Critical heat flux in a vertical tube at low and medium pressures. Pt. I. Experimental results

For pt.II see ibid., vol.193, p.91-103, 1999. This paper presents experimental CHF data obtained for vertical up flow in an 8 mm I.D. test section, for a wide range of exit qualities (5-75%) and exit pressures ranging from 5 to 40 bar. The experiments were carried out for heated lengths of 0.75, 1, 1.4, 1.8, 2.5 and 3.5 m. A number of different coordinate systems are used to present the experimental results, these include CHF as a function of the inlet subcooling, as a function of the outlet thermodynamic quality, and as a function of the boiling length. An analysis of these commonly used representations of the CHF, has shown, that for all of the cases studied, the critical heat flux is dependent in some manner on the heated length. This dependence greatly complicates the possibility of obtaining a universal correlation for the critical heat flux at low and medium pressures. (orig.)