PERFORMANCE EVALUATION OF TWO AIR DISTRIBUTION SYSTEMS. T Karimipanah 1 and H B Awbi 2

Transcription

1 PERFORMANCE EVALUATION OF TWO AIR DISTRIBUTION SYSTEMS T Karimipanah 1 and H B Awbi 2 1 Air Innovation AB, Sweden 2 University of Reading, UK ABSTRACT This paper focuses on evaluating the performance of a wall displacement ventilation system and a new impinging jet ventilation system. Ventilation efficiency, local mean age of air and other characteristic parameters were experimentally and numerically obtained for a mock-up classroom. The internal heat load of 25 person-simulators were set to represent a crowded classroom in order to investigate any indoor climate problems caused by increased cooling loads. Four ventilation strategies were compared in a previous paper [1] but here we have chosen only the two systems mentioned. In addition to a large number of costly experiments we used CFD simulations to study certain parameters in more detail and explore the results for other situations such as industrial ventilation. The results presented here are part of a larger search programme to develop alternative and efficient systems for new challenging situations of room airflow distribution. Keywords: Displacement, Impinging jet ventilation, classroom, industrial ventilation, measurements, CFD INTRODUCTION Although the traditional mixing systems show poor ventilation efficiency and less energy efficient but they still occupy a large portion of the market. When displacement ventilation was first introduced almost three decades ago, it seemed at the time to be a promising ventilation concept due to its high ventilation efficiency and stratification principle. To create an effective ventilation system in the occupied zone, there should be a balance between the momentum and thermal (buoyancy) forces. In this low momentum displacement flow, the buoyancy forces created by heat sources have a tendency to take over and thus often causing poor ventilation efficiency in some zones of the room [2]. Another disadvantage of a displacement system is that it can only be used for cooling and is not suitable for winter heating. To overcome this problem new systems like ceiling mounted textile bag supply and down-to-floor impinging supply have been developed, see reference [1]. A new method of air distribution known as the Air Queen (AQ) has been developed in Sweden, which is based on the impinging jet principle [3]. This method has the advantages of both the mixing and displacement ventilation systems without known disadvantages. Impinging jet ventilation (IJV) has lower momentum than mixing and higher momentum than wall displacement ventilation (WDV). Although higher momentum than WDV, IJV produces a similar flow field and has, therefore, promising applications [1, 4]. Ventilation parameters To assess the effectiveness of a ventilation system in measurement or CFD simulation, some well-known parameters are used [5]:

2 Ventilation Effectiveness for Heat Distribution or Removal (ε t )- This is similar to a heat exchanger effectiveness and is defined by: To Ti ε t = (1) T T i Ventilation Effectiveness for Contaminant Removal (ε c )- This is a measure of how effective the ventilation system is in removing internally produced contamination. It is defined by: Co Ci ε c = (2) C Ci In equations (1) and (2), T is temperature ( o C), C is the contaminant concentration in parts per million (ppm), the subscripts i and o refer to inlet and outlet respectively and ( ) represents the mean value for the occupied zone (to a height of 1.8 m). The values of εt and ε c is dependent on the method of room air distribution, room characteristics, heat and contaminant sources, etc. In addition, expressions for the predicted percentage of dissatisfied (PPD) and predicted mean vote (PMV) that are used here are defined by Fanger [6]. EXPERIMENTS The size of the mock-up was 8.4x7.2x3 m and 25 person-simulators were placed in the room representing a teacher and 24 students. A climate chamber attached to the room was used to simulate extreme winter and summer conditions, see Figure 1. The heat output from personsimulators were 2375 W and 525 W was considered for lighting giving a total load of 2 9 W (48 W/m 2 ). In all tests the air flow rate was 1 l/s per person and the inlet air temperature was kept constant at +15 C. The outdoor temperature was kept at 21 C to simulate winter conditions and at + 25 C to simulate summer conditions. The devices were tested by measuring the air temperature, air velocity and air quality (local mean age of air) in the occupied zone. The room area was divided into 12 zones and a stand placed in the middle of each zone was used to measure the required quantities at different heights. The local mean age of air was measured by using the tracer gas decay technique at 1.2 m above floor. Further details can be found in references [1 and 4]. CFD CALCULATIONS The CFD program VORTEX [7] has been used to predict the airflow properties in the classroom. This program has been developed for the simulation of airflow, heat transfer, mean age of air distribution, PPD and PMV in enclosures. The code uses the standard κ-ε turbulence model and has been developed for ventilation research, which may be more suitable to ventilation simulations than the more general-purpose codes. In the simulations, the measured mean surface temperatures of all six room surfaces have been used as boundary conditions. The number of nodes used were 49 x 39x 37 giving a resolution of.17 m in the horizontal plane and.81 m in the vertical direction. The distance from the floor of the impinging jet outlet was.9 m in both the measurements and the CFD simulations.

