Simulation of heat transfer in a flow of over-critical nitrogen and oxygen in a horizontal circular tube

In the framework of the k-s turbulence model with enhanced wall treatment, heat transfer in a flow of over-critical nitrogen and oxygen in a horizontal circular tube with heat boundary conditions of the third kind in the pressure range of 20H40 MPa, mass flow rates of 0.03÷0.12 kg/s, and heat transfer coefficients to the environment of 500÷2000 W/(m²K) is simulated. The distributions of the bulk temperature, calculated as the thermodynamic temperature of the average mass enthalpy of the supercritical fluid flow, are obtained. Analysis of the heat transfer coefficient of the flow showed that at pressures less than three times the critical one, a reduced heat transfer mode appears, so the near-critical region is not recommended for use in oxygen and nitrogen gasifiers. In the framework of the multiplicative model for the Nusselt number, insignificant simplices of thermophysical properties are established and correlations for oxygen and nitrogen in the over-critical region are constructed, which agree with the result of the computational experiment with an average relative accuracy of 1^1.5%. On the basis of the one-dimensional heat transfer equation, the adequacy of the obtained correlations is shown. It is established that when using the bulk temperature based on the mass average enthalpy, a separate account of the initial heat transfer section is not required.