Louver Pitch Research Articles

Numerical optimization of louvered fin heat exchanger with variable louver angles

Several studies of the louvered fin heat exchanger have already been done. Both experimental and numerical studies are available. Investigations to the optimal louver angle have been performed, many times in combination with other fin parameters such as louver pitch and fin thickness. Most studies assume a single louver angle for all the louvers in the heat exchanger. Hsieh and Jang [1] on the other hand studied the effect of a variable louver angle for 5 different cases with successively increasing or decreasing louver angles. Tube-fin interactions were not taken into account. In this study, a round tube and fin geometry with individually varying louver angles is analyzed. The thickness of the fin was neglected. Any interactions between the optimal louver angles and the fin thickness are hence not captured. A laminar and steady calculation was performed, with symmetric boundary conditions. For the Reynolds number on the hydraulic diameter (ReDh) of 535 that was studied, a Von Karman vortex street is present behind the last tube row of heat exchanger. The steady calculation is hence only an approximation of the reality, but is shown to give reasonable results. An ordinary kriging response surface model was used to explore the entire parameter space. Updates to the model were made on the basis of improving the Pareto front, visualizing the tradeoff between heat transfer and pressure drop. It is shown that the use of individually varying louver angles allows increasing the Colburn j factor by 1.3% for the same friction factor, with respect to the optimal uniform louvered fin configuration.

Optimal Design of Compact Heat Exchanger (Louver Fin-tube Heat Exchanger for High Heat Transfer and Low Pressure Drop)

The present work was conducted to get the best geometric information for the optimum design of the complex heat exchanger. The objective function for optimal design was expressed as a combination of pressure drop and heat transfer rate. The geometric parameters for the variables of louver pitch and height, tube width, etc., were limited to ranges set by manufacturing conditions. The optimum geometric parameters were calculated by using empirical correlations and theory. The sensitivity of the parameters and optimum values are shown and discussed. The weighting factor in the objective function is important in the selection of the louver fin-tube heat exchanger.

Heat Transfer and Flow Resistance Characteristics of Louver Fin Geometry for Automobile Applications

The present work was conducted to investigate the air-side pressure drop and heat transfer performance of the louver fin-tube heat exchanger for automobile applications. Fourteen kinds of louver fin geometries with different louver pitches and angles were tested in the present work. The f and j factors for plane and louver fin configurations were compared experimentally and numerically. The heat transfer and pressure drop characteristics of the plane fin showed the combined mode of the developing flow on a flat plate and the fully developed flow in the rectangular channel. The heat transfer coefficient of the louver fin was about twice as high as that of the plane fin. Empirical correlations proposed by previous researchers were compared with the present experimental data. Correlations of j and f factors were proposed for the present experimental data. The j and f factors were simply expressed as functions of the average louver pitch, fin pitch, louver angle, and Reynolds number based on the louver pitch. The present correlations of the heat transfer and pressure drop agreed well with the experimental data.

Experimental investigation of geometry effects on the performance of a compact louvered heat exchanger

Abstract In this study, air-side heat transfer and pressure drop characteristics of flow over louvered fins in compact heat exchangers have been investigated experimentally. The test samples consist of two types of fin configurations. A series of tests were conducted to examine the geometrical parameters of louver pitch, louver arrangement and number of louver regions. The calculated results indicate that a symmetrical arrangement of louvered fins provides a 9.3% increase in heat transfer performance and a 18.2% decrease in pressure drop than the asymmetrical arrangement of louvered fin. Also, for a constant rate of heat transfer and pressure drop, a 17.6% decrease of fin weight is observed for the symmetrical arrangement of fins and this is following by considerable decrease in total weight and cost of the heat exchanger. The results from this investigation indicate that the configuration of the louvered fins has the dominant influence on the heat transfer and pressure drop from that louver.

