Prof. P. N. Kioni - Academia.edu (original) (raw)

Papers by Prof. P. N. Kioni

Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 2010

Abstract—Laser micromachining has been widely applied in the fabrication, production and manufact... more Abstract—Laser micromachining has been widely applied in the fabrication, production and manufacturing of Micro Electro Mechan-ical Systems (MEMS). It uses photo thermal melting or ablation to fabricate a microstructure. The use of heat as a means of material removal has various negative effects on different materials. Distortion of the material is one of the negative effects, especially polymers. Polymers are often used in medical devices, microelectronic and sensor industries where high precision and high quality is required. During laser cutting of polymers, bulges are formed mainly due to resolidification of molten material in the working zone and temperature difference between the heat affected zone and the heat unaffected zone. A mathematical model has been developed using finite element analysis to model polymer material behavior during laser cutting. The model was run on both FEMLAB and MATLAB softwares. The

In this study, methane/air, methanol/air, and methyl formate/air stoichiometric mixtures have bee... more In this study, methane/air, methanol/air, and methyl formate/air stoichiometric mixtures have been numerically simulated at constant volume, low pressure of 2.7 atm, and temperature ranging from 1000 K to 1950 K with an aim to establish the impact of fuel oxygenation on NO formation. These conditions represent those behind a reflected shock in a shock tube, which is modeled as adiabatic homogeneous mixture with constant internal energy and constant volume. Various chemical kinetic mechanisms have been employed and extensively tested so as to ensure validity of the results. A comparison of NO profiles and other radicals- CH, HCN, N and N2- that are dominant in its formation have been done. Since the initial temperatures are high, the flame temperatures attained by all the mixtures are also high; from approximately 2800 to 3100 K for initial temperatures of 1000 and 1950 K respectively. Therefore, NO are formed mostly through thermal NO mechanism with prompt MO being less significant....

Journal of Energy Technologies and Policy, 2012

Methanolic transesterification of Croton megalocarpus seed oil to produce biodiesel was investiga... more Methanolic transesterification of Croton megalocarpus seed oil to produce biodiesel was investigated using Immobilized Candida antarctica lipase as a catalyst. The reactions were optimized by varying the temperature, amount of methanol and the weight of lipase. The transesterification process yielded 98.71 % biodiesel conversion at optimal conditions of 30 % enzyme (m/m), 50 0 C reaction temperature and oil to alcohol molar ratio of 1:4. Biodiesel from this process had a remarkably high acid value. All the other fuel properties measured were within the range stipulated in the American Society for Testing and Materials (ASTM) and International Standards Organization (ISO) standards. Keywords: Biodiesel, Croton megalocarpus , Transesterification, lipase

In this study we have examined the effects of fuel/air mixtures composition on NO formation in me... more In this study we have examined the effects of fuel/air mixtures composition on NO formation in methane/air, methanol/air, and methyl formate/air freely propagating flames. The flames have been numerically simulated at pressure of 1 bar and equivalence ratios () from 0.7 to 1.3. Various chemical kinetic mechanisms have been employed and extensively tested so as to ensure validity of the results. A comparison of NO profiles for different equivalence ratios have been done for the three flames. The role of temperature in NO formation is clearly demonstrated in these profiles, with the NO profiles having direct correlation with temperature profiles. It has been established that there is a significant dependency of temperature for NO formation in methane/air as compared to the other flames. This is attributed to dominance of reaction; CH + N 2 􀀀→ HCN + N, which results in large amount of N atoms taking part in NO formation in temperature dependent Zeldovich thermal NO mechanism. On the ot...

