Nusselt Number Correlations Determined Concurrently on the Hot and Cold Fluid Sides in a Helically Coiled Tube-in-Tube Heat Exchanger

Due to their advantageous heat transfer characteristics and compact design, helically coiled heat exchangers have found extensive application in HVAC systems and are also gaining importance in advanced nuclear technologies, particularly in Small Modular Reactor (SMR) concepts. Accurate correlations for heat transfer are essential for their effective design; however, reliable predictive relationships remain limited, particularly for flow in annular channels. This study presents a new combined experimental and numerical approach for the simultaneous identification ofheat transfer correlations for the hot and cold fluids in a helically coiled concentric-tube heat exchanger. A nonlinear least-squares approach was employed to identify the unknown correlation coefficients by minimising the sum of squared residuals between the experimental and predicted outlet temperatures of both fluids. Experimental investigations were conducted over a broad range of inlet temperatures and water flow rates using a computerised data acquisition system.