Doctor of Philosophy

dissertationThe coal combustion process simulated employing high-fidelity models in both gas and particle phase using an Euleraian formulation of One-Dimensional Model (ODT). The coal submodels including vaporization, devolatilization and char oxidation and gasification are described and implemented within the ODT framework. Two coal devolatilization models: a two-rate model based on the Kobayashi-Sarofim and the Chemical Percolation Devolatilization (CPD) are described and implemented. In the gas phase, new formulation of an infinitely fast chemistry (flame-sheet) is developed and implemented. The main aim of this dissertation is to apply ODT model to simulate a large/pilot scale coal combustor. To achieve this aim, the models are first challenged in much simpler cases. An experiment conducted on single particle combustion in laminar flow is simulated to challenge the gas phase and coal submodels. The effects of the thermochemical models from the turbulence models are isolated. Ignition delay reported by experiment is applied as a metric to measure the accuracy of simulation predictions. The predicted ignition delays indicate that simpler Kobayashi-Sarofim and flame-sheet models roughly capture general trends present in the experimental data, but fail to provide quantitative agreement. On the other hand, the CPD model paired with detailed gas-phase chemistry provides reasonable agreement with the experimental observations over all reported conditions. Oxy-coal combustion is among the promising technologies to reduce greenhouse gas emissions for stationary power generation. An oxy-coal combustor located at the University of Utah is simulated using the ODT model. Predictions of flame stand-off distance are compared with experimental results. The impacts of models complexity and parameters as well as system parameters on the flame stand-off prediction and flame stability are studied. The influence of gas models, detailed kinetic vs flame-sheet, and devolatilization models, CPD vs Kobayashi-Sarofim models on the prediction of flame stand-off distance are investigated. Furthermore, the impacts of mixing rate and radiative temperature on the flame stability and flame stand-off are studied. Increase in the mixing rate shrinks the flame stand-off Probability Distribution Function (PDF) and moves the mode of PDF to shorter distances, however, the minimum flame stand-off distance is relatively insensitive to mixing rate. Impact of radiative temperature on flame stand-off distance is significant where an increase in radiative temperature shifts the whole flame stand-off PDF to shorter distances and also decreases the width of PDF. Using flame-sheet calculation in the gas phase, decreases the flame stand-off PDF width and moves the mode of PDF to shorter distances. Nevertheless, the minimum flame stand-off distance is insensitive to use flamesheet model. It is shown that the devolatilization model dictates the minimum flame standoff distance

A comparison of various models in predicting ignition delay in single-particle coal combustion

pre-printIn this paper, individual coal particle combustion under laminar conditions is simulated using models with various levels of complexity for the particle and gas phase chemical kinetics. The mass, momentum and energy governing equations are fully coupled between the particle and the gas phase. In the gas phase, detailed chemical kinetics based on GRI3.0 and infinitely-fast chemistry are considered and compared. For the particle phase, models for vaporization, devolatilization and char oxidation/gasification are considered, and the Kobayashi-Sarofim devolatilization model is compared to the Chemical Percolation Devolatilization (CPD) model. Ignition delay is used as a quantitative metric to compare the simulation prediction with experimental data, with careful attention given to the definition of ignition delay in the simulations. The effects of particle size, coal type and gas-phase temperature on the ignition delay are studied and compared with experimental data

Experimental study on heat transfer from rectangular fins in combined convection

Combined natural and forced convective heat transfer arise in many transport processes in engineering devices and in nature, which is frequently encountered in industrial and technical processes, including electronic devices cooled by fans, heat exchangers placed in a low-velocity environment, and solar receivers exposed to winds. In this study, the effects of design parameters have been experimentally investigated for the air-side thermal performance under combined (natural and forced)convection of the rectangular plate heat sinks, and the values of optimum design parameters were sought. Many ideas for improving cooling methods have been proposed, one of which is the heat sink. In this work, the average Nusselt number (Nu) and thermal resistance of a simple base rectangular plate and five vertical rectangular plate heat sinks with different numbers of fins under natural and combined convection were experimentally investigated to obtain the maximum average Nu and minimum thermal resistance for various Reynolds numbers (Re) from 2300 to 40000, Rayleigh numbers (Ra) from 1300000 to 13000000, and Richardson numbers (Ri) from 0.4 to 3. Also, in this experiment, fin spacing (P) was varied from 2.8 mm to 14.6 mm and the dimensionless P/H ratio was varied from 0.1 to 0.49. The flow velocity varied in the range of 2 to 8 m/s under combined convection. Based on the effects of Ri and Re, two empirical equations for natural and also for combined convection heat transfer were derived to calculate the average Nu. The average deviation for these two equations is about 7%.The outcomes of this research can be beneficial for engineers who work on electronics cooling systems

Cooling Effect of 3D Oscillating Heat Pipe with Nanofluid on Photovoltaic Panel in Hot Climates

