Wenjun Kong

An investigation of the thermal sensitivity and stability of the β-NaYF4: Yb, Er upconversion nanophosphors

The upconversion luminescence (UCL) of the hexagonal (β)-phase NaYF4:Yb,Er nanophosphors as a function of temperature (300–450 K) was examined under 978 nm light excitation. The thermal sensitivity was evaluated based on the UCL intensity ratio between H211/2–I415/2 and S43/2–I415/2 transitions (RHS). Excitation power, particle sizes (37, 65, and 140 nm), ion doping concentrations, and with and without silica coating were studied. It was found that RHS is only dependent on temperature at the low excitation power, and has no direct relation with the particle size and surface effects. Silica coating was found to enhance the thermal stability significantly without altering the thermal sensitivity.

Forced forward smoldering propagation in horizontally oriented flexible polyurethane foam

A series of experiments was conducted in a small-scale wind tunnel to investigate the effect of forced flow on forward smoldering propagation in flexible polyurethane foam material located horizontally. In the wind tunnel, the air at a flow rate in the range of 0.01-1.5m/s passed over the foam layer surface. Foam samples of different lengths with the same cross-sectional area were studied. From the experimental results, it is seen that the foam length has no clear influence on the smolder propagation velocity. With an increase of air flow rate, firstly, the smolder propagation velocity increases and reaches a maximum at an air flow rate of around 0.8 m/s due to the increased mass transfer of oxygen into the smolder reaction region. It decreases with further increase of air flow rate as heat losses becomes dominant compared to the positive effect of the increase of oxygen transfer into the reaction zone. The variation of the steady smoldering temperature with the air flow rate follows the same trend as that of the smolder propagation velocity. In this paper, a simple analysis is also presented to analyze the co-effect of the mass transfer of oxygen to the smoldering reaction region and the heat loss to the surrounding on the smolder propagation under our current experimental conditions.

THE IMPORTANCE OF THERMAL RADIATION TRANSFER IN LAMINAR DIFFUSION FLAMES AT NORMAL AND MICROGRAVITY

The importance of radiation heat loss in laminar and turbulent diffusion flames at normal gravity has been relatively well recognized. There is currently lack of quantitative understanding on the importance of radiation heat loss in relatively small scale laminar diffusion flames at microgravity. The effects of radiation heat transfer and radiation absorption on the structure and soot formation characteristics of a coflow laminar ethylene/air diffusion flame at normaland micro-gravity were numerically investigated. Numerical calculations were conducted using relatively detailed combustion chemistry and complex thermal and transport properties, an acetylene based soot formation model, and a statistical narrow-band correlated-k non-grey gas radiation model. Radiation heat transfer and radiation absorption in the microgravity flame were found to be much more important than its counterpart at the normal gravity.

Burning Characteristics of Non-Spread Diffusion Flames of Liquid Fuel Soaked in Porous Beds

An experimental study was conducted to investigate the effects of sand size and sand layer depth on the burning characteristics of non spread diffusion flames of liquid fuel soaked in porous beds. Sand beds with sand sizes from 0.12 to 3.18 mm and sand layer depths from 50 to 80 mm were chosen as the porous beds. Pure methanol was used as the liquid fuel. The flame appearances and effects of sand sizes and sand layer depths on flame temperature profiles, locations of vapor/liquid interface, vapor region moving speed, combustion duration time, fuel consumption and amount of fuel residues in the porous beds were studied in the experiments. An approximate analytical model based on the assumption of a two-phase Stefan problem was employed to predict the fuel consumption rate and the interface location. This model can quantitatively predict the interface position of combustion of the liquid fuel in the porous bed. The predicted results can also confirm that heat transfer in the bed is the controlling mode at the beginning stage of the combustion. After that, capillary force acts as the dominant role for diffusion of the vapor upward.

Behavior of non-spread diffusion flames of combustible liquid soaked in porous beds

Experimental studies were conducted to investigate the behavior of non-spread diffusion flames of liquid fuel soaked in porous sand beds of different depths. Sand bed depths from 50 to 80 mm and sand size of 1.55 mm were chosen as the porous beds. Pure methanol was used as the liquid fuel. The effects of sand bed depth on flame temperature profile, position and thickness of the vapor/liquid coexisting region, vapor region moving speed, combustion duration time, fuel consumption, and amount of fuel residues in the porous beds were studied in the experiments. Theoretical analysis was conducted to account for the experimental results. The capillary effect and heat conduction are the controlling mechanisms of the processes. The capillary pressure decreases with increasing bed depth due to the effects of gravity. Thus, only when the fuel soaked in the ground is shallow enough, combustion can be applied for effective soil decontamination. The presented heat transfer model can quantitatively predict the interface position of the combustion of the liquid fuel in the porous bed and explain the appearing maximum value in the fuel consumption rate curves in the early stage of the combustion. These results confirmed that heat conduction is the dominant mode of heat transfer in the beginning stage of combustion.

