- Email: [email protected]
Internal electrification of diesel oil injectors
Journal of Electrostatics 51}52 (2001) 481}487
Internal electri"cation of diesel oil injectors H. Romat*, A. Badri Laboratoire d+Etudes Ae& rodynamiques (UMR 6609 CNRS), Boulevard Marie et Pierre Curie, Te& le& port 2-BP 30179, 86962 Futuroscope-Chasseneuil Cedex, France
Abstract The objective of our research is to "nd a process which is able to charge (electrically speaking) a diesel oil jet in order to modify its hydrodynamic characteristics (dispersion and size of the droplets). In this paper we present the results of a campaign of experiments on the electri"cation of diesel oil jets. The liquid is electri"ed inside the injector with a needle brought to a high voltage (up to 4 kV). The electric charge of the jet is measured in terms of the voltage, the distance between the needle and the counter electrode (from 0 to 2 mm) and the rate of #ow (from 2.4 to 3.1;10\ m/s). The highest level of the electric charge recorded is 3;10\ C/m, which is 100 times higher than the level we recorded before with another process of electri"cation. 2001 Published by Elsevier Science B.V. Keywords: Electri"ed jet; Electri"ed spray
1. Introduction A lot of systems involving electric "elds have already been tested in order to modify the hydrodynamic characteristics of the atomisation of fuels. One can charge (electrically speaking) a jet by induction (action of the electric "eld outside the injector) or by injection of electric charges with a needle placed inside the injector and brought to a high voltage. The electri"cation by induction gives signi"cant results with conductive liquids [1,2]. On the other hand, the electri"cation by injection of charges inside the injector is used for dielectric liquid [3,4]. We start this article with a general presentation of the experimental device and with a detailed presentation of the new injector we designed. Then we give the results of the campaign of experiments we did
* Corresponding author. E-mail address: [email protected] (H. Romat). 0304-3886/01/$ - see front matter 2001 Published by Elsevier Science B.V. PII: S 0 3 0 4 - 3 8 8 6 ( 0 1 ) 0 0 1 2 4 - 3
482
H. Romat, A. Badri / Journal of Electrostatics 51}52 (2001) 481}487
with explanations and comments. We conclude with a comparison between our results and the ones of Kelly [3] and other authors having worked on electri"ed jets.
2. The experimental device Fig. 1 gives a general view of the system. The #uid #ow through the injector is induced by a pump (1), the pressure in this part varying between 40 and 120 bars. The liquid is electri"ed inside the injector thanks to a needle brought to a high voltage provided by an H.V. source (2) (Fig. 2 gives more details on the injector). Then the liquid goes through the ori"ce of the nozzle whose diameter is 400 m, arrives in the collector vessel (3) and discharges there. We measure the current of the spray with a Keithley ammeter (4), the collector vessel being placed inside a Faraday cage. The leakage current is recorded with another Keithley ammeter (5). Fig. 2 gives details of the injector. The electrical part of the injector is composed of a needle (1) linked to the high voltage source (2) and a grounded counter electrode (3). The diameter of the needle is 0.5 mm and the radius of curvature of its point is 30 m. We measure the leakage current from the counter electrode which is linked to a Keithley ammeter protected by a 1 M resistance. The position of the needle is adjustable so that the distance x between its point and the counter electrode can be modi"ed. We consider that x equals 0 when the extremity of the point is just in the centre of the hole of the counter electrode. x is positive for positions of the point above the hole; we never worked with negative x but it could have been the case, the diameter of the point of the needle being smaller than the one of the hole. The nozzle of the injector and the part in which the needle is embedded are made of a nonconducting material. In that injector, and for an appropriate range of velocity, the charges produced by the needle cannot reach the counter electrode because they are swept away by the #ow. They cannot discharge on the wall of the nozzle made of non-conducting
Fig. 1. General view of the experimental device.
H. Romat, A. Badri / Journal of Electrostatics 51}52 (2001) 481}487
483
Fig. 2. The injector.
material either. Therefore, the only possibility for them is to remain within the liquid until they discharge on the wall of the collector vessel.
