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PIV experiment and evaluation of air flow performance of swirl diffuser mounted on the floor
Energy and Buildings 156 (2017) 58–69
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Energy and Buildings journal homepage: www.elsevier.com/locate/enbuild
PIV experiment and evaluation of air flow performance of swirl diffuser mounted on the floor Angui Li ∗ , Changqing Yang, Tong Ren, Xin Bao, Erwei Qin, Ran Gao School of Environmental and Municipal Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, China
a r t i c l e
i n f o
Article history: Received 16 May 2017 Received in revised form 28 August 2017 Accepted 16 September 2017 Available online 21 September 2017 Keywords: Flow visualization 2D-PIV Swirl diffuser Dimensionless velocity Centerline velocity decay coefficient Entrainment ratio
a b s t r a c t In order to obtain satisfactory effect of air conditioning and ventilation, the floor–supply displacement ventilation has widely applied in many civil buildings. Swirl diffusers mounted on the floor are widely used to provide fresh air into the occupied zone in the floor–supply displacement ventilation system. This paper studied the air distribution performance of swirl diffuser by 2D-PIV (particle image velocimetry). The experiment was implemented in a full-scale room to achieve reliable data of swirl diffusers. Flow visualizations have been carried out to determine the shape of the swirl jet. The whole velocity field was measured by 2D-PIV system, which is very useful to analyze air distribution above the swirl diffuser. With the aid of the flow visualizations and PIV test technology, the macro and micro structure of the air flow above the swirl diffuser is specified in more detail. Moreover, the centerline velocity decay coefficient (K value) and entrainment ratio were obtained. The research work of this paper provides detailed experimental data of air flow above the swirl diffuser, which provide the reference for selection of air terminal devices and a basis for follow-up numerical simulation study. © 2017 Published by Elsevier B.V.
1. Introduction Indoor air distribution determines the diffusion of ventilated air, therefore, affecting air quality and human comfort. Mixing ventilation (MV) and displacement ventilation (DV) are often used to describe air movement within closed environments [1]. MV systems are very widely used and have the large market share. However, MV always exhibits a poor ventilation performance [2] and have low energy efficiency [3]. Meanwhile, although some studies have shown that DV is an effective method of providing air conditioning buildings [4,5], DV also have some drawbacks such as the large vertical temperature difference, the big air supply inlet and the draft sensation [5,6]. Therefore, to avoid these problems, underfloor air distribution (UFAD) has been studied and used [7,8]. UFAD and DV systems are based on many of the same principles in cooling manner as they both supply conditioned air directly into the occupied zone and return it near the ceiling area [9]. The main difference between UFAD and DV is in the way that they supply conditioned air into the occupied zone. In current situation, these mentioned ventilation systems are respectively adopted in a suitable building to provide the comfortable interior environment.
∗ Corresponding author. E-mail address: [email protected] (A. Li). http://dx.doi.org/10.1016/j.enbuild.2017.09.045 0378-7788/© 2017 Published by Elsevier B.V.
Air diffusers are important components of ventilation and airconditioning systems. The comfort level of occupants and the efficient delivery of air in a building depend on, among other factors, properly designed and selected diffusers. Swirl diffusers are modern types of air diffusers which can provide high mixing effect due to making the air to swirl and using the centrifugal force to enhance induction effect. Based on computational fluid dynamics, S.C. Hu [10] studied the airflow features in a specific type of ceiling swirling diffuser, which shows that the influence of the stationary twists guide vanes on centerline decay coefficient and entrainment ratio. Sajadi B et al. [11] studied numerically the air distribution performance in a common type of ceiling swirl diffuser with different swirl blades angle and slots geometry. And the results of a parametric study demonstrate that the effect of swirl blades angle on the air distribution performance is more important than that of slots specifications. Raftery P et al. [9] performed laboratory experiments to study the stratification performance of swirl diffuser mounted on the floor that supply air horizontally. Kobayashi N et al. [12], Lau J and Chen Q [13] carried out the experiments and developed numerical models to predict the air distribution performance of a UFAD system clearly. The simulated temperature distribution were in excellent agreement with the experimental results. Meanwhile, although the agreements between the computed velocity and experimental results were reasonable, it is not as good as the temperature at some zones since the numerical methods, physi-
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Fig. 1. 2D-PIV measurement system.
Fig. 2. Experimental rigs.
cal models and mesh quality are not perfect and the measurement instrument error is difficult to avoid. Ehsan Tavakoli and Reza Hosseini [14] studied numerically the effect of swirl diffuser angle on airflow pattern and ventilation characteristics. In many air conditioning systems, swirl diffusers are commonly used to supply conditioned air directly. However, the previous researches have mostly focused on the other models, for example, supply grilles, flared nozzle, hemispherical nozzles or ceiling diffusers. Up to now, many relevant studies of floor swirl diffuser have mostly focused on the thermal comfort conditions of occupants, indoor air quality and energy consumption in the building. In addition, most experimental studies always use the single-point measurement data to analyze the airflow pattern of swirl diffuser, it is not feasible to measure the whole velocity vector field of swirl diffuser. Therefore, detailed airflow information including airflow pattern and the whole velocity vector field above the floor swirl diffuser has not been available. With the development of the computational fluid dynamics (CFD) techniques, the researchers can obtain large data about the airflow and the whole velocity vector field above the swirl diffuser without getting involved into the expensive. However, the complex geometry of the floor swirl dif-
fuser and the paucity of accurate experimental data impress the simulated results accuracy seriously. Moreover, detailed airflow information about floor swirl diffuser including jet centerline velocity decay coefficient (K value) and entrainment ratio is not available, which also brings certains difficulty to select the terminal devices effectively in the air conditioning systems. In this paper, with the aid of the flow visualizations and 2D-PIV test technology, the objectives are: to obtain the macro and micro structure of the air flow above the swirl diffuser in more detail experimentally, to obtain jet centerline velocity decay coefficient (K value) and entrainment ratio for selection of air terminal devices and provide the whole velocity vector field of swirl diffuser for follow-up numerical simulation study. 2. Experimental set up There are a number of methods available for measuring jet velocity vector field, each with its own advantages and disadvantages [15,16]. Specifically, PIV has been an increasingly popular and effective technique for indoor airflow field measurement during the last decade. Because the non-intrusive feature of PIV technique, PIV
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Fig. 3. Swirl diffuser mounted on the floor: (a) the front of diffuser; (b) the back of diffuser.
