Air-side Heat Transfer Performance Research Articles

Numerical Simulation of Heat Transfer Tube Characteristics of Refrigeration and Air Conditioning Based on Mixed Fractional Model

The main parameters affecting the heat transfer performance of heat transfer tube heat exchanges include fin shape, fin spacing, fin thickness, tube row arrangement, tube diameter, dry and wet bulb temperature and flow rate. The air side heat transfer performance of heat transfer tube heat exchange and the influence of velocity field and temperature field distribution on heat transfer effect have been the focus of domestic and foreign scholars. In this paper, based on the mixed fraction model, CFD software is used to simulate the absorption process of gravity falling film outside the heat transfer tubes of refrigeration and air conditioning, and to study the flow and heat transfer characteristics of the process. The results show that, for the heat transfer tubes with the selected structure, the heat transfer capacity increases with the increase of water flow velocity, and the heat transfer enhancement effect of turbulence is enhanced. The heat transfer tubes have better comprehensive heat transfer performance than smooth tubes with the same diameter.

Numerical analysis on heat transfer enhancement of wavy fin-tube heat exchangers for air-conditioning applications

• Heat transfer enhancement at constant pumping power fits closest to real scenarios. • Larger HTC does not necessarily lead to larger heat transfer rate. • Waffle height, wave length, and fin pitch should be optimized simultaneously. • The optimized geometries yield 23% improvement of HTC and 3% of heat transfer rate. • Proper Perforation on wavy fin leads to 34% enhancement of HTC. The air-side heat transfer performance of herringbone wavy fin-tube heat exchangers can be improved by optimizing fin geometries and introducing perforations to the wavy fin. Based on a typical herringbone wavy fin-tube heat exchanger used in the outdoor unit of an air conditioner with collar outside diameter of 7.3 mm, transverse tube pitch of 22 mm, and longitudinal tube pitch of 19.05 mm, the effects of fin pitch, wave length, slits height, and geometries of slits are simulated using CFD and the outcomes are discussed in the paper. We demonstrate that constant pumping power is the ideal criterion for the performance evaluation of any new designs. The wave length has no remarkable effects on the heat transfer coefficient while the lowest pressure drop appears with the symmetric wave configuration. The heat transfer coefficient can be improved by 23.4% when the waffle height, fin pitch, and wave length are simultaneously optimized. Furthermore, punching slits on top of the windward part of wavy fins leads to a 5.5% increase in the heat transfer coefficient. In the end, 34.2% improvement of the heat transfer coefficient is yielded when combining the slit perforations to the optimum fin pitch, wave length, and waffle height.

Field Study on the Air-Side Heat Transfer Performance of Copper Finned-Flat Tubes for Heavy-Duty Truck Radiators

Heavy-duty truck cooling systems have been given low importance in the enhancement and research of heat transfer performance since off-highway conditions are hard to evaluate in laboratory essays or CFD studies. The present work is performed to evaluate the heat transfer performance of copper finned-flat tubes used in heavy-duty truck radiators. Parameters were measured in the field of two heavy-duty truck engines cooling systems. In both vehicles water is used as the cooling fluid. The results showed that the Air convective heat transfer coefficient and Overall heat transfer coefficient on the air side decreases as the Reynolds Number decreases and increases as passing through the first row to the fourth row. Additionally, the mass air flow and heat transfer rate have very high values in comparison from normal automotive radiators' operative conditions, since heavy-duty truck radiators require a large heat transfer rate. The analysis presented in this paper was used for a heavy-duty truck radiator but can be extended to any equipment with finned flat tubes. A more accurate study should be done considering vibrations and different environmental conditions.

Improving air-side heat transfer performance in air-cooled power plant condensers

One method to reduce water consumption in cooling towers for steam condensation is to use dry-cooled technologies with air as the heat rejection medium. These air-cooled systems are beneficial where water is unavailable or uneconomical, but typically result in lower power plant efficiencies. The present study investigates the application of an A-frame air-cooled condenser (ACC) with various air-side enhancements to increase dry-cooled plant performance to levels approaching wet-cooled plants. A computational model was developed to predict the air-cooled condenser performance during the summer (inlet air at 30 °C and 25% relative humidity), when air-cooled systems typically perform the worst. Parametric studies were conducted to identify optimal condenser geometries with smooth, wavy, and louvered fins. The segmented condenser model was integrated into Rankine cycle and combined cycle power plant models to determine the impact of various condenser fin geometries on overall cycle performance. While ACC designs with wavy or louvered fins improved the overall cycle efficiencies over the baseline case, it was found that a reduction in fin spacing of the conventional, smooth fins resulted in the largest improvement in cycle efficiency (1.14% increase for the Rankine cycle, and a 0.84% increase for the Combined cycle).

