Thermal and flow performance of tilted oval tubes with novel fin designs

https://doi.org/10.1016/j.ijheatmasstransfer.2020.119621Get rights and content

Highlights

  • Heat exchangers with novel additive manufactured fins were devised, fabricated and experimentally studied for different tube tilt angle.

  • Nusselt number is higher for both novel heat exchanger designs compared to the conventional heat exchanger and increases with tube tilt angle.

  • The serrated integrated pin fin design gives the best.

  • Highest volumetric heat flux density was achieved for the circular integrated pin fin design with highest tube tilt angle.

  • The global performance of the finned oval tubes depends on the fin design, the Reynolds number and the tube tilt angle.

  • Correlations for the heat transfer from the novel heat exchangers were proposed for different tube tilt angle.

Abstract

We studied the thermal and flow performance of tube heat exchangers with novel fin designs for tube tilt angles of 0°, 20°, 30° and 40° to the horizontal. The novel fin designs target to enhance the conduction heat transfer within the fin and the convective heat transfer along the fin surface simultaneously. Tubes with three different fin designs, the circular plain fin (CPF), the circular integrated pin fin (CIPF) and the serrated integrated pin fin (SIPF), were additively manufactured by selective laser melting and experimentally investigated in an air flow channel for Reynolds number between 1800 and 7800. We analysed the performance evaluation criterion, the volumetric heat flux density and the global performance criterion. It was found, that the SIPF achieves highest performance evaluation criterion and the CPF performs worst. Thus, the SIPF is recommended, when the required surface area, the material cost and the weight of the finned tube heat exchanger are relevant. Highest heat transfer per volume heat exchanger and temperature difference was achieved for the CIPF at highest tube tilt angle. The value of the global performance criterion strongly depends on the fin design and the tube tilt angle. For the horizontal orientation the CPF reaches highest global performance and for the 40 tube tilt angle the CIPF gives best performance. From the experimental data we derived appropriate heat transfer correlations for Reynolds number, Prandtl number, tube tilt angle and fin designs.

Introduction

Finned tube heat exchangers are used in many energy engineering applications, such as e.g. air-conditioning, refrigeration, electronics cooling, power generation, solar thermal systems and more. The heat transfer rate of cross flow heat exchangers to ambient air is usually restricted by the thermal resistance on the gas side, which is typically 85 % of the total thermal resistance [1]. For that reason the heat exchanger surface is extended on the gas side by fins to increase the heat flux. Out of the many fin patterns the circular plain fin is the most common one, due to its high durability, simplicity and rigidity [2]. However, light weight, compactness and high performance are required in modern energy utilization systems to improve the manufacturing and operation of heat exchangers [3]. For that reason many other fin designs were developed, such as serrated fins, crimped fins, plain plate fins, wavy fins and plain fins with delta winglets [4]. Nevertheless, to the knowledge of the authors these fin designs do mostly not target to improve the heat conduction within the fin. Furthermore, in most applications the main flow direction on such heat exchangers is not perpendicular to the tube axis. Typically tubes are tilted to allow drainage of the heat transfer fluid inside the tube or to reduce the foot print [5]. In fact, the tilt angle of a tube has some effect on the air-side flow and consequently influences the heat transfer and flow performance of a heat exchanger. In the study presented here, we developed novel fin designs to improve the thermal conduction within the fin and studied their thermal and flow performance for various tube tilt angles. Selective laser melting (SLM), an additive manufacturing technology, was applied to materialize these designs. Before discussing our results we will in the following give a brief summary on the state-of-the-art of finned cross flow heat exchangers.

The heat transfer performance of finned tubes and tube bundle heat exchangers was studied by many researchers in the recent past. Thus, Gnielinski developed correlations for the heat transfer of heated plates, cylinders and spheres in a flow field by analysing numerous experimental data from literature [6]. The heat transfer and pressure drop of annular finned tubes in a one-row and two-row inline as well as staggered configuration were experimentally studied by Sparrow at al. [7]. For the two-row configuration a higher pressure drop compared to the one-row configuration was found. Furthermore, the two-row staggered configuration showed higher pressure drop and Nusselt number compared to the inline arrangement. Fin efficiency and flow distribution was studied by Watel et al. for a finned tube [8]. Thermography and particle image velocimetry were used in the mid-plane between the fins and the fin spacing was changed. Heat transfer on finned tubes with rising fin spacing was investigated by Chen et al. [9]. An increase of heat transfer coefficient at growing fin spacing up to 18 mm was observed. For higher spacing the heat transfer coefficient enhancement was found to become small and the fin efficiency to become almost constant. The effect of fin spacing on heat transfer was also studied by Mon et al. by numerical methods for a finned tube heat exchanger in staggered and inline arrangement [10]. It was found that the development of boundary layers as well as horseshoe vortices between the fins strongly depend on the ratio between the fin spacing and the fin height. An optimum ratio of 0.32 was determined by the authors. Another numerical analysis was performed by Kong et al. to describe the effect of several parameters, such as fin oblique angle, fin spacing, fin thickness and tube external diameter [11]. As an optimum the authors recommended a fin oblique angle of 30, a fin spacing of 10 mm, a fin thickness of 0.3 mm and an external diameter of 18 mm. Zhao et al. modelled the heat transfer from rectangular finned oval tubes having different tube row number, transversal and longitudinal tube pitch, fin pitch and fin thickness [12]. The thermal and flow performance was found to be mainly influenced by the transversal tube pitch and the fin efficiency was found to be dominated by the fin thickness and fin spacing. A four row finned oval tube heat exchanger in a chimney was numerically studied by Unger et al. [13]. In these simulations, the natural convection flow was found to become similar to a forced convection flow, when the chimney reaches a height of 11 m. An optimum geometry regarding heat transfer was found for a fin height of 17 mm, a fin spacing of 3 mm and a fin thickness of 1.5 mm.

