Effect of lateral fin profiles on turbulent flow and heat transfer performance of internally finned tubes

Heat transfer performance of internally finned tubes with blocked core-tube was numerically investigated by the realizable k–ε turbulence model with wall function method using a commercial software FLUENT. Three kinds of lateral fin profiles, that is, S-shape, Z-shape and V-shape, were studied and compared. The corresponding correlations of Nusselt number and friction factor for different-shape internally finned tubes were obtained. The comprehensive performances of the studied tubes were compared under identical mass flow rate, identical pumping power and identical pressure drop conditions. It was found that tubes with S-shape fins and Z-shape fins were superior to that with V-shape fins, and moreover, tube with Z-shape fins had the best performance. The fin outer curvature radius R near the inner surface of out-tube for the S-shape finned tube had appreciable effect on heat transfer, whereas the fin inner curvature radius r near the outer surface of blocked core-tube had little impact on heat transfer. Hence, when manufacturing the internally finned tube with S-shape fins, it is better to select the outer curvature radius R as smaller as possible.     

Improvement of heat transfer with perforated circular holes in finned tubes of air-cooled heat exchanger

The present study investigated the effect of perforated circular finned-tube (PCFT) on the convective heat transfer performance of circular finned-tube heat exchangers. The air-side convective heat transfer coefficients increased by 3.55% and 3.31% for 2-hole and 4-hole PCFT cases, respectively. The increase in the convective heat transfer coefficient was related to the reduction of the recirculation region by introducing the perforations at the flow-separation locations on the finned tube. The pressure drop across the finned-tube bundles increased by 0.68% and 2.08% for the 2-hole PCFT and 4-hole PCFT cases, respectively. The greater pressure drop in the case of the 4-hole PCFT might be due to excessive flow disturbances produced by multiple perforations. The fin factor defined as the ratio of the % increase of the convective heat transfer coefficient and that of the pressure drop was 5.19 for the 2-hole PCFT case, whereas that was 1.59 for the 4-hole PCFT case.     

Numerical computation of flow and heat transfer in finned and unfinned tube banks

A finite volume numerical scheme is utilized to predict fluid flow and heat transfer characteristics in inline tube banks. The effect of equipping the tubes with longitudinal fins on the pressure drop and heat transfer is studied. The governing equations for fluid flow and heat transfer are numerically solved, with the assumption of periodic, fully developed flow. The numerical methodology utilizes the stepped boundary technique to approximate the tube surface. The tubes are maintained at a constant temperature, and the calculations are carried out for laminar flow and for a large range of Reynolds and Prandtl numbers. The results for the unfinned tube case are compared with previously published experimental data. The numerical results agree well with the experimental measurements. Representative results for the case of the finned tubes indicate, surprisingly, a decrease in the heat transfer rate, and small changes in the pressure drop, as a result of finning. The decrease in the heat transfer rate probably occurs because the fins are placed in the stagnation regions at the front and rear of the tubes, and thus do not increase the heat transfer.     

Heat Transfer Characteristics of Laminar Flow in Internally Finned Tubes under Various Boundary Conditions

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Maximization of heat transfer density from radially finned tubes in cross-flow using the constructal design method

The maximization of volumetric heat transfer density from radially finned tubes in cross-flow is investigated in this study based on the constructal design method. A row of radially finned tubes is placed in cross-air flow. The tubes and the radial fins are heated at uniform temperatures and cooled by the air cross-flow. The cross-air flow is generated by a finite pressure difference. Two dimensionless pressure differences (Bejan number) are considered (Be = 10³ and Be = 10⁵). The objective function, the degrees of freedom, and the constraints in the constructal design method should be identified. The objective function is the maximization of the heat transfer density from the finned tubes. The degrees of freedom are; the fin tip-to-fin tip spacing, the number of fins, the tube diameter, the fin thickness, and the angle between the fins. The constraints are the length and height of the space occupied by the finned tubes. The pressure-driven flow and energy equations (steady, two-dimensional, and incompressible) are solved by means of the finite volume method. The ranges of the dimensionless fin tip-to-fin tip spacing are (0.2 ≤ S ≤ 1 for Be = 10³ and 0.05≤ S ≤ 0.3 for Be = 10⁵). The number of fins is changed as (N = 2, 4, 6, 8, 10, and 12). The dimensionless tube diameter is changed as (D = 0.25, 0.5, and 0.75). The dimensionless fin thickness is changed as (T = 0.001, 0.01, and 0.05). The results showed that for both (Be = 10³) and (Be = 10⁵), the highest value of the maximum volumetric heat transfer density is for (N = 2) and decreases as the number of fins increases. In addition, the minimum values of the maximum volumetric heat transfer density occur when the vertical fins exist at (N = 4, 8, and 12).     

