Numerical investigation of a transient free jet resembling a laser-produced vapor jet

In the present study, the transiently developing free jet emanating from a laser-impacted surface is considered. The jet velocity profiles are varied with time in connection with the vapor jet velocity profiles emanating from the laser-produced cavity. Consequently, jet expansion from the laser cavity situation is modelled in the simulations. The jet exiting profiles measured previously are employed in the present simulations. Since the thermophysical properties of the laser-produced vapor are unknown, air properties are used for the jet in the simulations. A numerical method employing control volume approach is introduced to discretize governing equations of flow and energy. It is found that in the early period, jet behavior is similar to slowly flowing jets as reported in the literature. The self-similar transient jet behavior occurs as the time progresses; in which case jet exit velocity profiles become similar.     

A three-dimensional numerical simulation of impinging jet arrays on a moving plate

The structure of the flow field and its effect on the heat transfer characteristics of a jet array system impinging on a moving heated plate are investigated numerically for Reynolds numbers between 100 and 400 and for steady state conditions. An array consisting of 24 square jets (3 rows × 8 columns) impinging on a moving heated flat surface is considered as a representative pattern.The simulations have been carried out for jet-to-jet spacing in the range 2D–5D and for nozzle exit to plate distance of 0.25D, where D is the jet width. The velocity ratios of the moving heated plat to the jet velocity (R_m = u_p/u_j) used are in the range 0.25–1.0. The obtained results were compared with published data for the case of fixed heated plate (R_m = 0.0). The results show that the streamwise profile of the Nusselt number exhibit strong periodic oscillations, spatially. The amplitude of the periodic oscillations of the Nusselt number is attenuated as one proceeds in the downstream direction. For such small nozzle-to-plate spacing used, the results show that the ratio R_m has no effect on the oscillations of Nusselt number.     

Visualization of hydrogen jet using deformation of the laser beam profile

Hydrogen has the widest flammable range, the fastest flame propagation speed, and the lowest ignition energy, so its safety needs special attention before the wide application of hydrogen energy. The main objective of this work is to propose a new method to evaluate hydrogen jet pressure by using a Helium–Neon laser through the jet. A mathematical model was proposed, which describes the deformation of the laser beam profile passing through an axisymmetric circular hydrogen jet pressure flow field in detail. This research attempts to apply the expression of density Gaussian distribution, ideal gas equation, and Gladstone Dale equation to disclose the deformation of laser beam profile under different outlet conditions. The experimental uncertainty is about 3 × 10^?3. A non-contact optical experimental system is established to visually measure the density gradient distribution of the gas jet. Our findings show that the hydrogen jet can be regarded as a gas-phase lens, and the deformation of the laser beam profile in the horizontal direction increases linearly with jet pressure. Finally, the preliminary results of calculations of the spot area with the theoretical model were presented and compared with the images of the laser beam profile passing through the jet in the experiment. The theoretical model gave similar results and the overall agreement with the experiment was satisfactory. Our technology exhibits high sensitivity in the measurement of hydrogen leakage pressure, providing a theoretical basis for non-contact, non-damaged, high-response, and high-sensitivity detection of hydrogen leakage.     

Experimental study of turbulent jet induced by steam jet condensation through a hole in a water tank

A turbulent jet induced by steam jet condensation in a water pool was investigated experimentally. An experimental apparatus equipped with a steam boiler, a single-hole steam sparger, and a water pool, etc. was used. For the measurements, a pitot tube and thermocouples were used for turbulent flow velocity and temperatures, respectively. Overall flow shapes of the turbulent jet by the steam jet condensation are similar to those of axially symmetric turbulent jet flows. The angular coefficients of turbulent rays are quantitatively comparable between the traditional turbulent jet flows and the turbulent jet flows induced by the steam jet condensation in this work. Although the turbulent flows were induced by the steam jet condensation, general theory of turbulent jets was found to be applicable to the turbulent flows of this work.     

