Brazed Aluminum Heat Exchangers Research Articles

Fabrication of a superhydrophobic surface with fungus-cleaning properties on brazed aluminum for industrial application in heat exchangers

Extensive research has been carried out concerning the application of superhydrophobic coating in heat exchangers, but little is known about the application of this technique to brazed aluminum heat exchangers (BAHEs). In this work, we describe a new superhydrophobic coating method, which is suitable for BAHE use on an industrial scale. We first render the BAHE superhydrophobic by fabricating micro/nanostructures using solution dipping followed by fluorination. After the complete removal of the silicon residue, we verify using surface analysis that the BAHE surface is perfectly superhydrophobic. We also studied the fungus-cleaning properties of the superhydrophobic surface by growing fungus for 4 weeks in a moist environment on BAHE fins with and without superhydrophobic coating. We observed that, whereas the fungus grown on the untreated fins is extremely difficult to remove, the fungus on the fins with the superhydrophobic coating can be removed easily with only a modest amount of water. We also found that the coated BAHE fins exhibit excellent resistance to moisture. The superhydrophobic coating method that we propose is therefore expected to have a major impact in the heating, ventilating and air conditioning industry market.

Upward branching of two-phase refrigerant in a parallel flow minichannel heat exchanger

Brazed aluminum heat exchangers are recently considered as evaporators of automotive or residential air conditioners. In this case, it is very important to distribute the two-phase refrigerant (especially the liquid) evenly into each tube. In this study, R-134a flow distribution was experimentally studied for a round header-ten flat tube test section simulating a brazed aluminum heat exchanger. Three different inlet orientations (parallel, normal, vertical) were investigated. Tests were conducted with upward flow for the mass flux from 70 to 130kgm−2s−1 and quality from 0.2 to 0.6. Tubes were protruded to center of the header. Results show that better distribution (both for liquid and gas) is obtained for vertical inlet configuration. Possible explanation is provided based on flow visualization results. Efforts were made to develop correlations to predict the liquid or gas distribution in a header with limited success. Refined correlations should consider both geometric and operating variables. Header pressure drop data are also provided.

Improvement of two-phase refrigerant distribution in a parallel flow minichannel heat exchanger using insertion devices

In this study, various insert devices (wire mesh, perforated plate, perforated tube) were investigated to obtain an improved flow distribution in a round header/ten flat tube test section using R-134a. To simulate an actual brazed aluminum heat exchanger, tubes were protruded to center of the header. Tests were conducted both for downward and upward flow for the mass flux from 70 to 130 kg m−2 s−1 and quality from 0.2 to 0.6. Of the investigated insert devices, perforated tube significantly improved the flow distribution both for downward and upward flow when the holes were located facing tubes. Preferred hole geometry was dependent on the flow direction. For downward flow, non-uniform hole configuration with decreasing hole sizes in flow direction yielded better flow distribution than uniform hole configuration. For upward flow, however, uniform hole configuration yielded better flow distribution. Possible reason was provided based on flow visualization results. Header pressure drop was obtained from the measured pressure drop by subtracting appropriate pressure drops. For no-insert configuration, header pressure drops were mostly negative, which implied that the pressure was recovered in the header. With insertion of the perforated tube, however, the header pressure drop increased significantly. Other insert devices (wire mesh, perforated plate) were not effective in improving the flow distribution.

Effect of inlet configuration on upward branching of two-phase refrigerant in a parallel flow heat exchanger

The refrigerant R-134a flow distributions were experimentally studied for a round header-10 flat tube test section simulating a brazed aluminum heat exchanger. Three different inlet orientations (parallel, normal, vertical) were investigated. Tests were conducted with upward flow for the mass flux from 70 to 130 kg m−2 s−1 and quality from 0.2 to 0.6. Tubes were flush-mounted in the header with no protrusion. Both for liquid and gas, the best flow distribution was obtained from vertical inlet configuration. Between other two inlet configurations, parallel inlet was better for liquid distribution, and normal inlet was better for gas distribution. In general, the effect of mass flux or quality on flow distribution was not significant. Correlations were developed to predict the fraction of liquid or gas taken off by downstream channel as a function of header liquid Reynolds number at immediate upstream. Header pressure drop was obtained by subtracting appropriate pressure drops from the measured pressure drop. The header pressure drops were mostly negative, which implied that the pressure was recovered in the header.

