An Investigation into Air Ejector with Pulsating Primary Flow Václav Dvořák and Petra Dančová

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1 An Investigation into Air Ejector with Pulsating Primary Flow Václav Dvořák and Petra Dančová Abstract The article deals with neumatic and hot wire anemometry measurement on subsonic axi-symmetric air ejector. Performances of the ejector with and without ulsations of rimary flow are comared, measuring of characteristic ressures and mass flow rates are erformed and ejector efficiency is evaluated. The ulsations of rimary flow are roduced by a synthetic jet erator, which is laced in the suly line of the rimary flow just in front of the rimary nozzle. The aim of the ulsation is to intensify the mixing rocess. In the article we resent: Pressure measuring of ulsation on the mixing chamber wall, behind the mixing chamber and behind the diffuser measured by fast ressure transducers and results of hot wire anemometry measurement. It was found out that using of rimary flow ulsations yields higher back ressure behind the ejector and higher efficiency. The rocesses in this ejector and influences of rimary flow ulsations on the mixing rocesses are described. T Keywords Air ejector, ulsation flow I. INTRODUCTION HE article deals with exerimental investigation of mixing in axi-symmetric subsonic ejector with included device erating ulsations of rimary flow. The aim of ulsations is to intensify the mixing rocess. The mixing can by intensified by many ways that can be divided into two grous, assive and active, as they were in ublication by authors Ginevsky, Vlasov and Karavosov []. Shaing of the rimary nozzle trailing edge belongs to the assive methods, erating of flow ulsation is the active method. The work [] deals with free streams from jets, number of works dealing with active or assive control of mixing in ejectors are quite limited. E.g. Havelka et al. in exerimental work [] used a device inserted into the rimary nozzle to add a tantial velocity comonent into the rimary stream. Measuring showed that the secondary mass flow rate is increased for certain range of tantial velocity and the shorter mixing chamber is satisfactory. Waitz et al. investigated intensification of mixing with the hel of a lobe nozzle in work []. Dvořák [] otimized the lobe nozzle for mixing and found out that the nozzle with low number of big lobes is advantageous for high efficiency of the ejector. Chang and Manuscrit received July,. This roject was realized with financial suort by the Foundation of the Academy of Science of the Czech Reublic, grant no. IAA768 and by the Czech Science Foundation, grant no. P//79. Václav Dvořák is with the Deartment of Power Engineering Equiment, Faculty of Mechanical Engineering, Technical University of Liberec, Studentská, 67 Liberec, Czech Reublic (hone: ; fax: + 8 6; vaclav.dvorak@tul.cz). Petra Dančová is with the Deartment of Power Engineering Equiment, Faculty of Mechanical Engineering, Technical University of Liberec, Studentská, 67 Liberec, Czech Reublic ( etra.dancova@tul.cz). Chen [] used a etal nozzle in a suersonic ejector and comared it with common diverging nozzle. They showed that the ejector with etal nozzle is better for higher area ratio A /A kr than the ejector with common nozzle. Tylor and Williamson [6] divided the mixing into two regions. In the initial region of mixing, the shear layer between the rimary and the secondary stream does not reach the mixing chamber wall or the boundary layer. In the main region of mixing, the shear layer sreads across the whole mixing chamber cross section. The momentum decay is slow in the initial region and static ressure changes only slightly. We can consider a free stream here. But the momentum decay and also static ressure rise are accelerated in the main region. Otimizations made by Dvořák in work [7] showed that only choosing the velocity ratio ω = c /c. can lead to the high efficiency of the ejector. Unfortunately, the length of the initial region of mixing is than relatively long (L D) in this case and causes high friction losses. The length of the main region deends less on the velocity ration. To intensify and accelerate the mixing rocesses, it is essential to erate fluctuations either at the beginning of the mixing chamber or even in front of it, i.e. in the rimary