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Design and development of a particulate emission monitor

Gerazounis, Stylianos; (2000) Design and development of a particulate emission monitor. Doctoral thesis (Ph.D), UCL (University College London). Green open access

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Abstract

In the last two decades, numerous studies have revealed that atmospheric particulates and especially those emitted by diesel engined vehicles pose a serious health and environmental hazard. This thesis describes the design and development of a co-axial capacitance transducer as well as the ancillary solids dispersion production unit for the on-line measurement of particulates concentration in air in real time basis. The primary application of the device is as a particulate monitor for diesel engine exhausts although the reported experimental results also evaluate and establish its feasibility for monitoring solids/gas dispersions during their pneumatic conveying. Briefly, the transducer comprises two different diameter metallic cylindrical electrodes placed co-axially within one another so that an annulus is formed. The latter constitutes the sensing volume of the capacitance transducer following the application of a voltage between the two electrodes. The principle of the operation of the device relies on the fact that the effective dielectric constant of a solids-gas dispersion driven between the electrodes is proportional to the concentration of the entrained solids. In practice therefore, the concentration of a test powder is determined by measuring capacitance and referring to a previously prepared calibration chart. The feasibility and reliability of the transducer have been verified by conducting a series of experiments investigating its performance characteristics in response to changes in a number of design and operating parameters in conjunction with different powders of various size, density, and electrical properties. The design parameters investigated include variations in electrode diameters, length and separation distance. The various operating parameters on the other hand deal with changes in air relative humidity (8 - 78 %), temperature (20 °C - 100 °C), flow velocity (6.5 - 15 ms-1), solids flow pattern (e.g. from homogeneous to slug flow) as well as variations in the frequency of the applied voltage (1-100 kHz). The results indicate that the transducer's sensitivity increases with a decrease in the separation distance between the two cylindrical electrodes, whereas the electrodes' length has no profound effect on it. On the other hand, the effective dielectric constant, ϵeff of all solids-gas dispersions tested was found to be directly proportional to the solids concentration and unaffected by variations in air humidity, air velocity, electric field frequency, and solids flow regime. However, ϵeff for dispersions of insulating powders, in contrast to that of conducting powders, was found to be dependent on the respective dielectric constant of the solid particles as well as their size. Furthermore, in the case of mixtures of two insulating powders simultaneously dispersed in air, ϵeff was found to be dependent not only to the total solids concentration but also on the volumetric ratio of the two powders in the mixture. The transducer's baseline capacitance (zero solids concentration) varied linearly with the average surface temperature of the cylindrical electrodes. Finally, a 'temperature capacitor coefficient' was calculated in order to account for the effect of temperature on capacitance. This was found to be in close agreement with the coefficient of thermal expansion of the electrodes' material of construction (c.f. 0.0001/°C with 0.00012/°C).

Type: Thesis (Doctoral)
Qualification: Ph.D
Title: Design and development of a particulate emission monitor
Open access status: An open access version is available from UCL Discovery
Language: English
Additional information: Thesis digitised by ProQuest.
Keywords: Earth sciences; Particulate matter
URI: https://discovery.ucl.ac.uk/id/eprint/10097926
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