Two-phase refrigerant distribution in a parallel flow minichannel heat exchanger having horizontal headers

The refrigerant R-134a flow distributions are experimentally studied for a round header-ten flat tube test section simulating a brazed aluminum heat exchanger. Three different inlet orientations (parallel, normal, vertical) are investigated. Tests are conducted with downward flow for the mass flux from 70 to 130kg/m2s and quality from 0.2 to 0.6. Tubes are protruded to the center of the header. It is shown that the preferred inlet configuration is dependent on mass flux or quality. At low mass flux or quality, vertical inlet yields the best flow distribution, followed by normal and then parallel inlet. As mass flux or quality increases, the optimum configuration switches to normal inlet configuration. As compared with the flush-mounted configuration, more uniform flow distribution is obtained for the present protruded configuration. Possible explanation is provided based on flow visualization results. Correlations are developed to predict the fraction of liquid or gas taken off by downstream channel as a function of header gas Reynolds number at immediate upstream.

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.

Effect of inlet configuration on the refrigerant distribution in a parallel flow minichannel heat exchanger

The refrigerant R-134a flow distribution was experimentally studied for a round header/ten flat tube test section simulating a brazed aluminum heat exchanger. Three different inlet configurations (parallel, normal, vertical) were investigated. Tests were conducted with downward flow for mass flux from 70 to 130 kg m −2 s −1 and quality from 0.2 to 0.6. Tubes were flush-mounted in the test section, with no protrusion into the header. It is shown that normal and vertical inlet yielded similar flow distribution. As mass flux or quality increased, however, better results were obtained for normal inlet configuration. The flow distribution was worst for the parallel inlet configuration. Possible explanation is provided based on flow visualization results. Correlations were developed to predict the fraction of liquid or gas taken off by downstream channel as a function of header gas Reynolds number at immediate upstream.

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.

Sealed Aluminum Cavity Reactions when Submerged in Pure O2 Reboiler Sump

Abstract Aluminum has a long history of safe service in cryogenic air separation units; however, there have been some rare instances of aluminum/O2 reactions. Aluminum ignition and propagation depends strongly on the oxygen purity, oxygen pressure, aluminum geometry, and type and energy of igniter. Over the years, the industry has experienced a particular type of aluminum/O2 reaction: cavity incidents. These incidents are characterized by the presence of a sealed cavity that is created when welding two metal items together. Such a sealed cavity can lead to aluminum ignition under certain specific conditions: (1) The sealed cavity is submerged in a bath of liquid oxygen. Over a long period of time, liquid cryogen can enter the sealed cavity through slight imperfections in the weld. (2) The sealed cavity is warmed over a short period of time, typically a few hours. (3) The liquid oxygen vaporizes, and the pressure inside the cavity builds to very high levels (potentially over 50 bars). (4) Ignition occurs in the high pressure, high purity oxygen environment, and the resulting aluminum/oxygen reaction burns through the relatively thick cavity walls. (5) The oxygen and reaction products exit the cavity through the hole, lowering the pressure and extinguishing the reaction. This paper discusses two such incidents, which have occurred since 2001. Both incidents took place in brazed aluminum heat exchanger (BAHX) reboiler support beam systems. In both cases, the BAHX reboiler was damaged, leading to a process leak, which required that the plant be repaired. In one case, the damage occurred while warming a plant. In the second case, the damage occurred during normal operation. This second case does not appear to follow the sequence of events outlined above; however, evidence is presented to support the scenario that the cavity incident occurred during a previous warming of the plant. This initial damage then impaired the normal operation of the reboiler, leading to a second hydrocarbon related reaction during normal operation. The paper discusses the potential ignition mechanisms, probable causes of the incidents, and methods to prevent future re-occurrences.

