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Mixing enhancement using a coil insert
Applied Thermal Engineering 21 (2001) 303±309
www.elsevier.com/locate/apthermeng
Mixing enhancement using a coil insert H.R. Rahai a,*, H.T. Vu a, M.H. Shojaee Fard b a
Mechanical Engineering Department, California State University, Long Beach, CA 90840, USA Mechanical Engineering Department, Iran University of Science and Technology, Tehran, Iran
b
Received 2 October 1999; accepted 3 April 2000
Abstract Mixing enhancement in a Bunsen burner with a coil insert, using air as a ¯uid is experimentally investigated. The coil has a ratio of pitch to the tube inside diameter ( p/D) as 1.0 and the ratio of coil diameter to the tube inside diameter (d/D) is 0.17. The results show that a coil insert with certain con®gurations can be used as a good mixing device. 7 2000 Elsevier Science Ltd. All rights reserved. Keywords: Mixing; Turbulent jet; Swirling motion
1. Introduction In combustion systems, mixing enhancement is obtained using strong swirls, which generate a shorter and more intense ¯ame. Results of Gupta et al. [3], Takagi and Okamoto [6] and Tangirala and Driscoll [7] have shown that in some cases increased mixing results in both, reduction of NOx formation and increase in combustion eciency. However, as Ho et al. [5] have shown, a very strong degree of swirl causes excessive entrainment of the surrounding air, resulting in a ¯ame blow out. The introduction of swirl on a turbulent jet results in increases in the jet half width, rate of entrainment, rate of decay of the jet and inducement of pressure ®elds to balance the centrifugal forces. Decay of swirl caused by shear and mixing with surrounding ¯uid results in * Corresponding author. Tel.: +1-562-9855132; fax: +1-562-9854408. E-mail address: [email protected] (H.R. Rahai). 1359-4311/01/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 4 3 1 1 ( 0 0 ) 0 0 0 5 4 - 5
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H.R. Rahai et al. / Applied Thermal Engineering 21 (2001) 303±309
Nomenclature D U Uc U0 X d p r r1/2 u 2 1=2 v 2 1=2 uv
tube inside diameter, m axial mean velocity, m/s centerline mean velocity, m/s maximum mean velocity at x/D = 0.1, tube outlet, m/s axial distance, m coil wire diameter, m pitch spacing, m radial distance, m jet half width, m axial turbulent velocity, m/s radial turbulent velocity, m/s time averaged turbulent shear stress, m2/s2
an adverse pressure gradient along the jet axis which displaces the location of the maximum mean velocity away from the jet axis. A weak degree of swirl has limited practical application. However, as Gupta et al. [4] have explained, it can be used in characterization of some geophysical phenomena such as ®re whirls, dust devils, tornadoes, hurricanes, and water spouts. In combustion, a weak degree of swirl causes increase in the ®re length, which has practical application in ®re whirls and tangentially ®xed boilers. Other studies relevant to the present investigation are those by Babikian et al. [1], and Chegier and Chervinsky [2]. Chegier and Chervinsky [2] performed experimental investigations on a series of axi-symmetric free turbulent jets with dierent degree of swirl ranging from 0.066 to 0.640. Their results show that for weak and moderate degrees of swirl, mean velocity and static pressure pro®les are similar, starting at approximately four jet diameter downstream in axial direction. For strong swirl, there was a vortex formation along the axis near the jet outlet resulting in displacement of maximum axial velocity from the jet axis. However, beyond 10 diameters downstream, the eect of vortex disappears and the velocity pro®le and static pressure ®elds become similar. Babikian et al. [1] present results of an experimental investigation of the velocity ®eld in the central region of a spirally ¯uted tube. Their results show that the presence of the ¯utes do not cause the ¯uid to be in solid body rotation. There is more than 10% increase in rms axial turbulent velocity away from the tube axis for the ¯uted tube than the corresponding published data for the smooth tube. In regions away from the tube axis, the normalized radial and azimuthal turbulent velocities are found to be considerably less than the corresponding values for the smooth tube. The ®nal results show that the ¯uted tube induces a swirl on the ¯ow, but the ¯ow is not in solid body rotation, where this rotation causes transfer of turbulent kinetic energy from the radial to the axial component. Rahai and Hoang [8] and Rahai and Wong [9] present results of extensive experimental investigations on the eects of coil inserts with variable coil wire diameters and pitch spacing
H.R. Rahai et al. / Applied Thermal Engineering 21 (2001) 303±309
305
on the mixing process in a turbulent jet from a round tube. Their results show that for ratio of pitch to the tube inside diameter ( p/D ) of 1.2 and the ratio of coil diameter to the tube inside diameter (d/D ) is 0.17. Their results also exhibit that there are signi®cant increases in swirl velocity, turbulent kinetic energy and jet half width which are indications of higher mixing process. The present experiments are an extension of these investigations with potential applications in pre-mixed burners. 2. Experimental procedure and techniques A Bunsen burner with air as the ¯uid is used in the experiment. Fig. 1 shows its con®guration. The burner has a planer jet of 1 mm diameter where a tube with dimensions of 7 mm ID and 90 mm in length is placed above it. There is a 10 mm open space between the planer jet and the tube. Compressed laboratory air is used in the experiments. The air is regulated and supplied at a constant volume ¯ow rate of 0.014 m3/s. A coil with d/D of 0.17 and p/D of 1.0 is inserted into the full length of the tube. The turbulent jet from the tube is investigated using a TSI X-hot wire probe model 1244 in conjunction with two channels of TSI IFA 100 intelligent ¯ow analyzers with corresponding
Fig. 1. The Bunsen burner with coil insert.
