Experimental investigation on heat transfer characteristics of smooth tube with downward flow

In the range of pressure from 11.5 to 28 MPa, mass flow rate from 450 to 1550 kg m−2 s−1, and heat flux at inner wall from 50 to 585 kW m−2, an experiment was conducted to investigate the heat transfer characteristics of water in a vertical downward tube. The temperature distribution in the tube wall was obtained. The effects of heat flux on heat transfer coefficient and wall temperature were analyzed and the corresponding empirical correlations were presented. The heat transfer coefficients of vertical downward flow were compared with that of vertical upward flow. The experimental results show that at subcritical and near critical pressure, dryout and departure from nucleate boiling (DNB) can occur in vertical downward tube. With the increase of heat flux, heat transfer deterioration occurs ahead and the peak wall temperature after heat transfer deterioration increases obviously. At supercritical pressure, the wall temperature increases slowly with fluid enthalpy when the bulk temperature is lower than the pseudo-critical temperature and increases considerably when the bulk temperature is higher than the pseudo-critical temperature. In the pseudo-critical enthalpy region, the heat transfer is enhanced.