Aalborg Universitet Exhaust System Reinforced by Jet Flow

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INSTITUTTET FOR BYGNINGSTEKNIK, DEPT OF BUILDING TECHNOLOGY AND STRUCTURAL ENGINEERING. AALBORG UNIVERSITETSCENTER AUC AALBORG DANMARK,INDOOR ENVIRONMENTAL TECHNOLOGY. PAPER NO 19,Presented at Ventilation 91 Cincinnati Ohio 1991. L G PEDERSEN P V NIELSEN,EXHAUST SYSTEM REINFORCED BY JET FLOW. DECEMBER 1991 I SSN 0902 7513 R 9147, The papers on INDOOR ENVIRONMENTAL TECHNOLOGY are issued for.
early dissemination of research results from the Indoor Environmental Technol. ogy 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. INSTITUTTET FOR BYGNINGSTEKNIK, DEPT OF BUILDING TECHNOLOGY AND STRUCTURAL ENGINEERING. AALBORG UNIVERSITETSCENTER AUC AALBORG DANMARK,INDOOR ENVIRONMENTAL TECHNOLOGY. PAPER NO 19,Presented at Ventilation 91 Cincinnati Ohio 1991. L G PEDERSEN P V NIELSEN,EXHAUST SYSTEM REINFORCED BY JET FLOW. DECEMBER 1991 ISSN 0902 7513 R9147,EXHAUST SYSTEM REINFORCED BY JET FLOW.
Lars Germann Pedersen Danish Technological Institute Denmark. Peter V Nielsen University of Aalborg Denmark,NOMENCLATURE. d mm diameter of the exhaust opening,D mm diameter of the flange. X m distance to exhaust,y m distance to longitudinal axis. ui m s average inlet velocity,uo m s average exhaust velocity. Ucr m s critical inlet velocity,u m s velocity in the distance x m.
ql m3 h inlet flow rate,qo m3 h exhaust flow rate,I ratio between inlet and exhaust momentum. K constant,n exponent,s mm height of inlet slot in radial jet. a capture efficiency,CO ppm concentration in exhaust. coc ppm mean background concentration,c ppm reference concentration. INTRODUCTION, The conventional exhaust is well known It is characterized by air flowing towards the.
exhaust opening equally from all directions in the space surrounding the exhaust A. conventional exhaust must therefore be described as non selective as the air is not. removed from a selected area If contaminants which are emitted from a restricted area. should be removed it cannot be avoided that clean air from the surrounding areas will. be evacuated, In practice the missing selectivity means that the velocity of the air in a flow towards. the exhaust opening decreases strongly with the increasing distance to the opening This. is convenient in case of comfort ventilation because it will be easy to install the exhaust. in normally ventilated rooms without causing draught In industrial environment it is. of great importance for the internal working environment that contaminants are removed. quickly and efficiently Local exhaust must therefore be located very close to the source. of contamination in order to be efficient In practice working routines will often prevent. an ideal position the consequence will be a reduced capture efficiency The prospects. of improving the capture efficiency of conventional exhaust e g by optimizing the. flanges and the geometry are limited,REEXS REINFORCED EXHAUST SYSTEM. Since 1985 the University of Aalborg and Nordfab A S have been working on an. exhaust principle which is quite different from traditional exhaust systems The REEXS. principle Reinforced Exhaust System which originally was designed for the. agricultural sector is particularly well suited for industrial ventilation purposes With. the REEXS principle it is possible to create a flow pattern in front of the exhaust. opening which will have a considerable influence on the general flow in a given room. The system is a double device system combining air inlet and exhaust Figure 1 shows. the principle of the system, Figure 1 The REEXS principle The inlet air is supplied along the edge of the. exhaust opening through a narrow slot, Geometrically the system may have various shapes The characteristic of the system is. that the flow pattern may be oriented towards a specific direction which increases the. range significantly The directional effect is created by radial air inlet along the edge. of a circular exhaust opening through a narrow slot The air is ejected with a relatively. high velocity but the operation of the system is determined by the ratio of the inlet air. and the exhaust air, The air does not flow towards the opening evenly from all directions but is concentrated.
