Centerline Velocity Decay Research Articles

Exploration of the effect of hot smoke generated by tunnel fire on the ventilation flow field of jet fan

Abstract To reduce the hazard of hot smoke from tunnel fires, ventilation becomes a necessary control strategy. Uniform ventilation is the main method used in the study of tunnel fires, but it has obvious differences from the widely used jet fan ventilation in practice, and the two have different performances in controlling smoke. Therefore, the distribution of the upstream flow field with jet fan ventilation will be studied in a tunnel fire. Considering several parameters such as the HRR and the velocity of fan exit, numerical simulations were conducted to monitor the fire development by using a Fire dynamics simulator (FDS). The results suggest that the center-line velocity decay of a jet fan can be categorized into three regions: linear decay, power function decay, and stabilization. The decay coefficient (n) related to power function decay correlates with HRR, and therefore, a new prediction for the center-line velocity distribution of the jet fan is proposed. The predicted results correlate well with the available experimental data.

Analysis of mixing enhancement ability of bypass dual throat nozzle with single/multi-tabs

Bypass dual throat nozzle (BDTN) represents an innovative fluidic thrust vectoring nozzle capable of adjusting thrust vectoring angles through self-adaptive flow control. This study explores the BDTN-single/multi-tabs configurations, ensuring that the nozzle’s outlet projected area remains constant while introducing one or more pairs of tabs coplanar with the convergent section of the cavity. This design accounts for aerodynamic performance, thrust vectoring capabilities, and mixing abilities of the BDTN. Through theoretical analysis, design implementation, numerical simulations, and wind tunnel experiments, findings reveal that, in the case of BDTN-single-tab configuration, an increase in tab angle (α) leads to heightened streamwise vortex intensity at the nozzle outlet, reduced high-temperature core area, and enhanced jet centerline velocity and temperature attenuation rate. BDTN-single-tab achieves up to a 90% reduction in centerline-temperature/velocity decay rate, implying stronger mixing enhancement with higher tab angles. Compared to α = 10° BDTN-single-tab, the area of the high-temperature core area decays by 75%. For the BDTN-multi-tab configuration, an increase in the number of tabs results in increased thrust coefficient and vectoring angle, alongside diminished streamwise vortex strength at the nozzle outlet, and a more complex streamwise vortex structure. However, the jet’s centerline velocity and temperature decay rate decreased as a consequence.

Experiments into the interaction of side dilution jets with bluff-body stabilised annular jets

Fundamental insights into the effects of side dilution jets on the flow behaviour and turbulence field in gas turbine combustors further advances the ability to use these jets to control (upstream) flame stability and (downstream) emissions. Despite this, the interaction of side dilution jets with typical flame anchoring mechanisms, such as bluff-bodies, and annular flows, both swirling and non-swirling, remains elusive due to the challenges of undertaking highly resolved laser diagnostics in such complex geometries. In this work, Particle Image Velocimetry (PIV) is used to investigate the interaction of turbulent side dilution jets (Red = 18,000 and 11,300) with bluff-body stabilised annular jets (Res = 35500 and 17800), under both swirling (S = 0.3) and non-swirling (S = 0) conditions. Boundary conditions for the cases studied are well-defined through Constant Temperature Anemometry, which in conjunction with the PIV resolved flow field means the results presented additionally serve as good benchmark cases for modelling (not undertaken here). The experiments are conducted in an optically accessible 700 mm long square-like (310 mm × 310 mm) enclosure equipped with four peripheral side dilution jets on the rounded corner supports.Results show that irrespective of the flow configuration used (swirling or non-swirling), side dilution jets considerably modify the overall flow field and lead to the development of characteristic recirculation zones on the periphery of the annular jet, named as Peripheral Recirculation Zones (PRZ). In the upstream region, PRZ significantly promote turbulence in the outer shear layer of the bluff-body stabilised annular jet, leading to a massive 176% (at x/D = 1.4) and 218% (x/D = 1.8) increase in peak turbulent kinetic energy. These high levels of turbulence further magnify closer to the location of the side jets. In the downstream (beyond x/D > 2) of the bluff-body stabilised annular jet, noticeable increases in centreline velocity decay (13%) and jet spreading (32%) are observed in the presence of a PRZ, which indicates better mixing and flow entrainment. The overall effects of side dilution jets vary directly and inversely with the Reynolds number of side dilution jets and annular jets, respectively. To resolve the flow dynamic behaviour independent of heat release, all tests are undertaken isothermally (non-reacting).

