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.     

Air-side thermal hydraulic performance of offset strip fin aluminum heat exchangers

Experimental studies on the air-side heat transfer and pressure drop characteristics for 16 types offset strip fins and flat tube heat exchangers were performed. Parameters including fin space s, fin height h, fin thickness t, fin length l and flow length d, a series of tests were conducted in region of air-side Reynolds number 500–7500, 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 heat transfer coefficients and pressure drop data with different fin space s, fin height h, and fin length l were reported in terms of frontal air velocity. The general correlations for Colburn j-factor and Fanning fraction f-factor were derived by regression analysis and F significance test. The correlations for j and f factors can predict 95% and 90% of the experimental data within  ± 10%. And the average deviations of predictive data for the j and f factors are 0.2% and 1.2%, mean deviations are 4.2% and 5.3%.     

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.     

Experimental investigation of mixed convection heat transfer from longitudinal fins in a horizontal rectangular channel

Mixed convection heat transfer from longitudinal fins inside a horizontal channel has been investigated for a wide range of modified Rayleigh numbers and different fin heights and spacings. An experimental parametric study was made to investigate effects of fin spacing, fin height and magnitude of heat flux on mixed convection heat transfer from rectangular fin arrays heated from below in a horizontal channel. The optimum fin spacing to obtain maximum heat transfer has also been investigated. During the experiments constant heat flux boundary condition was realized and air was used as the working fluid. The velocity of fluid entering channel was kept nearly constant (0.15 ? w_in ? 0.16 m/s) using a flow rate control valve so that Reynolds number was always about Re = 1500. Experiments were conducted for modified Rayleigh numbers 3 × 10⁷ < Ra¹ < 8 × 10⁸ and Richardson number 0.4 < Ri < 5. Dimensionless fin spacing was varied from S/H = 0.04 to S/H = 0.018 and fin height was varied from H_f/H = 0.25 to H_f/H = 0.80. For mixed convection heat transfer, the results obtained from experimental study show that the optimum fin spacing which yields the maximum heat transfer is S = 8–9 mm and optimum fin spacing depends on the value of Ra¹.     

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.     

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

The air-side performance of crimped spiral fin and tube heat exchangers at high Reynolds number (3000–13,000) is investigated in this study. The test heat exchangers have a new type of multipass parallel and counter cross-flow water flow arrangement which is a combination of parallel cross-flow and counter cross-flow. The test samples are made from copper and aluminium with different number of tube rows (N_row = 2, 3, 4 and 5). The effects of number of tube rows and fin material on the heat transfer and friction characteristics are studied. The results show that no significant effect for either number of tube rows or fin materials on the heat transfer performance is found at high Reynolds number. In addition, the correlation of the air-side performances of this type of the heat exchangers at high Reynolds number is developed for industrial applications.     

Effect of flow bypass on the performance of a shrouded longitudinal fin array

Theoretical and experimental studies were carried out to investigate the effects of duct velocity, fin density and tip-to-shroud clearance on the flow bypass and its impact on the pressure drop across a longitudinal aluminum fin array and its thermal performance. The clearance was varied parametrically, starting with the fully shrouded case and variations of the channel height giving partially shrouded configuration of different clearance ratios were also carried out. The flow bypass was found to increase with increasing fin density and insensitive to the air flow rate. This effect of fin density decreased as the clearance increased. The calculated total pressure was greatly affected by fin density. For fully-shrouded fin array, with H_f/S equals to 8 and 12.72, the pressure drop increased by a factor of 4.3 and 20 of that with H_f/S equals to 3.4, respectively. The total pressure drop and the average convective heat transfer coefficients corresponding to the fully and partially shrouded tested fin array of H_f/S = 3.4 were compared. Going from fully to partially shrouded one of the largest clearance ratio (C/H_f = 0.89), the total pressure drop is reduced by about 50%. For clearance ratios equal to 0.36, 0.56, and 0.89, the average heat transfer coefficients were reduced by about 12, 17, and 30% of that for the fully shrouded configuration at Re_D of about 3 × 10³. That percentage reduction in heat transfer coefficients are decreased with the increase of air flow rate.     

