Between Four Different Ventilation Systems

Proc. Roomvent 2002, pp.181-184, Copenhagen, 8-11 Sep. 2002 2002

A COMPARISON BETWEEN FOUR DIFFERENT VENTILATION SYSTEMS

Y Cho1, H B Awbi1 , T Karimipanah2 1The University of Reading, UK 2Air Innovation AB, Sweden [email protected] [email protected] [email protected]

Summary based using the mean velocity, comfort parameters, local air change index at a Measurements and CFD simulations from four breading zone and ventilation parameters systems are compared using the air change including the heat and contaminant removal index and a new Ventilation Parameter (VP). effectiveness. VP combined with thermal comfort and indoor air quality indices gives a good index for Experimental Set-up comparison of four systems. Impinging Jet is capable of achieving better air distribution in The experiments were carried out in the the space than the other systems (mixing, wall environmental chamber at the Univ. of displacement and floor displacement) Reading. The chamber used for the particularly at higher heat loads. experimental study has external dimensions 4.0m x 3.0m x 2.52m ceiling height. The Introduction dimensions of the test compartment are 2.78m (length) x 2.78m (width) x 2.3 m (height), i.e. Traditional mixing ventilation (MV) systems representing a small size office room. In the which are driven by high jet momentum force, case of the wall displacement ventilation (DV), still occupy a large portion of the market a semi circular wall unit of overall dimensions although in many cases has poor ventilation 0.5m x 0.5m with a radius of 0.25m was used efficiency and is less energy efficient. and the air is supplied through many small In displacement ventilation (DV) fresh air is diameter holes. For floor displacement usually supplied at floor level with an inlet ventilation (DF), two circular floor units of velocity < 0.5m/s and temperature ≥18 oC and overall diameter 150mm was used and the air the cool air rises as it encounters heat sources is supplied through radial slots to create a swirl in the room thus creating a temperature motion at an angle to the face of the diffuser. stratification. However, DV alone can only be For mixing ventilation, a slot with dimensions used for cooling. 400mm x 20mm was placed at a height of Recently, a new method of air distribution 2.16m above the floor projecting the air towards developed by Air innovation AB in Sweden that the ceiling with an angle of 36o in order to avoid is based on the impinging jet principle the dumping of cold jet onto the lower zones. (Karimipanah and Awbi 2002). As a medium For impinging jet ventilation, a semi circular momentum supply device (DV < IJV device with an inlet sectional area 0.0135m2 momentum < MV), impinging jet ventilation was used to supply air at a height of 0.84m (IJV) can combine the positive effects of both above the floor. mixing and displacement system. It produces To provide a realistic office situation a light higher momentum than displacement (36W fluorescent), a computer box (a 400mm ventilation and can result in the jet spreading cube with a 150W light bulb fitted inside), two evenly over the floor. As a result it can provide heated plates (2 x 500mm x 1000mm), a desk a clean air zone in the lower part of the and a chair were placed in the chamber. A occupied zone like displacement ventilation. heated mannequin (see Fig. 1) was made from In this paper CFD and experimental results are 1 mm aluminium sheet with the overall surface compared for four different ventilation systems area of 1.60m2. Heating elements inside the body, head and legs of the mannequin were model and room surface radiation. The controlled to provide a surface temperature calculated velocity, air temperature and radiant equal to that of a typical naked human temperature at each computational cell were (Olesen,1982). A polyurethane tube was used in Fanger’s comfort equation (Fanger, attached to a copper tube (the nose) inside the 1972) for calculating the PD and PPD at the head and fed through the torso and out to the cell. The calculation of local mean age of air, gas sampler. This location represented the  , in a room combines the air flow model with sampling point for the breathing zone. p Four-wire Platinum Resistance Thermometer the transport equation for age of air. The (PRT) sensors (accuracy = 0.15K) have been transport equation for the local mean age of air, used to measure the air temperature and the  , is: inside and outside surface temperatures of the    p   t    p  chamber. Other measuring devices used in the Ui      1 (2) x x     x tests were an accurate Wattmeter, DANTEC i i    i  omni-directional velocity sensors and a Brüel and Kjäer SF6 gas sampling system. The SF6 where Ui is the mean velocity component in xi gas analysis system incorporated a sampling direction,  p is the local mean age of air,  box, gas analyser and a computer with analysis and control software. Further details can be and  t are the laminar and turbulent kinematic found in Xing et al (2001). viscosity respectively,  is the laminar Scmidt

(Prandtl) number and  is the Scmidt number for age of air (=1.0).

