Air-side performance of herringbone wavy fin-and-tube heat exchangers under dehumidifying condition – Data with larger diameter tube

This study examines the airside performance of the herringbone wavy fin-and-tube heat exchangers in dehumidifying condition having a larger diameter tube (D_c = 16.59 mm) with the tube row ranging from 2 to 12. Test results are compared to that of dry conditions and plain fin geometry. Upon the influence of surface condition (dry or wet) on the heat transfer performance, the heat transfer performance in dehumidifying condition normally exceeds that in dry condition, and is more pronounced with the rise of tube row or reduction of fin pitch. By contrast, it is found that the heat transfer coefficient for plain fin geometry in dehumidifying condition is slightly lower than that in dry condition. The pressure drops in wet condition is much higher than that in dry condition. However, the difference in pressure drop amid dry and dehumidifying condition for wavy fin configuration is less profound as that of plain fin geometry.     

Airside performance of herringbone wavy fin-and-tube heat exchangers – data with larger diameter tube

This study examines the airside performance of the wavy fin-and-tube heat exchangers having a larger diameter tube (D_c = 16.59 mm) with the tube row ranging from 1 to 16. It is found that the effect of tube row on the heat transfer performance is quite significant, and the heat transfer performance deteriorates with the rise of tube row. The performance drop is especially pronounced at the low Reynolds number region. Actually more than 85% drop of heat transfer performance is seen for F_p ～ 1.7 mm as the row number is increased from 1 to 16. Upon the influence of tube row on the frictional performance, an unexpected row dependence of the friction factor is encountered. The effect of fin pitch on the airside performance is comparatively small for N = 1 or N = 2. However, a notable drop of heat transfer performance is seen when the number of tube row is increased, and normally higher heat transfer and frictional performance is associated with that of the larger fin pitch.     

Airside performance of fin-and-tube heat exchangers in dehumidifying conditions – Data with larger diameter

This study presents the airside performance of the fin-and-tube heat exchangers having plain fin geometry with a larger diameter tube (D_c = 15.88 mm) under dehumidifying condition. A total of nine samples of heat exchangers subject to change of the number of tube row and fin pitch are made and tested. It is found that the effect of fin pitch on the sensible j factor is, in general, diminished with the rise of tube row. However, there is a unique characteristic of fin pitch at a shallow tube row, the heat transfer performance is first increased at a wider pitch but a further increase of fin pitch lead to a falloff of heat transfer performance due to interactions amid flow development and bypass flow. The influence of tube row on the airside performance is rather small for both heat transfer and frictional characteristics at a fin pitch of 2.1 mm and when the Reynolds number is less than 4000. A slight deviation of this effect is encountered when fin pitch is increased to 2.54 mm or 3.1 mm due to condensate adhered phenomena.     

Parametric and design optimization investigation of a wavy fin and tube air heat exchanger using the T-G technique

The focus of this paper is to optimize the air-side performance of a wavy fin and tube heat exchanger at different design parameters on an individual target response using the Taguchi method. However, a statistical concept, gray relational analysis, is also studied for combined optimization, considering all target responses at a time. Based on the heat exchanger requirement, parametric study for the air-side is regarded as a more significant heat transfer and lower frictional factor. Experimental correlations were available and used for the 27 orthogonal runs. Investigation revealed the highest 47.06% fin pitch, 37.24% fin pitch, 25.46% air velocity, and 23.9% fin thickness contribution ratio for the target response of friction factor (TPF), heat transfer coefficient, and Colburn factor, respectively, with the application of the Taguchi method in a heat exchanger. GRG gives an optimum set of design parameters, A3B3C2D1E3F2G1, for wavy fin and tube of fin pitch of 6 mm, tube row number of 6, waffle height 1.8 mm, fin thickness 0.12 mm, and air velocity 5 m/s. Also, longitudinal tube pitch is 27.5 mm, and transverse tube pitch of 24.8 mm, at which TPF is maximum while the friction factor is minimal. The Colburn factor is the most significant, minor friction factor, and the heat transfer coefficient and TPF are the most considerable in GRG. Hence, an improved heat transfer performance design of a wavy fin and tube heat exchanger is achieved using the above techniques.     

