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Modelling of ions for seeding technique to electrify the atmosphere
Journal of Electrostatics 75 (2015) 19e26
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Modelling of ions for seeding technique to electrify the atmosphere N.A. Doshi a, *, S.D. Agashe b a b
Department of Electronics and Telecommunication, College of Engineering, Malegaon (BK), Baramati 413102, India Department of Instrumentation, College of Engineering, Pune 413002, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 January 2014 Received in revised form 20 November 2014 Accepted 18 February 2015 Available online 3 March 2015
There are number of ways in which weak electrification can affect the microphysics of clouds, with consequences for cloud lifetime, radiative properties, and precipitation efficiency. Kauffman [2011] suggested ions produced by direct current generators will add to and enhance the catalysing effects that cosmic ray ions are now known to produce in among other things, lowering nucleation barriers, stimulating charged particle growth and stability and increasing the scavenging rate in clouds. Thus to electrify the atmosphere ions can be generated artificially in abundance along with large electric field. Ions can be generated by the corona effect using Atmospheric electrifiers (a device used to generate negative ions) which makes use of corona discharge phenomenon to charge the air particles. Exact assessment of electric field and charge density distributions and the flow dynamics inside the electrifiers is essential to understand the particle behaviour inside the electrifiers. In this paper, a novel model of governing equations to evaluate the space charge density, electric field intensity and velocity of ionized airflow is suggested as a function of applied voltage. The Poisson and charge conservation equations are derived and hence can be used to estimate the electric field and charge density distributions. Navier stokes equation can be used to get the velocity of ionized airflow because of electric force on the air. Simulation is carried out to validate the proposed model and verify that velocity is function of input voltage and is proportional to it. © 2015 Elsevier B.V. All rights reserved.
Keywords: Atmospheric electrification Negative ions Corona discharge EHD flow Ionized wind Artificial rain
Introduction Svensmark and Friis-Christensen [1997] and Svensmark [1998] demonstrated correlations of cloud cover with Galactic cosmic rays (GCR) flux, and speculated that ionization processes could affect nucleation or the phase transitions of water vapour. Corona effect ions may have a role in catalysing atmospheric phenomena as suggested by R.G. Harrison and K.S. Carslaw in [2003].S.D Pawar and Kamra [2009] have shown that the space charge released into the atmosphere by corona currents from the ground can increase the air-conductivity of the atmosphere near to earth surface by more than an order of magnitude during the dissipation stage of thunderstorm. The overall process of natural electrification of atmosphere which is responsible for coalescence and precipitation is summarized [3e11,23] in Fig. 1. There are positive ions and free electrons present in the air as a result of “background radiation” or photon
* Corresponding author. Tel.: þ91 996 024 6549; fax: þ91 02112254424. E-mail address: [email protected] (N.A. Doshi). http://dx.doi.org/10.1016/j.elstat.2015.02.006 0304-3886/© 2015 Elsevier B.V. All rights reserved.
excitation occupying the atmosphere. This natural ionization is emitted from artificial and natural sources, such as radioactive elements and cosmic rays. The majority of the ionization is caused by cosmic radiation where the electric field is high enough. Radon gas released from the Earth's crust can cause radioactive ions to attach to airborne dust and other particles to form aerosols by nucleation. Attached aerosols coagulate to form condensation nuclei which further condense to give cloud condensation nuclei to form cloud. Similar process can be initiated and induced artificially by generating abundance of corona effect negative ions in the atmosphere. Small ions are produced continuously in the nature. But because of pollution, global warming, the so called modern life style of human beings is responsible and also due to other phenomena; adverse forces tend to destroy these ions as a result of which many natural processes are affected in the atmosphere. The major impact of this is on the natural rainfall hence Ion losses is also the major issue. Thus the actual percent-concentration of the electrons and ions in the free space can be disregarded for this study. The electron avalanche effect because of the artificial ions will ensure an exponential increase of electron generation, which allows for ionization to occur.
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Fig. 1. Role of corona effect ions in catalysing the atmosphere [suggested by R.G. Harrison and K.S. Carslaw in 2003].
When high density uni-polar ions are added in the atmosphere the recombination losses are negligible. As after recombination's with available opposite polarity ions, high density ions act as seed for further reactions with dust particles and droplet coalescence processes. The injection of a large number of DC corona effect ions will induce changes in cloud microphysics and cloud cover and, consequently modifications in weather conditions. It is expected that DC generated ions are going to be a more aggressive catalyser than cosmic rays as corona effect ions are hygroscopic and grow rapidly with increased humidity [1].
Corona effect to generate negative ions Direct current corona effect ionization starts to occur at ground level. The net effect of ionization will be to charge pre-existing aerosol or form new charged aerosol. Ions produced by direct current generators will add to and enhance the catalysing effects that cosmic ray ions are now known to produce in, among other things, lowering nucleation barriers, stimulating charged particle growth and stability and increasing the scavenging rate in clouds [1]. Thus modelling the corona ions is important as “clouds can be seeded by ions to produce rain from ground station by electrifying the atmosphere”. Negative ions are generated using corona discharge phenomenon. Corona discharges are relatively low power electrical discharges that take place at or near atmospheric pressure. The corona is invariably generated by strong electric fields using small diameter electrodes [2] (ion generator and shield electrode). When a negative high voltage exceeding the potential at which corona is found to originate is called corona threshold voltage. When this voltage is applied across an asymmetric geometric model with very small curvature ion generator electrode; because of corona effect the negative ions are generated.
Modelling of an ion The movement of air particles as a result of the electro hydrodynamic (EHD) phenomenon can be explained through the use of basic physics, including Coulomb's Law of Electrostatics, Conservation of Momentum, and Newton's Third Law. In order to model the ions for seeding technique to electrify the atmosphere, the governing equations necessary to represent EHD flow induced by corona discharge are described in Section 3.1.
Electrostatic equations The presence of corona phenomena complicates the mathematical models used to simulate the impact of the electric field and current density distributions in inter-electrode space. Hence a simplified model for analysis of direct current corona field and induced electro hydrodynamic airflow field in atmospheric electrifier system is presented.
Modelling assumptions The operating voltage range for corona discharge lies between the corona threshold voltage and the air gap breakdown voltage. Corona induced airflow is possible. The gap between ion generator electrodes and grounded electrode can be divided into two regions, the ionization and drift zones. When the ion generator electrode voltage is above the corona onset level, the corona discharge generates the EHD flow. When the radius of the ion generator electrode is much smaller than the distance between ion generator electrode and grounded electrodes, the ionization zone forms a uniform sheath over the coronating region of the ion generator electrode surface. The ionization zone exists in close proximity to the ion generator electrode, in which air ionization occurs, and both positive and negative ions exist. The drift region, located outside the ionization zone, contains ions of a single polarity that have been driven out of the ionization region by the electric field. Columbic force interactions between the ions and the electric field are responsible for ion acceleration. A simplified model of corona discharge is assumed in this study where the following apply [12e16]: 1. The mono polar corona discharge inside the atmospheric electrifiers is modelled under- All quantities are steady in time and - All ions and particulates are neglected except for those generated by corona discharge. 2. The ion mobility is constant and independent of the electric field's influence. 3. The corona is a discharge where ionization is non-thermal hence thermal diffusion of the ions is neglected. 4. When the flow is at steady-state, space charge density does not change with respect to time. 5. Under standard atmospheric pressure, the EHD flow has a negligible effect on the corona discharge, because the air velocity is much smaller than the velocity of moving ions.