3. Operating protocol and results First, we "x the distance x between the needle and the counter electrode, then we choose a #owrate (from 24 to 31 ml/s) and vary the voltage of the needle, the counter electrode remaining grounded through the protecting resistance and the ammeter. We measure the spray and the leakage currents for all the selected voltages (from 0 to 4.5 kV) and then we change the #owrate. Once the measurements are done for all the #owrates we "nally change the distance between the needle and the counter electrode. 3.1. Inyuence of the voltage and the yowrate In Fig. 3 we have plotted the electrical charge density against the #owrate for four voltages and for x"0. We can see that the charge density depends very little on the #owrate. Whatever the voltage, the variation of the charge density is not signi"cant. On the contrary the charge density depends on the voltage applied to the needle. It increases with the voltage, which is not surprising. However, the behaviour of the charge density for the two lowest voltages (0 and 1.5 kV) must be clari"ed. The level of the "rst one (0 kV) is not null, which means that the liquid charges itself without any voltage applied to the needle. This probably comes from the di!use double layers present inside the injector, the #ow sweeping them away and charging the liquid. The level of the charge density corresponding to the second voltage (1.5 kV) is similar. The conduction phenomena in that case are predominant and responsible for the charge of the liquid. In fact, the real understanding of the results needs more information on the conduction mechanisms within the liquid between the needle and the counter electrode. For that purpose, we did experiments with an applied voltage on the needle and no #ow. The current recorded was the leakage current that is to say the one measured
484
H. Romat, A. Badri / Journal of Electrostatics 51}52 (2001) 481}487
Fig. 3. In#uence of the #owrate for 4 V and x"0.
Fig. 4. Experiments with no #ow.
from the counter electrode. In Fig. 4 we plot the leakage current against the voltage for di!erent distances needle}counter electrode. We can see that the current increases when the voltage increases but for 1.5 kV the current is very low whatever the distance x (3;10\ A compared to the maximum of 50;10\ A). If we evaluate the electric "eld in the vicinity of the needle for x"0 and for 1.5 kV, we obtain 1.7;10 V/m which is enough to start a process of injection of electric charges from the needle. The process of injection of charges in the fuels starts generally when the electric "eld reaches 10 V/m (the breakdown in the fuels occurring at 10 V/m). For lower electric "elds, the regime of conduction is di!erent (natural or arti"cial dissociation inside the #uid) and the current is smaller. Once the injection starts the current increases rapidly
H. Romat, A. Badri / Journal of Electrostatics 51}52 (2001) 481}487
485
with the voltage. This is the case in Fig. 4 and because the calculus of the electric "eld in the vicinity of the needle is really approximate (we do not know the position of the needle precisely and we calculated the electric "eld for a cylindrical geometry), we think that for small values of the electric "eld we have a regime of dissociation and for voltages greater than 2 kV we have a regime of injection. We never met problems of breakdown. In fact we would need complementary experiments to determine the voltage for which the injection of charges starts exactly. If we come back to Fig. 3 and if transpose what we have just said to the real case (with #ow) we understand why we have a very low level of charge density for 1.5 kV: the injection of charges has probably just started. For 3 and 4.5 kV the level of charge density is much higher: the phenomenon of injection of charge has started. In all the cases the #owrate has a little in#uence on it. If we compare the mobility velocity of the charges, approximately 10\ m/s with 10\ m/Vs as an average value for the mobility of the ions, to the mean velocity of the liquid (150 m/s) we can understand easily that in this case (x"0) the velocity has no in#uence on the charge density. All the charges injected cannot reach the counter electrode and remain within the #uid. 3.2. Inyuence of the voltage and the distance needle}electrode In Fig. 5 we have plotted the charge density against the voltage for a "xed #owrate (2.4;10\ m/s) and for "ve di!erent needle}electrode distances. We observe a maximum of the charge density for x"0 mm irrespective of the voltage. Once again the intensity of the electric "eld determines the level of injection. When x increases we have a combination of two factors and both weaken the charge density: the electric "eld which diminishes and the local Reynolds number (in the vicinity of the point of the needle) which diminishes too. If we calculate this Reynolds number roughly, we have 3;10 for x"0 and 1.5;10 for x"2 mm. If we combine those two factors and the fact that geometrically the probability for the charges to reach the counter
Fig. 5. In#uence of the applied voltage for 5 distances x and for a #owrate of 2.4;10\ m/s.
486
H. Romat, A. Badri / Journal of Electrostatics 51}52 (2001) 481}487
electrode is greater for x"2 mm than for x"0 mm we understand easily why the in#uence of x is so important. 3.3. Inyuence of the distance needle}electrode and the yowrate In Fig. 6 we present all the results with the charge density against the #owrate for "ve distances x and for four voltages. The charge density for the "rst two "gures (a and b) is around 2;10\ C/m when the #owrate is below 2.9;10\ m/s but it increases drastically when the #ow rate is above 2.9;10\ m/s. For 3 kV we have the same behaviour but weaker and for 4.5 kV it disappears. The Reynolds number in the region of the hole of the counter electrode (500 m) is about 30 000. It does not change much when x varies and when the #owrate varies (within our range of velocities). In the vicinity of the needle, even if the Reynolds number does not change much, the regime of the #ow may change much because we are around 2000 (the diameter of the chamber is 7 mm). This is perhaps the reason why we have this increasing of the charge density for 2.9;10\ m/s in the "rst three "gures, the #ow changing its regime from laminar to turbulent. The injection of electric charges being more and more important when the voltage increases, the in#uence of the #owrate on the charge density diminishes with the voltage and is null for 4.5 kV.
Fig. 6. All the results.
H. Romat, A. Badri / Journal of Electrostatics 51}52 (2001) 481}487
487
4. Comparison with former works and conclusion Our goal was to test the technique of electri"cation by injection of charges on high velocity jets. This technique has given signi"cant results and the comparison to the ones we obtained with another technique (in#uence) [5] for the same range of velocity shows that it is 10 times more e$cient. Moreover, we did not use any additives for this technique whereas in the other one we added ASA 3 to the diesel oil to increase its conductivity. If we now compare our results to the ones of Kelly [3] we are much below the level of charge density he obtained. Typically, the charge density was 1 C/m, which is 20 times higher than ours. However, we think that it is possible to reach such levels, if we increase the voltage. We did not try voltages greater than 4.5 kV because of problems of electrical insulation of the needle with the body of the injector. We did not optimise the needle either. So, objectively, we think that we can improve the e$ciency of our electri"cation system, even for #owrates greater than 10 ml/s, and to get closer to 1 C/m, our "nal objective being to signi"cantly modify the hydrodynamic characteristics (dispersion, size of the droplets) of sprays of fuels in order to improve the combustion.
References [1] E.S. Law, Embedded-electrode electrostatic-induction spray-charging nozzle: theoretical and engineering design; Trans. ASAE 21 (1978) 1096}1104. [2] A. Baily, The theory and practice of electrostatic spraying, Atomisation Spray Technol. 2 (1986) 95}134. [3] A.J. Kelly, The electrostatic atomisation of hydrocarbons, J. Inst. Energy 2 (1984) 312}320. [4] W. Balachandran, A charge injection nozzle for atomisation of fuel oils in combustion applications, IEE-IAS Annual Meeting, 1994, pp. 1436}1441. [5] A. Selenou Ngomsi, Etude expeH rimentale sur la densiteH volumique de charge d'un jet a` grande vitesse soumis a` l'action d'un champ eH lectrique, Thesis, 1999.
Internal electri"cation of diesel oil injectors H. Romat*, A. Badri Laboratoire d+Etudes Ae& rodynamiques (UMR 6609 CNRS), Boulevard Marie et Pierre Curie, Te& le& port 2-BP 30179, 86962 Futuroscope-Chasseneuil Cedex, France
Abstract The objective of our research is to "nd a process which is able to charge (electrically speaking) a diesel oil jet in order to modify its hydrodynamic characteristics (dispersion and size of the droplets). In this paper we present the results of a campaign of experiments on the electri"cation of diesel oil jets. The liquid is electri"ed inside the injector with a needle brought to a high voltage (up to 4 kV). The electric charge of the jet is measured in terms of the voltage, the distance between the needle and the counter electrode (from 0 to 2 mm) and the rate of #ow (from 2.4 to 3.1;10\ m/s). The highest level of the electric charge recorded is 3;10\ C/m, which is 100 times higher than the level we recorded before with another process of electri"cation. 2001 Published by Elsevier Science B.V. Keywords: Electri"ed jet; Electri"ed spray
1. Introduction A lot of systems involving electric "elds have already been tested in order to modify the hydrodynamic characteristics of the atomisation of fuels. One can charge (electrically speaking) a jet by induction (action of the electric "eld outside the injector) or by injection of electric charges with a needle placed inside the injector and brought to a high voltage. The electri"cation by induction gives signi"cant results with conductive liquids [1,2]. On the other hand, the electri"cation by injection of charges inside the injector is used for dielectric liquid [3,4]. We start this article with a general presentation of the experimental device and with a detailed presentation of the new injector we designed. Then we give the results of the campaign of experiments we did
* Corresponding author. E-mail address: [email protected] (H. Romat). 0304-3886/01/$ - see front matter 2001 Published by Elsevier Science B.V. PII: S 0 3 0 4 - 3 8 8 6 ( 0 1 ) 0 0 1 2 4 - 3
482
H. Romat, A. Badri / Journal of Electrostatics 51}52 (2001) 481}487
with explanations and comments. We conclude with a comparison between our results and the ones of Kelly [3] and other authors having worked on electri"ed jets.
2. The experimental device Fig. 1 gives a general view of the system. The #uid #ow through the injector is induced by a pump (1), the pressure in this part varying between 40 and 120 bars. The liquid is electri"ed inside the injector thanks to a needle brought to a high voltage provided by an H.V. source (2) (Fig. 2 gives more details on the injector). Then the liquid goes through the ori"ce of the nozzle whose diameter is 400 m, arrives in the collector vessel (3) and discharges there. We measure the current of the spray with a Keithley ammeter (4), the collector vessel being placed inside a Faraday cage. The leakage current is recorded with another Keithley ammeter (5). Fig. 2 gives details of the injector. The electrical part of the injector is composed of a needle (1) linked to the high voltage source (2) and a grounded counter electrode (3). The diameter of the needle is 0.5 mm and the radius of curvature of its point is 30 m. We measure the leakage current from the counter electrode which is linked to a Keithley ammeter protected by a 1 M resistance. The position of the needle is adjustable so that the distance x between its point and the counter electrode can be modi"ed. We consider that x equals 0 when the extremity of the point is just in the centre of the hole of the counter electrode. x is positive for positions of the point above the hole; we never worked with negative x but it could have been the case, the diameter of the point of the needle being smaller than the one of the hole. The nozzle of the injector and the part in which the needle is embedded are made of a nonconducting material. In that injector, and for an appropriate range of velocity, the charges produced by the needle cannot reach the counter electrode because they are swept away by the #ow. They cannot discharge on the wall of the nozzle made of non-conducting
Fig. 1. General view of the experimental device.
H. Romat, A. Badri / Journal of Electrostatics 51}52 (2001) 481}487
483
Fig. 2. The injector.
material either. Therefore, the only possibility for them is to remain within the liquid until they discharge on the wall of the collector vessel.
3. Operating protocol and results First, we "x the distance x between the needle and the counter electrode, then we choose a #owrate (from 24 to 31 ml/s) and vary the voltage of the needle, the counter electrode remaining grounded through the protecting resistance and the ammeter. We measure the spray and the leakage currents for all the selected voltages (from 0 to 4.5 kV) and then we change the #owrate. Once the measurements are done for all the #owrates we "nally change the distance between the needle and the counter electrode. 3.1. Inyuence of the voltage and the yowrate In Fig. 3 we have plotted the electrical charge density against the #owrate for four voltages and for x"0. We can see that the charge density depends very little on the #owrate. Whatever the voltage, the variation of the charge density is not signi"cant. On the contrary the charge density depends on the voltage applied to the needle. It increases with the voltage, which is not surprising. However, the behaviour of the charge density for the two lowest voltages (0 and 1.5 kV) must be clari"ed. The level of the "rst one (0 kV) is not null, which means that the liquid charges itself without any voltage applied to the needle. This probably comes from the di!use double layers present inside the injector, the #ow sweeping them away and charging the liquid. The level of the charge density corresponding to the second voltage (1.5 kV) is similar. The conduction phenomena in that case are predominant and responsible for the charge of the liquid. In fact, the real understanding of the results needs more information on the conduction mechanisms within the liquid between the needle and the counter electrode. For that purpose, we did experiments with an applied voltage on the needle and no #ow. The current recorded was the leakage current that is to say the one measured
484
H. Romat, A. Badri / Journal of Electrostatics 51}52 (2001) 481}487
Fig. 3. In#uence of the #owrate for 4 V and x"0.
Fig. 4. Experiments with no #ow.
