Air-side Heat Transfer Research Articles

Experimental Study on Air Cooling Via a Multiport Mesochannel Cross-Flow Heat Exchanger

The air-side heat transfer and flow characteristics of cross-flow multiport slab mesochannel heat exchanger are investigated experimentally in this article. The multiport slab mesochannel heat exchanger consists of 15 finned aluminum slabs; each slab contains 68 flow channels of 1 mm circular diameter. The cold deionized water at a constant mass flow rate was forced to flow through the mesochannels, whereas the hot air at different velocities was allowed to pass through the finned passages of the heat exchanger core in cross-flow orientation. The heat transfer and fluid flow key parameters were examined in the region of the air-side Reynolds number in the range of 972–2758, with a constant water-side Reynolds number of 135. The effect of air-side Reynolds number on air-side Nusselt number was examined and a general correlation of Nusselt number with Reynolds number was obtained. The Nusselt number value was found to be higher in comparison with other research works for the corresponding Reynolds number range. The multiport mesochannel flat slab geometry has offered uniform temperature distribution into the core. This uniform temperature distribution leads to higher heat transfer over stand-alone inline flow tube bank.

Heat Transfer Enhancement of Spray Cooling on Flat Aluminum Tube Heat Exchanger

This study provides an experimental analysis on the heat transfer performance of a flat aluminum tube microchannel heat exchanger with spray cooling. The effects of water spraying rate, airflow rate, and relative humidity were investigated. The test results show that the heat transfer performance increased with increasing the water spraying rate but without the penalty of increased flow resistance at low spray conditions. This effect is further enhanced by increasing the water spraying rate. However, when the spraying rate is high, part of the nonevaporated drops attached to the fin surface and formed a liquid film, which caused the flow passage to become narrower. Further increase in the spraying rate resulted in part of the flow passages being blocked by the nonevaporated water drops and caused a region of poor heat transfer. The friction coefficient jumped drastically at this condition. This phenomenon deceased gradually with increasing airflow rate. High inlet air humidity resulted in the water accumulation phenomenon appearing at lower water spraying rates. The evaporative cooling effect decreased and flow friction increased. The test results just described show that the water spray is able to significantly improve the air-side heat transfer performance. The optimum spray rate for each airflow rate must be carefully determined.

Experimental Study on the Air-Side Performance of a Multipass Parallel and Counter Cross-Flow L-Footed Spiral Fin-and-Tube Heat Exchanger

This study conducted experiments on the air-side performance of novel L-footed spiral fin-and-tube heat exchangers that were faced with airflow at high Reynolds numbers (3500–13,000). The examined heat exchangers have a multipass parallel-and-counter cross-flow type of water flow arrangement. This flow arrangement is a combination of the parallel cross-flow and the counter cross-flow. This type of water flow arrangement may be the best fit for the reverse-flow system, because it can provide constant heat-exchange effectiveness for every flow reversal direction at the same airflow rate. Ambient air was used as a working fluid on the air side and hot water for the tube side. This way the effect of the number of tube rows on the heat transfer and friction characteristics of L-footed spiral fin-and-tube heat exchangers was clearly observed. The effect of the fin's outside diameter on the pressure drop was also studied. The results show that the number of tube rows has no significant effect on the air-side heat transfer or on friction characteristics at high Reynolds numbers. However, the fin's outside diameter shows a significant effect on the pressure drop. The pressure drop increases as the fin's outside diameter increases for the same number of tube rows.

Effect of a micro-grooved fin surface design on the air-side thermal-hydraulic performance of a plain fin-and-tube heat exchanger

In this study, the effectiveness of plain fin-and-tube heat exchangers constructed using anisotropic, micro-patterned aluminum fins has been explored. These fins which can more completely drain the condensate that forms on the heat transfer surface during normal operation were selected with the aim of improving the thermal-hydraulic performance of the heat exchanger. This study presents and critically evaluates the efficacy of four full-scale heat exchangers by measuring and comparing dry/wet air-side pressure drop and dry/wet air-side heat transfer data. The prototype fin surfaces were shown to reduce the wet air-side pressure drop from 9.3% to 53%, while at the same time having a negligible effect on the sensible heat transfer coefficient under both dry and wet conditions. That is to say, this novel fin surface design has shown the ability, through improved condensate management, to enhance the performance of the heat exchanger. Data pertaining to the durability of the alkyl silane coating used in this work are also presented.