Air-side Heat Transfer and Pressure Drop Characteristics of Louver-finned Aluminum Heat Exchangers at Different Inclination Angles

The effect of inclination angle on the heat transfer and pressure drop characteristics of brazed aluminum heat exchangers having louvered fins is experimentally investigated. Three samples having different fin pitches (1.25, 1,5 and 2.0 mm) were tested. Results show that heat transfer coefficients are not affected by the inclination angle. Friction factors, however, increase as the inclination angle increases, with negligible difference between the forward and the backward inclination. Both the heat transfer coefficient and the friction factor are the smallest at Fp = 1.5 mm, followed by 2.0 mm and 1.25 mm. Possible explanation is provided considering the louver layout. Comparison of the present data with existing correlations is also made.

Wet surface heat transfer and pressure drop of aluminum parallel flow heat exchangers at different inclination angles

The effect of inclination angle on the heat transfer and pressure drop characteristics of brazed aluminum heat exchangers was experimentally investigated under wet conditions. Three samples having different fin pitches (1.25, 1.5 and 2.0 mm) were tested. Results show that heat transfer coefficients are not affected by the inclination angle. However, friction factors increase as the inclination angle increases with negligible difference between the forward and backward inclination. The effect of fin pitch on the heat transfer coefficient and on the pressure drop is also discussed. Comparison of the dry and wet surface heat transfer coefficients reveals that dry surface heat transfer coefficients are significantly larger than wet surface heat transfer coefficients. Possible explanation is provided by considering the condensate drainage pattern. The data are also compared with the existing correlation.

Effect of Fluoride on Corrosion of Cooling System Metals in Ethylene Glycol-Based Antifreeze/Coolants

Abstract Automotive and light-duty vehicle heat exchangers such as radiators are now predominantly made of aluminum alloys. For over 20 years, a brazing technique known as “controlled atmosphere brazing (CAB) with noncorrosive fluxes” has become the preferred process for producing aluminum automotive heat exchangers for original equipment manufacturers and aftermarket suppliers. Fluoride-based fluxes are typically used in the CAB processes to dissolve the aluminum oxide layer on the surface and inhibit reoxidation during the brazing cycle to allow the clad alloy to flow properly. The flux is generally a mixture of potassium fluoroaluminate, (K3AlF6, K2AlF5, and KAlF4). The flux residue left on the internal surface of heat exchangers after brazing is thought to be insoluble and noncorrosive, hence no removal is necessary. Although external corrosion of brazed aluminum heat exchangers has been studied using salt spray tests, a study addressing the effect of interaction between different antifreeze and flux residues on internal corrosion has not been reported. To the authors' knowledge, few studies on the effect of fluoride in engine coolants on metal corrosion are available. In this paper, laboratory test data are provided to show that flux residue is soluble in commercially available coolants and can generate fluoride ions that enhance cooling system corrosion. Systematic study on the effect of fluoride on metal corrosion in cooling systems and its remedies are also presented and discussed.

Effect of inlet humidity condition on the air-side performance of an inclined brazed aluminum evaporator

The effect of air inlet humidity condition on the air-side heat transfer and pressure drop characteristics for an inclined brazed aluminum heat exchanger has been investigated experimentally. For a heat exchanger with a louver angle of 27°, fin pitch of 2.1 mm and flow depth of 27.9 mm, a series of tests are conducted for the air-side Reynolds numbers of 80–400, with variation of inlet humidity condition. The heat transfer data are obtained for wet condition only and the pressure drop data are measured for both dry and wet conditions. The inlet air temperature and relative humidity range are 12 °C and 60–90%, respectively. The inclination angles ( θ) from the vertical position are 0, 14, 45, and 67° clockwise (leeward direction). The inclination angles affect moderately the sensible heat transfer coefficient for wet condition, and the pressure drops for both dry and wet conditions increase systematically with the inclination angle. The heat transfer and pressure drop characteristics under wet condition are not influenced substantially by the air inlet humidity for θ ⩽45°. The effect of the louver directions at the inlet and outlet of the inclined heat exchanger on the performance is also addressed.

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.

