**1. Introduction**

According to Li et al. [1], two types of techniques have been proposed to improve the heat transfer of heat exchangers: the active and the passive. The active techniques require external power, such as vibration or magnetic fields, whereas the passive techniques require deformations on the tube surface, without external power, as well as on any surface where there is heat transfer. The passive technique was widely recommended by several authors because it considers bent tubes and its ability to compact the heat exchanger. When proposing a combination of passive improvements to improve the heat transfer in a piece of equipment, the problem of knowing the hydrodynamics and temperature profile in the fluid is presented. Passive improvements increase the heat transfer and consequently compact equipment is built.

The contribution of this research is to provide a Computational Fluid Dynamic (CFD) study of a tube-in-tube helical heat exchanger evaluated with two passive techniques implemented. Special interest is put in the effects of the fluid flow rate on the heat transfer while the geometry was changing in the analysis.

The background of this research is described with the following works. Pan et al. [2] described the heat transfer improvements assuming passive techniques, it produced by secondary flows and studied by [3] since 1927. The simple improvement techniques consist of the insertion of coils [4], twist tape [1], staggered tapes in straight tubes, the corrugation of the tube and the curved tube [5]. Another technique is the double improvement, which is a combination of two simple improvements [6], such as a curved tube with springs, corrugated or baffles inserts; or a straight tube with tape inserts and corrugated [7]. Previous research emphasizes the CFD study in the heat exchangers with the aim of validating the numerical results and optimization [8] and control strategies of tubular heat exchangers using neural-networks-based method [9]. A numerical study using CFD approach in a vertical cylindrical tube-in-tank thermal energy storage with helical coils design is presented by [10], in which the effect of the input temperature and flow rate values to the heat transfer device were analyzed. Sharifi et al. [11] presented the influence of coiled wire inserts on the Nusselt number, friction coefficient and overall efficiency in double-pipe heat exchangers using CFD software for their analysis, in which significant improvements are described with regard to the heat exchanger containing coiled wire inserts with pitch length equal to 69 mm, the Nusselt number was enhanced by 1.77 times as compared with the plain heat exchanger. Numerical investigation and fluid flow characteristics of spiral finned-tube heat exchangers considering the effects of location of perforations and RNG *k* − *ε* model was carried out by [12].

Kumar et al. [13] presented the heat transfer characteristics and hydrodynamics behavior of tube-in-tube helical heat exchanger, this research is the basis of our work. The motivation of this research is to present the CFD results to quantify the increase in heat transfer with passive improvements in the helical heat exchanger with the objective of knowing the hydraulic and thermal behavior of the fluid subjected to the proposed operating conditions and the proposed geometries.

This work has been organized as follows: first, the simulation code was developed, and it was carefully verified with numerical results presented by [13]. Second, the simulation of the heat exchanger with four ridges in the internal tube, and the simulation of three heat exchangers increasing the number of twists in the internal tube from one to five. Finally, the Nusselt number was calculated for each case with the aim of assessing the effect of each passive technique on the heat transfer.