Determination of underexpanded hydrogen jet flame length with a complex nozzle geometry

With the growing number of hydrogen-powered cars and hydrogen filling stations, it is essential to have accurate and reliable engineering models for this infrastructure. The length of a hydrogen jet flame resulting from a high pressure release will have an impact on its consequences. This work examined the effect that nozzle geometry has on hydrogen jet flame length. The geometry was modified by varying the diameter of the spouting nozzle downstream from the choked nozzle upstream. These experimental results were compared with an existing model for estimating jet flame length. Sensors upstream from the complex nozzle geometry measured the temperature, mass flow rate, and pressure for the released hydrogen. A high-speed camera recorded the hydrogen jet flame at a stable pressure and mass flow. Flame lengths were determined with an image processing tool used to analyze the high-speed video for each experiment. By analyzing the dataset with the image processing tool, the jet flame length distribution, minimum and maximum jet flame length, and the jet flame length standard deviation could be computed. Results showed that the nozzle geometry can increase the jet flame length by 62% compared to a single nozzle configuration with equal mass flow rate. With the upstream nozzle as input, the smallest average relative deviation from previously published models was 13%. The discharge coefficients for different nozzles were calculated and presented.     

Measurements of concentration distribution of hydrogen jet using deflection of center of the laser spot

The main purpose of this work is to propose a new method to evaluate the concentration distribution of the hydrogen jet by using a He–Ne laser through the jet. This research attempts to apply the expression of concentration Gaussian distribution, the refraction formula of inhomogeneous refractive index medium, and the concentration inversion function to disclose the displacement of the center of the laser spot at different heights in the gas jet. The spot images of the laser beam passing through the gas jet at three vertical heights z = 10d, 20d, 30d, and different radial positions are obtained. The radial spatial asymmetry of the gas jet is also found in the experiment. Finally, the calculated concentration distribution curve and the fluent simulation curve, it is found that the two results are very similar. Our findings show that the error between the concentration distribution of this method and the simulated concentration distribution reaches 2.43%.     

Experimental study on heat-transfer characteristics of circular water jet impinging on high-temperature stainless steel plate

Water jet impinging heat transfer is widely used in steel-making industry, nucleate power plants, and many other applications. In this study, an experimental study was carried out in an industry-scale facility. In this test facility, an insulating material was added to simplify the heat-transfer process. The data filter method was used to remove the noise in temperature measurements. Using inverse heat conduction analysis, surface heat flux was obtained, and the inverse heat conduction method was verified. The relative error between the calculated and measured temperature at the bottom surface of the test plate was less than 5%, and the relative error of the inner temperature of the test plate was less than 0.35%. Increasing the surface roughness provided better heat transfer in the stagnation region, which is in agreement with the reported results in the literature. By determining the surface temperature of the test plate, the boiling heat-transfer mode can be identified.     

Active control of jet premixed flames in a model combustor with manipulation of large-scale vortical structures and mixing

Active control of a bluff-body stabilized methane/air jet flame issued from a coaxial nozzle is made by using miniature magnetic flap actuators attached to the outer nozzle. CH radical chemiluminescence, and CO and NO_x emissions are measured to assess the flame characteristics. By changing the flapping Strouhal number, the flame stability and CO emission are drastically improved under different equivalence ratio (?) conditions. At ? = 0.72, the optimum Strouhal number for stable combustion is unity, since methane/air mixing is enhanced by large-scale vortices synchronized with the flap motion. On the other hand, at a lower equivalence ratio of ? = 0.48, the optimum Strouhal number is much larger than unity; with small-scale vortices, the premixed combustion is stabilized by stratified mixing. In addition to acetone and OH-PLIF, a new two-line OH-PLIF is employed for flame temperature measurement. The longitudinal flame temperature distribution is obtained from conditional-averaged OH fluorescence intensities at the OH front taken with two different excitation lines. CO and NO_x emission characteristics of the controlled flame are discussed on the basis of the local fuel and OH distributions and the flame temperature.     