Effect of Plasma Actuator on Velocity and Temperature Profiles of High Aspect Ratio Rectangular Jet

The turbulence jet centerline velocity and temperature decay intensely along the centerline flow direction. Thus, improving it could benefit engineering applications, such as air conditioners. However, active flow control techniques with high-aspect-ratio jets, especially for controlling the temperature, have not been widely investigated. This paper presents the velocity and temperature performance of a high-aspect-ratio rectangular jet controlled by two dielectric barrier discharge plasma actuators located on the longer sides of the nozzle and controlled by high-voltage and high-frequency pulse-width modulation sinusoidal waves. The scanning method was used to cover 362 cases as combinations of working parameters (modular frequency vs. duty vs. phase difference) for the velocity and temperature performances of the jets. Results show that plasma actuators can control both velocity and temperature distribution with minor input power compared with the rectangular jet’s kinetic energy and heat flux. The velocity increased up to 4% and decreased to 11%, measured at the interest position where x/h = 70 on the centerline. There were a 5% increase and a 4% decrease compared to the temperature-based case. Distinctive velocity and temperature distributions were observed under noteworthy cases, indicating the potential of the actuator to create various flow features without installing new hardware on the flow.

Effect of side dilution jets on the velocity field and mixing in swirl and bluff-body stabilised annular confined flows

Stationary and mobile power generation systems, as well as those for aero propulsion, continue to rely heavily on gas turbines. Operational envelopes which critically depends on (flame) jet stabilisation mechanisms, such as swirl, are also affected by considerations for emissions control. In this context, the evolving swirling flow domains feature injection of side (peripheral) dilution air. Despite this, the role of side dilution jets on modifying the flow, turbulence, and mixing fields of turbulent bluff-body (BB) stabilised (swirling) annular flows has not been investigated. This study uses three-dimensional simulations, supported with experimental data to establish boundary conditions, to investigate the interaction of side dilution jets (Red = 18,000) with both non-swirling (S = 0) and swirling (S = 0.3) turbulent BB stabilised annular flows (Res = 35,500). Constant Temperature Anemometry (CTA) measurements are conducted to resolve inlet boundary conditions. The Reynolds Stress Model (RSM) and species transport model (isothermal) are used with validations against benchmark datasets. Results show that the introduction of side dilution jets into a confined domain, whether swirling or not, induces (asymmetrical) Peripheral Recirculation Zones (PRZ). However, the strength, length, and location of PRZ varies with swirl and the downstream positioning of side dilution jets (H = 2.6D, 6.6D). Additionally, side dilution jets increase centreline velocity decay for both, non-swirling and swirling BB flows. In terms of turbulence characteristics, addition of side dilution jets leads to significant gains in shear layer turbulence with an increase of 72% and 35% in turbulent kinetic energy for non-swirling and swirling BB flows, respectively. Analysis of the mixing characteristics shows negligible impact of side dilution jets in the upstream region, however, from x/D > 3 considerable improvement in central jet (fuel) mixing is observed.