Nowadays, there has been a growing trend toward using photovoltaic systems due to their high efficiency and cost-effectiveness; nevertheless, an inevitable side of solar panels is that their efficiency and life span experience a reduction when they are exposed to high intensity of solar irradiance and warm-up. In other words, when they are cooled, the electrical efficiency is promoted. Using oscillating heat pipes is a creative and practical approach to enhance PV efficiency. Furthermore, applying nanofluid as a working fluid can play an important role in maximizing panel productivity. The main objective of this investigation is to explore the cooling effect of a three-dimensional oscillating heat pipe on a photovoltaic panel, while graphene oxide nanofluid and distilled water are used as coolants. For this purpose, a novel three-dimensional oscillating heat pipe has been designed. The significant result from the study is that the cooling system was able to lower the temperature and improve the power output of the PV by 8.6 °C and 2.78 (W), respectively, in the warmest time of the day, when graphene oxide nanofluid was used as a coolant

Thermal comfort differences between polycarbonate and opaque roofing material installed in bus stations of Malaysia

Bus shelter as a semi-outdoor space protectsindividuals from direct sun light, rain and wind. The roofing material of busstop has extreme influence on environmental and subjective conditions ofpersons within this space. In this study, the principal concentration isdistinguishing the differences between installation of Polycarbonate plasticroofing material and that of opaque protection cover as two widespread roofingmaterials in obtaining human thermal comfort in equatorial climate of Malaysia. Hence, two bus stops, one covered by Polycarbonate translucent plastic and onewith opaque concrete-based tile, were selected to evaluate their inner thermalcomfort condition by measurement of four main microclimatic parameters (i.e.air temperature, wind velocity, humidity and mean radiant temperature) as wellas subjective survey in a university campus using the Physiological EquivalentTemperature (PET) as thermal index. The study found that the Polycarbonateroofing material is not appropriate material for permanence in bus shelters ofMalaysia neither objectively nor subjectively comparing with opaque protectivecover. Additionally, it was revealed adoption greatly impacts individualthermal perception which should not be neglected in the examination of thermalcomfort in non-indoor spaces

A review of researches about human thermal comfort in semi-outdoor spaces

The role of human thermal comfort on sustainable architecture and urban design is substantial. To concentrate on outdoor thermal comfort subject as well as indoor thermal comfort is a key to solve the global warming andwasting energy problems for future years. Semi-opened spaces as favorable and frequent spaces for people in urban areas affect strongly the quality of life and well-being of persons. Although the assessments of outdoor thermal comfort have been increased in recent years, the number of researches that primarily focused on human thermal comfort in semi-outdoor spaces is few. This article reviews the studies about semi-opened spaces have been executed in various climate conditions since late years. Furthermore, it explained the general assessment methods of the semi-outdoor thermal comfort

Experimental investigation of using Graphene Oxide with Ethylene Glycol and Water mixture to improve the performance of a car radiator

Nanofluids offer a good alternative heat transfer mediums with approximately five-fold heat transfer enhancement. This paper presents the results from a research study carried out on the use of Graphene Oxide/ Ethylene glycol Mixture (GnO/H2O-EG) Nanofluid as the heat transfer medium in a car radiator. The radiator consisted of 30 vertical tubes with elliptical cross section. Air makes a cross flow inside the tube bank with constant speed. The system was tested with three different Nanofluid concentrations (0.1, 0.3 and 0.5% by weight). The tests were conducted for flow rates ranging between 2 to 5 lit/min which corresponds to Reynolds number between 14000 and 38000. The effect on fluid outlet temperature to the radiator was analyzed for different flow rates and constant inlet temperature. The data were compared to that obtained with potable water in the radiator. The result of the comparison revealed that the use of graphene oxide increased the temperature drop across can radiator by upto 22 % compared to 50% Glycol mixture

Novel cooling system for free-standing photovoltaic panels

Although photovoltaic (PV) technologies enjoy tremendous benefits and hold the huge potential to lower building overall energy consumption, there is a major drawback. PV efficiency is extremely sensitive to heat and significantly reduced by increasing setting temperature and solar irradiance; thereby, thermal management in PV collectors plays a significant role in generating electrical energy. Using oscillating heat pipes attached to the rear side of PV panels is considered a novel and useful approach to dissipating heat. In this study, a novel cooling system that consists of a newly designed spiral oscillating heat pipe is introduced, while DI water and 0.2 g/l graphene are used as working fluid and PV panels are located at tilt angles of 30° and 60°. The OHP efficiency is higher at 60°; however, the efficiency of PV is maximized at 30° since the panel is exposed to maximum solar irradiance. The research demonstrates that the cooling method proves highly effective, especially in the hottest time of the day and the power output improves considerably from 38 W to more than 42 W at 30°, while the value is about 39.7 W when water is used as a coolant