Heat transfer decreasing in a tube for up and down water flow

The experimental data on temperature conditions of the uniformly heated vertical tube of 9.75 mm internal diameter in case of water boiling in it under the up and down flow conditions are presented. The experiments have been conducted at pressures of 9.8-17.8 MPa, mass rates 350-1500 kg/(m2s) and specific heat fluxes-100-800 kW/m2. At 9.8 MPa critical steam contents, temperature fields and heat transfer coefficients in the post burnout zone are independent of the flow direction. While pressure growing the differences in temperature conditions at various flow directions increase, they decrease with the mass rate increase. At high pressures the heat transfer coefficients decrease with the heat flux increase