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Study of aerodynamic performances of different wind tunnel configurations and air inlet velocities, using computational fluid dynamics (CFD)
Computers and Electronics in Agriculture 125 (2016) 137–148
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Computers and Electronics in Agriculture journal homepage: www.elsevier.com/locate/compag
Original papers
Study of aerodynamic performances of different wind tunnel configurations and air inlet velocities, using computational fluid dynamics (CFD) Ester Scotto di Perta a,b, Maria Angela Agizza c, Giancarlo Sorrentino d, Lorenzo Boccia a, Stefania Pindozzi a,⇑ a
Department of Agriculture, University of Naples Federico II, Via Università 100, Portici (NA), Italy Department of Agriculture, Forestry, Nature and Energy (DAFNE), University of Tuscia, Via S. Camillo de Lellis s.n.c., 01100 Viterbo, Italy Interdepartmental Research Center For Environment, I.R.C. Env, Italy d Department of Chemical, Materials and Production Engineering, University of Naples Federico II, P.le Tecchio 80, Napoli, Italy b c
a r t i c l e
i n f o
Article history: Received 28 July 2015 Received in revised form 22 April 2016 Accepted 11 May 2016 Available online 19 May 2016 Keywords: Ammonia emission Wind tunnel Jet effect Flow distribution devices CFD
a b s t r a c t Livestock and agricultural activities contribute significantly to atmospheric ammonia emission in Europe. The volatilization process depends on many factors, especially wind speed and rainfall. The most important methods to evaluate ammonia volatilization are the wind tunnel and micrometeorological methods. The tunnels are more flexible and simple to use in every situation. Few studies have been carried out to determine, which conditions are established inside the chamber and how they influence the ammonia volatilization and measurement. The aim of this research was to investigate the effects of the wind tunnel configuration and flow inlet velocity, by means of CFD simulations and wind speed measurements, in order to achieve a better aerodynamic performance. The SST k–x model used for simulations was first validated in order to prove the consistency of the model itself. Several configurations were simulated and compared. In particular, in order to overcome the asymmetric flow conditions that occurred in all wind tunnel configurations, four flow distribution devices were proposed and simulated. The best setup was chosen with the purpose of reaching both the best uniform velocity distribution (to ensure homogeneous volatilization from the emitting surface) and easy transport for field applications. It consists of an emission chamber 40 cm wide, 25 cm high and 80 cm long, situated between a divergent diffuser and a convergent duct, respectively 50 cm and 25 cm long. Moreover, structures similar to honeycombs, namely guiding channels, were introduced in the divergent diffuser, because they showed the best aerodynamic performance. These 20 channels, located in the divergent diffuser, prevent flow from separating, by means of the reduction of the expansion angle, obtaining the desired flow conditions inside the wind tunnel. Finally, it was verified that CFD confirmed its usefulness as a decision-support instrument to design and simulate possible solutions, reducing design time. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction Livestock and agricultural activities contribute considerably to anthropogenic ammonia emission in Europe (Balsari et al., 2007). In particular, the major contribution, approximately 81% of total ammonia gaseous losses, result from animal waste (Buijsman et al., 1987). In general, these losses derive from each step of livestock manure management, that is to say from stables, manure storages or during/after application of manure as fertilizer ⇑ Corresponding author at: Department of Agriculture, University of Naples Federico II, Via Università 100, 80055 Portici (NA), Italy. E-mail address: [email protected] (S. Pindozzi). http://dx.doi.org/10.1016/j.compag.2016.05.007 0168-1699/Ó 2016 Elsevier B.V. All rights reserved.
(Olesen and Sommer, 1993), but more than half of these emissions comes from field-applied slurry (Loubet et al., 1999, Part 1). Large amounts of ammonia in the atmosphere can have damaging effects on human and animal health, because of its contribution to the deterioration of air quality and ecosystems (Rotz et al., 2014). Indeed, the volatilization of ammonia from field is known to be responsible for soil acidification, water eutrophication, and formation of fine particulates and secondary emissions of nitrous oxide. In addition to their environmental pollution problem, ammonia leaks can cause heavy economic losses to farms, because of the lack of available nitrogen that is indispensable for crop production (Martínez-Lagos et al., 2013). Consequently, it is quite clear why there is increasing interest in limiting ammonia
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volatilization, through the development of abatement technologies. The volatilization process depends on many factors such as soil conditions, agricultural practices, slurry characteristics and climate conditions, especially wind speed and rainfall (Génermont and Cellier, 1997; Sommer and Hutchings, 2001; Martínez-Lagos et al., 2013). Consequently to the complexity of this process, it is difficult to assess the atmospheric ammonia fluxes and there is no standard measurement for this purpose, because the emission rate is extremely variable due to the effects of different factors. Several studies have been carried out to evaluate ammonia losses, using different methods. The most important are dynamic flux chambers or wind tunnels and micrometeorological techniques. Even if the latter are often preferred in terms of accuracy, they suffer from many limitations due to the large number of samples and the extensive analysis required. Instead, the principle of the dynamic flux chamber approach is very simple. Although the wind tunnel method introduces microclimate modifications, which could affect the measurements, it is more flexible and simple to use, especially for small area sources (Parker et al., 2013). In particular, to simulate the wind action, the air is supplied with a fan, that allows obtaining a velocity, inside the main chamber, between 0.3 and 1.0 m/s. The emission rate depends on the air velocity selected during the measurements. In particular it can be assessed as the product of the flow rate and the concentration of volatile chemicals under a special cover, in which the aerodynamics and flow rates are controlled (Jiang et al., 1995). In literature, there are many examples of portable wind tunnels being placed on the emission surface. One of the most important is the Lindvall, 1969 hood that is characterized by a rectangular measurement section, with contraction and expansion sections. Another one is the Lockyer (1984) wind tunnel system, constructed with a transparent polycarbonate sheet and a steel circular duct (Gostelow et al., 2003). Few studies have been carried out to determine which conditions are established inside the tunnel and how they could influence the emission rate and the measurement. Since the aerodynamic performance of the tunnel is considered a critical parameter (Jiang et al., 1995), it is, therefore, important to understand precisely what aerodynamic conditions have been established inside the tunnel (Gostelow et al., 2003). However, it is not easy to make a comparison among several emission rates obtained with the different geometric dimensions of wind tunnels (Saha et al., 2009). Therefore, each geometric configuration must be considered separately. Previous studies under laminar conditions reported that the flow in a symmetric chamber configuration with a plane expansion is almost always symmetrical, but becomes asymmetrical at a critical Reynolds number and downstream of the sudden expansion. In these cases, an asymmetrical mean flow pattern is observed depending on the shape of the channel and the flow Reynolds number (Durst et al., 1974; Jiang et al., 1995; De Zilwa et al., 2000; Nabavi, 2010). On the other hand, the reduced length of the chamber needed to achieve ease of transport, requires a diverging diffuser with a short working length and a larger tunnel which can cover a greater emission area, in order to obtain a more significant results. The flow asymmetry could affect the turbulent mixing and the ammonia transfer rate, therefore uniform flow distribution at the outlet of diffuser should be obtained. This is something that needs to be achieved by means of the installation of flow distribution devices, which improve aerodynamic performance. The present study is framed in this background and its aim is to set up a wind tunnel, a simpler and economical procedure, with optimal aerodynamic performances, in order to accurately evaluate ammonia losses, obtaining replicable results. In other words, it is very important to avoid the eventual accumulation of compounds
inside the chamber and the possibility that the flow behaviour does not fit the atmospheric real case. For such purposes, several wind tunnel configurations were simulated and compared. In particular, it was verified the existence of asymmetric flow behaviour, that is very problematic for ammonia measurement. In order to overcome this phenomenon, called ‘‘jet effect”, and to obtain a fully developed uniform flow, four flow distribution devices were proposed, simulated and compared.
