Louvered Fin Heat Exchanger Research Articles

A performance analysis of porous graphite foam heat exchangers in vehicles

Due to the increasing cooling power and space limitation in vehicles, a new compact heat exchanger – graphite foam heat exchanger is proposed for vehicle cooling application. The graphite foam has high thermal conductivity (the effective thermal conductivity is 40–150 W/m K) and low density (0.2–0.6 g/cm3), but it has high flow resistance which is a problem in heat exchanger applications. In order to find a graphite foam heat exchanger with low flow resistance, four different configurations (baffle, pin-finned, corrugated, and wavy corrugated) of graphite foam fins are analyzed in terms of thermal performance and pressure drop by using a computational fluid dynamics approach. The simulation results show that the wavy corrugated foam presents high thermal performance and low pressure drop. Moreover, a comparative study between the wavy corrugated foam heat exchanger and a conventional aluminum louver fin heat exchanger is carried out to evaluate the performance of graphite foam heat exchangers in terms of coefficient of performance (removed heat/air pumping loss), power density (removed heat/mass of heat exchangers), and compactness factor (removed heat/volume of heat exchangers). Finally, this paper concludes that graphite foam heat exchangers should be further developed in vehicles, and presents several recommendations for how such development can be promoted.

Characteristics and performance evaluation of surface-treated louvered-fin heat exchangers under frosting and wet conditions

Hydrophilic, hydrophobic, and dual (hydrophilic and hydrophobic) coatings were applied to louvered-fins typically used for heat pump heat exchangers, and the characteristics and performance of the heat exchangers under both frosting and wet conditions were compared according to surface treatment. The hydrophilic heat exchanger had the highest air-side pressure drop under frosting conditions. The hydrophobic unit had a lower air-side pressure drop than the others (hydrophilic and dual), due to frost retardation, and the reduction of the heat transfer rate was also smaller. The dual-fin heat exchanger exhibited frost retardation only in the early stage of the experiment, and the heat transfer rate was slightly greater than that of the hydrophilic unit. In wet-condition experiments for evaluating the evaporating performance of surface-treated heat exchangers, the hydrophilic unit had a lower air-side pressure drop than the others, due to the thin film of water condensation on the fins. The differences in the heat transfer rates of the heat exchangers were not significant.

Numerical Study of Flow Deflection and Horseshoe Vortices in a Louvered Fin Round Tube Heat Exchanger

In louvered fin heat exchangers, the flow deflection influences the heat transfer rate and pressure drop and thus the heat exchanger’s performance. To date, studies of the flow deflection are two-dimensional, which is an acceptable approximation for flat tube heat exchangers (typical for automotive applications). However, in louvered fin heat exchangers with round tubes, which are commonly used in air-conditioning devices and heat pumps, the flow is three-dimensional throughout the whole heat exchanger. In this study, three-dimensional numerical simulations were performed to investigate the flow deflection and horseshoe vortex development in a louvered fin round tube heat exchanger with three tube rows in a staggered layout. The numerical simulations were validated against the experimental data. It was found that the flow deflection is affected by the tubes in the same tube row (intratube row effect) and by the tubes in the upstream tube rows (intertube row effect). Flow efficiency values obtained with two-dimensional studies are representative only for the flow behavior in the first tube row of a staggered louvered fin heat exchanger with round tubes. The flow behavior in the louvered elements of the subsequent tube rows differs strongly due to its three-dimensional nature. Furthermore, it was found that the flow deflection affects the local pressure distributions upstream of the tubes of the downstream tube rows and thus the horseshoe vortex development at these locations. The results of this study are important because the flow behavior is related to the thermal hydraulic performance of the heat exchanger.

