Performance of Supply Airflow Entrainment for Particles in an Underfloor Air. Distribution System

Transcription

1 Performance of Suly Airflow Entrainment for Particles in an Underfloor Air istribution System Can Li 1, Nianing Li 1 octoral Candidate Professor Associate Professor 1College of Civil Engineering, Hunan University, Changsha P.R.China, Hunan University of Technology, Zhuzhou P.R.China, Lc1999@16.com Abstract: Investigation and engineering rojects indicate that the underfloor air distribution (UFA) system is suerior to conventional HVAC systems in some asects. For examle, the former is more adative in rearranging the office sace to imrove indoor comfort conditions and energy conservation. However, the suly air outlet of UFA system is set on the floor, such that the suly airflow may entrain the dust articles settled on the floor or susended near the floor. This creates roblems that need to be studied, because airflow can bring theses article to the human breathing zone. In this aer, the erformance of the airflow field in a UFA system and the characteristics of suly air entrainment for articles are studied. A hysical and numerical model is established. Comarison of the numerical calculation results of the airflow field with the measured data is given. Results show that the model is valid and can redict the erformance of airflow entrainment for dust articles in different air suly conditions. Key words: under floor air distribution system, article, entrainment, simulation 1. INREOCCTION In many modern office buildings, raised floor were build to create floor voids for flexible and convenient cable management. with the installation of the raised floor, under floor air distribution (UFA) system, which make use of the floor void to distribute conditioned air, has been gaining concern among the industry and the number of alications have been growing since it was first introduced in the 1950s. With UFA system, the air diffuser convenient be arranged or oerated. Besides this, many researchers show that well-designed UFA systems can rovide such benefits as: erformed better than the traditional ceiling-based systems both in the energy asect and in the indoor air quality asect [1] ; reduced life cycle building coats [] ; imroved thermal comfort [3]. However, the UFA systems conduct different air distribution attern from traditional over-head air conditioned systems. For UFA systems, the article deosited under floor or susended on the air near floor may be entrained by air flow then be carried into the human s inhaled zone have to be considered. Exeriments are usually conducted in model ventilation room to measurement airflow filed and concentration of article due to these arameter in real room were comlicated and influenced by many factors [4,5]. Chao et al. measured airflow and air temerature distribution in the occuied region to an under floor ventilation system [6]. Lu et al. studied the model and measurement of airflow and aerosol article distribution in a ventilated chamber which is divided into two zones by a artition with a large oening [7]. With the alication of comutational Fluid ynamics (CF) in building environment has undergone extensive develoment in recent years, many reorts have been found in which air flows or aerosol article susend in ventilated room are simulated.shimada et al [8] and Cheong et al [9] have