Measurement of Supersonic Jet Noise with Optical Wave Microphone System

An optical wave microphone system is a new technique of sound measurement. This technique has been developed as a new plasma diagnostic technique to measure electron density fluctuations in the nuclear fusion research. Because the sound wave is a pressure or a density fluctuation, it is possible for this technique to measure the sound wave, too. The acoustical characteristics of the optical wave microphone system were examined by using a speaker as a first step. Next, feasibility of this device to measure jet noise was examined. It was found that the optical wave microphone system could measure the jet noise as well as a sound from speaker. Hence the optical wave microphone system can be considered one of the devices equivalent to condenser microphone. Because of these reason, this device is very convenient to scan the acoustic filed through jet flow from the inside to the out side and more preferable for not disturbing the observation field.     

Time-averaged characteristics of a reacting fuel jet in vitiated cross-flow

This paper describes an experimental study of reacting jets in a high-temperature (1775 K) vitiated crossflow at 6 atm. We present an extensive data set based on high speed chemiluminescence imaging and exhaust gas sampling showing the characteristics of the time-averaged trajectory, width of the flame, flame standoff (or ignition) location, and NO_x emissions over a momentum flux ratio range of 0.75 < J < 240. Key observations are: (1) Depending upon ignition times, reaction can initiate uniformly around the jet, initiate on the leeward side of the jet and spread around to the windward side farther downstream, or initiate further downstream. (2) The time-averaged trajectory generally follows nonreacting trajectories, but penetrates further in the far-field than for what would be expected of a nonreacting jet. (3) The width of heat release zone increases monotonically with downstream location, J, and flame flapping amplitude, but seems to be dominated by the size of the counter-rotating vortex pair. (4) The measured ignition locations were of the same order of magnitude as values based on calculated ignition time scales and mean jet exit velocities, but with some additional variability. (5) The incremental NO_x emissions were controlled primarily by the global temperature rise associated with burning the jet fuel (for the fixed crossflow conditions studied here), and the NO_x emissions increased roughly linearly with the temperature rise.     

采用射流泵利用城市供水管网的压力节能

介绍射流泵辅助二次加压供水系统的组成、工作过程及节能原理,并对系 统的特点及实际应用效果进行了说明。     

Experimental investigation of the effects of heat release on mixing processes and flow structure in a high-speed subsonic turbulent H2 jet

In this paper, we explore the effects of heat release on mixing and flow structure in a high-speed subsonic turbulent H₂ jet in an air coflow. Heat release effects are determined from the comparison of nonreacting and reacting jet behavior, boundary conditions being identical in both cases. Experiments are performed in a wind tunnel specifically designed for this purpose. Planar laser induced fluorescence on OH radicals and on acetone (seeded in the hydrogen jet) are used to characterize the cartography of scalars, and laser Doppler velocimetry is used to characterize velocity profiles in the far field of the H₂ jet. Results show significant effects of heat release on mixing and flow structure, indicating an overall reduction of mixing and entrainment in the reacting jet compared to the nonreacting jet. First, a change is observed in the orientation of coherent structures originating from Kelvin-Helmholtz type instabilities, and responsible for air entrainment within the jet, which appear “flatter” in the jet flame. Then, the flame length is increased over what would be predicted from the intersection of the mean stoichiometric contour with the centerline of the nonreacting jet. And finally, the longitudinal average velocity decrease along the jet axis is quicker in the nonreacting jet, and nondimensional transverse velocity fluctuations are about half as high in the reacting jet as in the nonreacting jet, indicating a reduction of the turbulence intensity of the flow in this direction in the jet flame.     

