Underground Coal Gasification (UCG) produces synthesis gas at extremely high temperatures and pressures, requiring efficient cooling systems to ensure safe transportation and energy recovery. Conventional gas cooling systems often experience limitations in heat transfer performance when handling compressible turbulent flows. Advanced heat exchanger configurations such as membrane helical coils provide promising solutions due to their enhanced mixing characteristics and compact geometry.
The present study investigates the thermo hydraulic and exergy performance of a membrane helical coil heat exchanger designed for high-pressure syngas cooling applications in underground coal gasification systems. A three-dimensional computational fluid dynamics model was developed using ANSYS Fluent to simulate turbulent compressible flow inside the helical tube. Temperature dependent thermo physical properties of syngas were incorporated to improve prediction accuracy. The influence of geometric parameters including coil diameter, pitch ratio, and tube diameter on heat transfer, pressure drop, and thermodynamic efficiency was systematically evaluated.
Results indicate that the curvature of the helical tube generates strong centrifugal forces that induce secondary vortices, significantly enhancing fluid mixing and convective heat transfer. The Nusselt number increases by approximately 22-31% compared with equivalent straight tube configurations. However, the friction factor also increases due to additional curvature induced resistance. Exergy analysis reveals that heat transfer irreversibility is the dominant contributor to exergy destruction at lower Reynolds numbers, whereas fluid friction becomes more significant at higher Reynolds numbers.
A multi objective optimization approach identified optimal geometric conditions corresponding to a coil diameter of 220 mm and pitch ratio of 2.5, resulting in a maximum thermo hydraulic performance factor of 1.28 with minimum exergy destruction. The findings demonstrate that membrane helical coil heat exchangers offer an efficient solution for high temperature syngas cooling and can significantly improve thermal efficiency in underground coal gasification energy systems.