2. Materials and methods 2.1. The wind tunnels The wind tunnels used for this research were constructed according to the geometry proposed by Jiang et al. (1995) and then by Balsari et al. (2007). The geometry is characterized by a main chamber located between a divergent and a convergent duct (respectively expansion and contraction section). The first one allows for the transition from a circular section to rectangular one, and the second duct vice versa (Fig. 1). Each element could be disassembled from the others for ease of transport. Two airsampling points are considered, respectively one at the beginning and the other at the end of the wind tunnel, in order to measure the concentration of ammonia in the air at both the inlet and outlet sections. The bottom of the main chamber is open and facing the emission surface, in particular the experimental area is 40 cm wide and 80 cm long, so it consists of 0.32 m2 (Jiang et al., 1995). In order to simulate the wind conditions, which influence the emission phenomenon, ventilation is provided by an axial fan, with a maximum flow rate of 298 m3/h. The fan is connected to the convergent duct. In order to have a fully developed flow, upstream of the divergent diffuser there is a 0.10 m diameter cylindrical duct, whose length is ten times the inlet diameter (1 m). This solution leads to an improved velocity distribution and a reduction of velocity fluctuations (Jiang et al., 1995). As non-uniform velocity and concentration profiles may lead to errors in emission rate estimation (Gostelow et al., 2003), different wind tunnels sizes and different diffuser configurations were considered and compared, in order to prevent the flow from separating and to obtain a fully developed flow and uniform distribution of velocity profiles. Indeed, the main issues concerning the use of dynamic chambers are the eventual accumulation of compounds inside the hood and the possibility that the flow behaviour does not fit the atmospheric real case. Since few studies are focused on such purposes, the dimensions of the tunnels used for this work were selected according to: – the studies of Durst et al. (1974), De Zilwa et al. (2000), Nabavi (2010), focused on how expansion ratio and the shape of the duct could influence the conservation of flow symmetry and the separation length; – the considerations about the effect of different heights of the chamber in aerodynamic characteristics, discussed in the studies by Saha et al. (2009) and Saha et al. (2011); – the suggestions of Jiang et al. (1995) about the possibility of modifying the chamber length. In particular, the design dimensions are listed in detail in Table 1.
2.2. CFD modelling The computational fluid dynamics (CFD) technique was used to simulate ventilation performances, to study the airflow in the
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Fig. 1. A sketch of the wind tunnel and a detailed view of the divergent diffuser.
Table 1 Wind tunnels and their design dimensions. Parameter
A25-25
B50-15
C50-25
D50-50
E50-25-160
F50-25-25
WT width (m) WT height (m) Main chamber length (m) Divergent length (m) Convergent length (m)
0.40 0.25 0.80 0.25 0.25
0.40 0.15 0.80 0.50 0.50
0.40 0.25 0.80 0.50 0.50
0.40 0.50 0.80 0.50 0.50
0.40 0.50 1.60 0.50 0.25
0.40 0.25 0.80 0.50 0.25
room and the contaminant dispersion from buildings across wide research areas (Saha et al., 2011; Rong et al., 2011; Wu et al., 2012). The basic concept of CFD is to solve a set of partial differential equations. The governing equations are the Navier–Stokes and continuity equations. The accuracy of CFD depends on many factors, such as boundary conditions, numerical methods, mesh and turbulence models (Rong et al., 2011). For this reason, an optimal choice of the correct parameters is mandatory. Commercial software Fluent 6.3.26 was used to solve the equations on the basis of finite volume method. This software allows the user to choose between two different main solvers, segregated or coupled. They both implement the Finite Volume Method as numerical method, but the approach towards the governing equations is different. The coupled solver is recommended for transonic and supersonic flows, while the segregated solver is much faster for low-speed flows and is thus more suitable for incompressible flows, showing good performances for subsonic compressible flows calculations. For these reasons, the segregated solver was used (Fluent, 2006). 2.2.1. Turbulence model Many researchers have discussed the application of the different turbulence models for indoor environments. Since the inlet Reynolds number is higher than 5000 in each tunnel configuration, the airflow is in turbulent conditions and therefore the choice of a proper turbulence model is mandatory. Although the k–e model is still widely used to simulate ventilation conditions, several studies had confirmed that the SST k–x model is more appropriate to simulate a mass transfer process in aerial boundary layer (Sparrow et al., 2009; Saha et al., 2011). In particular, this model, developed by Menter (1994), is governed by two equations, and results from a
combination of two models, respectively for calculations in the inner boundary layer and in outside of the boundary layer. One of the advantages of the k–x formulation is the near wall treatment for low-Reynolds number computations. Indeed, the model does not comprise the complex nonlinear wall damping functions required for the k–e model and it is therefore more correct and more consistent (Rong et al., 2011). The better performance of this model has been demonstrated in different validation studies (Rong et al., 2011; Saha et al., 2011). The SST model is recommended for high accuracy boundary layer simulations. For this reason, the SST model was applied in this study and compared with the experimental results. The convergence is assumed to be reached when residuals have stabilized at 10 6 for continuity and momentum equations. The number of iterations required to reach this value showed an obvious dependence on mesh resolution, dimension of the calculated domain and its complexity. Nevertheless, the target value was reached in every calculation shown below. 2.2.2. Discretization scheme The computational domain is the entire wind tunnel for each configuration. A tetrahedral volume mesh was used in this study, for all selected cases. The discretization was done on the basis of the smallest mesh resolution required for the RANS simulation. In particular, a mesh independency study was also adopted. To make the meshing strategy clear, technical details are reported in Table 2. 2.2.3. Boundary conditions Another important step for a correct use of the CFD technique, in order to have reliable results, is a proper definition of the
Table 2 Technical simulations details. Parameter
A25-25
B50-15
C50-25
D50-50
E50-25-160
F50-25-25
Grid number First node height (mm) Nodes number Iterations number
165,591 0.775 55,560 1050
386,409 1.006 143,598 8750
191,178 0.800 68,799 2460
149,671 0.849 103,104 1890
87,823 1.067 35,243 1000
372,226 0.652 143,598 8750
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Fig. 2. Comparison of velocity profiles between CFD simulations and experimental measurements published by Jiang et al. (1995).
Fig. 3. Comparison of velocity profiles between CFD simulations and measurements for two tunnel configurations: F50-25-25 (a) and A25-25 (b).
boundary conditions, according to the case study. Three different boundary conditions were used in this work: velocity inlet, pressure outlet and walls. Specifically, for all the simulations, the pressure outlet was considered equal to atmospheric pressure. Moreover, all the wind tunnels interior surfaces were simulated as wall boundary with no-slip condition. For the comparison between simulations and experiments the inlet air velocity was fixed at 5 m/s, according to the data obtained by experimental measurements of the flow rate provided by the axial fan. For the simulations that investigated the introductions of flow distributions devices, the inlet air velocity was varied among 1, 2.5 and 5 m/s.
