H.K. Ma

THE JET MIXING EFFECT ON REACTION FLOW IN A BLUFF-BODY BURNER

Abstract This study investigates the mixing effects of primary and secondary jets on flame stability as fuel/air is injected into a bluff-body burner. A two-dimensional spray combustion model, based upon a SIMPLER method, is used for numerical studies of combusting flow. The influence of the jet-to-air velocity ratio on the recirculation zone behind the bluff body, the center axial velocity and the temperature profiles is studied in detail. The results show that mixing between the two jets is controlled by two vortex eddies on the inside and outside of the bluff-body. With a proper bluff-body blockage ratio, cone angle and jet-to-air velocity ratio a more stable flame can be achieved.

Study of an LED device with vibrating piezoelectric fins

This study examines an innovative design of a proposed vibrating fin for thermal management of an LED device. The vibrating fin, coated with thin copper and composed of piezoelectric material, can vibrate and conduct heat from the finned base using the piezoelectric effect. This innovative heat-dissipation device can easily break the thermal boundary layer to enhance heat-transfer rate by changing the convective heat-transfer coefficient. In this study, three-dimensional transitional models for the vibrating fins are successfully built to investigate the major heat dissipation phenomena. Simulated results show that the performance of the vibrating fins is strongly affected by dimensions, vibrating frequency, pitch, and amplitude of fins. In addition, the thermal performance of fins can be further increased by the proper phase difference between the fins and the direction of gravity. The phase angles of the vibrations affect not only the flow field, but also the temperature distribution around the fins.

Numerical study on heat and mass transfer in a liquid-fueled gas turbine combustor

Abstract A two-dimensional spray combustion code is developed for investigating the spray flame in a gas turbine combustor. The modified K -e model is used to describe the turbulent flow field and the generalized Rosin-Rammler equation is used to evaluate the fuel droplet size distribution in the spray. The effects of the inlet turbulent kinetic energy K o and dissipation rate eo on the flow velocity and the length of recirculation zone are studied. Compared with results in the isothermal flow, the recirculation strength will increase, but the length of the recirculation zone will decrease in reacting flow case. Also, the influence of bluff-body size on velocity, temperature, fuel/oxygen concentration, and droplet distribution profiles is studied in detail.

Radiation blockage by the interaction of thermal radiation with conduction and convection in the combustion of condensed fuels

Abstract During the combustion process, numerous soot particles evolve from the fuel surface and act as strong absorbers to the flame radiation, thus the radiative heat flux will be substantially attenuated. A simple flame-sheet model is proposed to account for this blockage effect. The analysis considers the interaction of thermal radiation with conduction and convection heat transfer for a one-dimensional flame where soot particles dominate the absorbing-emitting process. The numerical results reveal that neglecting conductive heat transfer can cause overprediction of radiation blockage; however, neglecting the effect of convection results in an underestimation of radiation blockage.

AN EXPERIMENTAL INVESTIGATION OF COMBUSTION SYNTHESIS OF SILICON DIOXIDE (SiO2) PARTICLES IN PREMIXED FLAMES

An experimental study of gas-phase combustion synthesis of silicon dioxide (SiO2) particles was conducted in C3H8/air and H2/air premixed flames. Two types of organosilicon compounds, HMDSO and HMDSA. were used as the silicon precursors of synthesis processes. The objective of this investigation was to study the effects of HMDSO and HMDSA concentration, flame temperature, water vapor, and oxygen mole fraction on the synthesis of SiO2 particles. The entrained HMDSO and HMDSA concentrations were varied from 0.2 percent to 1.2 percent. The flame temperatures and oxygen mole fractions in the post-flame region ranged from 1350 to 1700 K and 0 to 8.5 percent, respectively. As a small amount of HMDSO or HMDSA was added into C3H8/air premixed mixtures, an orange-color secondary flame was observed. With the increase in HMDSO (or HMDSA) concentration, a bright yellow flame brush appeared following the tip of inner luminous cone of the flame. The flame brush could represent a particle-laden region. It was found that the flames with the same mole fraction of HMDSO and HMDSA exhibited comparable emission luminosity and similar flame structure.

