Air-side Flow Research Articles

Optimization of exergy destruction minimization in moving bed heat exchanger with airflow-assisted cooling

Introducing cooling airflow is an effective method for achieving the localized cooling rate required during waste heat recovery of blast furnace slag. However, the associated parasitic loads must be considered. This study investigates moving bed heat exchanger with airflow-assisted cooling from an exergy perspective. The effects of the granular side Peclet number, airflow side Reynolds number, and the height ratio of the airflow section on heat loss, exergy destruction, and its distributions were examined experimentally. The results show that the trends of temperature and pressure exergy destruction with each parameter tend to be opposite. Therefore, optimizing the parameters requires a trade-off between different types of destruction. Additionally, reducing exergy destruction while meeting the cooling rate requirement was considered. The optimal parameter set was determined by taking the cooling rate requirement as a constraint and minimizing the dimensionless temperature and pressure exergy destruction to obtain the Pareto front.

FLOW AND HEAT-TRANSFER CHARACTERISTICS IN SMALL-DIAMETER TUBE BUNDLES WITH A STAGGERED LAYOUT: AN EXPERIMENTAL STUDY

While past research has primarily focused on heat transfer in larger diameter tubes, the capabilities of small-diameter tubes have often been estimated using correction factors. However, with the evolution of compact heat exchangers, conducting dedicated studies on small-diameter tube bundles has become increasingly crucial to achieve more precise conclusions. An experimental research on flow and heat-transfer characteristics of staggered tube bundles with different tube diameters (2, 3, and 5 mm) is conducted. In the experiment, the number of rows (4-12), the mass rate of the air (0.06-0.18 kg/s), and the transverse tube pitch (<i>S</i><sub>1</sub>/<i>d</i> = 2, <i>S</i><sub>1</sub>/<i>d</i> = 3) are variables to study the characteristics of the airside flow resistance and heat transfer. The three main conclusions of the experimental results are as follows: (1) Under the same conditions, the smaller tube diameter leads to the larger airside convective heat-transfer coefficient. Besides, the deviation between the Nusselt number of the experiment and the empirical correlation of Žukauskas is in the range between -14 and -10%; (2) The effect of transverse distance on heat transfer is not obvious, but the convective heat-transfer coefficient increases significantly with the increase of row number; (3) The external pressure drop of the tube exhibits an exponential increase with the air-flow rate. Particularly in the experimental samples with smaller diameters, the outflow resistance of the tube is noticeably higher compared to other tubes. Finally, new empirical correlations of the airside convection heat transfer for the small-diameter staggered tube bundles are fitted according to the experimental data, and it is hoped to provide a reference for the more accurate design of tube-bundle heat exchangers.

Исследование динамической устойчивости модели крыла в потоке воздуха

Investigating the aircraft structure elements for compliance with dynamic stability is one of the most important ways to study their dynamic properties. The paper considers possibility of the wing model dynamic loss of stability (flutter type) in the form of a flat rod of variable geometry close to the real wing geometry in the airflow. Internal force factors manifested under the frontal aerodynamic force influence, as well as increment in the lift and aerodynamic moment arising during possible small model oscillations are taken into account from the oncoming airflow side. Due to these external forces increment, the so-called energy pumping from the airflow side occurs, and the problem becomes non-conservative. Behavior of the first three complex eigenvalues of small oscillations was numerically studied, and the critical flow velocities, at which the model could lose dynamic stability, were determined for various parameters of the external aerodynamic forces.

Experimental investigating the thermal and hydraulic performance of heat exchangers with helix wire finned tube

In this study, the helix wire finned tube is introduced as a novel design to increase the performance, reduce weight, and manufacturing price of the heat exchangers. The helix wire fin can make turbulence in the fin-side flow, creates a larger contact surface, more flow trapping and increment in the residence time for enhancement of the heat transfer. The effect of Reynolds number inside and outside of the finned tube and inter-tube distance on the thermo-hydraulic performance of the heat exchanger has been investigated experimentally. The results show that increasing the Reynolds outside the finned tube causes an 80% enhancement in the performance evaluation criterion of the heat exchanger. Also, helix wire finned tubes provide a maximum of 65% and 57% better thermal performance compared to spiral and serrated fins. Increasing the Reynolds number of the outside and inside tube flow causes a 40% and 50% enhancement in the Nusselt number of the fin-side flow, respectively. Moreover, increasing the longitudinal distance between the tube rows reduces the Nusselt number and friction factor of the fin-side flow by 20% and 45%, respectively. Finally, an experimental correlation is derived to calculate the Nusselt number of the air-side flow.