2.3. Experimental data for model validation In order to avoid mistakes in the simulations and to be sure that results describe correctly the phenomena in its reality, model validation is a required step, before investigating the geometries designed. For this purpose, data used for validation were the experimental velocities published by Jiang et al. (1995). In particular, the wind tunnel (UNSW wind tunnel) used is similar to F50-25-25 tunnel configuration, considered in this work, characterized by a main chamber with a width of 400 mm, a height of 250 mm and a length of 800 mm. Therefore, the expansion section is 500 mm long, while the contraction section is 150 mm. An inlet velocity of 0.33 m/s
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was considered, according to Jiang et al. (1995) experiment. The velocities used for validation were the vertical ones, obtained in the three cross-sections of the main chamber, at the position of x = 200 mm, x = 400 mm and x = 600 mm. 2.4. Experimental measurements Velocity and smoke tests were performed in order to characterize the aerodynamic conditions inside the tunnel and to carry out a qualitative analysis of the obtained results with CFD simulations. The measurements were performed in different positions, with a portable air velocity transducer (MODEL 8465, TSI) that has a resolution of 0.07 m/s and a measuring working range of 0.125–50 m/ s. The instrument gives the instantaneous value of the measured air speed. The operation has been repeated three times, after ten seconds, for each position. The average of three obtained velocity values was considered as the final result. Three cross-sections were chosen for the measurements and for calculations, at the position of 1/8 (x = 10 cm), 1/2 (x = 40 cm), 7/8 (x = 70 cm) of the length of the main chamber, respectively, that are generally indicated in the following figures with the letters o and v respectively, for profiles along z and y-axis. In order to monitor the velocity profiles inside the chamber, each cross-section was characterized by twenty-four measuring points; in particular six locations along the height were investigated: 4 cm, 7.5 cm, 12.5 cm, 17.5 cm, 21 cm, and 24 cm from the top of the tunnel to the bottom. For each height, 4 measuring points along z were considered. The measured velocity profiles (VP) were used for a qualitative CFD comparison. VP were compared with those obtained from CFD simulations, considering the same geometrical characteristic of A25-25 and F50-25-25 tunnel configurations. Moreover, the flow behaviour was better observed by following the path of a visible tracer in both the tunnel configurations (Jiang et al., 1995). For this purpose, an airflow visualization analysis via smoke studies was performed using incense smoke sticks. Each stick was inserted in 4 holes along z for each cross-section, the same used for velocity measurements with anemometer. This allowed the observer to investigate the flow pattern at different positions. 3. Results and discussion 3.1. Grid independency Ideally, the CFD simulation results should be independent on the computational grid. To fulfil this requirement, different meshes were considered for each geometry, varying the number of grid elements or the control points. For the A25-25 case, computational
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grids of 75,077, 82,942 and 165,591 elements were compared. Few differences among the solutions have been found, implying that a further refinement of the grid wouldn’t change the results significantly. It confirmed that the results were mesh-independent. This independency test was performed for every calculation shown in the following test. 3.2. CFD model validation In order to prove the consistency of the CFD method, the experimental velocity profiles considered for the validation were obtained from the experimental data of Jiang et al. (1995). The study was conducted in a wind tunnel of similar size. Moreover, similar flow characteristics and the same flow behaviour, called ‘‘jet effect” were observed. The comparison of the vertical velocity profiles confirmed the same shape for the three cross sections considered. Specifically, the coefficient of determination (R2) values of measurement and numerical results for the section x = 200 mm, x = 400 mm and x = 600 mm, were respectively 0.96, 0.95 and 0.96, as shown in Fig. 2b. The comparison between the CFD and experimental results (Fig. 2a) demonstrated that the SST model is suitable for simulating characteristics of airflow in this specific situation and predicts in a satisfactory manner the trend of the atypical aerodynamic behaviour. As a consequence of the reliability of the numerical model, it has been used to simulate the airflow behaviour in following different improved designs of wind tunnels, as suggested by Saha et al. (2011). 3.3. Qualitative comparison between simulations and experiments Measured and simulated velocity profiles along z axis in three cross sections have been compared for F50-25-25 and A25-25 configurations in Fig. 3, at 12.5 cm from the emitting surface. Despite possible measurement errors, it was verified that both results confirmed the anomalous asymmetric behaviours of the flow pattern. In particular, these apparatuses cause a three-dimensional phenomenon whose onset is located in the divergent diffuser, as shown in Fig. 4. In the region of asymmetry, the velocity value near the wall is greater than that close to the centre of the tunnel. These behaviours have been reported in different studies in which the nature of asymmetric flow conditions has been examined (Durst et al., 1974; De Zilwa et al., 2000; Nabavi, 2010). Durst et al. studied the nature of asymmetric flow conditions in a plane symmetric sudden expansion and observed that flow is perfectly symmetric at low inlet Reynolds numbers, with equal-sized separation bubbles forming on both sides just downstream of the
Fig. 4. Velocity streamlines predicted by CFD simulation of F50-25-25 (a) and A25-25 (b).
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Fig. 5. Velocity profiles along z and y-axis and streamlines predicted by CFD simulations of three tunnel configurations: (a) B50-15; (b) C50-25 and (c) D50-50.
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Fig. 5 (continued)
Fig. 6. Velocity streamlines predicted by CFD simulations F50-25-25 configuration with a chamber with a double length.
sudden expansion. As the Reynolds number gets over a critical value, the flow loses its symmetry and one of the separation zones becomes larger than the other. Therefore, such phenomena depend on Reynolds number and expansion ratio, which influence the length of separation regions. In particular, the expansion ratio and the shape of the duct could influence the conservation of symmetry and the separation length. The experimental investigations by De Zilwa et al. (2000) suggested that the flow asymmetry would persist up to Reynolds numbers at which the flow becomes turbulent. The asymmetry of the steady flow is due to an
asymmetry-breaking bifurcation, and that the larger separation zone may occur on either side of the channel without any preference. This flow asymmetry is sometimes attributed to the so-called ‘‘Coanda effect” (Sobey and Drazin, 1986). 3.4. Effect of main chamber height The simulated velocity profiles and velocity contours in the B50-15, C50-25 e E50-50 tunnels are shown in Fig. 5. In particular, velocity profiles along y-axis are taken 20 cm from the tunnel wall
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and profiles along z-axis from 12.5 cm from the tunnel bottom. By examining the simulation results, it was observed that even if the asymmetric flow behaviour occurred in all the cases; air velocity values in different sections of the tunnel were higher when the height decreases. Moreover, it was found that in all the cases a big recirculation zone is developed inside the chamber. This is due to the shape and dimensions of the expansion section as suggested by Jiang et al. (1995). The phenomenon of a gaseous flow that passes through the hood, generating a rotation region inside the chamber itself, is called ‘‘jet effect”. It becomes more significant if the width of the tunnel is larger than the height. Interesting phenomena were observed by looking at the simulated results of the E50-50 tunnel configuration. In this case, the asymmetric flow was found not in the xz plane, but in the xy one (Fig.5c). The explanation to this result could be related to the expansion angle of the diffuser in the xy plane. In fact, if the height of the chamber increases, also the expansion angle of the diffuser in the xy plane will become larger than the one in the xz plane. 3.5. Effect of the main chamber length Given the flow non-uniformity, attested in all the investigated geometry configurations, only one of them (F50-25-25) was further investigated in a second part of the work. This choice seems to fit in the best way the double requirement of potential aerodynamic performances and reduced space. Different potential answers to the problem were analysed and compared. A possible solution could be to increase the length of the chamber or to introduce flow distribution devices (Jiang et al., 1995). Simulated E50-25-160 tunnel geometry, characterized by a main chamber of double length (1.60 m) is shown in Fig. 6. By increasing the main chamber length, the flow path is directed first towards one side of the wind tunnel and afterwards towards the other side, before returning to the centre-line, just at the end of the chamber (Durst et al., 1974). As shown, two unequal recirculation zones appear: the larger separation region occurs in the first part of the tunnel and then on the other side, in correspondence of the inversion of the flow. All the rotations occur around a vertical axis. As can be seen, a higher distance from the inlet reduces the inlet expansion effect, so that it is possible to obtain a stable flow if the length of chamber is increased. On the other hand, this solution occupies too much space, and it is not useful in field applications, in which reduced space is an important requirement. 3.6. Effect of the introduction of flow distribution devices In order to reach repeatable conditions inside the tunnel, it is important to obtain a steady and uniform distribution of velocity profiles. For this reason, flow distribution devices must be introduced (Jiang et al., 1995) and therefore, four solutions were designed and simulated. Tetrahedral volume mesh was used also in these cases. An example of mesh distribution is shown in Fig. 7. Fig. 8 shows devices configuration and location. The optimum condition was chosen as a function of the aerodynamic performance. Moreover, each device was investigated at three airflow inlet velocities, as shown in Table 3, for two main reasons: wind speed is a useful and valid parameter to compare flux measured inside the tunnel and those measured in open field condition (Loubet et al., 1999, Part 2), and the intensity of the ‘‘jet effect” phenomenon grows by increasing the velocity (Jiang et al., 1995). 3.6.1. Wire gauzes Two wire gauzes were considered inside the divergent diffuser: one near the inlet section and the other in the middle of diffuser.