almost will not affect the velocity field, which is a key advantage of PIV over many other measurement methods. In addition, PIV has advantages of measuring a complete velocity field at each instant in time [17,18]. The whole velocity field of the swirl diffuser is quite helpful to study airflow performance. Therefore, in this paper, the velocity vector field above the swirl diffuser is measured by 2D-PIV system. 2.1. 2D-PIV measurement facilities The 2D-PIV measurement system is showed in Fig. 1. In this study, pulsed laser beam was produced by the double pulsed Nd:YAG laser (energy:200 mJ). The high quantum efficiency and low noise digital CCD camera (Nikon Camera, Type: 630157 Power view plus 2MP) was used to take the experimental pictures, which is the 1.6k × 1.2k pixel resolution. During our studies the 450 mm × 600 mm section is the biggest measuring area which can be shot by PIV technology. In addition, the laser pulse computer controlled synchronizer (Type: TSI610034) was used to control time sequence of camera and laser. To avoid the contingency of experimental results, many pictures were shot in certain experimental conditions because of specificity of PIV [19]. In the PIV experiment, the time difference of two laser pulses (t) has a very significant impact on the experimental results. Because the supply air velocity changes in different cases, it should adjust this time before each test. Besides, we know that t increases with the decrease of the maximum velocity (vmax ) of the shooting section in different cases. In previous PIV literature of authors’ research group [20,21], a relationship between t and vmax was obtained as follow: ln (t) = 5.5214 − ln (vmax )
(1)
vmax × t = 250
(2)
Based on Eq. (2), the most important parameter of the laser source light can be set accurately in PIV experimental research. During our studies the mean particle size of the tracer particle is 1 ∼ 2 m, which is the spherical droplets generated by smoke generator (Type: Rosco 1700). Such particles have a good following
and astigmatism, and the response time to the motion of the air is reasonably short to accurately follow the flow. 2.2. Physical model The size of the room where the experiment was conducted is 6.6 m × 6.6 m × 3.15 m (L × W × H). According to experimental requests, a big box was used as pressured plenum (Fig. 2). Size of the box is 1.6 m × 0.6 m × 0.55 m (L × W × H), in which inner net height of the box is 0.5m. The distance between diffuser and ceiling is more than 2.6m. There is enough space around the diffuser in order to ensure air flowing out from diffuser to flow freely. In addition, TROX swirl diffuser (Type: FBK 200) is used in this study (see Fig. 3). As shown in Fig. 3, the basic sizes of swirl diffuser are prescribed: R1 = 199.5 mm, R2 = 188 mm, c1 = 21 mm. The effective outlet area of the swirl diffuser in this study is 0.0056m2 . 2.3. Shooting section set up In this study, vertical and horizontal shooting sections were shown in Figs. 4 and 5. Those green blocks are just shooting section. Due to symmetry of the Trox swirl diffuser and fog pictures, we choose three pieces of the section with the same size for 450 mm × 600 mm. There are vertical section 1, vertical section 2 and vertical section 3. The right edge of section 2 and 3 is just middle of section 1. Section 1 locates in the middle of swirl diffuser. Due to camera performance and limit of ceiling height, we shoot horizontal sections with the size of 360 mm × 250 mm at three different heights (heights = 50 mm, 200 mm, 500 mm). 2.4. Flow visualization In addition to the 2D-PIV experimental study and velocity point measurements, this paper conducted two tests to visualize the jets issuing from the swirl diffuser and observe the introduction of ambient air into the jets. The fog from the swirl diffuser is used to observe the motion of the ambient air near the diffuser. A nontoxic liquid is pumped into the machine and atomized to a very fine
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2.6. Processing method of experimental results Because the supply airflow and the effective outlet area of the swirl diffuser in our experiment are different from that of others’ diffusers, therefore, the experimental results were processed to be dimensionless data for comparison. The ratio of the jet velocity and the effective outlet velocity of the diffuser is dimensionless velocity. It is defined as follow: U=
Fig. 4. Vertical shooting sections (green areas). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Case
Air flow rate
Indoor ambient air temperature
Temperature of air supply
1 2
100CFM 50CFM
27.5 ◦ C 27.4 ◦ C
28.3 ◦ C 28.2 ◦ C
fog, which can easily be produced in a large, controllable quantity. This is a simple and effective method to flow visualization, considering the stringent safety requirements that must be observed. A CCD camera, is positioned to the pressured plenum on one side when record the flow visualizations of vertical shooting sections. It is positioned above the center of the swirl diffuser when record the flow visualizations of horizontal shooting sections. In addition, the background of the test platform is covered by the black paper to provide proper contrast for most of the tests. 2.5. Test conditions The experiment was conducted in 100 CFM (cubic feet per minute, 0.00047m3 /s) and 50 CFM. The experimental conditions are given in Table 1. Temperature of air supply is about 0.8 ◦ C higher than indoor ambient air temperature because of heat output from fog generator and centrifugal fan.
(1)
Where, U is dimensionless velocity above the swirl diffuser, V is the jet velocity above the swirl diffuser (m/s) and V0 is the effective outlet velocity of the diffuser (m/s), it can be calculated based on the supply airflow and the effective outlet area of the swirl diffuser in this study. Because the diameter of swirl diffuser is different, dimensionless vertical height is defined as follow: h=
Table 1 Experimental conditions.
V V0
Z D
(2)
Where, h is the dimensionless vertical height, Z is the distance to centerline velocity VZ (m) and D is the diameter of swirl diffuser (m). Similarly, dimensionless horizontal distance is defined as follow: r l= (3) R Here, l is the dimensionless horizontal distance; r is the horizontal distance away from the central point of the swirl diffuser and R is the radius of the swirl diffuser. 3. Results and discussions 3.1. Visual pictures (Tracer particle pictures) Visual pictures of the two cases are shown in Fig. 6 and Fig. 7. These fog pictures were shot with the camera of 2D-PIV system we mentioned before the experiment was being conducted. Because of the rotational symmetry of the flow above the swirl diffuser, the images on the right were not original pictures, but were flipped from the left ones in vertical section 2 and 3 (see Fig. 6). Examples of the air motion near the diffuser jet for the swirl diffuser at the different flow rate are shown in Fig. 6 and Fig. 7. From Fig. 6 we can see the direction of air flow around the swirl diffuser of two
Fig. 5. Horizontal shooting sections (green areas): (a) three different heights; (b) horizontal section 1 (height = 50 mm); (c) horizontal section 2 and 3 (height = 200 mm and 500 mm). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 6. Tracer particle of vertical shooting section: (a) Case 1; (b) Case 2.
Fig. 7. Tracer particle of horizontal shooting section: (a) Case 1; (b) Case 2.
cases by the fog movement. Meanwhile, Fig. 6 and Fig. 7 show a strong rotation in the horizontal shooting section 1 at the swirl diffuser outlet area, but this rotational motion decayed to produce a straight radial flow within three dimensional of the swirl diffuser center in the vertical shooting section. As seen in Fig. 6 and Fig. 7, the visual pictures of swirl diffuser in Case 1 were similar to those of Case 2. However, Fig. 6 and Fig. 7 illustrate that the fog area of Case 2 is larger, indicating greater mixing in the horizontal shooting section 2 and 3 when compared with the Case 1 shown in Fig. 7. The results also show that the swirl diffuser and jet decay performance
depends on the flow rate per diffuser partly. In addition, from Fig. 6 and Fig. 7 we can see that outside edge of fog in almost every picture is not beyond the border of shooting sections except for horizontal shooting section 3. That is one reason why we set up the shooting section in Section 2.3 in this paper. 3.2. Experimental results and discussion The software “Insight 3G and Tecplot Focus” were selected to analyze the PIV experimental results. After processing fog pictures
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Fig. 8. Airflow pattern and the velocity field in vertical shooting section 1 at two cases: (a) Case1; (b) Case 2.
above by Tecplot specially designed for PIV, experimental results of all cases are shown in Figs. 8–13. To avoid the contingency of experimental results, more than 500 pictures were shot in certain experimental conditions because of specificity of 2D-PIV.
