[Objective

]

Using a CO

2

/H_2O mixture as the composite working fluid

the supercritical carbon dioxide(S-CO

2

) semi-closed Brayton cycle can achieve efficient carbon capture and power generation while maintaining operational flexibility. Accurate descriptions of the thermophysical properties of CO

2

/H_2O mixtures are essential for cycle analysis

component design

and system performance optimization. Therefore

it is necessary to develop an equation of state capable of reliably describing the thermophysical properties of CO

2

/H_2O mixtures over the operating range relevant to the S-CO

2

semi-closed Brayton cycle. [Methods

]

In this study

the Kiselev crossover method was adopted to establish crossover volume-trans

lation Soave-Redlich-Kwong(CVTSRK) equations of state

for pure CO

2

and pure H_2O. The accuracy of the proposed pure-fluid CVTSRK equations was verified through calculations of saturated properties and single-phase pvT properties. On this basis

a CVTSRK equation of state applicable to CO

2

/H_2O mixtures was further established by combining component-based mixing rules with symbolic regression. The proposed equation was applied to single-phase pvTx property calculations for CO

2

/H_2O mixtures

and its correlation accuracy

predictive capability

applicability

and computational performance were evaluated across the temperature–pressure and composition ranges relevant to the S-CO

2

semi-closed Brayton cycle. [Results

]

The results indicate that the proposed CVTSRK equation of state can accurately describe the single-phase pvT(x) properties of pure CO

2

pure H_2O

and CO

2

/H_2O mixtures from subcritical to supercritical conditions. For the experimental density data of pure CO

2

and pure H_2O

the average absolute relative deviations between the calculated densities and the experimental data are 0.91% and 1.73%

respectively. For CO

2

/H_2O mixtures

the average absolute relative deviation between the calculated densities and the experimental data is 0.86%. For the data excluded from model fitting

the prediction deviation is only 0.40%

indicating the favorable predictive capability of the proposed equation. Moreover

the equation requires only a few fitting parameters and can be readily integrated into engineering calculation programs. Within the parameter range involved in the S-CO

2

semi-closed Brayton cycle

namely temperatures of 298 ～ 2 000 K

pressures up to 30 MPa

and CO

2

mole fractions of 0.75 ～ 1.00

the constructed CVTSRK equation of state exhibits satisfactory applicability and stable computational performance. [Conclusion

]

The proposed CVTSRK equation of state maintains high accuracy over the range cov

ered by experimental data and provides stable and reliable extrapolative predictions under high-temperature conditions. It also shows reliable performance in data-sparse regions and provides a thermophysical property basis for component design and performance analysis of key processes in S-CO

2

semi-closed Brayton cycles

including compression

expansion

and recuperation. Furthermore

the proposed equation can support further studies on cycle mechanisms

system configurations

and performance optimization

and provide a basis for scientific research and engineering applications.