Thermal design evaluation of ribbed/grooved tubes: An entropy and exergy approach

https://doi.org/10.1016/j.icheatmasstransfer.2020.105048Get rights and content

Highlights

  • A thermal evaluation of modified internally ribbed/grooved tubes is discussed.

  • The effect of various design parameters for ribs/grooves is widely examined.

  • Detailed flow phenomena and generated vortices domination are detected.

  • Entropy, exergy, and PEC valuable analysis are conducted to clarify the proper mechanism.

  • Optimum design parameters are explained thermodynamically.

Abstract

The smooth circular tubes are typically used in air-cooled heat exchangers systems. In literature, various research toward improving the hydraulic/thermal performance by changing the tubes' inner surface topology is found. In the current work, fluid friction and heat transfer characteristics of air-cooled tube heat exchangers with installed ribs and grooves have been numerically compared. Initially, the numerically calculated data of the ribbed tubes are validated /compared to the performance results in the literature. Performance evaluation criteria (PEC), entropy and exergy are numerically investigated for a tube with a group of ribs and grooves over the range of Reynolds number Re = 6700–20,000. Rib radius (r), number of ribs in the same group (Ng), the distance between two adjacent ribs (SL), and pitch between two adjacent groups (p) are the ribs parameters selected to study their effects on flow and heat transfer criteria. Also, the same evaluation criteria are performed on grooved tube with optimal parameters to make a comparison between results. Moreover, streamline patterns and temperature/pressure/velocity/turbulence kinetic energy contours are visually introduced to explicate the physics of heat transfer and pressure drop phenomena and to assess the flow structures inside different ribbed/grooved tubes. For the tubes with high-performance evaluation criteria (PEC), lower and higher entropy and exergy efficiency are determined. Recapitulation results from the technical comparison supported the use of grooved-tubes instead of ribbed-tubes. This is mainly due to lower flow irreversibility accompanied by higher PEC and exergy efficiency than those of ribbed tubes at the same flow condition.

Introduction

Heat exchanger is considered as a vital component for many industries, such as power plants and many of well-established chemical factories. Therefore, to reach the heat exchanger's optimum design, the augmentation of heat transfer is crucial, and many studies are performed. Increasing the heat transfer rate and reducing heat exchanger cost and size are the avails of thermal improvement of the heat exchanger. Active and passive techniques are utilized for enhancing the process of heat transfer [[1], [2], [3], [4], [5]]. For example, Alam and Kim [1] gave a review of the different active methods and passive techniques that have been used to enhance the heat transfer rate in heat exchanger devices such as solar air heater and cooling blades of turbines. Also, Sheikholeslami et al. [2] carried out an extensive literature review of various turbulators, rough surfaces, and swirl flow devices for enhancing heat transfer in heat exchangers. Kareem et al. [3] presented an extensive review of numerical and experimental studies on heat transfer enhancement, which covers the laminar and turbulent flow regions in the corrugations, especially in corrugated tubes. Garg et al. [4] made a review of heat transfer augmentation using twisted tapes used in heat exchanger systems. Ahmed et al. [5] presented a review of published works on flow and heat transfer augmentation in tube heat exchangers. Typically, passive techniques are considered a standard method used in heat exchangers to enhance heat transfer and reduce the operational cost [6]. Passive techniques include roughness creation in the inner tube surface and tape insertion inside the tube. These techniques, as well as vortices generation, causes disturbances in the fluid flow that makes discontinuities in the fluid boundary layers resulting in proper mixing between the hot and cold regions. Twisted tapes, coiled wires, and vortex generation are common shapes that are inserted into the tube. Also, the use of ribs, grooves, and dimples on the surface are typical shapes of roughness modification.

