Air-side Pressure Drop Research Articles

CFD studies on different configurations of drying chamber for thin-layer drying of leaves

ABSTRACT The design of a single air pass drying chamber used in tray dryers for thin-layer solar drying of fragile, heat-sensitive food materials such as edible leaves is a challenging task. It is essential to obtain good air distribution in all drying chamber designs to ensure uniform drying of the product. Simultaneous achievement of once-through air-side pressure drop and increase the moisture mass transfer rates are also important objectives of any heat and mass exchanger design. For obtaining these objectives in the dryer design, Computational fluid dynamics (CFD) is used as a tool for designing and theoretically testing different suitable air flow drying chamber configurations in the tray dryer. In this investigative research work, four different optimized geometries of the drying chamber for drying leaves are designed and evaluated theoretically from predicted performance by using ANSYS FLUENT software. The limiting air-side pressure drops and achieved dried output from various configurations are checked, assessed and presented in this work. The best configuration and construction detail are also highlighted. This generalized, proven, CFD procedure can be used for checking the performance of any similar transit type inter mass exchanger, to predict the best configuration in terms of mass transfer and pressure drop.

Experimental study on the air-side flow resistance of different water collecting devices for wet cooling tower applications

This paper experimentally studies the air-side flow resistance of three different water collecting devices (WCDs) being U-type, V-type and rectangle-type. A wind tunnel was designed to simulate the airflow across the WCDs within air speeds of 0.5–3.0 m/s. The effect of dry/wet conditions on the air-side flow resistance was also investigated. The results show that the air-side flow resistance of the rectangle-type WCD is the highest among all while those of U-type and V-type are more or less identical. Therefore, the U-type WCD is recommended for industrial application as it can collect more water with low air-side flow resistance. Meanwhile, wet condition produces higher air-side flow resistance when compared with dry condition for all of the studied WCDs. Finally, a correlation is developed to express air-side pressure drop as a function of air speed, air density, water-to-air volumetric flow rate ratio and non-dimensional shape parameter which has been introduced for predicting the air-side flow resistance across WCDs.

Parametric study on heat transfer and pressure drop of twisted oval tube bundle with in line layout

A numerical research is conducted to investigate air side heat transfer and pressure drop performance of twisted oval tube bundles with in line layout in cross flow. The effects of five geometric parameters (the ratio of outer major axis to outer minor axis (A/B), twist pitch length (S), transverse tube pitch (St), longitudinal tube pitch (Sl) and number of longitudinal tube rows (Z)) on the air side heat transfer and pressure drop performance are examined in the range of Reynolds number (Re) from 500 to 23,000. The velocity field and temperature field are repeated periodically every S/2 along the length of the twisted oval tube, which reveals that the twisted oval tube is a streamlined tube that is independent of the angle of attack. Twisted oval tube bundles perform excellent heat transfer performance compared with other tube bundles (circular tube bundle with the same tube layout as the twisted oval tube bundle, elliptic tube bundle with staggered manner, spirally corrugated tube bundle with in line layout) in cross flow. And the staggered tube layout is beneficial to enhance heat transfer performance compared with in line tube layout. Correlations for Nusselt number (Nu) and Euler number (Eu) from this paper are presented.

Wind tunnel test on the flow resistance of U-type water collecting devices for natural draft wet cooling towers

Most thermal power plants reject waste heat by conventional wet cooling towers. As is different from conventional wet cooling towers, high-level water collecting cooling towers (HWCCTs) replace the rain zone of conventional towers by water collecting devices (WCDs). For a HWCCT, the WCDs affect its cooling performance to a large extent. The WCDs affect the HWCCT performance mainly through their air-side flow resistance. This paper is therefore to investigate the air-side flow resistance across different designs of WCDs. A wind tunnel was designed to simulate the airflow across the WCDs. Three slab angles and three slab spaces were tested at air speeds within the range of 0.5–3 m/s. The effects of wet/dry conditions on the flow resistance was investigated as well. Tests find that the air-side pressure drop across WCDs increases with the decrease in slab angles, and the same trend happens at slab spaces. Generally, the average pressure drop value at wet condition is 86% higher than that at dry condition within tested air speeds. Finally, a non-dimensional correlation for predicting the air-side pressure drop of WCDs is developed based on the test data.