Even if the annular tube shape is the most common one in industrial applications, the oval tube and fin shape received increasing attention, due to the lower pressure drop compared to the annular one. While the published studies clearly show a lower pressure drop for oval shape tubes, the findings for the relation between tube shape and heat transfer are generally inconsistent. Jang et al. found a reduction of heat transfer and pressure drop for four row heat exchangers with oval tubes compared to ones with circular tubes [14]. Other numerical studies dealing with circular, oval and wing shaped tubes also found the heat transfer from the circular tube being higher compared to oval and wing shaped tubes [15], [16]. However, several other articles report a higher heat transfer performance of oval tubes over annular tubes. For instance, a relative heat transfer enhancement up to 19 % and a material reduction of 32 % was reported for oval tubes compared to circular tube shapes in [17]. In the numerical study of Ibrahim et al. the axis ratio of a cross flow heat exchanger was varied between 1: 4, 1: 3, 1: 2 and 1: 1 [18]. In fact, the highest heat transfer performance occurred for the highest eccentricity of 1: 4. Oval and circular finned tube condensers were also numerically analysed by Sun et al. to determine their overall performance [19]. An improvement of heat transfer between 8.3-30.9 % as well as a reduction in pressure loss between 20.0-27.3 % was observed for the elliptical tubes compared to the circular ones. Kumar et al. studied air-cooled heat exchangers by varying the tube shape from annular to oval tubes [20]. The simulations show an enhancement of heat transfer and a reduction of pressure drop for increasing tube eccentricity. The oval tubes showed a flow separation further downstream and the thermal wake regions were smaller compared to the annular shaped tubes. A recent CFD study of Yogesh et al. dealt with the heat transfer and pressure drop of oval finned tubes having an axis ratio of 1: 1, 1: 1.25, 1: 1.43 and 1: 1.67 [21]. In the transitional flow regime the largest heat transfer and lowest pressure drop were found for an axis ratio of 1: 1.67.

Various knowledge does also exists for non-plain circular fins. A review paper on different extended heat transfer surfaces was published by Nagarani et al. [22]. Various fin and tube designs have been compared there, e.g. annular fins, elliptical fins and tubes, pin fins as well as longitudinal fins and it was observed, that both elliptical tubes and elliptical fins give better performance than annular ones. Another literature study on finned heat exchangers with different designs and geometrical parameters was performed by Bhuiyan et al. [2]. Their review of experimental and numerical work summarizes a greater heat transfer performance and pressure drop of wavy fins compared to plain fins as well as for rising fin pitches. Wavy fins were analyzed for the laminar flow case and for the turbulent flow case by Bhuiyan et al. [23], [24]. Higher heat transfer performance and higher pressure drop of wavy fins over plain fins was indicated by these studies. The ratio of heat transfer to pressure drop, indicated as efficiency index, was lower for wavy fins in laminar and turbulent flow regime compared to the plain fin type. This study was extended for different wavy fin angles and fin pitches [25]. The efficiency index was greatest for the lowest fin pitch and reduces with higher wavy angle. Plate fin and tube heat exchanger were modeled by Bhuiyan et al. for transitional and turbulent flow [26], [27]. For both cases the efficiency index increases with reducing fin pitch and increasing longitudinal and transversal tube pitch. Kumar et al. [4] made an extensive numerical investigation on the thermal-hydraulic characteristics of various circular and plate fin designs. They concluded that the circular fins give the best performance in terms of heat transfer coefficient and heat transfer per unit pumping power compared to the plate fins and the crimpled fins give higher volume goodness factor. Bošnjaković et al. introduced a needle fin design and studied this by numerical methods [28]. This new design reached 30 % higher Nusselt number, 23.8 % less heat exchanger surface and required 10 % more specific fan power compared to the circular fin at air flow velocities between 1 m/s and 5 m/s. Additive manufacturing was used in the study of Bacellar et al. to generate air foil shaped tubes for microchannel heat exchangers [29]. The air foil shaped tube heat exchanger was 20 % smaller, had 20 % less pressure drop and 40 % lower material volume. Oval tubes with circular plain fins, circular integrated pin fins and serrated integrated pin fins with fin spacing of 6 mm, 11 mm and 16 mm were investigated by our group using [30]. We found, that the Nusselt number reduces and the pressure drop rises for lower fin spacing. The circular integrated pin fin designs reached highest heat transfer per volume heat exchanger and the serrated integrated pin fin designs gave best overall performance. Novel jagged fins on air condenser tubes were introduced by Guo et al. [31]. In this analysis a single tube was equipped with flat fins as well as two, five and ten separated novel jagged fins and an increase of Nusselt number as well as friction factor with the number of fins was observed. In fact, the tube with ten gave highest heat transfer and pressure drop. In a recent article of Du et al. novel air cooled heat exchangers were fabricated by direct laser sintering [32]. The performance of this heat exchanger was up to 40 % higher compared to the plain plate fin heat exchangers.

