Browsing by Author "Sher, Ilai"

Now showing 1 - 6 of 6
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    ItemOpen Access
    Blending and spray atomization modelling for gasoline-ethanol fuels.
    (2017-04) Etebu, Ongoebi Maureen Orubide; Sher, Ilai
    To achieve the ever stringent ls of low emission and to further improve the fuel economy, a much greater control of atomization and spray processes is required in the atomizer design of spray systems. In order to achieve this l, modeling of atomization characteristics of gasoline-ethanol fuel blends, fuel breakup models and correlations between flow patterns and droplet characteristics were adopted using OpenFOAM Computational Fluid Dynamics (CFD) model for direct gasoline injector using a simple mesh structure at constant volume. The Rosin Rammler distribution model was used to generate the number of spray particles injected into the cylinder. The spray modeling and atomization involved blob sheet model and KH-RT model while the numerical technique for simulating atomization process by CFD included the use of governing equations such as Eulerian for gas phase, lagrangian for disperse phase and turbulence modeling. The evaluation of the effect of fuel blends, injection pressure, and ambient gas pressure and spray cone angle on the axial spray tip penetration, spray width, and overall Sauter Mean Diameter (SMD) were carried out. The SMD was discovered to be affected by varying the degree of injection cone angle. The spray tip penetration lengths were larger for higher injection cone angles while higher penetration lengths were obtained at higher injection pressures. One salient conclusion drawn from the modeling is that as the number of particle increased, the density of clusters became smaller.
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    Computational modelling of thermal spraying processes
    (Cranfield University, 2016-03) Mahrukh, Mahrukh; Gu, Sai; Sher, Ilai
    The main aim of this project is to model the effects of varied injection parameters on the gas dynamics and droplet dynamics of the HVSFS and SP- HVOFS processes for improving the droplet breakup and evaporation to enhance the nanoparticles heating and deposition efficiency. Thermal spraying processes are widely used to generate thermal-, corrosion-, and wear-resistant layers over the machine parts, to increase the durability of the equipment under severe environmental conditions. The liquid feedstock is used to achieve nanostructured coatings. It is used either in the form of a suspension or a solution precursor. The suspension is a mixture of solid nanoparticles suspended in a liquid medium consisting, for instance, of water, ethanol, or isopropanol. This dispersion mechanism in a liquid carrier provides adequate flowability to the nanoparticles, which cannot be handled by conventional gas- based feeding systems, whereas the solution precursor is mixed at the molecular level; hence, more uniform phase composition and properties are expected in the sprayed coatings as compared to the suspension and conventional powder spraying. Firstly, experiments are conducted to analyse the effects of different precursor concentrations, solvent types and injection nozzles on the size and morphology of synthesized nanoparticles. The results indicate that the particle size increased with increasing precursor concentration due to the variations in the physical properties of the mixture solution. The higher precursor concentrations had an adverse effect on the droplet atomization and evaporation process that led to bigger size particle formation. The use of aqueous solvent has some limits and with higher precursor concentration the surface tension increases that resulted in the reduction of droplets’ disintegration, and thus bigger size precursor droplets generate larger nanoparticles. A mixture of aqueous-organic solvents and pure organic precursors are preferred to improve the process efficiency of the nanoparticles size and morphology. Furthermore, the nanoparticles size can be controlled by using liquid feedstock atomization before injecting into the HVOF torch. A new effervescent injection nozzle is designed and compared to different types of existing injection nozzles, to see the variations in the droplet disintegration, and its effects on the performance of the HVOF torch processes. It is detected that the atomization would result in smaller size particles with homogeneous morphology. In a numerical study, different droplet injection types are analysed to see their effects on the gas and droplet dynamics inside the HVOF torch. The group-type injection (GTI) and effervescent-type atomization (ETI) are used effectively to overcome the heat losses and delays in the droplet evaporation. These approaches reduce the thermal and kinetic energy losses in the suspension-fed-HVOF torch, thereby improving the coating formation. The effects of using multicomponent water-ethanol mixture injection in the HVOF torch are also modelled, and its impact on the droplet breakup and evaporation are studied. The organic solvents have a low heat of vaporization and surface tension, and can effectively be used in the HVOF spraying process over the water-based solvents. Furthermore, nanoparticles are suspended in the liquid feedstock and injected into the HVOF torch. The effect of increasing nanoparticles’ concentration in the feedstock and its consequence on the gas dynamics, droplet breakup and evaporation are analysed. The augmentation in the nanoparticles loading in the suspension droplets can decrease the droplet breakup and evaporation rate because the required heat of vaporization increases significantly. Moreover, the size of injection droplet affects the droplet fragmentation process; bigger sized droplets observed a delay in their evaporation that resulted in coating porosity. The results suggest that smaller droplet sizes are preferred in coating applications involving a higher concentration of nanoparticles with high melting point. Further, the gas flow rates (GFRs) are regulated to control the droplet dispersion, atomization and evaporation inside the solution precursor fed-HVOF torch. The size of the droplet diameter is decreased by an increment in the GFR, as higher combustion rates increase the combustion flame enthalpy and kinetic energy. Moreover, the increase in the oxygen/fuel flow rates dilutes the injected precursor. It reduces ZrO2 concentration in the process and decreases the rate of particle collision; as a result, non-agglomerated nanoparticles can be obtained.
