Abstract: The flow-field,in the casing double-shell of spiral baffle heat exchanger,was reasonably approximated,mathematically formulated with Realizable k-ε Turbulence model,theoretically analyzed and numerically simulated with Fluent software.The influence of the inner-shell diameter on the distributions of pressure/velocity/temperature of flowing water and performance of the heat exchanger was investigated.The simulated results show that the inner-shell diameter has a major impact.To be specific,the optimized diameter of 108 mm is capable of reducing the total pressure-drop by 53.2%~55.4%,and increasing the heat-transfer coefficient by 4.32%~10.7% on the shell wall.However,a small diameter,<95 mm,rapidly increases the pressure-drop,and a large diameter,>120 mm,considerably decreases the heat-exchange efficiency.The simulated and measured results were in good agreement.An obvious limitation of casing double-shell scheme is that removal of a cylindrical bundle of tube surround the inner-shell significantly decreases the heat-exchange capacity.