An Investigation into Film Condensation of Saturated Steam on Tube Surfaces by a Gradient Heatmetry

Abstract

Gradient heat flux measurement (gradient heatmetry) is a modern technology for measuring heat flux per unit area using gradient-type sensors. Since 2015, gradient heatmetry has been used to study heat transfer in film condensation of saturated steam on the inner and outer surface of tubes. This measurement method offers greater information capabilities than the more widely used thermometry when the heat flux is calculated from the temperature measured with thermocouples. The advantage of gradient heatmetry results from abnormally fast response time of sensors which is about 10–8–10–9 s. Therefore, they may be considered almost inertia-less measuring devices. Direct measurement of heat flux per unit area reduces the total uncertainty in calculating local and average heat-transfer coefficients. Heat transfer in film condensation of saturated steam on the outer and inner surfaces of tubes was studied using gradient heatmetry. Gradient heat flux sensors (GHFS) made of single-crystal bismuth were used on the outer surface, while heterogeneous GHFSs made of Grade 12Kh18N9T steel + Ni composition were installed on the inner surface. In both cases, reference tests were performed on vertical tubes. Their results confirmed the excellent information capability of this approach and its applicability for estimating heat flux. A series of experiments was carried out to study heat transfer during film condensation of saturated steam on the outer and inner surfaces of inclined pipes. The highest heat-transfer coefficient of 6.94 kW/(m2 K) in condensation of saturated steam on the outer surface of a tube is observed for the tube inclined at an angle of 20° to the vertical. This value exceeds the heat-transfer coefficient on a vertical tube by 14.9%. The highest heat-transfer rate in condensation on the inner surface was observed for the tube inclined at 60° to the vertical.