Air-side Heat Transfer Enhancement Research Articles

Air-side heat transfer enhancement in fin-tube heat exchangers using forced vibrations under various conditions

Forced vibrations can occur in fin-tube heat exchangers (FTHEs) by compressors, fans, and the driving conditions of automobiles. Accordingly, the effects of forced vibrations on the heat transfer performance of FTHEs in refrigeration systems and heat pumps should be investigated in detail for their practical design. However, few studies have investigated the improvement in heat transfer by vibrations in FTHEs. In this study, air-side heat transfer enhancement in FTHEs by applying forced vibrations is experimentally investigated. The heat transfer performance of the FTHEs was measured by varying the air-side Reynolds number, vibrational frequency, vibrational amplitude, and fin pitch. For practical purposes, the heat transfer characteristics of the FTHEs were analyzed under constant vibrational power, force, and velocity. The heat transfer enhancement increased with an increase in the vibrational frequency, amplitude, and fin pitch, whereas it decreased with an increase in the air-side Reynolds number. A correlation for heat transfer enhancement using forced vibrations was developed as a function of nondimensional operating and geometrical parameters. The results can be used to predict heat transfer enhancement by vibrations and the FTHE design for heating, ventilating, and air-conditioning systems.

Using patterned surface wettability to enhance air-side heat transfer through frozen water droplet vortex generators-part I: Experimental study

• Frozen water droplets can serve as vortex generators (VGs) to increase air-side heat transfer. • The VGs are formed via the naturally-occurring processes of condensation and freezing. • Patterned surface wettability encourages coalescence in predetermined locations. • A circular moat helps concentrate water inside a circular region prior to freezing. • The average height of the frozen droplets after the third defrost cycle was 1.25–3.20 mm. • The VG height is greater than the boundary layer thickness in the entrance region. In this study, a novel technique for deploying hemispherical vortex generators (VGs) on a refrigerator evaporator for air-side heat transfer enhancement is investigated. The dome VGs are formed via the naturally-occurring processes of condensation and freezing. By using patterned surface wettability to collect condensate and encourage coalescence in predetermined locations, it was found that large frozen droplets could be formed in various configurations on the fin surface, which in turn could then serve as VGs during the early frost growth period. These findings were examined experimentally here using a flat hydrophobic-coated aluminum plate (Part I) and were then simulated numerically in a follow-up study (Part II). Various geometries of staggered circles were studied to test the efficacy of the idea. The circular regions had an inner diameter of 3.175 mm (0.125 in) and were surrounded by a microchannel “moat” to promote the coalescence and trapping of condensate into a single droplet inside the circle. Surface testing was performed inside an environmental chamber via a one-hour condensation period ( T w ∼1°C) followed by three one-hour frost growth periods with ten minutes of defrosting in between. The following experimental conditions were used during testing: RH=60–80%, T air = 20°C–24°C, and T w = −3°C to –12°C. By varying the microchannel width (∼100–500 µm) and the wettability of the inner area, large frozen droplets were formed which remained visible for 40–60 min during the third frost cycle. The average diameter and height of the frozen droplets at the end of the third cycle were 3.20–3.90 mm and 1.25–3.20 mm, respectively.

Air side heat transfer enhancement using radiantly arranged winglets in fin-and-tube heat exchanger

The enhancement of airside heat transfer in fin-and-tube heat exchanger is crucial for reducing the pumping power, saving materials and so on. In this paper, the authors propose to arrange the winglets radiantly around the tubes on the fin to enhance the airside heat transfer in fin-and-tube heat exchanger. The airside flow and heat transfer characteristic are numerically investigated. The position and the geometric dimensions of the vortex generators are also optimized. The simulation results show that the Colburn j-factor and the friction factor f both increase with VG number. The fin structure has the best comprehensive performance when its VGs are uniformly arranged both in the upwind and backwind regions of the tubes, and at the same time make the enhanced zones the widest in the tube radial direction. With the increase of the VG length, the comprehensive performance factor JF gets lower. The comprehensive performance of the proposed fin with five rows of tubes are demonstrated to be better than a reference fin used in industry with the same fin thickness, tube diameter, fin pitch and tube pitch.