A Comparison of Metal-Foam Heat Exchangers to Compact Multilouver Designs for Air-Side Heat Transfer Applications

High-porosity metal foams, with novel thermal, mechanical, electrical, and acoustic properties, are being more widely used in various industrial applications. In this paper, open-cell aluminum foam is considered as a highly compact replacement for conventional louver fins in brazed aluminum heat exchangers. A model based on the ϵ-NTU method is developed to compare the flat-tube, serpentine louver-fin heat exchanger to the flat-tube metal-foam heat exchanger. The two heat exchangers are subjected to identical thermal-hydraulic requirements, and volume, mass, and cost of the metal-foam and louver-fin designs are compared. The results show that the same performance is achieved using the metal-foam heat exchanger but a lighter and smaller heat exchanger is required. However, the cost of the metal-foam heat exchanger is currently much higher than that of the louver-fin heat exchanger, because of the high price of metal foams. If the price of metal foam falls to equal that of louver-fin stock (per unit mass), then the metal-foam heat exchanger will be less expensive, smaller, and lighter than the louver-fin heat exchanger, with identical thermal performance.

An experimental and computational study of approach air distribution for a finned-tube heat exchanger

The distribution of air flow approaching a finned-tube heat exchanger is one of the predominant factors influencing the heat exchanger's performance. This article describes a method for measuring and predicting the inlet air flow distribution using particle image velocimetry (PIV) and a computational fluid dynamics (CFD) model, highlighting the source and magnitude of air side maldistribution. The studied case was a single-slab, four-depth-row, louvered-fin heat exchanger installed vertically in a horizontal duct. The measured data showed that the air approaching this very simple test case generally maintained velocities of 1.25 ms−1 (4.1 fts−1) to 1.35 ms−1 (4.4 fts−1), but certain portions of the coil were completely obstructed, resulting in no air flow, and other portions realized velocities of over 1.7 ms−1 (5.6 fts−1). A CFD model of the air flow through this heat exchanger was developed based on a momentum resistance modeling approach. The CFD results agreed well with the PIV measurements, predicting the local velocities within 3% over most of the domain and within 10% in areas with the largest velocity gradients.

A Simple Air-Side Data Analysis Method for Partially Wet Flat-Tube Heat Exchangers

An air-side data analysis method is developed for flat-tube heat exchangers under partially wet conditions. In order to simplify the combined sensible and latent heat transfer, it is assumed that condensate drainage paths develop such that, at steady state, water does not spread to noncondensing surfaces, which therefore remain dry. The air dew point is compared to local fin-tip and fin-base temperatures, and a partially wet flat-tube heat exchanger is partitioned into fully wet, partially wet, and dry-fin regions, which are subsequently analyzed as separate heat exchangers. Using an enthalpy-based effectiveness–NTU (number of transfer units) method, average fin efficiency is calculated for each region, and the locations of region boundaries are determined iteratively. The proposed data analysis method is demonstrated with experimental data for a flat-tube louver-fin heat exchanger under various latent loads. The general approach presented can be extended to other heat exchanger geometries.

Thermo-hydraulic characterization of a louvered fin and flat tube heat exchanger

In the present study, a whole heat exchanger with a hydraulic diameter of 2.3 mm is tested, which is a minichannel heat exchanger according to the Kandlikar classification. This is a louvered fin and flat tube heat exchanger currently used in car cooling systems, also known as radiator. A glycol–water mixture (60/40 in volume) circulates through the tubes at flows ranging from 100 to 7800 l/h and at a supply temperature of 90 °C. This fluid is cooled with ambient air at a temperature of 20 °C and at frontal air velocities varying between 0.5 and 7 m/s. The thermohydraulic performance of the heat exchanger is compared with the classical correlations given in the literature for the heat transfer and the friction factor calculation. On the glycol–water side the heat exchanger is characterized for Reynolds numbers from 30 to 8000. A first comparison is carried out with the correlations available in the literature with a purely predictive model by obtaining a predictive value with a systematic under prediction lower than 10%. In a second step a semi-empirical model is considered to identify the experimental heat transfer coefficients for this application.

