Air-side Heat Transfer Research Articles

An Experimental Study on the Air-Side Heat Transfer Coefficient and the Thermal Contact Conductance in Finned Tubes

The objective of this experimental study is to evaluate the heat transfer coefficient outside a tube with annular transverse fins, derived from strips of copper mechanically bound and coupled outside. Water is used as the heating medium, in turbulent conditions and flowing at different temperatures inside the tube. Petukhov's correlation has been selected to calculate the water heat transfer coefficient in the tube. The experimental data obtained are compared with a correlation from literature, and a similar trend is observed. A fitting of the data provides a correlation for the three tubes of different external diameter (30 mm, 22 mm, and 15.6 mm) that agrees very well with the experimental values. The thermal contact conductance is identified as the main reason for the difference between data and the original Briggs and Young correlation. An estimation of the contact conductance between fins and tubes provides values between 3500 and 11000 W/m2-K, slightly increasing with the air Reynolds number (based on the external diameter of the tube), whose range is 2000 to 8000. The thermal contact resistance is estimated and its importance is confirmed, contributing 30 to 50% to the total air-side thermal resistance in the tubes used in the experiments.

Computational Fluid Dynamics Investigation of Air Cooled Heat Exchangers

Cooling of process streams is a standard operation in many industries, and where possible water cooling is the most cost effective solution. However in areas where water supply is limited Air Cooled Heat Exchangers (ACHE’s) are often the only alternative. The overall coefficient maybe limited by the air side heat transfer. To improve this coefficient the air flow should be evenly distributed and at as high velocity as possible over the whole bundle. The limits are set by the fan power consumption and the generated noise level. The required fan power forms a significant part of running costs. In order to keep these costs as low as possible knowledge concerning the flow distributions within the bundle can be important.Fluid bypass within the tube-side can be of similar importance. In design calculations an equal fluid flow velocity per tube is typically assumed from the total inlet mass flow. However if some of the fluid bypasses the tube bundle due to poor header design lower than expected fluid velocities in the tubes will be found, leading to underperforming ACHE’s, since heat transfer and fouling behaviour are influenced by the fluid velocity.Computational Fluid Dynamics (CFD) has been found to be a powerful tool to investigate how fluid flows through a defined geometry. In this paper CFD techniques will be used to investigate the tube side and air side flow of ACHE’s. The main focus will be on finding areas of fluid maldistribution within the air side and the effect of bypass on the tube-side of ACHE’s.As a benchmark the pressure drop results of CFD simulations were compared to correlations available in the public domain and agreed well, giving confidence in the fluid flow patterns produced by the CFD simulations.The effect of the tube-side bypass was found to reduce the duty of the ACHE by 27 % in the worst case scenario that was tested. Maldistribution of the air-side flow was found in the ACHE, with the number of fans and depth of the plenum influencing the amount seen.

Numerical simulation of heat transfer and fluid flow characteristics of composite fin

The composite fin is presented based on the advantage of longitudinal vortex generator and slit fin, respectively. The performance of air-side heat transfer and fluid flow is investigated by numerical simulation for Reynolds number ranging from Re=304 to 2130. Stepwise approximation method is applied on the mesh generation for the irregular domains of delta winglets and slit fins. The mechanism for augmenting heat transfer is also analyzed based on the local fluid field, field synergy principle and entransy dissipation principle. The computational results show that some eddies are developed behind the X-shaped slit and delta winglet, which produce some disruptions to fluid flow and enhance heat transfer; compared with plain fin and slit fin, it shows the composite fin has better heat transfer performance. By applying on the field synergy principle and entransy dissipation principle to analyze the composite fin, the computational results show that composite fin can improve the synergy of temperature gradient and velocity fields, and its equivalent thermal resistance is smaller and its irreversibility of heat transfer is lower.

Heat transfer enhancement of wavy finned flat tube by punched longitudinal vortex generators

Punched longitudinal vortex generators (LVGs) were employed to enhance air-side heat transfer on the wavy fin surface of flat tube used in direct air-cooled condenser. The heat transfer enhancement of four types of the longitudinal vortex generators with different attack angles were compared by numerical simulations. It was found that the delta winglet pair with attack angle 25° could reach the greatest performance evaluation criteria (PEC) under the conditions of the inlet air flow velocity varied from 1m/s to 5m/s. The influences of locations on the wavy fin surface and the row number of the longitudinal vortex generators were also discussed. One delta winglet pairs at the middle of the wavy fin surface and the minimum row number, n=1, with the average PEC is 1.23, has the best heat transfer performance of all conditions, which can be recommended for practical applications. Experimental study in wind tunnel with flow field visualization by Particle Image Velocimetry (PIV), as well as the numerical simulations verified that the delta winglet pairs can generate obvious longitudinal vortex pairs at the down-sweep zone, which can enhance the heat transfer between the cooling air flow and heated wall surface with acceptable pressure loss.