Ventilation Parameters

Local Air Change Index (ap )

 ap , the ratio of nominal time constant ( n ) to

the local mean age of air ( p ), is expressed as

 n ap  (3)  p Fig. 1: Mannequin and Computer box Ventilation Parameter (VP) Two cooling loads for the case I and case II was 36 W/m2, 60W/m2 respectively and air To assess the effectiveness of a ventilation change rate was 5ach, i.e. air supply rate of system in both measurement and CFD 25l/s. simulation, the effectiveness for heat removal ( Using tracer decay gas technique, SF6 gas t ) and contaminant removal ( c ) are used was injected from venturi in order to obtain the together with the predicted percentage of contaminant removal effectiveness and the age dissatisfied (PPD) for thermal comfort and of air at a number of points in the room. The percentage of dissatisfied (PD) for air quality. local age of air at any point in the room can be calculated using following expressions: t andc are defined by:  C tdt 0 p (1)  p  To  Ti Co  Ci C0 t  and c  (4) Tm  Ti Cm  Ci CFD Calculations In equations (4), T is temperature (oC), C is the contaminant concentration (ppm), subscripts o,i The CFD program VORTEX (Gan and Awbi, and m denote outlet, inlet and mean value for 1994) has been used to predict the air flow, heat transfer and mean age distribution in the the occupied zone (to a height of 1.8m). t is chamber. This is a three dimensional program similar to a heat exchanger effectiveness and is which solves, using a Cartesian grid, the a measure of the heat removing ability of the continuity equation, the Navier –Strokes system.  is a measure of how effectively the equation, the thermal energy equation, the c concentration of species equation, the two contaminant is removed. The values for t equations for k and  in the k   turbulence and c are determined by heat and The results for c are not correlated with t contaminant sources, the method of room air since  is affected by convection and is distribution, room characteristics, etc. However, c t high values do not always give a good mainly affected by convection and radiation as indication of the thermal comfort and air quality Heiselberg and Sandberg (1990) found. The in the occupied zone. heat removal effectiveness for all four Fanger (1972) has developed expressions for ventilation systems is generally satisfactory. the percentage of dissatisfied (PD) with the The mixing ventilation for high load (case II) indoor air quality and the predicted percentage has a high t , however PPD is also high of dissatisfied (PPD) with the thermal because of dumping of the cold jet into the environment given by Eqs. (5) and (6). occupied zone. Thus MV cannot guarantee thermal comfort and energy saving for large PD  395  exp(1.83 v˙ 0.25 ) (5) heat loads. PPD = 100 – exp -{0.03353 (PMV)4 + 0.2179 For the floor displacement ventilation, the air (PMV)2} (6) does not flow over the floor as expected but actually spreads from the unit at an angle to the floor. Thus the PPD is too high which also Where v˙ is the ventilation rate (ls-1) and PMV affects the ventilation parameter (VP) as shown is the Predicted Mean Vote as defined in ISO in Fig. 2. Also, the air entrained by the 7730(1994) and the recommended PPD limit mannequin is a mixture of the supply air and for ideal thermal environment is 10%, room air, hence the air change index at the corresponding to -0.5  PMV  0.5. Thus, low breathing zone for case I is lower than what is values for both indices guarantee a good indoor expected, see Fig.3. For wall displacement air quality and thermal comfort. 2 system, case I (36 W/m ), ap and VP are The comfort number, Nt , and the air quality 2 number, Nc , (Awbi 1998) combined with PPD generally good but for case II (60 W/m ) there and PD respectively are useful to examine the is difficulty in satisfying the thermal comfort quality of a ventilation system. These are criterion. defined as: The impinging jet acts as displacement ventilation and produces a higher velocity in a t c Nt  , Nc  (7) thin layer over the floor. It also gives the PPD PD highest values of VP and an air change index

which is similar to displacement systems (DV These two numbers can be combined into a and DF systems). single parameter which determines the effectiveness of an air distribution system in Conclusions providing air quality and thermal comfort in the form of a Ventilation Parameter defined as: 1. The new Ventilation Parameter (VP) can provide useful information on ventilation VP  Nt  Nc (8) performance of a system with respect to thermal comfort and indoor quality.