N.A. Doshi, S.D. Agashe / Journal of Electrostatics 75 (2015) 19e26
6. The electric field and charge transport in the corona simulation are independent of the gas flow. 7. As the outlet velocity of the ionized airflow is in the order of a few metres per second, the ions drift is usually much large and dominates, hence one-way coupling affectively takes place as for decoupled model, due to one-way coupling (electric field causes gas motion, but motion of air doesn't affect the corona discharge parameters. The velocity of the natural airflowing inside the atmospheric electrifier is assumed to be very small in magnitude in the order of 102 m/s) it is the end of the calculation. 8. The passage of current between the electrodes is the result of two simultaneous effects: Movement of charge relative to the main body of gas and transport of charge by, rather than through, the gas stream. 9. The design feature leads to the distribution of electric fields and design of electrode shapes most suitable for producing the airflow in the desired direction. 10. The effects of material and surface conditions on the electron distribution could not be explicitly illustrated by the model because the dependence of the yield of photo electrons on electrode material and surface conditions has not been quantified. Instead, the model uses total corona current to specify the electron number density at the outer boundary of the corona plasma. When high voltage is applied to the atmospheric electrifier system, a space charge is formed, and a continuous electric current flows between the two electrodes. Both, applied voltage and space charge, will contribute to the generation of electric field between electrodes. The space charge density (r) and electric field strength (E) distributions in domain between electrodes are essential for calculating particle charging and trajectory. The solution of r and E enables the prediction of the voltageecurrent (VI) relations as well as ionized airflow characteristics. The displacement current is zero as charge density is not varying with time. The electric field intensity (V/m) between the electrodes can be described by Gauss's law
V$E ¼ r=ε0
(1)
where r is the space charge density (C/m3), V is the scalar electric potential (kV), and ε0 is the permittivity of free space (8.854 1012 C/V m).The impressed voltage V is equal to the difference in potential across the two electrodes. The electric potential anywhere in the system where no net accumulation takes place is defined from electric field intensity E as
VV ¼ E
(2)
Substituting (2) with (1), we obtain Poisson's equation, which is defined as
V$VV ¼ r=ε0
(3)
The drift layer has a combination of conduction (motion of ions under electric field relative to entire airflow), convection (transport of charges with airflow), and diffusion. The ion diffusion is of negligible importance compared to conduction. Because the gradient in the electron number density distribution is small and the ion diffusion current is neglected, the velocity of ions diffusion is much smaller than that caused by the effect of electric field and wind flow. Convection term is also negligible as without the external force there will be no charge carriers. Hence only the conduction term is dominant in the system, Equation (4) simplifies to
J ¼ mE rE
V$J ¼ 0
(6)
After combining Equations (5) and (6), substituting for the electric field with potential (2), expanding the divergence, and then substituting Poisson's Equation (3) with the result, the charge transport equation can be derived to be
V$ðmE rVVÞ ¼ Vr$VV þ rV$VV ¼ 0 . Vr$VV ¼ r2 ε0
(7)
This resulting equation is a dot product between the vector space charge density and scalar electric potential. The electric problem of corona discharge is governed by a set of two partial differential equations: Poisson's Equation (3) with unknown potential, V, and the charge transport Equation (7) with the unknown space charge density, r. These equations can be solved by using many numerical methods. To get the values of r and V, the initial value of the space charge density can be adjusted such that the electric field strength will be equal to the breakdown electric field strength in air ¼ 3.23 103 kV/m in Equation (7) to get the value of V. This value of V can be then substituted in Equation (3) to check its validity and then the value of r can be iterated like wise to satisfy (3) and get the final value of V. Fluid dynamics equations
J ¼ mE Er þ UreDr
Modelling assumptions. The fluid which passes through the atmospheric air which gets ionized because of columbic force. made are [18,8]: The fluid has constant density and cosity, small temperature variations are there
where J-is the ionic current density, U- is the velocity vector of airflow, D-is the charge diffusion coefficients of ions (5.3 105 m2/ s), mE - is ion mobility of negative ions (2.7x10-4 m2/V s).
(5)
Under steady state conditions, the current density must satisfy the charge conservation equation or current continuity equation. Since the charges are indestructible, if there is an in flow of charges in some part of the surface of a conductor, then there would be an equivalent outflow elsewhere or there would be an accumulation of charge upon the conductor. Under steady e state conditions ruling out the possibility of charge accumulation, the total in flow of the current to any conductor must equal the total outflow, a condition must be true for any part of conductor. The electric current does not change with time; the charge density is also independent of time. Under this condition, the current density vector has no source, and the charges cannot be created at any point in a steady current flow [2]. Current continuity condition gives equation for current density
This Equation (3) relates the electric potential to the charge density and governs the electric field of the system. The ionic charges are accelerated by the Coulomb force and move towards the grounded electrode. The charge drift creates an electric current with a density defined by the current density equation.
(4)
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EHD flow is a particular domain of electro-hydrodynamics, which involves the study of corona discharge asymmetric geometry model for the purpose of particle movement in fluid mediums.
Electrifiers is Assumptions constant visi.e. uniform
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temperature where diffusion can be neglected, is a single phase fluid and hence rules out the possibility of sound or shock waves to occur, small pressure drop is there and hence is assumed to be incompressible Newtonian fluid. The continuity equation for such fluid reduces to
V$U ¼ 0
(8)
The conservation of momentum equation also known as the NaviereStokes equations (NSE) is a nonlinear PDE written for Newtonian fluids. Studying such fluids is simple because the viscosity model ends up being linear. As the ion generator electrode with the ionization zone cross section area is negligible in comparison with an overall area of the field, the ion generator electrode can be represented as a point. Coulomb force acting on the ions becomes an electric body force on the air molecules and gives rise to the EHD flow. This Coulomb force is the main contribution to the ionic airflow that causes the system to generate ionic wind. Actually the force due to EHD is the sum of contributions due to the permittivity of gradient force, dielectrophoretic force, and electrocostriction force. But as the air is a single phase fluid and the motion is not due to polarisation, the contribution due to these forces is not considered. The high frequency of collisions between the space charge and air bulk can be theoretically estimated as a full transfer of momentum, which contributes to the resultant electrical body force responsible for EHD flow. Considering the body force Fe on fluid motion, due to steady state electrostatics the NSE can be expressed in the simplified form as:
rair U$VU ¼ Vp þ mair V2 U þ Fe
(9)
where rair is the air density (1.23 kg/m3), p is the static air pressure (in Pascal), and mair is the air dynamic viscosity (1.8205 105 N/ m2). The LHS of Equation (9) represents the acceleration term and the RHS represents the different acting forces. Considering the above assumptions, the flow field is laminar and hence these basic governing equations for the conservation of the mass and momentum are sufficient. The mean field approximation of the electrostatic potential is described by the Poisson equation, which relates the electrical potential to the charge density in this simplified form.
V$ðε0 VVÞ ¼ r
(10)
The electrostatic body force density Fe as a function of the mean electrical potential (V) using the Poisson Equation (3) and Equation (2) can be expressed as function of applied voltage.
Fe ¼ rE ¼ rVV
(11)
r ¼ ε0 V2 V
(12)
Fe ¼ ε0 V2 V VV
(13)
Poisson's equation for electric potential is to be solved only inside the drifting zone with prescribed voltage on external boundary of ionization zone. Thus values of space charge density and electric field intensity obtained from Equations (7) and (3) will be used to find the body force from Equation (11). Equation (13) shows that the body electric force and the input voltage are linearly proportional, which in turn when substituted in Equation (9), gives the unknown value of velocity of the airflow. The numerical simulation of the proposed model can be used to predict the effect of electric corona discharge on the generation of ions and in turn on the airflow. All distributed and global
parameters, including both the electric field and the flow, can be calculated. Procedure of numerical analysis of the field may consist of two stages. The first stage would comprise of the numerical solution of the Poisson and charge conservation equations describing the direct-current corona discharge. The results of this stage then would be used for computation of the components of Coulomb force in each of the node of computational grid. The second stage can perform computation of electro hydrodynamic flow field by use of NaviereStokes and flow continuity equations to give the value of velocity proportional to the input applied voltage. Thus, the studied EHD flow model described by the system of Equations (3), (7), (9) and (13), with the appropriate boundary conditions along with the numerical simulation can be used as inputs for ion seeding to electrify the atmosphere.
Simulation Researchers have tried to solve problems long before for different applications. Several of them invoke a commercial software packages with finite element method FEMLAB, COMSOL or ANSYS [19,20] the other calculate with their made programs using numerical or analytical methods. It is clear from their results that in each corona discharge electrode system electric fields are quite different. It must be simulated separately to each application of corona discharge. 2-D model of Electrifier with two electrodes namely ion generator and shield electrode for analysis of direct current corona field and induced EHD airflow field is presented. To evaluate the values of the Space charge and voltage field following boundary conditions are set as shown in Fig. 2; Zero volts is applied to the shield electrode, variable negative voltage from -5 kV to 40 kV is applied to the ion generator electrode, no slip boundaries are used for all solid surfaces. The charge transport Equation (7) is also only applied to the drift layer. Two electrodes act as stationary electrodes where the components of the velocity vectors vanish. The outside boundary is defined with a pressure inlet and outlet with both inlet and outlet pressures equal to zero and outlet pressure in the direction of the back flow normal to the boundary. This means that the air is free to flow in both directions. The computation air space is set to an “open boundary,” which implies a zero normal stress (N/m2) and pressure component. Zero charge boundary condition at the exterior boundaries and at the interior boundaries; it means that no displacement field can penetrate the boundary and that the electric potential is discontinuous across the boundary is considered. No flux condition is boundary condition on the exterior boundaries, represents boundaries where no mass flows in or out of the boundary; such that the total flux is zero. A zero diffusive flux is imposed on all boundaries except the surface of the ionization layer because the diffusion term does not affect the outflow boundaries. Velocity component other than the inlet and outlet i.e. u and v are zero.At the inlet the velocity is assumed to be 0.01 m/s. The simulations are run using COMSOL Multi physics 4.2 software on a Intel(R) X5650 with Dual 2.8 GHz Quad-core Xeon processors and 64 GB of RAM. The electrostatics module, NaviereStokes module, and a generalized PDE coefficient form modules are used to represent the derived governing equations. Due to the generalized structure of COMSOL, the generalized PDE coefficient module are modified and used for the charge transport equation. A single 2-D electrifier cross-section Fig. 2 is modelled. The electrostatic module, PDE coefficient module for the charge transport equation, and NaviereStokes module were applied separately to the 2-D Electrifier model. The variables in each of these modules are heavily dependent and need to be solved separately before they are linked.
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P=0
Outlet
No Flux ,no slip Ion generator electrode (r=0.5mm) (-20kV)
50mm
Shield electrode (r=1mm) (0V)
V=0
Air inlet Input velocity =0.01m/s Fig. 2. Boundary conditions.
Radius of ion generator electrode is 0.5 mm and of shield electrode is 1 mm. The distance between electrodes is varied to find the safe distance. The ion generator voltage is varied to find combination at which maximum velocity is reached. Fig. 3(a) and (b) shows the potential and velocity distributions for a 2-D model. With a 20 kV applied voltage to the ion generator and the shield at
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ground, a smooth potential distribution is found in the majority of the air gap except near the electrodes. Notice that the graph does not reach the maximum 20 kV voltage because corona discharge occurs before this point and maintains the onset voltage (Vc) value along the external surface of the ionization layer. The velocity has a peak within the air gap and causes an electrical force that pushes the negative ions upwards. The calculation for this value influenced by the electric field never exceeds the electric field intensity for air breakdown [26]. Fig. 3(b) shows the airflow velocity distribution as a result of the NaviereStokes and charge transport modules. The airflow modelled in this simulation is mainly driven by the Coulomb force. The transfer of momentum increases the airflow and peaks at the edges of the ion generator electrode. The Coulomb force distribution is a product of the electric field and space charge density. The Coulomb force drives the air motion and is seen to be greatest between the ion generator and shield. The total velocity in this system is a direct result of the airflow caused by the Coulomb force. Simulation result A graph of output velocity versus ion generator voltage for fixed safe distance form the result of the simulation is plotted as seen in Fig. 4 the voltage is varied in the steps of 2e3 kV. The simulations are carried out first to find safe distance (before
Fig. 3. Simulation results (a) Electric potential (b) Velocity distribution.
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2.
Fig. 4. Simulation result for change in output velocity w.r.t. input voltage.
electric arc) between the electrodes. Well above the safe distance, i.e. at a distance of 50 mm between the electrodes, the velocity is seen to be increasing with the applied ion generator voltage which validates the model's output (i.e. voltage -20 kV- velocity -1.286 m/ s). Thus by increasing the size of the electrodes; the safe distance would also increase, the voltage to be applied would increase and finally the output velocity would increase. To electrify the atmosphere the velocity of the ionized airflow must be greater than 4 m/s [17] as this is maximum velocity of the horizontal winds which should be overcome. Depending upon the electrode radius chosen, the ion generator voltage can be accordingly changed to achieve the electric field intensity [26] and space charge density and thus the desired velocity.
3.
4.
5.
Development of ionic wind to electrify the atmosphere The ionized airflow/wind to electrify the atmosphere because of negative corona ions is treated in more detail. Many processes continue to occur after the corona discharge takes place. The ones that are often important for further physical and chemical processes are [21,22]: 1. An electron excited by an electric field E will ionize an atom or molecule of a gas as it has enough energy, called the ionizing potential of the particular gas. This implies that a free electron gains on average several eV of energy from the field in-between collisions. There will be electrons in the tail of the electron
6.
energy distribution function that have enough energy to cause ionization. Outside the ionization zone, the negative ions collide in the route with neutral air molecules. During these collisions momentum is transferred from ions to neutral molecules resulting in a bulk motion of the gas. Attachment is the formation of negative ions when low energy electrons combine with atoms or molecules. Not all particles form negative ions, examples are the noble gases and also nitrogen molecules. Some molecules that easily form negative ions are O2, H2O and CO2. The conductivity of air is strongly influenced by the capturing of low energy electrons: the light and mobile electrons are replaced by heavy ions. Therefore the field strength for sustaining a certain current is much higher in an electronegative gas. Recombination of negative ions leads back to the neutral gas as before the discharge pulse. The active particles in a corona discharge are formed in a thin channel. It is the intention to treat the whole gas volume. The EHD flow is the result of the interaction between drifting ions and the surrounding air molecules. Both electric field and fluid flow are present and interacting with each other. The high-energy electrons in the streamer head cause ionization and excitation to higher electronic states. Excited metastable states cannot lose their energy by emission of a photon. Another process to take away energy from them is by collisional quenching. It is also possible that they transfer all their energy to a different molecule. In this way quenching of a N2 metastable can cause dissociation of a water molecule. This process is considered to give an important contribution to the formation of OH radicals in humid air. Radicals are formed by electron impact dissociation of molecules in the streamer head region. The dissociation energy is usually somewhat lower than the ionization energy. Primary radicals are the ones created directly by these collisions, e.g. O, H, OH and N. They may react rapidly with molecules to form secondary radicals such as HO2 or O3. The applied electric field provides the driving force for the movement of these charged particles towards the lower potential electrode and thus generates the ionic wind i.e. ionised airflow, the velocity of which is linearly proportional to the amplitude of the applied voltages before corona is arrived at the state of burst-pulse streams [9,13]. This ionic wind occurs from the corona threshold voltage of corona discharge, and the airflow is accelerated from the tip of ion generator electrode. As a consequence, the produced ionic wind velocity is linearly
Fig. 5. Schematic illustrating GCReCNeCCN-Cloud hypothesis that if confirmed, might explain the correlation between variations of GCR flux and low cloud cover. The possible dominating species involved in the different phases of CN formation and growth processes are also indicated. The organics species and water molecules may play an important role in growing the CN into the size of CCN. (From Yu, 2002).
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controlled over the range and provides a steady uniform flow of ionic wind and would effectively be used for artificial electrification of atmosphere.
Effect of atmospheric electrification on the formation of artificial raindrop When a sufficient electric potential is supplied to the ion generator electrode it begins to display corona effect thus emitting electrons. The grounded shield electrode provides a stable outflow of the electrons emitted, thus maintaining the maximum possible level of emission from the ion generator electrode. The majority of the electrons emitted by the ion generator electrode and repelled by the negative background potential of the earth surface tend to rise upwards into the atmosphere forming a stable flow [25]. The growing of the negative ion into a raindrop is stepwise process as shown in Fig. 4. It has been demonstrated recently that air ions may play an important role in the production of new particles under typical tropospheric conditions [Yu and Turco, 2000. 2001]. When electrons combine with the air molecules, negatively charged ions are formed, which immediately begins to attract groups of molecules of the atmospheric water vapour. Essentially, the production of H2SO4 in the gas phase by photochemical reactions in the atmosphere results in supersaturated H2SO4 vapour. There is a competition between deposition of H2SO4 on pre-existing aerosols and on the ions. The competition results in a complicated dependence of the nucleation rates on the concentrations of pre-existing aerosols, ions, and H2SO4 vapour. The presence of water molecules, H2SO4, HNO3, **NH3 and other similar gases accelerate the flow of the ionized molecular complexes formed in the atmosphere. The charges molecular clusters, and thus can preferentially achieve stable, observable sizes. The proposed ion-mediated nucleation (IMN-((H₂SO₄)n and (H₂O)m ion))theory as shown if Fig. 5 can physically explain the enhanced growth rate (a factor of ~10) of sub-nanometre clusters as observed by Weber et al., [1997]. An increase in precipitation entails a decrease in the amount of cloud water that is available for evaporation into air being entrained into the air mass of a storm. The reduction of this diabatic (non-adiabatic) cooling is equivalent to a diabatic heating, causing the adiabatic volume expansion of the air surrounding the said complexes, where upon the air rises upwards while cooling down, and the water vapour contained in the air condenses under appropriate conditions. The low-pressure areas receive new portions of the surrounding air, which are also ionized by the maintained ion flow and diabatic heating rise upwards [24]. Since the direction of the ion flow is defined by the charge of the earth surface and the direction of the air stream is defined by the atmospheric pressure gradient, the overall direction of air stream will be substantially vertical under favourable conditions (e.g. in the absence of strong horizontal wind, this being typical for anticyclone areas) [25]. Thus, a stable upwards ionized air stream (or “current”) is formed in the atmosphere which is responsible for the rainfall. Conclusion There is a great deal of modelling that is needed in order to provide quantitative relationships between atmospheric electrification and ionization. However, proposed models of the electrostatics to govern corona ions and fluid mechanics for determining velocity of airflow to understand the effect of corona effects on the air in the atmosphere will be useful for validating the present observational results. It will provide accurate inputs into electrification of atmosphere.
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A numerical model along with governing equations for generating negative ions using an Electrifier is developed to evaluate the value of the space charge density, electric field intensity and ionized airflow velocity parameters. It is validated by simulation results. The Electrifier generates ions, which ionizes the air particles and gives output velocity to the airflow equal to 1.286 m/s for 20 kV with input velocity of 0.01 m/s. These negative ions can be used as seeds to electrify the atmosphere. From the model and the simulation results it is seen that the velocity of the ionized airflow is proportional to the applied voltage. Nucleation because of these ions helps the growing of the negative ions into the raindrop. Dc corona ions generated artificially function in a similar way as the ions which are generated by the cosmic ray and other natural resources. Artificial ions are in fact more powerful catalysers and hygroscopic than the natural ions. The particle flux changes directly affect the atmospheric concentration of ions, and indirectly affect the density of space charge in the troposphere. The changes in space charge and/or ion concentration are likely to affect properties of atmosphere to get artificial rainfall by electrifying the atmosphere. References [1] Kaufman, United States Patent Application publication, U. S. 2011/7,965,488 B2, A1eApparatus and Related Methods for Weather Modification by Electrical Processes in the Atmosphere, Jul.21, 2011. [2] Jen-shih Chang, Arnold J. Kelly, Joseph M. Crowley, Handbook of Electrostatics processes. [3] H. Svensmark, E. Friis-Christensen, Variation of cosmic ray flux and global cloud coverage, J. Atmos. Terr. Phys. 59 (1997) 1225e1240. [4] R.,G. Harrison, in: Atmospheric Electricity and Cloud Microphysics; Proceedings of the Workshop on Atmospheric Science, 2001. [5] K.S. Carslaw, R.G. Harrison, J. Kirkby, Cosmic rays, clouds and Climate, Sci. Rev. Geophys. 298 (2002) 1732e1737. [6] R.G. Harrison, K.S. Carslaw, Ion-aerosol-cloud processes in the lower atmosphere, Rev. Geophys. 41 (3) (2003) 1012e1027. [7] Hirsikko, T. Nieminen, S. Gagne, K. Lehtipalo, H.E. Manninen, M. Ehn, U. Heorrak, V.-M. Kerminen, L. Laakso, P.H. McMurry, A. Mirme, S. Mirme, €ja €1, H. Tammet, Vakkari, M. Vana, M. Kulmala, Atmospheric ions and T. Peta nucleation: a review of observations, Atmos. Chem. Phys. 11 (2011) 767e798. [8] Blair, Weather Elements a Text in Elementary Metrology, March 1994, 12print. [9] K.A. Nicoll Understanding the Electrical interactions between ions, aerosols and clouds. [10] Yu Nadykto, Uptake of neutral vapor molecules by charged clusters/particles: enhancement due to dipole-charge interaction, J. Geophys. Res. 108 (D23) (2003) 4717e4723. [11] S.D. Pawar, P. Murugavel, V. Gopalakrishnan, New particle formation by ioninduced nucleation during dissipation stage of thunderstorm, J. Earth Syst. Sci. 120 (5) (October 2011) 843e850. [12] Jack Wilson, Hugh D. Prekins, Willam K. Thompson, An investigation of ionic Wind Propulsion, NASA/TM- 2009-215822. [13] Lin Zhao, Kazimierz Adamiak, EHD flow produced by Electric Corona Discharge (Numerical and Experimental Studies, and Applications). [14] C. Kim, K.C. Noh, J. Hwang, Numerical investigation of corona plasma region in negative wire-to-duct corona discharge, Aerosol Air Qual. Res. 10 (2010) 446e455, http://dx.doi.org/10.4209/aaqr.2010.03.0019. Copyright © Taiwan Association for Aerosol Research ISSN: 1680-8584 print/2071-1409 online. [15] L. Zhao, K. Adamiak, Numerical simulation of the electrohydrodynamic flow in a single wireeplate electrostatic precipitator, IEEE Trans. Ind. Appl. 44 (3) (2008) 683e691. [16] Y.N. Chun, J. Chang, A.A. Berezin, J. Mizeraczyk, Numerical modeling of near corona wire electrohydrodynanmic flow in a wire-plate electrostatic precipitator, IEEE Trans. Dielectr. Electr. Insul. 14 (1) (February 2007) 119e124. [17] United States Patent Application Publication, US 2011/0174892 A1-Apparatus and related methods for weather modification by electrical processes in the atmosphere, Jul.21, 2011. [18] A. Castellanos, Coulomb-driven convection in electro hydrodynamics, IEEE Trans. Electr. Insul. 26 (6) (Dec. 1991) 1201e1215. [19] B. Benamar, E. Favre, A. Donnot, M.O. Rigo, Finite element solution for ionized fields in DC electrostatic precipitator, in: Excerpt from the Proceedings of The COMSOL Users Conference, 2007 (Grenoble). [20] S. Karpov, I. Krichtafovitch, Electrohydrodynamic flow modeling using FEMLAB, in: Excerpt from the Proceedings of the COMSOL Multiphysics User's Conference 2005, 2005. Boston. [21] E.M. van Veldhuizen, W.R. Rutgers, Corona discharges: Fundamentals and diagnostics Faculty of Applied Physics, Technical University Eindhoven.
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[22] E.M. van Veldhuizen (Ed.), Electrical Discharges for Environmental Purposes: Fundamentals and Applications, Nova Science Publishers, New York, 1999, ISBN 1-56072-743-8, p. 420. [23] Brian A. Tinsley, Fangqun Yu, Atmospheric ionization and Clouds as Links between Solar Activity and Climate.
[24] European Patent Application, EP 1652423A1, Method for breaking circulation and device for carrying out said method. May 2006. [25] United States Patent, 3,717,148, AEROIONIZER-Feb.20, 1973. [26] F.W. Peek, Dielectric Phenomena in High Voltage Engineering, McGraw-Hill, 1929.
Contents lists available at ScienceDirect
Journal of Electrostatics journal homepage: www.elsevier.com/locate/elstat
Modelling of ions for seeding technique to electrify the atmosphere N.A. Doshi a, *, S.D. Agashe b a b
Department of Electronics and Telecommunication, College of Engineering, Malegaon (BK), Baramati 413102, India Department of Instrumentation, College of Engineering, Pune 413002, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 January 2014 Received in revised form 20 November 2014 Accepted 18 February 2015 Available online 3 March 2015
There are number of ways in which weak electrification can affect the microphysics of clouds, with consequences for cloud lifetime, radiative properties, and precipitation efficiency. Kauffman [2011] suggested ions produced by direct current generators will add to and enhance the catalysing effects that cosmic ray ions are now known to produce in among other things, lowering nucleation barriers, stimulating charged particle growth and stability and increasing the scavenging rate in clouds. Thus to electrify the atmosphere ions can be generated artificially in abundance along with large electric field. Ions can be generated by the corona effect using Atmospheric electrifiers (a device used to generate negative ions) which makes use of corona discharge phenomenon to charge the air particles. Exact assessment of electric field and charge density distributions and the flow dynamics inside the electrifiers is essential to understand the particle behaviour inside the electrifiers. In this paper, a novel model of governing equations to evaluate the space charge density, electric field intensity and velocity of ionized airflow is suggested as a function of applied voltage. The Poisson and charge conservation equations are derived and hence can be used to estimate the electric field and charge density distributions. Navier stokes equation can be used to get the velocity of ionized airflow because of electric force on the air. Simulation is carried out to validate the proposed model and verify that velocity is function of input voltage and is proportional to it. © 2015 Elsevier B.V. All rights reserved.
Keywords: Atmospheric electrification Negative ions Corona discharge EHD flow Ionized wind Artificial rain
Introduction Svensmark and Friis-Christensen [1997] and Svensmark [1998] demonstrated correlations of cloud cover with Galactic cosmic rays (GCR) flux, and speculated that ionization processes could affect nucleation or the phase transitions of water vapour. Corona effect ions may have a role in catalysing atmospheric phenomena as suggested by R.G. Harrison and K.S. Carslaw in [2003].S.D Pawar and Kamra [2009] have shown that the space charge released into the atmosphere by corona currents from the ground can increase the air-conductivity of the atmosphere near to earth surface by more than an order of magnitude during the dissipation stage of thunderstorm. The overall process of natural electrification of atmosphere which is responsible for coalescence and precipitation is summarized [3e11,23] in Fig. 1. There are positive ions and free electrons present in the air as a result of “background radiation” or photon
* Corresponding author. Tel.: þ91 996 024 6549; fax: þ91 02112254424. E-mail address: [email protected] (N.A. Doshi). http://dx.doi.org/10.1016/j.elstat.2015.02.006 0304-3886/© 2015 Elsevier B.V. All rights reserved.
excitation occupying the atmosphere. This natural ionization is emitted from artificial and natural sources, such as radioactive elements and cosmic rays. The majority of the ionization is caused by cosmic radiation where the electric field is high enough. Radon gas released from the Earth's crust can cause radioactive ions to attach to airborne dust and other particles to form aerosols by nucleation. Attached aerosols coagulate to form condensation nuclei which further condense to give cloud condensation nuclei to form cloud. Similar process can be initiated and induced artificially by generating abundance of corona effect negative ions in the atmosphere. Small ions are produced continuously in the nature. But because of pollution, global warming, the so called modern life style of human beings is responsible and also due to other phenomena; adverse forces tend to destroy these ions as a result of which many natural processes are affected in the atmosphere. The major impact of this is on the natural rainfall hence Ion losses is also the major issue. Thus the actual percent-concentration of the electrons and ions in the free space can be disregarded for this study. The electron avalanche effect because of the artificial ions will ensure an exponential increase of electron generation, which allows for ionization to occur.
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Fig. 1. Role of corona effect ions in catalysing the atmosphere [suggested by R.G. Harrison and K.S. Carslaw in 2003].
When high density uni-polar ions are added in the atmosphere the recombination losses are negligible. As after recombination's with available opposite polarity ions, high density ions act as seed for further reactions with dust particles and droplet coalescence processes. The injection of a large number of DC corona effect ions will induce changes in cloud microphysics and cloud cover and, consequently modifications in weather conditions. It is expected that DC generated ions are going to be a more aggressive catalyser than cosmic rays as corona effect ions are hygroscopic and grow rapidly with increased humidity [1].
Corona effect to generate negative ions Direct current corona effect ionization starts to occur at ground level. The net effect of ionization will be to charge pre-existing aerosol or form new charged aerosol. Ions produced by direct current generators will add to and enhance the catalysing effects that cosmic ray ions are now known to produce in, among other things, lowering nucleation barriers, stimulating charged particle growth and stability and increasing the scavenging rate in clouds [1]. Thus modelling the corona ions is important as “clouds can be seeded by ions to produce rain from ground station by electrifying the atmosphere”. Negative ions are generated using corona discharge phenomenon. Corona discharges are relatively low power electrical discharges that take place at or near atmospheric pressure. The corona is invariably generated by strong electric fields using small diameter electrodes [2] (ion generator and shield electrode). When a negative high voltage exceeding the potential at which corona is found to originate is called corona threshold voltage. When this voltage is applied across an asymmetric geometric model with very small curvature ion generator electrode; because of corona effect the negative ions are generated.
Modelling of an ion The movement of air particles as a result of the electro hydrodynamic (EHD) phenomenon can be explained through the use of basic physics, including Coulomb's Law of Electrostatics, Conservation of Momentum, and Newton's Third Law. In order to model the ions for seeding technique to electrify the atmosphere, the governing equations necessary to represent EHD flow induced by corona discharge are described in Section 3.1.
Electrostatic equations The presence of corona phenomena complicates the mathematical models used to simulate the impact of the electric field and current density distributions in inter-electrode space. Hence a simplified model for analysis of direct current corona field and induced electro hydrodynamic airflow field in atmospheric electrifier system is presented.
Modelling assumptions The operating voltage range for corona discharge lies between the corona threshold voltage and the air gap breakdown voltage. Corona induced airflow is possible. The gap between ion generator electrodes and grounded electrode can be divided into two regions, the ionization and drift zones. When the ion generator electrode voltage is above the corona onset level, the corona discharge generates the EHD flow. When the radius of the ion generator electrode is much smaller than the distance between ion generator electrode and grounded electrodes, the ionization zone forms a uniform sheath over the coronating region of the ion generator electrode surface. The ionization zone exists in close proximity to the ion generator electrode, in which air ionization occurs, and both positive and negative ions exist. The drift region, located outside the ionization zone, contains ions of a single polarity that have been driven out of the ionization region by the electric field. Columbic force interactions between the ions and the electric field are responsible for ion acceleration. A simplified model of corona discharge is assumed in this study where the following apply [12e16]: 1. The mono polar corona discharge inside the atmospheric electrifiers is modelled under- All quantities are steady in time and - All ions and particulates are neglected except for those generated by corona discharge. 2. The ion mobility is constant and independent of the electric field's influence. 3. The corona is a discharge where ionization is non-thermal hence thermal diffusion of the ions is neglected. 4. When the flow is at steady-state, space charge density does not change with respect to time. 5. Under standard atmospheric pressure, the EHD flow has a negligible effect on the corona discharge, because the air velocity is much smaller than the velocity of moving ions.
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6. The electric field and charge transport in the corona simulation are independent of the gas flow. 7. As the outlet velocity of the ionized airflow is in the order of a few metres per second, the ions drift is usually much large and dominates, hence one-way coupling affectively takes place as for decoupled model, due to one-way coupling (electric field causes gas motion, but motion of air doesn't affect the corona discharge parameters. The velocity of the natural airflowing inside the atmospheric electrifier is assumed to be very small in magnitude in the order of 102 m/s) it is the end of the calculation. 8. The passage of current between the electrodes is the result of two simultaneous effects: Movement of charge relative to the main body of gas and transport of charge by, rather than through, the gas stream. 9. The design feature leads to the distribution of electric fields and design of electrode shapes most suitable for producing the airflow in the desired direction. 10. The effects of material and surface conditions on the electron distribution could not be explicitly illustrated by the model because the dependence of the yield of photo electrons on electrode material and surface conditions has not been quantified. Instead, the model uses total corona current to specify the electron number density at the outer boundary of the corona plasma. When high voltage is applied to the atmospheric electrifier system, a space charge is formed, and a continuous electric current flows between the two electrodes. Both, applied voltage and space charge, will contribute to the generation of electric field between electrodes. The space charge density (r) and electric field strength (E) distributions in domain between electrodes are essential for calculating particle charging and trajectory. The solution of r and E enables the prediction of the voltageecurrent (VI) relations as well as ionized airflow characteristics. The displacement current is zero as charge density is not varying with time. The electric field intensity (V/m) between the electrodes can be described by Gauss's law
V$E ¼ r=ε0
(1)
where r is the space charge density (C/m3), V is the scalar electric potential (kV), and ε0 is the permittivity of free space (8.854 1012 C/V m).The impressed voltage V is equal to the difference in potential across the two electrodes. The electric potential anywhere in the system where no net accumulation takes place is defined from electric field intensity E as
VV ¼ E
(2)
Substituting (2) with (1), we obtain Poisson's equation, which is defined as
V$VV ¼ r=ε0
(3)
The drift layer has a combination of conduction (motion of ions under electric field relative to entire airflow), convection (transport of charges with airflow), and diffusion. The ion diffusion is of negligible importance compared to conduction. Because the gradient in the electron number density distribution is small and the ion diffusion current is neglected, the velocity of ions diffusion is much smaller than that caused by the effect of electric field and wind flow. Convection term is also negligible as without the external force there will be no charge carriers. Hence only the conduction term is dominant in the system, Equation (4) simplifies to
J ¼ mE rE
V$J ¼ 0
(6)
After combining Equations (5) and (6), substituting for the electric field with potential (2), expanding the divergence, and then substituting Poisson's Equation (3) with the result, the charge transport equation can be derived to be
V$ðmE rVVÞ ¼ Vr$VV þ rV$VV ¼ 0 . Vr$VV ¼ r2 ε0
(7)
This resulting equation is a dot product between the vector space charge density and scalar electric potential. The electric problem of corona discharge is governed by a set of two partial differential equations: Poisson's Equation (3) with unknown potential, V, and the charge transport Equation (7) with the unknown space charge density, r. These equations can be solved by using many numerical methods. To get the values of r and V, the initial value of the space charge density can be adjusted such that the electric field strength will be equal to the breakdown electric field strength in air ¼ 3.23 103 kV/m in Equation (7) to get the value of V. This value of V can be then substituted in Equation (3) to check its validity and then the value of r can be iterated like wise to satisfy (3) and get the final value of V. Fluid dynamics equations
J ¼ mE Er þ UreDr
Modelling assumptions. The fluid which passes through the atmospheric air which gets ionized because of columbic force. made are [18,8]: The fluid has constant density and cosity, small temperature variations are there
where J-is the ionic current density, U- is the velocity vector of airflow, D-is the charge diffusion coefficients of ions (5.3 105 m2/ s), mE - is ion mobility of negative ions (2.7x10-4 m2/V s).
(5)
Under steady state conditions, the current density must satisfy the charge conservation equation or current continuity equation. Since the charges are indestructible, if there is an in flow of charges in some part of the surface of a conductor, then there would be an equivalent outflow elsewhere or there would be an accumulation of charge upon the conductor. Under steady e state conditions ruling out the possibility of charge accumulation, the total in flow of the current to any conductor must equal the total outflow, a condition must be true for any part of conductor. The electric current does not change with time; the charge density is also independent of time. Under this condition, the current density vector has no source, and the charges cannot be created at any point in a steady current flow [2]. Current continuity condition gives equation for current density
This Equation (3) relates the electric potential to the charge density and governs the electric field of the system. The ionic charges are accelerated by the Coulomb force and move towards the grounded electrode. The charge drift creates an electric current with a density defined by the current density equation.
(4)
21
EHD flow is a particular domain of electro-hydrodynamics, which involves the study of corona discharge asymmetric geometry model for the purpose of particle movement in fluid mediums.
Electrifiers is Assumptions constant visi.e. uniform
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temperature where diffusion can be neglected, is a single phase fluid and hence rules out the possibility of sound or shock waves to occur, small pressure drop is there and hence is assumed to be incompressible Newtonian fluid. The continuity equation for such fluid reduces to
V$U ¼ 0
(8)
The conservation of momentum equation also known as the NaviereStokes equations (NSE) is a nonlinear PDE written for Newtonian fluids. Studying such fluids is simple because the viscosity model ends up being linear. As the ion generator electrode with the ionization zone cross section area is negligible in comparison with an overall area of the field, the ion generator electrode can be represented as a point. Coulomb force acting on the ions becomes an electric body force on the air molecules and gives rise to the EHD flow. This Coulomb force is the main contribution to the ionic airflow that causes the system to generate ionic wind. Actually the force due to EHD is the sum of contributions due to the permittivity of gradient force, dielectrophoretic force, and electrocostriction force. But as the air is a single phase fluid and the motion is not due to polarisation, the contribution due to these forces is not considered. The high frequency of collisions between the space charge and air bulk can be theoretically estimated as a full transfer of momentum, which contributes to the resultant electrical body force responsible for EHD flow. Considering the body force Fe on fluid motion, due to steady state electrostatics the NSE can be expressed in the simplified form as:
rair U$VU ¼ Vp þ mair V2 U þ Fe
(9)
where rair is the air density (1.23 kg/m3), p is the static air pressure (in Pascal), and mair is the air dynamic viscosity (1.8205 105 N/ m2). The LHS of Equation (9) represents the acceleration term and the RHS represents the different acting forces. Considering the above assumptions, the flow field is laminar and hence these basic governing equations for the conservation of the mass and momentum are sufficient. The mean field approximation of the electrostatic potential is described by the Poisson equation, which relates the electrical potential to the charge density in this simplified form.
V$ðε0 VVÞ ¼ r
(10)
The electrostatic body force density Fe as a function of the mean electrical potential (V) using the Poisson Equation (3) and Equation (2) can be expressed as function of applied voltage.
Fe ¼ rE ¼ rVV
(11)
r ¼ ε0 V2 V
(12)
Fe ¼ ε0 V2 V VV
(13)
Poisson's equation for electric potential is to be solved only inside the drifting zone with prescribed voltage on external boundary of ionization zone. Thus values of space charge density and electric field intensity obtained from Equations (7) and (3) will be used to find the body force from Equation (11). Equation (13) shows that the body electric force and the input voltage are linearly proportional, which in turn when substituted in Equation (9), gives the unknown value of velocity of the airflow. The numerical simulation of the proposed model can be used to predict the effect of electric corona discharge on the generation of ions and in turn on the airflow. All distributed and global
parameters, including both the electric field and the flow, can be calculated. Procedure of numerical analysis of the field may consist of two stages. The first stage would comprise of the numerical solution of the Poisson and charge conservation equations describing the direct-current corona discharge. The results of this stage then would be used for computation of the components of Coulomb force in each of the node of computational grid. The second stage can perform computation of electro hydrodynamic flow field by use of NaviereStokes and flow continuity equations to give the value of velocity proportional to the input applied voltage. Thus, the studied EHD flow model described by the system of Equations (3), (7), (9) and (13), with the appropriate boundary conditions along with the numerical simulation can be used as inputs for ion seeding to electrify the atmosphere.
Simulation Researchers have tried to solve problems long before for different applications. Several of them invoke a commercial software packages with finite element method FEMLAB, COMSOL or ANSYS [19,20] the other calculate with their made programs using numerical or analytical methods. It is clear from their results that in each corona discharge electrode system electric fields are quite different. It must be simulated separately to each application of corona discharge. 2-D model of Electrifier with two electrodes namely ion generator and shield electrode for analysis of direct current corona field and induced EHD airflow field is presented. To evaluate the values of the Space charge and voltage field following boundary conditions are set as shown in Fig. 2; Zero volts is applied to the shield electrode, variable negative voltage from -5 kV to 40 kV is applied to the ion generator electrode, no slip boundaries are used for all solid surfaces. The charge transport Equation (7) is also only applied to the drift layer. Two electrodes act as stationary electrodes where the components of the velocity vectors vanish. The outside boundary is defined with a pressure inlet and outlet with both inlet and outlet pressures equal to zero and outlet pressure in the direction of the back flow normal to the boundary. This means that the air is free to flow in both directions. The computation air space is set to an “open boundary,” which implies a zero normal stress (N/m2) and pressure component. Zero charge boundary condition at the exterior boundaries and at the interior boundaries; it means that no displacement field can penetrate the boundary and that the electric potential is discontinuous across the boundary is considered. No flux condition is boundary condition on the exterior boundaries, represents boundaries where no mass flows in or out of the boundary; such that the total flux is zero. A zero diffusive flux is imposed on all boundaries except the surface of the ionization layer because the diffusion term does not affect the outflow boundaries. Velocity component other than the inlet and outlet i.e. u and v are zero.At the inlet the velocity is assumed to be 0.01 m/s. The simulations are run using COMSOL Multi physics 4.2 software on a Intel(R) X5650 with Dual 2.8 GHz Quad-core Xeon processors and 64 GB of RAM. The electrostatics module, NaviereStokes module, and a generalized PDE coefficient form modules are used to represent the derived governing equations. Due to the generalized structure of COMSOL, the generalized PDE coefficient module are modified and used for the charge transport equation. A single 2-D electrifier cross-section Fig. 2 is modelled. The electrostatic module, PDE coefficient module for the charge transport equation, and NaviereStokes module were applied separately to the 2-D Electrifier model. The variables in each of these modules are heavily dependent and need to be solved separately before they are linked.
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P=0
Outlet
No Flux ,no slip Ion generator electrode (r=0.5mm) (-20kV)
50mm
Shield electrode (r=1mm) (0V)
V=0
Air inlet Input velocity =0.01m/s Fig. 2. Boundary conditions.
Radius of ion generator electrode is 0.5 mm and of shield electrode is 1 mm. The distance between electrodes is varied to find the safe distance. The ion generator voltage is varied to find combination at which maximum velocity is reached. Fig. 3(a) and (b) shows the potential and velocity distributions for a 2-D model. With a 20 kV applied voltage to the ion generator and the shield at
23
ground, a smooth potential distribution is found in the majority of the air gap except near the electrodes. Notice that the graph does not reach the maximum 20 kV voltage because corona discharge occurs before this point and maintains the onset voltage (Vc) value along the external surface of the ionization layer. The velocity has a peak within the air gap and causes an electrical force that pushes the negative ions upwards. The calculation for this value influenced by the electric field never exceeds the electric field intensity for air breakdown [26]. Fig. 3(b) shows the airflow velocity distribution as a result of the NaviereStokes and charge transport modules. The airflow modelled in this simulation is mainly driven by the Coulomb force. The transfer of momentum increases the airflow and peaks at the edges of the ion generator electrode. The Coulomb force distribution is a product of the electric field and space charge density. The Coulomb force drives the air motion and is seen to be greatest between the ion generator and shield. The total velocity in this system is a direct result of the airflow caused by the Coulomb force. Simulation result A graph of output velocity versus ion generator voltage for fixed safe distance form the result of the simulation is plotted as seen in Fig. 4 the voltage is varied in the steps of 2e3 kV. The simulations are carried out first to find safe distance (before
Fig. 3. Simulation results (a) Electric potential (b) Velocity distribution.
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2.
Fig. 4. Simulation result for change in output velocity w.r.t. input voltage.
electric arc) between the electrodes. Well above the safe distance, i.e. at a distance of 50 mm between the electrodes, the velocity is seen to be increasing with the applied ion generator voltage which validates the model's output (i.e. voltage -20 kV- velocity -1.286 m/ s). Thus by increasing the size of the electrodes; the safe distance would also increase, the voltage to be applied would increase and finally the output velocity would increase. To electrify the atmosphere the velocity of the ionized airflow must be greater than 4 m/s [17] as this is maximum velocity of the horizontal winds which should be overcome. Depending upon the electrode radius chosen, the ion generator voltage can be accordingly changed to achieve the electric field intensity [26] and space charge density and thus the desired velocity.
3.
4.
5.
Development of ionic wind to electrify the atmosphere The ionized airflow/wind to electrify the atmosphere because of negative corona ions is treated in more detail. Many processes continue to occur after the corona discharge takes place. The ones that are often important for further physical and chemical processes are [21,22]: 1. An electron excited by an electric field E will ionize an atom or molecule of a gas as it has enough energy, called the ionizing potential of the particular gas. This implies that a free electron gains on average several eV of energy from the field in-between collisions. There will be electrons in the tail of the electron
6.
energy distribution function that have enough energy to cause ionization. Outside the ionization zone, the negative ions collide in the route with neutral air molecules. During these collisions momentum is transferred from ions to neutral molecules resulting in a bulk motion of the gas. Attachment is the formation of negative ions when low energy electrons combine with atoms or molecules. Not all particles form negative ions, examples are the noble gases and also nitrogen molecules. Some molecules that easily form negative ions are O2, H2O and CO2. The conductivity of air is strongly influenced by the capturing of low energy electrons: the light and mobile electrons are replaced by heavy ions. Therefore the field strength for sustaining a certain current is much higher in an electronegative gas. Recombination of negative ions leads back to the neutral gas as before the discharge pulse. The active particles in a corona discharge are formed in a thin channel. It is the intention to treat the whole gas volume. The EHD flow is the result of the interaction between drifting ions and the surrounding air molecules. Both electric field and fluid flow are present and interacting with each other. The high-energy electrons in the streamer head cause ionization and excitation to higher electronic states. Excited metastable states cannot lose their energy by emission of a photon. Another process to take away energy from them is by collisional quenching. It is also possible that they transfer all their energy to a different molecule. In this way quenching of a N2 metastable can cause dissociation of a water molecule. This process is considered to give an important contribution to the formation of OH radicals in humid air. Radicals are formed by electron impact dissociation of molecules in the streamer head region. The dissociation energy is usually somewhat lower than the ionization energy. Primary radicals are the ones created directly by these collisions, e.g. O, H, OH and N. They may react rapidly with molecules to form secondary radicals such as HO2 or O3. The applied electric field provides the driving force for the movement of these charged particles towards the lower potential electrode and thus generates the ionic wind i.e. ionised airflow, the velocity of which is linearly proportional to the amplitude of the applied voltages before corona is arrived at the state of burst-pulse streams [9,13]. This ionic wind occurs from the corona threshold voltage of corona discharge, and the airflow is accelerated from the tip of ion generator electrode. As a consequence, the produced ionic wind velocity is linearly
Fig. 5. Schematic illustrating GCReCNeCCN-Cloud hypothesis that if confirmed, might explain the correlation between variations of GCR flux and low cloud cover. The possible dominating species involved in the different phases of CN formation and growth processes are also indicated. The organics species and water molecules may play an important role in growing the CN into the size of CCN. (From Yu, 2002).
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controlled over the range and provides a steady uniform flow of ionic wind and would effectively be used for artificial electrification of atmosphere.