An experimental study on the air side heat transfer performance of the perforated fin-tube heat exchangers under the frosting conditions

In this paper, perforated fins were applied to enhance the air side heat transfer performance under frosting conditions for fin-tube heat exchangers with large fin pitches. The frost mass, frost thickness, heat transfer rate and heat transfer coefficient of the perforated fin-tube heat exchangers were measured and analyzed. Although, the frost mass for the perforated fin-tube heat exchangers was 46.7% more than that for the plain fin. The total heat transfer rate and the heat transfer coefficient of the perforated fin was increased by 38.9% and 31.8%. Moreover, under the same heat transfer rate, the heat exchangers with the perforated fins generated less frost mass compared with the plain fin. Two empirical correlations for predicting the j-factor and frost mass of the perforated fin-tube heat exchangers were developed respectively. The correlations were functions of the Reynold number, dimensionless fin pitch, dimensionless temperature, and humidity and Fourier number. The deviations between these predictions and the corresponding measured experimental data fell within ±15.0%.

Air-side heat transfer characteristics under wet conditions at lower ambient pressure of fin-and-tube heat exchanger

Air-side heat transfer performance of plain fin-and-tube heat exchanger under fully wet conditions was experimentally investigated at different ambient air pressure. Test results were compared with three available correlations to examine their applicability in lower pressure. Tested samples had different row numbers of 2–4 and the experimental ambient pressure had a range of 40–100 kPa. The experimental data at atmospheric pressure is basically consistent with the correlations. Air-side jh factors increased by 16.51–40.19% as Re number decreased from 4000 to 1000, and balloon to 112.04–141.37% when Re number declined from 1000 to 200. The row number effect still exists at lower ambient pressure and only become more apparent. Available correlations fails to describe experiment data at lower ambient pressure, maximum average error soar from 15.90% to 137.41% when ambient pressure drop from 100 kPa to 40 kPa. By adding the environmental pressure correction factor to the Re number in correlation, the refined correlation can describe 90.43% of experimental data within ±20%, and have a mean deviation of 1.46%.

Field study on the self-adaptive capacity of multi-split heat pipe system (MSHPS) under non-uniform conditions in data center

The multi-split heat pipe system (MSHPS) is an efficient cooling system, since it mainly belongs to two-phase heat transfer and closes to the heat source. The self-adaptive capacity including operation performance under various heating loads, refrigerant distribution characteristics and anti-failure capability are important factors to determine the thermal safety of the data center using MSHPS. This paper focused on the onsite test about self-adaptive capacity of a MSHPS in a real data center under 25%, 50%, 75% and 100% heating loads and various fan failures. The results show that the MSHPS abnormally operated under low heating loads, but it still met the cooling demands due to its superior self-adaptive capacity. It was also found that abnormal operation was deteriorated by refrigerant side unbalanced pressure and heat leak. Further, the influence of fan failures on the airside heat transfer performance was limited, but the influence on refrigerant side was obvious due to the deteriorating of heat leak. For racks, the air velocity reduction caused by fan failures will increase the risk of servers overheating. For the whole computer room, owing to its superior self-adaptive capacity, the MSHPS ensured the thermal safety under serious fan failures in single rack and mild fan failure in row racks. Those results can give good guidance to improve the operational availability and reliability of MSHPS application.