Модальный анализ модели крыла в потоке воздуха в консервативной постановке

Modal analysis of the elastic system dynamic properties is one of the most important types of research. The paper investigates eigen frequencies and oscillation modes of a wing model in the form of a flat rod of variable geometry, close to the real wing geometry in the air flow with zero angle of attack. Influence of the oncoming airflow is considered due to the internal force factors appearing under the frontal aerodynamic force action from the flow side. Aerodynamic forces and moments increment arising from the model small oscillations are not taken into account, i.e. the energy is not being “pumpe” at small oscillations from the airflow side, and the problem is solved in the conservative formulation. First three frequencies of natural oscillations and their corresponding forms are numerically determined; it is shown that all of them are purely bending. Dependence of the first three frequencies behavior on the oncoming flow speed was studied.

Numerical thermal performance analysis of a PCM-to-air and liquid heat exchanger implementing latent heat thermal energy storage

Due to the mismatches in energy supply and demand in thermal systems, employing latent heat thermal energy storage using phase change materials (PCMs) is a reliable and effective solution. In this regard, this paper introduces an innovative PCM-to-air and liquid heat exchanger to increase thermal system performance by providing a hybrid heat source to the airside. A novel numerical work based on multiphysics coupling of heat transfer and double fluid flow and phase change within a complex geometry is represented. Numerical heat transfer analysis is performed on the model based on three-dimensional computational fluid dynamics simulation. In order to make a thorough thermal performance assessment, the dynamic behaviour of the system is investigated for both PCM charging and discharging processes. Furthermore, the effect of airside flow variation on the thermal response of the system is studied, and the results are discussed based on fluids temperature, heat transfer rate, and the PCM phase transition procedure. It is demonstrated that the PCM can store excess heat from the working fluid during the charging process and releases it to the airside during the discharging process. The heating load of 323 kJ is stored during the charging process which has enabled the PCM to provide up to 6 min of extra airside heating time during discharging. The share of PCM latent component of the airside heat transfer is determined to be around 48 %. It is also observed that by increasing the airflow rate, the discharged heat load is decreased slightly, and the heating time is reduced notably. The presented thermal energy storage system offers a unique solution for the start-stop function implemented in many hybrid and electric vehicles. During short periods of engine shutdown, the system could provide passenger thermal comfort and enable effective energy savings.

Using inverting CO critical value to predict coal spontaneous combustion severity in mine gobs with considering air leakages – A case study

The mined-out space in underground coal mines increases due to the mining face advances. The airflow continuously could enter the mine gob via the air leakage channels, which may cause the coal in gob spontaneously ignite and combust. The gob is not accessible for personal direct gas composition measurement due to rock collapses and blockages. Thus, it is difficult to monitor its spontaneous combustion condition timely, which leads to the blindness of developing spontaneous combustion prevention measures. Therefore, it is important to understand the status of spontaneous combustion of residual coal in the gob in time. To more accurately predict the coal spontaneous combustion in gobs, this paper proposes a new model to predict the burning state of coal in the gob by using the gas-CO generated by coal spontaneous combustion with considering the action of air leakages in the gob. Then the reflected CO critical value in the corner of the longwall face in return airflow side can be derived. An example application is also presented in this paper, which shows that the proposed method could better reflect the spontaneous combustion state of gob, and the derived critical value can be used as an early warning message to prevent the coal spontaneous ignition in the gob and reduce the probability of coal spontaneous combustion accidents.

BIM Building HVAC Energy Saving Technology Based on Fractional Differential Equation

Abstract This article uses BIM technology to simulate HVAC in a commercial building. And use fractional differential equations to verify the set temperature, humidity, and other related parameters. The experiment takes an air-conditioned room model as the research object and uses the thermodynamic characteristics to establish a dynamic mathematical model of differential equations. Research shows that the multiple electric heat pump equipment aggregation groups discussed in this article are a good resource for user-side demand response. The air-side flow rate disturbance of the building HVAC has the greatest impact on the heat transfer of the surface cooler. As the air velocity increases, the rate of increase in heat exchange will decrease.

