Centerline Axial Velocity Research Articles

Effect of initial generating eddy height on temperature, velocity and air entrainment of fire whirl

In this paper, we present an experimental effort to reveal the effect of the initially generated eddy height on the thermal, velocity, and air entrainment properties of the fire vortex plume. Experiments were performed by a fixed-frame facility with variable channel wall height (h), using propane as the fuel (20 cm in burner exit diameter, 50.0–100.0 kW in heat release rate). Results show that the growth rates of centerline excess temperature and axial velocity tend to increase with h in the continuous flame below h. It is found that the radial distribution of excess temperature is always of hump-type and little affected by h at lower levels, while the radial profile of axial velocity changes from the plateau-type into the hump-type as h is increased. In the upper continuous and intermittent flames, the radial profile of excess temperature evolves from the hump type into the unimodal type, and that of axial velocity develops into the unimodal type from the plateau type as h is increased. The growth rates of excess temperature radius and axial velocity radius with the normalized height decrease with h for the continuous and intermittent flames. The axial position corresponding to the significant increase in the growth rate moves upstream with the increase of the heat release rate (Q˙). Under small h, the tangential velocity within the vortex core decreases with height, suggesting that the rotational strength becomes weakened. It is found that the air entrainment is weak at the flame bottom within the initial generating eddy height, and the entrained air is insufficient for complete combustion. The axial mass flow rate increases obviously as the channel wall disappears. With the increase of h, turbulence suppression is enhanced, and its role in flame elongation becomes more significant. The reduction of flame height is probably attributed to the greater turbulent mixing length under lower initial generating eddy height.

Fire plume characteristics of annular pool fire with different cylindrical obstacles in a ship engine room

A numerical study of oil pool fire with different cylindrical obstacles in a ship engine room was carried out. The characteristics of flame height, maximal temperature rise, and centerline axial velocity of fire plume were investigated and analyzed. The results showed that the flame height of the merged flame decreases with the increased radius ratio and obstacle height, while the plume maximal temperature rise and axial velocity decreases with a larger radius ratio. Under the broken flames, the flame height only decreases with a larger radius ratio, while the plume temperature rise and axial velocity decreases with the increase of both radius ratio and obstacle height. When the obstacle is smaller, the annular flames merge above the obstacle, while the annular broken flames surround the cylindrical obstacle when the obstacle is larger. And there is a competition between flame height enhancement effect and weakening impact when the flame could merge above obstacle. The empirical models of flame height, centerline temperature rise and axial velocity of annular pool fire with different cylindrical obstacles are obtained and validated by experimental test data. These results can offer an essential reference for fire prevention design and fire science research in the ship engine room.

Synthetic Jet Actuators with the Same Cross-Sectional Area Orifices-Flow and Acoustic Aspects

In this paper, synthetic jet actuators (SJAs) with three different orifice shapes (circular, square, and slot) with the same cross-section area were investigated. The SJA efficiency and the synthetic jet (SJ) Reynolds number were calculated based on the time-mean reaction force measurement. The momentum velocity was measured with hot-wire anemometry and additionally, the sound pressure level (SPL) was measured. The efficiency was equal maximally to 5.3% for each orifice shape, but the square orifice characterized the higher Reynolds number. The compared centerline (axial) velocities and the radial velocity profile at a distance of 112 mm were similar for each orifice type. The SPL measurement results were surprisingly constant in relation to each other. The square orifice generates the lowest SPL, approximately 2.8dB lower than the circular orifice, and approximately 4.2dB lower than the slot orifice, at each investigated real power. Finally, the differences to other papers and limitations of the approach to comparing orifices presented in the present paper were indicated.