Optimum length of tubes for heat transfer in turbulent flow at constant wall temperature

Maximum heat transfer per cross-sectional area of a tube with smooth wall in turbulent flow at constant wall temperature is determined for a given pressure loss. The dimensionless tube length is determined dependent on the pressure Reynolds number, Prandtl number and inlet local pressure loss coefficient. Limiting cases for short and long tubes are separately investigated. Semi-empirical equations are derived for both optimum dimensionless tube length and dimensionless maximum heat flow per cross-sectional area using numerically obtained values with a maximum deviation of ±6.6% and with a RMSE of 3.5%. The results can also be applied to the channels with non-circular cross-sectional area.     

Effects of hydrophilic surface treatment on evaporation heat transfer at the outside wall of horizontal tubes

An experimental study has been conducted to investigate the effects of hydrophilic surface treatment on evaporation heat transfer at the outside wall of various kinds of copper tubes. Plain, spiral, corrugated, and low-finned tubes were selected as test tubes. In this work, to increase the wettability of distilled water on copper tubes, a novel hydrophilic surface treatment method using plasma was employed. The experiments show that every kind of hydrophilic surface treated tube tested in the work exhibits superior evaporation heat transfer performance as compared with that of the same kind of untreated tube. It is found out that during the evaporation process, the high wettability of the surface obtained through hydrophilic treatment induces film flow on the tubes while sessile drops are formed on untreated tubes. The film has a smaller thickness as well as a greater heat transfer area than the sessile drops, and this yields higher heat transfer rate for hydrophilic surface treated tubes than that for untreated tubes.     

An experimental heat transfer study for helically flowing outside petal-shaped finned tubes with different geometrical parameters

The performances of shell-side heat transfer and pressure drop were experimentally studied in a helically baffled single tube heat exchanger, where water was used as a working medium. The tested tubes included one smooth tube and five petal-shaped fin tubes (PF tubes) with different geometrical parameters for improving the heat transfer of the shell side. It was shown that, compared with the smooth tube, five PF tubes significantly increased the values of Nusselt numbers. The Nusselt numbers increased with the fin height and decreased with the fin pitch. In the range of the present experiments, it was found that the Nusselt numbers for the PF tubes were increased by up to 233%, while the pressure drop was increased by less than 111%, as compared with that for the smooth tube. It is a promising route to use a PF tube instead of smooth tube for improving the performance of a helically baffled heat exchanger.     

Augmentation of heat transfer during filmwise condensation of steam and R-134a over single horizontal finned tubes

An experimental investigation has been carried out to augment the heat transfer rate by enhancing the heat transfer coefficient during the condensation of pure vapours of steam and R-134a over horizontal finned tubes. The study was conducted for plain tubes, circular integral-fin tubes (CIFTs), spine integral-fin tubes (SIFTs) and parially spined circular integral-fin tubes (PCIFTs). The SIFT out performed the CIFT for the condensation of R-134a by approximately 16%. However, the spines were found most effective in the bottom side of the CIFT. The PCIFTs with the spines only in the bottom side of the tube augmented the heat transfer coefficient by 20% and 11% for the condensation of steam and R-134a, respectively, in comparison to the CIFT.     