According to aerospace applications, screens are often used to reduce turbulence scale. Results, in Fig. 8a, show that flow behaviour is not so much different from the previous simulations, also varying inlet velocity, in fact turbulence is not reduce and the ‘‘jet effect” still occurs. This is probably related to the fact that a screenhoneycomb combination was not used, and screens alone are not enough. 3.6.2. Guide vanes Another possible solution that was investigated involves the introduction of horizontal and vertical guide vanes combination at the beginning of the divergent diffuser (Fig. 8b). Each vane has a width of 5 cm and they are running parallel to the xz plane initially and later to the xy plane of the wind tunnel, in the direction of the flow. Even in that case, three different velocity inlet values were simulated but no differences were obtained among them, in terms of flow behaviour. 3.6.3. Perforated plates As reported by Noui-Mehidi et al. (2005), sometimes to prevent the flow from separating and to achieve a good uniformity in the flow behaviour inside wide-angle diffuser, perforated plates are proposed. To this matter, two perforated plates were positioned within the divergent diffuser. The first one was located at 15 cm from the diffuser inlet, while the second plate was placed after 30 cm from the first one. The plates are characterized by circular holes of 2 cm diameter, located at a distance of 1 cm from each other, in order to obtain a porosity varying from 40% to 50% (Fig 8c). This configuration was designed in respect of WardSmith et al. (1991). They found that even with only two perforated plates, flow uniformity could be achieved, provided that appropriate porosity and plate positions have been chosen. Velocity profiles of three investigated inlet velocity are shown below (Fig. 9). Perforated plates represent the first solution that allows the ‘‘jet effect” to be eliminated. However, to test the development of the flow, velocity value of the three sections were analysed and correlated each other. Results are reported in the Section 3.6.5. 3.6.4. Guide channels Finally, two structures similar to honeycombs have been simulated. They consist of a series of horizontal and vertical planes that intersect each other, in order to obtain several channels, located in the divergent diffuser. These channels guide the flow direction up to the main chamber of wind tunnel and prevent flow from separating, by means of the reduction of the expansion angle. Different structures were considered, but only the one that gave the best results is reported in Fig. 8d. The structure is characterized by 4 vertical planes and 3 horizontal ones, in order to obtain in total 20 channels. These are extended through the diffuser up to 10 cm to its end, in order to reduce the possible velocity fluctuations due to the channels. Results of simulations, carried out with three different inlet velocity values, are shown as follows. The obtained velocity profiles are reported in Fig. 10. As for perforated plate configuration, guide channels lead to uniform fluid distribution, because ‘‘jet effect” does not occur. Thus, it is possible to suppose that the emitting surface under wind tunnel is completely involved in the volatilization process, since there are not stagnation areas. Moreover, as it is possible to observe, especially for lower velocity, the flow seems to be fully developed at the beginning of the main chamber, without increasing its length. Even if, based on the previous figures, for lower inlet velocity, parabolic velocity profiles are less dominant and a reduced number of velocity fluctuations can be observed, as found also by Jiang
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et al. (1995), the introduction of guide channels in the divergent allows the desired aerodynamic performances to be obtained, even for values of velocity lower than 5 m/s. This means that contrary to the velocity range usually considered for this kind of study, it is possible to use this kind of wind tunnel also in the regions, where the wind speed is relevant and higher than 1 m/s near the ground, with conditions inside the chamber that are closer to the real conditions study here considered. Indeed, inlet velocity is an
important parameter, which should be accurately chosen, but it is strongly dependent on the real case to be simulated. 3.6.5. Comparison between perforated plates and guide channels Simulations have shown that the best configurations are perforated plates and guide channels, in terms of uniform fluid distribution. In order to choose one of them as the best flow distribution device, the full flow development of each device was considered.
Fig. 7. Mesh distribution as an example: (a) Wire gauzes; (b) Guide vanes; (c) Perforated plates; (d) Guide channels.
Fig. 8. Devices configuration and velocity streamlines predicted by CFD simulations at the inlet velocity of 5 m/s: (a) Wire gauzes; (b) Guide vanes; (c) Perforated plates; (d) Guide channels.
Table 3 Flow devices, their simulated parameters and technical simulations details. Parameter
Wire gauzes
Inlet velocity Re (103) Device location Grid number Nodes number
1 2.5 5.5 13.9 X1 = 1 m; X2 = 1.25 m 271,837 88,062
5 27.8
Guide vanes
Perforated plates
Guide channels
1 2.5 5 5.5 13.9 27.8 From X = 1 m to X = 1.1 m 840,258 154,642
1 2.5 5 5.5 13.9 27.8 X1 = 1.15 m; X2 = 1.45 m 3,061,708 551,066
1 2.5 5 5.5 13.9 27.8 From X = 1 m to X = 1.4 m 180,495 38,619
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Fig. 9. Simulated velocity profiles for velocity of 5, 2.5 and 1 m/s: (a) along z axis and (b) along y axis.
Namely, in addition to the uniformity of air flow profiles, also the variability of the velocity profiles for consecutive cross sections was taken into account for both devices. In particular, to verify fully development of air flow, velocity profiles along y-axis, obtained by CFD modelling, for the three consecutive positions (x = 10 cm, x = 40 cm, x = 70 cm) were analysed and correlated each other. Although three inlet air velocities were investigated, only the correlation for the velocity along y-axis with an airflow velocity inlet of 5 m/s is reported in Fig. 11 for brevity. As it is possible to note, guide channels have R2 values which are higher than those for the perforated plates, about 0.98–0.99. This effectively proves that even with maximum inlet velocity values, they allow a fully developed flow to be obtained from the beginning of the main chamber. Instead, according to perforated plates, even if velocity profiles start to be reasonable, in terms of uniformity, more space is necessary to reach a fully developed flow. Results demonstrated that the wind tunnel configuration with guide channels represent the best solution, in order to have a fully developed symmetrical and uniform flow in a reduced space with replicable results. 4. Conclusions In this study, an assessment of the effects of the wind tunnel geometry with focus on the flow behaviour was performed with CFD simulations. In order to prove the consistency of the SST model, the experimental velocity profiles, obtained from the experiment of Jiang
et al. (1995), conducted with a wind tunnel of similar size, were considered for the validation. The comparison between the numerical and experimental results demonstrated that it is possible to consider that CFD simulates well the general trend of air velocity inside the tunnel. Therefore, it is a powerful tool not only to catch the complexity of the 3D asymmetry (which occurs with a wide expansion of the air flow in the three dimensions), but also to design and simulate possible solutions for separating flow and to reduce design time. In this sense, in order to obtain a steady and uniform distribution of velocity profiles, an optimal design of the flow distribution devices seems to be very important. Therefore, four solutions were designed, simulated and compared by means of CFD computations. Among several experimental devices designed and simulated, only the guide channels configuration results in the attainment, in reduced spaces, of the desired symmetrical and a fully developed flow inside the wind tunnel, which leads to replicable results. This work shows that it is possible to solve the problem of the stagnation areas and suppose that the emitting surface under wind tunnel is completely involved in the volatilization process, without using a wind tunnel with a main chamber longer than 1 m. This means that the device is lighter and easier to handle than the previous devices considered in other studies. Moreover, the introduction of guide channels inside the divergent allows for the application of the wind tunnels in regions where the wind velocity is higher than 1 m/s. Indeed, as the results highlight, up to 5 m/s the flow can still be considered as fully developed. In this way, aerodynamic conditions inside the chamber should be closer to the real conditions of the case study considered.
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Fig. 10. Simulated velocity profiles for inlet air velocity of 5, 2.5 and 1 m/s: (a) along z axis and (b) along y axis.
Fig. 11. From the left: correlation between CFD simulated velocity values along y axis of guide channels and perforated plates (respectively), with an airflow velocity inlet of 5 m/s.
Acknowledgments
References
This research was realized under the Project ‘‘Optimizing the use of manure as a resource” funded by the Agriculture Department of the Campania Region. Authors want to thank Dr. Dianna Pickens, for English revision of the manuscript, and the anonymous reviewers for their valuable contributions to improve the quality of this paper.