Air side pressure drop in plate finned tube heat exchangers

Previous researches have proved that newly introduced parameters such as void velocity and others can be successfully used for correlating the heat transfer and pressure drop parameters. In order to establish a reliable procedure for estimation of air pressure drop, experiments on plate finned tube heat exchangers have been conducted, as well as the extensive investigation of the open literature. Using own and previously published experimental data, new correlation for estimation of air pressure drop have been established, covering a wide range of parameters of industrial importance. New correlation is comprehensive and statistically superior to any other pressure drop correlation.

Numerical investigation of thermal and hydraulic performances of a condenser coil with oblique-shaped tubes

High efficiency condenser coil providing higher heat removal capacity with lower air-side pressure drop is desirable. However, coil designs with different fin patterns for higher heat transfer incur large air-side pressure drop and thus consume more energy. In this study, the thermal and hydraulic performances of the condenser coil design with plain fin, corrugated fin, and novel oblique-shaped tube are evaluated numerically with a 3-fin and 3-tube model in comparison with a conventional circular tube coil. This novel oblique-shaped tube design features smaller air recirculation zone behind the tube so as to increase effective heat transfer area and to lower air-side pressure drop. The numerical results show that, for the plain fin configuration, the shape of the oblique tube is important in reducing the recirculation zone on the fin, thus increasing effective heat transfer area and heat transfer amount up to 14% and reducing the air-side pressure drop up to 14% at the same inlet air velocity as compared with the circular tube coil. With corrugated fin guiding and promoting air speed, causing more flow separation zones, it is detrimental to the thermal-hydraulic performance of the oblique tube coil. The discussion of choice of materials for tube and fin, such as copper, brass, and aluminium, is provided with possible manufacturing processes required for realizing potential high performance coil design. Parametric optimisation on the best performing plain fin oblique tube design shows that, for the design of condenser coil with oblique tube, there is no optimal FPI, but it is recommended to keep the ratio of tube pitch to tube frontal length/diameter to about 3 to 3.5.

Heat transfer enhancement in a parallel, finless heat exchanger using a longitudinal vortex generator, Part B: Experimental investigation on the performance of finless and fin-tube heat exchangers

The air-side pressure drops and overall heat transfer coefficients of a finless heat exchanger with and without a vortex generator, as well as those of five all-aluminum parallel multi-port heat exchangers, were experimentally measured and compared under dry, wet, and frosting/defrosting conditions. The multi-port heat exchangers included three exchangers with louvered fins featuring fin pitches of 1.2 mm, 1.4 mm, and 1.6 mm, and two exchangers featuring slit fins with pitches of 1.2 mm and 1.4 mm. For the fin-tube heat exchangers, the effects of fin pitch and type on the air-side pressure drop and overall heat transfer performance were discussed. Under the dry condition, the heat transfer performance of a finless heat exchanger with a longitudinal vortex generator (LVG) is still approximately 40% less than that of a fin-tube heat exchanger. However, the pressure drop is similar. Under wet conditions, due to the excellent drainage performance of the finless heat exchanger, the heat transfer coefficient reached the same level. However, the pressure drop is lower. Under the frosting/defrosting condition, the heat transfer coefficient is similar, but the pressure drop is much lower. It only reaches 50 Pa at the end of each frosting period. Overall, the performance of a finless heat exchanger with a vortex generator is still lower than that of a fin-tube heat exchanger in dry conditions, but it demonstrates superiority in wet and frosting/defrosting conditions.

Experimental study on heat transfer and pressure drop of twisted oval tube bundle in cross flow

An experimental research on air side heat transfer and pressure drop performance of twisted oval tube bundle with staggered layout in cross flow is conducted. The comprehensive performance of present twisted oval tube bundle is larger than that of round tube bundle with the same layout by 25.5–33.3% during the experimental range of Remax (7500–18,000). The economic operation Remax among the experimental range is from 7500 to 13,000 for present twist oval tube bundle in cross flow. The correlations for Nusselt number and Euler number deduced from this paper are well compatible with experimental data, which could provide a theoretical reference of twisted oval tube bundle for industrial applications.