Study of junction flows in louvered fin round tube heat exchangers using the dye injection technique

Detailed studies of junction flows in heat exchangers with an interrupted fin design are rare. However, understanding these flow structures is important for design and optimization purposes, because the thermal hydraulic performance of heat exchangers is strongly related to the flow behaviour. In this study flow visualization experiments were performed in six scaled-up models of a louvered fin round tube heat exchanger. The models have three tube rows in a staggered layout and differ only in their fin spacing and louver angle. A water tunnel was designed and built and the flow visualizations were carried out using dye injection. At low Reynolds numbers the streakline follows the tube contours, while at higher Reynolds numbers a horseshoe vortex is developed ahead of the tubes. The two resulting streamwise vortex legs are destroyed by the downstream louvers (i.e. downstream the turnaround louver), especially at higher Reynolds numbers, smaller fin pitches and larger louver angles. Increasing the fin spacing results in a larger and stronger horseshoe vortex. This illustrates that a reduction of the fin spacing results in a dissipation of vortical motion by mechanical blockage and skin friction. Furthermore it was observed that the vortex strength and number of vortices in the second tube row is larger than in the first tube row. This is due to the thicker boundary layer in the second tube row, and the flow deflection, which is typical for louvered fin heat exchangers. Visualizations at the tube-louver junction showed that in the transition part between the angled louver and the flat landing a vortex is present underneath the louver surface which propagates towards the angled louver.

Thermal characteristics of louvered fins with a low-reynolds number flow

A heat recovery system is crucial for the effective use of energy where heat rejection from production processes is unavoidable and must be reused. The response of the louvered fins to the low-Reynolds number hot gas is yet to be reported in the literature for the application of a heat exchanger on low-speed hot plume arising from heat sources in production processes. This study focuses on the effects of the louvered fin heat exchanger’s design parameters, which include the louver pitch and louver angle, on the convective heat transfer, which defines the thermal interaction between the hot, buoyant, naturally-induced air and the louvered fins. The resulting Colburn factors (j) are compared with those derived under forced convection with a similar range of low Reynolds number (233 to 1024). All experiments are done on a 15:1 scaled-up model. The fin aspect ratios between the fin spacing and louver pitch are set at 0.75, 1, and 1.5, while the louver angles are set at 18°, 23°, 30°, 35°, and 40°. The Colburn factor strongly depends on the louver angle, especially at the lower range of the Reynolds number. The decreasing aspect ratio induces more hot buoyant air into the louver-formed channels, increasing the heat transfer rate. When the fin angle increases towards 30°, a larger Colburn factor is produced. However, the heat transfer characteristic drops as the angle goes beyond 30°. The highest j for the low speed flow is attained when the louver angle is 30° and the fin aspect ratio is 1.

Effect of inlet manifold structure on the performance of the heater core in the automobile air-conditioning systems

Brazed aluminum flat tube and louver fin heat exchanger is widely used as the heater core in automotive heat, ventilation and air-conditioning (HVAC) module. It was found that the temperature distribution in heater core surface is not equivalent, and it was considered that the flow maldistribution in the tubes responses for this phenomenon. The purpose of this work was to enhance the performance of the heater through optimizing the inlet manifold structure. The computational fluid dynamics (CFD) was adopting for investigating this phenomenon and finding some optimization schemes. In addition, the experiments were carried out for this optimization. In these experiments, two samples of heater core, before and after optimizing, were tested in an experimental facility, and the experimental results showed that the performance had improved by 1.03–3.98% through adding a deflector in the inlet manifold. The IR pictures also showed that the temperature distribution on the heater core surface was more homogeneous.

Air-Side Heat Transfer and Friction Correlations for Flat-Tube Louver-Fin Heat Exchangers

The objective of this study is to develop an accurate, reliable, and updated predictive model for the air-side performance of flat-tube louver-fin heat exchangers. Using the most comprehensive experimental database to date—consisting of 1030 heat-transfer and 1270 pressure-drop measurements, from nine independent laboratories for 126 sample heat exchangers—j- and f-factor correlations are developed to predict the air-side performance of heat exchangers. The database is analyzed, the form of the curve fits is explored, and the predictive performance of the correlations is evaluated. The j- and f-factor correlations predict the experimental data with rms errors of 11.5% and 16.1%, respectively. Multiple regressions for a locally linearized data model were used to estimate the confidence intervals and covariances of the regression constants. A comparison to prior correlations shows the proposed correlations to provide more accurate predictions and to span a much broader parameter space than prior work. Practical utility in design and optimization, and unavoidable limitations in developing such correlations are discussed.