flow suly ieline. The former work by authors Dvořák and Dančová [8] dealt with an ejector with only one synthetic jet. The aim of the study was to determine the influences of the synthetic jet (SJ), which was laced in the beginning of the mixing chamber. The urose of that work was to investigate the ossibility of using SJ to accelerate the mixing rocesses by the intensification of momentum and mass transfer, as it was shown by Trávníček and Vít in work [9]. In work [8], it was roved, that the influences of the oerating SJ on the flow in the ejector were follows: SJ accelerated the mixing rocess only negligibly, but for the regimes with high ejection ratio, SJ stabilizes the flow fluctuations in the diffuser and thus the higher back ressure and higher efficiency are achieved. SJ laced in the beginning of the mixing chamber influenced the flow in the diffuser ositively, but when laced at the end of the mixing chamber, the imrovements were reduced. Velocities of the rimary stream in the centre of the mixing chamber were affected during the oeration of SJ, but the secondary stream and the mixing shear layer were affected only in the immediate vicinity of SJ. The aim of current study is to use SJ actuator to erate ulsation in the rimary flow in front of the mixing chamber, i. e. in the rimary flow suly ieline. II. METHODS Dimensions of the synthetic jet actuator used for erating of rimary flow oscillations and its osition on the rimary 6

2 nozzle are obvious from Fig.. SJ actuator consists from a sealed cavity and two loudseakers (MONACOR SP-8/SQ) with nominal arameters: Ω, Wmax. These loudseakers have the same ower and diameters (Dc = 7mm, membrane diameter Dm = 68mm) and are arallel-connected. Loudseakers membranes have stiff cone shae and they can be considered to be istons, which control the jet. Actuator was fed with sinusoidal signal with electrical ower P = 9.W. Signal was erated from Tektronix AFG signal erator and was amlified with Omnitronic MPZ-8 amlifier. Generator of rimary flow ulsations Pressure chamber m 8 Primary nozzle R 9. Mixing chamber Diffuser (6 enlargement) m Fig. System of the ejector and the erator of rimary flow oscillations 97 8 t 9 d t,, x CTA Druck OM-link 9 cl 7 Fig. Exerimental arrangement: - comressor, - air dryer, - tank, - reduction valve, - filter, 6 - rotameter, 7 - Coriolis mass flow meter, 8 - stilling chamber, 9 - stilling riddles, - measuring of rimary stagnation ressure, - measuring of rimary mass flow rate, rimary flow suly tube, holder of rimary nozzle, rimary nozzle, - secondary nozzle, 6 - mixing chamber with static ressure tas, 7 diffuser, 8 - CTA robes, 9 outflow ie, - measuring of total mass flow rate, - suction ejector, - control valve, chocking, - bed, - CTA measuring, 6 neumatic measuring, 7 erator of rimary flow ulsations A circular converging nozzle with diameter d = 9. mm was used. The mixing chamber had diameter D = mm. The area ratio of nozzles was μ = A A =. and the relative length of the mixing chamber was L D = 8. A diffuser with 6 enlargement and with outlet diameter 7. mm was laced behind the mixing chamber. First ste of the measurement was the determination of the system nominal frequency i.e. frequency on which ejector works with the highest ower. Nominal frequency was found as f = 69. Hz. This low frequency is given by the length of the ieline behind the ejector, which ends by orifice used for measuring of mixed mass flow rate m, see osition in Fig.. For slow ressure measuring, we used ressure sensors Druck LP with range,, and Pa. These low ressure sensors with high accuracy % are slow, so only mean value of ressures were measured. Arrangement of the exeriments is obvious from Fig.. We also used very fast miniature ressure sensors Kulite XTL-B-9M with MDAQ-OR6-BRIDGE-D and PC card DEWE-ORION- 66- for fast ressure measuring with frequency of khz. 6

