ISSN 1063-7850, Technical Physics Letters, 2017, Vol. 43, No. 3, pp. 267–269. © Pleiades Publishing, Ltd., 2017. Original Russian Text © V.K. Baev, A.N. Bazhaikin, 2017, published in Pis’ma v Zhurnal Tekhnicheskoi Fiziki, 2017, Vol. 43, No. 5, pp. 68–74.
Features of Interaction of an Axisymmetric Gas Jet with a Barrier of High-Permeability Material V. K. Baev and A. N. Bazhaikin* Khristianovich Institute of Theoretical and Applied Mechanics, Siberian Branch, Russian Academy of Sciences, Novosibirsk, 630090 Russia *e-mail:
[email protected] Received October 25, 2016
Abstract—Complex diagnostics (shadow shooting, smoke imaging, gas analysis) has shown the formation of flows at inleakage of a carbon-dioxide gas jet onto a porous barrier on its surface, inside it, and behind it, as well as the flow circulating between the nozzle and the barrier. The spatial distribution of CO2 concentrations and the scheme of flows at jet interaction with a barrier are presented. DOI: 10.1134/S1063785017030026
Studying characteristics of flow when a jet interacts with a barrier is necessary for the development of resource-saving technologies and energy-related heatexchange process, as well as the application of various coatings, among other things. The relatively small cost of jet blowing of barriers, a significant intensification of heat and mass transfer, and beneficial effects associated with the flow restructuring and turbulization near the barrier, stimulated many researches. In [1], a scheme of flows when the jet is incident onto impermeable barrier is given. It consists of a zone of free jet, a zone of turning and a zone of flow creeping on the surface of barrier. Experimental data and calculations of the flow parameters in these zones are given, and the conditions of flow separation from the barrier and formation of an inverse jet flow are analyzed. In [2], experimental investigations of local turbulent characteristics, tangential stresses, and pressures on the barrier depending on the conditions of interaction of jet with barrier are described. In [3], numerical simulation of the vortex flow structure and heat transfer in the region of interaction with the barrier and the flow reversal is carried out. Technologies to obtain new permeable porous materials with high porosity values, specific surface, permeability, and heat-exchange characteristics have been developed [4]. These properties significantly expand the range of applications of jet technologies, which requires the study of flows arising from interaction of jets with permeable barriers. At jet inleakage onto a permeable barrier, the gas spreads across the surface (as in the case of an impermeable barrier) and inside the barrier, while a portion of gas passes through it. The structure of such complex flows remains poorly studied. In [5], investigations of interaction of a super-
sonic air jet with permeable barrier were described. In this case, shadow shooting of measurement process of static pressures on the barrier and numerical calculation of flows with the use of the Fluent software package were used. As a result of measurements and calculations, schemes of flows on the surface inside and behind permeable barrier were constructed. According to the schemes, in the axial zone before the barrier, a tear-off vortex zone is formed that is enveloped by the main flow with the formation of a zone of turning by 90°, after which the flow spreads on the surface of the barrier. Another part of the flow spreads inside the barrier with subsequent exiting either onto the front side of it into the spreading flow or onto the back side of barrier, forming the flow behind it. It should be noted that the schemes and parameters of flows can vary considerably depending on the velocity and properties of the inflowing gas and the barrier, the distance between the nozzle and the barrier, and the presence of other enclosing surfaces and conditions that match the goals of research. Therefore, further comprehensive study of this complex nonstationary process⎯jet inleakage onto a permeable barrier⎯s necessary. In this work, we present the results of experimental studies of interaction of a carbon-dioxide gas jet (CO2) with a barrier of high-permeable cellular-porous material (HPCPM) located perpendicular to its axis according to the scheme shown in Fig. 1a. The jet was injected from a cylindrical nozzle 1 with a diameter of 0.5 mm under pressures of 0.1–1 atm onto a barrier 2 at distance lc = 50–100 mm. Carbon dioxide gas was fed onto the nozzle from balloon 3 through reduction gear 4 and gas-flow meter (GSS-4) 5 with injection pressure-control manometer 6.
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Fig. 1. (a) The scheme of experiments and (b) the image of permeable barrier disks made of ceramics and nickel.
The diagnostics of the process included videotaping through an IAB-451 shadow device, visualization of flows using smoke from oiled nichrome wire 7 heated by current, and gas analysis of the environment using capillary tube 8 connected to a Test gas analyzer 9. Disk barriers made of HPCMP ceramic and nickel (see Fig. 1b) with a thickness of 20 mm, diameter of 150–200 mm, porosity (Por) of 83–96%, size of cellular pores (dp) of 10–30 ppi, and a permeability of 0.8– 5 × 10–8 m2 were used [4]. Experimental results were obtained under various injection conditions and properties of disks that affect the image of the process. Figure 2a exhibits a shadow image of the CO2 gas-jet inleakage injected under pressure P = 0.8 atm (velocity of injection U0 = 270 m/s, gas-consumption rate G = 0.1 × 10–3 kg/s) onto a barrier of nickel HPCMP (Por = 95%, dp = 20 ppi) at lc = 50 mm. A nonuniform picture is observed between the nozzle and barrier, on the background of which ring formations around the jet can be seen. Analysis of video recordings showed that the waves of inverse flows are constantly moving from the barrier (in the direction of the nozzle), forming, in total, a circular toroidal vortex with the center in the axial zone of jet. Behind the disk, on the contrary, the flow is calm, uniform, “sifted” through the pores and cells, forming a cone that is transformed into a cylinder. At small injection pressures P < 0.2 atm (U0 < 140 m/s and G < 0.05 × 10–3 kg/s), such a picture is not observed: the jet creeps along the barrier, and inverse flows are not formed. Figure 2b shows a map of flow visualization by means of smoke streams obtained by CO2 injection under similar conditions (Fig. 2a), where the nozzle is
Fig. 2. (a) Shadow image of interaction of a CO2 jet with a permeable barrier and (b) visualization of flows by means of smoke.