Zheng et al. [[7], [8], [9], [10], [11]], numerically, studied the effect of discrete ribs and grooves as inclined or straight on the friction factor (f) and Nusselt number (Nu). Several parameters for ribs/grooves were chosen in these studies, as listed in Table 1. They used performance evaluation criteria (PEC) to calculate thermal-hydraulic performance (THP) quickly. Results showed that ribs give higher PEC than the grooves or ribs/grooves combinations. Also, inclined ribs/grooves combinations were acting better than straight ones. Bilen et al. [12] claimed that the heat transfer rate in a tube with circular, trapezoidal, and rectangular grooves, respectively, increases by 63%, 58%, and 47% more than that of the smooth one. Moreover, the pressure drop (∆P) and the rate of heat transfer for different pitches helically grooved tubes were studied experimentally by Pirbastami et al. [13]. Outcomes indicated that a helically grooved tube improved the coefficient of heat transfer h by 34% more than the smooth one. The effects of ribs depth (e) and pitch (p) between two adjacent ribs on h and (∆P) were examined by Huang et al. [14]. They found that h increases to 3 times higher than the smooth one. Helically corrugated tubes with various (e) and (p) were experimentally and numerically studied in [15,16]. They stated that h increased by 3.8 times more than that of the smooth one. Wang et al. [17] showed that the use of an internal helically-finned tube increased h by 1.8 times that of the smooth one. García et al. [18] performed a comparison between corrugated, dimpled tubes and those with wire-coils insertion from a heat transfer point of view. This study showed that tubes with coiled-wires were recommended at Reynolds number of (2 × 102 ≤ Re ≤ 2 × 103) and corrugated and dimpled tubes were recommended at Re ≥ 2 × 103 because the tube with coiled-wire has higher ∆P. san et al. [19] inserted coiled-wire with different (p) and wire thickness (t) inside the tube and studied its effect on (∆P) and (h). Results indicated that Nu increased to more than or nearly twice the value for the smooth tube. Blossom shaped internal fins were numerically simulated by Duan et al.[20]. Results showed that increasing the number of fins above 3 had the same effect on heat transfer. Circular disk inserts can increase h by up to 4.45 times than that of the smooth tube, as shown in Kumar et al. [21]. Multi or single twisted-tapes perforated or drilled by different shapes were introduced in [[22], [23], [24]]. For instant, He et al. [22] investigated the heat transfer and flow characteristics of a tube fitted with cross hollow, twisted tape inserts. PEC varies from 0.87 to 0.98 under a Re of 5600–18,000. Bhuiya et al. [23]explored the effects of perforated double counter twisted tapes on heat transfer and fluid flow characteristics in a heat exchanger. PEC varies from 1.08 to 1.44 based on constant pumping power. Oni and Paul [24] carried out the thermal analysis for flow through a circular tube induced with different twisted tapes. They observed that PEC for the modified case is 1.35–1.43 times than that without modification. The use of a conical ring inside the tube increases h by up to 333% while adding twisted-tape with a conical ring increases the heat transfer rate by 367%, as shown in Promvonge and Eiamsa-ard [25]. Thianpong et al. [26] showed that h increased by up to 3 times than that of the smooth tube by using a combination of dimples and twisted tape. Double twisted-tape with helically ribbed tube increases h by about 2.2 times than that of the plain one. The helically-ribbed tube with double twisted-tape inserts gave a better improvement of h in comparison to plain or ribbed tubes, as stated by Promvonge et al. [27].

Bejan [28] gave a brief review of the requirements of thermodynamics optimization by using entropy generation minimization and exergy analysis. Keklikcioglu and Ozceyhan [29] experimentally investigated the influence of triangular cross-section coiled wire inserts on the entropy generation of a smooth circular tube. Results showed that the increase in Re and/or pitch ratio leads to an increase in entropy generation number. Entropy/exergy calculations were performed for experimentally tested equilateral triangle coiled-wire inserts in [30]. The results showed that the increase in Re leads to an increase in entropy generation number and a decrease in exergy. Recently, Sayed Ahmed et al. [31] numerically studied the effect of changing the ring-type ribs configurations in a circular tube using entropy and exergy analysis. Results indicated that PEC increases by up to 1.4 times compared to the smooth tube case.

