As the core equipment of supercritical carbon dioxide (S-CO₂) Brayton cycle, there is a lack of reliable evaluation and test verification of the overall performance. An in-depth simulation and performance analysis of one axial turbine are carried out, focusing on the impact of inlet and exhaust housings with experiment results for different operating conditions. The results show that the numerical calculation method and model are able to evaluate the performance at different load conditions more accurately. Compared with the test results, the maximum efficiency error is 1.77 percentage point and the flow rate error remains within 5.6%. The crown pattern can reduce leakage and mixing losses, and increase efficiency by 1.4 percentage point compared to the common top clearance pattern. Simulation results show that the efficiency of turbine unit is reduced compared to turbine stage, with a maximum reduction by 2.9 percentage point. The flow loss of inlet and exhaust housings is the main reason for the reduction. The research results can provide technical support for the design and performance simulation of S-CO₂ axial turbines.

A cooling system was designed for a supercritical carbon dioxide (S-CO₂) axial turbine. The dry gas seal, shaft and casing were cooled by extracting the low temperature S-CO₂ in the pipeline behind the compressor to ensure that the dry gas seal operating temperature was below 200 ℃. The flow and heat transfer characteristics of the cooling system were analyzed using the coupled heat transfer method, and the temperature distributions of solid domains such dry gas seal, shaft and casing of different cooling schemes were compared. The research shows that the temperature drop of the shaft reaches 220.3 ℃ when the_shaft cooling scheme is adopted, and the maximum temperature of the dry gas seal is 229.1 ℃. Further introduction of S-CO₂ with lower temperature and larger flow rate to cool the casing can inhibit the heating effect of the high temperature mainstream at the turbine inlet. The temperature drop of the shaft increases to 244.1 ℃, and the maximum temperature of the dry gas seal decreases to 181.2 ℃. Meanwhile, the reasonable temperature gradient of the cooled domains such as the dry gas seal, shaft and casing is achieved. The cooling system designed in this paper provides a solution for the safe and reliable operation of S-CO₂ axial turbines.

In view of the actual distribution of heat boundary conditions on the cooling wall of supercritical carbon dioxide (S-CO₂) coal-fired boiler, the heat transfer characteristics of supercritical CO₂ in a vertical circular tube under axial non-uniform heat flux were numerically studied by using SST k-ω low Reynolds number turbulence model. The influence of different heat flux distribution, mass flux on heat transfer performance and wall temperature distribution was analyzed. The results show that the axial non-uniform heat flux distribution has a significant effect on the heat transfer of S-CO₂. Compared with the uniform heat flux, the total heat transfer coefficient under the axial non-uniform heat flux increases by about 8%. The non-uniform distribution of axial heat flux can inhibit the heat transfer deterioration and effectively reduce the peak wall temperature. Under the condition of non-uniform heat flux, the heat transfer of S-CO₂ is mainly affected by the thickness of the gas-like film, the thermal conductivity of the gas-like film and the specific heat near the wall. The results provide theoretical guidance for the design of supercritical CO₂ boiler.

The abnormal heat transfer behavior of supercritical carbon dioxide (S-CO₂) with low mass fluxes in a horizontal tube was studied, the S-CO₂ heat transfer process in the horizontal tube under the condition of low mass fluxes was simulated with Fluent software, and the abnormal heat transfer behavior of heating and cooling conditions and the influence of heat flux on heat transfer were analyzed. The results show that when the thermal boundary conditions are P=8 MPa, G=200 kg/(m²·s) and q/G=0.2 kJ/kg, the temperature of top and bottom walls in the S-CO₂ tube decreases along the way during the flow cooling process. When the mainstream temperature of S-CO₂ reaches the pseudo critical temperature, the heat transfer coefficient of the top wall at 551.0 mm from the inlet has a sudden peak value, heat transfer enhancement occurs here. Under heating conditions, the temperature of the top wall first rises along the tube path, then drops to 395 K and then rises slowly. The temperature of the bottom wall drops briefly and then rises slowly. At the top wall 69.5 mm away from the inlet, the heat transfer coefficient has a valley value, and the heat transfer at this point deteriorates. The increase of heat flow density aggravates the deterioration of heat transfer under heating conditions, but has no obvious effect on cooling heat transfer. It can be seen that the thermal physical property distribution of the characteristic section is the main reason for the different heat transfer behaviors. Based on the low mass fluxes conditions, thermo-physical properties and buoyancy effects, a correlation equation for predicting supercritical heat transfer enhancement is constructed, which provides theoretical guidance for the design and operation optimization of supercritical fluid heat exchanger.

