## 1 Introduction  
Owing to the advanced enhancements on thermophysical property, transport property and heat transfer potentials in many industrial applications [1; 2; 3; 4; 5], nanofluids, first proposed by a research group in Argonne National Laboratory in the USA in 1995 [6], have attracted significant interest. A nanofluid is a uniform and stable suspension, composed of nanoparticles and base fluid. As the addition in nanofluid, the nanoparticles can be metal and/or nonmetal whose typical size is no larger than 100 nm. Many experimental and numerical investigations have been implemented on heat transfer enhancement of nanofluids in open literature. However, most of these works have used Newtonian fluids as the base fluid, while little attention has been paid on the nanofluids based on non-Newtonian fluids [7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20]. Non-Newtonian fluids, that is, against [PERSON]'s law of viscosity, have been widely used in many industries such as food, petrochemical, pharmaceutical industries, etc. However, the viscosity of non-Newtonian fluids is usually greater, involving low Reynolds number and high Prandtl number. Consequently, in the process of flow and heat transfer, the pressure drop is high while the heat transfer coefficient is considerably low [21]. To overcome the aforementioned disadvantages existing in non-Newtonian fluids, nanoparticles are added and form non-Newtonian nanofluids, whose thermophysical properties are improved. [PERSON] et al. [22] dispersed MWCNTs into an aqueous solution of carboxymethyl cellulose at a weight fraction of 3 wt % and obtained non-Newtonian nanofluids with 0.1 wt %, 0.5 wt %, 1 wt %, and 2 wt % MWCNTs. Results indicated that the thermal conductivity of all the nanofluids is higher than that of the base liquid and the thermal conductivity enhancement is as high as 14.6% for the nanofluid containing 2 wt % MWCNTs. [PERSON] et al. [23] dispersed \(\gamma\)-Al\({}_{2}\)O\({}_{3}\), TiO\({}_{2}\) and CuO nanoparticles into an aqueous solution of carboxymethyl cellulose to form three types of non-Newtonian nanofluids and compared their heat transfer performances flowing through a uniformly heated circular tube. Results showed that both Nusselt number and convective heat transfer coefficient were enhanced by nanoparticles and increase with an increase in the nanoparticle concentration. Up to now, researches and investigations mainly focus on the improvement of thermophysical properties or the flow and heat transfer characteristics in heat transfer equipment with extremely simple structure such as a single heated tube. As a necessary section in many industries, shell-and-tube heat exchangers are commonly used to heat or cool the process fluid. The flow and heat transfer performance of non-Newtonian fluids flowing in shell-and-tube heat exchangers is worth studying, but only a few investigations have been reported in open literature. In addition to disperse nanoparticles, improving the structure of shell-and-tube heat exchangers is another effective means of enhancing heat transfer rate and reducing flow resistance. The baffles are an important section in shell-and-tube heat exchangers because they not only support the tube bundles but also direct the working fluid flow. Compared with segmental baffles, helical baffles showed better performance. Experimental result obtained by [PERSON] et al. [24] indicated that the ratio of heat transfer coefficient to pressure drop in the shell side of heat exchanger with helical baffles is higher than that of heat exchanger with segmental baffles. Other than optimization of the baffles, utilizing enhanced tube is another way to improve heat transfer coefficient. [PERSON] et al. [25; 26; 27] have experimentally and numerically compared the shell side heat transfer performance of helical baffle heat exchanger with fin tubes and smooth tubes using oil and water as the working fluids. The results showed that the shell side heat transfer coefficient was obviously enhanced by finned tubes at the helical flow condition.