Fin Spacing Research Articles

Experimental and numerical investigations on enhanced and stabilized flow boiling of hydrocarbon fuel in micro-finned channel

Due to the high heat transfer coefficient, flow boiling has applied in various industrial fields. Previous literature mainly focuses on the enhancement methods of flow boiling for water or refrigerants, with few reports on the enhancement of flow boiling for hydrocarbon fuels. Therefore, in this study, we design different types of micro-finned surface with strong capillary force to increases the nucleate sites and keep the wall in a superwetting state during the flow boiling to postpone the appearance of “annular bubble” flow. The influence of fin width, fin height, fin spacing and coolant type on boiling heat transfer through experiments and numerical simulation. The results show that the wider the fin width, the lower the fin height, the smaller the fin spacing, the stronger the heat transfer enhancement effect, and the more stable the flow boiling. The best heat transfer performance of the finned surface is achieved with the average wall temperature 27 °C lower and heat transfer coefficient 1.9 times higher than the smooth surface. Moreover, the heat transfer coefficient of hydrocarbon fuel is 43.3% larger than that of deionized water.

An investigation of the thermal performance of functionally graded annular fins on a horizontal cylinder under natural convection

ABSTRACT In industrial applications, annular fins are commonly made of homogeneous materials such as aluminum, struggling to provide uniform temperature distribution along their length. Recognizing the potential of Functionally Graded Materials (FGMs) to enhance thermal performance by combining two different materials with a gradual variation, this study proposes their use as fin materials to achieve optimum performance. The study consists of three major components: (1) numerical analyses to determine the optimum volume composition of FG fins, (2) the fabrication process of aluminum and FG fins utilizing powder metallurgy and the hot-pressing technique, and (3) experiments to evaluate the thermal performance of FG and aluminum fin arrays attached to a horizontal cylinder of 250 mm length under natural convection. Heat ranging from 25 W to 150 W is applied to the cylinder during these experiments. Based on the experiments, the thermal performances of fins are evaluated in terms of net free convection heat transfer rate, the Nusselt number, and fin effectiveness. Overall, experimental results demonstrate that the net convection heat transfer rate depends on fin spacing, material, and the base-to-ambient temperature difference. Specifically, FG fin arrays enhance the net heat transfer rate by 59%, while aluminum fin arrays increase it by 33% compared to finless cylinders. Moreover, FG fins outperform aluminum fin arrays by 40% in convective heat transfer coefficient h. Due to being the first experimental study, this study sets itself apart.

Optimization of geometric configurations for enhanced thermal performance in microchannel finned plates

This study investigates the thermal performance of microchannel finned plates by comparing various geometric configurations and operating conditions. Twelve models (a to l) were evaluated by varying parameters such as fin height, fin spacing, and channel width, as well as operating factors like inlet velocity and heat flux. The results reveal that increasing the fin height and decreasing the fin spacing enhance heat transfer by expanding the surface area, though at the cost of increased pressure drops and higher pumping power requirements. Variations in channel width impact flow characteristics, with wider channels reducing temperature but potentially lowering convective efficiency. Higher inlet velocities improve convective cooling but increase pressure drops, while elevated heat fluxes lead to steeper temperature gradients. Model k was found to provide an optimal balance between heat transfer efficiency and pressure drop, while Model l’s novel fin geometry showed unique temperature distributions warranting further exploration. Temperature contours and volume renders demonstrated air temperature increases across the models, ranging from 4.78 K to 17.36 K, and surface heat flux measurements confirmed the significant influence of fin configuration on thermal performance. The findings offer valuable insights for optimizing the design of microchannel finned plates for enhanced heat transfer and effective thermal management in various applications.