Experimental investigation on the air-side flow and heat transfer characteristics of 3-D finned tube bundle

The heat exchanger is widely applied to many thermal systems, and its structure significantly affects the flow and heat transfer characteristics. 3-D finned tube is a new type of carbon steel heat exchanger element that is low-fin-tube geometry with integral fins, which has larger heat transfer area and higher heat exchange efficiency. The current researches on the 3-D finned tube mainly focus on the flow and heat transfer performance of single tube by changing the structure parameters of fin or the condition of working medium. However, a reasonable tube bundle arrangement must be considered to obtain a compact structure in the engineering applications. So in the study, experiments are performed to investigate the air side flow and heat transfer characteristics of 3-D finned tube bundle with different geometries. The effects of varied transversal tube pitches, longitudinal tube pitches, and fin heights on Nu, Eu, and j/f are presented and discussed. The predictive correlations of 3-D finned tube bundle for Nu and Eu are proposed based on the presented data, and they can provide a theoretical reference for the industrial applications of 3-D finned tube. Finally, the flow and heat transfer characteristics of 3-D finned tube bundle are contrasted with those of other types of finned tube bundle from extant studies, and the results show that the 3-D finned tube bundle heat exchanger has higher air-side heat transfer performance and lower pressure drop.

Experimental Study on Air-side Heat Transfer Performance by Pitch of Wave Heat Exchanger

In this study, the performance of a small - sized wave heat exchanger to be applied to the white smoke reduction system was experimentally confirmed. The heat transfer rate, drain and pressure drop were measured according to the air flow rate, water flow rate and relative humidity change of the wave heat exchanger for two kinds of pitch numbers. A constant temperature and humidity calorimeter and a constant temperature water bath were used to measure the performance of the wave heat exchanger. The heat transfer rate and drain increased gradually with changes of water flow rate. Case 2 showed more than 50% higher heat transfer rate and drain than Case 1. The increase of air heat transfer rate and drain according to air flow rate was greatly increased when the number of pitches was the same or increased, unlike the result of water flow rate change. In the temperature visualization using a thermal imaging camera, it can be seen that as the water flow rate and the number of pitches increase, the heat transfer becomes more effective in Case 2.

Numerical study on airside thermal-hydraulic performance of rectangular finned elliptical tube heat exchanger with large row number in turbulent flow regime

The objective of this study is to investigate the air side thermal–hydraulic characteristics of rectangular finned elliptical tube heat exchangers (RFETHXs) using 3D numerical simulations based on the validated standard k-ε turbulence model. First, fin efficiency of RFETHXs is investigated. The effects of various parameters such as row number, transverse tube pitch, longitudinal tube pitch, fin pitch, fin thickness and Re have been studied. Then the influences of structural parameters on the thermal-hydraulic performance of RFETHXs are examined, not only the characteristics including the effect of fin thermal resistance but also the pure convective performance excluded this effect. The contribution ratios of each structural parameter on fin efficiency and air side thermal-hydraulic performance are given. It is observed that, the most important structure factors for fin efficiency are fin thickness and fin pitch. For the RFETHXs with fully developed air side flow and heat transfer (N>5), from the third row to the last second row, the average fin surface Nu of each row is almost stable, and the value of the second row is 9% higher than the stable value. The results show that the air side heat transfer performance and friction factor of RFETHXs is mainly determined by the transverse tube pitch. For j/f1/2 at fixed Re, the contribution ratio of transverse tube pitch is 32% while the value is 24% at fixed frontal velocity.Moreover, two sets of multiple correlations, both including and excluding fin thermal resistance, are proposed to describe the air side heat transfer and friction characteristics of RFETHXs in turbulent flow regime.

Design and numerical parametric study of a compact air-cooled heat exchanger

Air-cooled heat exchangers are a main component in air-conditioning and heat pump systems and, therefore, a major research focus. Such heat exchangers, mainly of fin-and-tube and microchannel types, use fins to augment the airside heat transfer area. Recently, it has been shown that finless designs, referring to heat exchangers using < = 2 mm hydraulic diameter bare tube bundles, can deliver better airside heat transfer performance than conventional heat exchangers. In the current study, a novel air-cooled heat exchanger consisting of bifurcated bare tubes with two different tube diameters is proposed. The tubes in the first level split into two or more tubes at certain angles. The secondary tubes then merge back to the first level tube again. The heat transfer and pressure drop characteristics are numerically analyzed using a three-dimensional model and compared with baseline bare tube heat exchangers. The novel bifurcated bare tube heat exchanger was found to have 15% higher airside heat transfer coefficient and 4%∼12% lower airside pressure drop than baseline bare tube heat exchanger with the same diameter (0.8 mm), frontal area, volume, and air velocity (3.5∼5 m/s).