Airside characteristics of heat,mass transfer and pressure drop for heat exchangers of tube-in hydrophilic coating wavy fin under dehumidifying conditions

The airside heat, mass and momentum transfer characteristics of seven wavy fin-and-tube heat exchangers with hydrophilic coating under dehumidifying conditions were experimented. The test inlet air dry bulb temperatures were 20, 27 and 35 ^oC, the inlet relative humidity were 50%, 60%, 70% and 80%, and the air velocity were 0.5, 1.0, 2.0, 3.0 and 4.0 m s^?1. The test results indicate that both the Colburn j_m factor and the Colburn j_h factor decrease with the increase of fin pitch, and this phenomenon becomes more and more pronounced as Reynolds number decreases. The friction factor is very sensitive to the change of fin pitch, and the friction factor shows a cross-over phenomenon as fin pitch changes. The Colburn j_h factor decreases and the Colburn j_m factor increases when the number of tube rows increases, while the friction performance is insensitive to the change of the number of tube rows. The effects of inlet relative humidity on the heat transfer and friction performance can be omitted, but the Colburn j_m factor decreases with the increase of the inlet relative humidity. The predictive ability of the available state-of-the-art heat transfer and pressure drop correlations was evaluated with the experiment data of the present study. The new heat, mass and momentum transfer correlations were proposed to describe the present test results according to the multiple linear regression technique. The mean deviations of the proposed j_h, j_m and f correlations are 6.3%, 8.9% and 7.9%, respectively. Comparing to published data reduction method, the process line on psychrometric chart of fin-and-tube heat exchanger for partially wet conditions and more accurate overall heat transfer coefficient equation are put forward in this paper.     

Effect of fin pitches on the air-side performance of crimped spiral fin-and-tube heat exchangers with a multipass parallel and counter cross-flow configuration

This study investigates the effect of fin pitches and fin materials on the air-side performance of crimped fin-and-tube heat exchangers in the range of high Reynolds numbers (4000–13000). The test samples are made from copper and aluminium with different fin pitches (f_p = 3.2, 4.2 and 6.2 mm). It is found that the proposed simple average effectiveness equation from the pure counter and parallel circuitry arrangement can well represent the effectiveness-NTU relationship for the current z-shape arrangement. The experimental results reveal that the fin pitch casts insignificant effect on the heat transfer characteristics (Colburn j factor). However, a detectable rise of the friction factor is seen when the fin pitch is increased to f_p = 6.2 mm. On the other hand, the effect of fin material on the airside performance is negligible.     

Experimental study of forced convective heat transfer in a different arranged corrugated channel

In this study, heat transfer rate for sinusoidal corrugated channel has been experimentally investigated. Three different type sharp corrugation peak fins and a plain surface were used in the experiment. Results were carried out for constant heat flux of 616 W/m², varied Reynolds number Re 1500 to 8000 for the corrugation angle (27, 50 and 22/60°) and channel height of 5 and 10 mm. Nusselt number (Nu), convection heat transfer coefficient (h), Colburn factor (j) and enhancement ratio (E) against Reynolds number (Re) have been studied. The effects of the wavy geometry and channel height have been discussed. The increase of corrugated angle gave rise to a heat transfer rate.     

Numerical study of local heat transfer coefficient and fin efficiency of wavy fin-and-tube heat exchangers

Three-dimensional numerical simulations were performed for laminar flow of wavy fin-and-tube heat exchangers by using body-fitted coordinates (BFC) method with fin efficiency effect accounted. The prediction results of average Nusselt number, friction factor and fin efficiency were compared with the related experimental correlations [R.C. Xin, H.Z. Li, H.J. Kang, W. Li, W.Q. Tao, An experimental investigation on heat transfer and pressure drop characteristics of triangular wavy fin-and-tube heat exchanger surfaces, J. Xi'an Jiaotong Univ. 28 (2) (1994) 77–83] and Schmidt approximation [T.E. Schmidt, Heat transfer calculations for extended surfaces, Refrigerating Engineering (April 1949) 351–357]. For Reynolds numbers based on the tube outside diameter ranging from 500 to 4000, the mean deviation is 3.3% for Nusselt number, 1.9% for friction factor and 3.6% for fin efficiency. The distributions of local Nusselt number and fin efficiency on fin surface were studied at wavy angle equal to 0° (plain plate fin), 10° and 20° respectively. The local Nusselt number decreases along the air flow direction, but fin efficiency increases in general. The wavy angle can greatly affect the distributions of local Nusselt number and fin efficiency, and make the distributions present fluctuation along the flow direction. The result also shows that the fin efficiency at the inlet region of wavy fin is larger than that of plain plate fin at the same region. With the increase of Reynolds number, the effects of wavy angle on the distributions of local Nusselt number and fin efficiency are more and more significant.     