2 emloyed the commercial code FLUENT investigate the transort and disersion of contaminant article in a ventilated room or turbulent flow behavior. They have both conducted satisfactory agreement results with the exerimental data. In this aer, the re-normalization grou(rng) k- ε turbulence model of FLUENT is emloyed simulate the airflow velocity rofiles in a UFA system model room. The numerical calculate results are comared with measurements. And the article transort behavior with the air movement is investigated by emloying Particle iscrete Model (PM) simulating. In RNG k- ε model equations, effective viscosity that is calculated by ρ k d( ) = 1.7 εμ ˆ υ 3 ˆ υ 1+ C υ d ˆ υ Where ˆ υ = μ / μ and C υ = 100, eff μ eff denotes μ denotes gas hase turbulent viscosity, defined as μ = ρc ε. is the generation of μ ( k / ) turbulence kinetic energy due to the mean velocity gradient defined by P k P k = μ. js ij. COMPUTAL MOLE S ij 1 u ( i ui = + ) x x j i.1 Air Flow Model In this study, just isothermal air flow is investigated. The air is regarded as a continuous fluid, so the air movement in the room is governed by the Eulerian conservation equation ( ρ) + div( ρv ) div[ Γ grad] = S (1) t Continuity equation ( =1) Γ = 0, S = 0 () Momentum equation ( = reresents each of ui the three velocity comonents u v w ) u i Γ = μ, S Turbulent kinetic energy equation ( = k ) eff = x j (3) Γ = μ / σ, S = P ρε (4) eff k k issiation equation ( = ε ) ε Γ = μeff / σk, S = ( Cε P ) 1 k Cε ρε R (5) k is the rate-of-strain tensor. More details of turbulence arameters of RNG k-εmodel can be found in Ref.[10]. The boundary conditions which are secified for gas hase in this study are defined as follows: the inlet is defined as an oening with a uniform velocity; Neumann boundary conditions are alied at the outlet.that is, mass flow boundaries are secified to ensure the mass flow rate out of the domain the same as the mass flow rate into the flow domain; A non-sli condition at the solid wall is alied for velocities. Wall functions are alied to describe the turbulent flow roerties in the near wall region. The equations are discretized into algebraic equations by the Finite volume method (FVM). The discretization method is a second-order uwind scheme and SIMPLE algorithm is adoted. The Boussinesq model is emloyed to consider the buoyancy effect.. Particle Phase Model In FLUENT, a discrete trajectory aroach (Lagrangian method) is emloyed in the iscrete Phase Model (PM). The Lagrangian aroach slits the article hase into a reresentative set of individual articles and tracks these articles

3 searately through the flow domain by solving the equations of article movement. The following assumtions are used: all articles are inert articles in sherical solid shae; heat and mass transfer between air and articles are neglected; no articles coagulation in the article deosition rocess; all articles on solid surface are entraed by them, such as walls, floors and ceiling. This force balance equates the article inertia with the forces acting on the article, and can be written (for the direction in Cartesian coordinates) as du dt i = F ( ui ui) + gi( ρ ρ)/ ρ + F i (6) where F ( u u ) is the drag force er unit i i article mass, and F F is given by 18μ C Re = (7) ρ 4 u is the article velocity, ρ is the density of the article. The drag coefficient, C, is evaluated from exerimental-fitted exression of Morsi and Alexxander. C = a a a + + (8) 3 1 Re Re and virtual mass force caused by unsteady flow, Brownian force, lift force due to shear (Saffman s lift force),and Thermohoretic Force caused by temerature gradient. The article roerties (such as article size and density) and the air flow field influence the magnitude of these forces. Considering the article size and density in indoor environments, some of the forces are small enough to be negligible [11].Ref. [11] analyzed each of the force then found the Brownian force and Saffman s lift force may be relatively large for the fine articles and flow field to be studied. Therefore, only the Brownian force and Saffman s lift force are taken into account for this study since they may lay an imortant role in sub-micron articles motion near the walls, where the velocity gradient is large enough to make Saffman s lift force dominant. The Saffman s lift force is from Li and Ahmadi and is a generalization of the exression rovided by Saffman [1] : F si Kν ρd = ( u i u i ) (9) ρ 0.5 ij 0.5 ( dlkdkl) Where K =.594 and is the deformation tensor. The initial conditions for article tracking include the starting ositions and initial velocities of articles which have the density of the oil smoke article, i.e. ρ = kg / m 3 The starting ositions of article is assumed in the underground and uniformly distributed in that area with 0 initial velocities. d ij Where a1 a a3 are constants that aly to 3. EXPERIMENT PROCEUCES smooth sherical articles over several ranges of Re. The second term on the right-hand side of equation (6) is the gravitational force er unit article mass. And the third term, F i, reresents the additional acceleration force exerted on articles deending on the flow condition and article roerties. These forces include ressure gradient force, Basset force Exerimental work is carried out in a UFA model room. The room size used in the exeriment is L(X ) W(Y ) H(Z)=3 m 3 m.64 m., The height of floor lenum is 180mm. Air is entrained the floor lenum from three rectangular air duct and is sulied to room through a circular oening(100) on the floor. In the ceiling of room, 4 rectangular air return grill are arranged regularly, which size is L(X)

