Very High-Temperature Reactor (VHTR) is a leading candidate for the U.S. Department of Energy’s Next Generation Nuclear Power Plant (NGNP) project. It is a helium gas-cooled reactor with very high reactor core outlet temperatures (800-950°C) and offers high-efficiency electricity generation and a broad range of process heat applications, such as coal liquefaction, coal gasification, and oil recovery from shale. To efficiently transfer the core thermal energy to a secondary fluid, high-temperature and high integrity intermediate heat exchangers (IHXs) with high effectiveness are required. While there is no proven IHX concept for NGNP applications yet, a concept called printed circuit heat exchangers (PCHEs) appears most promising. The current research focuses on the design, fabrication, thermal-hydraulic performance testing, and modeling of PCHEs under high operating temperatures and pressures.
PCHEs are plate-type heat exchangers, fabricated by photochemical machining and diffusion bonding. In the current research work, both these fabrication techniques have been demonstrated on Alloy 617 plates, a high-temperature candidate material for VHTR structural components. Two counter-current flow PCHEs have been designed and fabricated using Alloy 617 plates and are installed in a small-scale high-temperature helium test facility (HTHF). The HTHF has been designed and constructed at The Ohio State University as part of this research to facilitate experiments at temperatures and pressures up to 800°C and 3 MPa, respectively.
Microstructural and mechanical characterizations studies performed on diffusion bonded Alloy 617 specimens are discussed. This study provided confidence from a safety view point insofar as the operation of the heat exchangers, under high temperature and pressure conditions in the HTHF. Performance testing of the two counter-flow PCHEs in the test facility has been completed at varied operating temperatures, helium pressures, and helium flow rates. The PCHE inlet temperature and pressure were varied from 85-390°C/1.0-2.7 MPa for the cold side and 208-790°C/1.0-2.7 MPa for the hot side, respectively, while the mass flow rate of helium was varied from 15 to 49 kg/h. The maximum helium temperature that has reached at the exit of the main heater is 823°C. This range of mass flow rates corresponds to PCHE channel Reynolds number of 950-4,100 for the cold side and 900-3,900 for the hot side (corresponding to laminar and laminar-to-turbulent transition flow regimes). The experimental data have been analyzed for the pressure drop and heat transfer characteristics of the heat transfer surface of the PCHEs and compared with the available models and correlations in the literature. In addition, numerical and theoretical treatment of hydrodynamically developing and hydrodynamically fully developed laminar flow through a semicircular duct is presented. In summary, the PCHE testing at high temperatures and pressures and the experience accumulated during the design and construction of the HTHF and the PCHEs will be of value to the high-temperature reactor research.