Shirshak K. Dhali

Two‐dimensional studies of streamers in gases

We present the results of two‐dimensional computer simulations of streamer initiation and propagation in atmospheric pressure N2. The simulation algorithm makes use of flux‐corrected transport techniques and was used as a tool to study the solutions of the transport equations under conditions suitable for streamers, for which realistic analytic solutions are not known. We present and discuss conclusions about streamer transport based on the results of these studies. Finally, we present a novel method of checking on the numerical accuracy with which the algorithm solves the transport equations.

Dielectric‐barrier discharge for processing of SO2/NOx

We report the results of an investigation of plasma assisted oxidation of SO2 to SO3. The SO3 is a more desirable byproduct of flue gas as opposed to SO2 because it can be dissolved in water easily and can be reacted with lime to form gypsum. A coaxial dielectric‐barrier discharge is used to generate an air plasma to carry out the reaction. The dielectric‐barrier discharge, which is an interrupted discharge, is very efficient in producing atomic oxygen and requires a simple form of excitation. We have observed a significant reduction (over 75%) in the SO2 concentration using this technique. The simulation results indicate that about 99% of the SO2 can be oxidized with UV‐irradiation along with plasma oxidation and NOx can be reduced to N2 in the presence of an air plasma. The results of varying the gas composition, the gas pressure, and the gas flow are also reported.

Non‐thermal plasma remediation of SO2/NO using a dielectric‐barrier discharge

Here we report the experimental and simulation results of removal of SO2 and NO from a simulated coal combustion gas using a dielectric‐barrier discharge. A Monte Carlo calculation is done to determine the transport properties of the flue gas. Following this, a detailed plasma chemistry calculation is done which takes into account the pulsed nature of the dielectric‐barrier discharge. The computed results are in good agreement with the experimental data. We find that in low concentrations (400 ppm for SO2 and 100 ppm for NO) the dielectric‐barrier discharge was capable of removing 99% of the SO2 and NO. Due to temperature dependence of the heavy particle reactions, there is a decrease in NO removal as the energy density is increased past an optimum value.

SIMULATION OF MICRODISCHARGES IN A DIELECTRIC-BARRIER DISCHARGE

The results of simulation of microdischarges in a dielectric-barrier discharge are presented. These simulations have been carried out in two dimensions with an adaptive grid. The results presented are for oxygen at atmospheric pressure. The dielectric has little effect on the propagation of the streamerlike discharge until the very last stage. The dielectric capacitance has been found to have very little effect on the radical production efficiency.

Streamer discharge simulation in flue gas

Streamer discharges are formed in a dielectric-barrier discharge used for nonthermal plasma generation. The results of simulation of streamer type discharge in a flue gas mixture is reported. A Monte Carlo simulation is done to obtain the transport and appropriate rate coefficients. The transport and rate coefficients calculated from the Monte Carlo is used to solve the conservation equations for electron, positive and negative ions, together with the Poissons equation. The G-factor (radicals produced per 100 eV of electrical energy input to the discharge) obtained for Townsend-type discharge is higher as compared to a streamer-type discharge. Also experimental results of the SO/sub 2/ removal efficiency is compared to theoretical predictions. >

Plasma oxidation of SO2

We report the results of an experimental investigation of plasma‐assisted oxidation of SO2 to SO3. A coaxial dielectric‐barrier discharge is used to generate atomic oxygen to carry out the reaction. The dielectric‐barrier discharge, which is an interrupted discharge, is very efficient in producing atomic oxygen and requires a simple form of excitation. We have observed a significant reduction (up to 90%) in the SO2 concentration using this technique. The results of varying the gas composition, the gas pressure, and the gas flow are also reported.

Electrical characteristics of a coaxial dielectric barrier discharge

The dielectric barrier is becoming popular as a source of atmospheric pressure non-thermal plasmas. For atmospheric pressure plasma processing, either coaxial or parallel plate geometry can be used. In this paper, experimental investigation of the electrical characteristics of a coaxial dielectric barrier discharge is reported. The ratio of the inner to outer electrode radius has large influence on the corona onset voltage and the generation of radicals. It was also found that low levels of ultraviolet irradiation increase the number of microdischarges considerably resulting in a more uniform discharge. Also the energy coupled to the discharge from the power source increases with ultraviolet irradiation.

Numerical simulation of streamers in SF6

The results of computer simulations of streamer development and propagation are reported in SF6. The two‐dimensional algorithm makes use of flux‐corrected transport techniques for the solution of transport equation of charged species under the action of the space‐charge electric field. This field is obtained from the solution of Poisson’s equation assuming cylindrical symmetry. The role of attachment, space‐charge field, secondary electrons, and the shape and size of the initiating charge have been determined. The results presented are for a uniform applied field‐to‐gas density ratios (E/N) of 355–435 Td for background neutral ionization densities of 104–106 cm−3. The results in SF6, which is an attaching gas, are compared with a nonattaching gas like nitrogen.

Laser-induced fluorescence of OH radicals in a dielectric barrier discharge

We discuss the results of laser-induced fluorescence measurements of OH radicals in a dielectric-barrier discharge. The discharge is in parallel plate geometry in atmospheric pressure air and argon. Although the air discharge consists of discrete microdischarges, two-dimensional images show the spatial uniformity of the OH radical. Results show that with increasing power, the OH production decreases due to gas heating and increased ozone production. The addition of O2 increases the OH production at low concentrations; however, at higher O2 concentration the OH concentration decreases due to increased electron attachment.

Optimal design of an offshore wind farm layout

The cost of offshore wind farms are about 30-60% higher than onshore wind farms of same capacity. The cost and efficiency of offshore wind farms are determined by variety of factors which include the type of electrical system (AC or DC), transmission length, transmission voltage, rated power, wind turbine type and the farm topology, and wind speed. Since the design variable space is large for such problems, computational optimization is necessary to find an optimal solution. We discuss the development of cost, loss and reliability models and the application of geometric program for optimization of the layout and configuration of an offshore wind farm. Preliminary calculations show that HVAC systems work better for short distances of the farm from the shore and the HVDC system performed better for longer distances.