MODELLING OF A DELUGEABLE FLAT BARE TUBE BUNDLE FOR AN AIR-COOLED STEAM CONDENSER

In this paper, a one dimensional model was developed for the analytical evaluation of the thermal performance of adelugeable flat tube bundle to be incorporated in the second stage of an induced draft hybrid (dry/wet) dephlegmator(HDWD) of a direct air-cooled steam condenser (ACSC). A one-dimensional model is analysed by using three methodsof analysis which are: Poppe, Merkel, and heat and mass transfer analogy. The model’s accuracy was validated througha comparison of solutions obtained from the above-mentioned methods of analysis. Satisfactory correlations between theresults were reached. However, heat transfer rate attained by Poppe method is higher by 2.89% and 9.87% than thatobtained by Merkel, and heat and mass analogy methods, respectively. The difference in air-side pressure drop obtainedby all the methods found to be insignificant. Furthermore, the best configuration of the flat tube bundle for the secondstage of induced draft HDWD was identified through the comparison of its performance to the round tube bundle. Theperformance of the round tube bundle is found to be around 2 and 1.5 times of that of flat tube bundle, when both bundles operate in wet and dry operating modes respectively.

Airside thermal-hydraulic characteristics for tube bank heat exchangers used to cool compressor bleed air in an aero engine

A numerical study has been carried out to investigate the airside heat transfer and pressure drop characteristics of bare tube bank and plain finned tube heat exchangers used to cool the compressor bleed air in an aero turbine engine. The exchangers use small diameter tubes (3.0 mm) with compact staggered tube layout and operate in high temperature circumstances with large temperature differences. Calculations are performed using the Realizable k-ɛ turbulence model with consideration of the air property variations caused by the air temperature and pressure changes. Calculations are implemented for 5–20 m/s frontal air velocities, 561–690 K and 0.5–1.0 MPa frontal air temperatures and pressures, which correspond to 15–30 compressor compression ratios, and 298 and 350 K tube wall temperatures. The airside thermal-hydraulic performance is evaluated using the heat transfer coefficient and pressure drop as well as their dimensionless forms - j and f factors. The results suggest that the heat transfer coefficient and pressure drop are strongly affected by the frontal air state whereas the j and f factors are less dependent on it. The effect of air property variability on j factor is limited while that on f factor is more notable.

Development and validation of a CO2 gas cooler moving-boundary model.

This paper presents the development of a CO2 gas cooler model using the moving-boundary method. The model aims to separate the gas cooler into two regions: supercritical gas and supercritical liquid by means of the CoolProp thermophysical property library (Bell et al., 2014). The model uses the latest correlations for refrigerant and air-side heat transfer coefficients and pressure drops. The experimental results from Ge and Cropper (2009) are used for model validation. A total of at 36 operating conditions have been reported. The model predicted the gas cooler heating capacity within an MAE of ±4.7% and refrigerant side outlet temperature within ±3 K. The present moving-boundary model also showed an improved agreement compared to the segment-by-segment model proposed by Ge and Cropper (2009).

Influence of wet-particle deposition on air-side heat transfer and pressure drop of fin-and-tube heat exchangers

Wet particles have higher propensity to deposit on fin surfaces than dry particles, and may cause a more serious deterioration of the air-side performance of fin-and-tube heat exchanger. The purpose of this paper is to experimentally investigate the influence of deposited wet particles on the air-side heat transfer and pressure drop of heat exchangers. Test samples cover three types of fins, i.e., plate fin, wavy fin and louver fin. Experimental conditions cover the inlet air relative humidity of 50–90%, the inlet air velocity of 1.0–3.0 m s−1, and the particle concentration of 2.1–10.8 g m−3. The results show that wet particles mostly deposit on the leading edge of fins as well as the front part of tubes, and the louver fin is almost completely blocked by the wet particulate fouling. Moreover, the increase of inlet air relative humidity, particle concentration and the decrease of fin pitch reduce the air-side heat transfer and increase the air-side pressure drop after wet particle deposition; the increase of inlet air velocity firstly deteriorates and then enhances the air-side performance; and louver fin is the most conducive to the deterioration of air-side performance.