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H.R. Rahai et al. / Applied Thermal Engineering 21 (2001) 303±309
signal conditioner units. Measurements are carried out at various values of the ratio of axial distance to tube inside diameter (x/D = 0, 2, 5, 10, and 15). At each axial position the probe is traversed in the radial direction to obtain the turbulent ¯ow statistics. At each measurement location, 50 records where each record consists of 2048 samples which corresponds to more than 100 000 samples are digitized at a sample rate of 6000 samples/s, using a Metra-Byte DAS-20 analog to digital converter, connected to a 80486 based micro-computer. The digitized data are analyzed, using software supplied by Data Ready Inc. For radial variations, only results at x/D = 5 are presented, since similar results are seen at further downstream locations. 3. Results and discussions Fig. 2 shows radial variation of the normalized axial mean velocity at x/D = 5 along with axial variation of the normalized axial mean velocity along the jet axis and the jet half width. There is a signi®cant decrease in the normalized maximum mean velocity of the tube inserted coil and substantial increase in the jet half width which are indications of higher mixing and entrainment with the surrounding air. The maximum normalized axial mean velocity is further away from the jet axis and there is a large dip in its value at the jet axis. The displacement of the maximum normalized mean velocity may be due to the presence of an adverse pressure gradient along the jet axis which is generated as a result of the decay of the swirl due to shear and mixing of the jet with the surrounding ¯uid. Fig. 3 shows radial variations of the normalized axial turbulent velocity, radial turbulent velocity and turbulent shear stress at x/D = 5. The presence of the coil results in an increase in the axial turbulent velocity along and near the jet axis. The turbulent velocity then decreases to a value less than the corresponding value for the smooth tube at the ratio of radial distance to tube inside diameter, r/D of 0.5 and then increases and becomes higher than the smooth tube value for r/D > 1. Results for the radial turbulent velocity show that except along the jet axis and at r/D = 0.5, where the values for the tube inserted coil are the same as the corresponding smooth tube values, at all other locations, the tube with coil insert has a higher radial turbulent velocity. As compared with the corresponding results for the smooth tube, the presence of the coil results in a positive shear stress near the jet axis (r/D < 0.5) and reduced negative shear stress afterwards. The eects of the coil on the initial conditions are shown as increased in the turbulent kinetic energy and the turbulent shear stress. These results suggest that the coil insert acts similar to a turbulator, causing generation of large scale structures in the initial region of the jet, resulting in increased entrainment and mixing processes. 4. Conclusions Experimental investigations of the eect of a coil insert with p/D = 1.0 and d/D = 0.17 on a round turbulent jet from a tube of a Bunsen burner are performed and results are compared with the corresponding results from the smooth tube. Results show that the coil causes
H.R. Rahai et al. / Applied Thermal Engineering 21 (2001) 303±309
307
Fig. 2. Radial variation of the axial mean velocity at x/D = 5, axial decay of the axial mean velocity along the jet axis, and axial variation of the jet half-width.
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H.R. Rahai et al. / Applied Thermal Engineering 21 (2001) 303±309
Fig. 3. Radial variations of the rms axial and radial turbulent velocities and turbulent shear stress at x/D = 5.