Based on the previous review, the contribution of the present work is in applying the considered numerical scheme to test the thermal performance of a new design type of internal tube surface alteration. The proposed surface modification is by creating a group of ribs or grooves, which has a discrete pitch between two consecutive groups and a discrete distance between two ribs/grooves in the same group. The grouped ribs/grooves are different in design from a classic ring-type rib, which has a continues distance between the two successive ribs along the tube length (single group). The preliminary results showed that these methods have a better enhancement performance than classic ring-type ribs. To the best of our knowledge, no publication has been found in the literature that attempted to compare thermal/hydraulic performance between the grooved and ribbed tube in grouping configuration and extending the evaluation toward the exergy and entropy analysis. Furthermore, with the help of the numerical simulation results, the physics behind the flow circulation and reattachment over the ribbed/grooved tubes has been explained. To further achieve the objective, geometrical design parameters of the grouped ribs/grooves are varied, such as the number of ribs in each group (Ng), the distance between two ribs in the same group (SL), the pitch between two groups (p), and rib radius (r). Furthermore, the entropy generation and exergy analysis for these configurations are assessed. Two approaches have been identified in the current study. The first approach is to build a reliable numerical model based on a commercial code ANSYS FLUENT 17.2 to assess the performance of a smooth/ribbed tube in a steady-state condition. Once the numerical method is confirmed by an extended comparison with experimental data available in the literature, the ribbed/grooved tubes configuration are optimized in term of thermal performance and entropy/exergy analysis criteria by studying the effects of changing the geometrical parameters. The optimum ribbed tube configuration that gives the maximum thermal performance is identified and replaced with a grooved tube for further comprehensive performance evaluations in terms of different longitudinal length (SL) and flow parameters. The merits of using different arrangements (ribbed/grooved) are assessed by comparing entropy generation and exergy efficiency associated with each case.

Section snippets

Physical model description and numerical methodology

Fig. 1 shows the schematic diagram of a circular tube with multi-repeated ring-type ribs (Fig. 1a) and a grooved tube (Fig. 1b). Ribs are uniformly arranged on the inner tube surface. The smoothed/ribbed/grooved tube has a length (L) of 1080 mm. Since the focus of the current study is to address the internal flow characteristics inside tubes, the outer tube diameter is not mentioned or neither considered, and all tubes have an inner diameter (D) of 13.8 mm. The shaded areas for the grooved tube

Governing equations

Two-dimensional numerical calculations are performed using CFD FLUENT 17.2 software to study the heat transfer and flow characteristics of the proposed ribbed/grooved tubes. During the simulations, ANSYS Design-Modeler software is used to create the geometry where the meshing tool has the ability to produce a high-quality mesh. Details about governing equations, grid independence, solution settings, and data reduction are presented in this section.

Reynolds averaged Navier Stokes (RANS)

Results and discussion

Numerical studies have been performed at various Reynolds number to investigate the thermal/hydraulic performance of different types of alteration of air-cooled heat exchanger tubes with varying geometric parameters associated with these configurations. The results of computational work are compared/validated against experimental data available in the literature. Next, the effect of the main geometrical parameters is systematically discussed. Performance evaluation criterion, friction factor,

Conclusions

In the current study, enhanced wall treatment with turbulence model RNG (k-ε) is chosen to assess the effect of different design parameters for groups of ribbed/grooved tubes on entropy, exergy, and thermal performance. Below, the following essential evaluations concerning the graphics and visual results are summarized:

  • For (e = r), the ribbed tube gives higher PEC than the tube with (r) lower or higher than (e). Also, the ribbed tube with Ng = 3, p/D = 0.29, (e = r = 1 mm), and SL = 1 and 3 mm

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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