Design and evaluation of an additively manufactured aircraft heat exchanger

Additive manufacturing (AM) technology has significant potential to improve heat exchanger (HX) performance through incorporation of novel geometries and materials, but there is limited understanding of AM HX functionality relative to conventionally manufactured components. This study compares the performance of conventionally-built plate-fin air–liquid crossflow heat exchangers (i.e., aircraft oil coolers) to additively manufactured heat exchangers of similar geometry. To replicate internal features, three dimensional X-ray computed tomography scans were performed on the conventionally-built heat exchanger. A baseline AM model of the conventional design was designed, as well as an AM model with additional enhancement features on the air side. The two AM heat exchanger geometries were constructed using a laser-based powder bed fusion process with AlSi10Mg aluminum-alloy powder. Visual inspection of the as-built AM HX indicated significant surface roughness and some cracks in the fin-tube joint, but only at the edges of the heat exchanger. Overall heat transfer was increased by about 10 percent for the baseline AM and by 14 percent for the enhanced AM heat exchanger when compared to the conventionally built baseline heat exchanger. Measured air-side pressure drop for the AM heat exchangers was double that of the conventionally built baseline heat exchanger. Overall, this study indicates potential for improved heat transfer and demonstrated functionality of AM HX in realistic applications.

Air side heat transfer and pressure drop of H type fin and tube bundles with in line layouts

A set of experiments to research the heat transfer and pressure drop performance of H type fin and tube bundles with in line layouts are conducted. The effects of different geometry parameters including fin width, fin height, fin pitch, transverse tube pitch, longitudinal tube pitch and Reynolds number on heat transfer and pressure drop characteristics are discussed. The correlations of Nusselt number and Euler number deduced by multiple regressions from this paper are well compatible with experimental data, which could provide a theoretical reference of H type finned tubes for industrial applications related to air or gas heat exchangers.

Numerical investigation of the effects of fin shape, antifreeze and nanoparticles on the performance of compact finned-tube heat exchangers for automobile radiator

In this paper, heat transfer and pressure drop of air in the radiator of an internal combustion engine automobile were investigated. First, three types of fins including louvered, triangular vortex generator and rectangular vortex generator were modeled and their performance were compared with a plain fin. Effects of adding antifreeze in volume ratios of 40, 50 and 60% on the performance of louvered and rectangular vortex generator fins were investigated. Finally, the effects of adding copper oxide and aluminum oxide nanoparticles on the heat transfer improvement of louvered and rectangular vortex generator fins were simulated. The results demonstrated that louvered fin had the highest heat transfer rate compared to the other fins and improved it up to 24.6% in comparison with plain fin; however, its air side pressure drop increased 67.7%. Adding antifreeze resulted in a decrease in heat transfer rate. For example, heat transfer rate decreased for louvered and rectangular vortex generator fins at 60% vol. antifreeze, 12.9 and 11.8% compared to pure water, respectively. Also, adding copper oxide and aluminum oxide nanoparticles improved heat transfer rate and compensated harmful effects of antifreeze and it increased heat transfer rate in comparison with pure water.

Analytical Performance Analysis of Cross Flow Louvered Fin Automobile Radiator

The objective of the present paper is to propose an analytical model for calculating performance parameter of a radiator having rectangular tube with louvered fins. The theoretical effectiveness, heat transfer rate, outlet temperatures of both air and coolant are determined using effectiveness-NTU method. The coolant and air side pressure drop is also calculated. The proposed procedure is validated with experimetal results available in the literature and the GT model. It is found that the maximum deviation in the heat transfer rate calculated from proposed model is 10.97%, the coolant and air outlet temperatures is 2.75% and variation in pressure drop is about 3.29%.