Heat transfer and pressure drop correlations for compact heat exchangers with multi-region louver fins

An experimental study on the air-side heat transfer and pressure drop characteristics for brazed aluminum heat exchangers with multi-region louver fins and flat tubes was conducted. A series of tests were conducted for heat exchangers with different numbers of louver regions at the air-side Reynolds numbers of 400–1600 based on the louver pitch. The air-side thermal performance data were analyzed by using the effectiveness-NTU method. The characteristics of the heat transfer and pressure drop for heat exchangers with different geometry parameters were presented in terms of the Colburn j factor and Fanning friction f factor as function of the Reynolds number.

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 some geometric parameters on performance of PF 2 heat exchangers in periodic frosting

The thermal performance under conditions of initial frost growth and subsequent refrosting after a defrost is experimentally studied for the parallel flow parallel fin (PF 2) heat exchanger, a new style of heat exchanger that uses louvered bent fins on flat tubes to enhance water drainage when the flat tubes are horizontal (typically outdoor heat exchanger in heat pump operation). This paper focuses on quantification of the effects of geometry (i.e. fin pitch 12–22 fpi and louver pitch 1.4–2.8 mm) on defrost and refrost times. Eight heat exchangers differing in louver pitch and fin spacing are studied. A series of tests are conducted in search for the best geometry. The effects of geometry on heat transfer (thermal performance) and pressure drop for air face velocities of 0.9, 2, and 3 m/s are determined and used for comparison. Characteristics of initial heat transfer coefficient and pressure drop during the first frosting cycle are reported in terms of Colburn j 0 factor and Fanning friction f 0 factor, as a function of Re L p and geometry. The newly developed air-side correlations for PF 2 heat exchangers (Colburn j 0 factor and Fanning friction f 0 factor) predict the test data within rms error of ±10%, and with mean deviation of 2.95% and 4.98%, respectively. The correlations are base on a very low Reynolds numbers in the range of 100–620, and 8 PF 2 heat exchangers using 48 experimental data.

Air-side performance of louver-finned flat aluminum heat exchangers at a low velocity region

The heat transfer and pressure drop characteristics of heat exchangers having louver fins were experimentally investigated. The samples had small fin pitches (1.0–1.4 mm), and experiments were conducted up to a very low frontal air velocity (as low as 0.3 m/s). Below a certain Reynolds number (critical Reynolds number), the fall-off of the heat transfer coefficient curve was observed. The critical Reynolds number was insensitive to the louver angle, and decreased as the louver pitch to fin pitch ratio (Lp/Fp) decreased. Existing correlations on the critical Reynolds number did not adequately predict the data. The heat transfer coefficient curves crossed over as the Reynolds number decreased. Possible explanation is provided considering the louver pattern between neighboring rows. Different from the heat transfer coefficient, the friction factor did not show the fall-off characteristic. The reason was attributed to the form drag by louvers, which offsets the decreased skin friction at low Reynolds numbers. The friction factor increased as the fin pitch decreased and the louver angle increased. A new correlation predicted 92% of the heat transfer coefficient and 94% of the friction factor within ±10%.

Heat transfer and pressure drop correlations for the multi-louvered fin compact heat exchangers

Abstract Experimental studies on the air side heat transfer and pressure drop characteristics for 20 types of multi-louvered fin and flat tube heat exchangers were performed. A series of tests were conducted for air Reynolds numbers of 200–2500 based on the louver pitch with different fin pitch, fin height, fin thickness, fin louver angle and flow length at a constant tube side flow rate of 2.8 m3/h.The air side thermal performance data were analyzed using the effectiveness-NTU method. Characteristics of the heat transfer and pressure drop for the different geometry parameters were reported in terms of Colburn j-factor and Fanning friction f-factor as a function of Re L p . The general correlations for the j and f factors were derived by regression analysis and F test of significance. The correlations of the j and f factors predict the test data within a rms error of ±10% and ±12%, and the mean deviations are 4.1% and 5.6%, respectively. In addition, two comparison methods were adopted to evaluate the air side thermal hydraulic performance of the multi-louvered fins.

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%.