3.2.1. Vertical shooting sections Figs. 8–10 shows the measured airflow pattern and the velocity field of the swirl diffuser in vertical shooting section 1, 2 and 3, including the nephogram of velocity, the streamlines of velocity and the velocity vector field in the vertical shooting section 1, 2 and 3 for case 1–2. As shown in Fig. 8(a-b), the swirl jet divide into three between h = 0 and h = 1.5 (Z = 0 mm–300 mm) in the vertical shooting section 1. Compared with the surrounding jet velocity, the air velocity is small at 0 < l < 0. 5 (X = 280 mm–370 mm) in the central area of the swirl jet, where it acts in the opposite direction to the surrounding jet velocity. Depending on where the streamlines of velocity was produced, it was pulled downward into the diffuser jet, clearly showing the motion of the air near the jet. This was caused mainly by that the central area of the swirl is a solid structure (See Fig. 3). Under the influence of surrounding spiral jet, it can produce a vortex shape and create a negative pressure zone in the
central area of the vortex region, which helps mix the surrounding air and supply air for a more complete and much quicker. Fig. 8 illustrates that the zone of max velocity was located in the range of h = 0.25–0.625 (Z = 50–125 mm) near the swirl outlet. The reduce rates for jet velocity of two cases have been 64.3% and 50.0% respectively from h = 0 to h = 2.25. In addition, the obvious contraction can be observed in the horizontal section of h = 1.25 (Z = 250 mm) in the swirl jet for case 1–2. From Fig. 9(a-b) we can see clearly that the velocity distribution for Case 1–2 in the vertical shooting section 2 are all very similar. In the same vertical shooting section, the jet velocity in the mainstream area for two cases decreased up to 50.0% and 42.9% respectively. Fig. 9(a-b) illustrates that the negative pressure zone in the central area of the swirl jet faded away gradually with increasing distance from swirl outlet. Meanwhile, the jet velocity of the mainstream area for Case 1–2 started to become uniform. From Fig. 10(a-b) we can see that the jet velocity in the mainstream area for Case 1–2 had dropped to 0.35 m/s or so although the air flow is different between Case 1 and Case 2. The negative pressure zone we mentioned above completely disappeared. Therefore, the whole velocity field in the mainstream area became
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Fig. 9. Airflow pattern and the velocity field in vertical shooting section 2 at two cases: (a) Case1; (b) Case 2.
very uniform. However, Fig. 10 illustrates that the supply air of Case 1 spread more broadly than that of Case 2 in the vertical shooting section 3. 3.2.2. Horizontal shooting sections Fig. 11(a-b) shows the measured airflow pattern and the velocity field of the swirl diffuser in horizontal shooting section 1, 2 and 3, including of the nephogram of velocity, the streamlines of velocity and the velocity vector field in the horizontal shooting section 1, 2 and 3 for case 1–2. Depending on the streamlines of velocity was produced in Fig. 11(a-b), a strong rotation in the horizontal shooting section 1 near the swirl diffuser outlet area can be seen, where the vortex in the central area can be observed obviously. The jet velocity at the red circle in the nephogram of velocity has the maximum value. In addition, the jet velocity at other areas is much smaller. Although the height difference between the horizontal shooting section 1 and 2 is only 150 mm, the reduce rates for maximum jet velocity of two cases have been 46.7% and 40.0% respectively. As seen in Fig. 11, the vortex still exists in horizontal shooting section 2. It is evident from the streamlines of velocity that jet interacts with the surrounding air. Compared with the velocity field of horizon-
tal shooting section 1, the jet velocity distribution becomes more uniform. Compared with the velocity field of horizontal shooting section 1 and 2, the negative pressure zone of jetting region disappeared in the horizontal shooting section 1 and 3. Fig. 13(a-b) illustrates that the reduce rates for maximum jet velocity of two cases have been 66.3% and 66.7% respectively, and the jet velocity distribution of the primary jet flow for two cases are remarkably even. From Figs. 11–13 we can see that supply air velocity affect only the velocity distribution near the swirl diffuser. Therefore, this swirl diffuser can reduce the risk of feeling a draught effectively even if the supply air velocity is considerably large. 3.2.3. Velocity distribution and evaluation indexes 3.2.3.1. Velocity distribution at vertical and horizontal directions. To study the air distribution performance of swirl diffuser, the three lines were selected at vertical and horizontal direction, respectively. The dimensionless velocity data of air flow above the swirl diffuser can be seen in Fig. 14(a-b). From Fig. 14 (a) we can see clearly that the velocity increased to the maximum and then decreased with the height from the out-
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Fig. 10. Airflow pattern and the velocity field in vertical shooting section 3 at two cases: (a) Case1; (b) Case 2.
let of swirl diffuser. And the maximum value of centerline velocity (l = 0) is slower than that of the line (l = 0.5). Fig. 14 (a) illustrates that the velocity of the line (l = 1.0) is pretty small, which has less influence on the surrounding environment. In addition, we can see that the dimensionless velocity dramatically decreased with the increasing of height away from the outlet. When dimensionless vertical height h > 4.0, the dimensionless velocity of measuring points for three lines are close to 0.075. From Fig. (b) we can see that the jet velocity distribution at three different heights (h = 0.25, 1.0, 2.5). The dimensionless velocity of the center of the swirl diffuser is small, which completely coincide with above PIV experimental results. Fig. 14(a-b) illustrate that the maximum value of dimensionless velocity for different height appear in the same area (0.5 < l < 0.75). In addition, the dimensionless velocity decreased to 0.025 when the dimensionless horizontal distance l increased to 2.0. Afterward, the dimensionless velocity of three different heights are almost constant with the dimensionless horizontal distance (l) increasing. 3.2.3.2. The jet centerline velocity decay coefficient and entrainment ratio.
In above figures the air jet traveled vertically, which belongs in the field of free jet. The jet centerline velocity decay coefficient (K value) can be used to analyze air flow performance of the swirl diffuser, which is defined in the previous literature [10,22] as Eq. (4). The jet centerline velocity decay coefficients of different diffusers in the literature [23] are obtained from the experimental data conducted 30 years ago [24].
VZ Z + Zp K= V0 A0
(4)
Where, VZ is jet centerline velocity at distance Z (m/s), K is jet centerline velocity decay coefficient, which is a dimensionless constant. Ao is the effective outlet area of the swirl diffuser (m) and Zp is the distance of the virtual origin (m). In order to get the K and xp values for the swirl diffuser more easily, Eq. (4) can be restated by the following form [10]: Zp V0 Z = + VZ K A0 K A0
(5)
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Fig. 11. Airflow pattern and the velocity field in horizontal shooting section 1 at two cases: (a) Case1; (b) Case 2.