Airside heat transfer and pressure drop of louver-finned parallel flow heat exchanger having a drainage channel

Airside heat transfer and pressure drop of louver-finned parallel flow heat exchanger having a drainage channel

Field measurement and modeling of UVC cooling coil irradiation for heating, ventilating, and air conditioning energy use reduction (RP-1738)—Part 1: Field measurements

Wet cooling coil surfaces can provide opportunities for microorganism growth. This biological fouling (biofouling) increases airside pressure drop and decreases airside heat transfer coefficient. Ultraviolet germicidal irradiation is one method to eliminate biofouling. The current article reports field measurements of changes in pressure drop and heat-transfer characteristics of fouled coils treated with ultraviolet germicidal irradiation designed for surface disinfection. Ultraviolet germicidal irradiation systems were installed in operating air-handling units with visibly fouled cooling coils at sites in Tampa, Florida and University Park, Pennsylvania. Pressure drop data were controlled for airflow and latent load, and overall heat-transfer coefficient data were controlled for heat exchanger entering conditions. Analysis of Tampa site data showed an initial mean 22.70% to 22.72% decrease (µ ± 2σ) in airside pressure drop and a mean 13.2% to 13.6% (µ ± 2σ) increase in overall heat-transfer coefficient. Longer-term improvement was 14.6% for the airside pressure drop and 15.7% for overall heat-transfer coefficient, though the decrease in pressure drop improvement was not able to be explained. The University Park, Pennsylvania site, cleaned by unaware maintenance personnel before data collection, had an initial mean 10.3% pressure drop improvement and 57.7% overall heat-transfer coefficient improvement, followed by a long-term 0% pressure drop improvement and 36.7% overall heat-transfer coefficient improvement.

System-level analysis of a novel air-cooled condenser using spray freezing of phase change materials

A comprehensive system-level analysis is performed for a novel air-cooled condenser based on spray freezing of phase change materials (PCMs). This novel air-cooled condenser uses PCMs to decouple the process of steam condensation and heat rejection to air in order to significantly improve air-side heat transfer and reduce steam condensation temperature as compared to conventional air-cooled condensers (ACCs). Melting of solid PCM particles in a two-phase PCM slurry flow anchors the steam condensation temperature close to the PCM melting point regardless of the change in ambient air temperature. Spray freezing of millimeter-sized liquid PCM droplets increases the air-side heat transfer coefficient by five times compared to the finned-tubed ACCs. A multiscale model, which directly captures the melting and settling of PCM particles at the microscopic level and accounts for phase change through energy source terms at the macroscopic level, has been developed to simulate the PCM slurry flow over heated tube bundles. Using this multiscale model, the effects of particle volume fraction, Reynolds number, and particle to steam tube diameter ratio on the averaged wall Nusselt number of the steam tubes are investigated. It is found that the averaged wall Nusselt number for a PCM slurry flow of 20% solid fraction achieves a 38% enhancement over the PCM single-phase flow of same Reynolds number. On the air side, the freezing/melting of PCM droplet/particle is approximated based on a 1-D transient heat conduction model and the air-side pressure drop across the PCM droplet array is determined using a 3-D k-ε turbulence model. The performance of this spray-freezing PCM ACC is compared against a baseline ACC of a 500 MWe power plant. It is found that, for comparable footprint area and ambient conditions, the spray-freezing PCM ACC reduces the initial temperature difference to as low as 16.8 °C and provides up to 10.8 MW net power production gain compared to the baseline ACC.

Thermal performance of peripheral-finned tube evaporators under frosting

This study presents an experimental and theoretical evaluation of frost formation on the moist air side of a peripheral finned-tube (PFT) heat exchanger. Previous studies of this compact geometry correlated the friction and heat transfer parameters above the dew-point temperature of the air (no frost or condensate formation). Here, for the first time, the thermal-hydraulic performance of a PFT heat exchanger is analyzed under frosting. A PFT heat exchanger prototype was evaluated experimentally in a closed-loop wind tunnel calorimeter to determine the influence of the tube wall temperature, air velocity and psychrometric properties (temperature and relative humidity) on the heat transfer rate, air-side pressure drop and frost buildup on the surface. The thermal and hydrodynamic behavior of the enhanced air-side surface was analyzed using a distributed heat exchanger model in which mass, momentum and energy balances are applied to one-dimensional control volumes in the air flow direction. The model treats the air flow path as a porous medium in which the porosity, equivalent particle diameter and thermal properties vary as a function of time due to the frost accumulation. Good agreements (within 20% average error) between the model predictions and the experiments for the air-side pressure drop and heat transfer rate have been found. The enthalpy effectiveness was found to drop from 0.68 for dry to 0.50 under severe frosting, which suggests that the PFT heat exchanger continues to be effective under frost.