A Novel Approach to Model the Air-Side Heat Transfer in Microchannel Condensers

The work presents a model (Fin1D×3) for microchannel condensers and gas coolers. The paper focusses on the description of the novel approach employed to model the air-side heat transfer. The model applies a segment-by-segment discretization to the heat exchanger adding, in each segment, a specific bi-dimensional grid to the air flow and fin wall. Given this discretization, the fin theory is applied by using a continuous piecewise function for the fin wall temperature. It allows taking into account implicitly the heat conduction between tubes along the fin, and the unmixed air influence on the heat capacity. The model has been validated against experimental data resulting in predicted capacity errors within ± 5%. Differences on prediction results and computational cost were studied and compared with the previous authors' model (Fin2D) and with other simplified model. Simulation time of the proposed model was reduced one order of magnitude respect the Fin2D's time retaining its same accuracy.

An engineering procedure for air side performance evaluation of flat tube heat exchangers with louvered fins

An accurate evaluation of possible air side heat transfer surface geometries is a prerequisite for optimal heat exchanger design. Aiming for practical engineering applicability a simplified and transparent analytical procedure for the assessment of louvered fin and flat tube heat exchanger geometries and the calculation of fin parameters that enable maximal performance for given boundary conditions has been developed. The proposed method comprises determining fins temperature profiles and effective heat transfer temperature difference, introduction of a relative heat transfer surface area, as well as the utilization of recent experimentally obtained heat transfer correlations confirmed for the observed range of boundary conditions. The proposed methodology is validated through comparison with experimental and numerical results of other authors.

Optimization of peripheral finned-tube evaporators using entropy generation minimization

The peripheral finned-tube (PFT) is a new geometry for enhanced air-side heat transfer under moisture condensate blockage (evaporators). It consists of individual hexagonal (peripheral) fin arrangements with radial fins whose bases are attached to the tubes and tips are interconnected with the peripheral fins. In this paper, experimentally validated semi-empirical models for the air-side heat transfer and pressure drop are combined with the entropy generation minimization theory to determine the optimal characteristics of PFT heat exchangers. The analysis is based on three independent parameters, i.e., porosity, equivalent particle diameter and particle-based Reynolds number. The total heat transfer rate is a fixed constraint. The optimal heat exchanger configurations, i.e., those in which the entropy generation number reaches a minimum, are calculated for constant heat flux and constant tube wall temperature boundary conditions. Performance evaluation criteria of fixed geometry, fixed face area and variable geometry were implemented. In all cases, it was possible to determine a combination of independent parameters that provided a minimum entropy generation rate.

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

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

Numerical model for microchannel condensers and gas coolers: Part I – Model description and validation

The present work presents a model (Fin1Dx3) for air-to-refrigerant microchannel condensers and gas coolers, with any refrigerant circuitry. The model applies a segment-by-segment discretization to the heat exchanger, adding in each segment a novel bi-dimensional discretization to the fluids flow, fin and tube wall. Fin1Dx3 introduces a new approach to model the air-side heat transfer by using a composed function for the fin wall temperature, which allows to take into account more fundamentally the heat conduction between tubes. The proposed model accounts for: 2D longitudinal heat conduction in the tube wall, the heat conduction between tubes along the fin, and the unmixed air influence on performance. The paper presents the heat exchanger discretization, the governing equations, the numerical scheme employed to discretize equations and the solving methodology. The model has been validated against experimental data for both a condenser and a gas cooler, resulting in predicted capacity errors within ±5%.