4 W(Y) =0.m 0.m. As be seen in Fig.1. The ositions of the velocity measurement oints are as x = 1000, 500,0,500,1000 y = 1000, 500,0,500,1000 z = 1100 and x = 0 y = 0 z = 100,500,1100,1500,1800 The origin of coordinates is set in the center of the floor diffuser, meantime, the central axis of the floor air diffuser is the central axis of the room. Exeriment erformances in two conditions as inlet air velocity is 1.1m/s and 1.3m/s resectively. Velocity of each measured sots is measured in both inlet velocity conditions. In fact, the value of the velocity is turbulent, it always fluctuate within in a range. The range is varied for different sot. So, to conveniently comare with numerical simulations, which are fixed, the average value of measured data within 30 seconds is extracted when rocessing data. emloyed in the calculation. That s, the main flow is defined as entering the inlet with a uniform velocity, i.e. (a) ux = 1.1 m/ s, uy = u z = 0 (for ACH 8h -1 ), and (b) ux = 1.3 m/ s, uy = u z = 0 (for ACH 9h -1 ). The quantities k and E at the inlet are normally based on the mean flow characteristics at the inlet. (a) Inlet velocity is 1.1m/s Fig.1 The structure of model room 4. RESULTS AN ANALYSIS 4.1Gas Phase Calculation The room considered here is divided into cells (i.e. control volumes). The inlet section is divided into cells, while cells are set u for the exhaust section To comare numerical calculation with exerimental data, the similar conditions are (b) Inlet velocity is 1.3m/s Fig. Air velocity contour as Z=1.1m Fig (a) and (b) show the calculated velocity contour in these two conditions as Z=1.1m. The corresonding measured values of velocity of each test oint are marked in the figure yet. Comarisons

5 of air velocity along the central axis of measured data with simulated results in two conditions are showed in Fig.3 (a) and (b). From fig., it can be found that in both inlet conditions reasonable and accetable of air velocity field room horizon lan is been simulated. Esecially the calculated velocity values for the oints some distant away from walls is good agreement with the measured values. However, the simulated results for less oint which set on the wall vicinity is disagreement with the measured data. The main reason may be the model can not treat the room wall erfect that can be seen in Ref.[8]. Fig.3 indicates that along the central axis of room the simulated calculation results of test oints are good agreement with the measured data in both inlet velocity conditions, although the calculated velocity of the oint near by the air suly(x=y=z=0) is smaller than the measured data. (a) Inlet velocity is 1.1m/s (b) Inlet velocity is 1.3m/s 4. Case of Particle Phase Calculation Many of simulations of contaminant article disersion and distribution within room using the Lagrangin article-tracking model have reorted. The similitude method is emloyed in this aer, which has been gained result good agreement with measured data [11] [13]. Based on the right airflow field calculated result, cases of movement of the article are simulated in this study. Consider the article which size under 10μm is interest for indoor air quality since they are known as inhaled article, the articles of the case are divided into three grous with 1,5 and 10 μ m diameter. To comare the article movement and distribution, articles are assumed to be uniformly carried by each size grou. The article mass carried by each samle grou is assumed to be 50μg. The articles are assumed to initially be on the floor, therefore the initial osition of articles are set u in the whole surface on the floor. Tra or escae tye boundary conditions are set for wall boundary conditions of the article transort model, i.e. there is no article rebound when it reaches the wall. Fig.4 gives the comarison of the tracks between three grou articles (PM1/PM5/PM10) when inlet air velocity is 1.3m/s. The calculation results reort and Fig.4 illustrate that the bigger articles (PM10) send few time susended in ventilation sace, while the smaller articles (PM1/PM5) send few time susended in there. At the other inlet velocity, the similar result is conduct. This means the smaller articles contributed more to the indoor ollutant concentration. Fig.3 Comarisons of air velocity along the central axis of measured data with simulated results