Effect of fin pitches on the optimum heat transfer performance of crimped spiral fin-and-tube heat exchangers

This study conducted experiments on the optimized fin pitch for crimped spiral fin-and-tube heat exchangers. The experiments covered a size range of 2.4–6.5 mm, which is the manufacturing limitation for this kind of fin. The water-flow arrangement used in this experiment combined the parallel cross-flow and the counter cross-flow in a two-row configuration. Ambient air was used as the working fluid on the air-side, and hot water was used on the tube-side. The effects of fin pitches on the heat transfer coefficient and pressure drop characteristics were studied. The results clearly showed that the convective heat transfer coefficient (h_o) for a fin pitch of 2.4 mm is relatively low compared with that of other fin pitches with the same air frontal velocity. Using larger fin pitches (i.e., 4.2, 6.2, and 6.5 mm) resulted in negligible differences in the pressure drop. Moreover, this work introduces the parameter of three performances indexes, which can be expressed as the ratio of the desired output to the required input, for optimization purposes. Due to the difference in optimum fin pitch obtained by these performance indexes, an intersection analysis was conducted. The results indicated that the optimum fin pitch is 4.2 mm for this work, which could be valuable for the effective design for industrial thermal-system applications.     

Heat transfer enhancement by flow bifurcations in asymmetric wavy wall channels

The enhancement characteristics of heat transfer, through a transition scenario of flow bifurcations, in asymmetric wavy wall channels, are investigated by direct numerical simulations of the mass, momentum and energy equations, using the spectral element method. The heat transfer characteristics, flow bifurcation and transition scenarios are determined by increasing the Reynolds numbers for three geometrical aspect ratios r = 0.25, 0.375, and 0.5, and Prandtl numbers 1.0 and 9.4. The transition scenarios to transitional flow regimes depend on the aspect ratio. For the aspect ratios r = 0.25 and 0.5, the transition scenario is characterized by one Hopf flow bifurcation. For the aspect ratio r = 0.375, the transition scenario is characterized by a first Hopf flow bifurcation from a laminar to a periodic flow, and a second Hopf flow bifurcation from a periodic to quasi-periodic flow. The periodic and quasi-periodic flows are characterized by fundamental frequencies ω₁, and ω₁ and ω₂, respectively. For all the aspect ratios and Prandtl numbers, the time-average mean Nusselt number and heat transfer enhancement increases with the Reynolds number as the flow evolves from a laminar to a transitional regime. For both Prandtl numbers, the highest increase in the Nusselt number occurs for the aspect ratio r = 0.5; whereas, the lowest increases happen to r = 0.25. The increase of the Nusselt number occurs at the expense of a higher pumping power, which, for both Prandtl numbers, grows as the aspect ratio increases from r = 0.25 to r = 0.5 for reaching a specific Nusselt number. This enhancement is obtained without the necessity of high volumetric flow rates associated with turbulent flow regimes, which demand much higher pumping powers. Significant heat transfer enhancements are obtained when the asymmetric wavy channel is operated in the appropriate transitional Reynolds number range.     

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.     

Numerical study of heat-transfer enhancement by punched winglet-type vortex generator arrays in fin-and-tube heat exchangers

The potential of punched winglet type vortex generator (VG) arrays used to enhance air-side heat-transfer performance of finned tube heat exchanger is numerically investigated. The arrays are composed of two delta-winglet pairs with two layout modes of continuous and discontinuous winglets. The heat transfer performance of two array arrangements are compared to a conventional large winglet configuration for the Reynolds number ranging from 600 to 2600 based on the tube collar diameter, with the corresponding frontal air velocity ranging from 0.54 to 2.3 m/s. The effects of different geometry parameters that include attack angle of delta winglets (β = 10 deg, β = 20 deg, β = 30 deg) and the layout locations are examined. The numerical results show that for the punched VG cases, the effectiveness of the main vortex to the heat transfer enhancement is not fully dominant while the “corner vortex” also shows significant effect on the heat transfer performance. Both heat transfer coefficient and pressure drop increase with the increase of attack angle β for the side arrangements; the arrays with discontinuous winglets show the best heat transfer enhancement, and a significant augmentation of up to 33.8–70.6% in heat transfer coefficient is achieved accompanied by a pressure drop penalty of 43.4–97.2% for the 30 deg case compared to the plain fin. For the front arrangements of VGs higher heat transfer enhancement and pressure drop penalty can be obtained compared to that of the side arrangement cases; the case with front continuous winglet arrays has the maximum value of j/f, a corresponding heat transfer improvement of 36.7–81.2% and a pressure drop penalty of 60.7–135.6%.     

New approach relevant to total heat transfer coefficient including the effect of radiation and convection at the ceiling in a cooled ceiling room

In this study, radiative and convective heat transfer coefficients at the ceiling are determined for a cooled ceiling room. Firstly, convective heat transfer is simulated numerically neglecting the radiative heat transfer at the surfaces (ε_f = ε_w = ε_c = 0), then, radiative heat transfer is calculated theoretically for different surface emissivities (ε_f = ε_w = ε_c = 0.5, 0.6, 0.7, 0.8 and 0.9) for different room dimensions (3 × 3 × 3, 4 × 3 × 4 and 6 × 3 × 4 m) and thermal conditions (T_f = 25 °C, T_w = 28–36 °C and T_c = 0–25 °C). Numerical data is compared with the results of correlations based on experimental data given in literature. New equations related to convective and total (including the effect of convection and radiation) heat transfer coefficients for ceiling are found in the current study.     