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Durst, F., Melling, A., Whitelaw, J.H., 1974. Low Reynolds number flow over a plane symmetric sudden expansion. J. Fluid Mech. 64 (01), 111–128. Fluent, Inc., 2006. FLUENT 6.3 User’s Guide. Génermont, S., Cellier, P., 1997. A mechanistic model for estimating ammonia volatilization from slurry applied to bare soil. Agric. For. Meteorol. 88 (1), 145– 167. Gostelow, P., Longhurst, P., Parsons, S.A., Stuetz, R.M., 2003. Sampling for measurement of odours. Scientific and Technical Report No. 17. IWA Publishing, London, UK. Jiang, K., Bliss, P.J., Schulz, T.J., 1995. The development of a sampling system for determining odor emission rates from areal surfaces: Part I. Aerodynamic performance. J. Air Waste Manage. Assoc. 45 (11), 917–922. Lindvall, T., 1969. On sensory evaluation of odorous air pollutant intensities. Nordisk hygienisk tidskrift (Suppl-2). Lockyer, D.R., 1984. A system for the measurement in the field of losses of ammonia through volatilisation. J. Sci. Food Agric. 35 (8), 837–848. Loubet, B., Cellier, P., Flura, D., Genermont, S., 1999a. An evaluation of the windtunnel technique for estimating ammonia volatilization from land: Part 1. Analysis and improvement of accuracy. J. Agric. Eng. Res. 72 (1), 71–81. Loubet, B., Cellier, P., Genermont, S., Flura, D., 1999b. An evaluation of the wind-tunnel technique for estimating ammonia volatilization from land: Part 2. Influence of the tunnel on transfer processes. J. Agric. Eng. Res. 72 (1), 83–92. Martínez-Lagos, J., Salazar, F., Alfaro, M., Misselbrook, T., 2013. Ammonia volatilization following dairy slurry application to a permanent grassland on a volcanic soil. Atmos. Environ. 80, 226–231. Menter, F.R., 1994. Two-equation eddy-viscosity turbulence models for engineering applications. AIAA J. 32 (8), 1598–1605. Nabavi, M., 2010. Three-dimensional asymmetric flow through a planar diffuser: effects of divergence angle, Reynolds number and aspect ratio. Int. Commun. Heat Mass Transfer 37 (1), 17–20. Noui-Mehidi, M.N., Wu, J., Šutalo, I.D., Grainger, C., 2005. Velocity distribution downstream of an asymmetric wide-angle diffuser. Exp. Therm. Fluid Sci. 29 (6), 649–657.
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Contents lists available at ScienceDirect
Computers and Electronics in Agriculture journal homepage: www.elsevier.com/locate/compag
Original papers
Study of aerodynamic performances of different wind tunnel configurations and air inlet velocities, using computational fluid dynamics (CFD) Ester Scotto di Perta a,b, Maria Angela Agizza c, Giancarlo Sorrentino d, Lorenzo Boccia a, Stefania Pindozzi a,⇑ a
Department of Agriculture, University of Naples Federico II, Via Università 100, Portici (NA), Italy Department of Agriculture, Forestry, Nature and Energy (DAFNE), University of Tuscia, Via S. Camillo de Lellis s.n.c., 01100 Viterbo, Italy Interdepartmental Research Center For Environment, I.R.C. Env, Italy d Department of Chemical, Materials and Production Engineering, University of Naples Federico II, P.le Tecchio 80, Napoli, Italy b c
a r t i c l e
i n f o
Article history: Received 28 July 2015 Received in revised form 22 April 2016 Accepted 11 May 2016 Available online 19 May 2016 Keywords: Ammonia emission Wind tunnel Jet effect Flow distribution devices CFD
a b s t r a c t Livestock and agricultural activities contribute significantly to atmospheric ammonia emission in Europe. The volatilization process depends on many factors, especially wind speed and rainfall. The most important methods to evaluate ammonia volatilization are the wind tunnel and micrometeorological methods. The tunnels are more flexible and simple to use in every situation. Few studies have been carried out to determine, which conditions are established inside the chamber and how they influence the ammonia volatilization and measurement. The aim of this research was to investigate the effects of the wind tunnel configuration and flow inlet velocity, by means of CFD simulations and wind speed measurements, in order to achieve a better aerodynamic performance. The SST k–x model used for simulations was first validated in order to prove the consistency of the model itself. Several configurations were simulated and compared. In particular, in order to overcome the asymmetric flow conditions that occurred in all wind tunnel configurations, four flow distribution devices were proposed and simulated. The best setup was chosen with the purpose of reaching both the best uniform velocity distribution (to ensure homogeneous volatilization from the emitting surface) and easy transport for field applications. It consists of an emission chamber 40 cm wide, 25 cm high and 80 cm long, situated between a divergent diffuser and a convergent duct, respectively 50 cm and 25 cm long. Moreover, structures similar to honeycombs, namely guiding channels, were introduced in the divergent diffuser, because they showed the best aerodynamic performance. These 20 channels, located in the divergent diffuser, prevent flow from separating, by means of the reduction of the expansion angle, obtaining the desired flow conditions inside the wind tunnel. Finally, it was verified that CFD confirmed its usefulness as a decision-support instrument to design and simulate possible solutions, reducing design time. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction Livestock and agricultural activities contribute considerably to anthropogenic ammonia emission in Europe (Balsari et al., 2007). In particular, the major contribution, approximately 81% of total ammonia gaseous losses, result from animal waste (Buijsman et al., 1987). In general, these losses derive from each step of livestock manure management, that is to say from stables, manure storages or during/after application of manure as fertilizer ⇑ Corresponding author at: Department of Agriculture, University of Naples Federico II, Via Università 100, 80055 Portici (NA), Italy. E-mail address: [email protected] (S. Pindozzi). http://dx.doi.org/10.1016/j.compag.2016.05.007 0168-1699/Ó 2016 Elsevier B.V. All rights reserved.
(Olesen and Sommer, 1993), but more than half of these emissions comes from field-applied slurry (Loubet et al., 1999, Part 1). Large amounts of ammonia in the atmosphere can have damaging effects on human and animal health, because of its contribution to the deterioration of air quality and ecosystems (Rotz et al., 2014). Indeed, the volatilization of ammonia from field is known to be responsible for soil acidification, water eutrophication, and formation of fine particulates and secondary emissions of nitrous oxide. In addition to their environmental pollution problem, ammonia leaks can cause heavy economic losses to farms, because of the lack of available nitrogen that is indispensable for crop production (Martínez-Lagos et al., 2013). Consequently, it is quite clear why there is increasing interest in limiting ammonia
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volatilization, through the development of abatement technologies. The volatilization process depends on many factors such as soil conditions, agricultural practices, slurry characteristics and climate conditions, especially wind speed and rainfall (Génermont and Cellier, 1997; Sommer and Hutchings, 2001; Martínez-Lagos et al., 2013). Consequently to the complexity of this process, it is difficult to assess the atmospheric ammonia fluxes and there is no standard measurement for this purpose, because the emission rate is extremely variable due to the effects of different factors. Several studies have been carried out to evaluate ammonia losses, using different methods. The most important are dynamic flux chambers or wind tunnels and micrometeorological techniques. Even if the latter are often preferred in terms of accuracy, they suffer from many limitations due to the large number of samples and the extensive analysis required. Instead, the principle of the dynamic flux chamber approach is very simple. Although the wind tunnel method introduces microclimate modifications, which could affect the measurements, it is more flexible and simple to use, especially for small area sources (Parker et al., 2013). In particular, to simulate the wind action, the air is supplied with a fan, that allows obtaining a velocity, inside the main chamber, between 0.3 and 1.0 m/s. The emission rate depends on the air velocity selected during the measurements. In particular it can be assessed as the product of the flow rate and the concentration of volatile chemicals under a special cover, in which the aerodynamics and flow rates are controlled (Jiang et al., 1995). In literature, there are many examples of portable wind tunnels being placed on the emission surface. One of the most important is the Lindvall, 1969 hood that is characterized by a rectangular measurement section, with contraction and expansion sections. Another one is the Lockyer (1984) wind tunnel system, constructed with a transparent polycarbonate sheet and a steel circular duct (Gostelow et al., 2003). Few studies have been carried out to determine which conditions are established inside the tunnel and how they could influence the emission rate and the measurement. Since the aerodynamic performance of the tunnel is considered a critical parameter (Jiang et al., 1995), it is, therefore, important to understand precisely what aerodynamic conditions have been established inside the tunnel (Gostelow et al., 2003). However, it is not easy to make a comparison among several emission rates obtained with the different geometric dimensions of wind tunnels (Saha et al., 2009). Therefore, each geometric configuration must be considered separately. Previous studies under laminar conditions reported that the flow in a symmetric chamber configuration with a plane expansion is almost always symmetrical, but becomes asymmetrical at a critical Reynolds number and downstream of the sudden expansion. In these cases, an asymmetrical mean flow pattern is observed depending on the shape of the channel and the flow Reynolds number (Durst et al., 1974; Jiang et al., 1995; De Zilwa et al., 2000; Nabavi, 2010). On the other hand, the reduced length of the chamber needed to achieve ease of transport, requires a diverging diffuser with a short working length and a larger tunnel which can cover a greater emission area, in order to obtain a more significant results. The flow asymmetry could affect the turbulent mixing and the ammonia transfer rate, therefore uniform flow distribution at the outlet of diffuser should be obtained. This is something that needs to be achieved by means of the installation of flow distribution devices, which improve aerodynamic performance. The present study is framed in this background and its aim is to set up a wind tunnel, a simpler and economical procedure, with optimal aerodynamic performances, in order to accurately evaluate ammonia losses, obtaining replicable results. In other words, it is very important to avoid the eventual accumulation of compounds
inside the chamber and the possibility that the flow behaviour does not fit the atmospheric real case. For such purposes, several wind tunnel configurations were simulated and compared. In particular, it was verified the existence of asymmetric flow behaviour, that is very problematic for ammonia measurement. In order to overcome this phenomenon, called ‘‘jet effect”, and to obtain a fully developed uniform flow, four flow distribution devices were proposed, simulated and compared.