The K and Zp values of the swirl diffuser can be derived by linear regression of the testing results. Fig. 15 shows centerline velocity decay coefficients measured by hotwire anemometer and simulated results of different supply airflow. There are four zones that could be identified for a free jet after entering a space [25,26]. Among them, axisymmetric decay region is usually referred to as the “fully developed flow region” for three-dimensional jets. In this region, the divergent angle of the jet remains the same, which depends on the type and structure of the air terminal devices. Therefore, the experimental data near the swirl diffuser are discarded in this study. The remaining data of axisymmetric decay region are used to calculate K and xp values by the linear regression in Fig. 15. In addition, the K values of the previous simulated and experimental results [10] are shown in Fig. 15 for comparative analysis. From Fig. 15 we can see that K values based on the effective outlet area in the paper are 1.938 and 1.953 respectively, which are greater than the simulated results based on the effective outlet
area [10] and lower than the experimental results based on the neck velocity [10,22]. And the K values of swirl diffuser in this study are greater than that of conventional diffuser. Because a higher K value will cause more ambient air to be mixed with its jet, the swirl diffuser create more induced air than that of the conventional diffuser for a given flow rate. For a given swirl diffuser and supply airflow, the entrainment ratio of the jet can indicate its induction effect effectively, which is defined as follow [23]:
Z + Zp QZ = CZ Q0 K A0
(6)
Where, CZ is the entrainment ratio, QZ is the airflow at the distance Z above the swirl diffuser (CFM), Qo is the supply airflow (CFM), and other parameters have been introduced in the above article.
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Fig. 12. Airflow pattern and the velocity field in horizontal shooting section 2 at two cases: (a) Case1; (b) Case 2.
The entrainment ratio Cx in the fully developed flow region can be calculated by Eq. (6) for different cases. Therefore, for Qo = 150 CFM and Qo = 100 CFM, the mean value of the entrainment ratio is 3.22 and 3.16 respectively. The results are similar to that of S.C. Hu [10] and much greater than that of ceiling diffusers [11,23], which indicates that the swirl diffuser have significant induction effect. Therefore, compared with the conventional diffuser, the swirl diffuser can effectively reduce the influence of draught for a given airflow rate. 4. Conclusions The airflow visualization and full-scale experimental results of swirl diffuser mounted on the floor are reported in this paper. The experimental data of the jet for swirl diffuser are obtained by 2DPIV experimental system under isothermal conditions, and some useful conclusions are obtained.
Experimental results show the macro and micro structure of the air flow above the swirl diffuser in detail. A strong rotation and the vortex in the central area near the swirl diffuser outlet can be observed obviously. Meanwhile, air velocity is small in the central area of the jet (−0.45 < l < 0.45 and 0 < h < 1.5), which acts in the opposite direction to the surrounding jet velocity. The maximum value of dimensionless velocity for different height appear in the same area (0.5 < l < 0.75). In addition, the obvious contraction can be observed at the horizontal section. The dimensionless velocity U dramatically decreased with the increasing of dimensionless vertical height h and dimensionless horizontal distance l when l > 1.0 and h > 4.0. Fig. 14 and 15 illustrate that the velocity is pretty small and when l > 2.0 or h > 5.5, which has less influence on the surrounding environment. For the swirl diffuser investigated, the K value of the swirl diffuser based on the effective velocity is 1.94, which is lower than that based on the neck velocity [11,22]. The average entrainment
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Fig. 13. Airflow pattern and the velocity field in horizontal shooting section 3 at two cases: (a) Case1; (b) Case 2.
Fig. 14. Dimensionless velocity distribution of Case 1: (a) at three vertical lines for different l; (b) at three horizontal lines for different h.
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Fig. 15. Centerline velocity decays coefficient K for different cases.
ratio CZ is 3.19 based on Eq. (6), much greater than that of ceiling diffusers. Therefore, the swirl diffuser exhibits an entrainment greater than that of the circular ceiling diffusers. For a given flow rate, the swirl diffuser can create more create more induced air and reduce the influence of draught effectively. Acknowledgment This research is sponsored by the Industrialization Project of Educational Commission of Shaanxi Province of China (Grant No. 15JF004) and National Natural Science Foundation of China (Grant No. 51478377, No. 51508442). References [1] ASHRAE, ASRAE Handbook: Fundamentals, American Society of Heating, Refrigeration and Air-Conditioning Engineers Inc., Atlanta, GA, 2001. [2] G.Y. Cao, H.B. Awbi, R.M. Yao, Y.Q. Fan, K. Siren, R. Kosonen, J.S. Zhang, A review of the performance of different ventilation and airflow distribution systems in buildings[J], Build. Environ. 73 (2014) 171–186. [3] O. SeppEnen, Ventilation strategies for good indoor air quality and energy efficiency[J], Int. J. Vent. 6 (4) (2008) 297–306. [4] L. Magnier-Bergeron, D. Derome, R. Zmeureanu, Three-dimensional model of air speed in the secondary zone of displacement ventilation jet[J], Build. Environ. 114 (2017) 483–494.
