[Objective

]

The supercritical carbon dioxide(S-CO

2

) semi-closed Brayton cycle has attracted widespread attention due to its high cycle efficiency and potential for near-zero carbon emissions. The working fluid in this cycle primarily consists of the CO

2

/H_2O mixture. Except during the compression and separation processes

the CO

2

/H_2O mixture operates in the low-to medium-density region for most of the cycle. Accurate prediction of thermophysical properties in this region is crucial for the power cycle. Due to the lack of experimental data for the mixture

the applic

able temperature range of multiparameter equations of state is limited

making it difficult to meet the demand for thermophysical property calculations at high temperatures. Computational models based on theoretical foundations have greater potential for extrapolation in the high-temperature region. Among these

semitheoretical cubic equations of state can reasonably describe the trends in thermophysical properties. However

they are limited by their structure and cannot achieve extremely high precision. In contrast

the virial equation of state has a rigorous theoretical foundation and provides an effective approach for predicting thermodynamic properties at high temperatures. On this basis

a truncated virial equation of state applicable to CO

2

/H_2O mixtures is developed to enable high-accuracy calculations of thermodynamic properties over a wide temperature range. [Methods

]

A predictive model that strictly satisfies physical consistency requirements is established for the cross second virial coefficient of the CO

2

/H_2O mixture based on ab initio data. Building upon this

a three-term truncated virial equation of state(VEOS3) is developed to calculate the thermodynamic properties of pure CO

2

pure H_2O

and their binary mixtures. The predictive performance of VEOS3 is systematically evaluated across a range of properties

including pvT(x)

isobaric heat capacity

internal energy

enthalpy

entropy

and speed of sound. The effective range of VEOS3 is determined by comparison with multiparameter equations of state within their respective validity domains. At a given temperature

the maximum density and the corresponding pressure within the region where the absolute relative deviation does not exceed 1% are defined as the effective density limit and effective pressure limit

respectively. The model performance is further validated using experimental data. [Results

]

The results show that the effective density limit of VEOS3 remains approximately constant in the high-temperature regi

on or increases linearly with temperature. As the temperature rises

the effective prediction range of VEOS3 can be extended to higher pressures. Meanwhile

VEOS3 reproduces available experimental data with high accuracy. Here

the average absolute relative deviation(AARD) is used to quantify prediction deviations. For pure CO

2

and pure H_2O

the AARDs of VEOS3 in density

isobaric heat capacity

and speed of sound are 0.14%/0.30%

0.28%/0.82%

and 0.02%/0.16%

respectively. For CO

2

/H_2O mixtures

the AARD in density is 0.43%. [Conclusion

]

VEOS3 can accurately calculate thermophysical properties in the low-to medium-density region

where the reduced density is less than or equal to 2/3. It also maintains stable and reliable extrapolation capability at high temperatures

providing model support for thermophysical property calculations in the S-CO

2

semi-closed Brayton cycle.