3 Figure 1. Plan view of the classroom with supply positions. All dimensions in meters. RESULTS Figure 2 (a,b) shows the air flow patterns from the CFD results for the two supply methods at a height of 4 mm from the floor. Although the air supply velocity of 1.56 ms -1 in the case of IJV was much higher than in the displacement case the velocity close to the floor decays very rapidly and it is only large at a short distance from the jet impinging point on the floor. The spread of the impinging jet over the floor produces a velocity near the floor which is similar in magnitude to that of the displacement ventilation system. However, the floor layer in this case is much thinner than that in the case of the WDV. (a) wall displacement ventilation (b) impinging jet ventilation Fig. 2 Velocity vectors in a horizontal plane 9 cm from floor and age of air contours in a vertical plane

4 The temperature gradients for the two cases are shown in Fig. 3. The agreement between the measurements and CFD simulations are good. The recommended temperature gradient of 3K or less is fulfilled. The mean velocity gradients are shown in Figure 4. Except for heights up to 15 cm above the floor the velocities are much lower than.15 m/s for both cases, indicating a comfortable environment. The velocity distribution over a horizontal plane 1 cm above the floor (see Fig.1) is plotted in Figure 5. Only the wall displacement system shows a tendency for a velocity higher than the recommended maximum of.15 cm/s closed to the walls. 22, ,5 21 Temprature [ o C] 2,5 2 19, ,5 18,1,9,42,74,89 1,19 1,5 2,7 2,38 2,46 2,61 2,85 Height [m] Fig. 3 Mean temperature profiles for two ventilation systems,3,25 Mean velocity [m/s],2,15,1,5,1,9,42,74,89 1,19 1,5 2,7 2,38 2,46 2,61 2,85 Height [m] Fig. 4 Mean Velocity gradients for two ventilation systems

5 Figure 6 shows the predicted percentage of dissatisfied (PPD. One can see from the figure that in a large portion of the room PPD is below 1% for both systems and this is acceptable for such large heat loads. The normalised mean age of air (i.e. the local age divided by that at the exhaust) is shown in Figure 7 for all the measuring points at a height of 1.2 m above the floor (breathing zone level). There is some agreement between the WDV and the IJV results at some of the points but not at others. This may be due to the difference between the momentum in the two systems and its interaction with the local buoyancy forces. The CFD results show similar trends to measurements.,25,2 Mean velocity [m/s].,15,1,5, 1,5 3,15 5,25 7,35 8, Distance from window, x[m] Fig. 5 Predicted and measured mean velocity distribution in a horizontal plane 1 cm above the floor for two ventilation systems PPD [%] ,1,9,42,74,89 1,19 1,5 2,7 2,38 2,46 2,61 2,85 Height [m] Fig. 6 Predicted PPD profiles for two ventilation systems

