Elliptical Tube Heat Exchangers Research Articles

Aero-thermal behavior and performance optimization of rectangular finned elliptical heat exchangers with different tube arrangements

Tube arrangement largely determines the flow structure as well as aero-thermal performance in finned tube heat exchangers (FTHXs). Most of the previous studies did not cover both overall performance and local thermal flow characteristics due to the physical complexity. This work is related to the comparison of various aero-thermal behaviors of rectangular finned elliptical tube heat exchangers (RFETHXs) with different tube arrangements in a turbulent flow region, from basic characteristics to optimal structures. Using 3-D numerical simulation based on the validated standard k-ε model, the local thermal flow characteristic as well as overall dimensional and dimensionless performances of three arrangements is investigated. Then, using variance analysis, the contribution of each structural parameter of the three arrangements on overall performance is compared after the screening of the factor of longitudinal tube pitch. Using the same CFD data as variance analysis, an elliptical basis function (EBF) model of each type of arrangements is constructed to understand the distribution of j/f0.5 as well as PEC in the parameter range of this work. Moreover, optimal structural parameters of each arrangement are provided. It is observed that, each tube arrangement presents its own characteristic of flow separation and heat transfer coefficient distribution on fins. For the in-line and staggered row arrangements, the contributions of all the main factors and the two-way interactions on various overall performances are close, while the situation of the staggered column is different. The influence of two-way interactions is marginal in the staggered column structure, while it cannot be neglected in the two other arrangements in most cases. For j/f0.5, the optimal structural parameter combination of the in-line and staggered row arrangements is fin thickness 0.6 mm, fin pitch 2 mm and transverse tube pitch 54 mm, whereas that of the staggered column arrangement is 0.6 mm, 2 mm and 70 mm in turn.

Numerical simulation of natural convection heat transfer for annular elliptical finned tube heat exchanger with experimental data

This study presents a hybrid method of three-dimensional computational fluid dynamics (CFD) commercial software and inverse method along with experimental data and various flow models to study the natural convection heat transfer and fluid flow characteristics of vertical annular elliptical finned tube heat exchangers for various fin spacings. The inverse method of the finite difference method along with the experimental temperature data is first applied to estimate the heat transfer coefficient on the fins. After that, CFD along with various flow models and estimated heat transfer coefficients are used to determine air temperature and air velocity profiles, fin surface temperature and heat transfer coefficient on the fins. More accurate numerical results, appropriate flow model and number of grid points, number of iterations and relative convergence criteria can be obtained when the resulting heat transfer coefficient and fin temperature are as close as possible to the inverse results and experimental temperature measurements, respectively. The results show that the zero-equation turbulence model is more suitable for this study than other flow models. The choice of relative convergence criteria, number of iterations and grid points are important for obtaining more accurate results. The commercial software version, the relative convergence criteria for momentum and energy equations, the number of iterations and the Nt value vary with the fin spacing. The effect of the round-off error on the numerical results obtained needs to be considered. The heat transfer coefficient increases with increasing fin spacing and approaches a constant. The best fin spacing is about 18 mm. The fin efficiency of the annular elliptical fins is higher than that of the annular circular fins. The proposed correlations between Nusselt number and Rayleigh number is in good agreement with the numerical and inverse results obtained.

Thermodynamic coupling characteristics in hybrid (dry/wet) cooling system

ABSTRACTA hybrid cooling system consisting of both a dry section and wet section is proposed in this paper as a means to conserving energy and water by combining the benefits of both dry and wet cooling modes. A new thermodynamic coupling characteristics computing model was established to identify the best combination of dry and wet cooling subsystems in the hybrid tower throughout year-round operation based on its air thermodynamic state under the “no plume” principle. A hybrid cooling tower in Inner Mongolia, China, consisting of an elliptical tube heat exchanger with rectangular fins and counter-flow wet packing, was designed as an example under the no plume principle. The minimum number of heat exchanger units in service and the corresponding thermodynamic operating parameters were obtained under a year-round operation. The tower exhibited notable advantages in regards to water conservation compared to the traditional evaporative cooling tower at an estimated yearly savings of 3.74 × 107 kg water.