Heat transfer mechanism of a swirling impinging jet in a stagnation region

Heat transfer characteristics of a swirling impinging jet have been experimentally examined using a combined particle image velocimetry (PIV) and laser‐induced fluorescence (LIF) technique for simultaneous measurement of velocity and temperature fields. The present study shows that the radial width of the jet stretches with increasing swirl intensity, and that the stretching phenomenon contributes to the maximum local heat transfer coefficient. At the stagnation region, the flow near the heated surface is mixed intermittently by reverse flows toward upstream, and spatial distributions of temperature are correlated with instantaneous velocity vector maps. The dynamic behavior of recirculation zones, attributed to swirl number Sw and impinging distance, mainly determines the turbulent heat transfer at the stagnation region. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(8): 663–673, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10120     

In situ measurements of water transfer due to different mechanisms in a proton exchange membrane fuel cell

Water management is of critical importance in a proton exchange membrane (PEM) fuel cell, in particular, those based on a sulfonic acid polymer, which requires water to conduct protons. Yet there are limited in situ studies of water transfer through the membrane and no data are available for water transfer due to individual mechanisms through the membrane in an operational fuel cell. Thus it is the objective of this study to measure water transfer through the membrane due to each individual mechanism in an operational PEM fuel cell. The three different mechanisms of water transfer, i.e., electro-osmotic drag, diffusion and hydraulic permeation are isolated by specially imposed boundary conditions. Therefore water transfer through the membrane due to each mechanism is measured separately. In this study, all the data is collected in an actual assembled operational fuel cell. The experimental results show that water transfer due to hydraulic permeation, i.e. the pressure difference between the anode and cathode is at least an order of magnitude lower than those due to the other two mechanisms. The data for water transfer due to diffusion through the membrane are in good agreement with some of the ex situ data in the literature. The data for electro-osmosis show that the number of water molecules dragged per proton increases not only with temperature but also with current density, which is different from existing data in the literature. The methodology used in this study is simple and can be easily adopted for in situ water transfer measurement due to different mechanisms in other PEM fuel cells without any cell modifications.     

A numerical simulation of the suppression of hydrogen jet fires on hydrogen fuel cell ships using a fine water mist

In this paper, in order to evaluate the reliability of a fine water mist for the suppression of fires on hydrogen fuel cell ships, the fire dynamics simulator (FDS) software was used to simulate the jet fire process and the action of a fine water mist on a fire caused by a hydrogen leakage in the hydrogen storage tank areas of hydrogen fuel cell ships. The fire scenario was classified into vertical or horizontal jet fires according to the location of the leakage in the hydrogen storage tank area, and the suppression effects of a fine water mist on hydrogen jet fires under a different droplet size, spray velocity, and ambient wind speed were compared and analyzed. The results indicate that a fine water mist is not effective in extinguishing hydrogen jet fires; however, by selecting suitable parameters (a spray velocity of 30 m/s and average droplet size of 30 μm), it can effectively reduce the fire field temperature of hydrogen jet fires and prevent the fire from developing further. Increasing the average droplet size of the fine water mist results in a gradual degradation of the suppression effect, while a higher spray velocity of the mist enhances the suppression effect to a certain extent. The ambient wind speed is an important factor that influences the suppression effect of a fine water mist on hydrogen jet fires, and when this speed is less than 4 m/s, a fine water mist with a higher spray velocity and smaller average droplet size is still a superior way of suppressing fires.     

Enhancing liquid water transport by laser perforation of a GDL in a PEM fuel cell

The presence of liquid water in a polymer electrolyte membrane fuel cell hinders gas diffusion to the active sites, which results in large concentration overpotentials and instability of the fuel cell performance. In this paper, a new customized gas diffusion layer (GDL) is presented that enhances liquid water transport from the electrode to the gas channels and therefore lowers mass transport losses of oxygen through the porous media. The GDL is systematically modified by laser-perforation with respect to the flow field design. The holes are characterized by SEM images. The performance of the laser-treated GDL was investigated in a small test fuel cell with a reference electrode by voltammetry and chronoamperometry measurements and compared to corresponding data with a non-modified GDL. Voltammetry experiments with different humidification levels of the inlet gases were conducted. In all cases, the cathode overpotential with the perforated GDL clearly shows reduced saturation which can be seen in a lower overpotential in the region limited by mass transport resulting in a higher limiting current density. The investigated current response of the chronoamperometry measurements clearly shows a better dynamic and overall performance of the test cell with the perforated GDL.     