6 The temperature and velocity profiles at all measuring points 1.2 m above the floor (breathing zone) are shown in Figures 8 and 9. Both systems show good temperature distributions but the WDV gives a higher values. Considering the velocity field for both cases one can see that the velocities are very low and there is little tendency to draught risk. An overall look to the all parameters studied show that both systems can handle the extreme situation they exposed for with some small differences in behaviours. 1,8 1,6 Normalised mean age εi [-] 1,4 1,2 1,8,6,4, Measuring point number Fig. 7 Predicted and measured normalised local mean age of air profiles at the breathing zone (1.2 m above floor) for two ventilation systems Temprature [ o C] Measuring point number Fig. 8 Predicted and measured local mean temperature profiles at the breathing zone (1.2 m above the floor) for two ventilation systems The predicted contaminant removal effectiveness (ε), was 14% for WDV and 13% for IJV indicating the ability of both systems to remove the contaminants in an effective way. The

7 predicted mean votes (PMV) were.63 and.61, which are still less than.5 that is suggested by Fanger [6] to be acceptable.,16 Mean velocity [m/s],14,12,1,8,6,4, Measuring point number Fig. 9 Predicted and measured local mean velocity profiles in breathing zone (1.2 m above the floor) for two ventilation systems Figure 1 shows four sequences of smoke visualisation for an impinging jet supply device. One can see that when the jet reaches the heat source (sitting person) a plume forms and due to sufficient momentum the jet continues along the floor. This is the advantage of impinging jet ventilation compared to a displacement system, in which case the flow sometimes has insufficient momentum to continue passed a heat source and its totally consumed by the plume. Two industrial application of impinging jet ventilation in Sweden are shown in Figure 11a & b. Figure 11a shows an installation of impinging jet supply in System 3R International, which was installed to replace displacement devices in a building where precision instruments/tools are manufactured. When displacement ventilation was used there were many complains from the workers and after their replacement with impinging jet systems no compliment was reported. Figure 11b shows the IJV installed in a factory for heavy metal industry. Another advantage of impinging jet ventilation is that the duct itself has a sound damping effect, which reduces aerodynamic noise at the supply terminal. CONCLUSIONS The results obtained here have shown that the floor level air distribution can handle a full room heat load in an acceptable manner. Although both the WDV and the IJV systems show similar tendencies, some small differences were observed in their performance. Because of better balance between buoyancy and momentum forces the IJV system show slightly better age and velocity distributions. Furthermore, the latter can also be used for both heating and cooling purposes. According to references [1 and 4] the impinging height has a significant

8 effect on the IJV performance but this is not considered here. Therefore, this new system may need further studies by ventilation researchers and designer to fully understand its performance at different conditions. Fig. 1. Smoke visualisation of impinging jet ventilation (a) System 3R International Factory (b) Heavy Metal Industries in Fagersta in Stockholm Fig. 11 Two industrial applications of impinging jet ventilation. Acknowledgements

9 The authors gratefully acknowledge Prof. Mats Sandberg, Leif Claesson (in BMG, Gävle), Mr Bengt Svensson and Lars Berthilson (VVESS Consulting AB, Gävle), Per-Johan Ohlsson, Johan Kostakis and Örjan Josefsson for their valuable contribution to this work. REFRENCES [1] Karimipanah, T, Sandberg, M and Awbi, HB (2), A comparative study of different air distribution systems in a classroom, Air Distribution in Rooms: Ventilation for Health and Sustainable Environment, Proc. ROOMVENT 2, HB Awbi (ed.), Vol. 2, pp , Elsevier, Oxford. [2] Etheridge D. and Sandberg M (1996), John Wiley & Sons Ltd., UK. Building Ventilation: Theory and Measurement, [3] Karimipanah, T (1996), Turbulent jets in confined spaces, PhD thesis, Royal Institute of Technology, Sweden. [4] Kostakis J and Josefsson, Ö (1999), Thermal Comfort and Air Quality in a classroom, University college of Gävle, B.Sc Thesis (In Swedish). [5] Awbi, H.B. (1998), Energy efficient room air distribution, Renewable Energy, Vol. 15, pp [6] Fanger, P.O. (1972). Thermal Comfort, McGraw-Hill, New York. [7] Gan, G and Awbi, HB (1994), Numerical simulation of the indoor environment, Building and Environment, Vol 29, No 4, pp