Investigations On The Characteristics Of The SCO₂ Brayton Cycle Near The Critical Region

With the advantages of high efficiency,compactness,and low water consumption,supercritical carbon dioxide（s CO₂）Brayton cycle is proposed for various energy systems,including the solar energy,the fossil fuel,the nuclear,the waste heat recovery and the geothermal energy.In the near-critical region,the dramatic change of physical properties leads to complex characateristics of the system.At present,the research of s CO₂ power cycle still lacks universal heat transfer correlations and accurate measurement method for critical parameters.At the same time,problems such as insufficient operating experience and low implementation efficiency of the demonstration system also need to be overcome.In this paper,studies were carried out from four aspects: heat transfer near the critical point,monitor for density near the critical point,experimental and numerical investigations for transcritical CO₂ dynamic system,selection and optimization of the system.Experimental investigations on the heat transfer characteristics of CO₂ in the horizontal and vertical tubes were carried out near the critical point.The effect of mass flow rate,working pressure and heat flux on the heat transfer coefficient（HTC）were studied,and the heat transfer characteristics of CO₂ were concluded from the experimental results.Greater mass flow rate leads to higher Re in the tube,which causes more significant heat transfer enhancement and higher peak value of HTC.With the pressure approaching the critical value,the change of physical properties strengths and the HTC increased.With the heat flux increasing from 100 k W·m-2 to 200 k W·m-2,the value of peak HTC is reduced by half at 7.6 MPa.The heat flux affects the HTC by affecting the distribution of fluid velocity and shear force in the thermal boundary layer.Correlations concluded from the experimental results could give a good predict of the HTC in experiments（the maximum deviation of horizontal tube and vertical tube are ± 20% and ± 10%）,which were used in the sebsequent dynamic model construction.The infrared absorption spectrum of s CO₂ density was investigated as a monitor applied in the stable and dynamic processes at the critical region.In the experiments,the absorption ratio of the 3400³500 nm bands were found to give a clear and fast feedback（< 1 s）to the density change of the CO₂,which could serve as an efficient signal of CO₂ state monitoring.70% reduction of the density and 60% reduction of the absorption ratio were observed at the steady state,when the temperature increased from 30 ℃ to 34 ℃ at 7 MPa.In different dynamic processes,densityrelated scattering phenomena were observed near the pseudo-critical points,which strengthened the non-transmissive ratio（up to 0.95）of CO₂,provided a strong signal for the density monitoring,and validated the sensitivity and feasibility of the Infrared spectrum measuring method.A dynamic model was developed with the modular modeling method,which was validated by the experimental results（deviations were between 0.3% and 3.55%）.Results from the transcritical CO₂ test loop showed that dynamic model could avoid the fluctuations and disturbances of the system components.The model successfully captured the dynamic characteristics at the pressure changing point,while the experiments failed due to the limitation of the sensor sensitivity.Increasing the working pressure from 8 MPa to 10 MPa led to a 40%overpressure and 2 times pressure drop in a short time.Turning the pump frequency from 20 Hz to 50 Hz led to a 22% flow overshot,a 40% overpressure and a 12% pressure drop overshot.The heating power should be adjusted before the mass flow rate and pressure to avoid overheat of heaters.The response time is linearly linked with the heat capacity of components.The structure and parameter optimization research of s CO₂ cycle were carried out based on inlet parameters of the compressor and turbine,pinch point temperature of the system.When the turbine and compressor inlet conditions are（500 °C,22 MPa）and（34 °C,7.5 MPa）,the optimized efficiency of the recompression s CO₂ Brayton cycle is 5% higher than the simple s CO₂ Brayton cycle.Reheat can improve 1.8% efficiency of the recompression s CO₂ Brayton cycle,which is promising for commercial applications.With the efficiency of turbomachinery from demonstrations,the simple s CO₂ Brayton cycle has a simpler layout,a higher efficiency（> 20%）,smaller sensitivity with critical parameter and higher output power than the recompression s CO₂ Brayton cycle,which is proposed for the small-scale demonstrations.This work is expected to provide a valuable reference for the research and development of the key components,and a reference scheme for the design,construction,operation and monitor of the s CO₂ Brayton cycle.