1. Jomo Kenyatta University of Agriculture and Technology (JKUAT) 2. Kenya Bureau of Standards (K... more 1. Jomo Kenyatta University of Agriculture and Technology (JKUAT) 2. Kenya Bureau of Standards (KEBS) 3. Dedan Kimathi University of Technology

AIMS Environmental Science, 2020

The increasing global demand for energy, the need to reduce green house gasses, and the depletion... more The increasing global demand for energy, the need to reduce green house gasses, and the depletion of fossil fuel resources have led for the need for renewable fuel sources such as biodiesel fuels. In the diesel engines, biodiesel fuels can also be used directly without comprehensive engine changes. Biodiesel relates to a diesel fuel that is based on vegetable oil or animal fat consisting of longchain of methyl, ethyl, or propyl esters. Methyl ester fuel burns more efficiently and has lower emissions of particulate matter, unburnt hydrocarbon, and carbon monoxide than fossil fuels. However, combustion of methyl ester fuel results in increased nitrogen oxides (NOx) emissions relative to fossil fuels. This study is concerned with characterizing the formation of NOx in the combustion of methyl formate under a counter diffusion flame. This was carried out in an Exhaust Gas Recirculation (EGR) system. Simulation of the process was done using Combustion Simulation Laboratory Software (COSILAB), and involved simulating the reactions of methyl formate fuel. The results obtained were compared to those of the methane/air diffusion flame, which is a well-characterized system. The extension validated the results obtained for the methyl formate/air diffusion flame. The reduction of NOx was found to be 26% and 14% in methane and methyl formate diffusion flame respectively from 0% to 29.5% of EGR. Increased EGR from 0% to 29.5% increased NOx reduction. Compared to methane/air diffusion flame, methyl formate/air diffusion flame with and without EGR had lower NOx emission. This was found to be true when examining the amount of other metrics viz. temperature, H, OH and N radicals associated with NOx. This showed that EGR system have an effect on NOx formation.

Energy, 2019

This is a PDF file of an article that has undergone enhancements after acceptance, such as the ad... more This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Fuel Processing Technology, 2018

This paper presents an investigation of wood combustion in a laboratory-scale fixed bed with an a... more This paper presents an investigation of wood combustion in a laboratory-scale fixed bed with an aim of establishing the effect of CO 2 environment on flame propagation speed and flame structures. Different oxy-fuel combustion atmospheres in which the composition of O 2 in CO 2 was varied from 21% to 50% by volume were tested and compared to air-fuel condition. Euler-Lagrange (Computational Fluid Dynamics-Discrete Element Method, CFD-DEM) approach which captures information of individual particle processes is used to model wood conversion in a packed bed. Results show that flame front propagation speed in oxy-fuel atmosphere reduced to 78% of that of the air-fuel condition with similar O 2 concentration. For oxy-fuel conditions, propagation speed increased with increase in O 2 concentration. The CFD-DEM model agrees very well with experimental values for mass loss, propagation speed and flame front positions. However, peak temperatures are poorly predicted at lower oxygen concentrations. The accuracy of temperature prediction improves at higher oxygen concentrations. During initial and devolatilization stage, mass fraction of tar predicted in CO 2 environment are smaller than in N 2 environment, while the amount of CO predicted is almost equal in both environments. However, during char combustion stage a high amount of CO is observed in oxy-fuel conditions.

International Journal of Energy and Environmental Engineering, 2018

Physical and thermal properties of briquettes produced by recycling charcoal dust under different... more Physical and thermal properties of briquettes produced by recycling charcoal dust under different processing conditions have been reported in this study. The main aim was to investigate the effects of the binder and processing conditions on the properties of briquettes. The effect of adding molasses binder on combustion properties of the briquette was first assessed. Then by fitting experimental data, mathematical models to predict gross calorific value, ash content, moisture content, relaxed density and shatter index with respect to binder mass fraction, drying temperature and compaction pressure were developed. All briquettes properties were predominantly affected by amount of molasses used. Molasses mass fraction increment in briquette results in significant increment in ash content, moisture content, relaxed density and shatter index and significant reduction in gross calorific value. Drying temperature did not have major influence on briquette properties except moisture content. Compaction pressure (50-150 MPa) used in this study had negligible influences on all briquette properties. Therefore, such high pressure which involves energy consumption is not necessary during production of charcoal dust briquettes. Optimized values of gross calorific value and shatter index were 29.031 MJ/kg and 80.363%, respectively, for 50 MPa compaction pressure, 29.512 °C drying temperature and 10% molasses mass ratio.