Heat transfer augmentation along the tube wall of a louvered fin heat exchanger using practical delta winglets

Delta winglets are known to induce the formation of streamwise vortices and increase heat transfer between a working fluid and the surface on which the winglets are placed. This study investigates the use of delta winglets to augment heat transfer on the tube surface of louvered fin heat exchangers. It is shown that delta winglets placed on louvered fins produce augmentations in heat transfer along the tube wall as high as 47% with a corresponding increase of 19% in pressure losses. Manufacturing constraints are considered in this study whereby piercings in the louvered fins resulting from stamping the winglets into the louvered fins are simulated. Comparisons of measured heat transfer coefficients with and without piercings indicate that piercings reduce average heat transfer augmentations, but significant increases still occur with respect to no winglets present.

An amendment of the generalized friction correlation for louver fin geometry

This study proposed an amendment to the previous correlation developed by Chang et al. [Y.J. Chang, K.C. Hsu, Y.T. Lin, C.C. Wang, A generalized friction correlation for louver fin geometry, Int. J. Heat Mass Transfer 43 (2000) 2237–2243] for the generalized frictional correlation for louver fin geometry. A total of 91 samples of louver fin heat exchangers are used in the regression analysis. The proposed amendment eliminates the discontinuity of the original correlation and gives a mean deviation of 9.11%, and it is shown that 83.91% of the frictional data can be correlated within ±15%.

Effects of winglets to augment tube wall heat transfer in louvered fin heat exchangers

The louvered fin heat exchanger, a type of compact heat exchanger, has been used heavily in the automotive and air conditioning industries for the last several decades. The majority of past research, aimed towards improving louvered fin exchanger efficiency, has focused on optimizing various parameters of the louvered fin. The experimental study presented in this paper concentrates instead on augmenting the heat transfer along the tube wall of the compact heat exchanger through the use of winglets placed on the louvers. The experiments were completed on a 20 times scaled model of an idealized louvered fin exchanger with a fin pitch to louver pitch ratio of 0.76 and a louver angle of 27°. The Reynolds numbers tested, based on louver pitch, were between 230 and 1016. A number of geometrical winglet parameters, including angle of attack, aspect ratio, direction, and shape, were all evaluated based on heat transfer augmentation, friction factor augmentation, and efficiency index (combination of both augmentations). In an attempt to optimize these winglet parameters, tube wall heat transfer augmentations as high as 39% were achieved with associated friction factor augmentations as high as 23%.

3-D thermal-hydraulic analysis for louver fin heat exchangers with variable louver angle

In the present study, successively increased or decreased louver angle patterns are proposed and 3-D numerical analysis on heat and fluid flow are carried out. Five different cases of successively increased or decreased louver angles (+2°, +4°, −2°, −4°, and uniform angle 20°) are investigated: case A (20°, 22°, 24°, 26°, 24°, 22°, 20°), case B (20°, 24°, 28°, 32°, 28°, 24°, 20°), case C (26°, 24°, 22°, 20°, 22°, 24°, 26°), case D (32°, 28°, 24°, 20°, 24°, 28°, 32°), case E (uniform angle 20°) . For case A (+2°), case B (+4°), case C (−2°) and D (−4°), the maximum heat transfer improvement interpreted by j/ j 0 are 115%, 118%, 109% and 107%, and the corresponding friction factor ratio f/ f 0 are 116%, 119% 110% and 108%, respectively, where j/ j 0 and f/ f 0 are the Colburn factor ratio and friction factor ratio between successively variable louver angles and uniform angle, respectively. It is also shown that the maximum area reduction for case B can reach up to 25.5% compared to a plain fin surface. The present results indicated the successively variable louver angle patterns applied in heat exchangers could effectively enhance the heat transfer performance.

Impact of leading edge delta-wing vortex generators on the thermal performance of a flat tube, louvered-fin compact heat exchanger

The effectiveness of delta-wing type vortex generators is experimentally evaluated by full-scale wind-tunnel testing of a compact heat exchanger typical to those used in automotive systems. The mechanisms important to vortex enhancement methods are discussed, and a basis for selecting a delta-wing design as a vortex generator is established. The heat transfer and pressure drop performance are assessed at full scale under both dry- and wet-surface conditions for a louvered-fin baseline and for a vortex-enhanced louvered-fin heat exchanger. An average heat transfer increase over the baseline case of 21% for dry conditions and 23.4% for wet conditions was achieved with a pressure drop penalty smaller than 7%. Vortex generation is proven to provide an improved thermal-hydraulic performance in compact heat exchangers for automotive systems.