Development of a Micro-diaphragm Pump with Piezoelectric Device

In this study, a new design of MaHou one-side actuating micro-diaphragm pump with piezoelectric device has been successfully developed by using its harmonic resonance of working fluid with system components (valve, diaphragm) in the pump chamber. The micro-diaphragm pump with two valves is fabricated in aluminum case by using highly accurate CNC machine, and cross-section dimensions are 5mm times 28mm. Both valves and diaphragm are manufactured from PDMS. The amplitude of vibration by a piezoelectric device produces an oscillating flow which may change the chamber volume by changing the curvature of a diaphragm. Several experimental set-ups for performance test in a single micro-diaphragm pump, isothermal flow open system, and a closed notebook water cooling system are designed and executed. The performance of one-side actuating micro-diaphragm pump is affected by the design of check valves, diaphragm, piezoelectric device, chamber volume, voltage and frequency. The measured maximum flow rate of present design is 72 ml/min at zero total pump head in the ranges of operation frequency 70-180 Hz. It can be applied in a liquid cooling system for notebook computer or in other related areas.

Solidification Models of an Impinging Metal Droplet

Two modified theoretical models for the spread of a molten metal droplet impinging on a flat solid surface, including considering the solidification, were derived in this study: maximum spread factor and splashing parameter. To simplify the models, we take account of only the physical and thermodynamic properties of the droplet and exclude those of the surface. By employing a solidified-time factor, the maximum spread factor improves its predictive precision to within ±11% error compared to experiments, in particular, only 3% error for Reynolds number > 17,000. In addition, a concise splashing parameter 1.5 gives a prediction of whether the spreading would splash or not. One can modify Reynolds and Weber numbers of an impinging droplet, e.g., regulating the impingement speed, to make its splashing parameter below 1.5 and have a spread without splashing. Simultaneously, the solidified spot size can be predicted by the maximum spread factor. In contrast, raising the splashing parameter can atomize the droplet. These models would be helpful in such areas as thermal spray and powder coating.

Mixing and Recirculation Characteristics of A Double Concentric Burner with Bluff-Body

The concentric bluff-body jet burner is widely used in industrial combustion systems. This kind of burner often generates a considerably complex recirculation zone behind the bluff body. As a result, the fuel often remains in the recirculation zone, achieving stability of flame. This study investigates, by means of experiments, the variations of the aerodynamics as the fluid is injected into a combustion chamber through a double concentric burner with a bluff-body. The observation and measurement of the aerodynamics in our experiment are conducted under a cold flow. The controlled parameters in our experiment are: variations in the blockage ratio of the center bluff body, the cone angle of the bluff body, and the velocity ratio (Us/Up) of the secondary jet and primary jet; the injection of helium bubbles into the primary and secondary jets to observe the recirculation zone behind the bluff body; using Tufts for observing the characteristics of corner recirculation zone in a combustion chamber, measuring the average velocity of each point within the aerodynamics by the 5-hole pitot tube; measuring the distribution of static pressure of the combustion chamber walls with a static pressure tap.

The Reduction of Thermal Nitrogen Oxides（NOx） Emission Using Staged Combustion in Furnace

The experimental study described in this paper is to investigate the control of thermal nitrogen oxides emissions from a 2.28 MW gas-fired test furnace. Tests, including changing axial or radial air flow rate, adding cooling water, and adding staged air, were performed to characterize and optimize the fuel-rich burning zone and the fuel-lean burnout zone independently. Detailed measurements of O2, CO2, CO, NO and NOx were made at the fuel-rich burning zone and furnace exit. The influence of forming CO, NO and NOx was examined. Results indicated that adding staged air in the fuel-rich burning zone (75 cm from burner) will reduce the maximum NO and NOx emissions. Adding cooling water in a right position may further lower the NO and NOx emissions. In addition, the least formation of thermal nitrogen oxides in the first stage fuel-rich burning zone will occur at the stoichiometric ratio’s inverse value, (φ1)−1, 0.65 to 0.7.

Numerical and experimental studies of a one-side actuating micropump with piezoelectric effect

In this study, a three-dimensional, transitional simulated model of a one-side actuating micropump, based on an actual design, has been successfully developed to predict the flow rates and pump heads by changing actuator displacements and the coupling effects of valves under different frequencies. The grid deformation with two moving boundaries, including the PDMS diaphragm and the piezoelectric device, is adopted in a CFD theoretical model for transition simulation. The simulated results indicate that the performance of the micropump strongly depends on the displacement and frequency of the actuator as well as the vibration of the valves. Compared with the maximum displacement of theoretical input data for the actuator at the frequency of 130 Hz, the simulated maximum flow rate is 1.6 ml/s at 140 Hz with the maximum pump head of 3.6 kPa due to the effect of valve vibration. In addition, the experimental results show that the pump with inlet/outlet+2 mm has the highest drain flow rate, but the pump with inlet/outlet+1 mm earns the highest pump efficiency due to the highest pump head.