H.R. Rahai et al. / Applied Thermal Engineering 21 (2001) 303±309
309
increases in the jet half width, turbulent kinetic energy, rate of decay and rate of entrainment, and displacement of the maximum mean velocity away from the jet axis. These results indicate that coils with certain con®gurations can be used as good mixing devices and can have potential application in premixed burners. References [1] D.S. Babikian, J.C. LaRue, H.R. Rahai, Characteristics of the velocity ®eld I the central region of a spirally ¯uted tube, Eighth Symposium on Turbulent Shear Flows, Munich, Germany, September, 1991. [2] N.A. Chegier, A. Chervinsky, Experimental investigation of swirling vortex motion in jets, Transaction of ASME, J. Appl. Mech. (1967) 443±451. [3] A.K. Gupta, M. Ramavajjala, M.R. Taha, The eect of swirl and nozzle geometry on the structure of ¯ames and nox emission, AIAA 30th Aerospace Science Meeting and Exhibit, Reno, Nevada, 1992a. [4] A.K. Gupta, D.G. Lilley, N. Syred, Swirl Flows, Abacus Press, Tunbridge Wells, UK, 1992b. [5] W. Ho, S. Yang, R. Yang, The Advanced Burner Design Project, The 14th Annual Conference on Theoretical and Applied Mechanics, Chung LI, Taiwan, ROC, 1990. [6] T. Takagi, T. Okamoto, Characteristics of combustion and pollutant formation in swirling ¯ames, Combust. Flame 43 (1981) 69±79. [7] V. Tangirala, J.F. Driscoll, Temperature within non-premixed ¯ames: eects of rapid mixing due to swirl, Combust. Sci. Technol. 69 (1988) 143±162. [8] H.R. Rahai, H.T. Hoang, Mixing enhancement in a round jet using ribbed tubes, Paper No. FEDSM99-F208, ASME Fluids Engineering Division Summer Meeting, San Francisco, CA, USA, July 18±22, 1999. [9] H.R. Rahai, T.W. Wong, Experimental investigation of turbulent jet from round ribbed tubes, Paper No. FEMSM98-4972, ASME Fluids Engineering Division Summer Meeting, Washington, DC, USA, June 21±25, 1998.
www.elsevier.com/locate/apthermeng
Mixing enhancement using a coil insert H.R. Rahai a,*, H.T. Vu a, M.H. Shojaee Fard b a
Mechanical Engineering Department, California State University, Long Beach, CA 90840, USA Mechanical Engineering Department, Iran University of Science and Technology, Tehran, Iran
b
Received 2 October 1999; accepted 3 April 2000
Abstract Mixing enhancement in a Bunsen burner with a coil insert, using air as a ¯uid is experimentally investigated. The coil has a ratio of pitch to the tube inside diameter ( p/D) as 1.0 and the ratio of coil diameter to the tube inside diameter (d/D) is 0.17. The results show that a coil insert with certain con®gurations can be used as a good mixing device. 7 2000 Elsevier Science Ltd. All rights reserved. Keywords: Mixing; Turbulent jet; Swirling motion
1. Introduction In combustion systems, mixing enhancement is obtained using strong swirls, which generate a shorter and more intense ¯ame. Results of Gupta et al. [3], Takagi and Okamoto [6] and Tangirala and Driscoll [7] have shown that in some cases increased mixing results in both, reduction of NOx formation and increase in combustion eciency. However, as Ho et al. [5] have shown, a very strong degree of swirl causes excessive entrainment of the surrounding air, resulting in a ¯ame blow out. The introduction of swirl on a turbulent jet results in increases in the jet half width, rate of entrainment, rate of decay of the jet and inducement of pressure ®elds to balance the centrifugal forces. Decay of swirl caused by shear and mixing with surrounding ¯uid results in * Corresponding author. Tel.: +1-562-9855132; fax: +1-562-9854408. E-mail address: [email protected] (H.R. Rahai). 1359-4311/01/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 4 3 1 1 ( 0 0 ) 0 0 0 5 4 - 5
304
H.R. Rahai et al. / Applied Thermal Engineering 21 (2001) 303±309
Nomenclature D U Uc U0 X d p r r1/2 u 2 1=2 v 2 1=2 uv