Schlieren and BOS velocimetry of a round turbulent helium jet in air

Seedless velocimetry is gaining interest in many industrial and research applications. We report on a comparative study of time-resolved optical velocimetry using traditional, mirror-type knife-edge schlieren optics versus Background-Oriented Schlieren (BOS) of subsonic round turbulent helium jets in air at Red = 5890 and 11,300. Digital images with 1024 pixels streamwise resolution (0 ≤ x/d ≤ 200) were captured at 6000 frames/s in large ensembles. Velocimetry was performed on these results by digital image correlation (DIC) using OpenPIV software, and by streak-schlieren analysis of x-t diagrams (Kymography). Limited PIV data were also collected for verification of the schlieren velocimetry results. Both BOS and traditional schlieren show partial success in measuring the mean-flow helium jet self-similarity in terms of the 1/x decay of centerline velocity, Gaussian-shaped radial velocity profiles, and linear spreading rate of the jet. Visualized turbulent eddies, used as tracers in schlieren velocimetry, are observed to last longer than is necessary for this purpose in the present helium jets. Also, the measured convective velocity appears to be sufficiently robust to sum to the jet mean velocity in some of the results. Kymography yields better overall results than DIC, which we attribute to Kymography's spatiotemporal “spectrum” of jet velocities, enabling the discrimination of fast eddies near the jet centerline from slower ones near the jet periphery. DIC and other analysis methods suffer from a path-averaging bias which negatively affects the results. The reduction of kymographic data for velocimetry was done manually and also by a Fourier-transform image-feature-orientation code, both yielding equivalent results.

Flow regime and Reynolds number variation effects on the mixing behavior of parallel flows

The hydraulic single-phase mixing of three parallel rectangular channels is experimentally investigated at various Reynolds numbers (Re) and flow regime combinations. Particle Image Velocimetry results for seven mixing cases are presented and discussed with varying Re combinations ranging from 1,824 to 20,844. While all cases result in the same Re ratio of ∼0.69 between the inner and outer flows, two cases represent multi-regime mixing with the inner-outer regime pair of laminar-transitional and transitional-turbulent, while the other 5 cases are all characteristic of turbulent mixing with varying levels of turbulence. The outer channels initially share characteristics with a backward facing step. The center channel is found to initially behave like a slot jet, but then sees a significant increase in velocity decay. This inner flow velocity decay increased dramatically in the laminar-transitional mixing case, whose centerline velocity decay was ∼6 times larger than the decay in the turbulent mixing cases. Second order statistics revealed a consistent mixing layer thickness of ∼0.1 hydraulic diameters for all the cases but showed more intense shearing in the multi-regime mixing cases. The combined point and thereby the mixing layer length is determined using centerline velocity decay profiles, which show a much more aggressive mixing in multi-regime flows. Multi-regime mixing demonstrated superior characteristics relative to turbulent mixing due to a more dramatic velocity decay in the inner flow and a shorter mixing length. The contributions of this work include communicating the benefits of multi-regime mixing and providing detailed characterization efforts that can serve future efforts for validating computational models. This research also lays the groundwork for future studies aimed at achieving high levels of mixing without a severe penalty in pressure drop.

Numerical analysis of non-excited and excited jets issuing from non-circular nozzles

This paper concentrates on the application of passive and active flow control methods to jet flows. The passive control is obtained by shaping a nozzle cross-section (hexagonal, hexagram, square, triangular) and the active one by an external excitation. The research is performed by applying the large eddy simulation method (LES). The obtained results are thoroughly validated by comparisons with theoretical, experimental and numerical data for non-excited jets. The presented results comply well with the reference data, both in terms of the mean velocity profiles, velocity fluctuations profiles and the Strouhal number of the preferred oscillation frequency (Stp). Based on its value, the excitation frequencies (Stf) are chosen such to cover the range of Stp and its subharmonics. The applied excitation is defined as a sinusoidal function, which enforces the axial velocity oscillations at the level of 10% of the bulk velocity. It is shown that the dynamics of the excited non-circular jets is significantly different from the dynamics of their non-excited counterparts. It is demonstrated that by changing Stf one may modify both the instantaneous as well as their time-averaged behaviour, i.e., the centreline velocity decay, turbulence intensity, directional spreading rate and the entrainment rate. It is found that not all jets are prone to excitations to the same extent. In the far-field, the dependence of the velocity decay rate on Stf is the most pronounced in the jets issuing from circular and hexagonal nozzles. In the near-field, regardless of Stf the excitation causes a faster velocity drop, shortening of the potential core and increase of the centreline fluctuations in all jets. However, in the hexagram jet, the fluctuations turn out to be the least sensitive to change of Stf. Apparently, in this case, the complex nozzle shape determines the velocity oscillations so strongly that a relatively low amplitude excitation has only a small influence. In this respect, the jets issuing from square and triangular nozzles show the opposite, as for them the impact of the excitation is the strongest. Depending on Stf three qualitatively different scenarios of the fluctuations growth are identified by either manifesting their smooth increase or the occurrence of local maxima and minima. The entrainment in the square and triangular jets also turned out to be more dependent on the excitation rather than by the nozzle geometry. In the near-field, the excitation with Stp/2<Stf<Stp leads to almost doubling the entrainment maximum found for the non-excited jets.

