Experimental Study on Sharp Increase of Wall Temperature in Vapor Generator for Organic Rankine Cycle

ORC (Organic Rankine Cycle) is a promising way to efficiently utilize renewable energy and waste heat. This paper focuses on CHF (Critical Heat Flux) causing sharp increase of wall temperature in vapor generator which should be carefully avoided due to its negative effects of heat transfer deterioration and instability of working fluids, aiming to provide guidance for vapor generator's operation and cycle design of ORC system. Experiments were conducted in a 10.3 mm horizontal smooth tube covering a wide pressure range of 0.61 to 0.97 Pc. The occurrence of CHF at bottom wall is found to be the mechanism leading to sharp increase of wall temperature. Pressure is a significant factor changing the CHF type from dryout type to DNB type, which makes near-critical region with higher pressures more sensitive in triggering of CHF. For near-critical region which has not been well studied in existing literature, experimental results showing CHF values for various conditions are provided showing mass flux is important in enhancing CHF level and compensating for negative impact of increasing pressure, accompanied by a newly proposed correlation for predicting CHF at bottom wall in horizontal tube. The predicting CHF values agree well with experimental results with a mean deviation of 7.3% and prediction accuracy is not sensitive to the change of operating conditions.