확관을 고려한 불균일 내면가공관의 전열특성

A plate-fin heat exchanger is a type of heat exchanger widely used in air conditioners, and tubes and fins are tightly assembled by the mechanical expansion process of tubes. The tube expansion process deforms the grooves inside the tube, and the groove shapes also affect the adhesion between tubes and fins. In this study, the adhesion and heat transfer performance affected by the tube expansion of the non-uniform groove shape tube with different heights are investigated by both analysis and experiments. From the analysis method, it was shown that the contact pressure of non-uniform groove tube is higher than that of the uniform groove tube, and the most appropriate high groove number of the non-uniform groove tube is designed for the maximum contact pressure. From the experimental results, the decreasing rate of the condensation heat transfer coefficient is smaller in the non-uniform groove tube with different heights, compared to the conventional uniform groove tube. Also, the air-side heat transfer coefficient of the non-uniform groove tube with different heights is higher than that of the uniform groove tubes.

An experimental and numerical investigation of air side heat transfer and flow characteristics on finned plate configuration

In this paper, a new type of finned plate heat exchanger (FPHE) is presented to recover the waste heat from exhaust flue gases. A finned plate configuration causes low pressure drop and it is especially appropriate for heat transfer at the flue gas side. Meanwhile, this paper presents a detailed experimental and numerical study of convection heat transfer and pressure drop of the new structure. Three-dimensional numerical simulation results using the CFD code FLUENT6.3 were compared with experimental data to select the best model. The heat transfer and pressure drop with different geometry pattern was then studied numerically using the selected model. And the velocity field and temperature distribution of air flow in the finned plate channel are presented with different geometry patterns. These results provide insight into improved designs of FPHEs.

Air Side Heat Transfer Characteristics of Wavy-Finned-Tube in a Forced Draft Direct Air-Cooled Steam Condenser

An experimental study was performed on wavy-finned-tube used in forced draft direct air-cooled steam condenser (DACSC) under actual working conditions of the power generating unit. Tests were carried out to study the air side heat transfer characteristics of wavy-finned-tube in actual operating conditions of DACSC, such as: air temperature, air face velocity, environmental temperature, exhaust pressure of steam turbine and temperature of exhaust steam. The air-side heat transfer characteristics of wavy-finned-tube heat exchangers were tested and analyzed by varying air face velocity .One empirical correlations for predicting the h-factor was developed.

Air cooling by multiport slabs heat exchanger: An experimental approach

While numerous studies have been undertaken to characterize the heat transfer and flow characteristics in microchannels, literature investigating the air-side performance of microchannel heat exchangers is scarce. Air-side heat transfer and flow characteristics were studied through an air to de-ionized water cross-flow serpentine microchannel heat exchanger through air cooling. In the single phase, twenty distinct operating conditions were maintained within the range of air and de-ionized water-side Reynolds number 283⩽Rea⩽1384 and 105⩽Rew⩽159 respectively, retaining the constant inlet temperatures of both fluids (Ta,i=38±0.5°C and Tw,i=9±0.5°C). Heat transfer and fluid flow key parameters were evaluated from experimental data. Heat balance performance was observed within 4% throughout the experiment, and major thermal resistance was found in the air-side, ranging from 89% to 94% of overall thermal resistance. The effects of Rew and Rea on air-side Nusselt number (Nua), friction factor (fa), and other parameters were examined, and general correlations were achieved.

Heat transfer and pressure drop characteristics of peripheral-finned tube heat exchangers

The peripheral-finned tube is a new geometry aimed at avoiding moisture-condensate blockage hindering of the air-side heat transfer, by allowing for robust air flow pathways. It consists of a porous structure formed by periodic, radial-hexagonal fin arrangements of different radial extents mounted with a 30° offset from its neighboring level. Here, the air-side pressure drop and the heat transfer characteristics of five different heat exchanger prototypes with different geometric characteristics, such as the radial fin length, fin distribution, and heat exchanger length, were evaluated experimentally in an open-loop wind-tunnel calorimeter. The results demonstrate the effective performance, i.e., the pressure drop and heat transfer characteristics, of this new heat exchanger. A one-dimensional theoretical model based on the porous media treatment was also developed to predict the thermal-hydraulic behavior of the heat exchanger. The model incorporates the actual fin geometry into the calculation of the air-side porosity. The air-side permeability is calculated according to the Kozeny–Carman model and the particle-diameter based analysis. The model predicts the experimental data within a few percent RMS, depending on the correlations used for the friction coefficient and interstitial Nusselt number.