located at the bottom left. Smoke jets between the nozzle and barrier visualize streamlines induced by the air jet [6]. They are directed to the axis of the jet, but they do not reach it and abruptly break off, moving around a fictional spherical body that probably represents a circular vortex. For 5–10 mm before the barrier, the trajectories of jets dramatically unfold radially from the axis of the jet and merge with the general stream creeping along the barrier, beyond which the flow expands and “attracts” the jets formed behind the barrier. The smoke visualization complements the data of shadow videotaping of the jet structure and surrounding air in the region of laminar flows, where the turbulence does not destroy visualization jets. Gas analysis allowed quantitative information to be obtained on the composition of the formed СО2 mixture and air. Sampling of the mixture was carried out with capillary tube 8 along the radius of cross sections perpendicular to the jet axis at different distances from the nozzle, including the front and back surfaces of the barrier. Figure 3a shows a three-dimensional map of the CO2 concentration distribution (C CO2 ) along the length of the jet in various sections of 60-mm radius obtained under conditions analogous to shadow shooting. One can see a sharp CO2 peak on the axis of the jet (in its core) near the rear of the nozzle (curve 1), followed by a plateau with concentrations of 2–3%. The concentration maximum of CO2 on the axis in the cross section located a distance 20 mm from the nozzle significantly decreases (curve 2), while the values C CO2 on the plateau are nearly constant, similar to the subsequent cross sections before the barrier (not shown in Fig. 3a). The results show that carbon dioxide is present not only in the core and in the zone of jet mixing, but also in the outer region significantly exceeding the jet radius in the given cross sections. Therefore, one can suggest that CO2 (diluted by air due to the ejection of jet) is present in this region as a result of inverse gas flow (recirculation) from the barrier to the nozzle with subsequent movement to the barrier along with the jet. The obtained results and suggestions made in this work are important for describing the structure of flows and for practical applications, for example, in matters of burning (CO2
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Fig. 3. (a) Concentration distribution of CO2 at jet inleakage onto a barrier and (b) and the scheme of flows: (1) nozzle, (2) barrier, (3) flow behind the barrier, (4) spreading before the barrier, (5) spreading inside the barrier, (6) toroidal vortex, and (7) air ejection.
density is close to that of propane–butane mixtures). On the barrier (shaded curve 3), a nonuniform picture is observed with distinct minima, maxima, and sharp fluctuations of C CO2 in the points of measurement (the average values of C CO 2 are shown on the curves), indicating significant turbulent flows. On the back side of the barrier (curve 4), all inhomogeneities are smoothed and C CO2 is gradually reduced to the periphery from a constant maximum value in the axial zone of the jet. In the cross sections removed from the barrier (curve 5), C CO2 also gradually decreases from the axis to the edge of the flow and the sizes of the jet behind the barrier are reduced. Therefore, complex diagnostics showed that a few parts (flows) are formed from the jet incident onto the
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permeable porous barrier. One part passes through the barrier, followed by movement in the axial direction. Another part spreads on the front surface of the barrier and goes beyond it. Part of the jet spreads inside the barrier, followed by exiting either onto the front surface of it, merging with the spreading flow, or onto the back surface, connecting with the jet behind the barrier. Another part of the flow spreading before the barrier branches off from the latter and forms the inverse flow circulating between the nozzle and barrier and back, forming a toroidal vortex. In this case, surrounding air is ejected into the region of circulation followed by the formation of a gas–air mixture of a certain composition. Figure 3b shows a map of flows based on the obtained results and literature data. The flows shown in the scheme were observed in nearly all regimes of the experiments indicated above (except P < 0.2 atm). The analysis showed that, with increasing velocity of collision of a jet with a barrier and a decrease in lc, the contact surface area of the jet with the barrier, the diameter of the flow behind it, and the level of CO2 concentration grow. With increasing permeability of the barrier, the level of C CO 2 in the flow behind the barrier increases, while, correspondingly, it decreases in the flows in front of it. It becomes possible to control the structure, intensity, and other characteristics of flows by changing the conditions of interaction of the jet with the barrier depending on the practical purpose of application of the described process. REFERENCES 1. G. N. Abramovich, Theory of Turbulent Jets (Nauka, Moscow, 1984) [in Russian]. 2. S. V. Alekseenko, V. V. Kulebyakin, D. M. Markovich, et al., Inzh.-Fiz. Zh. 69, 615 (1996). 3. K. N. Volkov, J. Appl. Mech. Tech. Phys. 48, 44 (2007). 4. V. N. Antsiferov and V. D. Khramtsov, Perspekt. Mater., No. 5, 56 (2000). 5. V. I. Zapryagaev, I. N. Kavun, and A. V. Solotchin, J. Appl. Mech. Tech. Phys. 56, 406 (2015). 6. A. N. Bazhaikin, Tech. Phys. Lett. 38, 979 (2012).
Translated by G. Dedkov
