Hafiiz Osman | National University of Singapore (original) (raw)

Papers by Hafiiz Osman

Journal of physics, Oct 1, 2019

Ultraviolet (UV) reactor for ballast water treatment is investigated in this paper. Experimental ... more Ultraviolet (UV) reactor for ballast water treatment is investigated in this paper. Experimental and numerical simulations are performed for a base reactor named LBW850e. B. Pumilus is chosen as a challenge organism in the experiments. Simulation is carried out based on the commercial software ANSYS FLUENT with user defined functions implemented. The effects of water flow rate and UV transmittance (UVT) on the UV reactor performance in terms of reduction equivalent dose (RED) are studied. The results show that the increase of water flow rate reduces RED. While RED increases with the increase of UVT. The experimental and simulation results show reasonable agreement with each other. With this achieved, the numerical model developed in the current work can be applied to other reactors.

Journal of Visualization, Jul 5, 2020

We present an experimental study on hydrodynamic cavitation generated by accelerating liquid thro... more We present an experimental study on hydrodynamic cavitation generated by accelerating liquid through a series of constrictions in the presence of gas bubbles and explore its possible applications in water treatment with particular example in aquaculture industry. The formation of intense cavitation bubbles is visualized using a high-speed photography. The cavitation is initiated when a gas bubble moves towards a narrow cylindrical constriction where it accelerates, expands and then splits into smaller bubbles owing to the sharp pressure gradient of the liquid flow inside the constriction section. As the bubbles emerge downstream from the constrictions, they are exposed to a higher pressure region and collapse violently forming a cloud of bubbles. Smaller and more dispersed bubbles are produced by further passing the bubbles through a second series of constrictions. By introducing gas bubbles that serve as cavitation nuclei prior to the constriction, it is unnecessary to force the liquid flow below its vapor pressure to produce intense cavitation, thus enhancing the cavitation activities. We also present experimental evidences of a significant reduction of gram-negative Escherichia coli concentration after exposing them to the cavitation bubbles. Yet, the cavitation bubbles are found to be not sufficiently strong to lyse endospores Bacillus subtilis that are widely used in aquacultures. Keywords Hydrodynamic cavitation Á Bubble Á Disinfectant Á High-speed visualization 1 Introduction Water disinfection techniques, ranging from the simplest technique by simply boiling to the most advance systems by using both chemical (e.g. chlorination, electro-chlorination, ozonation, peracetic acid, and chlorine dioxide) and physical treatments (e.g. ultraviolet irradiation, ultraviolet with TiO 2 , deoxygenation, Electronic supplementary material The online version of this article (

Physics Procedia, 2016

Ultrasonic disinfection involves the application of low-frequency acoustic energy in a water body... more Ultrasonic disinfection involves the application of low-frequency acoustic energy in a water body to induce cavitation. The implosion of cavitation bubbles generates high speed microjets >1 km/s, intense shock wave >1 GPa, localized hot spots >1000 K, and free-radicals, resulting in cell rupture and death of microorganisms and pathogens. Treatment of marine ballast water using power ultrasonics is an energy-intensive process. Compared with other physical treatment methods such as ultraviolet disinfection, ultrasonic disinfection require 2 to 3 orders of magnitude more energy to achieve similar rate of microorganism mortality. Current technology limits the amount of acoustic energy that can be transferred per unit volume of fluid and presents challenges when it comes to high-flow applications. Significant advancements in ultrasonic processing technology are needed before ultrasound can be recognized as a viable alternative disinfection method. The ultrasonic resonator has been identified as one of the areas of improvement that can potentially contribute to the overall performance of an ultrasonic disinfection system. The present study focuses on the design of multiple-orifice resonators (MOR) for generating a well-distributed cavitation field. Results show that the MOR resonator offers significantly larger vibrational surface area to mass ratio. In addition, acoustic pressure measurements indicate that the MOR resonators are able to distribute the acoustic energy across a larger surface area, while generating 2-4 times higher pressures than existing ultrasonic probes.

In this paper, we report both experimental and computational studies of hydrodynamic cavitation g... more In this paper, we report both experimental and computational studies of hydrodynamic cavitation generated by accelerating liquid through a series of constrictions. The detailed process of cavitation generation is visualized using a high-speed photography. The cavitation is initiated when a gas bubble moves towards the constrictions. The gas bubble initially accelerates, expands and then splits into smaller bubbles when it moves along the constriction. As these bubbles migrate into a large liquid compartment, they collapse violently to form a bubble cloud, owing to a sudden jump in liquid pressure in the compartment. The experimental observation is further confirmed using computational fluid dynamics (CFD) simulations. We also present experimental evidence showing a significant reduction in gram-negative Escherichia coli concentration after it passes through the constrictions.