2. Materials and methods 2.1. The wind tunnels The wind tunnels used for this research were constructed according to the geometry proposed by Jiang et al. (1995) and then by Balsari et al. (2007). The geometry is characterized by a main chamber located between a divergent and a convergent duct (respectively expansion and contraction section). The first one allows for the transition from a circular section to rectangular one, and the second duct vice versa (Fig. 1). Each element could be disassembled from the others for ease of transport. Two airsampling points are considered, respectively one at the beginning and the other at the end of the wind tunnel, in order to measure the concentration of ammonia in the air at both the inlet and outlet sections. The bottom of the main chamber is open and facing the emission surface, in particular the experimental area is 40 cm wide and 80 cm long, so it consists of 0.32 m2 (Jiang et al., 1995). In order to simulate the wind conditions, which influence the emission phenomenon, ventilation is provided by an axial fan, with a maximum flow rate of 298 m3/h. The fan is connected to the convergent duct. In order to have a fully developed flow, upstream of the divergent diffuser there is a 0.10 m diameter cylindrical duct, whose length is ten times the inlet diameter (1 m). This solution leads to an improved velocity distribution and a reduction of velocity fluctuations (Jiang et al., 1995). As non-uniform velocity and concentration profiles may lead to errors in emission rate estimation (Gostelow et al., 2003), different wind tunnels sizes and different diffuser configurations were considered and compared, in order to prevent the flow from separating and to obtain a fully developed flow and uniform distribution of velocity profiles. Indeed, the main issues concerning the use of dynamic chambers are the eventual accumulation of compounds inside the hood and the possibility that the flow behaviour does not fit the atmospheric real case. Since few studies are focused on such purposes, the dimensions of the tunnels used for this work were selected according to: – the studies of Durst et al. (1974), De Zilwa et al. (2000), Nabavi (2010), focused on how expansion ratio and the shape of the duct could influence the conservation of flow symmetry and the separation length; – the considerations about the effect of different heights of the chamber in aerodynamic characteristics, discussed in the studies by Saha et al. (2009) and Saha et al. (2011); – the suggestions of Jiang et al. (1995) about the possibility of modifying the chamber length. In particular, the design dimensions are listed in detail in Table 1.
2.2. CFD modelling The computational fluid dynamics (CFD) technique was used to simulate ventilation performances, to study the airflow in the
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Fig. 1. A sketch of the wind tunnel and a detailed view of the divergent diffuser.
Table 1 Wind tunnels and their design dimensions. Parameter
A25-25
B50-15
C50-25
D50-50
E50-25-160
F50-25-25
WT width (m) WT height (m) Main chamber length (m) Divergent length (m) Convergent length (m)
0.40 0.25 0.80 0.25 0.25
0.40 0.15 0.80 0.50 0.50
0.40 0.25 0.80 0.50 0.50
0.40 0.50 0.80 0.50 0.50
0.40 0.50 1.60 0.50 0.25
0.40 0.25 0.80 0.50 0.25
room and the contaminant dispersion from buildings across wide research areas (Saha et al., 2011; Rong et al., 2011; Wu et al., 2012). The basic concept of CFD is to solve a set of partial differential equations. The governing equations are the Navier–Stokes and continuity equations. The accuracy of CFD depends on many factors, such as boundary conditions, numerical methods, mesh and turbulence models (Rong et al., 2011). For this reason, an optimal choice of the correct parameters is mandatory. Commercial software Fluent 6.3.26 was used to solve the equations on the basis of finite volume method. This software allows the user to choose between two different main solvers, segregated or coupled. They both implement the Finite Volume Method as numerical method, but the approach towards the governing equations is different. The coupled solver is recommended for transonic and supersonic flows, while the segregated solver is much faster for low-speed flows and is thus more suitable for incompressible flows, showing good performances for subsonic compressible flows calculations. For these reasons, the segregated solver was used (Fluent, 2006). 2.2.1. Turbulence model Many researchers have discussed the application of the different turbulence models for indoor environments. Since the inlet Reynolds number is higher than 5000 in each tunnel configuration, the airflow is in turbulent conditions and therefore the choice of a proper turbulence model is mandatory. Although the k–e model is still widely used to simulate ventilation conditions, several studies had confirmed that the SST k–x model is more appropriate to simulate a mass transfer process in aerial boundary layer (Sparrow et al., 2009; Saha et al., 2011). In particular, this model, developed by Menter (1994), is governed by two equations, and results from a
combination of two models, respectively for calculations in the inner boundary layer and in outside of the boundary layer. One of the advantages of the k–x formulation is the near wall treatment for low-Reynolds number computations. Indeed, the model does not comprise the complex nonlinear wall damping functions required for the k–e model and it is therefore more correct and more consistent (Rong et al., 2011). The better performance of this model has been demonstrated in different validation studies (Rong et al., 2011; Saha et al., 2011). The SST model is recommended for high accuracy boundary layer simulations. For this reason, the SST model was applied in this study and compared with the experimental results. The convergence is assumed to be reached when residuals have stabilized at 10 6 for continuity and momentum equations. The number of iterations required to reach this value showed an obvious dependence on mesh resolution, dimension of the calculated domain and its complexity. Nevertheless, the target value was reached in every calculation shown below. 2.2.2. Discretization scheme The computational domain is the entire wind tunnel for each configuration. A tetrahedral volume mesh was used in this study, for all selected cases. The discretization was done on the basis of the smallest mesh resolution required for the RANS simulation. In particular, a mesh independency study was also adopted. To make the meshing strategy clear, technical details are reported in Table 2. 2.2.3. Boundary conditions Another important step for a correct use of the CFD technique, in order to have reliable results, is a proper definition of the
Table 2 Technical simulations details. Parameter
A25-25
B50-15
C50-25
D50-50
E50-25-160
F50-25-25
Grid number First node height (mm) Nodes number Iterations number
165,591 0.775 55,560 1050
386,409 1.006 143,598 8750
191,178 0.800 68,799 2460
149,671 0.849 103,104 1890
87,823 1.067 35,243 1000
372,226 0.652 143,598 8750
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Fig. 2. Comparison of velocity profiles between CFD simulations and experimental measurements published by Jiang et al. (1995).
Fig. 3. Comparison of velocity profiles between CFD simulations and measurements for two tunnel configurations: F50-25-25 (a) and A25-25 (b).
boundary conditions, according to the case study. Three different boundary conditions were used in this work: velocity inlet, pressure outlet and walls. Specifically, for all the simulations, the pressure outlet was considered equal to atmospheric pressure. Moreover, all the wind tunnels interior surfaces were simulated as wall boundary with no-slip condition. For the comparison between simulations and experiments the inlet air velocity was fixed at 5 m/s, according to the data obtained by experimental measurements of the flow rate provided by the axial fan. For the simulations that investigated the introductions of flow distributions devices, the inlet air velocity was varied among 1, 2.5 and 5 m/s.