Parametric study on rectangular finned elliptical tube heat exchangers with the increase of number of rows

The objective of this study is to investigate the performance of rectangular finned elliptical tube heat exchangers (RFETHXs) of low number of rows (N) in turbulent region, with an emphasis on the characteristics with the increase of N. Using 3D numerical simulations based on the validated standard k-ε turbulence model, the average thermal-hydraulic characteristics of each row as well as the overall performance along with N are investigated. Then the contribution of each structural factor and the interaction effects between various parameters on the performance are revealed using variance analysis in 4 sub-row-regions. The most significant interaction effect on the performance evaluation criterion (PEC) between structural parameters is confirmed using Response Surface Method (RSM). It is observed that, for the RFETHXs with N > 2, the heat transfer coefficient of the second row of fins is the largest, while those of the first and the last rows remain the smallest two. A 2-row RFETHX has better performance compared to the RFETHXs with larger N. In most cases, the flow and heat transfer of RFETHXs enters fully developed state from the fifth row, but as the transverse tube pitch (Pt) increases or the fin thickness declines, such state is postponed to deeper rows. It demonstrates that significant impact of N on PEC exists in the sub-row-range of 2–3. With the incline of N, the effect of fin pitch (Fp) to j and f decreases while that of Pt increases, and the interaction effect between Pt and Fp to j and PEC becomes more pronounced. This interaction reduces with the increase of frontal velocity. The reason for such interaction is discussed. At last, results of RMS analysis further depict that the interaction effect between Pt and Fp on PEC of a 6-row RFETHX is more significant than that of a 2-row RFETHX.

Experimental Investigation of Cross Flow Staggered Elliptic Tube Heat Exchanger at 455 Angle of Attack

Thermal and hydraulic of performance of cross flow heat exchanger using elliptical shape tube in staggered arrangement at 45˚ angle of attack is carried out. Hot water is passed in a zigzag manner through 25 tubes of length 0.15 m and cold air is flowing over the tubes in cross flow heat exchanger of size 1×0.15×0.165 m^3. Elliptic shaped tubes array is having transverse and longitudinal pitches of 0.052 m with 45o angle of attack and minor to major axis ratio of 0.5. From the literature it was observed that the selected 45o angle of attack gives the highest thermal performance for a heat exchanger. Effect of varying mass flow rate of air on Nusselt number, Reynolds number, Prandtl number, coefficient of heat transfer, friction factor and pressure drop is calculated. Thermal performance of elliptical shape tube heat exchanger increases 30% - 75% as compare to circular shape tubes in the Reynolds number range 4500 - 14500. Friction factor (f) decreases from 1.96 to 1.38 as Reynolds number increased. The new correlation is developed for staggered tube array as, 〖Nu〗_a=0.106×〖Re〗_a^(0.773)×〖Pr〗_a^(0.36).

Heat transfer and pressure drop characteristics of inclined elliptical fin tube heat exchanger of varying ellipticity ratio using CFD code

A three-dimensional computational fluid dynamics (CFD) investigation of the friction and heat transfer characteristics of a fin tube heat exchanger having elliptical tubes of various ellipticity ratio and orientation of these tubes to the horizontal are analysed in this study. Air in the transitional regime is considered as the working fluid in this present study. Numerical validation of the predicted results by ANSYS Fluent 15.0 is carried out by comparing the simulated results with the numerical results a obtained by Bhuiyan et al. (2013). Reynold’s number at inlet is varied in the range of 1300–1600 and Standard k-ω turbulence model is used for turbulent closure. The ellipticity ratio of the tubes utilized for the analysis ranges between 0.6 and 0.8 and the effect of rotation of these tubes on Colburn factor, friction factor and efficiency index has been analysed. The Colburn factor increased initially reaching a peak, after which it gets lowered with increase in tube inclination. Increase in ellipticity and Reynold’s number decreased the Colburn factor. Friction factor varies in direct proportion to tube inclination and ellipticity ratio, whereas it decreases with the increase in Reynold’s number. The efficiency index parameter for all the models has been evaluated.

