Muh-Rong Wang

Experimental and Theoretical Study on Hollow-Cone Spray

A theoretical and experimental investigation has been conducted to study the two-phase turbulent structure in an isothermal hollow-cone spray. Mean and fluctuating velocity components, drop number density, as well as drop-size distribution were measured with a nonintrusive diagnostic tool, a two-component phase Doppler particle analyzer. Complete initial conditions required for theoretical calculations were also provided with measurements. Theoretical calculations were made with an Eulerian-Lagrangian formulism. Turbulent dispersion effects were numerically simulated using a Monte Carlo method. Turbulence modulation effects were also taken into account in the modeling. The well-defined experimental data were used to assess the accuracy of the resultant Eulerian-Lagrangian model. Comparisons showed that the theoretical predictions, based upon the Eulerian-Lagrangian model, yielded reasonable agreement with the experimental data. The improvements made by inclusion of the selected turbulence modulation model were insignificant in this work.

Theoretical and Experimental Study on Two-Phase Structure of Planar Mixing Layer

A theoretical and experimental investigation is conducted to study the two-phase turbulent structure in a planar mixing layer with polydispersed drops. Mean and fluctuating velocity components, drop number density, as well as drop size distribution were measured with nonintrusive diagnostics of the two-component phase Doppler particle analyzer. Complete initial conditions required for theoretical calculation were also provided with measurements. Theoretical investigation is made with the Eulerian-Lagrangian formulation. Turbulent dispersion effects are numerically simulated using the Monte Carlo method. The well-defined experimental data have been used to assess the accuracy of the Eulerian-Lagrangian model. The comparisons show that the theoretical predictions, based on the Eulerian-Lagrangian model, yield reasonable agreement with the experimental data.

Controlling mechanisms and performance of coal burning rijke type pulsating combustors

This paper describes an investigation which has been concerned with the determination of the performance and mechanisms which control the operation of coal burning, Rijke type pulsating combustors. The combustor consisted of a vertical tube open at both ends. Air entered the combustor through its lower end and unpulverized coal was burned on a metal grid located at the center of its lower half. Heat released by the combustion process excited the fundamental acoustic mode of the combustor. The interaction between the acoustic oscillations and the combustion process resulted in high combustion efficiencies and high combustion intensities. Furthermore, the presence of pulsations in the flow increased convective heat transfer rates to the combustor walls. Results obtained in this study showed that the amplitude of the pulsations increased as the steady temperature ratio between the combustion products and the cold air, TH/Tc, at the combustion bed increased. Also, the amplitude of pulsations depended upon the acoustic losses in the combustion bed. For a given air/fuel ratio, most of the combustion occurred in the bed for low coal feed rates. However, as the coal feed rate increased, a fraction of the coal particles burned in spouting type combustion above the bed. Combustion efficiencies higher than 95% were attained when the combustor was operated with 13% excess air; performance which compares very favorably with other combustors (e.g., stokers) which burn unpulverized coal. Measurements indicated that NOx production in the developed combustor could be controlled by combustion stating. Finally, the simplicity of the combustor, its high combustion intensity, high convective heat transfer rate and low excess air operation suggest that the utilization of this combustor would involve low capital investment and low operating costs.

Study on reduction of SO2 and NOX emissions in a pulsating combustor burning petroleum coke

We have investigated the SO2 and NOX emissions of petroleum-coke burning in a Rijke-type pulsating combustor. Petroleum coke has a higher heating value and lower ash content, as well as a much higher sulfur content, than coal. Significant SO2 reduction has been achieved by adding limestone or dolomite particles to the pulsating combustor. Control of NOx emissions can be obtained by using the low-excess-air-firing and staged-combustion techniques.

Agglomeration Processes and Mechanisms of CO2 Snow Inside a Tube

This article experimentally investigates the agglomeration mechanism of CO2 primary particles inside a tube. The results show that a complicated particle motion in the upper portion of the tube is responsible for the formation of large snow particles. The high speed and complicated motion of the snow particles inside the tube provide both the opportunities and time for the collision of particles, which implies that only particle deposition and re-entrainment cannot completely describe the phenomenon of particle agglomeration. The results also show the mechanisms of particle agglomeration inside a tube, which include primary particle agglomerate in jet vortexes, agglomerated particles flowing upward into the recirculation region, particle clusters growing in the recirculation flow, and finally particles being released with the jet flow. A minimum tube length (30 mm in this case) is needed to ensure the complete formation of the agglomeration mechanisms with recirculation flow, and thus the formation of considerable amounts of agglomerated particles. The results of this study thus improve current understanding of the agglomeration process and mechanisms of CO2 snow formation inside a tube. Copyright 2014 American Association for Aerosol Research

MECHANISMS AND CHARACTERISTICS OF SPRAY FORMATION WITH FLOW FOCUSING IN A NEW AIR-ASSIST MICRO-ATOMIZER

The atomization mechanisms and characteristics of a new air-assist micro-atomizer (AMA) fabricated via MEMS machining are described. Observations show that the breakup regimes can be described by the mechanisms involved with the laminar jet disintegration, aerodynamic disintegration and turbulent mode based on the atomizing pressure and Weber number. A new phenomenon of flow branching of the liquid jet emanating from the atomizer was also observed. The ratio of liquid-to-gas flow rate determines the spreading angle and droplets size of the branched liquid streams. It is also found that the flow focusing mechanism resulted in the mono-dispersed droplets stream. Furthermore, the micro spray with mean droplet size of 5µm can be obtained by adjusting the atomization pressure. It is very useful in the applications of sample introduction in inductively coupled plasma mass spectrometry (ICP-MS) and medical inhalation devices.

