The main aims of the current three-dimensional numerical study are scrutinizing the effects of various twisted tapes on hydraulic-thermal performance and optimizing the geometrical parameters of a twisted tape geometry via response surface methodology. This study is carried out in three various phases. In the 1st phase, heat transfer characteristics of testing fluid through a tube which have a simple twisted tape is investigated. In this phase, numerical, experimental, and theoretical results are compared to each other. Then, the effects of different inserts on three key parameters, i.e., Nusselt (Nu) number, friction fraction (f), and thermal performance factor are investigated. After determining the superior twisted tape from the thermal performance factor aspect, in the final phase, an optimization study via response surface methodology is performed based on different objectives. According to the results, amidst the different inserts, the clockwise-counterclockwise and alternate axis twisted tapes ranked first from the highest thermal performance factor (1.46) and Nu number (34.27) standpoints, respectively. The lower friction fraction belonged to clockwise-counterclockwise type (f = 0.4068). The optimum value for twisted ratio, connection angle, and number of clockwise-counterclockwise strips, in order, is 3.02, 44.55°, and 1. Compared to tubes with and without simple insert, the Nu number increased by 46.2% and 221%, respectively, utilizing optimized clockwise-counterclockwise insert.

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