Heat-Transfer and Friction Characteristics for the Louver-Fin Heat Exchanger

The present work focuses on the effects of heat-transfer and friction characteristics for the brazed aluminum louver-fin heat exchanger used in a air conditioner with a relatively low air velocity. The heat-transfer and friction characteristics were experimentally studied for the different louver angles and fin pitches of the louver-fin heat exchanger, which are supposed to he the important parameters. Test conditions were varied by four louver angles, three fin pitches, and various Reynolds numbers of air, which ranged from 110 to 800. The experimental results show that the heat transfer is mainly affected by the fin pitch rather than the louver angle, whereas the pressure drop is affected by the louver angle rather than fin pitch. The performance evaluation criterion of heat exchangers is adopted so that the major operational variables, such as heat-transfer coefficient and pumping power, can be considered simultaneously. The f/f 1/3 ratios decrease as the louver angle increases. The optimal value of the j/f 1/3 ratio exists around 20-25 deg of the louver angle. For relatively high louver angles such as 30 and 35 deg, the j/f 1/3 ratios have almost the same value under the condition that the Lp/Fp = 1.03, 1.20, and 1.43.

Air-side performance of brazed aluminum heat exchangers under dehumidifying conditions

An experimental study for air-side thermal-hydraulic performance of brazed aluminum heat exchangers under dehumidifying conditions has been performed. For 30 samples of louvered fin heat exchangers with different geometrical parameters, the heat transfer and pressure drop characteristics for wet surface were evaluated. The test was conducted for air-side Reynolds number in the range of 80–300 and tube-side water flow rate of 320 kg/h. The dry- and wet-bulb temperatures of the inlet air for heat exchangers were 27 and 19 °C, respectively and the inlet water temperature was 6 °C. The air-side thermal performance data for cooling and dehumidifying conditions were analyzed using effectiveness- NTU method for cross-flow heat exchanger with both fluids unmixed. The test results are reported, compared with those for the dry surface heat exchangers, in terms of sensible j factor and friction factor f, as functions of Reynolds number based on louver pitch. The correlations for j and f factors are developed within rms errors of ±16.9 and ±13.6%, respectively.

Heat transfer and friction characteristics of fin-and-tube heat exchangers

An experimental study has been carried out to investigate the heat transfer and pressure drop characteristics of fin-and-tube exchangers with plate, wavy and louvered fin surfaces. In all, 36 samples of heat exchangers, including 12 plate-fin, 12 wavy-fin and 12 louvered-fin geometries, were tested. Results are presented as plots of friction factor f and Colburn j factor against Reynolds number in the range 300–2000. Additionally, the dimensional heat transfer coefficient and pressure drop are also presented against frontal air velocity. Finally, various comparison methods were adopted to evaluate the air side performance of the plate, wavy and louver fin heat exchangers.

Entry region of louvered fin heat exchangers

Abstract The dominant thermal resistance for most compact heat exchangers occurs on the gas side and as such an understanding of the gas side flowfield is needed before improving current designs. Louvered fins are commonly used in many compact heat exchangers to increase the surface area and initiate new boundary layer growth. For this study, detailed flowfield measurements were made in the entry region of several louvered fin geometries whereby the louver angle, ratio of fin pitch to louver pitch, and Reynolds number were all varied. In addition to mean velocity measurements, time-resolved velocity measurements were made to quantify unsteady effects. The results indicated larger fin pitches resulted in lower average flow angles in the louver passages and longer development lengths. Larger louver angles with a constant ratio of fin pitch to louver pitch resulted in higher average flow angles and shorter development lengths. As the Reynolds number increased, longer development lengths were required and higher average flow angles occurred as compared with a lower Reynolds number case. Time-resolved velocity measurements indicated some flow periodicity behind the fully developed louver for a range of Reynolds numbers. The Strouhal number of these fluctuations was constant for a given louver geometry, but the value increased with increasing fin pitch.