tube inside diameter, m axial mean velocity, m/s centerline mean velocity, m/s maximum mean velocity at x/D = 0.1, tube outlet, m/s axial distance, m coil wire diameter, m pitch spacing, m radial distance, m jet half width, m axial turbulent velocity, m/s radial turbulent velocity, m/s time averaged turbulent shear stress, m2/s2
an adverse pressure gradient along the jet axis which displaces the location of the maximum mean velocity away from the jet axis. A weak degree of swirl has limited practical application. However, as Gupta et al. [4] have explained, it can be used in characterization of some geophysical phenomena such as ®re whirls, dust devils, tornadoes, hurricanes, and water spouts. In combustion, a weak degree of swirl causes increase in the ®re length, which has practical application in ®re whirls and tangentially ®xed boilers. Other studies relevant to the present investigation are those by Babikian et al. [1], and Chegier and Chervinsky [2]. Chegier and Chervinsky [2] performed experimental investigations on a series of axi-symmetric free turbulent jets with dierent degree of swirl ranging from 0.066 to 0.640. Their results show that for weak and moderate degrees of swirl, mean velocity and static pressure pro®les are similar, starting at approximately four jet diameter downstream in axial direction. For strong swirl, there was a vortex formation along the axis near the jet outlet resulting in displacement of maximum axial velocity from the jet axis. However, beyond 10 diameters downstream, the eect of vortex disappears and the velocity pro®le and static pressure ®elds become similar. Babikian et al. [1] present results of an experimental investigation of the velocity ®eld in the central region of a spirally ¯uted tube. Their results show that the presence of the ¯utes do not cause the ¯uid to be in solid body rotation. There is more than 10% increase in rms axial turbulent velocity away from the tube axis for the ¯uted tube than the corresponding published data for the smooth tube. In regions away from the tube axis, the normalized radial and azimuthal turbulent velocities are found to be considerably less than the corresponding values for the smooth tube. The ®nal results show that the ¯uted tube induces a swirl on the ¯ow, but the ¯ow is not in solid body rotation, where this rotation causes transfer of turbulent kinetic energy from the radial to the axial component. Rahai and Hoang [8] and Rahai and Wong [9] present results of extensive experimental investigations on the eects of coil inserts with variable coil wire diameters and pitch spacing
H.R. Rahai et al. / Applied Thermal Engineering 21 (2001) 303±309
305
on the mixing process in a turbulent jet from a round tube. Their results show that for ratio of pitch to the tube inside diameter ( p/D ) of 1.2 and the ratio of coil diameter to the tube inside diameter (d/D ) is 0.17. Their results also exhibit that there are signi®cant increases in swirl velocity, turbulent kinetic energy and jet half width which are indications of higher mixing process. The present experiments are an extension of these investigations with potential applications in pre-mixed burners. 2. Experimental procedure and techniques A Bunsen burner with air as the ¯uid is used in the experiment. Fig. 1 shows its con®guration. The burner has a planer jet of 1 mm diameter where a tube with dimensions of 7 mm ID and 90 mm in length is placed above it. There is a 10 mm open space between the planer jet and the tube. Compressed laboratory air is used in the experiments. The air is regulated and supplied at a constant volume ¯ow rate of 0.014 m3/s. A coil with d/D of 0.17 and p/D of 1.0 is inserted into the full length of the tube. The turbulent jet from the tube is investigated using a TSI X-hot wire probe model 1244 in conjunction with two channels of TSI IFA 100 intelligent ¯ow analyzers with corresponding
Fig. 1. The Bunsen burner with coil insert.
