Aalborg Universitet. Evaporation Controlled Emission in Ventilated Rooms. Topp, Claus; Nielsen, Peter V.; Heiselberg, Per. Publication date: 1997

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1 Downloaded from vbn.aau.dk on: April 13, 2019 Aalborg Universitet Evaporation Controlled Emission in Ventilated Rooms Topp, Claus; Nielsen, Peter V.; Heiselberg, Per Publication date: 1997 Document Version Publisher's PDF, also known as Version of record Link to publication from Aalborg University Citation for published version (APA): Topp, C., Nielsen, P. V., & Heiselberg, P. (1997). Evaporation Controlled Emission in Ventilated Rooms. Aalborg: Dept. of Building Technology and Structural Engineering. ndoor Environmental Technology, No. 76, Vol.. R9739 General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.? Users may download and print one copy of any publication from the public portal for the purpose of private study or research.? You may not further distribute the material or use it for any profit-making activity or commercial gain? You may freely distribute the URL identifying the publication in the public portal? Take down policy f you believe that this document breaches copyright please contact us at vbn@aub.aau.dk providing details, and we will remove access to the work immediately and investigate your claim.

2 NSTTUTTET FOR BYGNNGSTE<:NK DEPT. OF BULDNG TECHNOLO GY AND STRUCTURAL ENG NEERNG AALBORG UNVERSTET AAU o AALB ORG DANMARK = aj u E o.oo 1s aj 0 u Floo r, no wall -B- Floor, wa ll -e- Cei li ng, no wa ll --7- Ceili ng, wall --*- Tes t eh amb er,!! ffi----1-:7' ?.lt :Y:;;..-c----1 = 0':3 '- 0': /--.Lf------::;/ Velo city (m s) NDOOR ENVRONME N TAL T E C H NOLOGY PAPER N O. 76 Proceed ings of H ealthy Buildings/AQ '97. Glob al ssues a nd Regional Solut io ns, Washingt on DC, USA, Vol. 3, p p , Septem.ber 27 - Octob er 2, 1997 C. T OPP, P.V. N ELSEN, P. HESELBERG E VAPORAT ON CONTROLLED EMSSON N V ENT LATED ROOMS NOVEMBER 1997 SSN R 9739

3 The papers on NDOOR ENVRONMENTAL TECHNOLOGY are issued for early dissemination of research results from the ndoor Environmental Technology Group at the University of Aalborg. These papers are generally submitted to scientific meetings. conferences or journals and should therefore not be widely distributed. Whenever possible reference should be given to the final publications (proceedings, journals, etc.) and not to the paper in this series. Printed at Aalborg University

4 NSTTUTTE T FOR BYGN NGSTEKN K DEPT. OF BULDNG TECHNOLOGY AND STRUCTURAL ENGNEERNG AALBORG UNVERSTET AAU AALBORG DANMARK NDOOR ENVRONMENTAL TECHNOLOGY PAPER NO. 76 Proceedings of Healthy Buildings/AQ '97. Global ssues and Regional Solutions, Washington DC, USA, Vol. 3, pp , September 27 - October 2, 1997 C. TOPP, P.V. NELSEN, P. HESELBERG EVAPORATON CONTROLLED EMSSON N VENTLATED ROOMS NOVEMBER 1997 SSN R9739

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6 EVAPORATON CONTROLLED EMSSON N VENTLATED ROOMS C. Topp, P.V. Nielsen, P. Heiselberg Department of Building Technology and Structural Engineering, Aalborg University, Denmark ABSTRACT Emission of volatile organic compounds (VOCs) from materials is traditionally determined from tests carried out in small-scale test chambers. However, a difference in scale may lead to a difference in the measured emission rate in a small-scale test chamber and the actual emission rate in a full-scale ventilated room when the emission is fully or partly evaporation controlled. The ;>bjective of the present research work has been to investigate the change of emission rates from small-scale experiments to full-scale ventilated rooms and to investigate the influence of the local air velocity field near the source. A series of CFD experiments has been carried out for different set-ups and different air change rates. The results provide a method to compare mass transfer coefficients found in different scales and different geometries for evaporation controlled emission processes. Furthermore, the results show that for a given set-up the mass transfer coefficient increases with velocity and turbulence intensity. NTRODUCTON Traditionally, emission from surfaces is characterized as emission controlled by diffusion through the material or emission controlled by evaporation from the surface. The emission from a material is often determined from tests carried out in a small-scale test chamber. For a given pollutant a difference in scale may lead to a difference in the emission rate measured in the test chamber when the emission is controlled by evaporation and depends on environmental parameters such as temperature, pollutant concentration, humidity, air flow pattern, air velocity and turbulence. This paper deals with evaporation controlled emission influenced by the air flow pattern near the source. t is assumed that emission of volatile organic compounds (VOCs) from a surface is limited by molecular diffusion through the boundary layer at the surface-air interface see figure 1, and can be described by Pick's law (equation 1). The driving force for the emission from a surface is the difference in concentration between the surface and the air. de E=-Ddy E = kjc,. -c) D k =- c 8 D (1) (2) (3) where E=emission rate (mg/sm D=molecular diffusion coefficient (m 2 /s), c=concentration, kc=mass transfer coefficient (m/s), C 5 =concentration at surface (mg/m 3 ), c=background concentration (mg/m 3 ), 8 0 =thickness of diffusion boundary layer (m).