Numerical investigation of fluid flow and heat transfer over louvered fins in compact heat exchanger

Numerical investigation of fluid flow and heat transfer characteristics over louvered fins and flat tube in compact heat exchangers is presented in this study. Three-dimensional simulations of single and double row tubes with louvered fins have been conducted. Simulations are performed for different geometries with varying louver pitch, louver angle, fin pitch and tube pitch and for different Reynolds number. Conjugate heat transfer and conduction through the fins are considered. The air-side performance of heat exchanger is evaluated by calculating Stanton number and friction factor. The results are compared with experiment and a good agreement is observed. The local Nusselt number variation along the top surface of the louver is calculated and effects of geometrical parameters on the average heat transfer coefficient is computed. Design curves are obtained which can used to predict the heat transfer and the pressure drop for a given louver geometry.

Fin–tube junction effects on flow and heat transfer in flat tube multilouvered heat exchangers

Three-dimensional simulations of four louver–tube junction geometries are performed to investigate the effect on louver and tube friction and heat transfer characteristics. Three Reynolds numbers, 300, 600 and 1100, based on bulk velocity and louver pitch are calculated. Strong three-dimensionality exists in the flow structure in the region where the angled louver transitions to a flat landing adjoining the tube surface, whereas the flow on the angled louver far from the tube surface is nominally two-dimensional. Due to the small spatial extent of the transition region, its overall impact on louver heat transfer is limited, but the strong unsteady flow acceleration on the top louver surface augments the heat transfer coefficient on the tube surface by over 100%. In spite of the augmentation, the presence of the tube lowers the overall Nusselt number of the heat exchanger between 25% and 30%. Comparisons with correlations derived from experiments on full heat exchanger cores show that computational modeling of a small subsystem can be used reliably to extract performance data for the full heat exchanger.

저속 영역에서 루버휜이 장착된 평판관형 알루미늄 열교환기의 공기측 전열 성능에 대한 실험적 연구

The heat transfer and pressure drop characteristics of heat exchangers with louver fins were experimentally investigated. The samples had small fin pitches (1.0 mm to 1.4 mm), and experiments were conducted up to a very low frontal air velocity (as low as 0.3 m/s). At a certain Reynolds number (critical Reynolds number), the flattening of the heat transfer coefficient curve was observed. The critical Reynolds number was insensitive to the louver angle, and decreased as the louver pitch to fin pitch ratio (L_p/F_p) decreased. Existing correlations on the critical Reynolds number did not adequately predict the data. It is suggested that, for proper assessment of the heat transfer behavior, the louver pattern in addition to the flow characterization need to be considered. The heat transfer coefficient increased as the fin pitch decreased. At low Reynolds numbers, however, the trend was reversed. Possible explanation is provided considering the louver pattern between neighboring fins. Different from the heat transfer coefficient, the friction factor did not show the flattening characteristic. The reason may be attributed to the form drag by louvers, which offsets the decreased skin friction at a low Reynolds number. The friction factor increased as the fin pitch decreased and the louver angle increased. A new correlation predicted 92 % of the heat transfer coefficient an d 90 % of the friction factor within ± 10%.

Air-side thermal hydraulic performance of multi-louvered fin aluminum heat exchangers

An experimental study on the air-side heat transfer and pressure drop characteristics for multi-louvered fin and flat tube heat exchangers has been performed. For 45 heat exchangers with different louver angles (15–29°), fin pitches (1.0, 1.2, 1.4 mm) and flow depths (16, 20, 24 mm), a series of tests were conducted for the air-side Reynolds numbers of 100–600, at a constant tube-side water flow rate of 0.32 m3/h. The inlet temperatures of the air and water for heat exchangers were 21 and 45°C, respectively. The air-side thermal performance data were analyzed using effectiveness-NTU method for cross-flow heat exchanger with both fluid unmixed conditions. The heat transfer coefficient and pressure drop data for heat exchangers with different geometrical configurations were reported in terms of Colburn j-factor and Fanning friction factor f, as functions of Reynolds number based on louver pitch. The general correlations for j and f factors are developed and compared to other correlations. The f correlation indicates that the flow depth is one of the important parameters for the pressure drop.