Effect of inclination on the air-side performance of a brazed aluminum heat exchanger under dry and wet conditions

This study presents the effect of an inclination angle from the vertical position on the air-side thermal hydraulic performance for a multi-louvered fin and flat tube heat exchanger. For a heat exchanger with a louver angle of 27°, fin pitch of 1.4 mm and flow depth of 20 mm, a series of tests for dry and wet surface conditions were conducted for the air-side Reynolds numbers of 100–300. The inclination angles from the vertical position were 0°, ±30°, ±45°, and ±60° clockwise. The heat transfer performance for both dry and wet conditions was neither influenced significantly by the inclination angle (−60°< θ<60°), nor by the presence or absence of an upstream duct, while the pressure drops increased consistently with the inclination angle. The heat transfer coefficients and the pressure drops for the wet conditions revealed the importance of the role of condensate drainage.

HEAT TRANSFER AND FRICTION IN SMALL DIAMETER CHANNELS

Applications have been developed for heat transfer in small-diameter channels of circular or rectangular shape. This article discusses the ability of existing accepted correlations to predict single-phase and two-phase heat transfer and friction in channels having small hydraulic diameters (e.g., between 0.1 and 2.0 mm). Existing equations predict the single-phase heat transfer and friction data for multiport circular and rectangular-crosssection channels having hydraulic diameters between 0.96 and 2.13 mm. However, a 1994 study by Wang and Peng on single-phase flow in multiport rectangular channels found that the same equations considerably overpredicted their data. The present investigators show that their recently developed ''equivalent Reynolds number model'' will predict the condensation coefficient of various refrigerants in tube diameters as small as 2.13 mm. However, the 1979 Shah equation begins to fail for R-134a at p p cr 0.44. The present study is important for applications where very small channel hydraulic diameters are important, such as brazed aluminum heat exchangers or in electronic equipment cooling. More data are needed on channel diameters below 1.0 mm.

Conduite automobile et lésion médullaire. Le permis de conduire passé à l'hôpital est-il utilisé ?

Aluminum brazed heat exchangers are used widely in automobile air conditioning systems. Usually, this kind of heat exchanger is made of a flat tube with several independent passages in the cross-section, and formed into a serpentine or a parallel flow geometry. In these heat exchangers, a multitude of corrugated fins with louvers are inserted into the gaps between flat tubes. As a result of their high performance, some companies are starting to use extruded brazed aluminum heat exchangers in heating, ventilating and air conditioning applications. The purpose of this investigation is to provide the basic condensing heat transfer data for the extruded aluminum flat tube. Four different flat tubes and one micro fin tube were employed in this series of experimental studies. HFC-134a and HCFC-22 were the working fluids used. Our results were closest to Shah's correlation in the low quality region.

Condensing heat transfer characteristics of aluminum flat tube

Aluminum brazed heat exchangers are used widely in automobile air conditioning systems. Usually, this kind of heat exchanger is made of a flat tube with several independent passages in the cross-section, and formed into a serpentine or a parallel flow geometry. In these heat exchangers, a multitude of corrugated fins with louvers are inserted into the gaps between flat tubes. As a result of their high performance, some companies are starting to use extruded brazed aluminum heat exchangers in heating, ventilating and air conditioning applications. The purpose of this investigation is to provide the basic condensing heat transfer data for the extruded aluminum flat tube. Four different flat tubes and one micro fin tube were employed in this series of experimental studies. HFC-134a and HCFC-22 were the working fluids used. Our results were closest to Shah's correlation in the low quality region.

Air Side Performance of Brazed Aluminum Heat Exchangers

Extensive experiments on the heat transfer and pressure drop characteristics of brazed aluminum heat exchangers were carried out. In the present study, 27 samples of louvered fin heat exchangers with different geometrical parameters, including tube width, louver length, louver pitch, fin height, and fin pitch were tested in an induced draft open wind tunnel. Results are presented as plots of friction factor, f, and Colburn j factor against Reynolds number based on louver pitch in the range of 100 to 1000. Comparisons between the Sahnoun and Webb model and the present test data are reported and good agreements were found. By introducing “area ratio” parameters, a simpler correlation of the Colburn j factor and friction factor f were obtained. It is shown that 85% of the experimental data of heat transfer and friction data were correlated within ± 10%.