,and great application potential,due to it eliminates the mechanical pumping devices.In this paper,supercritical CO₂ is taken as the research object,and the natural circulation experiment and numerical simulation of supercritical CO₂ are systematically carried out to analyze its flow and heat transfer characteristics,and to develop high-precision deterioration criterion and heat transfer correlation.The research results can provide theoretical guidance for the design and optimization of supercritical CO₂ power cycle.The main contents are as follows:1.Firstly,a high-pressure CO₂ closed loop natural circulation experimental system was built,and the effects of pressure,cooling temperature,filling rate and heating power on natural circulation heat transfer were discussed in detail.The heat transfer mechanism in the loop under subcritical pressure and supercritical pressure was analyzed.From subcritical pressure to supercritical pressure,the heat transfer mechanism of the loop changes from two-phase boiling to single-phase convection,and the heat transfer characteristics are obviously different.2.To further reveal the flow and heat transfer characteristics of CO₂ natural circulation under supercritical pressure,the experimental system was improved to meet the requirements of more in-depth data analysis,and the effects of mass flow rate,CO₂ inlet and outlet temperature and pressure on heat transfer were discussed.The supercritical CO₂ is sensitive to system operating parameters.Two heat transfer modes can be observed,normal heat transfer and heat transfer deterioration.3.Different heat transfer modes were analyzed,and the existing supercritical fluid deterioration criteria were evaluated.By introducing the dimensionless parameter SBO,a heat transfer deterioration criterion suitable for supercritical CO₂ natural circulation was developed,which can accurately distinguish between normal heat transfer and heat transfer deterioration.4.By comparing and analyzing several typical supercritical CO₂ heat transfer correlations,the K number was introduced into the natural circulation heat transfer correlation as a correction term,and a heat transfer correlation suitable for supercritical CO₂ natural circulation was developed.5.A numerical model of supercritical CO₂ natural circulation was established.The loop was operated to a parameter range that could not be covered by the experiment,and the natural circulation flow and heat transfer characteristics under a wider parameter range were explored.The improved heat transfer correlation was evaluated.The relationship between the inlet fluid temperature and the pseudo-critical temperature significantly affects the loop flow and heat transfer characteristics.