Heat transfer enhancement and pressure drop for fin-and-tube compact heat exchangers with wavy rectangular winglet-type vortex generators

In the current work, heat transfer enhancement and pressure loss penalty for fin-and-tube compact heat exchangers with the wavy-up and wavy-down rectangular winglets as special forms of winglet are numerically investigated in a relatively low Reynolds number flow. The rectangular winglets were used with a particular wavy form for the purpose of enhancement of air side heat transfer performance of fin-and-tube compact heat exchangers. The effect of Reynolds numbers from 400 to 800 and angle of attack of 30° of wavy rectangular winglets are also examined. The effects of using the wavy rectangular winglet, conventional rectangular winglet configuration and without winglet as baseline configuration, on the heat transfer characteristics and flow structure are studied and analyzed in detail for the inline tube arrangements. The results showed that the wavy rectangular winglet can significantly improve the heat transfer performance of the fin-and-tube compact heat exchangers with a moderate pressure loss penalty. In addition, the numerical results have shown that the wavy winglet cases have significant effect on the heat transfer performance and also, this augmentation is more important for the case of the wavy-up rectangular winglet configuration.

Air-Side Heat Transfer Performance of Louver Fin and Multitube Heat Exchanger for Direct Methanol Fuel Cell Cooling Application

The present work is performed to evaluate the heat transfer performance of a heat exchanger used in a direct methanol fuel cell. Because of material constraints and performance requirements, a louver fin heat exchanger is modified for use with conventional microchannel tubes and also with multiple small-diameter tubes (called multitubes). Prototype heat exchangers are tested, and the air-side heat transfer, pressure drop, and fan power are measured in a wind tunnel and simulated using a commercial code. The air-side pressure drop and heat transfer coefficient of the multitubes show similar trends to those of the flat-tube heat exchanger if the contact resistance is negligible. The tube spacing of the prototype multitube heat exchangers has a small effect on the pressure drop and heat transfer, but it has a profound effect on the air-side heat transfer performance because of the contact resistance between the tubes and louver fins. The air-side pressure drop agrees well with an empirical correlation for flat tubes.

Flat tube heat exchangers – Direct and indirect noise levels in heat pump applications

In the outdoor unit of an air-source heat pump the fan is a major noise source. The noise level from the fan is dependent on its state of operation: high air-flow and high pressure drop often result in higher noise levels. In addition, an evaporator that obstructs an air flow is a noise source in itself, something that may contribute to the total noise level. To be able to reduce the noise level, heat exchanger designs other than the common finned round tubes were investigated in this study. Three types of heat exchanger were evaluated to detect differences in noise level and air-side heat transfer performance at varying air flow. The measured sound power level from all the heat exchangers was low in comparison to the fan sound power level (direct effect). However, the heat exchanger design was shown to have an important influence on the sound power level from the fan (indirect effect). One of the heat exchangers with flat tubes was found to have the lowest sound power level, both direct and indirect, and also the highest heat transfer rate. This type of flat tube heat exchanger has the potential to reduce the overall noise level of a heat pump while maintaining heat transfer efficiency.

Numerical Study on the Effect of Tube Rows on the Heat Transfer Characteristic of Dimpled Fin

The dimpled fin has excellent heat transfer performance and has attracted a lot of attention to apply on the fin and tube heat exchanger. A study presents to investigate the effects of number of tube rows on the air-side heat transfer characteristics of dimpled fin for velocity ranging from 1 to 3 m/s. The Q/Δ P and Q/((Δ P × V)) are used to evaluate the heat transfer performance of the heat exchanger. The results show that the dimpled arrangement can change the mainstream direction, increase the disturbance, and enhance the heat transfer. With the increase of the number of tube rows, the average Nusselt number decreases and Q/Δ P and Q/((Δ P × V)) increase gradually. Compared with the multipipe tube rows, the performance of two-row tube is better.