Radial mass flow in electrohydrodynamically-enhanced forced heat transfer in tubes

Experimental results of an electrohydrodynamically-enhanced oil heater of annular cross-section are presented. When a high d.c. voltage (typically 30 kV) was applied across the annular gap, it induced a very strong radial motion of the fluid resulting in a heat transfer increase of more than 20 fold over the fully-developed laminar flow, yet the pressure drop only increased 3 fold. The electrohydrodynamically-enhanced Nusselt number correlates well with electrical current passed, which is typically a few millionths of an ampere per metre of tube.     

Water pool boiling heat transfer enhancement for modified surface tubes

This paper deals with the method of decreasing the size of heat exchanger surfaces by increasing the heat transfer coefficients and the importance of heat transfer enhancement for vaporization. We report an experimental study on surfaces modified by passive methods applied to heat transfer surfaces mechanically processed, covered with sleeves made by metallic tissues or covered with metallic porous layers performed using welding procedures. Experiments are made to investigate the heat transfer coefficient on copper tubes with a 22 mm external diameter using heat from inner source to outer vaporizing liquid. There are developed specific heat transfer correlations for each group of enhanced surfaces. The experimental data and new proposed correlations are compared with well known correlations. The results are in best agreement with the Cornwell–Houston correlation.     

Single-phaseexperimental analysis of heat transfer in helically finned heat exchanger

A methodology for designing helically serrated finned tube heat exchanger based on the logarithmic Mean Temperature Difference (LMTD) method is validated with experimental tests. The method uses semi-empirical correlations for calculating convective coefficients both inside and outside staggered tube bundles. Equipment was designed, built, and installed in a paper factory in order to validate the methodology. Comparisons between predictions and experimental data show a precision of approximately 96% in heat transfer and approximately 90% in pressure drop for Reynolds numbers upper to 10,000.     

Turbulent fluid flow,heat transfer and onset of nucleate boiling in annular finned passages

An analytical model has been formulated for fully-developed turbulent flow and heat transfer in finned annuli using a modified mixing-length turbulence model. The model accounts for the conjugate heat transfer in the fluid and the solid, and the finite thickness of the fins. Solutions were obtained using the finite element method adopting a mesh that exactly fits the solution domain with fine elements near the solid boundaries. Predictions of the model have been compared with experimental results for smooth and finned annuli with generally good agreement between data and predictions. The model has been extended to predict the conditions at the onset of nucleate boiling using the criterion of Davis and Anderson. Again, these predictions agreed well in magnitude and trend with experimental data of finned annuli.     

Saturated flow boiling heat transfer and critical heat flux in small horizontal flattened tubes

This paper presents experimental results for flow boiling heat transfer coefficient and critical heat flux (CHF) in small flattened tubes. The tested flattened tubes have the same equivalent internal diameter of 2.2 mm, but different aspect height/width ratios (H/W) of ¼, ½, 2 and 4. The experimental data were compared against results for circular tubes using R134a and R245fa as working fluids at a nominal saturation temperature of 31 °C. For mass velocities higher than 200 kg/m²s, the flattened and circular tubes presented similar heat transfer coefficients. Such a behavior is related to the fact that stratification effects are negligible under conditions of higher mass velocities. Heat transfer correlations from the literature, usually developed using only circular-channel experimental data, predicted the flattened tube results for mass velocities higher than 200 kg/m²s with mean absolute error lower than 20% using the equivalent diameter to account for the geometry effect. Similarly, the critical heat flux results were found to be independent of the tube aspect ratio when the same equivalent length was kept. Equivalent length is a new parameter which takes into account the channel heat transfer area. The CHF correlations for round tubes predicted the flattened tube data relatively well when using the equivalent diameter and length. Furthermore, a new proposed CHF correlation predicted the present flattened tube data with a mean absolute error of 5%.     