Numerical study of flow patterns of compact plate-fin heat exchangers and generation of design data for offset and wavy fins

Thermo-hydraulic design of compact heat exchangers (CHEs) is strongly dependent upon the predicted/measured dimensionless performance (Colburn factor j and Fanning friction factor f vs. Reynolds number Re) of heat transfer surfaces. Also, air (gas) flow maldistribution in the headers, caused by the orientation of inlet and outlet nozzles in the heat exchanger, affects the exchanger performance. Three typical compact plate-fin heat exchangers have been analyzed using Fluent software for quantification of flow maldistribution effects with ideal and real cases. The headers have modified by providing suitable baffle plates for improvement in flow distribution. Three offset strip fin and 16 wavy fin geometries used in the compact plate-fin heat exchangers have also been analyzed numerically. The j and f vs. Re design data are generated using CFD analysis only for turbulent flow region. For the validation of the numerical analysis conducted in the present study, a rectangular fin geometry having same dimensions as that of the wavy fin has been analyzed. The results of the wavy fin have been compared with the analytical results of a rectangular fin and found good agreement. Similarly, the numerical results of offset strip fin are compared with the correlations available in the open literature and found good agreement with most of the earlier findings.     

On the heat transfer characteristics of heat sinks: Influence of fin spacing at low Reynolds number region

This study examines the thermal–hydraulic performance of heat sinks having plate, slit, and louver fin patterns. Comparison of the associated heat transfer performance and the effect of fin spacing are made. The results indicate that the enhanced fin patterns like louver or slit fin operated at a higher frontal velocity and at a larger fin spacing is more beneficial than that of plain fin geometry. The heat transfer performance of louver fin is usually better than that of slit fin but accompanies with higher pressure drops. However, it is found that the pressure drops for slit fin is comparable to the louver fin geometry when the fin spacing is reduced to 0.8 mm. This is associated with the appreciable rise of entrance/exit loss (form drag) caused by the slit fin geometry. The test results also reveal a significant drop of heat transfer performance at a low Reynolds number and at a small fin spacing, or the so-called “maximum” phenomenon of Colburn j factor. This is applicable to all the tested geometries. By a careful examination of the test results, it is concluded that this phenomenon is related to the developing/fully developed flow characteristics. In fact, the maximum point occurred roughly at x⁺ = 0.1 where fully developed and developing flow is separated.     

Three-dimensional numerical study of wavy fin-and-tube heat exchangers and field synergy principle analysis

In this paper, 3-D numerical simulations were performed for laminar heat transfer and fluid flow characteristics of wavy fin-and-tube heat exchanger by body-fitted coordinates system. The effect of four factors were examined: Reynolds number, fin pitch, wavy angle and tube row number. The Reynolds number based on the tube diameter varied from 500 to 5000, the fin pitch from 0.4 to 5.2 mm, the wavy angle from 0° to 50°, and the tube row range from 1 to 4. The numerical results were compared with experiments and good agreement was obtained. The numerical results show that with the increasing of wavy angles, decreasing of the fin pitch and tube row number, the heat transfer of the finned tube bank are enhanced with some penalty in pressure drop. The effects of the four factors were also analyzed from the view point of field synergy principle which says that the reduction of the intersection angle between velocity and fluid temperature gradient is the basic mechanism for enhance convective heat transfer. It is found that the effects of the four factors on the heat transfer performance of the wavy fin-and-tube exchangers can be well described by the field synergy principle.     