Journal of Clean Energy Technologie, 2018

The effects of pressure on NO x formation and flame structures in diffusion flames of methyl form... more The effects of pressure on NO x formation and flame structures in diffusion flames of methyl formate is investigated. The flames are simulated from the conservation equations for mass, momentum, energy, chemical species and equation of state. The results obtained, are validated by comparison with results obtained from the extensively studied methane/air and methanol/air flames.

Sustainable Research and Innovation Proceedings, Aug 22, 2013

Remarkable developments and improvements in laser source technology for industrial manufacturing ... more Remarkable developments and improvements in laser source technology for industrial manufacturing have taken place recently. Different technologies have continued to compete with each other on the benefit of the manufacturing industry. These include the full utilization of fibre laser toward becoming the dominant technology in laser manufacturing. This paper reviews the recent trends in the development and industrial use of high power fibre, diode, ultrafast, and other industrial laser sources. It is seen that these lasers are slowly replacing CO2 and Nd-solid-state lasers that have dominated the laser manufacturing industry for decades.

Lasers play a major part in the processing of the numerous materials used in engineering and manu... more Lasers play a major part in the processing of the numerous materials used in engineering and manufacturing. The range of processes in which lasers are involved is ever increasing. One of the areas in which laser has recently found application is in energy systems. With the dwindling non-renewable sources of energy such as fossil fuels and increasing demand of electricity, there is increasing interest in addressing the energy problem through development of systems for renewable energy alternatives such as solar energy. Though solar energy is readily available and has no adverse effects on environment such as pollution and greenhouse effects, it has not been fully exploited due to a number of challenges. Some of these challenges include high capital cost and the low conversion efficiency. Therefore photovoltaic industry has taken advantage of the benefits inherent in laser technology, namely accuracy, cost-efficiency, and flexibility that are critical in manufacturing today. This pape...

Journal of Clean Energy Technologies, 2013

Methane/air, methanol /air and methyl formate /air have been numerically simulated in three diffe... more Methane/air, methanol /air and methyl formate /air have been numerically simulated in three different flow configurations: homogeneous system; freely propagating flame; and diffusion flame. These simulations have been done with an aim of establishing the influence of fuel oxygenation on generation of pollutant. Various chemical kinetic mechanisms have been employed and extensively tested so as to ensure validity of the results. For each of the three configurations, a comparison of temperature, NO and its immediate dominant precursor species (CH and N) concentration profiles in the three fuels have been done. It has been established that, under the different flow configurations considered, CH 4 has high amount of total NO present in the flame region as compared to the oxygenated fuels (CH 3 OH and CH 3 OCHO). The temperatures attained in freely propagating and diffusion flames are relatively low (approximately ≤ 2000 K). This temperature favours prompt-NO formation, and therefore, a significant difference of the amount of NO (one order of magnitude higher) is observed in CH 4 as compared to oxygenated fuels due to low values of CH and N observed in these fuels (CH 3 OH and CH 3 OCHO). High flame temperatures (approximately 2900 K) due to high initial temperatures are observed in the homogeneous system. Therefore, in homogeneous system it was observed that the amount of NO produced by the three fuels is within the same order of magnitude due to availability of the O atoms and nitrogen molecules (important species in thermal NO mechanism (Zel'dovich mechanism)).

Combustion and Flame, 1993

In the present paper we investigate flame spread in laminar mixing layers both experimentally and... more In the present paper we investigate flame spread in laminar mixing layers both experimentally and numerically. First, a burner has been designed and built such that stationary triple flames can be stabilised in a coflowing stream with well defined linear concentration gradients and well defined uniform flow velocity at the inlet to the combustion chamber. The burner itself as well as first experimental results obtained with it are presented. Second, a theoretical model is formulated for analysis of triple flames in a strained mixing layer generated by directing a fuel stream and an oxidizer stream towards each other. Here attention is focused on the stagnation region where by means of a similarity formulation the three-dimensional flow can be described by only two spatial coordinates. To solve the governing equations for the limiting case in which a thermal-diffusional model results, a numerical solution procedure based on self-adaptive mesh refinement is developed. For the thermal-diffusional model, the structure of the triple flame and its propagation velocity are obtained by solving numerically the governing similarity equations for a wide range of strain rates.

Journal of Agriculture, Science and Technology, 2008