Air-side heat transfer and pressure drop in spiral wire-on-tube condensers

The air-side thermal-hydraulic performance of spiral wire-on-tube condensers is investigated experimentally in this paper. Sixteen prototypes have been manufactured and tested in an open-loop wind tunnel calorimeter. The influence of the following geometric parameters has been evaluated: the number of tube passes, the radial and longitudinal tube spacings and the wire spacing. Measurements of the air-side thermal conductance and pressure drop were carried out for air flow rates ranging from 70 to 220 m3 h−1. The data were correlated using empirical relationships for the Colburn j-factor and the Darcy friction factor. The agreement with the experimental data presented RMS deviations of 0.9% for the air-side heat transfer and 1.3% for the frictional pressure drop dimensionless parameters. A quantitative analysis based on the core volume goodness factors for heat transfer and pressure drop is presented to provide the most viable configuration from the point of view of application in a refrigerator.

핀관 열교환기에서 확관율이 접촉열전달계수에 미치는 영향

The plate fin and tube type of heat exchanger is widely used in air conditioner, and the heat exchanger is assembled by the mechanical expansion of copper tubes and fastening the aluminum fin. The objective of the present study is to investigate how the mechanical expansion of copper tube affects on the heat transfer performance of a plate fin and tube type heat exchanger. This study has been performed by experimental and numerical methods. The numerical and experimental results show that the tube expansion ratio has a influence on the heat transfer performance. Within the tested expansion ratio, the contact pressure shows the peak value and it decreases as the expansion ratio increases. Air-side heat transfer coefficient increases until the expansion ratio reaches 1.23, and then decreases with the similar pattern to the contact pressure. Also, contact heat transfer coefficient shows the maximum when the contact pressure is highest as well as the air-side heat transfer coefficient.

Experimental investigation of air side heat transfer and fluid flow performances of multi-port serpentine cross-flow mesochannel heat exchanger

Air side force convective heat transfer and flow characteristics of cross-flow mesochannel heat exchanger are investigated experimentally. A series of experiments representing 36 different operating conditions have been conducted on a finned mesochannel heat exchanger through the fully automated dynamic single-phase experimental facility which is capable of handling a wide variety of working fluids in air-to-liquid cross-flow orientation. The mesochannel heat exchanger is made of 15 aluminum slabs with arrays of wavy fins between slabs; 68 one millimeter circular diameter port located at each slab, and the air side frontal area of 304-mm×304-mm. The ethylene glycol–water mixture as the working fluid in the liquid side was forced to flow through mesochannels maintaining constant inlet temperature and flow rate at 74°C and 0.0345kg/s respectively whereas the inlet flowing air into the arrays of wavy fins was changed at four different temperature levels from 28°C to 43°C. Frontal air velocity was altered in nine steps from 3m/s to 11m/s at each temperature level corresponding range of Reynolds number 752<Rea<3165. The air side heat transfer and flow characteristics of mesochannel heat exchanger were evaluated during air heating, and heat transfer and fluid flow correlations were derived accordingly. The air side Nusselt number (Nua) and Colburn factor (ja) were found higher in comparison with other studies.

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.

Air-side Heat Transfer Characteristic Evaluation of Automobile Heat Exchanger

Air-side Heat Transfer Characteristic Evaluation of Automobile Heat Exchanger

Performance of microchannel condensers with metal foams on the air-side: Application in small-scale refrigeration systems

Abstract The thermal-hydraulic performance of microchannel condensers with open-cell metal foams to enhance the air-side heat transfer is investigated in this paper. Three different copper metal foam structures with distinct pore densities (10 and 20 PPI) and porosities (0.893 and 0.947) were tested. A conventional condenser surface, with copper plain fins, was also tested for performance comparison purposes. The experimental apparatus consisted of a closed-loop wind tunnel calorimeter and a refrigerant loop, which allowed the specification of the mass flow rate and thermodynamic state of R-600a at the condenser inlet. The experiments were performed at a condensing temperature of 45 °C. The air-side flow rate ranged from 1.4 × 10 −3 to 3.3 × 10 −3 m 3 /s (giving face velocities in the range of 2.1–4.9 m/s). The heat transfer rate, the overall thermal conductance, the Colburn j -factor, the friction factor and the pumping power were calculated as part of the analysis.