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    Energetic and exergetic study for cross-corrugated membrane-based total recovery exchanger for ventilation
    (2017-10) Abduljabbar, Ahmed A.; Sher, Ilai; Nishino, Takafumi
    Indoor air quality is an important component of the air conditioning of buildings due to its major effect on the health of the occupants, thus the air supplied to these buildings by the ventilation system should be sufficient, clean and healthy. A most promising development was the heat recovery system which offers better thermal energy efficiency and comfort with adequate fresh air. An energetic and exergetic analysis has been conducted on a cross-corrugated membrane based total heat exchanger core for ventilation of single dwellings. In order to enhance the sensible and latent effectiveness of the heat and mass transfer intensification was achieved by selecting Polyethersulfone for the membrane material, and a cross-corrugation arrangement of different dimensions for the primary surface exchanger. The design was tested against a ventilation air volume flow rate for an individual household; from 85 to 100 m³/hr. The dimensions of the exchanger were based on the polymer core being developed by Redring-Xpelair, Peterborough UK, with core dimensions of width and length both 250 mm, and a range of heights 100 – 500 mm. The cross-corrugated design of the test core had triangular openings with pitch lengths of 5, 10 and 25 mm. The ambient conditions were for a cold and humid winter in the UK. The ambient temperature test values were 2, 4, 6, 8 and 10 °C, and the inlet air velocities in the core were 0.5, 1.0, 1.5 and 2 m/s, with Reynolds numbers not exceeding 2200. CFD studies were conducted to investigate the thermal-fluid performance of the core, the Transition-SST model was used in the simulations within ANSYS Fluent 17.1 software and was validated using experimental data in the literature. The proposed model performed successfully in this study and proved that it was compatible with the test conditions. The exergetic analysis was conducted using the IPSEpro modelling software, by creating a system consisting of membrane core, a domestic dwelling, fresh air and exhaust fans. The energetic analysis results were the basis of the IPSEpro modelling to determine the exergy, the exergetic efficiency and exergy destruction in the system. The study concluded from both the energetic and the exergetic analysis that the membrane based exchanger core showed promising performance as a total heat and moisture recovery application with sensible and latent effectiveness values varying from 65% to 82%; and exergetic efficiency values varying from 30% to 60%, depending on core geometry and ambient conditions. The chemical exergy was the dominant factor in the performance in all cases, and the membrane core had the highest exergy destruction percentage comparing to the other system components. Decreasing the pitch length of the exchanger core intensified its performance, the 5 mm case showed the best performance, but there are likely to be difficulties in manufacturing such a compact core. But, and more directly, its use would mean unpleasant compromises due to the extremely higher pressure drop across such a core even at low Reynolds numbers. The 10 mm case gave a better performance than the 25 mm, but not substantially different, therefore, the optimum choice lies between the better heat and mass transfer performance of the 10 mm case and the lower pressure drop and relative ease of manufacture of the 25 mm.