Dynamic characteristics of supersonic turbulent free jets from four types of circular nozzles

Abstract The effects of nozzle structures and working pressure on the dynamic characteristics of supersonic turbulent free jets have been investigated numerically. Four types of nozzles (namely Laval, pipe, contraction I, and contraction II, respectively) and four pressure conditions (namely K = 0.8, 1, 1.5, and 2, respectively) were considered. A Standard k-ε model was utilized for the calculation of the supersonic turbulent free jets. Validation of the model was performed on the Laval jet by comparing it with the experiment and large-eddy simulation (LES). A perfect agreement was achieved in terms of the centerline and radial axial velocity profiles. The jets issuing from the Laval and the pipe had a longer potential core and a larger centerline axial velocity with the same outlet momentum. The length of the potential core was proportional to the working pressure, but variations of the centerline axial velocity decay rate were inverse for all nozzles. The effects of nozzle structures and work pressure on the spreading rates of the jets were insignificant. No obvious change trend could be observed on the kinematic and geometric virtual origins. The study can provide references for the nozzle and working pressure selection in practical application.

Characterization of Flow-Focusing and Flow-Blurring Modes of Atomization

This paper presents a characterization of flow-blurring (FB) and flow-focusing (FF) modes of atomization using a new atomizer, which enables continuous transition from FB to FF regimes by controlling the recess of a liquid tube. The atomizer has optical access and features flexibility in controlling the Weber number (We) and the air/liquid mass flow ratio (ALR). The Phase Doppler Particle Analyzer (PDPA) technique is employed here along with long-distance microscopic and direct imaging to observe and characterize the atomization regime. Experiments were carried out with water and air as the working fluids at room temperature and in an atmospheric pressure environment. The results show that operation in the FB regime generates smaller droplet sizes in the spray core compared to the FF regime. However, the benefit of FB decays with increasing We, and eventually the difference between both regimes decreases. Among the distinctive features of FB sprays found only in the near-field region are: a more noticeable change in the mean droplet size with the radial direction, more pronounced polydispersity, larger centerline axial velocity and larger axial and radial velocity fluctuations. Liquid tube recess was also found to be an important parameter in controlling the spray behavior in the FB regime. It was found to impact the droplet size, the values and shape of the axial velocity variation curves, as well as the range of radial velocity values. It is also shown that, at a high enough We, effective differentiation between FB and FF atomization regimes became harder and rather limited to the visual confirmation of the presence of a bubble in the liquid tube.

Evolution from conical to cylindrical fire whirl: An experimental study

This paper presents the first experimental effort on the evolution of the flow field from conical to cylindrical fire whirl, by using a small-scale rotating screen facility with finely controlled imposed circulation (Γ) under a fixed heat release rate of 4.0 kW. Stereo particle image velocimetry (SPIV) technique was employed to measure the instantaneous and mean velocity fields. Two-celled vortex structure and central recirculation zone are found to form and move downwards with increasing Γ in the plume region of cylindrical fire whirl. Within the mean flame height, the mean vortex core radius decreases slightly with Γ for conical fire whirl, while expands linearly after the onset of cylindrical fire whirl. The radial profiles of tangential velocity have self-similarity within the mean flame height for both conical and cylindrical fire whirls. In the plume region of the cylindrical fire whirl, the radial distribution of tangential velocity is similar to that of the Rankine vortex, and the decay of maximum tangential velocity and the expansion of the vortex core are faster than those of the conical fire whirl. With increasing Γ, the unimodal profile of axial velocity is found to change into the plateau profile and the wakelike profile in the plume region of the conical fire whirl and the transition location advances upstream steadily. For both conical and cylindrical fire whirls, the centerline axial velocities of certain levels and the mean flame height have consistent variation trends with Γ. Within the mean flame height for cylindrical fire whirl, the centerline axial velocity falls sharply at the axial position where the radial profile of axial velocity changes from the plateau to the wakelike profile. The height of this axial position is found to decrease steadily with the increase of Γ.

Numerical simulations of buoyancy-driven flows using adaptive mesh refinement: structure and dynamics of a large-scale helium plume