Research on battery heat generation characteristics and thermal management system applied to a typical eVTOL

Electric Vertical Takeoff and Landing (eVTOL) aircraft, as a highly promising future transportation mode, offers new possibilities for solving traffic congestion. Its unique flight mode and high power requirements present significant challenges for battery thermal management system design. Compared to electric vehicles, eVTOLs have totally different usage scenarios and operating conditions, leading to the fact that the thermal management system of conventional electric vehicles does not apply to eVTOLs. This paper proposes a battery thermal management design method for eVTOLs. The energy and power requirements for a short-distance eVTOL flight are determined through theoretical calculations. Additionally, experimental studies explore the thermal characteristics of a 61.5Ah lithium-ion battery under flight discharge conditions. Moreover, a battery thermal management system, which combines flat heat pipes and ram air, is analyzed through numerical simulations. Furthermore, this study investigates the effects of various air temperatures, air inlet mass flow rates, and fin spacing on the performance of the thermal management system. Finally, under the flight discharge cycle of a 61.5 Ah lithium-ion battery, with the environment temperature of 20 °C, the temperature can be controlled within 38.46 °C. The temperature difference can be cooled within 3.85 °C by the passive cooling system. Even at an extreme environment temperature of 40 °C, the battery temperature can still meet the standard. Meanwhile, comparing from multiple dimensions, this solution has a better overall performance than the traditional liquid cooling solution, which provides a foundation for designing a battery thermal management system for eVTOLs.

Investigation on the performance of a novel heat transfer structure based on a new twisted airfoil fin array

Printed circuit heat exchanger (PCHE) is a type of micro-channel heat exchanger with high performance, which has a broad application in microelectronics, solar energy, and aerospace. Airfoil fins are widely recognized due to their exceptional thermo-hydraulic performance, but there are few studies on their shape adjustment in three dimensions. In this study, a novel twisted airfoil fin was proposed and applied to the heat exchanger. The heat transfer and flow resistance of the twisted airfoil fin PCHE with the traditional PCHE are compared using the numerical simulation method, and the influence of the twist angle, fin spacing, and arrangement is investigated. The results show that the new fins have excellent flow and heat transfer performance. Under the multiple Re number conditions, the Nu number is improved up to 52%, and the performance evaluation criteria is increased up to 16%. Also, increasing the twist angle of the fins, decreasing the fin spacing, and staggering the fins according to diverse twist directions can improve the performance of the heat exchanger.

Experimental study on the characteristics of frost formation and defrosting water retention between vertical double surfaces with different wettability

Frost formation and defrosting water retention directly affect the heat transfer process of outdoor finned tube heat exchanger, which in turn influences the energy efficiency of air source heat pumps for building heating. Three typical wettability surfaces were prepared, including a bare copper hydrophilic surface (BCS), a hydrophobic copper surface (HCS) and a superhydrophobic copper surface (SHCS). The above three surfaces are combined in pairs to construct three different wettability combinations, which are Combination 1: HCS-BCS, Combination 2: BCS-SHCS and Combination 3: HCS-SHCS, respectively. Frosting-defrosting experiments between vertical double surfaces are carried out on the above combinations. The results illustrated that the combinations with SHCS exhibit excellent frost inhibition properties. Compared with Combination 1, the channel filling rate of Combination 2 and Combination 3 were reduced by 16.79 % and 30.09 % respectively at a frosting time of 60 min. The defrosting time of the combination is determined by the slower defrosting of the two surfaces. Compared with Combination 2, the defrosting rate of Combination 3 is improved by up to about 12.93 %. Avoiding the formation of large-sized liquid films and droplets or utilizing the “absorption” phenomenon caused by wettability differences between surfaces are important measures to reduce the possibility of bridge formation during the defrosting process. And the greater the wettability difference, the more obvious the “absorption” phenomenon became. Spherical droplets on the SHCS can be completely absorbed by the liquid film on BCS under the action of differential surface tension. Neither Combination 2 nor Combination 3 formed a liquid bridge at a surface spacing of 2 mm. The research will provide theoretical support for the selection of fin spacing and fin coating type under different working conditions in practical engineering.

Performance of pin-fin structures on pool boiling heat transfer

The present study investigates pool boiling heat transfer of Novec 649 on structured copper surfaces. Structures with varying height and spacing, in the range of 400–1200 µm, were machined on the boiling surface using electro-discharge machining (EDM). Pool boiling experiments have been performed to estimate the heat transfer performance of pin-fin structures using Novec 649 as the cooling fluid. The heat transfer coefficient (HTC) and the critical heat flux (CHF) limit of the structured surfaces have been estimated and compared to those of the unstructured ones, both in the horizontal and vertical orientation. In this work, the effects of pin fin heights and spacing, with dimensions similar to the capillary length scale, on the heat transfer coefficient and critical heat flux limit are examined. The experimental results demonstrate that adding pin-fin structures significantly enhances boiling heat transfer. Specifically, the HTC and CHF increases by factors of up to 10 and 2.8, respectively, compared to the smooth surface. This enhancement is primarily driven by the wetted area increase due to the pin-fin structures. The HTC per unit wetted area was found to be independent of geometry during the early stage of nucleate boiling and only exhibited a weak dependence on geometry in the later stages. Notably, the HTC per unit wetted area deteriorates when the pin fin spacing is less than the capillary length scale.

Fully developed flow through shrouded-fin arrays: exact and asymptotic solutions

The flow resistance, i.e. friction factor times Reynolds number ( $\,f\,{Re}$ ), of longitudinal-fin heat sinks with or without clearance between a shroud and the tips of the fins is an important parameter in thermal design. This is because it dictates the caloric resistance of the heat sink, i.e. change in bulk temperature of the fluid flowing through it. When there is no clearance and the common and oft-valid assumption of negligible fin thickness is invoked, $f\,{Re}$ corresponds to simply that of a rectangular duct. However, with clearance, only numerical results are available as per the well-known study by Sparrow, Baliga and Patankar (ASME J. Heat Transfer, vol. 100, 1978). We develop analytical formulae for $f\,{Re}$ for fully developed flow with clearance. The exact solution is provided by an integral formula derived via conformal mappings. Additionally, simple formulae are derived via asymptotic expansions in three cases: (1) the fin spacing is small compared to the fin height and clearance; (2) the clearance is small compared to the fin spacing, which is small compared to the fin height; (3) the same as case (2) but valid for larger clearances. The different asymptotic formulae are compared to the exact formula, and together cover almost the entire relevant parameter range (for fin spacing and clearance) with errors of less than 15 %. A formula for the limiting case of no clearance is shown to be more accurate, for any fin spacing, than a widely used correlation from the literature.

Investigations of Silica/MOF composite coating and its dehumidification performance on a desiccant-coated heat exchanger

Desiccant materials have an important impact on the dehumidification performance of desiccant-coated heat exchanger (DCHE). In this paper, a desiccant-coated heat exchanger based on a composite desiccant material of metal organic frameworks (MOFs) and silica gel was proposed. The coating was experimentally prepared and tested for its hygroscopic performance under different binder types (PVP, PVA, and PVB), different binder mass concentrations, and different MOF mass concentrations in the composite adsorbent. Further simulations were performed to investigate the effects of key structural and operational parameters on the performance of DCHE. Experimentally, the optimal hygroscopic performance of the composite adsorbent was obtained at 15 wt% PVP and 20 wt% MOF, which was 51.71 % higher than that of silica coating, and the cost of the composite adsorbent drops 78.04 % by comparing to the pure MOF for laboratory fabrication, and based on the novel economic efficiency index proposed in this paper, the mass concentration of the MOF should not exceed 32.8 wt%. Simulation results showed that under the conditions as follows: 1.2 mm (fin spacing), 25 mm (fin height), 0.28 mm (coating thickness), 25 °C (cooling water temperature), and 0 % (initial moisture content), the corresponding moisture removal capacity (MRC) and of DCHE was 6.740 g/kg, which was 149.63 % higher than that of silica coating.

Correlations for the prediction of natural convection heat transfer of hollow hybrid fin arrays

A hollow hybrid fin (HHF) array is a staggered array of hollow pin fins concatenated with radially-extruded plate fins. Preceding research for a decade shows that natural convection HHF arrays could yield up to 30 % superior mass-based thermal performance and less orientation dependence compared to natural convection pin fin arrays. Hence, the HHF array could be a potential alternative to conventional pin fin arrays for lightweight high-performance thermal management of microelectronics in natural convection. To demonstrate potential and limitation of the natural convection HHF array, optimization is inevitable, and thus correlations for the prediction of natural convection heat transfer around the HHF arrays should be developed to provide design guidelines for the optimization. Nu correlations are developed for HHFs, HHF arrays, and oriented HHF arrays. Due to broad ranges of physical conditions, 168 cases for the HHF, 48 cases for the vertical HHF array, and 24 cases for the oriented HHF arrays are calculated and validated by the measurement and the literature data. Utilizing CFD thermal data, Nu correlations are formulated. Discrepancies between correlation-predicted and CFD-evaluated Nu values are examined. The results show that average discrepancies are 2.9 % for the vertical HHFs, 4.8 % for the vertical HHF arrays, and 5.5 % for oriented HHF arrays. These reasonable discrepancy values demonstrate the correlation validity. Validated Nu correlations are used to analyze parametric effects on convection heat transfer coefficients, h. It is seen that h increases asymptotically with the increase of fin spacing, S, mainly due to the porosity increase. The parametric study finds that h declines till a certain internal diameter, Di. They are 2 mm for external diameter, Do, of 4 mm, 4 mm for Do of 7 mm, and 6 mm for Do of 10 mm. The parametric study shows that after such Di, h consistently increases until the widest Di. The parametric study for the orientation shows that for the sideward orientation, h increases with the increase of porosity. The result also shows that h of the downward case is greater than that of the sideward case, and the difference is greater at lower Ra values.

Exergetic performance evaluation of louvered finned solar air heater: an experimental investigation

Solar energy is a promising source of renewable energy, and solar air heaters are an important application for utilizing this energy source. This study investigates the exergetic performance of a louvered fin solar air heater (LFSAH) through experimental analysis conducted during day time from 9 am to 3 pm under identical operating and metrological parameters. Exergy analysis based on the 2nd law of thermodynamics is used to assess the quality of exergy losses and useful energy output and is deemed suitable for LFSAH design. The study experimentally examines the exergetic performance of LFSAH with various fin spacings and compares the results with those of a plane solar air heater (PSAH). The exergy efficiency is optimized by considering the fin spacing (ranging from 2 to 5 cm) and mass flow rate (MFR) (ranging from 0.007 to 0.0158 kg/s) through an exergy analysis of internal and exterior exergy losses. Based on the result analysis, the LFSAH’s maximum exergy efficiency was achieved at 3.31% for a fin spacing of 2 cm and a MFR of 0.007 kg/s. The study found that incorporating louvered fins into the SAH substantially improves its exergetic performance compared to the PSAH.

Establishment of Colburn ‘j’ factor and Fanning friction factor ‘f’ correlations for a compact heat exchanger having perforated wavy fins using Computational Fluid Dynamics

Wavy fins find widespread application in compact heat exchangers, due to their ability to achieve high heat transfer coefficients and lower friction factors, resulting in reduced pumping power requirements and improved overall efficiency. Despite the extensive literature available on these fins, no previous study has explored the influence of perforations on wavy fins. This study aims to establish accurate correlations for evaluating the Colburn ‘j’ factor and Fanning friction factor ‘f’ in wavy fins incorporating perforations, specifically tailored for compact heat exchangers. The Colburn ‘j’ factor guides the determination of the heat transfer coefficient, while the Fanning friction factor ‘f’ is instrumental in assessing pressure drop across the fin, both essential considerations in the rating and sizing of compact heat exchangers. The numerical model involves a perforated wavy fin constructed from ‘aluminum’ with ‘air’ as the fluid medium. To derive correlations, 1458 cases were simulated using ANSYS Fluent, systematically varying geometric parameters such as fin height (h), fin spacing (s), fin thickness (t), hole diameter (d), and fin pitch (p). The study spans Reynolds numbers from laminar (200≤Re≤1500) to turbulent (2000≤Re≤10000). ‘j’ &‘f’ factor values were validated using experimental data for wavy fins without perforations, followed by additional analysis incorporating perforations. The analysis showed a 19%–35% increase in the ‘j’ factor and a 21%–33% increase in the ‘f’ factor for perforated wavy fins compared to plain wavy fins. New ‘j’ and ‘f’ factor correlations were established, with over 96.3% and 95.0% of the points within ±16% and ±18% bounds, respectively. These innovative correlations applicable across the entire spectrum of operating and design conditions are expected to facilitate a more streamlined design process for compact heat exchangers featuring this unique fin geometry, enabling designers to reduce iterations during the design phase.

PARTICLE SWARM OPTIMIZATION AND CFD ANALYSIS OF A HEAT SINK FOR FORCED CONVECTIVE COOLING OF A DC/AC INVERTER

Power inverters play a crucial role in converting DC voltage and current to AC voltage and current. Its efficient operation relies on effective thermal management of the semiconductor devices, such as the IRF 3205 MOSFET chip which facilitate the switching operations to prevent thermo-mechanical stress that could to failure. This research developed and optimize a heat sink for an IRF 3205 MOSFET chip in a 3.5 KVA power inverter, using the Particle Swarm Optimization technique and experimented with a 3.5 KVA 24 V power inverter operating a 1.5HP air-conditioning unit. The heat sink geometry has 26 fins, 4 mm fin thickness, 40 mm fin height, 3 mm fin spacing, 2 mm fin base thickness, 179 mm length, 80 mm width and a thermal resistance of 60.75 °C/W. The thermal performance was evaluated using ANSYS Computational Fluid Dynamics (CFD) and results showed maximum base temperature of 65.85°C and experimental temperature of 64°C after two hours of steady operation dissipating heat of 45W. The results from both numerical and experimental data demonstrate the effectiveness of the optimized heat sink in maintaining the chip's temperature below its maximum working threshold of 170°C.

Design and simulation of a new type of fin-and-tube heat exchanger with trapezoidal slit fins

Slit fin-and-tube heat exchangers are extensively utilized in industry. The design of fin structure is crucial for improving the performance of heat exchanger and reducing energy losses. In this study, a new type of fin-and-tube heat exchanger with trapezoidal slit fins was proposed in this study. To determine the performance of the fins, physical and mathematical models of heat exchanger with the fin spacing Fp of 2.65 mm were established. The effects of structural parameters on heat exchanger performance were investigated by numerical simulation, which includes the slit height Sh, slit width Sw, transverse tube spacing Pt, and longitudinal tube spacing Pl. The effects of the four structural parameters on the comprehensive performance were analyzed by the evaluation index. The results revealed that the comprehensive performance of trapezoidal slit fins had increased by 9.6 % compared to flat fins. Furthermore, the influence degree of structural parameters on heat transfer performance was obtained based through the orthogonal experiments. The order of influence degree was as follows: Pl > Pt > Sh > Sw. Additionally, the optimal parameter combination was also obtained as follows: Sh = 1.47 mm, Pt = 42 mm, Pl = 27.5 mm, and Sw = 4 mm.

Thermal performance and optimization of an integrated collector–storage solar air heater based on lap joint-type flat micro-heat pipe arrays

This study seeks to optimize the performance of an integrated collector–storage solar air heater (ICSSAH) based on lap joint-type (LJT) flat micro-heat pipe arrays (FMHPAs) and latent thermal storage through numerical simulation. A novel dynamic heat transfer model is developed by incorporating the heat transfer correlation describing the thermal characteristics of phase change material (PCM) in narrow and elongated confined spaces within plate fins and validated using experimental data to address the issue of deviation caused by the fixed thermal resistance of the PCM in conventional numerical simulation methods. The validated model is used to simulate and predict the dynamic performance of LJT-FMHPA-ICSSAH under different heat transfer structures. Several optimization parameters, including the air gap thickness, the optical properties of the collector and glass cover plates, and fins structure, are considered. Simulation results indicate that the heat lost to the external environment through the glass cover plate accounted for over 70 % of the total heat dissipation. The optimal combination included a 50 mm gas layer; copper-based blue titanium coating; ultra-white glass; and fin spacing, thickness, height, and length of 5, 0.2, 60, and 195 mm, respectively. Under autumn/winter operating conditions, the optimized thermal performance is 1.13–1.35 times higher than the performance prior to optimization.

Performance evaluation and optimization of compact tube-fin latent heat storage system in phase change nanoemulsion discharge mode

The low thermal conductivity of phase change materials (PCMs) and the reduced heat transfer temperature difference along a tube both limit the thermal performance of latent heat thermal energy storage (LHTES). Herein, a compact tube-fin LHTES system based on phase change nanoemulsions was developed and the heat transfer process of nanoemulsion and PCM was numerically investigated. The effects of the heat transfer structure of the heat exchanger and the thermophysical parameters of the heat transfer fluid (HTF) on the cold energy discharge performance of the system were quantified. The optimized model reduced the discharge time by 24.4 % in nanoemulsion mode relative to water. Heat transfer improvement on the PCM side could be achieved by decreasing the fin spacing and tube spacing and by increasing the fin thickness. Such improvement reinforced the heat transfer enhancement of the nanoemulsion. The heat transfer mechanism was explained by the temperature distributions of the HTF and PCM along the tube. The phase-change thermostatic properties of the nanoemulsion and PCM contributed to the formation of dual-phase-change coupled heat transfer (i.e., the heat transfer process when the fluid inside the tube and the PCM filled outside the tube undergo a simultaneous phase change) within the LHTES, increasing the heat transfer temperature difference and the magnitude of the driving force. Increasing the phase change temperature difference accelerated the onset of dual-phase-change heat transfer, while increasing the PCM content prolonged the process. Briefly, this study provided operational and design assistance for LHTES systems in nanoemulsion mode with the intention to accelerate heat transfer rates.

Experimental investigation of heat transfer and aerodynamic drag of novel heat sinks with lamellar fins

Heat transfer and aerodynamic drag of novel small-sized heat sinks with lamellar fins, designed for electronic cooling, were experimentally investigated under conditions of forced convection in the range of Reynolds numbers 1 250–10 500. It was found that a gradual reduction in the fin spacing from 6 mm to 3 mm with a 29° angle of taper between the outermost fins leads to an increase in the heat transfer intensity by 15–32% with a significant increase in aerodynamic drag compared to the surface with a constant fin spacing of 6 mm. Incomplete cross-section cutting of fins at the relative depth of 0.6 in addition to the gradual reduction in the fin spacing provides aerodynamic drag decrease by 5–20% and increase of heat transfer intensity by 18–20% in comparison with the similar heat sink without fins cutting. Proposed novel designs of heat sinks enabled us to decrease by 7°С–16°С the maximum overheating of the heat sink's base in the flow speed range from 2.5 m/s to 7.5 m/s at constant heat load. To ensure a constant value of maximum overheating of the heat sink base the inlet flow velocity for the surface with constant fin spacing should be 1.6–2 times higher than that for the heat sink with 29° taper angle between outermost fins and partially fins cutting. In this case, the aerodynamic drag for the latter will be higher only by 1.6–2.7 times, which is quite acceptable.

Performance of a modified built-middle photovoltaic-thermocatalytic Trombe wall system by fins

The photovoltaic-thermocatalytic Trombe wall is attracting attention due to the ability to generate electricity, heat and purify indoor air, while it has the disadvantages of low electrical and degradation performance. Also the overall performance evaluation method of multi-functional Trombe walls still needs to be further improved. To fill these gaps, a modified built-middle photovoltaic-thermocatalytic Trombe wall by vertical fins was proposed in this study. Based on the first and second laws of thermodynamics, the effects of fins spacing and fins height under different solar radiation intensities on the performance were investigated by numerical simulation method. In addition, the performance of this modified system was compared with other relevant systems. The results reveal that the existence of an optimal fins spacing or fins height can maximize the thermal efficiency. The narrower fins spacing and higher fins can enhance electrical efficiency and degradation rate. Without sacrificing or even improving thermal performance, the presence of fins with suitable structural parameters leads to higher electrical and purification performance. Compared with the system with no fins, when the solar radiation intensity is 400 W/m2 and 800 W/m2 respectively, the thermal efficiency, electrical efficiency, degradation rate, and overall exergy efficiency gain relative enhancements of 1.81 % and 2.71 %, 2.47 % and 5.18 %, 2.27 times and 2.19 times, and 3.53 % and 6.67 %; and the overall exergy efficiency reaches 12.26 % and 12.20 % at the fins spacing of 31.25 mm and fins height of 30 mm. The introduction of exergy concept provides an effective way for overall performance evaluation of multi-functional Trombe walls with different grades of energies (products). The obtained thermal exergy and electrical exergy dominate the overall exergy efficiency, and the diffusion exergy accounts for a negligible proportion due to the relatively low concentration of pollutant. In comparison with other relevant systems, the modified system shows the best comprehensive utilization of solar energy, and has great application potential. This study provides a new direction for the performance improvement and assessment of multi-functional Trombe wall systems.

Experimental comparison of finned tube heat exchangers for heat rejection under natural convection conditions

This article presents experimental analysis of heat removal capabilities of a wavy finned tube heat exchanger operating under natural convection conditions Two types of heat exchangers are considered in this work: standard construction exchangers, most often found in waste heat rejection devices and exchangers based on a novel geometry, which supports intensification of heat transfer processes under natural convection. The aim of the research was to experimentally verify the novel conception and to determine the thermal efficiency of heat exchangers under the influence of selected design parameters. The factors analyzed in the research include: temperature difference between the heat transfer surface and the surrounding air, different angles of the units transferring heat and the height of the exchanger casing. Four variants of the casing height were considered in this work. The conducted research also allowed to determine the influence of the fin spacing on the performance of the heat exchangers under natural convection conditions. The test results obtained show that the novel geometry allows for significance improvement of heat transfer efficiency. The proposed solution allowed to increase the density of the heat flux from 12% in (fin spacing of 2.8 mm) up to 24% (fin spacing of 5.6 mm). It was also observed that the height of the casing has significant impact on the heat transfer efficiency. For each of the analyzed cases, the heat exchanger without a casing was characterized by the lowest thermal efficiency, while the highest heat flux was removed from the exchanger with the highest casing (up to four times higher heat transfer rate). It was determined that the largest amount of heat was removed from exchangers located horizontally, while the exchanger with an angle of 60°, was characterized by the lowest rate of heat removal. Another factor which was determined to have significant impact on the performance of the heat exchangers is the spacing between the fins. The research results obtained shows that increasing the spacing of the finned surface from 2.8 mm to 5.6 mm resulted in an average twofold increase in thermal efficiency in the case of four-row exchangers and an almost threefold increase in the case of two-row exchangers. The obtained results can be used as basic guide for the development of finned tube heat exchangers operating under natural convection conditions.

Numerical study of heat transfer and fluid flow in the offset strip-fin heat exchanger: A fin-by-fin analysis

Heat transfer and fluid flow in the offset strip-fin heat exchanger are studied numerically, fin by fin. The impact of the offset strip-fin geometry on j/f performance is also evaluated. The mathematical model is validated against j- and f-factor correlations from the literature. The numerical analysis revealed that the offset strip-fin heat exchanger achieves best j/f performance in the laminar flow region (Redh < 700). At higher Reynolds numbers, the j/f factor deteriorates under the impact of flow recirculations, oscillatory wakes and the contribution of form drag. Compared to the continuous rectangular fin, the offset strip-fin increases the air-side heat transfer coefficient by 50% at Redh = 100 and by 120% at Redh = 4000 while the pressure drop increases by a factor of 3–5. Generally, the offset strip-fin achieves j/f performance between 0.20 and 0.30, with maximum values found in the laminar flow region (Redh 〈1000). Offset strip-fin geometries with reduced fin space and increased wall thickness strongly affect the form drag, which reduces the j/f performance, especially for Redh > 1000. The fin height weakly influences the performance, and the j/f factor is between 0.22 and 0.23 for a wide range of fin heights.