306
H.R. Rahai et al. / Applied Thermal Engineering 21 (2001) 303±309
signal conditioner units. Measurements are carried out at various values of the ratio of axial distance to tube inside diameter (x/D = 0, 2, 5, 10, and 15). At each axial position the probe is traversed in the radial direction to obtain the turbulent ¯ow statistics. At each measurement location, 50 records where each record consists of 2048 samples which corresponds to more than 100 000 samples are digitized at a sample rate of 6000 samples/s, using a Metra-Byte DAS-20 analog to digital converter, connected to a 80486 based micro-computer. The digitized data are analyzed, using software supplied by Data Ready Inc. For radial variations, only results at x/D = 5 are presented, since similar results are seen at further downstream locations. 3. Results and discussions Fig. 2 shows radial variation of the normalized axial mean velocity at x/D = 5 along with axial variation of the normalized axial mean velocity along the jet axis and the jet half width. There is a signi®cant decrease in the normalized maximum mean velocity of the tube inserted coil and substantial increase in the jet half width which are indications of higher mixing and entrainment with the surrounding air. The maximum normalized axial mean velocity is further away from the jet axis and there is a large dip in its value at the jet axis. The displacement of the maximum normalized mean velocity may be due to the presence of an adverse pressure gradient along the jet axis which is generated as a result of the decay of the swirl due to shear and mixing of the jet with the surrounding ¯uid. Fig. 3 shows radial variations of the normalized axial turbulent velocity, radial turbulent velocity and turbulent shear stress at x/D = 5. The presence of the coil results in an increase in the axial turbulent velocity along and near the jet axis. The turbulent velocity then decreases to a value less than the corresponding value for the smooth tube at the ratio of radial distance to tube inside diameter, r/D of 0.5 and then increases and becomes higher than the smooth tube value for r/D > 1. Results for the radial turbulent velocity show that except along the jet axis and at r/D = 0.5, where the values for the tube inserted coil are the same as the corresponding smooth tube values, at all other locations, the tube with coil insert has a higher radial turbulent velocity. As compared with the corresponding results for the smooth tube, the presence of the coil results in a positive shear stress near the jet axis (r/D < 0.5) and reduced negative shear stress afterwards. The eects of the coil on the initial conditions are shown as increased in the turbulent kinetic energy and the turbulent shear stress. These results suggest that the coil insert acts similar to a turbulator, causing generation of large scale structures in the initial region of the jet, resulting in increased entrainment and mixing processes. 4. Conclusions Experimental investigations of the eect of a coil insert with p/D = 1.0 and d/D = 0.17 on a round turbulent jet from a tube of a Bunsen burner are performed and results are compared with the corresponding results from the smooth tube. Results show that the coil causes
H.R. Rahai et al. / Applied Thermal Engineering 21 (2001) 303±309
307
Fig. 2. Radial variation of the axial mean velocity at x/D = 5, axial decay of the axial mean velocity along the jet axis, and axial variation of the jet half-width.
308
H.R. Rahai et al. / Applied Thermal Engineering 21 (2001) 303±309
Fig. 3. Radial variations of the rms axial and radial turbulent velocities and turbulent shear stress at x/D = 5.
H.R. Rahai et al. / Applied Thermal Engineering 21 (2001) 303±309
309
increases in the jet half width, turbulent kinetic energy, rate of decay and rate of entrainment, and displacement of the maximum mean velocity away from the jet axis. These results indicate that coils with certain con®gurations can be used as good mixing devices and can have potential application in premixed burners. References [1] D.S. Babikian, J.C. LaRue, H.R. Rahai, Characteristics of the velocity ®eld I the central region of a spirally ¯uted tube, Eighth Symposium on Turbulent Shear Flows, Munich, Germany, September, 1991. [2] N.A. Chegier, A. Chervinsky, Experimental investigation of swirling vortex motion in jets, Transaction of ASME, J. Appl. Mech. (1967) 443±451. [3] A.K. Gupta, M. Ramavajjala, M.R. Taha, The eect of swirl and nozzle geometry on the structure of ¯ames and nox emission, AIAA 30th Aerospace Science Meeting and Exhibit, Reno, Nevada, 1992a. [4] A.K. Gupta, D.G. Lilley, N. Syred, Swirl Flows, Abacus Press, Tunbridge Wells, UK, 1992b. [5] W. Ho, S. Yang, R. Yang, The Advanced Burner Design Project, The 14th Annual Conference on Theoretical and Applied Mechanics, Chung LI, Taiwan, ROC, 1990. [6] T. Takagi, T. Okamoto, Characteristics of combustion and pollutant formation in swirling ¯ames, Combust. Flame 43 (1981) 69±79. [7] V. Tangirala, J.F. Driscoll, Temperature within non-premixed ¯ames: eects of rapid mixing due to swirl, Combust. Sci. Technol. 69 (1988) 143±162. [8] H.R. Rahai, H.T. Hoang, Mixing enhancement in a round jet using ribbed tubes, Paper No. FEDSM99-F208, ASME Fluids Engineering Division Summer Meeting, San Francisco, CA, USA, July 18±22, 1999. [9] H.R. Rahai, T.W. Wong, Experimental investigation of turbulent jet from round ribbed tubes, Paper No. FEMSM98-4972, ASME Fluids Engineering Division Summer Meeting, Washington, DC, USA, June 21±25, 1998.