Simulation of an Aluminum Foam Heat Exchanger Using the Volume Averaging Technique

Open cell metal foams show a high potential for heat transfer applications. In this paper the thermal and hydraulic performance of a metal foam heat exchanger with two tube rows in a staggered layout is studied. Two-dimensional simulations are performed using a bulk model based on the volume averaging technique. A louvered fin heat exchanger with the same tube layout and overall volume is considered as reference case. Velocities range from 1.2 m/s to 3.2 m/s. An experimental validation illustrates the quality of the simulation results. For the same fan power, the 10 ppi and 20 ppi foams show a smaller heat transfer than the louvered fin heat exchanger. This is mainly due to the larger airside heat transfer surface area of the louvered fin heat exchanger (because of the small fin pitch) compared to the foams. 30 and 35 ppi foams, having a specific surface area above 1000 m2/m3, show a better thermal hydraulic performance than the louvered fin design.

Determination of air side heat transfer coefficient in a mini-channel heat exchanger using Wilson Plot method

In this study, the air side heat transfer coefficient of an aluminium mini-channel heat exchanger was investigated for single-phase flow in the mini-channel, with water in the tubes and air on the outside. Research methods included hydraulic tests on a single mini-channel tube, Wilson Plot experiments and experiment validation. Results obtained from the hydraulic test showed that turbulent flow occurred in the tube at a Reynolds number of 830. Wilson Plot experiments were conducted to determine air side heat transfer coefficient of the heat exchanger. The tube side Reynolds number was maintained above 1000 to ensure turbulent flow and tube side heat transfer coefficient was calculated using Gnielinski equation for turbulent flow. The air side heat transfer coefficients obtained from the Wilson Plot experiments were in good agreement with known correlations. The outcome of this study is to use the air side heat transfer coefficient to calculate the performance of refrigerant condensers for different tube pass ratios and flow pass configurations.

Heat transfer and pressure performance of a plain fin with radiantly arranged winglets around each tube in fin-and-tube heat transfer surface

A radiantly arranged LVGs enhanced fin-and-tube structure is numerically investigated in this paper to enhance air side heat transfer. The arrangement of LVGs is totally different from existing publications. In the proposed structure there exist 12 winglets around each tube. The attack angles are 50, 50, 50, 50, 70 and 110°, respectively. The height ends of the winglets are further away from the tube, while the closed point ends of winglets are close to the tube wall. Heat transfer and pressure drop performance is compared with other three structures: an arc-shaped wavy fin-and-tube surface, a common-flow-down LVGs enhanced fin-and-tube surface and a plain plate fin-and-tube surface. The simulation results show that the 12 winglets form five local passages which can guide the moving fluid from the main flow to the tube wall, leading to some impinging effect or reducing the wake region behind the tube. The performance evaluation of the four structures is conducted by using the newly proposed ln (Nue/Nuo) vs. ln (fe/fo) plot based on energy saving. It is found that the proposed radiantly arranged LVGs enhanced fin-and-tube surface is the best. The field synergy principle is adopted to analyze the four structures and it is found that the domain averaged synergy angle of the proposed radiantly arranged LVGs enhanced structure is significantly less than that of other three cases. Finally characteristics of the proposed fin-and-tube surface with five tubes are investigated at five fin pitches and compared with the wavy structure of six tubes at the same other conditions. It is found that the proposed structure of five tubes can replace the wavy structure of six tubes.

Effect of Tube Location Change on Heat Transfer Characteristics of Plain Plate Fin-and-Tube Heat Exchangers

Three-dimensional numerical simulations are conducted for the air-side steady laminar flow and heat transfer characteristics of the plate fin-and-tube heat exchanger element. The effects of Reynolds number (Re), tube center location (Lc/D), and fin pitch (H/D) are examined. The Re based on the tube diameter and the air inlet velocity varies from 100 to 5000, the tube center location from 0.75 to 2, and the fin pitch from 0.2 to 0.5. The numerical results show that the increase of Re leads to the increase of heat transfer parameters (such as the span-averaged Nu¯(x), the overall average Nusselt number (Nu¯f, Nu¯f,t) of the fin as well as fin-and-tube, the fin surface average heat bulk q¯) and the averaged pressure drop on cross-sectional P¯(x), and the decrease of the friction factor (f) and the fin efficiency (ηf); moreover, the larger Re is, the smaller the influence on these parameters except the P¯(x). Regarding the effect of tube location (Lc/D), the f is almost insensitive to the Lc/D change, and the overall average Nusselt numbers of the fin as well as fin-and-tube increase with increasing Lc/D, but the q¯ and the ηf first gradually increase and then decrease. And so consequently, there exist the optimum Lc/D values corresponding to the largest averaged q¯ on the fin surface and the largest ηf, respectively. The investigation ascertains that the region achieving the highest average Nu¯(x) is not always holding the largest q¯(x) and/or the largest ηf for a conjugate heat transfer problem. The H/D has a positive impact on the ηf, namely the larger the H/D, the higher the ηf.

Experimental Investigation of Thermal and Hydraulic Performance of V-Shape Corrugated Carbon Foam

In air-cooled heat exchangers, air-side thermal resistance is usually the largest compared to conduction and liquid-side thermal resistances. Thus, reducing the air-side thermal resistance can greatly improve overall cooling performance. The performance of an air-cooled heat exchanger is usually characterized by the rate of heat which can be transferred and the pumping power required to convect the heat away. This paper presents a method of utilizing V-shape corrugated carbon foam to improve thermal performance. The air-side heat transfer coefficient and the pressure drop across the foam have been investigated using different V-shape foam geometrical configurations obtained by varying its length and height. Based on design considerations and availability, the foam length has been chosen to be 25.4, 38.1, and 52.1 mm, while its height is 4.4, 6.8, and 11.7 mm, resulting in nine different test pieces of foam with different heights and lengths. A total number of 81 experiments were carried out with different air face velocities (0.7-9m/s) and heat fluxes at the heater surface (0.5-2W/cm2). The pressure drop across the V-shape corrugated carbon foam as well as inlet air, exit air, foam, and ambient temperatures were measured. Of the nine V-shape configurations, the foam with the shortest length and tallest height gives the best performance. The present results are also compared with the results of prior work using different carbon foam geometries. It is shown that V-shape corrugated carbon foam provides better heat transfer coefficient and the overall performance.

Air-Side Heat Transfer And Friction Characteristics Of Fin-and-Tube Heat Exchangers With Various Fin Types

Tube bank fi n heat exchangers have widespread applications, they are transferring enormous amounts of energy, and also consuming very large amount of mechanical energy. Because the dominant thermal resistance in such heat exchangers is usually on the air side in practical applications, and therefore the use of the fi n surfaces on the air side is very common to effectively improve their overall thermal performance. Energy costs and environmental considerations continue to motivate attempts to derive better performance over the existing designs. As a result, during the past few years, there are also many investigations to the slit fi n pattern and to the longitudinal vortex generator fi ns. Because it involves a huge cost and a strenuous work to compare the heat transfer performances of these fi n patterns, up to now there are no enough data to summarize the comparisons thoroughly. However, there are a few reported results now available to make a brief summary on the airside performances of the various fi n patterns with aims to provide information for some readers to utilize the available data, to fithe work need to be done further, and fi nd out new fi n pattern having good heat transfer performance. Therefore, in this chapter the heat transfer performances of fifi n patterns of circular tube bank fi n heat exchangers are compared. In addition, the heat transfer performances of three fi n patterns of fl at tube bank fi n heat exchangers are also compared. Three sets of criteria, namely, the identical mass fl ow rate, pressure drop, and the identical pumping power, are used in judging. The results show that for the circular tube bank fi n heat exchangers, with the Reynolds number ranging from 4,000 to 10,000,

Thermodynamic model of a thermal storage air conditioning system with dynamic behavior

A thermodynamic model was developed to predict transient behavior of a thermal storage system, using phase change materials (PCMs), for a novel electric vehicle climate conditioning application. The main objectives of the paper are to consider the system’s dynamic behavior, such as a dynamic air flow rate into the vehicle’s cabin, and to characterize the transient heat transfer process between the thermal storage unit and the vehicle’s cabin, while still maintaining accurate solution to the complex phase change heat transfer. The system studied consists of a heat transfer fluid circulating between either of the on-board hot and cold thermal storage units, which we refer to as thermal batteries, and a liquid–air heat exchanger that provides heat exchange with the incoming air to the vehicle cabin. Each thermal battery is a shell-and-tube configuration where a heat transfer fluid flows through parallel tubes, which are surrounded by PCM within a larger shell. The system model incorporates computationally inexpensive semi-analytic solution to the conjugated laminar forced convection and phase change problem within the battery and accounts for airside heat exchange using the Number of Transfer Units (NTUs) method for the liquid–air heat exchanger. Using this approach, we are able to obtain an accurate solution to the complex heat transfer problem within the battery while also incorporating the impact of the airside heat transfer on the overall system performance. The implemented model was benchmarked against a numerical study for a melting process and against full system experimental data for solidification using paraffin wax as the PCM. Through modeling, we demonstrate the importance of capturing the airside heat exchange impact on system performance, and we investigate system response to dynamic operating conditions, e.g., air recirculation.

