Particle Image Velocimetry Laser-induced Fluorescence Research Articles

Effects of gas flow rate on rising bubble chains and induced flow fields: An experimental study

This paper reports an experimental investigation of the characteristics of bubble chains and their induced flow fields in a quiescent fluid at gas flow rates of 5, 10, 50, and 100 ml/min. The shadowgraphy and laser-induced-fluorescence particle image velocimetry (LIF-PIV) were used to analyze quantitatively the bubble morphology, kinematics, and induced flow fields. The results show that with increasing gas flow rate, the bubbles exhibit a larger average diameter, aspect ratio, and more-abundant morphology. The bubble motion includes a rectilinear rise followed by an oscillating rise, and the position where the bubble deviates from the linear trajectory is associated with the point of maximum bubble aspect ratio. The amplitude of bubble oscillation and the position of the instability both increase with the gas flow rate, except for the case of 50 ml/min. The bubble-induced flow field at higher gas flow rates exhibits stronger entrainment, which reflects the ability of the flow to entrain the surrounding fluid. The instantaneous flow-field results show that bubble-induced wake shedding is the main cause of the change in bubble rise state and trajectory instability. The direction of bubble deflection is related directly to the sequence of wake shedding: when the clockwise (resp. counterclockwise) vortex in the wake is the first to shed, the bubble deflects to the left (resp. right). The study also reveals that the simultaneous shedding of counterclockwise and clockwise vortices at 50 ml/min results in the bubbles continuing to rise rectilinearly for a period of time, and the position of trajectory destabilization has a maximum height.

Measurements of Air Entrainment into Diesel Spray by Laser Induced Fluorescence-Particle Image Velocimetry Method

Measurements of Air Entrainment into Diesel Spray by Laser Induced Fluorescence-Particle Image Velocimetry Method

Experimental Investigation of Magnetic Particle Movement in Two-Phase Vertical Flow under an External Magnetic Field Using 2D LIF-PIV

In this study, we experimentally investigated magnetic particle movement in two-phase flow under an external magnetic field. According to Faraday’s law, the alignment of a magnet is important for power generation. For high generation, it is important to understand how magnetic particles move in two-phase flow. The rotationality could be determined by observing a single particle; however, this is impossible due to the flow conditions. In this study, we estimated nonrotationality based on the vorticity. To eliminate scattered light and improve the signal-to-noise ratio, the laser-induced fluorescence particle image velocimetry technique was used. The solenoid nozzle has a hydraulic diameter of 3 mm. Its surface is covered with a coil with a diameter of 0.3 mm. The average diameter of a magnetic particle is 1.2 μm. The excitation and emission wavelengths are 532 and 612 nm, respectively. A thin laser sheet setup was configured. The laser sheet was illuminated on both sides to prevent shadows. The images were captured at 200 μm away from the wall and center of the nozzle. To estimate the decrease in vorticity, the theoretical and single-phase non-magnetic and magnetic particles are compared. The vorticity of magnetic particles is reduced by the external magnetic field.

Droplet size distribution and evaporation characteristics of fuel spray by a swirl type atomizer

Spray atomization and evaporation play extremely important roles in mixture formation and combustion processes of direct injection (DI) gasoline engines. In this study, the fundamental characteristics of a swirl spray injected into a constant volume vessel are investigated by means of several laser diagnostic techniques including the laser diffraction-based method for droplet size distribution, the laser induced fluorescence–particle image velocimetry for velocity distributions of droplets and spray-induced ambient air flow, and the two-wavelength laser absorption–scattering technique for concentration distributions of liquid and vapor phases in the spray. The results show that the droplets at outer zone of the spray exhibit larger diameter than those at inner zone under both ambient pressures 0.1 and 0.4 MPa. While this can be partially attributed to the effect of spray-induced ambient air flow, the strength of ambient air flow become small when increasing the ambient pressure from 0.1 to 0.4 MPa, indicating the strong influence of spray dynamics on the droplet size distribution. In the evaporating spray, there are higher vapor concentrations near the spray axis than at peripheral zones. At 4.0 ms after start of injection, spray droplets almost completely evaporate under ambient temperature 500 K and pressure 1.0 MPa, but there are significantly amount of fuels with equivalence ratio below 0.5 in the spray. Reduction in ambient pressure promotes the air entrainment and droplet evaporation, but lowered ambient pressure results in more fuel vapor of equivalence ratio above 1.3 along the spray axis.