Experimental study on the effects of fin pitches and tube diameters on the heat transfer and fluid flow characteristics of a fin punched with curved delta-winglet vortex generators

The effects of fin pitches and tube diameters on the air side heat transfer performance of a tube bank fin heat exchanger using curved delta-winglet vortex generators (CDVGs) were experimentally investigated, and the counterpart plain fin was selected as a reference for optimizations of fin geometries. Totally 18 samples with three different fin pitches and three different tube diameters were studied for the fin with/without CDVGs. Based on the experimental data and their corresponding fitting formula, the effects of the fin pitches and tube diameters on heat transfer performance of the fin with or without CDVGs were discussed, and the correlations of air side Nusselt number and friction factor with Reynolds numbers, fin pitches and tube diameters were obtained. The results indicate that the fin patterns with CDVGs could effectively enhance heat transfer performance compared with the counterpart plain fin under identical pump power constraint, and their heat transfer enhancement performances are dependent on the flow condition and the fin’s geometry parameters. In the range of studied geometry parameters and Reynolds numbers, the best performance could be achieved with the combination of the largest fin pitch and the smallest tube diameter, and the optimum parameters are less influenced by the Reynolds number.

A numerical study on the air-side heat transfer of perforated finned-tube heat exchangers with large fin pitches

For a finned-tube heat exchangers (FHEXs) with large fin pitches, the enhancement of air-side heat transfer performance by using perforated fins has been numerically investigated in this paper. The effects of the perforation size and number on the air-side j factor and heat transfer rates of the FHEX are analyzed in detail at different large fin pitches. Numerical simulation results indicate that an optimal perforation design can be obtained to realize maximum increase in the j factor for the perforated FHEX compared with those of the plate FHEX without perforations. It is also found that the enhancement of the j factor increases with the rising air-side Reynolds number from 750 to 2350. For the perforated FHEX with fin pitch of 10.0mm, the j factor increases by 0.3% at Re=750 and 8.1% at Re=2350, respectively, with the optimal perforation design. In addition, the results present that the j factor increase of the perforated FHEX compared with that of the plain FHEX is more obvious at smaller fin pitches. When the fin pitch varies from 20.0mm to 7.5mm, the increase in the j factor varies from 2.7% to 9.2% at Re=2350. However, the total heat transfer surfaces of the perforated FHEX are reduced by perforations, its heat transfer rates may be decreased. The results show that the air-side heat transfer rate is reduced by 6.3% at Re=750 when the fin pitch is 7.5mm. For this case, two methods are further proposed to compensate total surfaces for the perforated FHEX in order to obtain higher air-side j factor while ensuring identical heat transfer rate.

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.

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.

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.

The Influence of the Inlet Conditions on the Airside Heat Transfer Performance of Plain Finned Evaporator

The Influence of the Inlet Conditions on the Airside Heat Transfer Performance of Plain Finned Evaporator

Performance of Aluminum and Carbon Foams for Air Side Heat Transfer Augmentation

The air side heat transfer performance of three aluminum foam samples and three modified carbon foam samples are examined for comparison with multilouvered fins often found in compact heat exchangers. The aluminum foam samples have a bulk density of 216 kg/m3 with pore sizes of 0.5, 1, and 2 mm. The modified carbon foam samples have bulk densities of 284, 317, and 400 kg/m3 and machined flow passages of 3.2 mm in diameter. The samples were placed in a forced convection arrangement using a foil heater as the heat source and ambient air as the sink. A constant heat flux of 9.77 kW/m2 is applied throughout the experiments with the mean air velocity ranging from 1 to 6 m/s as the control parameter. The steady volume-averaged momentum equation and a two-equation nonequilibrium heat transfer model are employed to extract the volumetric heat transfer coefficients. Pressure drop measurements are correlated with the Darcy–Forcheimer relation. Empirical heat transfer correlations for the aluminum and carbon foam samples are provided. Using a hypothetical heat exchanger considering only the thermal resistance between the ambient air and the outer tube wall, the air side performance for each sample is modeled based on the local heat transfer coefficients and friction factors obtained from experiments. The performance of each sample is evaluated based on a coefficient of performance (COP, defined as the ratio of the total heat removed to the electrical input of the blower), compactness factor (CF, defined as the total heat removed per unit volume), and power density (PD, defined as the total heat removed per unit mass). Results show the carbon foam samples provide significant improvement in CF but the COP and PD are considerably lower than that for comparable multilouvered fin heat exchangers.