7 Velocity Concentration Do Figure 1 Velocity and concentration profiles in the turbulent boundary layer. Du and 8 0 are the thickness of the velocity and diffusion boundary layer, respectively. Analogous to heat transfer the emission can be expressed in terms of a mass transfer coefficient, kc (equation 2), where kc is defined as the molecular diffusion coefficient, D, divided bythe thickness of the diffusion boundary layer, 8 0 (equation 3). The thickness of the diffusion boundary layer, 8 0, depends on the conditions in the air flow near the source such as velocity and turbulence intensity. Consequently, the thickness of the diffusion boundary layer must be accounted for when describing the emission from a surface. This has been done by Tichenor et al. for small-scale test chambers and large rooms (1 ). t has been the objective of the present research work to provide a tool that links emission rates found in a small-scale test chamber and in a full-scale ventilated room. A model that expresses the mass transfer coefficient in terms of the air velocity near the surface has been developed for different geometries and different source locations, i.e. different local air flow patterns. METHODS A series of numerical experiments made by Computational Fluid Dynamics (CFD) has been carried out for a small-scale test chamber and a full-scale ventilated room including different source locations as well as different geometries. The full-scale room is chosen similar to the room used in the nternational Energy Agency, Annex 20 programme (3) and the different set-ups are shown in Figure 2. Here, the emitting surface is located either at the ceiling or at the floor. To avoid disturbances from inlet conditions and reattachment to the floor an area of 3 m from the inlet or the outlet respectively is not emitting. -l L l_ -, l - l Emitting surface Figure 2 Outline of the four different set-ups in a test room with two-dimensional flow (3). n two of the cases a wall (height 1.5 m) is located at x = 6 m. The length of the room is 9 m, the height of the room is 3 m, the height of the inlet is m and the height of the outlet is 0.48 m. 2

8 The test chamber is chosen as two emitting surfaces with typical length and spacing as used in the CLMPAQ unit (2), see Figure 3. Emitting surface Emitting surface Figure 3 The length of the test chamber is 0.8 m and the distance between the two emitting surfaces is 0.02 m (2). n all cases a hypothetical source is used. The concentration at the surface is c 5 = 1000 mg/m 3 and the molecular diffusion coefficient is D = m 2 /s. Usually, the k- turbulence model, where k is the turbulent kinetic energy and is the dissipation of energy, is used to calculate room air flow (4). The model is based on the assumption that the flow is fully turbulent which is not true in the near wall region where the local Reynolds number is so small that the transport equations for the turbulent quantities, k and, do not apply. When modelling flow in the wall region the problem can be accounted for either by adding molecular diffusion terms to the k- model (Low Reynolds Number model, LRN model) or by using a wall function (4). n terms of computational time the LRN model is expensive because it requires a high resolution of the boundary layer. The wall function describes the profile close to the wall and connects the wall boundary conditions with the properties in the fully turbulent layer. Consequently, grid points are not needed in that region. n the present case the interest is focused on the near wall region and the LRN model is therefore used. RESULTS The results from the CFD experiments are evaluated at x = 3 m for the floor and at x = 6 m for the ceiling. To investigate the influence of the velocity field near the source of emission a reference velocity is required. The reference velocity used for the test room is the local maximum velocity, Umax, in the x-direction and for the test chamber the inlet velocity is used. Typical velocity profiles from the test room are shown in figures 4 and 5. E. "" '-"._... = e 0... "" '-" c!:l '... l1, --Ceiling, no wall 11. ' ---Ceiling, wall -.fknowa ll fk: wall 1 11 ' r,,/ Velocity (m /s), {k (m /s) Figure 4 Velocity profiles and turbulence levels at the ceiling in the two-dimensional test room at x=6m and an air change rate of 0 h e 0.8 "" '-"... = 0.6 e "" '-" c!:l ! -- Floor, no wa ll Floor, wa ll r ---- lk.now all --- lk,wall.. f.., - / ) Velocity (m /s), {k (m /s) Figure 5 Velocity profiles and turbulence levels at the floor in the two-dimensional test room at x=3m and an air change rate of 0 h -. 3

