**1. Introduction**

The application of heat transfer enhancement technologies provides the potential to minimize cost, produce smaller units, and increase the reliability of heat exchangers. Enhanced tubes with passive enhancement can enhance the thermal performance while producing a small increase (when compared to a smooth tube) to pressure drop for most conditions. Therefore, widely used enhancement structures (such as micro-fin, herringbone, corrugated tubes, etc.) draw considerable attention in various industrial applications (i.e., air conditioning and refrigeration applications). According to Webb and Kim [1], three-dimensional enhanced tubes are good choices for heat transfer argumentation; three-dimensional enhanced tubes are considered here. Enhancement in these tubes is achieved by (i) increasing turbulence and surface area, (ii) producing fluid mixing and secondary flows, and

(iii) interrupting boundary layers. Additionally, when compared to traditional two-dimensional enhancement techniques, the enhanced three-dimensional tubes provide better condensate drainage effects, and an increase in nucleate site densities can also be achieved using the unique characteristic structures that make up the enhancements (i.e., dimples, grooves, etc.).

Since these three-dimensional tubes are novel, only a few investigations were carried out into various condensation and evaporation heat transfer characteristics. Additional experimental, numerical, and optimization studies need to be performed in order to better understand the thermal potential of these three-dimensional enhanced tubes. Several previous investigations were conducted into the heat transfer performance of similar dimpled tubes. Wang et al. [2] experimentally studied the heat transfer and flow properties of a dimpled tube. The results showed that the Nusselt number was enhanced (when higher compared to the equivalent smooth tube) by 26.9–75% (for ellipsoidal dimpled tubes) and 32.9–92% (spherical dimpled tubes). Ellipsoidal dimples on the inner surface can lower the laminar-to-turbulent transitional Reynolds number to a value less than 1000. Li et al. conducted numerical works [3] and geometrical optimizations [4] on the dimpled tubes, and they concluded that three-dimensional surfaces enhanced by dimples can significantly promote the thermal performance of heat exchangers; furthermore, the shape, depth, and arrangement of dimples significantly influence the thermal performance. Vicente et al. [5] investigated the heat transfer and pressure drop for low Reynolds flow in dimpled tubes. Similar experimental works were also reported by Kukulka et al. [6] to investigate the thermal performance of three-dimensional surfaces during single-phase flows. An overall parametric study and optimization was carried out recently by Lei et al. [7] based on a response surface methodology and desirability approach, while a combination of CuO/water nanofluid and dimpled tubes was investigated by Suresh et al. [8]. Therefore, additional optimization studies need to be performed in order to maximize the heat transfer efficiency. When considering two-phase flow application, limited previous investigations were found for the individual enhancement structures. Guo et al. [9] performed an experimental study that compared the convective heat transfer coefficient for a herringbone tube and a three-dimensional enhanced surface tube during condensation and evaporation of R22, R32, and R410A; they found that the herringbone tube provided a heat transfer coefficient increase of 200–300% when compared to a smooth tube during condensation; the heat transfer coefficient of the 1EHT (enhanced three-dimensional surface) tube was 1.3–1.95 times larger than that of the smooth tube. In addition, the 1EHT tube provided the best heat transfer performance during evaporation for the three working fluids. Li et al. [10] conducted experimental investigations to explore tube-side condensation and evaporation characteristics of two different 2EHT (a differently structured three-dimensional surface) enhanced tubes. Although negligible area enhancements were provided by these two enhanced tubes, the heat transfer coefficient ratio (when compared to an equivalent plain tube) was in the range of 1.1–1.43. Aroonerat and Wongwises conducted a series of experiments that were performed in order to determine the thermal performance of dimpled tubes [11], and the effect of dimple depths [12], helical angle, and dimple pitches [13] on the condensation heat transfer coefficient and pressure drop of R134a flowing in dimpled tubes. The results showed that the dimpled tube with the largest depth provided the highest heat transfer coefficient, as well as the largest pressure drop penalty (an unexpected pressure drop increase up to 892% higher than that of the smooth tube was reported). Sarmadian et al. [14] measured and analyzed the condensation heat transfer coefficient and frictional pressure drop of R600a in a helically dimpled tube. Their experimental results indicated that the heat transfer coefficients of the dimpled tube were 1.2–2 times higher than those found in an equivalent smooth tube with a pressure drop penalty ranging from 58% to 195% (when compared to smooth tubes). Their visualization showed that the dimples could accelerate the transition between annular and stratified flows. Shafaee et al. [15] performed a saturated flow boiling experiment and reported that the heat transfer performance was substantially improved because of the enhancement design. Additional enhancement structure design analyses were investigated by Ayub et al. [16], and their results show that, under similar operating conditions, the enhanced tube

