Investigation of flow and heat transfer of supercritical water in double-pipe heat exchanger

  Objective  The flow and heat transfer of supercritical fluid is complex, and most of the existing research is based on the conditions of constant wall temperature and constant heat flux. However, in practical engineering, heat transfer often occurs under the conditions of variable heat flux. In order to better understand the relevant performance, this paper studies the coupled flow and heat transfer of supercritical fluid in a double-pipe heat exchanger.

  Method  The experimental section of this paper focuses on a double-pipe heat exchanger. The flow and heat transfer characteristics of supercritical water in the double-pipe heat exchanger are analyzed by collecting temperature and pressure data from the temperature and pressure measuring points arranged on the inside and outside of the double pipe.

  Results   The experimental results show that great changes in physical parameters near pseudo-critical points are the main reasons for the phenomenon of supercritical water heat transfer, and they differ from those of ordinary heat transfer. There is a difference between the peak heat transfer coefficient temperature and pseudo-critical temperature which increases with the increase of pressure. This phenomenon is caused by the temperature gradient and variations in thermal-physical properties in the heat transfer section. The heat transfer coefficient of the outer pipe increases with the increase in water temperature at the inlet of the inner pipe when the mean bulk temperature is lower than the pseudo-critical temperature due to the buoyancy effect decreasing at the same time, while the buoyancy effect is almost independent of the water temperature at the inlet of the inner pipe when the mean bulk temperature is higher than the pseudo-critical temperature and the buoyancy effect is non-significant.

  Conclusion  The results can provide valuable references for the design of double-pipe heat exchangers and research and mechanism analysis of the coupled flow and heat transfer of supercritical fluid in double-pipe heat exchangers.