Investigation of heat transfer performance of double-walled pipes of heat exchanger for power plant applications

Heat transfer between contacting surfaces, is a key factor affecting the thermal performance of heat exchangers in power plants and other applications. Double-walled pipes, used in the heat exchangers, can significantly improve the safety factor of the system. In this research work, the thermal contact conductions (TCC) at the interfaces of double-walled stainless steel 316 L pipes under various contact pressures were investigated. These pipes were manufactured with the shrink-fitting method using servo-hydraulic universal testing machine. A numerical model, verified by X-ray diffraction analysis, was developed to investigate the contact pressure variation at the interfaces according to loading conditions. The contact pressure between the inner and outer walls was controlled in the range of 22.78 MPa to 25.70 MPa, by varying the tensile loading conditions. A phasesensitive transient thermal method was developed to measure the thermal conductivity (TC) and TCC of doublewalled pipes. TC was observed to be in the range of 15.13 W/mK to 15.84 W/mK while TCC at the interface was observed to increase sharply with the contact pressure, reaching 17 x 103 W/m2K under 25.70 MPa contact pressure.