6 (a) Inlet velocity is 1.1m/s Colored by PM concentraction(kg/m 3 ) Fig.4. Tracks of articles (the to one is trace of the article of PM10, the middle one is tracks of the article of PM5, the bottom one is tracks of the article of PM1) The comarison of contours of PM concentration of surface x=0 between different inlet velocity condition can be seen in Fig.5. It can be seen that the PM concentration of the occuied region decrease with the ventilation rate growth in current study. 5. CONCLUTIONS A CF analysis of airflow atterns and aerosol article movement erformance in a UFA system is been resented in this aer. Comarisons of air flow velocity field of comutations with exeriments have also been given. The calculated velocities of almost testing oint are accordance with the measured data. The following conclusions emerge. the model of UFA system resented in this study is valid to both exerimental ventilation (b) Inlet velocity is 1.3m/s Fig.5. Contours of PM concentration of surface x=0 conditions and the boundary conditions being set is roer. the model of PM model emloyed in current study can redict the erformance of the article entrained with the airflow The ventilation rate influences the article moving erformance in current study, the concentration of article decrease with the growth of ventilation rates the smaller articles send more time susended in ventilated saces and so have significant contribution to the ollutant concentration of indoor air Further work is required in this area, e.g. the study of airflow distribution in room with non-thermal jet in UFA SYSTEM, and the article movement under buoyancy influence.

7 ACKNOWLEGMENTS The financial suorted rovided by the Secialized Research Fund for the octoral Program of High Education (SRFP) I is grateful acknowledged. REFERENCES [1] K.J. Loudermilk, Underfloor air distribution solutions for oen office alications, ASHRAE Transactions [J] 105 (1999) [] F. Bauman, T. Webster, Outlook for underfloor air distribution, ASHRAE [J] 43 (6) (001) [3] T. Webster, F. Bauman, J. Reese, Underfloor air distribution: thermal stratification, ASHRAE [J] 44 (5) (00) [4] Thatcher TL, Lai Alvin CK, Moreno-Jackson R, Sextro RG, Nazaroff WW. Effects of room furnishings and air seed on article deosition rates indoors. Atmosheric Environment [J] 00; 36: [5] Koonen IK, Asmi A, Keronen P, Puhto K, Kulmala M. Indoor air measurement camaign in Helsinki, Finland 1999 the effect of outdoor air ollution on indoor air. Atmosheric Environment [J].001;35: [6] C.Y.H. Chao, M.P. Wan. Airflow and air temerature distribution in the occuied region of an under!oor ventilation system. Building and Environment[J] 004 ;39: [7] Weizhen LU, Howarth AT, Adam N, Riffat S. Modeling and measurement of airflow and aerosol article distribution in a ventilated two-zone chamber. Building and Environment [J] 1996;31: [8] Shimada M,Okuyama K,Okazaki S,Asai T,Matsuku ra M, Ishizu Y. Numerical simulation and exeriment on the transort of fine articles in a ventilated room. Aerosol Science and Technology [J] 1996;5:4 55. [9] Cheong KW,junaed y E,Poh TK,Tham KW, Sekhar SC, Wong NH,Ulah MB. Measurements and comutations of contaminant s distribution in an office environment. Building and Environment [J] 003;38: [10] FLUENT user s guide. Version 6. FLUENT incororated, Lebanon,NH, 003. [11] Bin Zhao; Ying Zhang, Xianting Li et al. Comarison of indoor aerosol article concentration and deosition in different ventilated rooms by numerical method. Building and Environment [J] 004; [1] Saffman PG. The lift on a small shere in a slow shear flow. Journal of Fluid Mechanics [J]1965; : [13] Z.F. TIAN, J.Y. TU,G.H. YEOH, R.K.K. YUEN. On the numerical study of contaminant article concentration in indoor airflow. Building and Environment [J]; 005;