Numerical study on airside thermal-hydraulic performance of rectangular finned elliptical tube heat exchanger with large row number in turbulent flow regime

The objective of this study is to investigate the air side thermal–hydraulic characteristics of rectangular finned elliptical tube heat exchangers (RFETHXs) using 3D numerical simulations based on the validated standard k-ε turbulence model. First, fin efficiency of RFETHXs is investigated. The effects of various parameters such as row number, transverse tube pitch, longitudinal tube pitch, fin pitch, fin thickness and Re have been studied. Then the influences of structural parameters on the thermal-hydraulic performance of RFETHXs are examined, not only the characteristics including the effect of fin thermal resistance but also the pure convective performance excluded this effect. The contribution ratios of each structural parameter on fin efficiency and air side thermal-hydraulic performance are given. It is observed that, the most important structure factors for fin efficiency are fin thickness and fin pitch. For the RFETHXs with fully developed air side flow and heat transfer (N>5), from the third row to the last second row, the average fin surface Nu of each row is almost stable, and the value of the second row is 9% higher than the stable value. The results show that the air side heat transfer performance and friction factor of RFETHXs is mainly determined by the transverse tube pitch. For j/f1/2 at fixed Re, the contribution ratio of transverse tube pitch is 32% while the value is 24% at fixed frontal velocity.Moreover, two sets of multiple correlations, both including and excluding fin thermal resistance, are proposed to describe the air side heat transfer and friction characteristics of RFETHXs in turbulent flow regime.

Infrared thermography for local Nusselt number estimation of an elliptical fin with a transient method

In this paper, a transient method involving an infrared (IR) set-up is used to investigate local heat transfer distribution over the fin of a staggered elliptical finned tube heat exchanger assembly. An experimental test bench was designed to record data during the fin cooling. The procedure of data preprocessing including IR camera calibration is presented. It allows extracting the temperature field evolution of the fin. These temperatures feed a numerical model that works with thermally thin material and takes into account lateral heat conduction and radiation with the surrounding. The heat transfer coefficient field is determined by integrating the model over time intervals that depend on space. Distributions of Nusselt number over the fin and their uncertainties are presented for several Reynolds numbers. The high resolution of the whole method and set-up allows detecting thermal imprints of developing horseshoe vortices.

Pumping Power Minimization in Staggered Finned Circular and Elliptic-Tube Heat Exchangers in Turbulent Flow

An experimental study aimed at minimizing pumping power required to supply air through a finned tube bundle configuration is presented in this article. Results were obtained for the Reynolds number based on the smaller ellipse axis (RE2b) ranging from 2,650 to 10,600, i.e., in turbulent flow. In the turbulent regime, pressure drops are expected to vary with eccentricity. Experimental optimization results for finned circular and elliptic tubes show that pumping power can be minimized with respect to tube spacing and eccentricity. In comparison with values obtained for circular tubes, the optimal elliptical arrangements show pumping power reductions from 5% to 10%.

Performance Analysis for Elliptical Tube Longitudinal Vortex Generators

In this paper, three-dimensional numerical simulation is conducted on the flow and heat transfer process for elliptical tube heat exchanger with rectangular vortex generators and triangular vortex generator by Fluent software, respectively, and the performance for elliptical tube heat exchanger with plate fin is compared. The results show that the Nusselt number increases with the increase in Reynolds number for the inlet velocity range from 1 to 5m/s, friction coefficient decreases with the increase in Reynolds number; at the same Reynolds number, Nusselt number for fin with Vortex Generators and friction coefficient are higher than that of elliptical tube with plate fin ; the vortex generator shape and angle of attack have a great impact on the comprehensive performance within a simulated attack angle and the velocity, at the same pump power, the comprehensive performance for heat exchanger with the rectangular vortex generator increase up to 10%.

Thermal performance criteria of elliptic tube bundle in crossflow

In this work, the thermofluid characteristics of the elliptic tube bundle in crossflow have been investigated. Experimental and numerical investigations of the turbulent flow through bundle of elliptic tubes heat exchanger are carried out with a particular reference to the circular tube bundle. The investigation covers the effects of key design parameters of Reynolds numbers (5600–40,000), minor-to-major axis ratios (0.25, 0.33. 0.5 and 1) and flow angles of attack (0–150°). Five bundles of elliptic tube heat exchangers with different axis ratios were designed and manufactured in staggered manner. Numerical CFD modeling using finite volume discretization method was conducted to predict the system performance extensively. Four methods were presented to resort a metric that expresses the thermal performance criteria of the elliptic tube bundle. The results indicated that, increasing the angle of attack clockwise until 90° enhances the convective heat transfer coefficient considerably. The maximum thermal performance under constraint of a fixed pumping power or a mass flow rate was obtained at a zero angle of attack and the minimum thermal performance occurred at an angle of attack equals 90°. The best thermal performance of the elliptic tube heat exchanger was qualified with the lower values of Reynolds number, axis ratio and angle of attack.