The physical characteristics and evolution of a large-scale helium plume are examined through a series of numerical simulations with increasing physical resolution using adaptive mesh refinement (AMR). The five simulations each model a 1-m-diameter circular helium plume exiting into a $$(4~\hbox {m})^3$$ domain and differ solely with respect to the smallest scales resolved using the AMR, spanning resolutions from 15.6 mm down to 0.976 mm. As the physical resolution becomes finer, the helium–air shear layer and subsequent Kelvin–Helmholtz instability are better resolved, leading to a shift in the observed plume structure and dynamics. In particular, a critical resolution is found between 3.91 and 1.95 mm, below which the mean statistics and frequency content of the plume are altered by the development of a Rayleigh–Taylor (RT) instability near the centerline in close proximity to the plume base. Comparisons are made with prior experimental and computational results, revealing that the presence of the RT instability leads to reduced centerline axial velocities and higher puffing frequencies than when the instability is absent. An analysis of velocity and scalar gradient quantities, and the dynamics of the vorticity in particular, show that gravitational torque associated with the RT instability is responsible for substantial vorticity production in the flow. The grid-converged simulations performed here indicate that very high spatial resolutions are required to accurately capture the near-field structure and dynamics of large-scale plumes, particularly with respect to the development of fundamental flow instabilities.

Computational investigation of slot jet impinging on a heated flat plate using low-Reynolds number modeling

The present work reports the computational investigation of turbulent slot jet impinging on a heated plate subjected to isothermal and isoflux boundary conditions. The eddy viscosity based 2-equation low-Reynolds number k − ε model of Yang and Shih has been considered. Low-Reynolds number (LRN) modeling employ near-wall integration which solves the entire boundary layer using very find grid near the wall to handle the sharp gradient of variables prevailing in these regions. The Reynolds number of jet at the inlet is 9900 and the non-dimensional distance between the top and bottom plate i.e. H/w is 7.5. The variation of centerline axial velocity, variation of local maximum axial velocity, profile of non-dimensional wall shear stress along impingement and confinement wall, distribution of local Nusselt number along impingement wall and confinement wall, and distribution of Reynolds shear stress in the domain are discussed.

Effect of imposed circulation on temperature and velocity in general fire whirl: An experimental investigation

This paper presents experimental effort to reveal the independent effect of imposed circulation on the evolutions of thermal and velocity fields in general fire whirl. Experiments were performed by using a small-scale rotating screen facility with finely controlled imposed circulation (Γ) under fixed heat release rate of Q˙ = 7.5 kW. It was found that the centerline excess temperature and axial velocity varied consistently with the flame shape. The buoyant regime and circulation-controlled regime were distinguished in specific ranges of Γ. For the buoyant fire whirl, the centerline axial velocity increased by a 1/2 power law of height in the entire continuous flame. For the circulation-controlled fire whirl, the centerline axial velocity depended on height by a 1/3 power law at lower levels and differed slightly with height in the rest continuous flame. At a certain level, the variations of centerline excess temperature and axial velocity with Γ varied noticeably in three consecutive axial regions. In the bottom region, the behavior of centerline axial velocity was closely related to the occurrence of a special jump in flame size and the drop of its height with increasing Γ. In the upper region, the centerline axial velocity decreased first, then increased and decreased, and finally increased quickly again with Γ. It was found that the imposed circulations at the interval boundaries increased steadily with height. The centerline axial velocity increased a little faster than the tangential velocity with Γ in the circulation-controlled regime. The radial distributions of excess temperature and axial velocity varied significantly with Γ and height, and unimodal profile, plateau-type profile and hump-type profile were found. The radial profiles of tangential velocity had self-similarity within the mean flame height. The Richardson number increased with Γ and height in the buoyant fire whirl and exhibited some decrease in the circulation-controlled fire whirl.

Near-field merging and penetration of triple starting plumes from volumetric heat sources in a calm environment

This paper investigates the buoyancy effect on near-field evolution of triple equal starting thermal plumes from volumetric heat sources, with a focus on merging process, velocity evolution, and stream-wise penetration. Instantaneous velocity fields and corresponding vorticity distribution, temporal evolution of centerline axial velocities, temporal penetrating rates and heights are examined by 2-D particle image velocimetry (PIV) at three different source heat strengths of 180W, 90W, and 30W.A four-stage merging process is demonstrated to be independent of heat strength. Normalized starting and terminating times of each developing stage are also unified regardless of the heat strength, i.e., t/t∗=1.7–2.5 for stage i (relatively isolated development), t/t∗=2.5–3.3 for stage ii (bending wall flows), t/t∗=3.3–3.9 for stage iii (development after a self-merged state), and t/t∗ > 3.9 for stage iv (evolution with global merging).The axial velocity at a specific centerline point usually involves three distinct developing periods: a “centerline-silent” period ending at a normalized transportation time tt_n when heat is just transported to the measuring points, a fast rising period, and a fluctuating period starting at a normalized endurance time te_n. The buoyancy-independent tt_n is approximately 1.5, 2.2, 3.0, and 3.2 at a normalized downstream distance of 1.5, 2.5, 3.5, and 4.5, respectively. The time te_n, independent of buoyancy and axial height, is about 3.6 for all measuring points in this study.The maximum axial velocity above the middle heat source exhibits three linearly developing periods with different acceleration rates successively: a quicker rate as the wall flow develops, a slower rate due to enhanced lateral interaction, and the fastest rate as the global merging happens. In different periods, the proportional coefficients of linear fitted functions vary from 0.48 to 2.98. The downstream distance where axial velocity is maximum shows similar power-law increasing during t/t∗=1.5–3.2 at Q=180 and 90W, but exhibits significant fluctuations at Q=30W. Normalized overall penetration based on global velocity fields is notably faster and higher at the lowest buoyancy (Q=30W) than that at the higher buoyancy (Q=180 and 90W), probably due to the weaker lateral interaction and turbulent mixing together with the more dedicated vertical rise at Q=30W.

Effects of inlet boundary conditions in an axial hydrocyclone

Due to the reduced space in platform, compact oil–water separators are essential in offshore oil production. In this sense, a compact design is provided by the in-line axial segregator, taking advantage of the pipe geometry where oil–water mixture flows. Numerical simulation is conducted for laminar swirling flow in an axial hydrocyclone. To study the sensitivity of swirling flow inside the hydrocyclone to the inlet boundary condition, different inlet deflection angle and annular gap width are employed. Validation of the numerical results was achieved by comparing the numerical predictions of pressure drop in swirling flow with experimental values collected in 5 cm size 3 m long transparent pipe using a glycerine–water solution of 54 mPa s viscosity and 1210 kg/m3 density as working fluid, at several flow rates. The numerical results show that the centerline axial velocity, centerline pressure distribution, friction coefficient and swirl number are influenced by inlet boundary conditions. Variation in deflection angles and gap width brings about a considerable increase in friction factor coefficient and decaying of swirl. Vortex breakdown is analyzed too and occurs to inlet swirl numbers larger than 2.5. A correlation to predict the length of developed flow is proposed as a function of Reynolds number and swirl number at inlet.

Numerical analysis of characteristics of multi-orifice nozzle hydrothermal jet impact flow field and heat transfer

Hydrothermal jet technology is a new drilling method in which the rocks are broken by the combination of thermal spallation and high velocity impact effect. This technology uses a high temperature and high velocity jet to break rocks and has the potential to be more economically advantageous than conventional techniques for geothermal well drilling. Previous related studies primarily examine numerical simulation on one hydrothermal jet flow field, and to the best of our knowledge, no specific study addresses the thermo-physical interaction between wellbore fluid and ambient rock. This paper presents a multi-orifice nozzle model to investigate the features of a flow field with multiple hydrothermal jets and the heat transfer to ambient rocks. A transient impact flow field with multiple hydrothermal jets is analyzed in terms of axial temperature, bottomhole temperature, and bottomhole pressure. Also, downhole velocity field is specifically investigated. Next, influences of time, jet temperature, and jet pressure difference on the flow field and heat transfer between wellbore fluid and ambient rocks are predicted and compared. The results indicate that the bottomhole central temperature and pressure are higher than the two sides under multiple hydrothermal jets conditions, which is similar to the flow pattern with a single jet. Moreover, the distribution of axial velocity has three peaks at cross-sections, and the centerline axial velocity is the highest. There is a negative relationship between the maximum radial velocity and the ratio between axial distance and nozzle diameter (L/D). Also, the position of the maximum radial velocity moves to the side wall as the ratio (L/D) increases. Second, the bottomhole temperature increases uniformly with increase in jet temperature. The bottomhole temperature becomes decreasingly sensitive to the variance of pressure difference, and the bottomhole pressure exhibits a ladder distribution. Finally, while an increase in either jet temperature or pressure difference can enhance the multiple hydrothermal jets heat transfer effect, increasing the jet temperature is considerably more effective. All of these results are beneficial to the parameters design for the hydrothermal jet drilling technology.

