Air-side Heat Transfer Coefficient Research Articles

COP-based optimization of accelerated flow evaporators for household refrigeration applications

This paper reports an investigation on the impact of an alternative evaporator design, the so-called Accelerated Flow Evaporator (AFE), on the performance of household refrigerators. In this novel evaporator concept, the air-side cross-section area decreases with the distance from the air flow inlet, accelerating the air as it flows across the tubes and hence improving the air-side local heat transfer coefficient. An AFE heat transfer and pressure drop calculation method proposed elsewhere [1] has been incorporated into an overall system simulation model [2] to assess the impact of the evaporator geometry on the system COP. The results were compared with experimental data obtained in a top-mount refrigerator. The predictions of working pressures, power consumption, cooling capacity and COP agreed with the experimental data to within ±10% error bands. The model was subsequently used in an optimization exercise of the AFE geometry that considered both the system COP and the evaporator mass of material.

Effect of salt spray corrosion on air-side hydrophilicity and thermal-hydraulic performance of copper-fin heat exchangers

The effect of salt spray corrosion on the air-side hydrophilicity and the thermal-hydraulic performance of copper-fin heat exchangers were experimentally investigated. Artificial accelerated method of salt spray corrosion on the copper-fin heat exchangers was used for simulating the actual corroded heat exchangers. The experimental results show that, the contact angles increase with the increase of salt spray corrosion hours, which results in the degradation of the hydrophilicity of copper fin. The air-side heat transfer coefficients decrease and pressure drops increase with the increase of corrosion hours. The effect of salt spray corrosion on the heat transfer coefficients and pressure drops become more obvious with the increase of inlet air velocity. The heat transfer coefficients of the corroded copper-fin heat exchangers decrease by 4.4–34.0% and the pressure drop increase by 5.2–26.1% comparing with those of the uncorroded copper-fin heat exchanger at the inlet air velocity ranging from 0.5 to 2.0 m/s.

Effect of Salt Spray Corrosion on Air-Side Performance of Finned-Tube Heat Exchanger with Hydrophilic Coating Under Dehumidifying Conditions

In this study, the effect of salt spray corrosion on the air-side performance of finned-tube heat exchangers with hydrophilic coating under dehumidifying conditions was experimentally investigated. Artificial accelerated method of salt spray corrosion on the hydrophilic coated heat exchangers was used for simulating the corrosion process of the actual heat exchangers. The experimental results show that the contact angles of hydrophilic coated aluminum fins increase with the increase of salt spray corrosion hours, which results in the degradation of hydrophilicity of fins; the heat transfer is enhanced at lower inlet air velocity and degraded at high inlet air velocity for the pitting corroded heat exchanger with hydrophilic coating; compared with the uncorroded finned-tube heat exchanger with hydrophilic coating at the inlet air velocity ranging from 0.5 to 2.0 m/s (5905.5 to 23622.0 ft/h), the effects of salt spray corrosion on the air-side heat transfer coefficient and on the air-side pressure drop are approximately within the range of −20.5%~8.7% and 1.7%~13.1%, respectively.

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.

Theoretical and experimental examination of air conditioner heat exchangers

Heat and mass transfer characteristics of finned-tube heat exchangers under dehumidifying and dry conditions are examined both theoretically and experimentally in the present study. Six types of heat exchangers were tested on an experimental facility. It was determined that there is less than 1 °C of outlet air temperature difference between computer program results and literature for water and R22 as a refrigerant. The outlet air specific humidity difference between them is less than 6.44% for water and 9% for R22. According to the experimental results, louvers on fin cause an increase of airside heat transfer coefficient of about 50% or 100%.