Air-side flow and heat transfer characteristics research and empirical correlations of wavy fins in negative gauge pressure environment

ABSTRACT When the vehicle cooling system works in negative gauge pressure environment of plateau, it will face the problem of reduced heat transfer performance, which will lead to an increase in vehicle energy consumption. So, there’s important meaning in studying the thermal-hydraulic performance of heat exchangers and optimizing the design of which for negative gauge pressure environments. In this article, experimental study of wavy fin heat exchanger is carried out in negative gauge pressure environment, and then the numerical simulation model is established. The effects of fin size parameters (fin pitch, fin height and fin amplitude) on the temperature field and flow field distribution of wavy fins are studied. Empirical correlations of Colburn j factor and Friction factor f are obtained according to 336 groups of numerical simulation results. The heat transfer coefficient of air side at −44kPa negative pressure is decreased by 34% on average compared with atmospheric pressure, and the friction factor f is increased by 61.5%. The average deviations of correlations for j and f are 2.3% and 2.7%, respectively. The results could be used in engineering practice of design and optimization of wavy fin heat exchangers for vehicle cooling systems operate in plateau environment.

Parametric analysis of artificial rib roughness for the enhancement of thermohydraulic performance of solar air heater: A review

Solar air heaters (SAHs) convert incident solar radiation to thermal energy on an absorber plate. Smooth absorber plates have a low convective heat transfer rate in traditional ducts. Artificial rib roughness is applied to the bottom sides of the absorber plate in order to improve heat transfer in SAHs. However, when the rib roughness is employed over the absorber plate of the airflow side, it also increases the pressure loss due to friction. The purpose of this work is to provide researchers with a scientific perspective on the geometry, structure of ribs, flow directions, and laminar zone in order to maximize heat transfer and reduce friction. This review article discusses different roughness parameters, various artificial roughness geometries, and thermohydraulic performance. Nusselt number and friction factor correlations have also been incorporated in this review article which is developed by various scientists based on their respective experimental results for various roughness parameters. The paper also provides a thorough analysis of current research, summarises prior developments, and speculates on potential future routes for developing new heat transfer systems. Furthermore, this review paper will assist researchers and academics in developing a sound understanding of the effects of roughness on the absorber plate.

Forced Convection Heat Transfer for Air Flow Across Fat Tubes Heat Exchanger (Dept.M)

Forced convection heat transfer for air side flow across flat tubes heat exchanger is investigated numerically and experimentally. The effect of inclination of flat tubes on the amount of heat transfer is investigated. Fluent 6.1 software program is used to solve this problem. Three proposed cases for inclination of tubes are studied. This first case is to make convergent and divergent channels for air flow (Case 1). The other two cases are tilting of flat tubes in forward (Case 2) and back ward (Case 3). Velocity and temperature distribution around the flat tubes for the studied cases are obtained. In turn, convection heat transfer coefficient and pressure drop for air side around flat lubes are calculated. An experimental apparatus is designed and constructed. Air is drawn through the wind tunnel from the surroundings to flow across one column of the tested tubes with different velocities. Two separated heating fluid units are used in the experimental work to perform different wall temperatures. The first unit is a refrigeration circuit, using R406A, as a working fluid. The air cooled condenser for this unit is considered the test section for this work and fixed inside the wind tunnel. The second unit is an open circuit, using steam as working fluid where steam is condensed inside the tested tubes. Three proposed cases for inclination of tubes in the theoretical work are manufactured tested and in the experimental work. The apparatus is equipped with the required sensors. The experimental and numerical results showed that the optimum angle of inclination for the proposed first and second cases is 4°. For convergent divergent construction of one row coil without fins (case 1), the obtained enhancement of convection heat transfer coefficient is about 46.9 % meanwhile in pressure drop increased by about 112%. The second proposed construction of tilting forward alt cubes in parallel with respect to horizontal, the enhancement in convection heat transfer coefficient is about 46 % against an increase in pressure drop by about 95%. There is an acceptable agreement between the numerical and experimental results. Empirical correlations to calculate average Nusselt number from the experimental work as a function of Reynolds number at optimum inclination angle or 4° are obtained. It is found that in case of convergent-divergent channels (inclination one tube toward clockwise direction and the next in counter-clockwise direction), and tilting all tubes in the forward direction considerable improvements are obtained but in case of tilting all tubes in the backward direction there is no improvement.