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[5] A.K. Melikov, G. Pitchurov, K. Naydenov, et al., Field study of occupants thermal comfort in rooms with displacement ventilation[J], Indoor Air 15 (3) (2005) 205–214. [6] X. Shan, J. Zhou, V.W.C. Chang, et al., Comparing mixing and displacement ventilation in tutorial rooms: students’ thermal comfort, sick building syndromes, and short-term performance[J], Build. Environ. 102 (2016) 128–137. [7] K. Zhang, X. Zhang, S. Li, et al., Review of underfloor air distribution technology[J], Energy Build. 85 (85) (2014) 180–186. [8] Y.J.P. Lin, T.Y. Tsai, An experimental study on a full-scale indoor thermal environment using an Under-Floor Air Distribution system[J], Energy Build. 80 (2014) 321–330. [9] P. Raftery, F. Bauman, S. Schiavon, et al., Laboratory testing of a displacement ventilation diffuser for underfloor air distribution systems[J], Energy Build. 108 (10) (2015) 82–91. [10] S.C. Hu, Airflow characteristics in the outlet region of a vortex room air diffuser[J], Build. Environ. 38 (4) (2003) 553–561. [11] B. Sajadi, M.H. Saidi, A. Mohebbian, Numerical investigation of the swirling air diffuser: parametric study and optimization[J], Energy Build. 43 (6) (2011) 1329–1333. [12] N. Kobayashi, Q. Chen, R.W. Herrick, et al., Floor-supply displacement ventilation in a small office[J], Indoor Built Environ. 12 (4) (2003) 281–291. [13] J. Lau, Q. Chen, Floor-supply displacement ventilation for workshops[J], Build. Environ. 42 (4) (2007) 1718–1730. [14] E. Tavakoli, R. Hosseini, Large eddy simulation of turbulent flow and mass transfer in far-field of swirl diffusers[J], Energy Build. 59 (4) (2013) 194–202. [15] M.H. Hosni, B.W. Jones, Development of a particle image velocimetry for measuring air velocity in large-scale room airflow applications [J], ASHRAE Transactions 108 (2002) 1164. [16] Q. Chen, Ventilation performance prediction for buildings: a method overview and recent applications[J], Build. Environ. 44 (4) (2009) 848–858. [17] V. Weitbrecht, D.G. Seol, E. Negretti, et al., PIV measurements in environmental flows: recent experiences at the Institute for Hydromechanics in Karlsruhe[J], J. Hydro-environ. Res. 5 (4) (2011) 231–245. [18] X. Cao, J. Liu, N. Jiang, et al., Particle image velocimetry measurement of indoor airflow field: a review of the technologies and applications[J], Energy Build. 69 (2014) 367–380. [19] A. Li, P. Tao, X. Bao, et al., PIV measurements of air distribution in a reduced-scale model-Ventilation of a busbar corridor in a hydropower station[J], Int. J. Vent. 12 (1) (2013) 81–98. [20] A. Li, E. Qin, B. Xin, et al., Experimental analysis on the air distribution of powerhouse of Hohhot hydropower station with 2D-PIV[J], Energy Conver. Manage. 51 (1) (2010) 33–41. [21] A. Li, X. Wang, Y. Zhang, 2D-PIV experiment analysis on the airflow performance of a floor-based air distribution with a novel mushroom diffuser (FBAD-MD)[J], Energy Build. 121 (2016) 114–129. [22] S. Shakerin, P.L. Miller, Experimental Study of Vortex Diffusers. NREL Report/TP-472-7331, National Renewable Energy Laboratory, Golden, Colorado, 1995. [23] ASHRAE, Handbook—Fundamentals, Atlanta American Society of Heating, Refrigerating & Air-Conditioning Engineers, 1997 (chapter 32). [24] A. Koestel, Path of horizontally projected heated and chilled air jets. Transactions of the ASHRAE 61, ASHRAE Res. Rep. 1534 (1955) 213–232. [25] H.B. Awbi, Ventilation of Buildings, E&FN Spon, London, 1991. [26] E. Djunaedy, K.W.D. Cheong, Development of a simplified technique of modelling four-way ceiling air supply diffuser[J], Build. Environ. 37 (4) (2002) 393–403.
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Energy and Buildings journal homepage: www.elsevier.com/locate/enbuild
PIV experiment and evaluation of air flow performance of swirl diffuser mounted on the floor Angui Li ∗ , Changqing Yang, Tong Ren, Xin Bao, Erwei Qin, Ran Gao School of Environmental and Municipal Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, China
a r t i c l e
i n f o
Article history: Received 16 May 2017 Received in revised form 28 August 2017 Accepted 16 September 2017 Available online 21 September 2017 Keywords: Flow visualization 2D-PIV Swirl diffuser Dimensionless velocity Centerline velocity decay coefficient Entrainment ratio
a b s t r a c t In order to obtain satisfactory effect of air conditioning and ventilation, the floor–supply displacement ventilation has widely applied in many civil buildings. Swirl diffusers mounted on the floor are widely used to provide fresh air into the occupied zone in the floor–supply displacement ventilation system. This paper studied the air distribution performance of swirl diffuser by 2D-PIV (particle image velocimetry). The experiment was implemented in a full-scale room to achieve reliable data of swirl diffusers. Flow visualizations have been carried out to determine the shape of the swirl jet. The whole velocity field was measured by 2D-PIV system, which is very useful to analyze air distribution above the swirl diffuser. With the aid of the flow visualizations and PIV test technology, the macro and micro structure of the air flow above the swirl diffuser is specified in more detail. Moreover, the centerline velocity decay coefficient (K value) and entrainment ratio were obtained. The research work of this paper provides detailed experimental data of air flow above the swirl diffuser, which provide the reference for selection of air terminal devices and a basis for follow-up numerical simulation study. © 2017 Published by Elsevier B.V.
1. Introduction Indoor air distribution determines the diffusion of ventilated air, therefore, affecting air quality and human comfort. Mixing ventilation (MV) and displacement ventilation (DV) are often used to describe air movement within closed environments [1]. MV systems are very widely used and have the large market share. However, MV always exhibits a poor ventilation performance [2] and have low energy efficiency [3]. Meanwhile, although some studies have shown that DV is an effective method of providing air conditioning buildings [4,5], DV also have some drawbacks such as the large vertical temperature difference, the big air supply inlet and the draft sensation [5,6]. Therefore, to avoid these problems, underfloor air distribution (UFAD) has been studied and used [7,8]. UFAD and DV systems are based on many of the same principles in cooling manner as they both supply conditioned air directly into the occupied zone and return it near the ceiling area [9]. The main difference between UFAD and DV is in the way that they supply conditioned air into the occupied zone. In current situation, these mentioned ventilation systems are respectively adopted in a suitable building to provide the comfortable interior environment.
∗ Corresponding author. E-mail address: [email protected] (A. Li). http://dx.doi.org/10.1016/j.enbuild.2017.09.045 0378-7788/© 2017 Published by Elsevier B.V.
Air diffusers are important components of ventilation and airconditioning systems. The comfort level of occupants and the efficient delivery of air in a building depend on, among other factors, properly designed and selected diffusers. Swirl diffusers are modern types of air diffusers which can provide high mixing effect due to making the air to swirl and using the centrifugal force to enhance induction effect. Based on computational fluid dynamics, S.C. Hu [10] studied the airflow features in a specific type of ceiling swirling diffuser, which shows that the influence of the stationary twists guide vanes on centerline decay coefficient and entrainment ratio. Sajadi B et al. [11] studied numerically the air distribution performance in a common type of ceiling swirl diffuser with different swirl blades angle and slots geometry. And the results of a parametric study demonstrate that the effect of swirl blades angle on the air distribution performance is more important than that of slots specifications. Raftery P et al. [9] performed laboratory experiments to study the stratification performance of swirl diffuser mounted on the floor that supply air horizontally. Kobayashi N et al. [12], Lau J and Chen Q [13] carried out the experiments and developed numerical models to predict the air distribution performance of a UFAD system clearly. The simulated temperature distribution were in excellent agreement with the experimental results. Meanwhile, although the agreements between the computed velocity and experimental results were reasonable, it is not as good as the temperature at some zones since the numerical methods, physi-
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Fig. 1. 2D-PIV measurement system.
