Numerical Studies on Thermal and Hydrodynamic Characteristics of LNG in Helically Coiled Tube-in-Tube Heat Exchangers

As compact and efficient heat exchange equipment, helically coiled tube-in-tube heat exchangers (HCTT heat exchangers) are widely used in many industrial processes. However, the thermal-hydraulic research of liquefied natural gas (LNG) as the working fluid in HCTT heat exchangers is rarely reported. In this paper, the characteristics of HCTT heat exchangers, in which LNG flows in the inner tube and ethylene glycol-water solution flows in the outer tube, are studied by numerical simulations. The influences of heat transfer characteristics and pressure drops of the HCTT heat transfers are studied by changing the initial flow velocity, the helical middle diameter, and the helical pitch. The results indicate that different initial flow velocities in the inner tube and the outer tube of the HCTT heat exchanger have little influence on the secondary flow of the fluid in the helical tubes, and the overall flow characteristics tend to be stable. The smaller helical middle diameter of the HCTT heat exchanger leads to the shorter fluid flow length, the smaller resistance along the tubes and the increase of initial pressure under the condition of constant inlet velocity, which promotes the occurrence of secondary flow. The axial flow of fluid promotes the destruction of heat transfer boundary layer and gains strength of the turbulence and heat transfer efficiency. With the increase of the helical pitch of the HCTT heat exchanger, the turbulent intensity and the heat transfer efficiency are also increased. Moreover, the improvement of the flow state of the HCTT exchanger in a longer helical pitch also enhances the heat exchange efficiency.