9 n mtxmg ventilation the recirculating flow is similar to a wall jet and the level of turbulence can be expressed by.jk "" 1.1 RMS where k is the turbulent kinetic energy and RMS is the Root Mean Square value of the velocity fluctuations in the flow direction (5). The parameter.fk has been included in the figures to illustrate the level of turbulence. Both at the ceiling and at the floor the velocity is reduced by the wall but the level of turbulence is increased. Figures 6 and 7 show typical concentration profiles in the two-dimensional test room. Close to the wall in the laminar part of the boundary layer, the emission is controlled by molecular diffusion due to the concentration gradient. At the ceiling the concentration gradient is steeper than at the floor due to the higher velocity = <.1 «< e <:>... <.1 = «< , --Ceiling, no wall Ceiling, wall ' <.1 «<... = e <:> <.1 = Floor, no wall Floor. wall :... "" Concentration (mg/m 3 ) Figure 6 Concentration profiles at the ceiling in the two-dimensional test room at x=6 m and an air change rate of 1 0 h. Concentration (mg/m 3 ) Figure 7 Concentration profiles at the floor in the two-dimensional test room at x=3 m and an air change rate of 10 h- 1 The concentration distributions are also outlined in figure 8 together with the flow field. t can be seen that the free wall affects the flow field and, consequently, the concentration distribution. The larger influence from the wall occurs when the emission source is located at the floor... i c/c 5 =0. c L Figure 8 Concentration distributions in the two-dimensional test room for the four different set-ups corresponding to an air change rate of 10h" 1 4

10 The mass transfer coefficient, kc, is obtained from equation 2 with the emission rate, E, the surface concentration, C 5, and the background concentration, c, as input. E and c 5 are fixed but c is not well defined. n the present case the local concentration at y = 1.5 m has been chosen as the background concentration. n figure 9 the relation between the reference velocity and the mass transfer coefficient, kc, is shown for the 4 set-ups as well as for the CLMPAQ unit for different air change rates Floo r. no wall -- E -B- Floor, wall e- Ceiling, no wall - = Ceiling, wall... 0:: ' E- Test eh amber.., 0... '... =.., !::: OJO Velocity (m /s) Figure 9 Relation between mass transfer coefficient and velocity based on the local background concentration at y= 1.5 m for different air change rates ir====::r=:=====:::;----, -8- Floor, wall Ceiling, wall.._ 0 Exhaust + Exhaust E 6 Zero X Zero.., ===+====+-:L- 0:: '....., fifr-1---t---:;;L---t Velocity (m/s) Figure 10 Examples of the relation between mass transfer coefficient and velocity based on different definitions of the background concentration. Two other definitions of the background concentration based on different assumptions have been investigated. f the VOCs in the room were fully mixed which they are not in the present case, the background concentration, c, would be defined as the concentration in the exhaust air. Another way of handling c is to neglect the term but this is not correct according to the concentration profiles. The influence of the background concentration is illustrated in figure 0 where the relation between the mass transfer coefficient and the reference velocity is shown for the three definitions of the background concentration. DSCUSSON The influence of local air flow conditions on the molecular diffusion through a laminar boundary layer has been investigated. t is shown that velocity, turbulence and background concentration, due to different geometries give highly different mass transfer coefficients at a given velocity. Consequently, when testing a material in a small-scale test chamber the air velocity should not be the expected velocity in a full-scale ventilated room. f, in the present case, Umax = 0.15 m/s at the ceiling in the empty room the material should be tested at 0.08 m/s in the test chamber to obtain the actual mass transfer coefficient at kc = m/s. Furthermore, the CFD experiments show that for a specific set-up the mass transfer coefficient increases with velocity and turbulence. At a given air change rate the velocity levels at the floor and the ceiling respectively, are much different but the difference in mass transfer coefficient is not as significant. This is due to the higher turbulence level at the floor. 5