Results and Discussions 2. Although mixing ventilation can remove a high heat load, the thermal comfort may not be Tests with the four types of system (MV, DV, adequate as high velocity cold air can produce DF, IJV) were carried out for 5ach and two heat draught and a high PPD. loads (36 W/m2 without heated plates and 60W/m2 with heated plates). 3. The impinging jet system produced higher Table1 summarises the test conditions and the values of the ventilation parameter (VP) than results obtained from the tests and the CFD the other systems examined. It is still capable simulations at the same conditions for the 8 of achieving better air distribution in the space case. The overall agreement between the than the other three systems at the higher heat measured and predicted (CFD) are generally load. good. The discrepancies between the measured and CFD can be due to a limited References number of measuring points, a poor accuracy in measuring velocities less than 0.1m/s and the Awbi, H B (1998) Energy Efficient Room changes in shape of air inlets when a Cartesian Air Distribution, Renewable Energy, Vol.15, pp293- grid is used in the CFD solutions. 299. Fanger, P.O.(1972) Thermal comfort. Pv Tin Vm T0.1 T PP PD N N VP 1.1 t c t c D (%) ( % (%) (%) ) I 36W/m2 DV EXP 24 18 0.07 23.9 26.0 - - CFD 0.03 24.4 25.9 12 11 7.8 6.4 16. 18.0 17. 6 4 1 0 DF EXP 31 18 0.05 23.8 25.0 - - CFD 0.05 24.1 24.9 10 12 12.4 6.6 8.5 18.7 12. 6 4 7 MV EXP 53 13 0.10 24.5 25.3 95 99 CFD 0.08 25. 93 10 7.7 6.4 12. 15.7 13. 24.5 1 1 1 8 IJV EXP 35 18 0.07 23.3 25.3 11 11 5 1 CFD 0.06 23.5 25.2 12 11 7.1 6.5 17. 17.4 17. 1 3 0 2 II 60W/m2 DV EXP 4 18 0.06 26.2 27.9 - 3 - CFD 0.05 25.6 27.3 13 97 13.9 6.5 9.8 14.9 12. 7 1 DF EXP 4 18 0.05 26.3 28.8 - - 1 CFD 0.04 25.4 29.2 15 10 28.3 6.5 5.6 15.7 9.4 9 2 MV EXP 6 12 0.12 26.9 28.1 97 10 7 1 CFD 0.09 26.6 27.6 10 10 22.2 6.4 4.6 16.4 8.7 3 5 IJV EXP 3 18 0.07 23.3 26.1 11 10 7 7 8 CFD 0.06 24.3 26.9 11 11 7.9 6.5 14. 17.2 16. 8 2 9 0 McGraw-Hill New York. Gan, G and Awbi, HB (1994): Numerical simulation of the indoor environment, Building and Environment, Vol. 29, No. 4, pp 449-459. Heiselberg, P. and Sandberg, M (1990) Convection from a slender cylinder in a ventilated room, Roomvent 90’,Norway ISO/CEN 7730 (1994) Moderate thermal environments: Determination of PMV and PPD indices and specification of the conditions for thermal Table 1: Data for Experiments and CFD comfort. Karimipanah, T. and Awbi, H.B. (2002) 2 o Theoretical and experimental investigation of Pv =Ventilation load(W/m ) Ti = Inlet temp. ( C) impinging jet ventilation and comparison with wall Vm = Mean velocity in the occupied zone (m/s) displacement ventilation, to be published in Building and Environment. Olesen, B.W. (1982) Thermal comfort, Bruel and Kjaer Technical Review, No 2 Xing, H Hatton, A. and Awbi H.B. (2001) A study of air quality in the breathing zone in a room with displacement ventilation, Building and Environment, 2001;Vol 36: 809-820. Fig 2: Ventilation Parameters (VP) at different systems

Fig 3: Local air change index ( ap ) at the breathing zone at 36W/m2