Effect of atmospheric electrification on the formation of artificial raindrop When a sufficient electric potential is supplied to the ion generator electrode it begins to display corona effect thus emitting electrons. The grounded shield electrode provides a stable outflow of the electrons emitted, thus maintaining the maximum possible level of emission from the ion generator electrode. The majority of the electrons emitted by the ion generator electrode and repelled by the negative background potential of the earth surface tend to rise upwards into the atmosphere forming a stable flow [25]. The growing of the negative ion into a raindrop is stepwise process as shown in Fig. 4. It has been demonstrated recently that air ions may play an important role in the production of new particles under typical tropospheric conditions [Yu and Turco, 2000. 2001]. When electrons combine with the air molecules, negatively charged ions are formed, which immediately begins to attract groups of molecules of the atmospheric water vapour. Essentially, the production of H2SO4 in the gas phase by photochemical reactions in the atmosphere results in supersaturated H2SO4 vapour. There is a competition between deposition of H2SO4 on pre-existing aerosols and on the ions. The competition results in a complicated dependence of the nucleation rates on the concentrations of pre-existing aerosols, ions, and H2SO4 vapour. The presence of water molecules, H2SO4, HNO3, **NH3 and other similar gases accelerate the flow of the ionized molecular complexes formed in the atmosphere. The charges molecular clusters, and thus can preferentially achieve stable, observable sizes. The proposed ion-mediated nucleation (IMN-((H₂SO₄)n and (H₂O)m ion))theory as shown if Fig. 5 can physically explain the enhanced growth rate (a factor of ~10) of sub-nanometre clusters as observed by Weber et al., [1997]. An increase in precipitation entails a decrease in the amount of cloud water that is available for evaporation into air being entrained into the air mass of a storm. The reduction of this diabatic (non-adiabatic) cooling is equivalent to a diabatic heating, causing the adiabatic volume expansion of the air surrounding the said complexes, where upon the air rises upwards while cooling down, and the water vapour contained in the air condenses under appropriate conditions. The low-pressure areas receive new portions of the surrounding air, which are also ionized by the maintained ion flow and diabatic heating rise upwards [24]. Since the direction of the ion flow is defined by the charge of the earth surface and the direction of the air stream is defined by the atmospheric pressure gradient, the overall direction of air stream will be substantially vertical under favourable conditions (e.g. in the absence of strong horizontal wind, this being typical for anticyclone areas) [25]. Thus, a stable upwards ionized air stream (or “current”) is formed in the atmosphere which is responsible for the rainfall. Conclusion There is a great deal of modelling that is needed in order to provide quantitative relationships between atmospheric electrification and ionization. However, proposed models of the electrostatics to govern corona ions and fluid mechanics for determining velocity of airflow to understand the effect of corona effects on the air in the atmosphere will be useful for validating the present observational results. It will provide accurate inputs into electrification of atmosphere.
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A numerical model along with governing equations for generating negative ions using an Electrifier is developed to evaluate the value of the space charge density, electric field intensity and ionized airflow velocity parameters. It is validated by simulation results. The Electrifier generates ions, which ionizes the air particles and gives output velocity to the airflow equal to 1.286 m/s for 20 kV with input velocity of 0.01 m/s. These negative ions can be used as seeds to electrify the atmosphere. From the model and the simulation results it is seen that the velocity of the ionized airflow is proportional to the applied voltage. Nucleation because of these ions helps the growing of the negative ions into the raindrop. Dc corona ions generated artificially function in a similar way as the ions which are generated by the cosmic ray and other natural resources. Artificial ions are in fact more powerful catalysers and hygroscopic than the natural ions. The particle flux changes directly affect the atmospheric concentration of ions, and indirectly affect the density of space charge in the troposphere. The changes in space charge and/or ion concentration are likely to affect properties of atmosphere to get artificial rainfall by electrifying the atmosphere. References [1] Kaufman, United States Patent Application publication, U. S. 2011/7,965,488 B2, A1eApparatus and Related Methods for Weather Modification by Electrical Processes in the Atmosphere, Jul.21, 2011. [2] Jen-shih Chang, Arnold J. Kelly, Joseph M. Crowley, Handbook of Electrostatics processes. [3] H. Svensmark, E. Friis-Christensen, Variation of cosmic ray flux and global cloud coverage, J. Atmos. Terr. Phys. 59 (1997) 1225e1240. [4] R.,G. Harrison, in: Atmospheric Electricity and Cloud Microphysics; Proceedings of the Workshop on Atmospheric Science, 2001. [5] K.S. Carslaw, R.G. Harrison, J. Kirkby, Cosmic rays, clouds and Climate, Sci. Rev. Geophys. 298 (2002) 1732e1737. [6] R.G. Harrison, K.S. Carslaw, Ion-aerosol-cloud processes in the lower atmosphere, Rev. Geophys. 41 (3) (2003) 1012e1027. [7] Hirsikko, T. Nieminen, S. Gagne, K. Lehtipalo, H.E. Manninen, M. Ehn, U. Heorrak, V.-M. Kerminen, L. Laakso, P.H. McMurry, A. Mirme, S. Mirme, €ja €1, H. Tammet, Vakkari, M. Vana, M. Kulmala, Atmospheric ions and T. Peta nucleation: a review of observations, Atmos. Chem. Phys. 11 (2011) 767e798. [8] Blair, Weather Elements a Text in Elementary Metrology, March 1994, 12print. [9] K.A. Nicoll Understanding the Electrical interactions between ions, aerosols and clouds. [10] Yu Nadykto, Uptake of neutral vapor molecules by charged clusters/particles: enhancement due to dipole-charge interaction, J. Geophys. Res. 108 (D23) (2003) 4717e4723. [11] S.D. Pawar, P. Murugavel, V. Gopalakrishnan, New particle formation by ioninduced nucleation during dissipation stage of thunderstorm, J. Earth Syst. Sci. 120 (5) (October 2011) 843e850. [12] Jack Wilson, Hugh D. Prekins, Willam K. Thompson, An investigation of ionic Wind Propulsion, NASA/TM- 2009-215822. [13] Lin Zhao, Kazimierz Adamiak, EHD flow produced by Electric Corona Discharge (Numerical and Experimental Studies, and Applications). [14] C. Kim, K.C. Noh, J. Hwang, Numerical investigation of corona plasma region in negative wire-to-duct corona discharge, Aerosol Air Qual. Res. 10 (2010) 446e455, http://dx.doi.org/10.4209/aaqr.2010.03.0019. Copyright © Taiwan Association for Aerosol Research ISSN: 1680-8584 print/2071-1409 online. [15] L. Zhao, K. Adamiak, Numerical simulation of the electrohydrodynamic flow in a single wireeplate electrostatic precipitator, IEEE Trans. Ind. Appl. 44 (3) (2008) 683e691. [16] Y.N. Chun, J. Chang, A.A. Berezin, J. Mizeraczyk, Numerical modeling of near corona wire electrohydrodynanmic flow in a wire-plate electrostatic precipitator, IEEE Trans. Dielectr. Electr. Insul. 14 (1) (February 2007) 119e124. [17] United States Patent Application Publication, US 2011/0174892 A1-Apparatus and related methods for weather modification by electrical processes in the atmosphere, Jul.21, 2011. [18] A. Castellanos, Coulomb-driven convection in electro hydrodynamics, IEEE Trans. Electr. Insul. 26 (6) (Dec. 1991) 1201e1215. [19] B. Benamar, E. Favre, A. Donnot, M.O. Rigo, Finite element solution for ionized fields in DC electrostatic precipitator, in: Excerpt from the Proceedings of The COMSOL Users Conference, 2007 (Grenoble). [20] S. Karpov, I. Krichtafovitch, Electrohydrodynamic flow modeling using FEMLAB, in: Excerpt from the Proceedings of the COMSOL Multiphysics User's Conference 2005, 2005. Boston. [21] E.M. van Veldhuizen, W.R. Rutgers, Corona discharges: Fundamentals and diagnostics Faculty of Applied Physics, Technical University Eindhoven.
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[22] E.M. van Veldhuizen (Ed.), Electrical Discharges for Environmental Purposes: Fundamentals and Applications, Nova Science Publishers, New York, 1999, ISBN 1-56072-743-8, p. 420. [23] Brian A. Tinsley, Fangqun Yu, Atmospheric ionization and Clouds as Links between Solar Activity and Climate.
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