from the counter electrode. In Fig. 4 we plot the leakage current against the voltage for di!erent distances needle}counter electrode. We can see that the current increases when the voltage increases but for 1.5 kV the current is very low whatever the distance x (3;10\ A compared to the maximum of 50;10\ A). If we evaluate the electric "eld in the vicinity of the needle for x"0 and for 1.5 kV, we obtain 1.7;10 V/m which is enough to start a process of injection of electric charges from the needle. The process of injection of charges in the fuels starts generally when the electric "eld reaches 10 V/m (the breakdown in the fuels occurring at 10 V/m). For lower electric "elds, the regime of conduction is di!erent (natural or arti"cial dissociation inside the #uid) and the current is smaller. Once the injection starts the current increases rapidly
H. Romat, A. Badri / Journal of Electrostatics 51}52 (2001) 481}487
485
with the voltage. This is the case in Fig. 4 and because the calculus of the electric "eld in the vicinity of the needle is really approximate (we do not know the position of the needle precisely and we calculated the electric "eld for a cylindrical geometry), we think that for small values of the electric "eld we have a regime of dissociation and for voltages greater than 2 kV we have a regime of injection. We never met problems of breakdown. In fact we would need complementary experiments to determine the voltage for which the injection of charges starts exactly. If we come back to Fig. 3 and if transpose what we have just said to the real case (with #ow) we understand why we have a very low level of charge density for 1.5 kV: the injection of charges has probably just started. For 3 and 4.5 kV the level of charge density is much higher: the phenomenon of injection of charge has started. In all the cases the #owrate has a little in#uence on it. If we compare the mobility velocity of the charges, approximately 10\ m/s with 10\ m/Vs as an average value for the mobility of the ions, to the mean velocity of the liquid (150 m/s) we can understand easily that in this case (x"0) the velocity has no in#uence on the charge density. All the charges injected cannot reach the counter electrode and remain within the #uid. 3.2. Inyuence of the voltage and the distance needle}electrode In Fig. 5 we have plotted the charge density against the voltage for a "xed #owrate (2.4;10\ m/s) and for "ve di!erent needle}electrode distances. We observe a maximum of the charge density for x"0 mm irrespective of the voltage. Once again the intensity of the electric "eld determines the level of injection. When x increases we have a combination of two factors and both weaken the charge density: the electric "eld which diminishes and the local Reynolds number (in the vicinity of the point of the needle) which diminishes too. If we calculate this Reynolds number roughly, we have 3;10 for x"0 and 1.5;10 for x"2 mm. If we combine those two factors and the fact that geometrically the probability for the charges to reach the counter
Fig. 5. In#uence of the applied voltage for 5 distances x and for a #owrate of 2.4;10\ m/s.
486
H. Romat, A. Badri / Journal of Electrostatics 51}52 (2001) 481}487
electrode is greater for x"2 mm than for x"0 mm we understand easily why the in#uence of x is so important. 3.3. Inyuence of the distance needle}electrode and the yowrate In Fig. 6 we present all the results with the charge density against the #owrate for "ve distances x and for four voltages. The charge density for the "rst two "gures (a and b) is around 2;10\ C/m when the #owrate is below 2.9;10\ m/s but it increases drastically when the #ow rate is above 2.9;10\ m/s. For 3 kV we have the same behaviour but weaker and for 4.5 kV it disappears. The Reynolds number in the region of the hole of the counter electrode (500 m) is about 30 000. It does not change much when x varies and when the #owrate varies (within our range of velocities). In the vicinity of the needle, even if the Reynolds number does not change much, the regime of the #ow may change much because we are around 2000 (the diameter of the chamber is 7 mm). This is perhaps the reason why we have this increasing of the charge density for 2.9;10\ m/s in the "rst three "gures, the #ow changing its regime from laminar to turbulent. The injection of electric charges being more and more important when the voltage increases, the in#uence of the #owrate on the charge density diminishes with the voltage and is null for 4.5 kV.
Fig. 6. All the results.
H. Romat, A. Badri / Journal of Electrostatics 51}52 (2001) 481}487
487
4. Comparison with former works and conclusion Our goal was to test the technique of electri"cation by injection of charges on high velocity jets. This technique has given signi"cant results and the comparison to the ones we obtained with another technique (in#uence) [5] for the same range of velocity shows that it is 10 times more e$cient. Moreover, we did not use any additives for this technique whereas in the other one we added ASA 3 to the diesel oil to increase its conductivity. If we now compare our results to the ones of Kelly [3] we are much below the level of charge density he obtained. Typically, the charge density was 1 C/m, which is 20 times higher than ours. However, we think that it is possible to reach such levels, if we increase the voltage. We did not try voltages greater than 4.5 kV because of problems of electrical insulation of the needle with the body of the injector. We did not optimise the needle either. So, objectively, we think that we can improve the e$ciency of our electri"cation system, even for #owrates greater than 10 ml/s, and to get closer to 1 C/m, our "nal objective being to signi"cantly modify the hydrodynamic characteristics (dispersion, size of the droplets) of sprays of fuels in order to improve the combustion.
References [1] E.S. Law, Embedded-electrode electrostatic-induction spray-charging nozzle: theoretical and engineering design; Trans. ASAE 21 (1978) 1096}1104. [2] A. Baily, The theory and practice of electrostatic spraying, Atomisation Spray Technol. 2 (1986) 95}134. [3] A.J. Kelly, The electrostatic atomisation of hydrocarbons, J. Inst. Energy 2 (1984) 312}320. [4] W. Balachandran, A charge injection nozzle for atomisation of fuel oils in combustion applications, IEE-IAS Annual Meeting, 1994, pp. 1436}1441. [5] A. Selenou Ngomsi, Etude expeH rimentale sur la densiteH volumique de charge d'un jet a` grande vitesse soumis a` l'action d'un champ eH lectrique, Thesis, 1999.