Three-dimensional numerical study and field synergy principle analysis of wavy fin heat exchangers with elliptic tubes

Three dimensional numerical studies were performed for laminar heat transfer and fluid flow characteristics of wavy fin heat exchangers with elliptic/circular tubes by body-fitted coordinates system. The simulation results of circular tube were compared with the experiment data, then circular and elliptic ( e = b/ a = 0.6) arrangements with the same minimum flow cross-sectional area were compared. A max relative heat transfer gain of up to 30% is observed in the elliptic arrangement, and corresponding friction factor only increased by about 10%. The effects of five factors on wavy fin and elliptic tube heat exchangers were examined: Reynolds number (based on the smaller ellipse axis, 500 ∼ 4000), eccentricity ( b/ a, 0.6 ∼ 1.0), fin pitch ( F p /2 b, 0.05 ∼ 0.4), fin thickness ( F t /2 b, 0.006 ∼ 0.04) and tube spanwise pitch ( S 1/2 b, 1.0 ∼ 2.0). The results show that with the increasing of Reynolds number and fin thickness, decreasing of the eccentricity and spanwise tube pitch, the heat transfer of the finned tube bank are enhanced with some penalty in pressure drop. There is an optimum fin pitch ( F p /2 b = 0.1) for heat transfer, but friction factor always decreases with increase of fin pitch. And when F p /2 b is larger than 0.25, it has little effects on heat transfer and pressure drop. The results were also analyzed from the view point of field synergy principle. It was found that the effects of the five factors on the heat transfer performance can be well described by the field synergy principle.

Measurements of the overall heat transfer from combustion gases confined within elliptical tube heat exchangers

Measurements of the overall heat transfer for elliptical cylinders in cross-flow are presented for various flatness ratios and incidence angles. The measurements included the internal convective transfer of confined flowing (hot) gases combined with the external convective transfer. The internal hot gases were created using natural gas flames whereby combustion was completed prior to entering the elliptical test section. Results are reported for three flatness ratios, 0.20, 0.31, and 0.52, in addition to three incidence angles, 0, 45, and 90 degrees. The measurements are compared to round tube measurements and previous studies on elliptical cylinders in cross-flow. Results indicate that a significant flatness (0.30 or less) must be achieved to realize any appreciable (greater than 10 percent) change in the overall heat transfer coefficient. The effect of flattening heat exchanger tubes is presented in terms of the heat transfer enhancement gained without any consideration of the increase in the internal pressure losses.

Optimization study and heat transfer comparison of staggered circular and elliptic tubes in forced convection

In this study, a two-dimensional (2-D) heat transfer analysis was performed in circular and elliptic tube heat exchangers. The finite element method was used to discretize the fluid flow and heat transfer governing equations and a 2-D isoparametric, four-noded, linear element was implemented for the finite element analysis program, FEAP (O.C. Zienkiewicz, R.L. Taylor, The Finite Element Method, vol. 1, McGraw-Hill, London, 1989, Chapter 15). The numerical results for the equilateral triangle staggering configuration, obtained with the new element were then validated qualitatively by means of direct comparison to previously published experimental results for circular tubes heat exchangers (G. Stanescu, A.J. Fowler, A. Bejan, Int. J. Heat Mass Transfer 39 (2) (1996) 311–317). Next, a numerical geometric optimization was conducted to maximize the total heat transfer rate between the given volume and the given external flow both for circular and elliptic arrangements, for general staggering configurations. The results are reported for air in the range 300⩽ Re L⩽800, where L is the swept length of the fixed volume. Circular and elliptical arrangements with the same flow obstruction cross-sectional area were compared on the basis of maximum total heat transfer. The effect of ellipses eccentricity was also investigated. A relative heat transfer gain of up to 13% is observed in the optimal elliptical arrangement, as compared to the optimal circular one. The heat transfer gain, combined with the relative pressure drop reduction of up to 25% observed in previous studies (H. Brauer, Chem. Process Eng., August (1964) 451–460; S.N. Bordalo, F.E.M. Saboya, Determinação experimental dos coeficientes de perda de carga em trocadores de calor de tubos circulares e elípticos aletados, in: Proceedings of the 13th COBEM, Congresso Brasileiro de Engenharia Mec a ̂ nica (in Portuguese), Belo Horizonte, Brasil, 1995) show that the elliptical arrangement has the potential for a considerably better overall performance than the traditional circular one.