Waviness effect of a wavy circular cylinder on the heat transfer at a Reynolds number of 300

The present study investigates three-dimensional characteristics of fluid flow and heat transfer around a wavy cylinder which has the sinusoidal variation in the cross sectional area along the spanwise direction. The three different wavelengths of π/4, π/3 and π/2 at the fixed wavy amplitude of 0.1 have been considered to investigate the effect of waviness on especially the forced convection heat transfer around a wavy cylinder when the Reynolds and Prandtl numbers are 300 and 0.71, respectively. The numerical solution for unsteady forced convective heat transfer is obtained using the finite volume method. The immersed boundary method is used to handle the wavy cylinder in a rectangular grid system. The present computational results for a wavy cylinder are compared with those for a smooth cylinder. The fluid flow and heat transfer around the wavy cylinder depends on both the location along the spanwise direction and the wavelength. The time- and total surface-averaged Nusselt number for a wavy cylinder with λ = π/2 is larger than that for a smooth cylinder, whereas that with λ = π/4 and π/3 is smaller than that for a smooth cylinder. However, because the surface area exposed to heat transfer for a wavy cylinder is larger than that for a smooth cylinder, the total heat transfer rate for a wavy cylinder with different wavelengths of λ = π/4,π/3 and π/2 is larger than that for a smooth cylinder.     

Pool boiling of R-123/oil mixtures on enhanced tubes having different pore sizes

The effect of enhanced geometry (pore diameter, gap width) is investigated on the pool boiling of R-123/oil mixture for the enhanced tubes having pores with connecting gaps. Tubes having different pore diameters (and corresponding gap widths) are specially made. Significant heat transfer degradation by oil is observed for the present enhanced tubes. At 5% oil concentration, the degradation is 26–49% for T_sat = 4.4 °C. The degradation increases 50–67% for T_sat = 26.7 °C. The heat transfer degradation is significant even with small amount of oil (20–38% degradation at 1% oil concentration for T_sat = 4.4 °C), probably due to the accumulation of oil in sub-tunnels. The pore size (or gap width) has a significant effect on the heat transfer degradation. The maximum degradation is observed for d_p = 0.20 mm tube at T_sat = 4.4 °C, and d_p = 0.23 mm tube at T_sat = 26.7 °C. The minimum degradation is observed for d_p = 0.27 mm tube for both saturation temperatures. It appears that the oil removal is facilitated for the larger pore diameter (along with larger gap) tube. The highest heat transfer coefficient with oil is obtained for d_p = 0.23 mm tube, which yielded the highest heat transfer coefficient for pure R-123. The optimum tube significantly (more than 3 times) outperforms the smooth tube even with oil. The heat transfer degradation increases as the heat flux decreases.     

Convective heat transfer on a rotating finned cylinder with transverse airflow

The present experimental investigation relates to the convective heat transfer determination around annular fins mounted on a rotating cylinder with air crossflow. The mean convective heat transfer coefficient can be identified by solving the inverse conduction heat transfer problem during the fin cooling process. We used an inverse method, based on the mean squared error, to develop a model of mean convective heat transfer, taking lateral conduction into account. Tests were carried out for rotational Reynolds numbers Re_ω between 2150 and 17,200, air crossflow Reynolds numbers Re_U between 0 and 39,600, and fin spacings u in the range 10 mm to ∞, u = ∞ corresponding to the single disk case. For each fin spacing, the relative influences of the rotational and airflow forced convections on the heat transfer were analyzed and correlations of the mean Nusselt number on the fin, relative to both Reynolds numbers, are proposed. Moreover, an efficiency definition, that allows optimal geometrical configurations of the finned cylinder to be identified for the given operating conditions, is proposed.     