2.3. Experimental data for model validation In order to avoid mistakes in the simulations and to be sure that results describe correctly the phenomena in its reality, model validation is a required step, before investigating the geometries designed. For this purpose, data used for validation were the experimental velocities published by Jiang et al. (1995). In particular, the wind tunnel (UNSW wind tunnel) used is similar to F50-25-25 tunnel configuration, considered in this work, characterized by a main chamber with a width of 400 mm, a height of 250 mm and a length of 800 mm. Therefore, the expansion section is 500 mm long, while the contraction section is 150 mm. An inlet velocity of 0.33 m/s
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was considered, according to Jiang et al. (1995) experiment. The velocities used for validation were the vertical ones, obtained in the three cross-sections of the main chamber, at the position of x = 200 mm, x = 400 mm and x = 600 mm. 2.4. Experimental measurements Velocity and smoke tests were performed in order to characterize the aerodynamic conditions inside the tunnel and to carry out a qualitative analysis of the obtained results with CFD simulations. The measurements were performed in different positions, with a portable air velocity transducer (MODEL 8465, TSI) that has a resolution of 0.07 m/s and a measuring working range of 0.125–50 m/ s. The instrument gives the instantaneous value of the measured air speed. The operation has been repeated three times, after ten seconds, for each position. The average of three obtained velocity values was considered as the final result. Three cross-sections were chosen for the measurements and for calculations, at the position of 1/8 (x = 10 cm), 1/2 (x = 40 cm), 7/8 (x = 70 cm) of the length of the main chamber, respectively, that are generally indicated in the following figures with the letters o and v respectively, for profiles along z and y-axis. In order to monitor the velocity profiles inside the chamber, each cross-section was characterized by twenty-four measuring points; in particular six locations along the height were investigated: 4 cm, 7.5 cm, 12.5 cm, 17.5 cm, 21 cm, and 24 cm from the top of the tunnel to the bottom. For each height, 4 measuring points along z were considered. The measured velocity profiles (VP) were used for a qualitative CFD comparison. VP were compared with those obtained from CFD simulations, considering the same geometrical characteristic of A25-25 and F50-25-25 tunnel configurations. Moreover, the flow behaviour was better observed by following the path of a visible tracer in both the tunnel configurations (Jiang et al., 1995). For this purpose, an airflow visualization analysis via smoke studies was performed using incense smoke sticks. Each stick was inserted in 4 holes along z for each cross-section, the same used for velocity measurements with anemometer. This allowed the observer to investigate the flow pattern at different positions. 3. Results and discussion 3.1. Grid independency Ideally, the CFD simulation results should be independent on the computational grid. To fulfil this requirement, different meshes were considered for each geometry, varying the number of grid elements or the control points. For the A25-25 case, computational
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grids of 75,077, 82,942 and 165,591 elements were compared. Few differences among the solutions have been found, implying that a further refinement of the grid wouldn’t change the results significantly. It confirmed that the results were mesh-independent. This independency test was performed for every calculation shown in the following test. 3.2. CFD model validation In order to prove the consistency of the CFD method, the experimental velocity profiles considered for the validation were obtained from the experimental data of Jiang et al. (1995). The study was conducted in a wind tunnel of similar size. Moreover, similar flow characteristics and the same flow behaviour, called ‘‘jet effect” were observed. The comparison of the vertical velocity profiles confirmed the same shape for the three cross sections considered. Specifically, the coefficient of determination (R2) values of measurement and numerical results for the section x = 200 mm, x = 400 mm and x = 600 mm, were respectively 0.96, 0.95 and 0.96, as shown in Fig. 2b. The comparison between the CFD and experimental results (Fig. 2a) demonstrated that the SST model is suitable for simulating characteristics of airflow in this specific situation and predicts in a satisfactory manner the trend of the atypical aerodynamic behaviour. As a consequence of the reliability of the numerical model, it has been used to simulate the airflow behaviour in following different improved designs of wind tunnels, as suggested by Saha et al. (2011). 3.3. Qualitative comparison between simulations and experiments Measured and simulated velocity profiles along z axis in three cross sections have been compared for F50-25-25 and A25-25 configurations in Fig. 3, at 12.5 cm from the emitting surface. Despite possible measurement errors, it was verified that both results confirmed the anomalous asymmetric behaviours of the flow pattern. In particular, these apparatuses cause a three-dimensional phenomenon whose onset is located in the divergent diffuser, as shown in Fig. 4. In the region of asymmetry, the velocity value near the wall is greater than that close to the centre of the tunnel. These behaviours have been reported in different studies in which the nature of asymmetric flow conditions has been examined (Durst et al., 1974; De Zilwa et al., 2000; Nabavi, 2010). Durst et al. studied the nature of asymmetric flow conditions in a plane symmetric sudden expansion and observed that flow is perfectly symmetric at low inlet Reynolds numbers, with equal-sized separation bubbles forming on both sides just downstream of the
Fig. 4. Velocity streamlines predicted by CFD simulation of F50-25-25 (a) and A25-25 (b).
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Fig. 5. Velocity profiles along z and y-axis and streamlines predicted by CFD simulations of three tunnel configurations: (a) B50-15; (b) C50-25 and (c) D50-50.
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Fig. 5 (continued)
Fig. 6. Velocity streamlines predicted by CFD simulations F50-25-25 configuration with a chamber with a double length.
sudden expansion. As the Reynolds number gets over a critical value, the flow loses its symmetry and one of the separation zones becomes larger than the other. Therefore, such phenomena depend on Reynolds number and expansion ratio, which influence the length of separation regions. In particular, the expansion ratio and the shape of the duct could influence the conservation of symmetry and the separation length. The experimental investigations by De Zilwa et al. (2000) suggested that the flow asymmetry would persist up to Reynolds numbers at which the flow becomes turbulent. The asymmetry of the steady flow is due to an
asymmetry-breaking bifurcation, and that the larger separation zone may occur on either side of the channel without any preference. This flow asymmetry is sometimes attributed to the so-called ‘‘Coanda effect” (Sobey and Drazin, 1986). 3.4. Effect of main chamber height The simulated velocity profiles and velocity contours in the B50-15, C50-25 e E50-50 tunnels are shown in Fig. 5. In particular, velocity profiles along y-axis are taken 20 cm from the tunnel wall
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and profiles along z-axis from 12.5 cm from the tunnel bottom. By examining the simulation results, it was observed that even if the asymmetric flow behaviour occurred in all the cases; air velocity values in different sections of the tunnel were higher when the height decreases. Moreover, it was found that in all the cases a big recirculation zone is developed inside the chamber. This is due to the shape and dimensions of the expansion section as suggested by Jiang et al. (1995). The phenomenon of a gaseous flow that passes through the hood, generating a rotation region inside the chamber itself, is called ‘‘jet effect”. It becomes more significant if the width of the tunnel is larger than the height. Interesting phenomena were observed by looking at the simulated results of the E50-50 tunnel configuration. In this case, the asymmetric flow was found not in the xz plane, but in the xy one (Fig.5c). The explanation to this result could be related to the expansion angle of the diffuser in the xy plane. In fact, if the height of the chamber increases, also the expansion angle of the diffuser in the xy plane will become larger than the one in the xz plane. 3.5. Effect of the main chamber length Given the flow non-uniformity, attested in all the investigated geometry configurations, only one of them (F50-25-25) was further investigated in a second part of the work. This choice seems to fit in the best way the double requirement of potential aerodynamic performances and reduced space. Different potential answers to the problem were analysed and compared. A possible solution could be to increase the length of the chamber or to introduce flow distribution devices (Jiang et al., 1995). Simulated E50-25-160 tunnel geometry, characterized by a main chamber of double length (1.60 m) is shown in Fig. 6. By increasing the main chamber length, the flow path is directed first towards one side of the wind tunnel and afterwards towards the other side, before returning to the centre-line, just at the end of the chamber (Durst et al., 1974). As shown, two unequal recirculation zones appear: the larger separation region occurs in the first part of the tunnel and then on the other side, in correspondence of the inversion of the flow. All the rotations occur around a vertical axis. As can be seen, a higher distance from the inlet reduces the inlet expansion effect, so that it is possible to obtain a stable flow if the length of chamber is increased. On the other hand, this solution occupies too much space, and it is not useful in field applications, in which reduced space is an important requirement. 3.6. Effect of the introduction of flow distribution devices In order to reach repeatable conditions inside the tunnel, it is important to obtain a steady and uniform distribution of velocity profiles. For this reason, flow distribution devices must be introduced (Jiang et al., 1995) and therefore, four solutions were designed and simulated. Tetrahedral volume mesh was used also in these cases. An example of mesh distribution is shown in Fig. 7. Fig. 8 shows devices configuration and location. The optimum condition was chosen as a function of the aerodynamic performance. Moreover, each device was investigated at three airflow inlet velocities, as shown in Table 3, for two main reasons: wind speed is a useful and valid parameter to compare flux measured inside the tunnel and those measured in open field condition (Loubet et al., 1999, Part 2), and the intensity of the ‘‘jet effect” phenomenon grows by increasing the velocity (Jiang et al., 1995). 3.6.1. Wire gauzes Two wire gauzes were considered inside the divergent diffuser: one near the inlet section and the other in the middle of diffuser.