Study on the air-side flow and heat transfer characteristics of corrugated fin under low-pressure environment

ABSTRACT Many troubles appear when vehicles work in the plateau region, and the main reason is that the heat-transfer capability of radiator in the vehicle cooling system reduces under the plateau low air pressure environment. Hence, the research on the flow and heat transfer characteristics of radiators in plateau low air pressure environment is necessary. In this paper, the experimental study on the thermal-hydraulic characteristics of corrugated fin radiator is conducted on a wind tunnel test-bed with low pressure function. The results illustrate that the air-side convective heat transfer coefficient of the corrugated fins decreases with the decrease of the ambient pressure, and the Colburn factor (j) and Friction factor (f) increase simultaneously. When the gauge pressure is −44 kPa, the air-side convective heat transfer coefficient is 34% lower than that under normal pressure on average, and the Colburn factor and Friction factor increase by an average of 21.1% and 55.6%, respectively. On this basis, the entransy dissipation and field synergy angle distribution of the corrugated fin radiator under low-pressure environment are studied. It is showed that the entransy dissipation number at −44 kPa gauge pressure reduces by 65% on average. The air-side field synergy angle increases with the decrease of the ambient pressure. Finally, the experimental correlations of the Colburn factor and Friction factor are performed, and the average deviations of the calculated results are 2.8% and 3.3%, respectively.

Particle Image Velocimetry (PIV) experiment of the buoyant flow field of a thermal chimney model designed for geothermal power plants

ABSTRACT To enhance the air-cooling process in geothermal power plants for economical utilization of the exhaust steam from expansion, a natural-draft thermal chimney design was proposed and studied here in this paper. In view of the necessity of accurate velocity field measurements which would provide further insight into the physics behind the evolving plumes above heated horizontal cylinders, Particle Image Velocimetry (PIV) was employed to experimentally investigate the buoyant flow in the thermal chimney system. Two configurations have been tested to understand the flow induced by the horizontally heated cylinders inside the thermal chimney. Firstly, flow field above a single row of cylinders was tested while they were isothermally heated to simulate of an air-cooled condenser. After that, a second row of cylindrical heaters (air-heater) was added above the first row to enhance the buoyant flow, aiming at enhancing the air side flow of the air-cooled condenser. Flow characteristics and velocity enhancement were studied for both configurations. The results show that significant flow unsteadiness occur near the cylindrical heaters because of the non-steady crossing flows between adjacent cylinders, and the unsteadiness attenuates in the downstream. The effects of cylinder row distance, surface temperature as well as downstream distance on the flow field were then analyzed. Flow velocity is increased by the air-heater as the buoyancy force is enhanced, proving the idea of flow enhancement of the thermal chimney configuration. It is also observed that the velocity fluctuation, turbulent kinetic energy and vorticity change significantly after adding the second row of heaters. The present study provides further insight into natural convection flow theory of heated cylinders for a Rayleigh number range of 1.3E4 to 2.2E4, which is fundamental for the flow enhancement designing of the proposed natural-convection-driven cooling system.

Effect of Air Gap on thermohydraulic performance of finned tube bundles

The air gap between the consecutive tube bundle row fins is one of the most important parameters that have not been evaluated in terms of their impact on the performance of air-cooled condensers. In this paper, the effect of this air gap on the thermo-hydraulic performance of the tube bundle air-side flow is investigated. The importance of this parameter lies in the fact that it can change the heat transfer coefficient and pressure drop in the tube bundle air-side flow. Moreover, changing the air gap will vary the overall length of the condenser and the dimensions of the main cooling system and consequently imposes a change in the costs of construction and operation. In this study, the effect of inlet air velocity on fin and ambient functioning temperature conditions is investigated at various air gaps on the thermo-hydraulic performance of the tube bundle air-side flow. The results indicate that the air gap for the proper thermo-hydraulic performance of the tube bundle air-side flow is independent of the air inlet velocity of the tube bundle and the ambient temperature conditions of the power plant. Moreover, it was found that the tube bundle air-side flow at a 1-mm air gap will have better thermo-hydraulic performance. The behavior of tube bundle air-side flow of the condenser of a 484 MW power plant was also studied. The results indicated that the tube bundle air-side flow at this state will also have better performance at a 1-mm air gap such that the overall length of the condenser in this state will increase by 1.7% compared to the state where the air gap is zero