11 The background concentration, Coo, also influences the mass transfer coefficient and three definitions of Coo have been investigated. Defining Coo as the concentration in the exhaust air is based on the assumption that the VOCs in the room are fully mixed which is rarely the situation. Another way of handling the background concentration is to neglect the influence and let Coo = 0 which is not true either. Here, the local background concentration at y= 1.5m has been used. When the floor is emitting, the source is located far from the outlet and the local background concentration is higher than the concentration in the exhaust air. Therefore, the mass transfer coefficient based on the local background concentration is the higher one. The deviation from the mass transfer coefficient based on the local background condition is 0-10 % if Coo is based on the concentration in the exhaust air and 5-15 % if Coo is neglected. f the source is located at the ceiling, it is closer to the outlet and the order of the concentrations are reversed. Consequently, the mass transfer coefficient based on the concentration in the exhaust air is the higher one. n this case the deviation from the mass transfer coefficient based on the local background condition will be 0-5 % if Coo is based on the concentration in the exhaust air and 0-10 % if Coo is neglected. ACKNOWLEDGEMENTS This research work has been supported financially by the Danish Technical Research Council (STVF) as a part of the research programme "Healthy Buildings". REFERENCES 1. Tichenor, B.A.; Guo, Z. and Sparks, L.E "Fundamental Mass Transfer Model for ndoor Air Emissions from Surface Coatings." ndoor Air Vol. 3, pp Gunnarsen, L.; Nielsen, P.A. and Wolkoff, P "Design and Characterization of the CLMPAQ, Chamber for Laboratory nvestigations of Materials, Pollution and Air Quality." ndoor Air Vol. 4, pp Nielsen, P.V "Specification of a Two-Dimensional Test Case." nternal Report for the nternational Energy Agency, Annex 20, Aalborg University Denmark. 4. Launder, B.E. and Spalding, D.B "The Numerical Computation of turbulent Flows. "Computer Methods in Applied Mechanics and Engineering. Vol. 3, pp Nielsen, P.V "Airflow Simulations Techniques- Progress and Trends. "Proceedings of the loth AVC Conference, Espoo, Finland. 6. Nielsen, P.V "Healthy Buildings and Air Distribution in Rooms. "Proceedings of Healthy Buildings '95, Milan (taly), Vol. 2, pp

12 PAPERS ON NDOOR ENVRONMENTAL PAPER NO. 43: H. Brohus, P. V. Nielsen: Personal Exposure in a Ventilated Room with Concentration Gradients. SSN R9424. PAPER NO. 44: P. Heiselberg: nteraction between Flow Elements in La1:qe Enclos'lJ,re.s. SSN R9427. PAPER NO. 45 : P. V. Nielsen: Prospects jot Computational Fluid Dynamics in R.oom Air Contaminant ContTOl. SSN R9446. PAPER NO. 46 : P. Heiselberg, H. Overby, & E. Bj0rn: The Effec t of Obstacles on th e Boundary LayeT Flow at a Vertical S'!1.1jace. SSN R9454. PAPER NO. 47: U. Madsen, G. Aubertin, N. 0. Breum,.J. R. Fon'taine & P. V. Nielsen: TraceT Gas Technique vetsus a Control Box Method for Estimating Direct Capi'!l,re Efficiency of Exha ust Systems. SSN R9457. PAPER NO. 48 : Peter V. Nilsen: Verti cal Temperature DistTib ut?:on in a, Room with D 1:.5placement Ventilation. SSN R9509. PAPER NO. 49: Kjeld Svidt & Per Heiselberg: CFD Ca.lc'll,lntio ns of th e Air Flow along a Cold Vertical Wall with an Obstacle. SSN R9510. PAPER NO. 50: Gunnar P. Jensen & Peter V. Nielsen: Tra.n,gfer of Emission Test Da.ta. jto m Smo,ll Scale to Pull Sca.le. SSN R PAPER NO. 51: Peter V. Nielsen: Healthy B 1J.ildings a.nd Air Dis ttib ution 1:n R.oom... SSN R9538. PAPER NO. 52: Lars Davidson & Peter V. Nielsen: Calwlation of the Two Dimen.5ional A irfiow in Facial Regions and N asnl Cavity using an Un8t1"/tctured P.inite Volume Solver. SSN R9539. PAPER NO. 53: Henrik Brohus & Peter V. Nielsen : Personal Expo s1j.te to Co n taminant So ntces in n Uniform Velocity Field. SSN R9540. PAPER NO. 54: Erik Bj0rn & Peter V. Nielsen: Merging Th ermal PZ.u.mes in th e l ndoot Envimnment. SSN R9541. PAPER NO. 55: K. Svidt, P. Heiselberg & 0. J. HPndriksen: Natnml Ventilation in Atria - A Cnse St11.dy. SSN R9647. PAPER NO. 56: K Svidt & B. Bjerg: ComputeT Predi ction of AiT Quality in Livestock Buildings. SS N R9648. PAPER NO. 57: J. R. Nielsen, P. V. Nielsen & K Svidt: Obstacles in the Occupied Zone of a R.oom with Mixing Ventilation. SSN R9649. PAPER NO. 58: C. Topp & P. Heiselberg: Obstacles, an Energy-Efficient Method to Red11.ce Downdraught from Larg e Glazed Surfaces. SSN R9650. PAPER NO. 59: L. Davidson & P. V. Nielsen: Larg e Eddy Simulations of th e Flow in a ThTee-Dimensional Ventilated Room. SSN R9651. PAPER NO. 60: H. Brohus & P. V. Nielsen: CFD Mod els of PeTsons Evaluated by Full-Scale Wind Chann el Experiments. SSN R9652.

