Structural optimization of double-tube once-through steam generator using Pontryagin's Maximum Principle

• A heat transfer model and a pressure drop model for a double-tube once-through steam generator are developed.
• The inner helical tube pitch is optimized with Pontryagin's Maximum Principle.
• An objective function which consists of normalized pressure drop and normalized tube length is constructed with linear weighting method.

The heat transfer performance of a double-tube once-through steam generator (DOTSG) is studied in this paper. The double-tube consists of an outer straight tube and an inner helical tube. To minimize the DOTSG volume and reduce the pressure drop, an objective function is constructed with normalized tube length and pressure drop with linear weighted method. The pitch optimization proceeds from superheated region to boiling region and then to sub-cooled region in sequence, using Pontryagin's Maximum Principle. The results show that the optimal pitch keeps constant along the axial direction in the sub-cooled region and superheated region, but varies in the boiling region. In the boiling region, the tube length is 6.4% longer while the pressure drop is 36.3% smaller compared with those obtained from minimum tube length optimization; and the pressure drop is 30.7% larger while the tube length is 4.6% smaller compared with those obtained from minimum pressure drop optimization. The theoretical analysis and analytical results show the feasibility of Pontryagin's Maximum Principle used for structural optimization in this paper.