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    ItemOpen Access
    Modified coupled-mode model for thermally chirped polymer Bragg gratings
    (Osa Optical Society of America, 2010-04-05T00:00:00Z) Sher, Ilai; Han, B.; Bar-Cohen, A.
    A modified coupled-mode (CM) model is proposed for the optical behavior of thermally chirped Bragg gratings. The model accounts for the axial gradient in the modulation wavenumber, which has been ignored in the classical CM model. The model is used to characterize the optical behavior of a polymethyl methacrylate- based polymer Bragg grating subjected to nonisothermal conditions. The validity of the proposed method is verified by comparing the results of the modified CM model with those obtained from the exact numerical solution.
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    Numerical analysis of the effects of using effervescent atomization on solution precursor thermal spraying process
    (American Chemical Society, 2017-09-09) Mahrukh, Mahrukh; Kumar, Arvind; Nabavi, Seyed Ali; Gu, Sai; Sher, Ilai
    The solution precursor thermal spraying (SPTS) process is used to obtain nano-sized dense coating layers. During the SPTS process, the in situ formation of nanoparticles is mainly dependent on combustion gas-temperature, gas-pressure, gas-velocity, torch design, fuel type, and Oxygen-Fuel (O/F) mixture ratios, precursor injection feeding ratio and flow rates, properties of fuel and precursor and its concentration, and the precursor droplets fragmentation. The focus of the present work is the numerical study of atomization of pure solvent droplets streams into fine droplets spray using an effervescent twin-fluid atomizer. For better droplet disintegration appropriate atomization techniques can be used for injecting the precursor in the CH-2000 high-velocity oxygen fuel (HVOF) torch. The CFD computations of the SPTS process are essentially required because the internal flow physics of HVOF process cannot be examined experimentally. In this research for the first time, an effervescent twin-fluid injection nozzle is designed to inject the solution precursor into the HVOF torch, and the effects on the HVOF flame dynamics are analyzed. The computational fluid dynamics (CFD) modeling is performed using Linearized Instability Sheet Atomization (LISA) model and validated by the measured values of droplets size distribution at varied Gas-to-Liquid flow rate Ratios (GLR). Different nozzle diameters with varied injection parameters are numerically tested, and results are compared to observe the effects on the droplet disintegration and evaporation. It is concluded that the effervescent atomization nozzle used in the CH-2000 HVOF torch can work efficiently even with bigger exit diameters and with higher values of viscosity and surface-tension of the solution. It can generate smaller size precursor droplets (2 µm
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    Theoretical limits of scaling-down internal combustion engines
    (Elsevier, 2010-10-06) Sher, E.; Sher, Ilai
    Small-scale energy conversion devices are being developed for a variety of applications; these include propulsion units for micro aerial vehicles (MAV). The high specific energy of hydrocarbon and hydrogen fuels, as compared to other energy storing means, like batteries, elastic elements, flywheels and pneumatics, appears to be an important advantage, and favors the ICE as a candidate. In addition, the specific power (power per mass of unit) of the ICE seems to be much higher than that of other candidates. However, micro ICE engines are not simply smaller versions of full-size engines. Physical processes such as combustion and gas exchange, are performed in regimes different from those that occur in full-size engines. Consequently, engine design principles are different at a fundamental level and have to be re-considered before they are applied to micro-engines. When a spark-ignition (SI) cycle is considered, part of the energy that is released during combustion is used to heat up the mixture in the quenching volume, and therefore the flame-zone temperature is lower and in some cases can theoretically fall below the self-sustained combustion temperature. Flame quenching thus seems to limit the minimum dimensions of a SI engine. This limit becomes irrelevant when a homogeneous-charge compression-ignition (HCCI) cycle is considered. In this case friction losses and charge leakage through the cylinder-piston gap become dominant, constrain the engine size and impose minimum engine speed limits. In the present work a phenomenological model has been developed to consider the relevant processes inside the cylinder of a homogeneous-charge compression-ignition (HCCI) engine. An approximated analytical solution is proposed to yield the lower possible limits of scaling-down HCCI cycle engines. We present a simple algebraic equation that shows the inter-relationships between the pertinent parameters and constitutes the lower possible miniaturization limits of IC engines.