Critical heat flux of vertical upward water flow in uniformly heated round tube

It has long been known that the heat transfer and CHF can be enhanced using devices such as rifled tubes, twisters, springs and various grooved, ribbed, and corrugated tubes. An internally rifled surface is usually used to enhance flow boiling heat transfer in once-through boilers. However, the state of the art of enhanced heat transfer inside rifled tubes is much less advanced than that of conventional boiling heat transfer. In addition, the heat transfer and CHF enhancement in the rifled tubes largely depends on the detailed rifled tube geometry such as number of ribs, rib pitch, rib height, rib width and helical angle. Evaporator tubes used in once-through boilers have a long heated length and large inside diameters, while conventional CHF tests are concentrated to relatively small heated length and small inside diameters. Currently, KAERI is performing CHF tests in order to develop a CHF design correlation for the rifled tube. A series of CHF test is planned for smooth tubes with uniform and non-uniform circumferential heat fluxes, and rifled tube with non-uniform circumferential heat flux. From these three tests, a rifled tube CHF correlation will be developed using a conventional CHF prediction method for smooth tubes, the modification factor for circumferential heat flux distribution, and the CHF enhancement factor by rifled tube. As a first step, the present study provides water CHF test results in vertical tube with smooth inner surface having large diameter and investigates the prediction reliability of the conventional CHF prediction methods. This CHF data using round tube with smooth inner surface will be used as a reference for future CHF experiments and the development of the rifled tube CHF correlation