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In the present study, condensation heat transfer and frictional pressure drops of refrigerant R-600a (iso-butane) inside a helically dimpled tube and a plain tube of internal diameter 8.3 mm were measured and analyzed. All tests were performed at different vapor qualities up to 0.82 and average saturation temperatures ranging between 38 and 42℃. Refrigerant mass fluxes varied in the range of 114-368 kg/m 2 s. The inner surface of the helically dimpled tube has been designed and reshaped through three-dimensional material surface modifications consists of both shallow and deep protrusions which are placed evenly in helical directions on the tube wall. The experimental results show that the heat transfer coefficients of the dimpled tube are 1.2-2 times of those in smooth tube with a pressure drop penalty just ranging between 58% and 195%. The highest heat transfer coefficient is occurred at vapor quality of 0.53 and mass flow rate of 368 kg/m 2 s. On the other hand, the maximum increase of pressure drop takes place at vapor quality of 0.55 and mass flow rate of 368 kg/m 2 s. Nomenclature ݉ሶ mass flow rate (kg/s) A C p surface area (m ଶ ) specific heat (kJ/kg k)

The layout design problem of a propulsion system is complex and time-consuming process. This is mainly due to geometrical and performance constraints and system requirements. In addition, layout design optimization of a space propulsion system is non-linear, non-convex, and multimodal, which makes it difficult to implement conventional optimization methods to this class of design problems. This paper presents a hybrid optimization algorithm using genetic algorithm and sequential quadratic programming for optimal layout design of a space propulsion system. Previous research works mainly focused on the layout design components with constant parts. However, the approach adopted in this paper involves both variable mass component and hybrid optimization algorithm (GA-SQP) of a space propulsion system. The proposed hybrid optimization algorithm explores globally the design search space to locate the most promising region using genetic algorithm, whereas gradient-based sequential quadratic programming algorithm is used to reduce the computational time with a high degree of accuracy. The results obtained show that the proposed method provides an effective way of solving layout design optimization problem using both variable mass components method and a hybrid optimization for optimal layout design of a space propulsion system.