Numerical simulation of direct contact condensation of subsonic steam injected in a water pool using VOF method and LES turbulence model

The direct-contact condensation (DCC) of subsonic steam injected into a subcooled water pool has been simulated using the Volume of Fluid (VOF) multiphase flow model and Large Eddy Simulation (LES) turbulent flow model of FLUENT. A thermal equilibrium model has been introduced to simulate the phenomenon of DCC. The condensation model is a two resistance model which takes care of the heat transfer process on both sides of the interface. The numerical results for the steam plume can be divided into four stages: the initial stage, the maximum stability stage, the oscillatory stage and the detachment stage. Fairly good qualitative agreement was obtained with the available experimental data for the plume shape and evolution characteristics of the steam plume. In addition, it was found that the centerline axial temperature increases with the decrease of the centerline axial velocity at the pipe exit and the pressure oscillation is mainly affected by the steam velocity, steam condensation and subcooled water hydrostatic pressure.

Temperature, velocity and air entrainment of fire whirl plume: A comprehensive experimental investigation

Comprehensive measurements were performed to examine the temperature, velocity (in axial and tangential directions) and air entrainment of propane fire whirl plume in a medium-scale fixed-frame facility. It is found that the radial profile of excess temperature varies consistently with the continuous flame shape and the radial decay rate of excess temperature decreases in the intermittent flame and plume. The centerline axial velocity depends on both the buoyancy and the axial pressure gradient related to the tangential velocity. A weak annular reverse flow is found outside the upward core at z/D⩽1.0, while the absolute axial velocity slightly increases with fire size. The growth of axial velocity core is found to be constrained by the reverse flow. The mean axial velocity radius is confirmed to be greater than the mean excess temperature radius in the continuous flame. The radial profile of tangential velocity varies steadily with height, and is little affected by heat release rate. The radial swirl flow consists of the vortex core, the quasi-free vortex and the near wall region, and the active reaction occurs in the vortex core. The plume growth rate increases with decreasing RiB (Richardson number) and three distinct regions are identified in different RiB ranges. The air entrainment is strong at the bottom inflow boundary layer, while the mass flow rate increases slowly with height as compared to buoyant pool fires. The suppression of air entrainment may be attributable to the radial force balance and stable stratification in the centrifugal acceleration filed. The entrainment coefficient in the continuous flame is about one to two orders of magnitude lower than that in a buoyant pool fire, and it generally increases with height in the plume of fire whirl, approaching the value for pure buoyant plume. The mean entrainment coefficient depends on the mean RiB by −1 power law for z⩽Hf.

Hydraulic characteristics of turbulent circular jets under surface confinement

Experimental study is carried out to investigate the hydraulics of turbulent circular jet under surface confinement. The effect of surface confinement on the jet centerline velocity and axial velocity profiles under different surface confinements and jet exit velocities has been studied. The decay of jet centerline velocity is high under surface confinement compared to the free jet. The effect of surface confinement on the velocity profile in the vertical axis is negligible near the jet exit; however, it increases downstream of the jet exit. Flux entrainment increases linearly along the jet axis and decreases as surface confinement increases, while the kinetic energy decreases linearly along the jet axis and increases as surface confinement increases.

Experimental investigation of acoustic streaming in a cylindrical wave guide up to high streaming Reynolds numbers

Measurements of streaming velocity are performed by means of Laser Doppler Velocimetry and Particle Image Velociimetry in an experimental apparatus consisting of a cylindrical waveguide having one loudspeaker at each end for high intensity sound levels. The case of high nonlinear Reynolds number ReNL is particularly investigated. The variation of axial streaming velocity with respect to the axial and to the transverse coordinates are compared to available Rayleigh streaming theory. As expected, the measured streaming velocity agrees well with the Rayleigh streaming theory for small ReNL but deviates significantly from such predictions for high ReNL. When the nonlinear Reynolds number is increased, the outer centerline axial streaming velocity gets distorted towards the acoustic velocity nodes until counter-rotating additional vortices are generated near the acoustic velocity antinodes. This kind of behavior is followed by outer streaming cells only and measurements in the near wall region show that inner streaming vortices are less affected by this substantial evolution of fast streaming pattern. Measurements of the transient evolution of streaming velocity provide an additional insight into the evolution of fast streaming.