Effect of Cellular Instability on the Initiation of Cylindrical Detonations

The direct initiation of detonations in one-dimensional (1D) and two-dimensional (2D) cylindrical geometries is investigated through numerical simulations. In comparison of 1D and 2D simulations, it is found that cellular instability has a negative effect on the 2D initiation and makes it more difficult to initiate a sustaining 2D cylindrical detonation. This effect associates closely with the activation energy. For the lower activation energy, the 2D initiation of cylindrical detonations can be achieved through a subcritical initiation way. With increasing the activation energy, the 2D cylindrical detonation has increased difficulty in its initiation due to the presence of unreacted pockets behind the detonation front and usually requires rather larger source energy.

Large eddy simulation of methane/air lifted flame with hot co-flow

ABSTRACT The lifted flame with hot co-flow can represent typical combustion features in the practical systems with recirculation of the combustion product. In this study, the methane/air lifted flame was simulated by large eddy simulation. Regarding the special stabilization mechanism in the lifted flame, two different one-step methane oxidation mechanisms were used: (i) the conventional mechanism widely used in engineering simulations and (ii) a modified mechanism considering the effects of the equivalence ratio. By comparing the simulation results with the experimental data, both mechanisms could predict the liftoff phenomenon; however, the simulation using the modified mechanism provided more reasonable results.

Effect of Confinement on Combustion Characteristics in Lean Direct Injection Combustion System

Confinement has direct influence on the dome reference velocity and indirect influence on the combustor’s performance, emissions, operability, liner and dome temperature levels and gradients. Understanding the effects of the confinement is crucial to conventional combustor design.Recently, lean direct injection (LDI) combustor has been popularly employed to reduce nitrogen oxides (NOx) emissions from gas turbine. Compared to conventional combustor, LDI combustor has a much larger amount of air entering into the dome. Therefore, combustion characteristics and confinement effects in LDI combustor are quantitatively or even qualitatively different from those in conventional combustor. As a result, some design criteria for conventional combustor are no longer applicable for LDI combustor and it is essential to investigate the effects of confinement level in LDI combustor.In the literature, several studies have been conducted to understand how the confinement levels affect the characters of swirling non-reactive flow. However, there are few studies on the reactive flow and thereby the effects of confinement on combustion characteristics are not well understood. Due to its important role in combustor design, the confinement effect on spray combustion in LDI mode is systematically investigated in the present study.The experimental setup is established to study the confinement effects. Experimental data on flame characteristics, centerline temperature distribution, and pollutant emissions are obtained. Experiments at five different confinement ratios, 2.8, 6.8, 10.6, 19.5, and 28.5, are conducted. The results show that the confinement level has an important impact on the flame characteristics, centerline temperature, and pollution emissions. The optimal confinement ratio is determined, at which the pollution emissions become the lowest.Copyright

Study on the pre-ignition temperature variations of wire insulation under overload conditions in microgravity by the functional simulation method

A functional simulation method was applied to study the pre-ignition temperature variations of wire insulation under overload conditions in normal gravity. A simplified heating mode was proposed to theoretically investigate the wire heating process. Results of the temperature variations of wire insulation prior to ignition in normal gravity, but under reduced pressures, were obtained and compared with microgravity experimental results in order to investigate the effects of pressure and current on the pre-ignition thermal features of the wire insulation. Results show that the functional simulation method was effective to simulate the natural convection heat transfer effect on the wire insulation, which is a useful alternative approach to predict the pre-ignition thermal features of wire insulation by overload over a long duration in a microgravity environment.

Design of Sealing Structure and Analysis of the Flow Field in Micro Internal Combustion Swing Engine

A multistage groove sealing structure in Micro Internal Combustion Swing Engine (MICSE) was designed. By machining several grooves on the center-swing, it provides non-contacting control of internal leakage. The sealing principle of this structure may be described as follows. Gas at high pressure enters through the clearance of adjacent grooves to accelerate, then expands isentropically in the grooves. Once crossing several such grooves, gas emerges at the other end of the sealing structure at significantly reduced pressure. Numerical simulations were utilized to analyze the influences of pressure ratio and groove depth on leakage characteristics. The results demonstrate that at pressure ratio smaller than 2.0, turbulence eddy dissipation process in grooves cannot dissipate kinetic energy obtained during the throttling process completely, which results in poor sealing effect, at pressure ratio lager than 2.5, the two processes reach dynamic balance and sealing effect no longer changes. There is an optimal value in groove depth, undersize or oversize may lead to imperfect throttling effect or turbulence eddy dissipation process.