Surface modification of transversely twisted-turbulator using perforations and winglets: An extended study

Abstract

Twisted-turbulators are effective passive techniques for enhancing the thermal performance of heat exchangers. Although numerous studies have been conducted on longitudinal forms, there are very scarce studies on transverse forms. In this study, particular attention is paid on the effects of perforation, winglet, and a combination of both, as modifications, on the performance of transversely twisted-turbulators (TTTs) through a rectangular channel. Firstly, experimental tests are performed on the TTTs with three different twist-angles of 0°, 90°, and 180° for the Reynolds number range from 1643 to 4929. Then, numerical simulations are carried out on these models. It is found that the effects of perforations and winglets depend on the twist-angle of TTTs. In the case of 0° TTTs, introducing both perforations and winglets leads to lower values of h and ∆p. The results show that the solid model is more effective in fluid dispersion towards the hot walls and generate stronger vortices inside the channel. Based on the experimental data, for the case of 0° TTTs, the average reduction of h and ∆p between the solid model and other models are as follows, 9.8% and 16.4% for the perforated model, 16.2% and 16.9% for the winged model, and 23.6% and 41.8% for the combined model. However, in the cases of 90° and 180° TTTs, creating winglets enhances these parameters, leading to better overall performance. The highest overall performance indexes of 1.55, 1.66, and 1.65 are recorded for the solid 0° TTT, winged 90° TTT, and winged 180° TTT models at the Reynolds number of 1643.

Introduction

Heat transfer is a common process in many industries, from food to oil, gas, and petrochemical. Hence, improving the performance of heat exchange devices is critical for both economic and environmental viewpoints. Heat transfer enhancement techniques can be divided into three general categories: active, passive, and compound. The active technique involves the disturbance in fluid flow using external power, such as magnetic fields [1] and pulsating flow [2]. In contrast, passive techniques require no external power source and employ adding solid particles [3], geometry modification [4], and insertion of swirl flow devices, which is discussed in the following. A combination of both techniques is utilized in compound techniques. Often passive techniques are preferred due to the merits of simplicity, low-cost, and good enhancement with a reasonable penalty in pressure drop [5].

Twisted-turbulator is a type of swirl flow device that has attracted many researchers in recent years. Fig. 1 shows the trend of publications on this swirl flow device in the period range from 2000 to 2020, i.e., in the last twenty years. The most important motive for using twisted-turbulators is to create secondary rotational flows by inducing radial velocity components. It leads to better mixing between fluid layers, thereby increasing heat transfer rates.

Manglik and Bergles [6,7] experimentally investigated the effect of fitting typical twisted-turbulators on the thermohydraulic performance of a tube with constant wall temperature. They introduced correlations for the Nusselt number and friction factor of laminar, transition, and turbulent flow regime. They found that fitting twisted-turbulators can effectively enhance the heat transfer in the tube. However, because of the increased contact area between the fluid and the twisted tape, the pressure loss also augments significantly. Many studies were conducted on twisted-turbulators to obtain the maximized heat transfer enhancement, while the pressure loss being controlled as low as possible. Chakroun and Al-Fahed, [8] studied the effect of twisted-turbulator width on the thermohydraulic performance of fully developed laminar flow. Saha and Dutta [9] studied the effect of twist-ratio, length, and free space ratio of twisted-turbulators on the heat transfer and pressure drop of laminar flow. Eiamsa-ard et al. [10] compared the heat transfer and pressure loss of a typical full-length twisted-turbulator with that of a twisted tape with various free space ratio.

Several surface modifications have also been investigated on longitudinally twisted-turbulator (LTTs). It has been concluded that some of these surface modifications can further improve heat transfer enhancement due to the presence of swirl flows in the flowing fluid adjacent to solid walls. Eiamsa-ard et al. [[11], [12], [13]] experimentally studied delta-winglet-cut, peripherally-cut, and serrated twisted tape to enhance the degree of turbulence near the walls of a circular tube. Wongcharee and Eiamsa-ard [14] investigated the influence of triangular, rectangular, and trapezoidal wings formed on LTTs mounted in a circular tube and found that the trapezoidal wings had better performance. Promvonge et al. [15] equipped the LTTs with rectangular winglet vortex generators and achieved 17% higher OPI in comparison with typical LTTs. Bhuiya et al. [16] examined the effect of perforation on LTTs. They investigated the effect of four different porosities in a turbulent flow regime and obtained 28% to 59% higher overall performance index in comparison with a plain tube. Thianpong et al. [17] experimentally investigated the combined effect of perforation and parallel winglets on a LTT fitted in a heated tube in order to both induce extra turbulence near the walls and reduce pressure loss. They achieved a maximum overall performance index of 1.32. Rahimi et al. [18] compared the thermohydraulic performance of a typical LTT with that of perforated, notched, and jagged twisted tape and reported that the performance of the jagged twisted tapes was higher than other ones. Kumar et al. [19] introduced V-cuts on perforated LTTs and reported the maximum overall performance index of 1.58. Saysroy [20] numerically investigated the thermal performance of a tube fitted with square-cut twisted tape inserts and reported that compared to typical LTTs, the overall performance index is up to 1.32 times greater. Lin et al. [21] proposed a newly designed LTT with parallelogram winglets in order to decrease the contact area between the tape and the working fluid and subsequently reduced the pressure drop.

Although several works have been conducted on the LTTs with original and modified shapes [[22], [23], [24]], there are very scarce studies in the open literature on the transversely twisted-turbulator (TTT) [[21], [22], [23]]. Nanan et al. [25,26] examined the TTTs inside circular tubes, both experimentally and numerically, and Rashidi et al. [27] numerically studied these geometries in a square channel. In this work, an experimental study is carried out on employing TTTs inside a rectangular channel for the first time. Likewise, to the best of our knowledge, modifications on TTTs have not been investigated in the past, and this is the main motivation behind this study. Firstly, perforations and delta-winglets are introduced on the TTTs, then the combination of these techniques is examined. This study may help to understand basic physical processes for improving this passive technique (TTTs), particularly when combinations of two other techniques (perforations and delta-winglets) are employed.