Use of oscillating heat pipe technique as extended surface in wire-on-tube heat exchanger for heat transfer enhancement

This paper presents the performance of a wire-on-tube heat exchanger of which the wire is an oscillating heat pipe. The experiments for this heat exchanger were performed in a wind tunnel by exchanging heat between hot water flowing inside the heat exchanger tubes and air stream flowing across the external surface. R123, methanol and acetone were selected as working fluids of the oscillating heat pipe. The inlet water temperature was varied from 45 to 85 °C while the inlet air temperature was kept constant at 25 °C. The results show that the oscillating heat pipe technique for all working fluids could increase around 10% of the heat transfer rate obtained from that without the refrigerant flow in the capillary tube when the inlet water temperature is at 60 °C. Moreover, the heat transfer models for evaluating the heat exchanger effectiveness and the air-side heat transfer coefficient, are also developed in this work. These results of the models agreed very well with the experimental data.

Long-term performance of air-side heat transfer and pressure drop for finned tube evaporators of air conditioners under intermittent operation conditions

In this study, the effects of long-term intermittent operations on the air-side heat transfer and pressure drop performance of finned tube evaporators of air conditioners were investigated by experiments on an aluminum-fin evaporator and a copper-fin evaporator. In order to simulate intermittent operations of on–off controlled air conditioners, the temperatures of the two evaporators changed to 5 ± 0.5 °C at first and then to 27 ± 0.5 °C repeatedly. The repetition number was up to 4800, and the air-side heat transfer and pressure drop of the two evaporators were tested after every 300 repetitions. The test results indicate that after long-term intermittent operations, the air-side heat transfer coefficient decreases and the pressure drop increases. The variations of the heat transfer coefficient and the pressure drop are more obvious at lower inlet air velocity, and the influence of long-term intermittent operations on the aluminum-fin evaporator is greater than that on the copper-fin evaporator.

Air side heat transfer coefficients of discrete plate finned-tube heat exchangers with large fin pitch

The objective of this study is to investigate the heat transfer characteristics of discrete plate finned-tube heat exchangers with large fin pitches. Thirty-four heat exchangers were tested with variations of fin pitches, the number of tube rows, fin alignment, and vertical fin space. The j-factor of the discrete plate finned-tube exchanger was analyzed as a function of coil geometry and then compared with that of the continuous plate finned-tube heat exchanger. For fin pitches of 7.5–15 mm, the j-factors of the discrete plate finned-tube heat exchangers were 6.0–11.6% higher than those of the continuous plate finned-tube heat exchangers. Two separate correlations for the j-factor were developed for the inline and the staggered fin alignment in the discrete plate finned-tube heat exchangers to predict the measured data within a relative deviation of 2.9%.

Three-Dimensional Numerical Study of Flow and Heat Transfer Enhancement Using Vortex Generators in Fin-and-Tube Heat Exchangers

In this paper, a three-dimensional numerical investigation was performed for heat transfer characteristics and flow structure of full scale fin-and-tube heat exchangers with rectangular winglet pair (RWP). For the Reynolds number ranging from 500 to 880, the baseline configuration (without RWP) is compared with three enhanced configurations (with RWP): inline-1RWP case, inline-3RWP case, and inline-7RWP case. It was found that the air-side heat transfer coefficient improved by 28.1–43.9%, 71.3–87.6%, and 98.9–131% for the three enhanced configurations, with an associated pressure drop penalty increase of 11.3–25.1%, 54.4–72%, and 88.8–121.4%, respectively. An overall performance comparison was conducted by using the London area goodness factor. It is revealed that among the three enhanced configurations, the inline-1RWP case obtains the best overall performance, and the inline-3RWP case is better than the inline-7RWP case. The numerical results were also analyzed on the basis of the field synergy principle to provide fundamental understanding of the relation between local flow structure and heat transfer augmentation. It was confirmed that the reduction in the average intersection angle between the velocity vector and the temperature gradient was one of the essential factors influencing heat transfer enhancement. The analysis also provides guidelines for where the enhancement technique is highly needed.

Air side heat transfer characteristics of plate finned tube heat exchangers with slit fin configuration under wet conditions

An experimental study was performed on compact finned tube heat exchangers under wet conditions. Eight different finned tube heat exchangers having slit fins with hydrophilic coatings were tested. The effects of tube diameter, the number of tube rows, and inlet air relative humidity on air side heat transfer and pressure drop characteristics were investigated. Air side heat transfer coefficients were calculated using the log mean enthalpy difference method. The effects of the number of tube rows and the tube diameter on the Colburn j-factor and the f-factor were larger compared with those of the inlet air relative humidity. The Colburn j-factor and the f-factor of the single-row heat exchanger were larger than those of two- or three-row heat exchangers. The j-factor for the 5.30 mm tube diameter was compared with those for 7.35 mm and 9.95 mm tube diameters at 46% RH and was found to be 33% and 55% larger, respectively.