According to aerospace applications, screens are often used to reduce turbulence scale. Results, in Fig. 8a, show that flow behaviour is not so much different from the previous simulations, also varying inlet velocity, in fact turbulence is not reduce and the ‘‘jet effect” still occurs. This is probably related to the fact that a screenhoneycomb combination was not used, and screens alone are not enough. 3.6.2. Guide vanes Another possible solution that was investigated involves the introduction of horizontal and vertical guide vanes combination at the beginning of the divergent diffuser (Fig. 8b). Each vane has a width of 5 cm and they are running parallel to the xz plane initially and later to the xy plane of the wind tunnel, in the direction of the flow. Even in that case, three different velocity inlet values were simulated but no differences were obtained among them, in terms of flow behaviour. 3.6.3. Perforated plates As reported by Noui-Mehidi et al. (2005), sometimes to prevent the flow from separating and to achieve a good uniformity in the flow behaviour inside wide-angle diffuser, perforated plates are proposed. To this matter, two perforated plates were positioned within the divergent diffuser. The first one was located at 15 cm from the diffuser inlet, while the second plate was placed after 30 cm from the first one. The plates are characterized by circular holes of 2 cm diameter, located at a distance of 1 cm from each other, in order to obtain a porosity varying from 40% to 50% (Fig 8c). This configuration was designed in respect of WardSmith et al. (1991). They found that even with only two perforated plates, flow uniformity could be achieved, provided that appropriate porosity and plate positions have been chosen. Velocity profiles of three investigated inlet velocity are shown below (Fig. 9). Perforated plates represent the first solution that allows the ‘‘jet effect” to be eliminated. However, to test the development of the flow, velocity value of the three sections were analysed and correlated each other. Results are reported in the Section 3.6.5. 3.6.4. Guide channels Finally, two structures similar to honeycombs have been simulated. They consist of a series of horizontal and vertical planes that intersect each other, in order to obtain several channels, located in the divergent diffuser. These channels guide the flow direction up to the main chamber of wind tunnel and prevent flow from separating, by means of the reduction of the expansion angle. Different structures were considered, but only the one that gave the best results is reported in Fig. 8d. The structure is characterized by 4 vertical planes and 3 horizontal ones, in order to obtain in total 20 channels. These are extended through the diffuser up to 10 cm to its end, in order to reduce the possible velocity fluctuations due to the channels. Results of simulations, carried out with three different inlet velocity values, are shown as follows. The obtained velocity profiles are reported in Fig. 10. As for perforated plate configuration, guide channels lead to uniform fluid distribution, because ‘‘jet effect” does not occur. Thus, it is possible to suppose that the emitting surface under wind tunnel is completely involved in the volatilization process, since there are not stagnation areas. Moreover, as it is possible to observe, especially for lower velocity, the flow seems to be fully developed at the beginning of the main chamber, without increasing its length. Even if, based on the previous figures, for lower inlet velocity, parabolic velocity profiles are less dominant and a reduced number of velocity fluctuations can be observed, as found also by Jiang
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et al. (1995), the introduction of guide channels in the divergent allows the desired aerodynamic performances to be obtained, even for values of velocity lower than 5 m/s. This means that contrary to the velocity range usually considered for this kind of study, it is possible to use this kind of wind tunnel also in the regions, where the wind speed is relevant and higher than 1 m/s near the ground, with conditions inside the chamber that are closer to the real conditions study here considered. Indeed, inlet velocity is an
important parameter, which should be accurately chosen, but it is strongly dependent on the real case to be simulated. 3.6.5. Comparison between perforated plates and guide channels Simulations have shown that the best configurations are perforated plates and guide channels, in terms of uniform fluid distribution. In order to choose one of them as the best flow distribution device, the full flow development of each device was considered.
Fig. 7. Mesh distribution as an example: (a) Wire gauzes; (b) Guide vanes; (c) Perforated plates; (d) Guide channels.
Fig. 8. Devices configuration and velocity streamlines predicted by CFD simulations at the inlet velocity of 5 m/s: (a) Wire gauzes; (b) Guide vanes; (c) Perforated plates; (d) Guide channels.
Table 3 Flow devices, their simulated parameters and technical simulations details. Parameter
Wire gauzes
Inlet velocity Re (103) Device location Grid number Nodes number
1 2.5 5.5 13.9 X1 = 1 m; X2 = 1.25 m 271,837 88,062
5 27.8
Guide vanes
Perforated plates
Guide channels
1 2.5 5 5.5 13.9 27.8 From X = 1 m to X = 1.1 m 840,258 154,642
1 2.5 5 5.5 13.9 27.8 X1 = 1.15 m; X2 = 1.45 m 3,061,708 551,066
1 2.5 5 5.5 13.9 27.8 From X = 1 m to X = 1.4 m 180,495 38,619
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Fig. 9. Simulated velocity profiles for velocity of 5, 2.5 and 1 m/s: (a) along z axis and (b) along y axis.
Namely, in addition to the uniformity of air flow profiles, also the variability of the velocity profiles for consecutive cross sections was taken into account for both devices. In particular, to verify fully development of air flow, velocity profiles along y-axis, obtained by CFD modelling, for the three consecutive positions (x = 10 cm, x = 40 cm, x = 70 cm) were analysed and correlated each other. Although three inlet air velocities were investigated, only the correlation for the velocity along y-axis with an airflow velocity inlet of 5 m/s is reported in Fig. 11 for brevity. As it is possible to note, guide channels have R2 values which are higher than those for the perforated plates, about 0.98–0.99. This effectively proves that even with maximum inlet velocity values, they allow a fully developed flow to be obtained from the beginning of the main chamber. Instead, according to perforated plates, even if velocity profiles start to be reasonable, in terms of uniformity, more space is necessary to reach a fully developed flow. Results demonstrated that the wind tunnel configuration with guide channels represent the best solution, in order to have a fully developed symmetrical and uniform flow in a reduced space with replicable results. 4. Conclusions In this study, an assessment of the effects of the wind tunnel geometry with focus on the flow behaviour was performed with CFD simulations. In order to prove the consistency of the SST model, the experimental velocity profiles, obtained from the experiment of Jiang
et al. (1995), conducted with a wind tunnel of similar size, were considered for the validation. The comparison between the numerical and experimental results demonstrated that it is possible to consider that CFD simulates well the general trend of air velocity inside the tunnel. Therefore, it is a powerful tool not only to catch the complexity of the 3D asymmetry (which occurs with a wide expansion of the air flow in the three dimensions), but also to design and simulate possible solutions for separating flow and to reduce design time. In this sense, in order to obtain a steady and uniform distribution of velocity profiles, an optimal design of the flow distribution devices seems to be very important. Therefore, four solutions were designed, simulated and compared by means of CFD computations. Among several experimental devices designed and simulated, only the guide channels configuration results in the attainment, in reduced spaces, of the desired symmetrical and a fully developed flow inside the wind tunnel, which leads to replicable results. This work shows that it is possible to solve the problem of the stagnation areas and suppose that the emitting surface under wind tunnel is completely involved in the volatilization process, without using a wind tunnel with a main chamber longer than 1 m. This means that the device is lighter and easier to handle than the previous devices considered in other studies. Moreover, the introduction of guide channels inside the divergent allows for the application of the wind tunnels in regions where the wind velocity is higher than 1 m/s. Indeed, as the results highlight, up to 5 m/s the flow can still be considered as fully developed. In this way, aerodynamic conditions inside the chamber should be closer to the real conditions of the case study considered.
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Fig. 10. Simulated velocity profiles for inlet air velocity of 5, 2.5 and 1 m/s: (a) along z axis and (b) along y axis.
Fig. 11. From the left: correlation between CFD simulated velocity values along y axis of guide channels and perforated plates (respectively), with an airflow velocity inlet of 5 m/s.
Acknowledgments
References
This research was realized under the Project ‘‘Optimizing the use of manure as a resource” funded by the Agriculture Department of the Campania Region. Authors want to thank Dr. Dianna Pickens, for English revision of the manuscript, and the anonymous reviewers for their valuable contributions to improve the quality of this paper.
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