in a zone in the longitudinal axis of the exhaust In this way the velocity is increased. and the exhaust achieves greater working depth Figure 2 shows the basic difference. between a traditional flanged exhaust and the REEXS principle The areas with high. capture efficiency have markedly very different shapes and working depth. I The REEXS,Uo 20 a 80 I,Ol principle,Traditional,U0 20 a 80 0 I flanged. Figure 2 The basic difference between a traditional flanged exhaust and the. REEXS system The dashed line illustrates the capture zone 1 of U 0. In front of the system an area A 2 3 is created where the air will flow directly. towards the exhaust opening with an efficiency close to 100 The area is surrounded. by a volume where the flow is entrained into the radial jet and the exhaust efficiency. in this volume is approximately 0 The form and working depth of the efficient area are. determined by the ratio I between the momentum flow in the inlet and the momentum. flow in the exhaust where, The face velocities and U are determined as average velocities based on the area and. the air flow, The REEXS principle was to our knowledge invented and first described by Aaberg 11. CRITICAL INITIAL VELOCITY OF THE RADIAL JET, The critical velocity of inlet air ucr is defined as the velocity needed to prevent the inlet. air from being captured by the exhaust opening 2 In other words it is the minimum. initial inlet velocity which determines whether the system will short circuit or it will. work as a REEXS Below the critical velocity the efficiency of the system is lower than. of a conventional exhaust system, The critical velocity is directly proportional to the exhaust flow rate Figure 3 shows.
the critical velocity as a function of the exhaust flow rate at different values of the slot. QQ QQ O S 0 5 mm,ooooo S 1 0 mm,t S 2 5 mm,U ll S 4 0 mm. Exhaust flow rete q m h, Figure 3 Critical velocity of inlet air as a function of exhaust flow rate The slot. height is the parameter, The critical velocity has a hysteresis effect because two different values of ucr may. create the same flow pattern The upper critical value Ucr u is defined as the velocity. needed to break the short circuit in a system where the inlet air is drawn into the. exhaust opening The lower critical velocity llcr H is defined as the velocity where a. short circuit takes place if the inlet velocity is reduced. ucr u is higher than ucr I In practice it is necessary to use ucr u only In this case it will not. matter whether the ventilator used for the inlet air is started before or after the exhaust. ventilator,CAPTURE VELOCITY, Figure 4 shows the development of the centre line velocity decay in front of a 103 mm. REEXS The velocity is measured in a free flow The exhaust flow rate is 600m3 hand. the slot height is 2 5 mm,ooooo I 0 0 q1 0 m3 h,coocc I 0 1 q1 87 m3 h.
I 0 9 q1 261 m3 h,00000 I 2 0 q1 384 m3 h,Distance x d. Figure 4 Measured decay of the dimensionless centre line velocity for a 103 mm. REEXS qc 600 m3 h S 2 5 mm, As it appears the exhaust velocity depends on the momentum ratio between inlet air and. exhaust air An I value less than 0 1 is the lowest possible if short circuit is to be. It also appears that the velocity is significantly higher for I 0 1 than for I 0 0. which corresponds to a 103 mm traditional exhaust opening with flange D 223 mm. At a distance of e g 6 x d the velocity may vary between 0 009 and 0 027 or a factor. 3 By way of comparison it may be mentioned that the exhaust flow rate in a traditional. exhaust in theory should be increased by the factors 4 and 11 respectively in order to. obtain the same velocity, Looking at figure 4 the centre line velocity decay in front of the REEXS may be. expressed as, where K is a constant and n is the geometric inclination which can be read from figure. The exponent depends on the momentum ratio At a constant exhaust flow rate it can. be varied by changing the inlet flow rate A high momentum ratio will cause a high. velocity and consequently a small n value A series of experiments has indicated that. the non dimensional velocity decay is independent of the height of the slot S and the. diameter of the exhaust opening if the momentum of the inlet air remains unchanged. It is necessary for the proper functioning of the system that the inlet air does not hit. obstacles of any kind In that connection it makes no difference whether the value of. the inlet slot S is high or low since the structure of the flow in a radial jet only depends. on the momentum not on the velocity of the inlet air. If the exhaust is placed at a table or similar with the x axis parallel to the surface the. flow changes from axis symmetric flow axi to a three dimensional flow 3D The. velocity can still be expressed according to formula 2 but the exponent changes as the. 3D flow is identical to an axis symmetric flow with twice the area of the exhaust. opening The same thiD g happens if the inlet air is supplied along a plane surface 3D. radial wall jet Figure 5 on page 7 shows how the exponents depend on the momentum. ratio when placed in 3 different positions,1 4 Axis symmetric flow.