Effects of biofouling on air-side heat transfer and pressure drop for finned tube heat exchangers

Experimental investigations on the effects of biofouling on air-side heat transfer and pressure drop for three biofouled finned tube heat exchangers and one clean finned tube heat exchanger were performed. Artificial accelerated method of microorganism growth on the fin surface was used for simulating the biofouled finned tube heat exchangers. Experimental results indicate that the effects of biofouling on the air-side heat transfer coefficient decreases 7.2% at 2.0 m/s when the biofouled area ratio is 10%, while it decreases 15.9% at 2.0 m/s when the biofouled area ratio is 60%, and biofouling causes a 21.8% ∼ 41.3% increase in pressure drop when the air velocity is between 0.5 and 2.0 m/s. The increase of inlet air velocity is helpful to improve the long-term performance of finned tube heat exchanger. Biofouling makes the hydrophilic coating failure, and the condensation water easily converges on the fin surface where biofouling grows.

Heat transfer characteristics of a new helically coiled crimped spiral finned tube heat exchanger

In the present study, the heat transfer characteristics in dry surface conditions of a new type of heat exchanger, namely a helically coiled finned tube heat exchanger, is experimentally investigated. The test section, which is a helically coiled fined tube heat exchanger, consists of a shell and a helical coil unit. The helical coil unit consists of four concentric helically coiled tubes of different diameters. Each tube is constructed by bending straight copper tube into a helical coil. Aluminium crimped spiral fins with thickness of 0.5 mm and outer diameter of 28.25 mm are placed around the tube. The edge of fin at the inner diameter is corrugated. Ambient air is used as a working fluid in the shell side while hot water is used for the tube-side. The test runs are done at air mass flow rates ranging between 0.04 and 0.13 kg/s. The water mass flow rates are between 0.2 and 0.4 kg/s. The water temperatures are between 40 and 50°C. The effects of the inlet conditions of both working fluids flowing through the heat exchanger on the heat transfer coefficients are discussed. The air-side heat transfer coefficient presented in term of the Colburn J factor is proportional to inlet-water temperature and water mass flow rate. The heat exchanger effectiveness tends to increase with increasing water mass flow rate and also slightly increases with increasing inlet water temperature.

Performance analysis of air-cooled microchannel absorber in absorptionbased miniature electronics cooling system

Theoretical analysis and simulation of performance of an air-cooled microchannel absorber is reported in this study. It is shown that the air-cooled microchannel absorber can be integrated into an absorption-based miniature electronics cooling system by which the chip junction temperature can be maintained near room temperature, while removing 100 W of heat load. Water/LiBr pair is used as the working fluid and refrigerant vapor is intended to counter-currently flow against aqueous LiBr solution flow. Parametric study is carried out to determine the effects of several operating parameters, including inlet temperature and mass flow rate of the coolant, and inlet temperature of LiBr solution. To facilitate the air-cooling of microchannel absorber, an offset-strip-fin array is adopted, by which enhanced air-side heat transfer coefficient and large heat transfer area are obtained. The performance of the air-cooled absorber is compared to liquid-cooled absorber.

Prediction and Measurement of Apparent Heat Transfer Coefficient by Condensation in Finned-Tube Heat Exchangers

The apparent heat transfer coefficient by condensation in finned-tube heat exchangers is determined experimentally and numerically in this paper. The film method is used to predict the partial or total condensation of the water vapor contained in the humid air over the smooth or finned tube-heat recuperators. Based on this method, a computer code is developed here. The mathematical formulation is validated by our experimental results, using tube bundles in staggered and aligned arrangements. The determination of the fin portion, which functions in wet regime, is carried out by the prediction of thermal field over a circular fin. The condensation of the water vapor contained in the humid air is done preferentially with the last rows of the heat exchanger. The heat flux is predicted in a range of 20% and 5% in wet and dry regimes, respectively. The apparent heat transfer coefficient by condensation can exceed 10 times the value of the air-side heat transfer coefficient in dry regime.