Reynolds Number Effects on Fully Developed Pulsed Jets Impinging on Flat Surfaces

A systematic study of the effect of the Reynolds number on the fluid dynamics and turbulence statistics of pulsed jets impinging on a flat surface is presented. It has been suggested that the influence of the Reynolds number may be somewhat different for a jet subjected to pulsation when compared to an equivalent steady jet. A comparative study of both steady and pulsating jets is presented for a Reynolds number range from to . All the other factors that affect the flowfield are kept constant, which are , , and . It was found that for the range of the Reynolds numbers tested, pulsation results in a shortening of the jet core, the centerline axial velocity component declines more rapidly, and higher values of the radial velocity component for are observed. As the Reynolds number increases, the jet spreads more rapidly, the turbulent kinetic energy and nondimensional turbulent fluctuations decrease, and the flowfield near the impinging surface changes drastically, which is evident with the development of a turbulent momentum exchange interaction away from the wall for .

Approximation approach to multiple singularities of flow through a porous pipe with decelerating wall

The multiple singularity behaviour of flow through a porous pipe with decelerating wall is numerically studied in the present paper. The behaviour of the Riccati equation is introduced as a model problem. Then the steady axisymmetric flow of a viscous incompressible fluid driven along a pipe by the combined effect of the wall deceleration and suction is investigated. Our approach uses the power series in order to observe the instability of the problems. The series is then summed by using various generalizations of the Pade´-Hermite approximants. Analysis based on approximate method suggests that the convergence of the series of stream-function, skin friction and centerline axial velocity in powers of Reynolds number is limited by a number of singularities. The location and nature of the singularities in the real plane are presented. The bifurcations of skin friction and centerline axial velocity are also depicted graphically. DOI: http://dx.doi.org/10.3329/jname.v9i1.9283 Journal of Naval Architecture and Marine Engineering 9(2012) 35-42

Two distinct regions in a pulsed-axisymmetric jet

This research concerns on the axial measurement of a pulsed-axisymmetric jet to investigate the decay of inverse of mean centerline axial velocity with the use of a single normal hot-wire anemometer. The measurement is performed by controlling mean exit velocity and pulsing frequency. The axial measurement reveals that the pulsation method leads to the appearances of two distinct regions namely the pulsed dominated-and the high turbulence steady jet-regions. However, the existence of the two regions can be controlled by adjusting the mean exit velocity and pulsing frequency. At a high mean exit velocity and low pulsing frequency, the high turbulence steady region is not seen, hence the transition region can not be found. Furthermore, at a high pulsing frequency and a constant mean exit velocity, the pulse dominated region becomes shorter. The present study has provided a comprehensive knowledge in that the trend of the decay rate is very clear under the wide range of pulsation method imposed on the jet. This inclusive understanding becomes a substantial consideration for mixing processes as well as can be used to verify the k-e turbulence model to predict the decay of normalized mean centerline-axial velocity.

Steady and unsteady flow patterns in the supraglottal region of excised larynges.

2‐D‐particle image velocimetry was used to obtain time averaged and phase locked planar flow mapping of the near field supraglottis in the mid‐coronal plane for five excised canine larynges. Time averaged flow fields indicate that jet was either symmetric about the axis or was found to skew to one side. Further investigation using phase locked measurements indicate that the near field supraglottal jet was found to be mostly straight during the entire opening phase of the glottal cycle, whereas during the closing phase of the glottal cycle, two different behaviors were observed. The supraglottal jet was found either to skew to one side or flap like a flag. The direction of skew was found to be arbitrary. Either of these behaviors were found to be exhibited during one phonation trial in a larynx. The angle of skew was found to increase toward the end of closing as compared to early closing. Axial and tangential velocity profiles along with jet spreading rate and centerline axial velocity decay are also compared with synthetic vocal fold model data published by other investigators.