Thermal characteristics of a premixed impinging circular laminar-flame jet with induced swirl

A swirling flow has been induced in a premixed gas-fired impinging circular flame jet by adding two tangential air flows to the main axial air/fuel flow. The flame jet system was considered to be small-scale and operated under low-pressure, laminar flow conditions. The effects of Reynolds number of the air/butane mixture and nozzle-to-plate distance on the heating performance of the flame were studied and compared with the heat-flux distributions on an impingement plate under different operating conditions. The whole investigation was conducted under the stoichiometric air/fuel condition (i.e., equivalence ratio, Φ = 1) with the Reynolds number being varied from 800 to 1700, and nozzle-to-plate distance being selected between 1.5 and 4.0. The introduction of swirl to small-scale, low-pressure, laminar premixed gas-fired impinging circular flame jets is the method for enhancing their thermal performances. The heat-flux distribution on the impingement plate was more uniform and the flame temperatures essentially higher when compared with a similar flame jet system without induced swirl.     

Calculation of temperature field in a thin moving sheet heated with laser beam

A temperature field in a thin moving sheet heated with a laser beam is calculated. The power density in the beam is distributed according the Gauss function. Cooling effects caused by the free or forced convection in ambient gas is taken into account. The problem becomes two dimensional by averaging the temperature field over the sheet thickness. Two-dimensional integral Fourier transformation on space coordinates is applied to solve the problem. It gives an analytical solution of the problem in the Fourier space. The inverse Fourier transformation is fulfilled and the solution is represented via an integral having exponential asymptotes at infinity.     

Auto-ignition during instationary jet evolution of dimethyl ether (DME) in a high-pressure atmosphere

The auto-ignition process during transient injection of gaseous dimethyl ether (DME) in a constant high-pressure atmosphere is studied experimentally by laser-optical methods and compared with numerical calculations. With different non-intrusive measurement techniques jet properties and auto-ignition are investigated at high temporal and spatial resolution. The open jet penetrates a constant pressure oxidative atmosphere of up to 4 MPa. During the transient evolution, the fuel jet entrains air at up to 720 K. The subsequent auto-ignition of the ignitable part of the jet occurs simultaneously over a wide spatial extension. The ignition delay times are not affected by variation of the nozzle exit velocity. Thus, the low-temperature oxidation is slow compared with the shorter time scales of mixing, so that chemical kinetics is dominating the process. The typical two-stage ignition is resolved optically with high-speed shadowgraphy at a sampling rate of 10 kHz. The 2D fields of jet velocity and transient mixture fraction are measured phase-coupled with Particle Image Velocimetry (PIV) and Tracer Laser Induced Fluorescence (LIF) during the time-frame of ignition. The instationary Probability Density Functions (PDF) of mixture fraction are described very well by Beta functions within the complete area of the open jet. Additional 1D flamelet simulations of the auto-ignition process are computed with a detailed reaction mechanism for DME [S. Fischer, F. Dryer, H. Curran, Int. J. Chem. Kinet. 32 (12) (2000) 713-740; H. Curran, S. Fischer, F. Dryer, Int. J. Chem. Kinet. 32 (12) (2000) 741-759]. Calculated ignition delay times are in very good agreement with the measured mean ignition delay times of 3 ms. Supplemental flamelet simulations address the influence of DME and air temperature, pressure and strain. Underneath a critical strain rate the air temperature is identified to be the most sensitive factor on ignition delay time.     

Height effect on heat transfer characteristics of sintered porous blocks with a confined slot jet

This work numerically investigates the heat transfer of a sintered porous block under a confined slot air jet. The width of the jet nozzle (W) is 5 mm; the ratio of the porous block length to the jet nozzle width (L/W) is 12, and the Prandtl number (Pr) is 0.7. Variable parameters are the ratio of the porous block height to the jet nozzle width (H/W) and the Reynolds number (Re). The findings reveal that the cooling performance with the sintered porous block was better than that with an aluminum foam block, and was at least 5.8 times as large as that without it. The Nusselt number increased as the H/W fell. The effect of Reynolds number on the heat transfer was negligible at Re ≤ 1000 but considerable at Re > 1000.