c Three dimensional flow,Three dimensional flow,ooooo Axis symmetric flow. ooooo Three dimensional flow,Three dimensional flow radial wall jet. 4 1 1 r w l r r r r r r f,J lw lllel,0 0 o s 1 0 2 0. Momentum ratio I, Figure 5 The exponent as a function of the momentum ratio for a 103 mm. REEXS at 3 different positions, When comparing measured data for the 3 different positions it appears that.
The above connection between the momentum ratios is valid when the radial jet is. geometrical fitted to the shape of the area to be exhausted If the exhaust is located. close to a plane surface it is necessary to shield the bottom half of the supply slot facing. the surface, Formulas 2 and 3 also apply to other geometrically identical REEXS systems To. a certain extent they also apply to non geometrically identical shapes but in such cases. the exponent will be different,CAPTURE EFFICIENCY, The position of the A curve the limit between high and low capture efficiency does. not appear from the measurement of the velocity of the air flowing towards the exhaust. For distances more than 5 x d the velocity in a cross section in front of the exhaust. will be approximately constant Whether contaminants will be removed by the exhaust. or be entrained into the radial jet will not be shown directly For this reason. measurements of the capture efficiency of the exhaust have been added to the velocity. measurements, The capture efficiency of the exhaust system is measured by tracer gas technique. sulphurhexafluoride SF6 The direct capture efficiency of the exhaust is defined as. The tracer gas is induced at a constant flow and the reference concentration C is. determined by capturing the tracer gas 100 The gas is induced through a perforated. sphere The capture efficiency of a number of positions in front of the exhaust is. determined by moving the sphere, For a conventional circular unflanged exhaust it can roughly be assumed that the. capture efficiency of the exhaust is equal in all directions The REEXS system creates. its own field of efficiency An area around the longitudinal axis has a high capture. efficiency while it decreases strongly in the remaining area The experiments mentioned. were carried out without interference from the surroundings In other words the results. represent the maximum theoretical capture efficiency for a given source In practice the. size of the disturbances e g cross draught etc must be part of the calculations of the. capture efficiency, From figure 6 on page 9 it is seen that different values of the momentum ratio result.
in different profiles of the capture efficiency in the region in front of the exhaust. opening As it appears the system has a wide profile and a longer range at a low value. of the momentum ratio From this it seems to be expected that it always would be an. advantage to choose the low level of momentum ratio but the capture velocity. necessary to catch the contaminants must be sufficient the higher momentum ratio the. higher capture velocity,Capture efficiency for I 1 5. 0 0 0 2 0 4 0 6 0 8 1 0 1 2 1 4 1 6,Capture efficiency for I 0 7. 0 0 0 2 0 4 0 6 0 8 1 0 1 2 1 4, Figure 6 Capture efficiency profiles for an axis symmetric REEXS The exhaust. is located in x 0 0 and y 0 0 The longitudinal axis of the exhaust. is the x direction qe 600 m3 h in both cases, The distance from the theoretical A curve to the longitudinal axis of the exhaust can be. calculated from, where 11x is the measured velocity in the cross section in the distance x to the exhaust.
The connection between the capture efficiency profile and the A curve can be seen from. figure 7 Due to the fact that the flow pattern to the exhaust opening circles around the. The system is a double device system combining air inlet and exhaust Figure 1 shows the principle of the system Figure 1 A curve s 0 025d D 223 mm d 103 mm A The REEXS principle The inlet air is supplied along the edge of the exhaust opening through a narrow slot Geometrically the system may have various shapes The

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