Additively-manufactured metallic porous lattice heat exchangers for air-side heat transfer enhancement

In this study, two novel porous lattice air-cooled heat exchangers (Lattice 1 and Lattice 2) were fabricated by the selective laser melting (SLM) technique from an aluminum alloy (AlSi10Mg) powder. Repetitions of the Rhombi-Octet unit cells of different cell sizes were used to form the porous matrices. Experiments were carried out in a wind tunnel to evaluate the thermal-hydraulic performances of the heat exchangers. The thermal performance indicators such as the overall thermal conductance (UA), air-side thermal resistance (Ra), air-side heat transfer coefficient (ha) and volumetric heat flux density (q˙v) of the porous lattice heat exchangers were determined and comparisons were made against two conventional fin-tube heat exchangers (Fin-tube 1 and Fin-tube 2). In addition, the pressure drops across the heat exchangers were also measured. Based on our investigations, it was determined that Lattice 1 exhibited approximately 40%–45% higher UA and ha than Lattice 2. However, the pressure drop across Lattice 1 was also higher than Lattice 2. At the same mass flow rate of air (m˙a), it was found that the ha values of the porous lattice heat exchangers were more than 2 times those of the fin-tube heat exchangers. The significantly higher ha values of the porous lattice are mainly attributed to the presence of interconnected pores and the formation of eddies downstream of the ligaments that improved fluid mixing. For the same pumping power (W˙/H), the use of the porous lattice heat exchangers also resulted in consistently higher ha values than the fin-tube heat exchangers. These results demonstrated the potential of using SLM to fabricate a new generation of commercial-scale compact heat exchangers made of porous lattices. These new porous lattice structures have enhanced the thermal performances of the heat exchanger with no penalty in pumping power.

A novel caudal-fin-inspired hourglass-shaped self-agitator for air-side heat transfer enhancement in plate-fin heat exchanger

A novel caudal-fin-inspired hourglass-shaped self-agitator installed inside a plate-fin heat exchanger has been experimentally investigated to characterize its thermo-hydraulic performance. Because of fluid-structure interaction between the air and the caudal-fin-inspired hourglass-shaped self-agitator, there is periodic conversion between fluid kinetic energy and elastic strain energy. Vortices generated by the self-sustained vibration of the self-agitator enhance flow mixing and thus heat transfer performance. Experiments were conducted at a wide range of Reynolds numbers from 400 to 10,000, which covered the laminar-transitional-turbulent regime. Experimental correlations of pressure drop as a function of a dimension parameter, friction factor, and Nusselt number have been proposed. Mutual coupling motions and effects of multiple-row flapping caudal-fin-inspired hourglass-shaped self-agitators in parallel and tandem configurations were studied by using a high-speed camera. A stereo Particle Image Velocimetry system was used to conduct detailed flow field measurements to quantify the flow mixing level. Parameter studies for the stream-wise distance (i.e., the pitch) between the neighboring caudal-fin-inspired hourglass-shaped self-agitators have been investigated to exploit the best performance of the heat exchanger equipped with caudal-fin-inspired hourglass-shaped self-agitators. For the chosen plate-fin heat exchanger and assigned working conditions, the best heat transfer performance was obtained with twelve-row self-agitators with a pitch of 12.5 mm, which caused a 200% increase in the Nusselt number over the clean channel at the same Reynolds number. However, the best overall performance was obtained with six-row self-agitators with a pitch of 25 mm, which caused a 68% enhancement in thermal-hydraulic characteristics compared to the clean channel at the same Reynolds number.

Air-side heat transfer enhancement in plate-fin channel with an airfoil-based self-agitator

Airfoil-based self-agitators (AFAs) were installed in multiple plate-fin channels integrated with cantilever beams. The heat transfer enhancement and pressure drop characteristics of this AFA design were experimentally investigated and compared with the clean channel case. We found that the AFA vibrates periodically and generates vortices, which enhance flow mixing and thus heat transfer performance. A rectangular shaped airfoil-based self-agitator (RAFA) and a delta shaped airfoil-based self-agitator (DAFA) with variable chord lengths were designed to investigate shape and size effects on thermal performance. Measurements were also carried out for DAFAs with different layouts, such as multiple rows of DAFAs installed inside a single channel. For the chosen heat sink and assigned working conditions, the best heat transfer performance was obtained with four rows of DAFAs, which caused an 80% increase in overall Nusselt number over the clean channel at the same Reynolds number and a 50% rejected heat increase at the same pumping power due to the strong longitudinal vortices generated by the presence of the AFAs.

Airfoil-shaped self-agitator for convective heat transfer enhancement

A novel airfoil-shaped self-agitator is developed for airside heat transfer enhancement within rectangular heatsink channels. Due to fluid-structure interaction, the airfoil-shaped self-agitator oscillates and generates vortices that improve mixing within the channel and thus improve the heat transfer. The fluid-structure interaction is simulated with the fully coupled method of remeshing. Numerical results show that the self-agitator can generate out-of-phase heaving and rotating motions to enhance heat transfer by 27% without additional power costs and the Nusselt number can be improved by 53% at the same Reynolds number. To investigate the fundamental aspects of the convective heat transfer enhancement with vortex generation, modal analysis is performed for the vorticity and temperature fields with dynamic modal decomposition. Instantaneous results can be constructed with steady mode and six dominant unsteady modes, while the steady mode results can be used to predict the average thermal performance. A prototype of the self-agitator is fabricated and tested experimentally for concept demonstration and validation. The experimental results show the self-agitator can improve the air-side thermal performance in the rectangular channel significantly without additional pumping power.

A flapping vortex generator for heat transfer enhancement in a rectangular airside fin

This paper presents a two-dimensional numerical study of a novel flapping vortex generator mounted on a heatsink fin for airside heat transfer enhancement. The proposed vortex generator is made with a thin elastic sheet bonded to the inner wall of the heatsink channel with an inclined angle. Our investigations are focused on the effects of the Young’s Modulus of the vortex generator on the oscillations of the elastic sheet, vorticity fields, and heat transfer performances. The results are compared with the heat transfer performances of conventional rigid agitators at two different flow velocities (Reynolds nunbers). Our numerical results demonstrate that the vortex generator with a Young’s Modulus of 1 MPa has the best performance among the other three choices and can enhance the rejected heat by 140% at the same velocity and 87% at the same total pumping power. The developed flapping vortex generator can improve the average Nusselt number by 200% compared with a clean channel with the same Reynolds number. Modal analysis is performed with transient temperature and vorticity results using dynamic modal decomposition where it is found that a steady modal behavior directly influences the thermal performance of the system. Furthermore, creating more discrete patterns near the boundaries of the steady mode in the vorticity field can enhance the internal convective heat transfer rate. The numerical results presented can help to guide the design of the flapping vortex generators in future high-performance airside fins.

Air-Side Heat Transfer Enhancement Utilizing Design Optimization and an Additive Manufacturing Technique

This paper focuses on the study of an innovative manifold microchannel design for air-side heat transfer enhancement that uses additive manufacturing (AM) technology. A numerical-based multi-objective optimization was performed to maximize the coefficient of performance and gravimetric heat transfer density (Q/MΔT) of air–water heat exchanger designs that incorporate either manifold-microchannel or conventional surfaces for air-side heat transfer enhancement. Performance comparisons between the manifold-microchannel and conventional heat exchangers studied under the current work show that the design based on the manifold-microchannel in conjunction with additive manufacturing promises to push the performance substantially beyond that of conventional technologies. Different scenarios based on manufacturing constraints were considered to study the effect of such constraints on the heat exchanger performance. The results clearly demonstrate that the AM-enabled complex design of the fins and manifolds can significantly improve the overall performance, based on the criteria described in this paper. Based on the current manufacturing limit, up to nearly 60% increase in gravimetric heat transfer density is possible for the manifold-microchannel heat exchanger compared to a wavy-fin heat exchanger. If the manufacturing limit (fin thickness and manifold width) can be reduced even further, an even larger improvement is possible.

Comparison of metal foam and louvered fins as air-side heat transfer enhancement media for miniaturized condensers

Experimental work was conducted to compare the thermal–hydraulic performances of cross-flow microchannel condensers using louvered fins and metal foams as extended surfaces. Three copper foam surfaces with pore densities of 10 and 20 pores per inch (PPI) and porosities of 89.3 and 94.7%, and three aluminum louvered fins with lengths of 27 and 32 mm (in the flow direction) and heights of 5 and 7.5 mm were evaluated. The experiments were carried out in a closed loop wind-tunnel calorimeter equipped with a R-600a refrigeration loop. A condensing temperature of 45 °C was used in all tests, with face velocities ranging from 2.1 to 7.7 m/s. A comparison based on the thermal conductance and air-side pumping power showed that the surfaces enhanced with louvered fins performed better than the metal foams under all conditions investigated.

Study on heat transfer enhancement of discontinuous short wave finned flat tube

The typical configuration adopted by air-cooled condenser (ACC) in coal-fired power generating unit is the wave finned flat tube. The development of boundary layer between wave fins along long axis of flat tube can suppress the air-side heat transfer enhancement to a great extent. It has been proved that the serrated fins can enhance heat transfer obviously by breaking the development of boundary layer periodically. In the present study, the discontinuous short wave fin was introduced to the flat tube to enhance the air-side heat transfer of ACC. Two different types of arrangements, i.e. staggered and in-line for discontinuous short wave fins on the flat tube, were designed. By numerical simulation, the heat transfer and flow performances of short wave fins were studied under different arrangements (in-line, staggered), and the influences on heat transfer and flow characteristics of rows of short wave fin and interrupted distance between discontinuous short wave fins were revealed numerically. The results indicated that, compared with the original continuous wave fin, the discontinuous short wave fin effectively improved the air-side heat transfer of flat tube under the air flow velocities in the practical application of engineering. Moreover, the increment of pressure loss of air-side flow was restricted for the discontinuous short wave fins because of the reduction of contact areas between the air flow and fin surface.

Air-Side Heat-Transfer Enhancement by a New Winglet-Type Vortex Generator Array in a Plain-Fin Round-Tube Heat Exchanger

The impact of a vortex-generation technique for air-side heat-transfer improvement is experimentally investigated through full-scale wind-tunnel testing of a plain-fin round-tube heat exchanger under dry-surface conditions. Inspired by the formation locomotion of animals in nature, a new vortex generator (VG) array deployed in a “V” is proposed in the present work, aiming to create constructive interference between vortices. The array is composed of two delta-winglet pairs and placed at an attack angle of 10 deg or 30 deg. Its effectiveness is compared with a baseline configuration and two conventional single-pair designs placed at 30 deg, a small pair with half the area of the array and a large pair with the same area as the array. The frontal air velocity considered ranges from 2.3 m/s to 5.5 m/s, corresponding to a Reynolds number range based on the hydraulic diameter of 1400–3400. The experimental results show little impact of the 10 deg array and a moderate heat-transfer improvement of up to 32% for the small pair, both introducing additional pressure loss of approximately 20–40%. For the 30 deg array and the large pair, similar augmentation of 25–55% in air-side heat-transfer coefficient is obtained accompanied by average pressure drop penalties of 90% and 140%, respectively. Performance evaluation using the criteria of the modified area goodness factor and the volume goodness factor indicates the superiority of the heat exchanger enhanced by the 30 deg array among all the investigated VGs. The VG array is found more effective at comparatively low Reynolds numbers, representative of many heating, ventilation, air-conditioning, and refrigeration applications and compact heat-exchanger designs.

Heat transfer enhancement by winglet-type vortex generator arrays in compact plain-fin-and-tube heat exchangers

The potential of winglet type vortex generator (VG) arrays for air-side heat transfer enhancement is experimentally evaluated by full-scale wind-tunnel testing of a compact plain-fin-and-tube heat exchanger. The effectiveness of a 3VG alternate-tube inline array of vortex generators is compared to a single-row vortex generator design and the baseline configuration. The winglets are placed in a common-flow-up orientation for improved tube wake management. The overall heat transfer and pressure drop performance are assessed under dry-surface conditions over a Reynolds number range based on hydraulic diameter of 220 ≤ Re ≤ 960. It is found that the air-side heat transfer coefficient increases from 16.5% to 44% for the single-row winglet arrangement with an increase in pressure drop of less than 12%. For the three-row vortex generator array, the enhancement in heat transfer coefficient increases with Reynolds number from 29.9% to 68.8% with a pressure drop penalty from 26% at Re = 960 to 87.5% at Re = 220. The results indicate that vortex generator arrays can significantly enhance the performance of fin-tube heat exchangers with flow depths and fin densities typical to those used in air-cooling and refrigeration applications.

Measurement of Corona Wind Velocity and Calculation of Energy Conversion Efficiency for Air-Side Heat Transfer Enhancement in Compact Heat Exchangers

Corona discharge utilizes the effect of electrically induced secondary motions to enhance the air-side heat transfer coefficients between high-density fins in a fin structure. The objective of this study was to measure the velocity associated with the air jet generated by corona discharge for parametric ranges of interest to air-side heat transfer enhancement in compact heat exchangers. This is the first study of its kind where the nonintrusive laser Doppler velocimetry (LDV) technique was used to measure the velocity, with careful attention paid to correction for the electric field effects on the particles and the resulting slip velocity. Experiments were conducted with a prototypical wire-to-plate geometry (positively charged wire and grounded plate). In two series of experiments, the flow was seeded with 2.5 μm glass microspheres and 0.5 μm polystyrene nanospheres. The corrected velocity measurements were used to calculate the kinetic energy flux to the fluid and the resulting efficiency of the electric-to-kinetic energy conversion.

97/03177 Evaluating the potential of vortex-enhanced evaporator performance for refrigeration applications

In many refrigeration systems, overall evaporator performance is limited by the heat transfer coefficients are inherently lower for single-phase gas flow than for the two-phase refrigerant flow. Therefore, it is important to identify air-side performance enhancements suitable for the low-Reynolds-number frosting conditions found in these applications. In this paper the results of a preliminary assessment of vortex-induced air-side heat transfer enhancement for refrigeration applications are reported. The results indicate that longitudinal vortices may significantly improve air-side thermal performance, leading to smaller heat exchangers for a fixed duty or to better thermal performance for a fixed heat exchanger envelope. Further research is recommended to evaluate the to-scale performance of vortex generators under frosting conditions.