with a rod insert provided a three-fold higher heat transfer coefficient than the plain tube; additionally, the corresponding pressure drop penalty was even lower for low mass fluxes.

Several experimental investigations were conducted for two-phase heat transfer performance of annular (tube in tube) flows to evaluate the enhancement characteristics of two-sided, three-dimensional, dimple tubes. Li et al. [17] performed an experimental investigation on the shell-side flow condensation of R410A on horizontal tubes, at mass fluxes values in the range from 5–50 kg/(m2·s). Their results indicated that the smooth tube exhibited superior thermal performance over other enhanced tubes (herringbone and EHT tubes); this strange trend might be attributed to the liquid inundation at the lower portion in the annular, which results from surface tension effects. Tang and Li [18] carried out an experimental study on two horizontal enhanced tubes, as well as the enhanced surfaces that were made up of dimples, protrusions, and grooves. Mass flux and vapor qualities were varied in order to explore the possible mechanisms of the enhanced surfaces; results showed that the proposed enhanced tubes seemed to show a worse thermal performance than the smooth tube for *<sup>G</sup>* <sup>&</sup>lt; 150 kg/(m2·s), while one of the enhanced tubes provided an enhanced heat transfer coefficient ratio in the range from 1.03–1.14 for *G* = 200 kg/(m2·s). All EHT tubes employed by Tang and Li [18] and Li et al. [17] had the same outer diameter of 12.7 mm; future investigations should determine the effect of tube diameter on the heat transfer performance.

In this study, the three-dimensional enhanced (1EHT) tube and an equivalent plain tube with the same outer diameter of 19.05 mm were employed for shell-side condensation and evaporation heat transfer performance evaluation. Unlike traditional enhanced tubes such as micro-fin tubes, herringbone tubes, and typical dimpled tubes (as discussed in Aroonerat and Wongwises [19]), the 1EHT tube has a composite enhancement structure, which is made up of helically arranged dimples and petal arrays. The special surface structure of the 1EHT tube is shown in Figure 1. Dorao and Fernandino [20] suggested that the improvement in enhanced surfaces is connected to the improvement observed in the single-phase flow. The same article indicated that the delta-T-dependent region is related to improved mixing at the wall. Given this theory, the primary deep dimples were designed to increase fluid turbulence, enhance fluid mixing, and produce secondary flows, while a roughness was produced from the staggered petal arrays of shallow dimples that were designed for boundary layer interruption and nucleate site argumentation. As shown in Figure 1, the primary dimples of the 1EHT tube had a height of 1.71 mm and a typical projected diameter of 4.4 mm. In addition, they were helically arranged on the external surface of tubes, with a pitch of 9.86 mm and a helical angle of 60◦, and with an enhanced surface area ratio of 1.20 (internal surface area of the enhanced tube compared to that of the smooth tube). Detailed parameters of tested tubes are listed in Table 1. The objective of this study was to investigate the effect of mass flux, annulus gap, and enhancement structures on the heat transfer performance during condensation and evaporation of R134a in the annulus of a tube-in-tube heat exchanger.

**Figure 1.** The external surface structure of the 1EHT tube.


**Table 1.** Details of the test tubes.