表面强化管外池沸腾传热特性研究

为实现节能降耗,开发了多种强化沸腾传热的高效换热管。以水为工质,在0.1MPa下对垂直光管、烧结多孔管和T槽管进行了池沸腾传热实验研究,并分析了沿管子轴向的温度分布。实验结果表明,烧结多孔管与T槽管能显著降低起始沸腾过热度、强化沸腾传热:烧结多孔管和T槽管的起始沸腾过热度比光管的低1.5K左右;烧结多孔管和T槽管的核态沸腾传热系数分别为光管的2.4～3.2倍和1.6～2.0倍。此外,烧结多孔管和T槽管能降低相同热流密度下的壁面温度,且有利于降低管子轴向的温差。     

Simple power-type heat transfer correlations for turbulent pipe flow in tubes

Heat transfer in the turbulent flow of fluid in a pipe is analyzed. Nusselt number as a function of the Reynolds and Prandtl number is given. Power-type correlations were proposed within a wide range of Reynolds and Prandtl number. Relationships for the Nusselt number compare well with experimental data. The paper presents three power-type correlations of a simple form, which are valid for Reynolds numbers range from 3·103 ≤ Re ≤ 106, and for three different ranges of Prandtl number: 0.1 ≤ Pr ≤ 1.0, 1.0 Pr ≤ 3.0, and 3.0 Pr ≤ 103. Heat transfer correlations developed in the paper were compared with experimental results available in the literature. The comparisons performed in the paper confirm the good accuracy of the proposed correlations. They are also much simpler compared with the relationship of Gnielinski, which is also widely used in the heat transfer calculations.     

The heat transfer heterogeneities of bends in flow boiling of hairpin tubes

A series of visual experiments were conducted for liquid– vapor two‐phase flow in hairpin tubes, and it was observed that most of the nucleation sites were located at the outer tube wall of the bend. From the simulation, it was concluded that the uneven velocity distribution in the bend induced the heat transfer heterogeneity. Furthermore, the nucleation of both the inner and outer tube wall of the bend and the wall temperature distribution were discussed to understand the physical phenomena. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20269     

Numerical study of flow and heat transfer characteristics in outward convex corrugated tubes

The flow and heat transfer characteristics in convex corrugated tubes have been investigated through numerical simulations in this paper. Two kinds of tube types named as symmetric corrugated tube (SCT) and asymmetric corrugated tube (ACT) are modeled and studied numerically based on the k-ε model. The heat transfer working fluid at shell and tube sides are nitrogen and helium gases respectively. 2D axisymmetric model is adopted to simplify 3D model in order to reduce the computation cost greatly. Numerical simulation results for flow and heat transfer performances in SCT and ACT with various geometrical parameters, including corrugation pitch, corrugation height and corrugation trough radii are systematically analyzed. The mechanisms behind the improvement of overall performances of the simulated outward convex corrugated tube are discussed through investigating the details of turbulent velocity fields at both tube and shell sides. Compared to SCT, ACT exhibits 8–18% higher overall heat transfer performance.     

Forced flow and convective melting heat transfer of clathrate hydrate slurry in tubes

In this study, we presented the forced flow and convective melting heat transfer characteristics of tetrabutylammonium bromide (TBAB) clathrate hydrate slurry (CHS) flowing through straight circular tubes. Pressure drops of TBAB CHS with the volume fraction in the range of 0–20.0 vol% were measured, and the influence of the volume fraction was analyzed. Power-law equation was used to describe the flow behavior of CHS, and the corresponding indicators including flow behavior index and fluid consistency coefficient indicated that TBAB CHS behaved as a pseudo plastic fluid. Based on the modified Reynolds number, it was found that the empirical correlation could be used to predict the flow friction factor of TBAB CHS. Heat transfer experiments were conducted to measure the local heat transfer coefficients of TBAB CHS flow under constant wall heat flux, which were found to be enhanced by the latent heat involved in the solid–liquid phase change. Local heat transfer coefficient decreased along the heating section until the volume fraction was smaller than a certain small value, then it increased in the following section due to the increase of Reynolds number. The effects of heating power, flow velocity, volume fraction on the local heat transfer coefficients were discussed. Correlations for predicting TBAB CHS heat transfer performance were proposed based on the experimental data in laminar, transitional and turbulent flow regions, respectively.