Comparison of External Surface Heat Transfer Coefficients for Circular and Oval Tubes

Experimental measurements were carried out to compare the air-side heat transfer coefficients of three oval tubes (axis ratio 2, 3, and 4) with those of an equivalent circular tube (o. d. 18 mm). The tubes were tested as single tubes in a cross-flow of air. The range of the investigated Reynolds numbers Re D was between 1000 and 11000. The effects of the area blockage and the free stream turbulence were taken into consideration in the evaluation of the thermal performance. The measurement results indicate that Nusselt numbers Nu D for the tested circular and oval tubes are close at the lower range of the tested Reynolds numbers (Re D < 4000) corresponding to an air velocity < 4 ms −1 in this work, which is the air velocity for most air conditioning applications. For Re D > 4000, the Nu D for the circular tube are higher than those for the oval tubes, and the Nu D for the oval tubes decrease with the increase of the axis ratio.

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.

Effect of electric field on heat transfer performance of automobile radiator at low frontal air velocity

The effect of electric field on the performance of automobile radiator is investigated in this work. In this experiment, a louvered fin and flat tube automobile radiator was mounted in a wind tunnel and there was heat exchange between a hot water stream circulating inside the tube and a cold air stream flowing through the external surface. The electric field was supplied on the airside of the heat exchanger and its supply voltage was adjusted from 0 kV to 12 kV. From the experiment, it was found that the unit with electric field pronounced better heat transfer rate, especially at low frontal velocity of air. The correlations for predicting the air-side heat transfer coefficient of the automobile radiator, with and without electric field, at low frontal air velocity were also developed and the predicted results agreed very well with the experimental data.

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.

An analytical study on heat transfer performance of radiators with non-uniform airflow distribution

Heat exchangers used in modern automobiles usually have a highly non-uniform air velocity distribution because of the complexity of the engine compartment and underhood flow fields; hence ineffective use of the core area has been noted. To adequately predict the heat transfer performance in typical car radiators, a generalized analytical model accounting for airflow maldistribution was developed using a finite element approach and applying appropriate heat transfer equations including the ε-NTU (effectiveness - number of heat transfer units) method with the Davenport correlation for the air-side heat transfer coefficient. The analytical results were verified against a set of experimental data from nine radiators tested in a wind tunnel and were found to be within +24 and −10 per cent of the experimental results. By applying the analytical model, several severe non-uniform velocity distributions were also studied. It was found that the loss of radiator performance caused by airflow maldistribution, compared with uniform airflow of the same total flowrate, was relatively minor except under extreme circumstances where the non-uniformity factor was larger than 0.5. The relatively simple set of equations presented in this paper can be used independently in spreadsheets or in conjunction with computational fluid dynamics (CFD) analysis, enabling a full numerical prediction of aerodynamic as well as thermodynamic performance of radiators to be conducted prior to a prototype being built.

The Fin-on-Plate Heat Exchanger: A New Configuration for Air-Cooled Power Plants

The objective of this work was to study performance of a new design for air-cooled heat rejections units to be used in power plants or process industries. This heat exchanger was specially designed to provide a true counter-current flow path for the vapor (process fluid) and the air. In this new design, offset strip fins were used along the path of the air, thus providing a low pressure drop path for air while periodically breaking the thermal and hydraulic boundary layers at each fin strip. The air-side pressure drop was lower in this configuration compared to the conventional tube-and-fin heat exchangers because the drag force was significantly reduced. Lower pressure drops allowed air at higher velocities to be passed through the fins, hence achieving higher air-side heat-transfer coefficients by as much as 80% for the same pressure drop. In this work, we experimentally investigated the processes of heat and mass transfer in film condensation of steam in forced-flow through the plate side of this new heat exchanger.