An experimental investigation of air cooling thermal module using various enhancements at low Reynolds number region

This study examines the airside performance of heat sinks having fin patterns of plate fin (Type I), interrupted fin geometry (Type II), dense vortex generator (Type III), and loose vortex generator (Type IV). Test results indicate that the heat transfer performance is strongly related to the arrangement of enhancements. The interrupted and dense vortex generator configurations normally contribute more pressure drop penalty than improvements of heat transfer. This is especially pronounced when operated at a lower frontal velocity. Actually the plain fin geometry outperforms most of the enhanced fin patterns such as of Type II and Type III at the fully developed region. This is because a close spacing prevents the formation of vortex, and the presence of interrupted surface may also suffer from the degradation by constriction of conduction path. The results suggest that the vortex generators operated at a higher frontal velocity is more beneficial than that of plain fin geometry. In association with the VG-1 criteria (same pumping power and same heat transfer capacity), the results show that effective reduction of surface area can be achieved when the frontal velocities are at 3–5 m s^?1 and the fin patterns are triangular, triangular attack, or two-groups dimple. The result from the present experiment suggests that the asymmetric combination such as using loose vortex generator (Type IV) can be quite effective. The triangular attack vortex generator is regarded as the optimum enhancement design for it could reduce 12–15% surface area at a frontal velocity around 3–5 m s^?1. The asymmetric design is still applicable even when the fin pitch is reduced to 1 mm.     

A comparison of the airside performance of the fin-and-tube heat exchangers in wet conditions; with and without hydrophilic coating

An experimental study was performed on compact fin-and-tube heat exchangers in wet conditions. Airside performance for both hydrophilic coated and un-coated surface is examined. It is found that the effect of inlet relative humidity on the heat transfer performance is small. For un-coated surfaces, the effect of inlet relative humidity has a pronounced effect on pressure drops. It is likely that this phenomenon is related to the condensate flow pattern along the fin surface. The heat transfer performance for the hydrophilic coating surface is lower than the corresponding un-coated surface tested at the same wet condition. Further, the degradation of heat transfer performance may be up to 20% for fin pitches of 1.2 mm. The pressure drops for the hydrophilic coated surface are also lower than the corresponding un-coated surfaces. A maximum 40% reduction is observed for plain fin geometry. The effect of inlet condition on frictional performance is more pronounced in the enhanced slit geometry.     

Heat-transfer enhancement in fin-and-tube heat exchanger with improved fin design

This work presents information of an experimental design on the elements of the fin-and-tube heat exchanger. In this study, three different fins (plate fin, wavy fin, and compounded fin) were investigated in a wind tunnel. The heat transfer coefficient, the pressure drop of the air side, the Colburn factor (j), and fanning friction factor (f) against air velocity (1–3 m/s) and Reynolds number (600–2000) have been discussed. In order to shed light on the fluid flow phenomena, flow visualization was also realized to observe the detailed fluid flow characteristics. The results of the wavy fin to the flat fin show that the pressure drop, heat transfer coefficient, f factor and j factor increase about 10.9–31.9%, 11.8–24.0%, 2.2–27.5% and 0.5–2.7%, respectively. In addition, the results of the compounded fin compared to the flat fin show that the pressure drop, heat transfer coefficient, f factor and j factor increase about 33.5–63.1%, 27.0–45.5%, 6.9–71.1% and 9.4–13.2%, respectively. In summary, this study strongly suggests the use of the compounded fin constructed for heat exchanger.     

Heat transfer and pressure drop correlations for the wavy fin and flat tube heat exchangers

A total of 11 cross-flow heat exchangers having wavy fin and flat tube were studied experimentally. A series of tests were conducted for air side Reynolds number in the range of 800–6500 with different fin pitches, fin lengths and fin heights, at a constant tube-side water flow rate of 2.5 m³/h. The air side thermal performance data were analyzed using the effectiveness-NTU method. The characteristics of heat transfer and pressure drop for different geometry parameters were reported in terms of Colburn j-factor and Fanning friction factor f, as a function of Re. The effects of fin pitch, fin height and fin length on the performance of heat transfer and pressure drop were examined. The general correlations for j and f factors were derived by multiple linear regression analysis and F test of significance. The correlations for j and f factors can predict 95% of the experimental data within ±10%.     