QUANTITATIVE ANALYSES OF FUEL SPRAY-AMBIENT GAS INTERACTION BY MEANS OF LIF-PIV TECHNIQUE

The in-cylinder fuel-ambient gas mixing property in a direct injection (D.I.) diesel engine significantly influences the ensuing combustion and exhaust emission performance. In this study, the interaction of nonevaporating diesel spray with the surrounding gas was analyzed quantitatively in the quiescent condition at room temperature and with ambient gas pressure of 1 MPa by means of the laser induced fluorescence-particle image velocimetry (LIF-PIV) technique. Particularly, this study focused on the calculation of gas mass flow rate entrained through the entire spray region (spray side periphery and tip region) and total entrained gas-fuel ratio by using the gas velocity data obtained by the LIF-PIV technique. Another focus of this study was the gas entrainment characteristics of diesel spray under a wide range of injection pressures (100, 200, and 300 MPa) and the micro-hole nozzle (0.08mm) condition. The results indicate that the entrained gas mass flow rate at the spray tip region is prominent in the whole periphery and the proportion of gas entrainment at the side surface region increases as the spray develops Higher injection pressure significantly enhances the total entrained gas mass; however the increase of ambient gas/fuel mass ratio becomes moderate gradually as the injection pressure increases. The calculation model proposed by this work is capable of illustrating the ambient gas flow characteristics of the diesel spray accurately.

Spatial distribution of the gas phase in an axisymmetric submerged impact jet

Experimental data on the spatial distribution of the gas phase in an axisymmetric impact jet are obtained by the particle image velocimetry/laser-induced fluorescence (PIV/LIF) method. It is shown that the distribution of bubbles in the flow is determined by the dynamics of vortex structures.

Experiments on gas transfer at the air–water interface induced by oscillating grid turbulence

The gas transfer process across the air–water interface in a turbulent flow environment, with the turbulence generated in the water phase far away from the surface, was experimentally investigated for varying turbulent Reynolds numbersReTranging between 260 and 780. The experiments were performed in a grid-stirred tank using a combined particle image velocimetry – laser induced fluorescence (PIV-LIF) technique, which enables synoptic measurements of two-dimensional velocity and dissolved gas concentration fields. The visualization of the velocity and concentration fields provided direct insight into the gas transfer mechanisms. The high data resolution allowed detailed quantification of the gas concentration distribution (i.e. mean and turbulent fluctuation characteristics) within the thin aqueous boundary layer as well as revealing the near-surface hydrodynamics. The normalized concentration profiles show that asReTincreases, the rate of concentration decay into the bulk becomes slower. Independent benchmark data for the transfer velocityKLwere obtained and their normalized values (KLSc0.5/uHT) depend onReTwith exponent −0.25. The spectra of the covariance termc′w′ indicate that the contribution ofc′w′ is larger in the lower-frequency region for cases with smallReT, whereas for the other cases with higherReT, the contribution ofc′w′ appears to be larger in the higher-frequency region (small eddies). These interrelated facts suggest that the gas transfer process is controlled by a spectrum of different eddy sizes and the gas transfer at different turbulence levels can be associated with certain dominant eddy sizes. The normalized mean turbulent flux$\overline{c^\prime w^\prime}$profiles increase from around 0 at the interface to about 1 within a depth of approximately 2δe, where δeis the thickness of the gas boundary layer. The measured turbulent flux (c′w′) is of the same order as the total flux (j), which shows that the contribution ofc′w′ to the total flux is significant.