A coupled CFD approach for performance prediction of fin-and-tube condenser

This article developed a novel numerical model for accurately predicting the thermal performance of fin-and-tube condenser, which coupled heat transfer between the air and refrigerant side. The heat exchange calculation between the air and refrigerant side was alternatively performed by coupling the tube wall temperatures. Meanwhile, the air side flow and heat transfer characteristics among the fin plates were achieved by porous media approach. Based on this model, the numerical studies were conducted for investigating the effects of air inlet temperature and frontal velocity on the thermal performance of such heat exchanger. Results showed that the predicted overall heat transfer rate was within the range of ±10% compared with the experimental data. It indicated that the porous media model was able to predict the thermal and flow characteristics of air side in a fin-and-tube condenser, and furthermore it could be used for the optimal design of such heat exchanger.

Air side heat transfer enhancement using radiantly arranged winglets in fin-and-tube heat exchanger

The enhancement of airside heat transfer in fin-and-tube heat exchanger is crucial for reducing the pumping power, saving materials and so on. In this paper, the authors propose to arrange the winglets radiantly around the tubes on the fin to enhance the airside heat transfer in fin-and-tube heat exchanger. The airside flow and heat transfer characteristic are numerically investigated. The position and the geometric dimensions of the vortex generators are also optimized. The simulation results show that the Colburn j-factor and the friction factor f both increase with VG number. The fin structure has the best comprehensive performance when its VGs are uniformly arranged both in the upwind and backwind regions of the tubes, and at the same time make the enhanced zones the widest in the tube radial direction. With the increase of the VG length, the comprehensive performance factor JF gets lower. The comprehensive performance of the proposed fin with five rows of tubes are demonstrated to be better than a reference fin used in industry with the same fin thickness, tube diameter, fin pitch and tube pitch.

Effect of heat exchanger design on seasonal performance of heat pump systems

This paper presents the effect of heat exchanger design on heat pump performance based on partial load operating conditions. 3-D numerical analysis was conducted to calculate face velocity profiles for each outdoor heat exchanger design (rectangular, cylindrical, and trapezoidal) in 10 different operating conditions. Heat exchanger circuits were modified considering heat exchanger face velocity distributions, and seasonal heat pump performances were calculated with modified heat exchanger design. The maximum seasonal performance enhancement of 7.07% was achieved with a modified heat exchanger design. Air-side flow maldistribution could affect significantly refrigerant path design and heat exchanger performance as well as system performance. The analysis results also revealed that smaller refrigerant circuits at the upper part of the heat exchanger interacting with higher air velocity could further enhance the annual system performance.

Experimental and CFD analysis of solar air heater duct roughened with multiple broken transverse ribs: A comparative study

Lower thermal performance of conventional solar air heaters (SAH) can be enhanced by implementing small sized roughness elements on the air-flow side of the absorber plate. This article presents the results of a comparative study of two novel ribs arrangement namely multiple broken transverse ribs and square wave shaped ribs for performance enhancement in SAH’s. The investigation incorporates rib parameters as P/e = 10, e/D = 0.043 and W/w = 7 for both arrangements and investigated at six levels of Reynolds number ranging 3000–18,000. The effect of two different multiple shaped ribs on heat transfer and friction factor characteristics has been investigated using experimental and numerical investigation. The optimum parameters based on thermo-hydraulic performance has been evaluated. The CFD methodology has been validated for the smooth and roughened duct. Maximum thermal enhancement of 2.50 times with corresponding pumping power penalty of 3.92 times is obtained for square wave shaped ribs whereas in case of multiple broken ribs, the enhancements are 3.24 and 3.85 times respectively. The experimental results are illustrated and justified with the visualization of flow pattern and contours of temperature, pressure and turbulence.

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.

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.