In many applications the main flow direction for the finned tubes is not perpendicular to the tube axis due to geometrical restrictions or required fluid drainage inside the tube. However, only few studies analysed the influence of incident flow direction on heat transfer and friction. Two-row and three-row finned oval tube heat exchangers were experimentally investigate by Du et al. [33]. The entire heat exchangers were tilted towards the incoming air flow in different angles. For the two-row tube bundle higher Nusselt number and smaller friction factor were measured compared to the three-row tube bundle. The tube tilt angle of finned oval tubes was experimentally investigated by Unger et al. for forced convection [34] and natural convection [35]. It was found that the heat transfer increases with tube tilt angle from horizontal to 40 for forced convection and reduces for natural convection. However, for forced convection the pressure drop rises at higher tube tilt angles due to greater blockage effect. In the experiments of Kanematsu et al. three heat exchangers, an inline tube with uncut fins, staggered tubes with cut fins and staggered tubes with uncut fins and 0, 45, 60 and 80 inclination angle were studied [36]. At an inclination angle of 80 the heat transfer strongly reduces due to the formation of large wake regions. In the simulation of Guo et al. heat transfer of multi-louvered fin heat exchangers with different tube inclination angles was investigated [37]. The angle of attack was  ± 25,  ± 45 as well as 0 and the considered fin pitches were 1, 1.5 and 2. The effect of inlet flow angle reduces with fin pitch and a positive inlet flow angle results in highest performance. Şahin et al. considered 7 different fin inclinations (0, 5, 10, 15, 20, 25 and 30) for heat transfer and pressure drop analysis in a numerical simulation [38]. A maximum value of both, heat transfer and pressure drop, was reached at 30. Most researchers studied the change of angle of attack with respect to the rotation axis of the tube, while the tube tilt angle of the tube axis has not been studied yet. Furthermore, most articles describe fin designs that disturb the boundary layer development along the fin surface to improve the convective heat transfer.

Thus, the present investigation aims to introduce two novel fin designs, which enhance the local conductive heat transfer within the fin by material strengthening as well as the convective heat transfer, due to higher flow mixing. Furthermore, we intend to analyse the thermal performance of these fin designs for different orientations. Based on the experimental outcome empirical heat transfer correlations were derived for these novel fin designs for various tube tilt angles.

Section snippets

Experimental setup and measurement techniques

Experiments were performed in a 6.5 m long PMMA flow channel with an inner cross section of 0.27  ×  0.127 m2 (Fig. 1). Three sieve flow straighteners with smaller getting mesh size in downstream direction are installed upstream the heat exchanger structure to uniform the incoming air flow. Furthermore, a honeycomb flow straightener is installed to reduce the transversal air flow. Downstream of the test section there is an out-feed section of about 2.2 m to avoid any backlash from the flow

Data processing and analysis

As one of the most relevant parameters we determine the heat transfer coefficient from the relationshiph=Q(At+Af·η)·ΔTLM.

Here, Q is the air-side heat transfer rate, which is given by the heating power input, Af is the fin surface area, At is the tube surface area and ΔTLM is the logarithmic mean temperature differenceΔTLM=(TinTt)(ToutTt)ln(TinTtToutTt).

The temperatures are the test section inlet temperature Tin, the outlet temperature Tout and the tube wall temperature Tt. All temperature

Results and discussion

The influence of the conventional circular plain fins (CPF), integrated pinned fins (CIPF) and serrated integrated pinned fins (SIPF) as well as the tube tilt angle of these designs on the heat transfer and flow characteristics was analysed. Furthermore the thermal and flow performance criteria performance evaluation criterion, volumetric heat flux density and global performance criterion were determined from the measurements to assess the quality of the finned oval tubes. A validation of the

Conclusion

Two novel fin designs were introduced which enhance the conduction heat transfer from the fin base to the fin tip and improve the convective heat transfer along the fin surface simultaneously. An additively manufacturing process was applied to generate the new find designs and an experimental investigation was performed in a flow channel. In the experiments we studied the flow and heat transfer for different Reynolds numbers, tube tilt angles of α=0, α=20, α=30 and α=40 for the CPF, CIPF

Credit author statement

The authors are responsible and ensure an accurate description. All authors contributed to scientific content and the editorial content of the present paper. The order of the authors represents the share of the contribution to the article.

Declaration of Competing Interest

None.

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