Experimental study of R-134a evaporation heat transfer in a narrow annular duct

An experiment is carried out here to investigate the evaporation heat transfer and associated evaporating flow pattern for refrigerant R-134a flowing in a horizontal narrow annular duct. The gap of the duct is fixed at 1.0 and 2.0 mm. In the experiment, the effects of the duct gap, refrigerant vapor quality, mass flux and saturation temperature and imposed heat flux on the measured evaporation heat transfer coefficient h_r are examined in detail. For the duct gap of 2.0 mm, the refrigerant mass flux G is varied from 300 to 500 kg/m² s, imposed heat flux q from 5 to 15 kW/m², vapor quality x_m from 0.05 to 0.95, and refrigerant saturation temperature T_sat from 5 to 15 °C. While for the gap of 1.0 mm, G is varied from 500 to 700 kg/m² s with the other parameters varied in the same ranges as that for δ = 2.0 mm. The experimental data clearly show that the evaporation heat transfer coefficient increases almost linearly with the vapor quality of the refrigerant and the increase is more significant at a higher G. Besides, the evaporation heat transfer coefficient also rises substantially at increasing q. Moreover, a significant increase in the evaporation heat transfer coefficient results for a rise in T_sat, but the effects are less pronounced in the narrower duct at a low imposed heat flux and a high refrigerant mass flux. Furthermore, the evaporation heat transfer coefficient increases substantially with the refrigerant mass flux except at low vapor quality. We also note that reducing the duct gap causes a significant increase in h_r. In addition to the heat transfer data, photos of R-134a evaporating flow taken from the duct side show the change of the dominant two-phase flow pattern in the duct with the experimental parameters. Finally, an empirical correlation for the present measured heat transfer coefficient for the R-134a evaporation in the narrow annular ducts is proposed.     

An experimental study of pressure loss and heat transfer in the pin fin-dimple channels with various dimple depths

An experimental study was conducted to investigate the effects of dimple depth on the pressure loss and heat transfer characteristics in a pin fin-dimple channel, where dimples are located on the endwall transversely between the pin fins. The pin fin-dimple channels considered consist of ten rows of pin fin-dimple combined structure. The pin fin transverse spacing-to-diameter ratio S/D = 2.5, the streamwise spacing-to-diameter ratio X/D = 2.5, the pin fin height-to-diameter ratio H/D = 1.0. The dimples have a print diameter the same with the pin fins, but have three different dimple depth-to-diameter ratios, i.e. δ/D = 0.1, 0.2 and 0.3. The experimental results, mainly the average Nusselt number and friction factor, for the pin fin-dimple channels with various dimple depths have been obtained and compared with each other for the Reynolds number range of 8200–50,500. The study showed that, compared to the baseline pin fin channel, the pin fin-dimple channels have further improved convective heat transfer performance by up to 19.0%, and the pin fin-dimple channel with deeper dimples shows relatively higher Nusselt number values. The study still showed dimple depth-dependent pressure loss behaviors for the pin fin-dimple channels compared to the pin fin channel, and the pin fin-dimple channel with shallower dimples shows relatively lower friction factors by up to 17.6% over the studied Reynolds number range. Furthermore, three-dimensional conjugate computations have been carried out for similar experimental conditions, and the computations showed the detailed characteristics in the distribution of the velocity and turbulence level in the flow, which revealed the underlying mechanisms for the associated dimple depth-dependent pressure loss and heat transfer characteristics in the pin fin-dimple channels.     

3-D Numerical study on the correlation between variable inclined fin angles and thermal behavior in plate fin-tube heat exchanger

In this study, the heat transfer enhancement and pressure drop values of seven different fin angles with plain fin-tube heat exchangers were investigated. The numerical simulation of the fin-tube heat exchanger was performed by using a three dimensional (3-D) numerical computation technique. Therefore, a CFD computer code, the FLUENT was used to solve the equation for the heat transfer and pressure drop analyses in the fin-tube heat exchanger. The model drawing was created and meshed by using GAMBIT software. The heat transfer and pressure drop values of the vertical fin angle (θ = 0°) were provided to compare with variable inclined fin angles (θ = 5°, 10°, 15°, 20°, 25°, 30°). The heat transfer values were normalized to compare all cases. For inclined fin angle θ = 30°, which is the optimum angle, the maximum heat transfer enhancement per segment was obtained 1.42 W (the normalized value 105.24%), the maximum loss power associated with pressure drop per segment was only 0.54 mW.     

The particle thermal conductivity influence of nanofluids on thermal performance of the microtubes

The paper presents the numerical analysis on microchannel laminar heat transfer and fluid flow of nanofluids in order to evaluate the suitable thermal conductivity of the nanoparticles that results in superior thermal performances compared to the base fluid. The diameter ratio of the micro-tube was D_i/D_o = 0.3/0.5 mm with a tube length L = 100 mm in order to avoid the heat dissipation effect. The heat transfer rate was fixed to Q = 2 W. The water based Al₂O₃, TiO₂ and Cu nanofluids were considered with various volume concentrations ϕ = 1,3 and 5% and two diameters of the particles d_p = 13 nm and 36 nm. The analysis is based on a fixed Re and pumping power Π, in terms of average heat transfer coefficient and maximum temperature of the substrate. The results reveal that only the nanofluids with particles having very high thermal conductivity (λ_Cu = 401 W/m K) are justified for using in microcooling systems. Moreover, the analysis is sensitive to both the comparison criteria (Re or Π) and heat transfer parameters (h_ave or t_max).     

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.