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.

A numerical investigation of louvered fin-and-tube heat exchangers having circular and oval tube configurations

A numerical investigation of the airside performance for fin-and-tube heat exchanger having circular and oval configuration is presented in this study. The geometrical parameters of louver angle, louver pitch, and louver length are examined. For a fixed louver length (6.25 mm) and louver angle (14°), a 10% decrease of heat transfer performance is observed and a 41% decrease of pressure drop is seen for oval tube configuration. The calculated results indicate that the pressure drops always with increase in louver angle. However, an optimal value of heat transfer performance is observed due to the difference in the development of thermal boundary layer on the upper and lower louver surface. Both the heat transfer and friction performance increase with louver length. However, the rate of the increase of heat transfer performance is about the same as the increase of pressure drops.

Geometry effects on flow transition in multilouvered fins – onset, propagation, and characteristic frequencies

The effect of fin pitch, louver angle and thickness, and flow depth on the onset, propagation, and characteristic frequencies of instabilities in multilouvered fins is investigated. The initial instability appears in the wake of the exit louver. Subsequently, instabilities are established in the interior of the louver bank which spread upstream as the Reynolds number increases. It is shown that the interior instabilities are completely independent of the exit wake instability. On the other hand, the exit wake instability is not only dependent on the exit louver geometry but also on the internal geometry of the louver bank. Increasing louver thickness and angle increases the propensity of the flow to become unstable. Increasing fin pitch, louver angle, and thickness enhances the propagation of instabilities into the louver bank. It is shown that the characteristic frequencies in the interior of the louver bank, scale with the fin pitch rather than the louver thickness or louver pitch. The exit wake frequencies, on the other hand scale with the projected length of the exit louver in the flow direction. A simplified model for estimating the onset of instabilities in the interior of the louver bank is proposed.

Flow transition in a multilouvered fin array

The paper describes the detailed transition mechanism from steady to unsteady flow in a multilouvered fin geometry. The initial instability appears in the wake of the exit louver at a Reynolds number of 400 with a characteristic non-dimensional frequency (based on inflow velocity and louver pitch) of 0.84. Between a Reynolds number of 700 and 800, power spectra in the interior of the array indicate an increase in energy in the vicinity of the first harmonic of the initial exit wake instability. By a Reynolds number of 900, free shear layer or Kelvin–Helmholtz type instabilities develop on the leading edge shear layers of louvers near the exit. These instabilities have a characteristic non-dimensional frequency of 1.7. As the Reynolds number increases further, instabilities move upstream into the array. By a Reynolds number of 1300, most of the louvers exhibit unsteadiness, except for the entrance louver and the first two louvers following it. By this Reynolds number, the flow in the downstream half of the array exhibits a chaotic behavior.

Heat transfer and friction correlation for compact louvered fin-and-tube heat exchangers

General heat transfer and friction correlations for louver fin geometry having round tube configuration were proposed in the present study. A total of 49 samples of louvered fin-and-tube heat exchangers with different geometrical parameters, including louver pitch, louver height, longitudinal tube pitch, transverse tube pitch, tube diameter, and fin pitch were used to develop the correlations. The proposed correlation describes 95.5% of the Coburn j and 90.8% of the friction factors within ±15%.

A generalized heat transfer correlation for Iouver fin geometry

A generalized heat transfer correlation for louver fin geometry is developed with the aid of a large data bank. This data bank consists of 91 samples of louvered fin heat exchangers with different geometrical parameters, including louver angle, tube width, louver length, louver pitch, fin length and fin pitch. For the corrugated louver fin geometry, it is shown that 89.3% of the corrugated louver fin data are correlated within ± 15% with mean deviation of 7.55%. The inclusion of the plate-and-tube louver fin data in the heat transfer correlation (equation (Al)) results in a mean deviation of 8.21%.