Fig. 2. Experimental rigs.
cal models and mesh quality are not perfect and the measurement instrument error is difficult to avoid. Ehsan Tavakoli and Reza Hosseini [14] studied numerically the effect of swirl diffuser angle on airflow pattern and ventilation characteristics. In many air conditioning systems, swirl diffusers are commonly used to supply conditioned air directly. However, the previous researches have mostly focused on the other models, for example, supply grilles, flared nozzle, hemispherical nozzles or ceiling diffusers. Up to now, many relevant studies of floor swirl diffuser have mostly focused on the thermal comfort conditions of occupants, indoor air quality and energy consumption in the building. In addition, most experimental studies always use the single-point measurement data to analyze the airflow pattern of swirl diffuser, it is not feasible to measure the whole velocity vector field of swirl diffuser. Therefore, detailed airflow information including airflow pattern and the whole velocity vector field above the floor swirl diffuser has not been available. With the development of the computational fluid dynamics (CFD) techniques, the researchers can obtain large data about the airflow and the whole velocity vector field above the swirl diffuser without getting involved into the expensive. However, the complex geometry of the floor swirl dif-
fuser and the paucity of accurate experimental data impress the simulated results accuracy seriously. Moreover, detailed airflow information about floor swirl diffuser including jet centerline velocity decay coefficient (K value) and entrainment ratio is not available, which also brings certains difficulty to select the terminal devices effectively in the air conditioning systems. In this paper, with the aid of the flow visualizations and 2D-PIV test technology, the objectives are: to obtain the macro and micro structure of the air flow above the swirl diffuser in more detail experimentally, to obtain jet centerline velocity decay coefficient (K value) and entrainment ratio for selection of air terminal devices and provide the whole velocity vector field of swirl diffuser for follow-up numerical simulation study. 2. Experimental set up There are a number of methods available for measuring jet velocity vector field, each with its own advantages and disadvantages [15,16]. Specifically, PIV has been an increasingly popular and effective technique for indoor airflow field measurement during the last decade. Because the non-intrusive feature of PIV technique, PIV
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Fig. 3. Swirl diffuser mounted on the floor: (a) the front of diffuser; (b) the back of diffuser.
almost will not affect the velocity field, which is a key advantage of PIV over many other measurement methods. In addition, PIV has advantages of measuring a complete velocity field at each instant in time [17,18]. The whole velocity field of the swirl diffuser is quite helpful to study airflow performance. Therefore, in this paper, the velocity vector field above the swirl diffuser is measured by 2D-PIV system. 2.1. 2D-PIV measurement facilities The 2D-PIV measurement system is showed in Fig. 1. In this study, pulsed laser beam was produced by the double pulsed Nd:YAG laser (energy:200 mJ). The high quantum efficiency and low noise digital CCD camera (Nikon Camera, Type: 630157 Power view plus 2MP) was used to take the experimental pictures, which is the 1.6k × 1.2k pixel resolution. During our studies the 450 mm × 600 mm section is the biggest measuring area which can be shot by PIV technology. In addition, the laser pulse computer controlled synchronizer (Type: TSI610034) was used to control time sequence of camera and laser. To avoid the contingency of experimental results, many pictures were shot in certain experimental conditions because of specificity of PIV [19]. In the PIV experiment, the time difference of two laser pulses (t) has a very significant impact on the experimental results. Because the supply air velocity changes in different cases, it should adjust this time before each test. Besides, we know that t increases with the decrease of the maximum velocity (vmax ) of the shooting section in different cases. In previous PIV literature of authors’ research group [20,21], a relationship between t and vmax was obtained as follow: ln (t) = 5.5214 − ln (vmax )
(1)
vmax × t = 250
(2)
Based on Eq. (2), the most important parameter of the laser source light can be set accurately in PIV experimental research. During our studies the mean particle size of the tracer particle is 1 ∼ 2 m, which is the spherical droplets generated by smoke generator (Type: Rosco 1700). Such particles have a good following
and astigmatism, and the response time to the motion of the air is reasonably short to accurately follow the flow. 2.2. Physical model The size of the room where the experiment was conducted is 6.6 m × 6.6 m × 3.15 m (L × W × H). According to experimental requests, a big box was used as pressured plenum (Fig. 2). Size of the box is 1.6 m × 0.6 m × 0.55 m (L × W × H), in which inner net height of the box is 0.5m. The distance between diffuser and ceiling is more than 2.6m. There is enough space around the diffuser in order to ensure air flowing out from diffuser to flow freely. In addition, TROX swirl diffuser (Type: FBK 200) is used in this study (see Fig. 3). As shown in Fig. 3, the basic sizes of swirl diffuser are prescribed: R1 = 199.5 mm, R2 = 188 mm, c1 = 21 mm. The effective outlet area of the swirl diffuser in this study is 0.0056m2 . 2.3. Shooting section set up In this study, vertical and horizontal shooting sections were shown in Figs. 4 and 5. Those green blocks are just shooting section. Due to symmetry of the Trox swirl diffuser and fog pictures, we choose three pieces of the section with the same size for 450 mm × 600 mm. There are vertical section 1, vertical section 2 and vertical section 3. The right edge of section 2 and 3 is just middle of section 1. Section 1 locates in the middle of swirl diffuser. Due to camera performance and limit of ceiling height, we shoot horizontal sections with the size of 360 mm × 250 mm at three different heights (heights = 50 mm, 200 mm, 500 mm). 2.4. Flow visualization In addition to the 2D-PIV experimental study and velocity point measurements, this paper conducted two tests to visualize the jets issuing from the swirl diffuser and observe the introduction of ambient air into the jets. The fog from the swirl diffuser is used to observe the motion of the ambient air near the diffuser. A nontoxic liquid is pumped into the machine and atomized to a very fine
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2.6. Processing method of experimental results Because the supply airflow and the effective outlet area of the swirl diffuser in our experiment are different from that of others’ diffusers, therefore, the experimental results were processed to be dimensionless data for comparison. The ratio of the jet velocity and the effective outlet velocity of the diffuser is dimensionless velocity. It is defined as follow: U=
Fig. 4. Vertical shooting sections (green areas). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Case
Air flow rate
Indoor ambient air temperature
Temperature of air supply
1 2
100CFM 50CFM
27.5 ◦ C 27.4 ◦ C
28.3 ◦ C 28.2 ◦ C
fog, which can easily be produced in a large, controllable quantity. This is a simple and effective method to flow visualization, considering the stringent safety requirements that must be observed. A CCD camera, is positioned to the pressured plenum on one side when record the flow visualizations of vertical shooting sections. It is positioned above the center of the swirl diffuser when record the flow visualizations of horizontal shooting sections. In addition, the background of the test platform is covered by the black paper to provide proper contrast for most of the tests. 2.5. Test conditions The experiment was conducted in 100 CFM (cubic feet per minute, 0.00047m3 /s) and 50 CFM. The experimental conditions are given in Table 1. Temperature of air supply is about 0.8 ◦ C higher than indoor ambient air temperature because of heat output from fog generator and centrifugal fan.
(1)
Where, U is dimensionless velocity above the swirl diffuser, V is the jet velocity above the swirl diffuser (m/s) and V0 is the effective outlet velocity of the diffuser (m/s), it can be calculated based on the supply airflow and the effective outlet area of the swirl diffuser in this study. Because the diameter of swirl diffuser is different, dimensionless vertical height is defined as follow: h=
Table 1 Experimental conditions.
