Tube diameter size effects on heat transfer of supercritical R134a in horizontal tubes for vapor generator design in organic rankine cycles

Supercritical Organic Rankine Cycle (ORC) is a promising technology to convert the renewable energy and waste heat into power. This paper focused on the diameter size effect on the heat transfer characteristics of supercritical pressure R134a in horizontal tubes, aiming to provide guidance for the vapor generator design in ORC systems. Heat transfer experiments were conducted in two tubes with different inner diameters of 10.3 mm and 16 mm. Results showed that the heat transfer deterioration on the top surface is more serious with larger tube diameter. Heat transfer mechanisms were studied by analysing the buoyancy effect, thermal acceleration effect and the property variation effect quantitatively with various dimensionless numbers. The tube diameter size significantly affects the buoyancy effect in horizontal tubes. A larger tube diameter leads to stronger buoyancy effect and weaker thermal acceleration effect. However, the tube diameter size seems to have no effect on the radial property variations.