Combination of Spray Forming and Metal Powder Productions by the Internal Mixing Atomizer with a Substrate

This paper describes the performance of an atomizer coupled with a substrate which produces metal powder and spray forming materials simultaneous in the spray chamber. Ultra fine metal powders are produced from this process. The melt is atomized by a twin-fluid atomizer with internal mixing mechanisms. The molten spray injected from the swirling chamber of the atomizer is then impinged upon the substrate to form the two phase impinging flow. The deposition rate of the molten spray on the substrate is controlled by the diameter of the substrate, the height of the substrate ring and the distance of the substrate from the outlet of the atomizer. This in turn determines the powder production rate of the spraying processes. Experimental results indicate that the deposition rate of the spray forming material decreases as the distance between the substrate and the atomizer increases. For example, the deposition rate decreases from 48% to 19% as the substrate is placed at a distance from 20cm to 40cm. On the other hand, the metal powder production rate and its particle size increases as the substrate is placed far away from the atomizer. The production of metal powder with mean particle size as low as 3μ m level has been achieved, a level which is not achievable by the conventional gas atomization processes.

Effects of Substrate Geometry on Performance of Twin-Fluid Atomizer in Metal Powder Production

Uniform and spherical metal powders have been widely used in many industrial applications. This paper investigates the control of particle size and size distribution by the impingement of the molten spray on the substrate with different geometries. The idea is to combine the atomization process with the classification process. Result shows that a significant reduction of the particle size occurred when the substrate was placed in the spray jet. The mean particle size was lowered to 8.0μm with a low transmission ratio of the spray jet in the existence of the ring type substrate. The particle size increased from 8.78μm to 12.67μm as the transmission ratio was increased from 13.92% to 75.80%. The reduction in particle size was due to the effect of the blockage of the substrate on the spray. The particle size increased from 6.72μm to 6.98μm when the disk-type substrate was placed at Z = 150mm and 200mm, respectively. The particle size of this case was smaller than the case with ring type substrate because the transmission ratio of the disk type substrate was lower. The percentage of small particles (i.e., V15-) were higher than 60% and the percentages of V25-45 were 4.19% and 0.37% when the disk-type substrate was placed at Z=150mm and Z=200mm, respectively, indicating that almost all of the particles were below 25μm under these conditions. Hence this technique is very effective in controlling the particle size in the metal powder production.

Comparison of gas atomization on A6060 Al alloy powders produced by internal and external mixing type nozzles

MAM (Metal additive manufacturing) is a technology that produces three-dimensional parts layer by layer from metal powders. Most of metal powders for MAM are produced by gas atomization processes due to its flowability and purity. Related studies have shown the benefits of using gas-atomized metal powder in recent years. Although gas atomization process are commercially used in metal powder production for century, studies for gas-atomized Al powder are quite rare. The goal of this study was to design a novel internal-mixing type atomizer for producing Al alloy powders, and to compare it performance with that of external-mixing type nozzle. In this paper, a nozzle designed with internal prefilming mechanism is first proposed and characterized. Comparison of performance of internal and external-mixing nozzle on Al alloy powder production is then further introduced. To compare the performance of the nozzle design of external and internal-mixing, HPGA (High Pressure Gas Atomization) nozzle is used as external-mixing nozzle in this study. All the gas-atomized metal powders in this paper were screened by a 60-mesh sieve, and then were measured by Beckman Coulter LS230 particle sizer. Results show that the particles size of the gas-atomized Al alloy powders decreases as the atomization pressure was increased. Mean particle size (Dv50) of 18.0 μm is achieved in the test with Al alloy. The SEM micrograph of the metal powder shows that the sphericity of the particle is better than that produced by external-mixing type nozzle.

VELOCITY MEASUREMENTS OF TURBULENT WAKE FLOW OVER A CIRCULAR CYLINDER

There are two general concerns in the velocity measurements of turbulence. One is the temporal characteristics which governs the turbulent mixing process. Turbulence is rotational and is characterized by high levels of fluctuating vorticity. In order to obtain the information of vorticity dynamics, the spatial characteristics is the other concern. These varying needs can be satisfied by using a variety of diagnostic techniques such as invasive physical probes and non-invasive optical instruments. Probe techniques for the turbulent measurements are inherently simple and less expensive than optical methods. However, the presence of a physical probe may alter the flow field, and velocity measurements usually become questionable when probing recirculation zones. The non-invasive optical methods are mostly made of the foreign particles (or seeding) instead of the fluid flow and are, thus, of indirect method. The difference between the velocities of fluid and foreign particles is always an issue to be discussed particularly in the measurements of complicated turbulent flows. Velocity measurements of the turbulent wake flow over a circular cylinder will be made by using two invasive instruments, namely, a cross-type hot-wire anemometry (HWA) and a split-fiber hot-film anemometry (HFA), and a non-invasive optical instrument, namely, particle image velocimetry (PIV) in this study. Comparison results show that all three employed diagnostic techniques yield similar measurements in the mean velocity while somewhat deviated results in the root-mean-squared velocity, particularly for the PIV measurements. It is demonstrated that HFA possesses more capability than HWA in the flow measurements of wake flow. Wake width is determined in terms of either the flatness factor or shear-induced vorticity. It is demonstrated that flow data obtained with the three employed diagnostic techniques are capable of yielding accurate determination of wake width.