A method is developed by means of which it is possible to determine the influence of the pipe orientation on the location of the boiling crisis as well as on the heat transfer in the unwetted region. Wall temperature excursions during pressure and enthalpy transients are interpreted with the help of the boiling curve and the Leidenfrost phenomenon.

The heat transfer in the unwetted region was newly formulated taking into account thermal nonequilibrium between the water and steam phases. Based on these experiments a method of predicting CHF for a vertical upflow evaporator tube was developed. The experimental work consists of the following: Occurrence of critical heat flux (CHF) and post-CHF heat transfer in a vertical upflow evaporator tube influence of pressure and enthalpy transients on heat transfer in the unwetted region influence of pipe orientation on heat transfer and two phase flow pressure loss in wetted and unwetted region. Included in the analysis are quantities which determine the wetting state in steady and transient flow. The influence of the wetting state of a heated surface on heat transfer and pressure loss in an evaporator tube was investigated for a parameter range occurring in fossil-fired steam generators.

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