V V0
Z D
(2)
Where, h is the dimensionless vertical height, Z is the distance to centerline velocity VZ (m) and D is the diameter of swirl diffuser (m). Similarly, dimensionless horizontal distance is defined as follow: r l= (3) R Here, l is the dimensionless horizontal distance; r is the horizontal distance away from the central point of the swirl diffuser and R is the radius of the swirl diffuser. 3. Results and discussions 3.1. Visual pictures (Tracer particle pictures) Visual pictures of the two cases are shown in Fig. 6 and Fig. 7. These fog pictures were shot with the camera of 2D-PIV system we mentioned before the experiment was being conducted. Because of the rotational symmetry of the flow above the swirl diffuser, the images on the right were not original pictures, but were flipped from the left ones in vertical section 2 and 3 (see Fig. 6). Examples of the air motion near the diffuser jet for the swirl diffuser at the different flow rate are shown in Fig. 6 and Fig. 7. From Fig. 6 we can see the direction of air flow around the swirl diffuser of two
Fig. 5. Horizontal shooting sections (green areas): (a) three different heights; (b) horizontal section 1 (height = 50 mm); (c) horizontal section 2 and 3 (height = 200 mm and 500 mm). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 6. Tracer particle of vertical shooting section: (a) Case 1; (b) Case 2.
Fig. 7. Tracer particle of horizontal shooting section: (a) Case 1; (b) Case 2.
cases by the fog movement. Meanwhile, Fig. 6 and Fig. 7 show a strong rotation in the horizontal shooting section 1 at the swirl diffuser outlet area, but this rotational motion decayed to produce a straight radial flow within three dimensional of the swirl diffuser center in the vertical shooting section. As seen in Fig. 6 and Fig. 7, the visual pictures of swirl diffuser in Case 1 were similar to those of Case 2. However, Fig. 6 and Fig. 7 illustrate that the fog area of Case 2 is larger, indicating greater mixing in the horizontal shooting section 2 and 3 when compared with the Case 1 shown in Fig. 7. The results also show that the swirl diffuser and jet decay performance
depends on the flow rate per diffuser partly. In addition, from Fig. 6 and Fig. 7 we can see that outside edge of fog in almost every picture is not beyond the border of shooting sections except for horizontal shooting section 3. That is one reason why we set up the shooting section in Section 2.3 in this paper. 3.2. Experimental results and discussion The software “Insight 3G and Tecplot Focus” were selected to analyze the PIV experimental results. After processing fog pictures
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Fig. 8. Airflow pattern and the velocity field in vertical shooting section 1 at two cases: (a) Case1; (b) Case 2.
above by Tecplot specially designed for PIV, experimental results of all cases are shown in Figs. 8–13. To avoid the contingency of experimental results, more than 500 pictures were shot in certain experimental conditions because of specificity of 2D-PIV.
3.2.1. Vertical shooting sections Figs. 8–10 shows the measured airflow pattern and the velocity field of the swirl diffuser in vertical shooting section 1, 2 and 3, including the nephogram of velocity, the streamlines of velocity and the velocity vector field in the vertical shooting section 1, 2 and 3 for case 1–2. As shown in Fig. 8(a-b), the swirl jet divide into three between h = 0 and h = 1.5 (Z = 0 mm–300 mm) in the vertical shooting section 1. Compared with the surrounding jet velocity, the air velocity is small at 0 < l < 0. 5 (X = 280 mm–370 mm) in the central area of the swirl jet, where it acts in the opposite direction to the surrounding jet velocity. Depending on where the streamlines of velocity was produced, it was pulled downward into the diffuser jet, clearly showing the motion of the air near the jet. This was caused mainly by that the central area of the swirl is a solid structure (See Fig. 3). Under the influence of surrounding spiral jet, it can produce a vortex shape and create a negative pressure zone in the
central area of the vortex region, which helps mix the surrounding air and supply air for a more complete and much quicker. Fig. 8 illustrates that the zone of max velocity was located in the range of h = 0.25–0.625 (Z = 50–125 mm) near the swirl outlet. The reduce rates for jet velocity of two cases have been 64.3% and 50.0% respectively from h = 0 to h = 2.25. In addition, the obvious contraction can be observed in the horizontal section of h = 1.25 (Z = 250 mm) in the swirl jet for case 1–2. From Fig. 9(a-b) we can see clearly that the velocity distribution for Case 1–2 in the vertical shooting section 2 are all very similar. In the same vertical shooting section, the jet velocity in the mainstream area for two cases decreased up to 50.0% and 42.9% respectively. Fig. 9(a-b) illustrates that the negative pressure zone in the central area of the swirl jet faded away gradually with increasing distance from swirl outlet. Meanwhile, the jet velocity of the mainstream area for Case 1–2 started to become uniform. From Fig. 10(a-b) we can see that the jet velocity in the mainstream area for Case 1–2 had dropped to 0.35 m/s or so although the air flow is different between Case 1 and Case 2. The negative pressure zone we mentioned above completely disappeared. Therefore, the whole velocity field in the mainstream area became
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Fig. 9. Airflow pattern and the velocity field in vertical shooting section 2 at two cases: (a) Case1; (b) Case 2.
very uniform. However, Fig. 10 illustrates that the supply air of Case 1 spread more broadly than that of Case 2 in the vertical shooting section 3. 3.2.2. Horizontal shooting sections Fig. 11(a-b) shows the measured airflow pattern and the velocity field of the swirl diffuser in horizontal shooting section 1, 2 and 3, including of the nephogram of velocity, the streamlines of velocity and the velocity vector field in the horizontal shooting section 1, 2 and 3 for case 1–2. Depending on the streamlines of velocity was produced in Fig. 11(a-b), a strong rotation in the horizontal shooting section 1 near the swirl diffuser outlet area can be seen, where the vortex in the central area can be observed obviously. The jet velocity at the red circle in the nephogram of velocity has the maximum value. In addition, the jet velocity at other areas is much smaller. Although the height difference between the horizontal shooting section 1 and 2 is only 150 mm, the reduce rates for maximum jet velocity of two cases have been 46.7% and 40.0% respectively. As seen in Fig. 11, the vortex still exists in horizontal shooting section 2. It is evident from the streamlines of velocity that jet interacts with the surrounding air. Compared with the velocity field of horizon-
tal shooting section 1, the jet velocity distribution becomes more uniform. Compared with the velocity field of horizontal shooting section 1 and 2, the negative pressure zone of jetting region disappeared in the horizontal shooting section 1 and 3. Fig. 13(a-b) illustrates that the reduce rates for maximum jet velocity of two cases have been 66.3% and 66.7% respectively, and the jet velocity distribution of the primary jet flow for two cases are remarkably even. From Figs. 11–13 we can see that supply air velocity affect only the velocity distribution near the swirl diffuser. Therefore, this swirl diffuser can reduce the risk of feeling a draught effectively even if the supply air velocity is considerably large. 3.2.3. Velocity distribution and evaluation indexes 3.2.3.1. Velocity distribution at vertical and horizontal directions. To study the air distribution performance of swirl diffuser, the three lines were selected at vertical and horizontal direction, respectively. The dimensionless velocity data of air flow above the swirl diffuser can be seen in Fig. 14(a-b). From Fig. 14 (a) we can see clearly that the velocity increased to the maximum and then decreased with the height from the out-
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Fig. 10. Airflow pattern and the velocity field in vertical shooting section 3 at two cases: (a) Case1; (b) Case 2.