Optimum design parameters of a heat exchanger

In this study, the effects of the longitudinal and lateral seperations of consecutively enlarged-contracted arranged fin pairs, widths of the fins, angle of attack, heights of fins and flow velocity on the heat and pressure drop characteristics were investigated using the Taguchi experimental-design method. Nusselt number and friction factor were considered as performance parameters. An L₁₈(2¹13⁷) orthogonal array was selected as an experimental plan for the eight parameters mentioned above. First of all, each goal was optimized, separately. Then, all the goals were optimized together, considering the priority of the goals, and the optimum results were found to be fin width of 15 mm, angle of attack of 15°, fin height of 100 mm, span-wise distance between fins of 20 mm, stream-wise distance between fins of 10 mm, span-wise distance between slices of 20 mm, stream-wise distances between slices of 20 mm at a flow velocity of 4 m/s.     

Numerical study on air-side performance of an integrated fin and micro-channel heat exchanger

A new type of aluminum heat exchanger with integrated fin and micro-channel has been proposed. The air-side heat transfer and flow characteristics of the integrated fin and micro-channel heat exchanger are systematically analyzed by a 3D numerical simulation. The effect of flow depth, fin height, fin pitch and fin thickness at different Reynolds number is evaluated by calculating Colburn factor j and Fanning friction factor f. A parametric study method is used to analyze the fin designed parameters affecting the performance of the heat exchanger. The results show that the contribution ratio of the fin geometries in descending order is flow depth, fin pitch, fin height and fin thickness. The air-side performance of the integrated fin and micro-channel heat exchanger is compared with that of the multi-louver fin micro-channel heat exchanger and the wavy fin micro-channel heat exchanger.     

Experimental and numerical investigation of thermal -hydraulic performance in wavy fin-and-flat tube heat exchangers

To the more deeply understand the enhancement heat transfer mechanism and optimization design for wavy fin-and-flat tube heat exchangers, three-dimensional numerical simulations and experimental investigation of air flow and heat transfer characteristics over the wavy fin heat exchangers are presented in this study. The numerical simulation results compared with the wind tunnel test data, the results show that the numerical simulation results are in good with the test. The experimental results show that, in the range of Re = 1000–5500, the standard k-ε mode (SST) is more suitable to predict the air flow and heat transfer of wavy fin. The waviness amplitude has the distinct effect on the heat transfer and pressure drop of wavy fin, while the wavy fin profile (Triangular, Sinusoidal and Triangular round corner) has little effect on the heat transfer performance. In additional, the enhancement heat transfer mechanism of wavy fin is explained in view of field synergy principle. Reduction the synergy angle between velocity and temperature gradient will induce to the heat transfer coefficients increase of wavy fin.     

CONVECTIVE HEAT AND MASS TRANSFER IN TUBE-FIN EXCHANGERS UNDER DEHUMIDIFYING CONDITIONS

The article analyzes convective heat and mass transfer in the flow passages of tube-fin exchangers, adopting a simplified two-dimensional approach. The flow structure on the airside of these devices is spatially periodic, with fully developed conditions prevailing a short distance from the entrance. In numerical simulations, symmetric and/or antisymmetric periodicity in pressure, velocity components, temperature, and mass concentration of the water vapour are taken into account to reduce the computational domain. Using a finite-element discretization velocity, temperature and mass concentration fields are computed within wavy, offset-strip, and louver fin surfaces. Quantitative results are also obtained for friction factors, Nusselt numbers, and Colburn factors for heat and mass transfer.     

Experimental study on heat transfer enhancement of wavy finned flat tube with longitudinal vortex generators

The performance of direct air-cooled condensers in power plant is affected significantly by air-side flow and heat transfer characteristics of the wavy finned flat tube. Experimental investigations were conducted for air-side flow and heat transfer with and without delta winglet pairs punched on the surface of the wavy fin. The different temperature fields of the heated wavy fin surface with and without delta winglet pairs were obtained by the infrared thermography technology. Both experiments and numerical simulations showed that a substantial increase in the heat transfer with six delta winglet pair generators on the wavy fin was obtained with the Reynolds number varying from 1500 to 4500, which was the range of the air flow in practical direct air-cooled condensers. The average Nusselt number increased by 21–60% with the Reynolds number varying from 1500 to 4500 and the average friction factor increased by 13–83% with the Reynolds number varying from 500 to 4500 in experiments. The average performance evaluation criteria, PEC, can be up to 1.31 with six delta winglet pairs punched on the wavy fin surface, indicating the high potential of heat transfer enhancement to direct air-cooled condensers by longitudinal vortex generators.