Calculation of Heat Transfer Coefficient of Wavy Fin-and-Tube Heat Exchanger of Air Source-Heat Pump

Air side heat transfer coefficient is a key problem to study thermal regime of air-cooled units. Five commonly used equations for calculating the heat transfer coefficient of wavy finand tube heat exchanger of air source heat pump and scope of application were analyzed. According to the typical example, all the solutions of equation are calculated for the five methods. Comparing between theoretical data and experimental data, the experimental results are basically consistent with the theoretical solution J factor method and McQuiston method is recommended as the optimal method for calculating the heat transfer coefficient of wavy fin and tube heat exchanger. It is provided theoretical basis on optimal design and thermal characteristics.

Performance factors for ground-air thermoelectric power generators

The daily variation in air temperature is large compared with the temperature changes a short distance below the surface of the ground. In theory, a heat engine can be arranged to produce electricity from this temperature difference. In practice, the thermal efficiency of such a device will be low because of the small temperature differences involved. One example of such an energy harvesting device that can produce a small amount of electrical power uses a thermoelectric generator operating between the air and ground temperatures. The low thermal efficiency means that accurately predicting thermal resistances throughout the device and at the air-side and ground-side heat exchangers is critical to the creation of a useful device. Advantages of this device include high reliability, no acoustic emissions, low visibility, significant night-time power production, ruggedness, and long life. With appropriate external power conditioning components, the device could be used to power remote sensors and communications systems. The design of a pair of milliwatt-scale ground source heat engines is described. The devices were fabricated using custom heat exchangers and off-the-shelf thermoelectric modules and other supplies. Both systems were tested over an extended period in order to quantitatively assess effects of sunlight and precipitation on system performance and life. Exhaustive analysis of air-side average heat transfer coefficients, system thermal resistances, and ground-side thermal resistances provides quantitative design information for future applications. Finned and unfinned versions of otherwise identical prototypes permits assessment of fin performance on both ground-side and air-side heat transfer.

Numerical simulation of flow and heat transfer characteristics in wavy fin-and-tube heat exchanger with combined longitudinal vortex generators

The air-side heat transfer and fluid flow characteristics of wavy fin-and-tube heat exchanger punched with combined rectangular winglet pairs (CRWPs) were numerically analyzed. The influences of several main parameters of the CRWP on heat transfer and friction were examined and compared to rectangular winglet pairs (RWP). The numerical results showed that the wavy fin-and-tube heat exchanger has a better heat transfer performance when the winglets punched on the suction side of the wavy fin. For the wavy fin punched with CRWPs, the Nusselt number and friction factor increase with increases of secondary attack angle, accessory winglet length or accessory winglet width. In comparison with RWP, the vortices generated by CRWP are more, larger and stronger. Hence, CRWP can more effectively improve heat transfer and increase friction.

Effect of fin pitches on the air-side performance of L-footed spiral fin-and-tube heat exchangers

The purpose of this experimental investigation is to investigate the effects of fin pitch (i.e., fp of 2.4, 3.2, and 4.2mm) on the air-side heat transfer performance and frictional characteristics of L-footed spiral fin-and-tube heat exchangers at high Reynolds numbers (Redc) of 4000–15,000. A determinant of the parallel cross-flow and the counter cross-flow is the flow arrangement of the test heat exchangers. Ambient air and hot water are used as a working fluid on the air- and the tube-side, respectively. The results indicate that the air-side heat transfer coefficient and Colburn factor are independent of fin pitch. However, fin pitch does have an influential effect on the average heat transfer rate, pressure drop, and friction factor. In terms of industrial applications, the correlation of the Colburn factor and friction factor are proposed for practical applications.

Effect of Attack Angles on Air Side Thermal and Pressure Drop of the Cross Flow Heat Exchangers with Staggered Tube Arrangement

The experimental investigation of the air side heat transfer and pressure drop characteristics at steady state for the cross flow heat exchangers having flat tube configuration with staggered arrangement are presented. The effect of attack angles on the air side is major to study. In addition, the effect of tubes aspect ratios was also studied. Nineteen heat exchangers with four aspect ratios (0.18, 0.39, 0.66 and 1) and six attack angles (0, 30, 60, 90, 120 and 150) are investigated. The water temperature of 75°C and the air at ambient condition flowed through the heat exchangers with velocity range from 2 to 6 m/s. The results show that the thermal and the pressure drop characteristics had governed by the attack angles and the tube aspect ratios. For all values of the air velocity of each tube aspect ratios, to increasing the attack angles (0 90) the heat transfer rate and the pressure drop were increased, while the thermal hydraulic performance was decreased. When considered effect of tubes aspect ratios on thermal and pressure drop characteristics, the results indicated that to change of the attack angles of lower tube aspect ratios had effected on thermal and pressure drop more than attack angle changed of higher tube aspect ratios.