13 ';' PAPERS O N NDOOR ENVRONMENTAL TECH N OLOGY PAPER NO. 61: H. Brohus, H. N. Knudsen, P. V. Nielsen, G. Clausen & P. 0. Fanger: P etceived A ir Quality in a. D isplacement Ventilated Room. SSN R9653. PAPER NO. 62: P. Heiselberg, H. Over by & E. Bj 0rn: Energy-Efficient M ea.sures to Avoid D owndmft from Larye Glazed Fa cades. SS N R9654. PAPER NO. 63: 0. J. Hendriksen, C. E. Madsen, P. Heiselberg & K. Sviclt: n door Climate of Large Glazed Spaces. SS N R9655. PAPER NO. 64: P. Heiselberg: A nalysis an d Prediction T echniques. SSN R9656. PAPER NO. 65 : P. Heiselberg & P. V. Nielsen: Flow Elemen t Models. SSN R9657. PAPER NO. 66: Erik Bj0rn & P. V. Nielsen : Exposure d1w to l ntemct?:ng Azr Flows betwee n Two P etso;.s. SS N H9658. PAPER NO. 67: P. V. Nielsen: Temperature Distribution in a Displacement Ventilat ed Room. SSN R9659. PAPER NO. 68 : G. Zhang, J. C. Bennetsen, B. Bjerg & K. Sviclt: Analysis of Air Movement M ea.s1/,ted in a. Ventilated Enclos 1m :. SSN R9660. PAPER NO. 69 : E. Bj0rn, P. V. Nielsen: Passive Smoking in a. Displacem ent Ventila.t f'.d Room. SSN R9714. PAPER NO. 70: E. Bj 0rn, M. Mattsson, M. Sandherg, P. V. Nielsen: Displacemen t Ven tilatwn - Effects of Movement and E xhalation. SSN R P APEH NO. 71 : M. Ma.ttsson, E. Bj0rn, M. Sandberg, P. V. Ni elsen : Sirwu.lating P eo ple M ovr>ng zn Displacement Ventilat ed R ooms. SSN R9729. PAPER NO. 72: H. Brohus: CFD-Simulation of P etsonal E xpos11.te to Contaminant So utces in Ventilated Rooms. SSN R PAPEH NO. 73 : H. Brohus: Mea.mrement of P ersonal Expos11.re 1Lsing a Breathing TheTmal Manikin. SSN R9735. PAPER NO. 74: H. Brohus, C. E. Hyldga.a.rd : T he Us e of Tracer Gas Measurem ents in D etection and Solution of ndoor Air Q1wldy P To blems in a. Danish Twon Hall. SS N R9736. PAPER NO. 75: C. E. Hyldgaa.rd, H. Brohus: D etection and Solntion of ndoor Ar.r Q11.alzt y PTOblems in a, D anish T won Hall. SSN R9737. PAPER NO. 76 : C. Topp, P.V. Nielsen, P. Heiselberg: E vaporation ConiTOlled Emission in Ventdat erl R ooms. SSN R9739. PAPER NO. 77 : P. Lengweiler, P.V. Nielsen, A. Maser, P. Heiselberg. H. Taka.i: Deposr:tion and Resuspension of Particles. SS N R9740. Department of Build ing Technology a n d Structur al E ngineer ing Aalborg University, Sohngaardsholmsvej 57. D K 9000 A a lb org Telephone: Telefax: