The thesis describes the characterisation and application of the laser induced incandescence technique for making soot measurements in practical devices. Laser induced incandescence is the phenomenon whereby particulates such a soot absorb laser radiation and are heated to a temperature much higher than the bath gas. The broadband incandescence signal from the hot particles can be detected and the signal is proportional to volume fraction. The technique was used to study soot in partially premixed counterflow ethylene air flames, iso-octane explosion flames, and to image soot in a D. I. Diesel engine. Mie scattering, OH-LIF and absorption were used a complementary diagnostics. Appropriate ratios of LII and Mie images allowed the relative particle size and number density to be imaged. The counter flow burner measurements were used to study the effects of strain on soot formation while the bomb work demonstrated soot production in hydro-dynamically unstable cellular flames. The Diesel engine measurements are a demonstration of optical diagnostics in a real device. In order to characterise the LII signal behaviour two types of carbon aerosol generators were built. The liquid dispersion device produces almost spherical sub-micron carbon black particles. The device was used to characterise the soot field response, laser fluence response, signal decay and spectrum of the LII signal.

abstract: Electrophoretic exclusion is a counter-flow gradient focusing method that simultaneously separates and concentrates electrokinetic material at a channel entrance utilizing electric and fluid velocity fields. However, its effectiveness is heavily dependent on the non-uniform field gradients about the entrance. This work assesses the capability of electrophoretic exclusion to capture and enrich small molecules and examines the channel entrance region both quantitatively and qualitatively to better understand the separation dynamics for future design. A flow injection technique is used to experimentally evaluate electrophoretic exclusion of small molecules. Methyl violet, a cationic dye, and visible spectroscopy are used to monitor flow and electrophoretic dynamics at the entrance region resulting in successful capture and simultaneous enrichment of methyl violet at the channel interface. Investigation of the entrance region is performed using both experiment data and finite element analysis modeling to assess regional flow, electric fields, diffusion, convection, and electrophoretic migration. Longitudinal fluid velocity and electric field gradient magnitudes near the channel entrance are quantified using Particle Tracking Velocimetry (PTV) and charged fluorescent microspheres. Lateral studies using rhodamine 123 concentration monitoring agree qualitatively with simulation results indicating decreased gradient uniformity for both electric and fluid velocity fields closer to the channel wall resulting in a localized concentration enhancement at lower applied voltages than previously observed or predicted. Resolution interrogation from both a theoretical assessment and simulation construct demonstrate resolution improvement with decreased channel width and placement of an electrode directly at the interface. Simulation resolution predictions are in general agreement with early experimental assessments, both suggesting species with electrophoretic mobilities as similar as 10-9 m2/(Vs) can be separated with the current design. These studies have helped evolve the understanding of the interface region and set the foundation for further interface developments. / Dissertation/Thesis / Doctoral Dissertation Chemistry 2015

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Electrically assisted combustion has been studied in order to control or improve flame characteristics, and emphasizing efficiency and emission regulation. Many phenomenological observations have been reported on the positive impact of electric fields on flame, however there is a lack of detailed physical mechanisms for interpreting these. To clarify the effects of electric fields on flame, I have investigated flame structure, soot formation, and flow field with ionic wind electrical current responses in nonpremixed counterflow flames. The effects of direct current (DC) electric field on flame movement and flow field was also demonstrated in premixed Bunsen flames. When a DC electric field was applied to a lower nozzle, the flames moved toward the cathode side due to Lorentz force action on the positive ions, soot particles simultaneously disappeared completely and laser diagnostics was used to identify the results from the soot particles. To understand the effects of an electric field on flames, flow visualization was performed by Mie scattering to check the ionic wind effect, which is considered to play an important role in electric field assisted combustion. Results showed a bidirectional ionic wind, with a double-stagnant flow configuration, which blew from the flame (ionic source) toward both the cathode and the anode. This implies that the electric field affects strain rate and the axial location of stoichiometry, important factors in maintaining nonpremixed counterflow flames; thus, soot formation of the counterflow flame can also be affected by the electric field. In a test of premixed Bunsen flames having parallel electrodes, flame movement toward the cathode and bidirectional ionic wind were observed. Using PIV measurement it was found that a created radial velocity caused by positive ions (i.e. toward a cathode), was much faster than the velocity toward the anode. Even in a study of alternating current (AC) electric fields, bidirectional ionic wind could be observed, regardless of applied frequencies. Therefore, the effect of ionic wind cannot be considered negligible under both DC and AC electric fields. Detailed explanations for electrical current, flame behavior, and flow characteristics under various conditions are discussed herein.

Soot formation is one of the most complex phenomena in combustion science and an understanding of the underlying physico-chemical mechanisms is important. This work adopted both experimental and numerical approaches to study soot formation in laminar counterfl ow diffusion flames. As polycyclic aromatic hydrocarbons (PAHs) are the precursors of soot particles, a detailed gas-phase chemical mechanism describing PAH growth upto coronene for fuels with 1 to 4 carbon atoms was validated against laminar premixed and counter- flow diffusion fl ames. Built upon this gas-phase mechanism, a soot model was then developed to describe soot inception and surface growth. This soot model was sub- sequently used to study fuel mixing effect on soot formation in counterfl ow diffusion flames. Simulation results showed that compared to the baseline case of the ethylene flame, the doping of 5% (by volume) propane or ethane in ethylene tends to increase the soot volume fraction and number density while keeping the average soot size almost unchanged. These results are in agreement with experimental observations. Laser light extinction/scattering as well as laser induced fluorescence techniques were used to study the effect of strain rate on soot and PAH formation in counterfl ow diffusion ames. The results showed that as strain rate increased both soot volume fraction and PAH concentrations decreased. The concentrations of larger PAH were more sensitive to strain rate compared to smaller ones. The effect of CO2 addition on soot formation was also studied using similar experimental techniques. Soot loading was reduced with CO2 dilution. Subsequent numerical modeling studies were able to reproduce the experimental trend. In addition, the chemical effect of CO2 addition was analyzed using numerical data. Critical conditions for the onset of soot were systematically studied in counterfl ow diffusion ames for various gaseous hydrocarbon fuels and at different strain rates. A sooting temperature index (STI) and a sooting sensitivity index (SSI) were proposed to present the sooting tendencies of different fuels and their sensitivities to strain rates.

Propulsion engine combustor design and analysis require experimentally verified data on the chemical kinetics of limiting fuel combustion rates. Among the important data is the combustion extinction limit as measured by the maximum global strain rate on a laminar, counterflow, non-premixed flame. The extinction limit relates to the ability to maintain combustor operation, and the extinction limit data for pure fuel versus air systems provide a relative reactivity scale for use in the design of flame holders. Extinction limit data were obtained for nine fuels by means of a laminar flame experiment using an opposed jet burner (OJB). The OJB consists of two axi-symmetric tubes (for fuel and oxidizer separately), which produce a flat, disk-like, counterflow diffusion flame. This paper presents results of experiments conducted in an OJB that measured extinction limits at one atmosphere for vaporized n-heptane, the Air Force-developed fuels JP-7, and JP-10, as well as methane, ethane, ethylene, propane, butane, and hydrogen. In hypersonic aircraft development it is desirable to design a Scramjet engine that is operated on hydrocarbon fuel, particularly JP-7 due to its distinct properties. This study provides key data for JP-7, for which very limited information previously existed. The interest in n-heptane is twofold. First, it has undergone a significant amount of previous flame structure and extinction limit study. Second, n-heptane (C7H16) is a pure substance, and therefore does not vary in composition, as does JP-7, which is a variable mixture of several different hydrocarbons. These two facts allow a baseline to be established by comparing the new OJB results to those previously taken. Additionally, the existing data for n-heptane, for mixtures up to 26 mole percent in nitrogen, is extended to 100% n-heptane, reaching an asymptotic limit. Extinction limit data for the two fuels are given with a comparison to hydrogen and several other gaseous hydrocarbon fuels. Complete experimental results are included. / Master of Science

Syngas, a gaseous mixture of H2, CO and diluents such as N2, CO2, is a clean fuel generated via gasification of coal or biomass. Syngas produced via gasification typically has low calorific values due to very high dilution levels (~60% by volume). It has been recognized as an attractive energy source for stationary power generation applications. The present work focuses on experimental and numerical investigation of syngas-air counterflow diffusion flames with varying composition of syngas. Laser-based diagnostic techniques such as Particle Imaging Velocimetry, Rayleigh thermometry and Laser-induced fluorescence have been used to obtain non-intrusive measurements of local extinction strain rates, temperature, quantitative OH and NO concentrations, respectively, for three different compositions of syngas. Complementing the experiments, numerical simulations of the counterflow diffusion flame have been performed to assess the performance of five H2/CO chemical kinetic mechanisms from the literature. The first part of the work involved determination of local extinction strain rates for six H2 /CO mixtures, with H2:CO ratio varying from 1:4 to 1:1. The extinction strain rates were observed to increase from 600 sec-1 to 2400 sec-1 with increasing H2:CO ratio owing to higher diffusivity and reactivity of the H2 molecule. Numerical simulations showed few mechanisms predicting extinction conditions within 5% of the measurements for low H2:CO ratios, however, deviations of 25% were observed for higher H2 :CO ratios. Sensitivity analyses revealed that the chain branching reactions, H+O2 <=>O+OH, O+H2 <=>H+OH and the third body reaction H+O2 +M<=>HO2 +M are the key reactions affecting extinction limits for higher H2:CO mixtures. The second phase of work involved quantitative measurement of OH species concentration in the syngas-air diffusion flames at strain rates varying from 35 sec-1 to 1180 sec-1. Non-intrusive temperature measurements using Rayleigh thermometry were made in order to provide the temperature profile necessary for full quantification of the species concentrations. The [OH] is observed to show a non-monotonous trend with increasing strain rates which is attributed to the competition between the effect of increased concentrations of H2 and O2 in the reaction zone and declining flame temperatures on the overall reaction rate. Although the kinetic mechanisms successfully captured this trend, significant deviations were observed in predictions and measurements in flames with H2:CO ratios of 1:1 and 4:1, at strain rates greater than 800 sec-1 . The key reactions affecting [OH] under these conditions were found to be the same reactions identified earlier during extinction studies, thus implying a need for the refinement of their reaction-rate parameters. Significant disagreements were observed in the predictions made using the chemical kinetic mechanisms from the literature in flames with high H2 content and high strain rate. The final phase of work focused on measurement of nitric oxide (NO) species concentrations followed by a comparison with predictions using various mechanisms. NO levels as high as ~ 48 ppm were observed for flames with moderate to high H2 content and low strain rate. Quantitative reaction pathway diagrams (QRPDs) showed thermal-NO, NNH and prompt-NO pathways to be the major contributors to NO formation at low strain rates, while the NNH pathway was the dominant route for NO formation at high strain rates. The absence of an elaborate CH chemistry in some of the mechanisms has been identified as the reason for underprediction of [NO] in the low strain rate flames. Overall, the quantitative measurements reported in this work serve as a valuable reference for validation of H2/CO chemical kinetic mechanisms, and the detailed numerical studies while providing an insight to the H2:CO kinetics and reaction pathways, have identified key reactions that need further refinement.

Thesis (PhD)--Stellenbosch University, 1997 / ENGLISH ABSTRACT: An experimental investigation on gas-liquid counterflow in inclined rectangular ducts is conducted. The pressure drop across the sharp-edged gas inlet and the pressure gradient inside the duct are measured. Combinations of water, methanol, propanol, air, argon, helium and hydrogen are tested. The duct height and width are varied from 50 mm to 150 mm and 10 mm to 20 mm respectively. The emphasis is on high void fraction flow, i.e. low liquid flow rates as encountered in air-cooled reflux condensers. At low to moderate gas flow rates the pressure gradient is gas Reynolds number related while it becomes dependent on the superficial densimetric gas Froude number as the gas flow is increased. According to experiment the hydraulic diameter is the required length dimension in the gas Reynolds number while the duct height becomes the characteristic dimension in the Froude number regime. Flooding curves are generated for duct inclinations from close to the horizontal to the vertical. The data correlate in terms of the phase Froude numbers and a dimensionless liquid property parameter containing the hydraulic diameter, density, surface tension and the viscosity. The flooding gas velocity is found to be strongly dependent on the duct height, the phase densities and the duct inclination. The liquid viscosity has a stronger effect than the surface tension. Both these properties however playa secondary role. Flooding is not related to the gas Reynolds number. A theoretical model, based on the phenomenological findings of the adiabatic counterflow investigation, is derived to evaluate the performance of an air-cooled reflux condenser. Field tests are conducted on a full scale reflux condenser and the measured performance is compared to the model prediction. The reflux condenser is found to achieve only 60% of the predicted heat rejection rate due to the existence of so-called cold or dead zones. Indications are that excessive entraiment in the bottom header and the subsequent accumulation of condensate in the finned tubes causes a maldistribution of the steamside flow. In the process noncondensable gases accumulate and form dead zones, causing ineffective performance. Flooding as found in single-ducts does not appear to contribute to the formation of the dead zones. / AFRIKAANSE OPSOMMING: Die teenvloei van gas en vloeistof in reghoekige skuins buise is eksperimenteel ondersoek. Die drukverlies oor die skerp gasinlaat en die drukval in die buis is gemeet vir verskillende kombinasies van water, propanol, metanol, lug, argon, helium en waterstof. Buishoogtes en breedtes van 50 mm tot 150 mm en 10 mm tot 20 mm respektiewelik is getoets. Die klem van die ondersoek is op lae vloeistofvloeitempos soos teenwoordig tydens kondensasie van stoom in lugverkoelde teenvloeikondensors. Vir lae tot matige gasvloeitempos is die drukval afhanklik van die gas Reynolds-getal terwyl die densimetriese gas Froude-getal die heersende parameter word soos die gasvloei toeneem. Die hidrouliese diameter verteenwoordig die dimensie in die Reynolds-getal maar die buishoogte word die karakteristieke dirnensie in die Froude-getal gebied. Vloedingskurwes is vir 'n reeks van buishoeke gegenereer. Die vloedingdata korreleer in terme van die Froude-getal en 'n dimensielose parameter bestaande uit die hidrouliese diameter, oppervlakspanning, vloeistofdigtheid en die vloeistofviskositeit. Die vloeidingsnelheid is primêr van die buishoogte, vloeierdigthede en die buishoek afhanklik. Die vloeistofviskositeit-effek is sterker as die van die oppervlakspanning. Beide die eienskappe speel egter 'n sêkondere rol. Die gas Reynolds-getal beïnvloed nie die vloeidingsproses nie. Die fundamentele bevindinge van die teenvloeiondersoek is toegepas om die werkverigting van 'n lugverkoelde teenvloeikondenser teoreties te modelleer. Werkverigtingstoetse is uitgevoer op 'n volskaal teenvloeikondenser. Die toetsresultate word vergelyk met die teoretiese voorspelling. Die teenvloeikondensor behaal slegs sowat 600% van die voorspelde warmteoordrag omdat van die gevinde buise gedeeltelik by omgewingstemperatuur is. Hierdie verskynsel heet koue of dooie sones. Dit blyk dat die kondensaat in die onderste spruitstuk nie vrylik kan dreineer nie en in die vorm van druppels deur die stoom opgesleur word. Gevolglik versamel kondensaat binne die buise en sodoende kan nie-kondenseerbare gasse nie effektief uit die teenvloeikondensor verwyder word nie. Soos die gasse versamel word koue sones gevorm. Dit blyk dat vloeding soos waargeneem in enkelbuise nie tot die vorming van koue sones bydra nie.