Ultrasonic water treatment is based on the ability of an ultrasonic device to induce cavitation i... more Ultrasonic water treatment is based on the ability of an ultrasonic device to induce cavitation in the liquid, generating physical and chemical effects that can be used for biological inactivation. Effective treatment requires the ultrasonic device to generate intense cavitation field in a large treatment volume. Most conventional ultrasonic radiators fulfil only the first of these two requirements, rendering such devices highly unsuitable for use in high-volume, high-flow liquid processes. The present research investigates the design and performance of a new type of radial resonator in terms of their electromechanical characteristics, nonlinear behaviour, and their ability to treat synthetic ballast water with lower power consumption and short treatment times. The radial resonators were designed using finite element (FE) modelling, and the best designs related to their predicted modal behaviour and vibration uniformity were selected for fabrication and experimental evaluation. Experimental modal analysis (EMA) of the radial resonators showed excellent correlation with the FE models, deviating by only 0.3% at the tuned mode. Impedance analysis showed that the mechanical quality factor of the radial resonators are 28-165% higher than the commercial high-gain probe, but their coupling coefficients are 40-45% lower. Harmonic response characterisation (HRC) revealed shifts in the resonance frequencies at elevated excitation voltages. Duffing-like behaviour were observed in all resonators. RP-1 exhibited the Duffing-like behaviour to a far greater extent compared to the RPS-16 and RPST-16 multiple orifice resonators, indicating the influence of geometric parameters on the overall stiffness of the structure. Finally, experiments with Artemia nauplii and Daphnia sp. showed excellent biological inactivation capability of the radial resonators. Comparison with previous studies showed that 90% reduction in Artemia nauplii can be achieved with up to 33% less energy and using just one radial resonator compared to the dozens of conventional resonators used in precedent investigations. The present research have successfully demonstrated the use of FE modeling, EMA, and HRC to develop, validate, and characterise a new type of radial resonator. Experimental analysis showed that the radial resonators exhibited promising electrical, mechanical, and acoustical characteristics that has the potential to be cost-efficient, scalable, and a viable alternative water treatment method. v Acknowledgement Foremost, I express my sincerest gratitude to Mr. Simon Kuik for initiating this programme and encouraging me to take up the challenge. I also express my sincere appreciation to Mr. Prakash and Mr. Chew Tee Tank for rendering support whenever sought, often without question. Thank you all, for extending this rare opportunity to me, and granting me full autonomy in the conduct of the research.

Proceedings of the 32nd International Symposium on Shock Waves (ISSW32 2019), 2019

Applied Thermal Engineering, 2017

Journal of Physics: Conference Series, 2019

Drying Technology, 2011

... archival literature, the patent literature does not provide scientific or engineering data or... more ... archival literature, the patent literature does not provide scientific or engineering data or techno-economic analysis. ... inlet port and progress to the outlet by means of a pressure gradientassisted by the ... coal-in-tube dryer is designed to reduce the moisture content of 3-mm coal ...

In this paper, we report on both experimental and computational studies of hydrodynamic cavitatio... more In this paper, we report on both experimental and computational studies of hydrodynamic cavitation generated by accelerating liquid through a series of constrictions. The detail process of cavitation generation is visualized using a high-speed photography. The cavitation is initiated when a gas bubble is moving towards the constriction. The gas bubble accelerates, expands and splits into a few smaller bubbles when moving along the constriction. As they are released into a large liquid compartment, the bubbles collapse violently to form a bubble cloud due to a sudden jump of pressure in the compartment. The result is further confirmed using computational fluid dynamics (CFD) simulations. Lastly, we present experimental evidences showing a significant reduction of gram-negative Escherichia coli concentration after passing through the constrictions.

Water Science and Technology, 2019

Performance of UV reactors for water disinfection is investigated in this paper. Both experimenta... more Performance of UV reactors for water disinfection is investigated in this paper. Both experimental and numerical studies are performed on base reactor LP24. Enterobacteria phage MS2 is chosen as the challenge microorganism in the experiments. Experiments are conducted to evaluate the effect of different parameters, i.e. flow rate and UV transmission, on the reactor performance. Simulation is carried out based on the commercial software ANSYS FLUENT with user defined functions (UDFs) implemented. The UDF is programmed to calculate UV dose absorbed by different microorganisms along their flow trajectories. The effect with boundary layer mesh and without boundary layer mesh for LP24 is studied. The results show that the inclusion of boundary layer mesh does not have much effect on the reactor performance in terms of reduction equivalent dose (RED). The numerical results agree well with the experimental measurements, hence validating the numerical model. With this achieved, the numerica...