let of swirl diffuser. And the maximum value of centerline velocity (l = 0) is slower than that of the line (l = 0.5). Fig. 14 (a) illustrates that the velocity of the line (l = 1.0) is pretty small, which has less influence on the surrounding environment. In addition, we can see that the dimensionless velocity dramatically decreased with the increasing of height away from the outlet. When dimensionless vertical height h > 4.0, the dimensionless velocity of measuring points for three lines are close to 0.075. From Fig. (b) we can see that the jet velocity distribution at three different heights (h = 0.25, 1.0, 2.5). The dimensionless velocity of the center of the swirl diffuser is small, which completely coincide with above PIV experimental results. Fig. 14(a-b) illustrate that the maximum value of dimensionless velocity for different height appear in the same area (0.5 < l < 0.75). In addition, the dimensionless velocity decreased to 0.025 when the dimensionless horizontal distance l increased to 2.0. Afterward, the dimensionless velocity of three different heights are almost constant with the dimensionless horizontal distance (l) increasing. 3.2.3.2. The jet centerline velocity decay coefficient and entrainment ratio.
In above figures the air jet traveled vertically, which belongs in the field of free jet. The jet centerline velocity decay coefficient (K value) can be used to analyze air flow performance of the swirl diffuser, which is defined in the previous literature [10,22] as Eq. (4). The jet centerline velocity decay coefficients of different diffusers in the literature [23] are obtained from the experimental data conducted 30 years ago [24].
VZ Z + Zp K= V0 A0
(4)
Where, VZ is jet centerline velocity at distance Z (m/s), K is jet centerline velocity decay coefficient, which is a dimensionless constant. Ao is the effective outlet area of the swirl diffuser (m) and Zp is the distance of the virtual origin (m). In order to get the K and xp values for the swirl diffuser more easily, Eq. (4) can be restated by the following form [10]: Zp V0 Z = + VZ K A0 K A0
(5)
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Fig. 11. Airflow pattern and the velocity field in horizontal shooting section 1 at two cases: (a) Case1; (b) Case 2.
The K and Zp values of the swirl diffuser can be derived by linear regression of the testing results. Fig. 15 shows centerline velocity decay coefficients measured by hotwire anemometer and simulated results of different supply airflow. There are four zones that could be identified for a free jet after entering a space [25,26]. Among them, axisymmetric decay region is usually referred to as the “fully developed flow region” for three-dimensional jets. In this region, the divergent angle of the jet remains the same, which depends on the type and structure of the air terminal devices. Therefore, the experimental data near the swirl diffuser are discarded in this study. The remaining data of axisymmetric decay region are used to calculate K and xp values by the linear regression in Fig. 15. In addition, the K values of the previous simulated and experimental results [10] are shown in Fig. 15 for comparative analysis. From Fig. 15 we can see that K values based on the effective outlet area in the paper are 1.938 and 1.953 respectively, which are greater than the simulated results based on the effective outlet
area [10] and lower than the experimental results based on the neck velocity [10,22]. And the K values of swirl diffuser in this study are greater than that of conventional diffuser. Because a higher K value will cause more ambient air to be mixed with its jet, the swirl diffuser create more induced air than that of the conventional diffuser for a given flow rate. For a given swirl diffuser and supply airflow, the entrainment ratio of the jet can indicate its induction effect effectively, which is defined as follow [23]:
Z + Zp QZ = CZ Q0 K A0
(6)
Where, CZ is the entrainment ratio, QZ is the airflow at the distance Z above the swirl diffuser (CFM), Qo is the supply airflow (CFM), and other parameters have been introduced in the above article.
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Fig. 12. Airflow pattern and the velocity field in horizontal shooting section 2 at two cases: (a) Case1; (b) Case 2.
The entrainment ratio Cx in the fully developed flow region can be calculated by Eq. (6) for different cases. Therefore, for Qo = 150 CFM and Qo = 100 CFM, the mean value of the entrainment ratio is 3.22 and 3.16 respectively. The results are similar to that of S.C. Hu [10] and much greater than that of ceiling diffusers [11,23], which indicates that the swirl diffuser have significant induction effect. Therefore, compared with the conventional diffuser, the swirl diffuser can effectively reduce the influence of draught for a given airflow rate. 4. Conclusions The airflow visualization and full-scale experimental results of swirl diffuser mounted on the floor are reported in this paper. The experimental data of the jet for swirl diffuser are obtained by 2DPIV experimental system under isothermal conditions, and some useful conclusions are obtained.
Experimental results show the macro and micro structure of the air flow above the swirl diffuser in detail. A strong rotation and the vortex in the central area near the swirl diffuser outlet can be observed obviously. Meanwhile, air velocity is small in the central area of the jet (−0.45 < l < 0.45 and 0 < h < 1.5), which acts in the opposite direction to the surrounding jet velocity. The maximum value of dimensionless velocity for different height appear in the same area (0.5 < l < 0.75). In addition, the obvious contraction can be observed at the horizontal section. The dimensionless velocity U dramatically decreased with the increasing of dimensionless vertical height h and dimensionless horizontal distance l when l > 1.0 and h > 4.0. Fig. 14 and 15 illustrate that the velocity is pretty small and when l > 2.0 or h > 5.5, which has less influence on the surrounding environment. For the swirl diffuser investigated, the K value of the swirl diffuser based on the effective velocity is 1.94, which is lower than that based on the neck velocity [11,22]. The average entrainment
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Fig. 13. Airflow pattern and the velocity field in horizontal shooting section 3 at two cases: (a) Case1; (b) Case 2.
Fig. 14. Dimensionless velocity distribution of Case 1: (a) at three vertical lines for different l; (b) at three horizontal lines for different h.
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Fig. 15. Centerline velocity decays coefficient K for different cases.
ratio CZ is 3.19 based on Eq. (6), much greater than that of ceiling diffusers. Therefore, the swirl diffuser exhibits an entrainment greater than that of the circular ceiling diffusers. For a given flow rate, the swirl diffuser can create more create more induced air and reduce the influence of draught effectively. Acknowledgment This research is sponsored by the Industrialization Project of Educational Commission of Shaanxi Province of China (Grant No. 15JF004) and National Natural Science Foundation of China (Grant No. 51478377, No. 51508442). References [1] ASHRAE, ASRAE Handbook: Fundamentals, American Society of Heating, Refrigeration and Air-Conditioning Engineers Inc., Atlanta, GA, 2001. [2] G.Y. Cao, H.B. Awbi, R.M. Yao, Y.Q. Fan, K. Siren, R. Kosonen, J.S. Zhang, A review of the performance of different ventilation and airflow distribution systems in buildings[J], Build. Environ. 73 (2014) 171–186. [3] O. SeppEnen, Ventilation strategies for good indoor air quality and energy efficiency[J], Int. J. Vent. 6 (4) (2008) 297–306. [4] L. Magnier-Bergeron, D. Derome, R. Zmeureanu, Three-dimensional model of air speed in the secondary zone of displacement ventilation jet[J], Build. Environ. 114 (2017) 483–494.
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