3 III. RESULTS The results for ejector with ulsation erator switched OFF and ON are in Fig.,, and 6. The efficiency of the ejector is defined by relation κ κ m η = m T κ T κ, () where m is mass flow rate, static ressure, stagnation ressure, T stagnation temerature and κ ratio of secific heats. Subscrit denotes rimary flow, secondary flow, mixed flow and state behind the ejector, i.e. behind the diffuser. For incomressible fluid, or for comressible fluid when T = T and ( )/ <. the relation () can be simlified to η = Γ, () where ejection ratio Γ is used. The ejection ratio is given by relation m c = A ρ Γ =, () m c A ρ where c is velocity, A area of inlet nozzle and ρ density. A. Results of Slow Pressure Measuring. Efficiency [ ] Ejection ratio Γ = m /m [] Fig. Results of slow ressure measuring, ejector efficiency. The evaluation of the ejector efficiency from measured data is in Fig.. We can see that with ulsation erator turned ON the efficiency are higher. It means that higher back ressure and ejection ratio are obtained. This is visible also in Fig., where the relative back ressure is carried out. We can also see that the fluctuations of back ressure are not decreased while ulsation erator is oerating. These results contrast with work [8], where fluctuations of back ressure were strongly suressed with oerating synthetic jet actuator. ( - ) / ( - ) [] Ejection ratio Γ = m /m [] Fig. Results of slow ressure measuring, relative back ressure ( - ) / ( - ). The relative suction ressure in the beginning of the mixing chamber is in Fig.. We can see that during oeration of the erator the curve changes it moves towards higher ejection ratios, while the suction ressure decreases only negligibly. These results are rather surrising because suction ressure, which is measured in the beginning of the mixing chamber, is given by ejection ratio, see [8]. It is because the suction ressure determines the inlet velocity of both flows and thus, for used inlet area ratio μ = A A, the ejection ratio is given. It means that all measured data should fall into the single curve in Fig.. But in work [8], the SJ erator was laced behind the beginning of the mixing chamber, i.e. behind the lace where ressure is measured, while in this work, the erator is laced in front of this lace. ( - ) / ( - ) [] Ejection ratio Γ = m /m [] Fig. Results of slow ressure measuring, relative suction ressure ( - ) / ( - ). It indicated that during deceleration and acceleration of the rimary flow, the velocity ratio ω = c c changed while the effective inlet area ratio had to be constant. For given exansion ressure, which is measured on the mixing chamber wall, the velocity of secondary flow c should be 6

4 almost constant. Velocity c will oscillate because of ressure changes, but the mean velocity c should be lower. These required further investigation with the hel of fast ressure transducers and hot wire anemometry. The differences are also obvious on Fig. 6, where mixing ressure, measured behind the mixing chamber, is carried out. Again, higher ejection ratio is obtained with almost the same mixing ressure while the erator of ulsations is oerating. With the same static ressure, the dynamic ressure and mass flow rate behind the mixing chamber are higher and higher back ressure behind the mixing chamber is obtained. To further understand to the flow rocesses, we also erformed very fast measurements of the ressures. ( - ) / ( - ) [] Ejection ratio Γ = m /m [] Fig. 6 Results of slow ressure measuring, relative mixing ressure ( - ) / ( - ). B. Results of Fast Pressure Measuring ie.6.8. Fig. 7 Results of fast neumatic measuring, erator turned OFF, instantaneous curves, T = / 69. s. ressure behind the ie stilling chamber in the beginning of the rimary flow suly ieline, ressure near the erator Results of fast neumatic measuring are carried out on Fig. 7, 8, 9 and. We measured only one articular regime characterized by the highest efficiency. In Fig. 7 and 8, results for erator turned OFF are carried out. We can see that the flow is not steady, but there is some eriodic fluctuations in the rimary suly tube with the frequency of 6 Hz, while the fluctuations close to the erator are of the half frequency of 78 Hz. These fluctuations are erated in the beginning of the rimary flow suly ieline. Theirs frequencies are obviously given by the length of free sace in the stilling chamber, see ositions 8 and 9 in Fig., i.g. it is the length between the stilling riddles and the end of the stilling chamber. It is obvious from Fig. 8 that these fluctuations sread downstream slightly at the beginning and more significantly at the end of the mixing chamber, see courses of suction ressure and mixing ressure and values in Table ie.6.8. Fig. 8 Results of fast neumatic measuring, erator turned OFF, instantaneous curves, T =/ 78 s. Results for erator turned ON are carried out in Fig. 9 and. We can see that some courses change raidly, but courses of ressure in the rimary flow suly ieline ( ) ie and in the stilling chamber ( ) are not affected by the oerating erator. The former recorded oscillations of frequencies 78 Hz and 6 Hz are still resent and are suerosed on courses affected by oerating erator, see courses of and. TABLE I RESULTS OF FAST PNEUMATIC MEASURING FOR GENERATOR OFF AND ON, MEAN, RMS AND AMPLITUDE VALUES ) /( ) ie ( mean OFF amlit Hz hase mean ON amlit Hz Phase First, we discuss the influences of oerating erator on the suction ressure in the beginning of the mixing chamber. By comarison of Fig. 7 and 9, we can see that the 6

5 fluctuations of are increased only negligibly. The amlitude of is only and the hase after the erator is., see table. It means that for the highest ressure near the erator, the lower exansion ressure is obtained. It should be caused by the highest rimary velocity c from the nozzle and accordingly the strongest effect of suction of surrounding air. The working frequency of 69. Hz is not almost evident on the course for, which is caused by the fact that the suction ressure is measured in the beginning of the mixing chamber, for x =, i.e. almost in unconfined sace. Thus, the secondary flow, which is entrained into the mixing chamber, should not change its velocity during the working eriod of the erator. Changes of the rimary flow velocity c and alternatively secondary flow velocity c were investigated with the hel of hot wire anemometry ie.6.8. Fig. 9 Results of fast neumatic measuring, erator turned ON, instantaneous curves, T = / 69. s ie.6.8. Fig. Results of fast neumatic measuring, erator turned ON, averaged curves, T = / 69. s The mixing ressure and the back ressure are influenced significantly by the ulsating rimary flow. By comarison in Table, the amlitude of ulsations are increased from 9 to 8 for and from 8 to.96 for. The delay after the erator is in both cases. It means that while the rimary velocity is reaching its highest values, both ressures, and, are still rising. C. Static ressure distribution on the mixing chamber wall Results of neumatic measuring of static ressure distribution on the mixing chamber wall obtained by the hel of both slow and fast ressure transducers are carried out in Fig. for erator switched OFF and in Fig. for erator switched ON. By comarison of both curves of mean values, we can see that the static ressure distribution is influenced by the oerating erator only negligibly X=x/D [] Fig. Static ressure distribution on the mixing chamber wall, erator turned OFF. Mean values with shown amlitudes On the other hand, the amlitudes of ulsations, which are also carried out in Fig. and, are increased rather significantly. The amlitudes are from 6 to for erator turned OFF, while the frequency is 78 Hz, and do not change notably along the mixing chamber, see Fig X=x/D [] Fig. Static ressure distribution on the mixing chamber wall, erator turned ON. Mean values with shown amlitudes For erator turned ON, the amlitude is in the beginning of the mixing chamber and it increases fluently to 8 at the end of the mixing chamber, see Fig.. The frequency is of 69. Hz. More detailed measuring for 6

6 erator turned ON is in Fig., where the curve for. Is carried out too. We can observe that for hase. and.8 the mixing can be almost considered as a constant ressure mixing. For the hase of, the static ressure even decreases during the mixing, while for the hase., the maximal static ressure gradient is obtained x/d= Fig. Results of fast neumatic measuring on the mixing chamber wall, erator turned ON, averaged curves for various ositions in the mixing chamber, T = / 69. s 6 x/d=. D. Results of hot wire anemometry Results of erformed hot wire anemometry measuring are resented in Fig. for erator turned OFF and in Fig. for erator turned ON. We measured inlet velocity c of the rimary flow just behind the rimary flow nozzle and in its axis, inlet velocity c of the secondary flow in the beginning of the mixing chamber and the velocity c at the end of the mixing chamber of the mixed flow. c, c [m/s] c.6.8. Fig. Results of hot wire anemometry, erator turned OFF, T = / 69.s c c (-)/(-) [] c [m/s] As we can see in Fig., for erator turned OFF, the rimary flow velocity c and secondary flow velocity c are almost stationary and the velocity ratio is ω = c c =. 