In order to explore the similarities and differences of the loss characteristics of carbon dioxide and steam in turbine cascades, the flow characteristics of the two kinds of working fluids in stator cascade and stage were studied by numerical methods. And the optimal Mach number for efficient operation of turbine stage under subsonic condition was obtained. The results show that with the increase of Mach number, the flow loss first increases and then decreases. When the Mach number is lower, the diffuser has a large range and is easy to backflow, which makes the wall boundary layer thicken and separate, and increases the overall flow loss. When the Mach number is higher, the strength of the secondary vortex in passage is larger, and the shock wave will be generated near the trailing edge of blades. The reason for the larger flow loss is the secondary flow and shock wave. Compared with steam, the dynamic viscosity of carbon dioxide is slightly higher, and its density is about twice that of steam. Under the same Mach number condition, the mainstream velocity is lower, the boundary layer is thicker, and the overall loss is larger. When the Mach number is lower than 0.30, the total-total efficiency of turbine stages with carbon dioxide is lower. While the Mach number is higher than 0.50, the efficiency of carbon dioxide is slightly higher than that of steam. When the optimal outlet Mach number of balde is about 0.60, the efficiency of both is the highest. The research results will provide a reference for further improving the design level for axial flow turbines of steam and carbon dioxide, and further understanding the loss characteristics of different medium in turbine stage.

The supercritical carbon dioxide cycle has many advantages, such as high cycle efficiency, small equipment size, convenient transportation and installation, and easy to reach the critical point. Considering the huge cold energy of LNG, it can not only be used as coolant in the combined cycle system, but also the natural gas after heat transfer can be used as fuel input in the combined cycle, and the rest can be supplied to urban users. A gas turbine/supercritical carbon dioxide combined cycle system based on the utilization of LNG cold energy is proposed in this paper. Select the appropriate cost formula to calculate and analyze the investment cost, operating income and recovery cycle of the circulating power generation system in detail. The influence of some key parameters (such as maximum temperature, maximum pressure, minimum temperature, minimum pressure and shunt ratio) on the power generation characteristics and economy of the supercritical carbon dioxide cycle in the combined cycle system was studied. The results show that with the increase of each single parameter, the cost of equipment investment will first increase and then decrease, but the effect of power generation on income is dominant. Taking the yield as the measurement standard, the higher the maximum temperature, the better, the lower the minimum temperature, the better. Under other parameters, there are optimal values to maximize the yield.The key parameters were optimized by genetic algorithm to maximize the cumulative income. After optimization, the recovery cycle was 5.86 years, and the cumulative income (20 years) was 2.287 billion yuan.

Centrifugal compressor is one of the key components in supercritical carbon dioxide (S-CO₂) cycle system, which plays a decisive role in the efficiency and stable operation of the system. Different from the traditional air compressor, the unique physical properties of S-CO₂ working medium make the internal flow field of the compressor more complex. The loss model established based on the physical characteristics of air also needs to be modified specifically to meet the performance prediction requirements of S-CO₂ centrifugal compressor. Therefore, numerical simulation is needed to investigate the internal flow field characteristics of the compressor, so as to improve the compressor performance prediction method accordingly. Firstly, one-dimensional aerodynamic parameters of the compressor were designed, and a three-dimensional model was established based on the one-dimensional design parameters to analyze the characteristics of the internal flow field of the compressor. It was found that the shunt blade had a great influence on the internal flow field, and changes in the internal flow field of the impeller under varying working conditions would also cause changes in the outlet flow Angle. Based on this, The sliding factor and the calculated blade number of the compressor under off-design conditions were corrected, and the surface friction coefficient was improved to predict the performance of the compressor under off-design conditions. The numerical simulation results show that the prediction error of the improved model is significantly reduced, and the average efficiency error decreases from 2.03% to 0.16% under off-design conditions.

In order to effectively prevent the slagging of a supercritical 650 MW opposed firing boiler, the influence of different coal types on the flue gas temperature at the furnace outlet and the heat load of the boiler was checked by thermodynamic checking calculation, and the influence of the air distribution in the furnace and the cone-expanding angle of the burner on the flue gas dynamic field and flue temperature of the furnace was analyzed by CFD simulation. The results of the thermodynamic checking calculation showed that the boiler and the coal type were not the main reason for slagging. The variable air volume simulation showed that adjusting the ratio of internal and external secondary air could have effect on the dynamic field of flue gas. But in operation, changing the air volume did not solve the slagging problem well. The simulation results pointed out that the heat load of the water wall area could be reduced by reducing the cone-expanding angle of the burner from 45°to 30°, so as to inhibit the slagging. In the actual adjustment, after changing the cone expansion angle of the burner from 45°to 30°and adjusting the air distribution, the slagging situation of the boiler was greatly improved.The calculation results of variable SOFA wind showed that the appropriate reduction of SOFA wind proportion could reduce the flame height and the flue gas outlet temperature. The simulation of variable secondary air rotation showed that the secondary air rotation had a significant influence on the flow field and the slagging risk would significantly increase if the rotation of the burner was not arranged according to designed value. Further adjustments to SOFA wind ratio and secondary wind swirl could be considered in following adjustments.

The operating parameters of coal mills directly affect the safety and combustion performance of coal-fired units. Therefore, it is of great significance to carry out real-time monitoring of CO volume fraction at the outlet of coal mills and accordingly have safety early warning based on the monitored data. In this work, the measurement accuracy and detection limit of wavelength modulation direct absorption spectroscopy(WM-DAS) method were verified based on tunable diode laser absorption spectroscopy(TDLAS) and Herriott multi-pass cell. The experimental results showed that the measured CO volume fraction agree very well with the preset values in the range of 1 to 10 μL/L, and the detection limit can be as low as 5.2×10^–3 μL/L(300 s), which suggests the extremely high measurement sensitivity and accuracy of this method. Subsequently, based on this prototype experiment, an on-line monitoring system for the trace CO volume fraction was developed. Then, combined with the high-fidelity pretreatment technology of fuel gas based on the principle of constant-flow dilution, an on-line monitoring system for trace CO volume fraction was developed and applied to the outlet of coal mill of thermal power unit. The on-line CO monitoring and safety warning at the outlet of 5 coal mills of a single thermal power unit were realized by using multi-point alternating measurement strategy. With the monitored CO volume fraction and temperature of the coal mill outlet, the temperature of the primary air is adjusted to improve the boiler efficiency while ensuring safe operation of coal mill.

During signal sampling process of steam turbine digital electric hydraulic control system (DEH) tests, situations that the instrument can not meet the requirements of Nyquist sampling due to the frequency of the primary components is over high may occur. To solve this problem, the envelope of the signal is calculated through Hilbert transform, and the valve closing time is calculated according to the envelope signal. However, the calculation accuracy is limited due to the time interval of the peak points. To improve the calculation accuracy, the undersampled signal is reconstructed, the primary frequency is analyzed by using fast Fourie transform (FFT), and the primary frequency of the signal is determined based on the nature of the frequency domain. On this basis, the initial phase of the AC signals is calculated using the initial value, and the undersampled signals is reconstructed according to the frequency and initial phase. The closing time is calculated according to the difference between the reconstructed and sampled signals based on Akaike information criterion (AIC). In comparison with the Hilbert transform method, the reconstruction method can improve the calculation accuracy. The reconstruction and analysis method can be used in all kinds of undersampled periodic signals, which can make up the shortcomings of hardware in DEH tests.