9. The mixed flow velocity c fluctuates as a result of mixing rocesses and high turbulence intensity at the end of the mixing chamber. Values of measured velocities are in Table. c, c [m/s] c.6.8. Fig. Results of hot wire anemometry, erator turned ON, T = / 69.s As we can see in Fig., for erator turned ON, the rimary flow velocity c and secondary flow velocity c have nonzero amlitude of fluctuations. Again, the mixed flow velocity c fluctuates as a result of mixing rocesses and high turbulence intensity at the end of the mixing chamber, while the eriodic comonent of the velocity c ~ is small. The velocity ratio increased to the value ω =., though the increase is not as was exected according to results of slow neumatic measuring, see ejection ratio in Fig. and 6. This does not agree with our observation that ejection ratio increases more significantly while the erator is oerating. c c TABLE II RESULTS OF HOT WIRE ANEMOMETRY MEASURING c c c ω [m/s] [m/s] [m/s] [] OFF mean ON mean Hz amlitude IV. CONCLUSION Ejector with rimary flow oscillation erator was investigated with the hel of low and fast neumatic measurements. We comared ratio of mass flow rates ejection ratio, exansion ressure in the beginning of the mixing chamber, mixing ressure behind it, back ressure behind the diffuser and ejector efficiency for both erator turned OFF and ON. It was found out that for erator turned ON, the back ressure and the efficiency are higher. Of course we should kee in mind that oerating erator adds extra energy to the ejector. During that, the ejection ratio is higher while exansion and mixing ressures do not change. It indicated that velocity ratio could be affected due to the c [m/s] 66

7 oerating erator. Therefore, we also erformed a reliminary hot wire anemometry measuring, but our resumtion was not confirmed. We also erformed slow and fast neumatic measuring of static ressure distribution on the mixing chamber wall and the results illustrate how the ressure field in the mixing chamber ulsates. We have found out that the mixing rocesses are not significantly faster when the ulsation erator is oerating, which was the main urose of our investigation. Still, we do not fully understand to the mechanisms how the rimary flow ulsations influence the flow rocesses in the ejector. Further and more detailed investigation with the hel of hot wire anemometry will follow and the changes of velocity and turbulence rofiles will be insected. Measuring of more nozzles with various inlet area ratio A A and regimes with various velocity ratio c c are lanned. REFERENCES [] A. S. Ginevsky, Y. V. Vlasov, R. K. Karavosov, Acoustic Control of Turbulent Jets, Sringer-Verlag Berlin Heidelberg, Germany. [] P. Havelka, V. Linek, J. Sinkule, J. Zahradnik, M. Fialova, Effect of the ejector configuration on the gas suction rate and gas hold-u in ejector loo reactors, Chemical Engineering Science, Vol., No., 997, [] I. A. Waitz, Y. J. Qiu, T. A. Manning, A. K. S. Fung, J. K. Elliot, J. M. Kerwin, J. K. Krasnodebski, M. N. O'Sullivan, D. E. Tew, E. M. Greitzer, F. E. Marble, C. S. Tan and T. G. Tillman II, Enhanced Mixing with Flowwise Vorticity, Proy. Aerosace Sci., Vol., 997,. -. [] V. Dvořák, Study of otimization of lobed nozzle for mixing, Colloquium Fluid Dynamics, Institute of Thermomechanics AC CR, Prague, Czech Reublic, 7, 7-8. [] Y. J. Chang, Y. M. Chen, Enhancement of a steam-jet refrigerator using a novel alication of the etal nozzle, Exerimental Thermal and Fluid Science,,. -. [6] R. A. Tyler, R. G. Williamson, Confined mixing of coaxial flows, Aeronautical reort LR-6, NRC no. 88, Division of Mechanical Engineering, Ottawa, Canada 98. [7] V. Dvořák, Shae Otimization and Comutational Analysis of Axisymmetric Ejector, 8th International Symosium on Exerimental and Comutational Aerothermodynamics of Internal Flows, July -, 7 - Ecole Centrale de Lyon, France, 7, [8] V. Dvořák, P. Dančová, Exerimental Investigation into Flow in an Ejector with Perendicular Synthetic Jet, In: Exerimental Fluid Mechanics 9, Liberec. 7. November 9.. [9] Z. Trávníček, T. Vít, Hybrid synthetic jet intended for enhanced jet imingement heat/mass transfer, In.: Proc. th International Heat Transfer Conference IHTC-, Sydney, NSW Australia 6. 67