Numerical Investigation of Jet Control Using Two Pulsed Jets under Different Amplitudes

A turbulent jet with a Reynolds number of 71,000, controlled by a pair of pulsed jets operating 180° out of phase, is investigated by large-eddy simulation. The effect of the forcing amplitude on jet development and generated coherent structures is investigated through six cases: an unforced case and five cases at different amplitudes. With this forcing mode, the jet bifurcates in the far field that promotes the spread of the shear layer. It was found that the jet yields the largest bifurcation angle at medium amplitude, while the bifurcation is restrained at lower or higher amplitudes. As the amplitude increases, the streamwise vortex pair generated by the control jet is stronger and penetrates deeper into primary jet. This phenomenon reduces the inclination of vortex ring when mutual interaction between vortex ring and streamwise vortex pair occurs. Meanwhile, at higher amplitudes, the inclined vortex ring is stronger and unevenly distributed, prompting the jet bifurcation. In order to investigate the coupling effect of amplitude and frequency, different forcing frequencies were also simulated. The results demonstrated that the optimal frequency based on centerline velocity decay rate decreases with increasing amplitude; however, it eventually saturates around StD = 0.1.

Numerical simulation of reynolds number effect of a free incompressible isothermal turbulent coaxial jet

This paper studies the effect of Reynolds number on a two-dimensional free incompressible isothermal coaxial turbulent jet over a range of high Reynolds numbers. This is necessary because of its application in noise control and mixing. The Reynolds numbers at the nozzle exit were 9824, 19648, 29472, 39296 and 49120. The models were designed in ANSYS Design Modeler and the numerical simulation was done using a finite volume based Computational Fluid Dynamics (CFD) in ANSYS FLUENT using the two-dimensional Realizable turbulence model. The Governing equations were discretized using the finite volume method with the solution based on the PISO algorithm. The decay of centerline velocity, turbulent kinetic energy profile, the radial profile of axial velocity and similarity profile were investigated along the flow direction. Contour plot indicates that the velocity is high at the jet exit and decreases downstream due to the rapid mixing of the inner and outer jet and the surrounding fluid. It is found generally that Reynolds number plays significant role especially before self-similarity region. The result shows that increasing the Reynolds number give rise to more turbulence which in turn decreases the potential core length, turbulent kinetic energy and enhances the mixing of the fluid. However, at the jet exit, the flow with the lowest Reynolds number has the highest turbulent kinetic energy because it suffers the greater shear. The spreading of the jet was more or less independent of the Reynolds number beyond the self-similarity region. It is also found that the velocity profile is brought to congruence at about z/D=25 for the Reynolds numbers considered

Parametric study of a frequency-modulated pulse jet by measurements of flow characteristics

Recently, pulsed jet actuators have been discovered as an efficient and promising technique for flow separation control. In this paper, a systematic experimental investigation is carried out using a flow control approach, in order to study the flow field characteristics of a baseline single pulsed jet actuator, under different excitation conditions. The findings are used to provide a better understanding of pulsed blowing actuator operation and the associated flow physics. In these experiments, two types of signal waveforms have been implemented to produce the pulsed excitation of jet; a simple square-wave excitation signal for the first case and a burst modulated excitation signal for the second case. All measurements were conducted using a single hot-wire system to an axial distance x/D ≤ 30 along the centerline. The effects of electrical parameters on properties of a single pulsed jet, including the excitation frequency from 10 up to 220 Hz and the duty cycle from 15% up to 80% for the first case, and different combinations of the carrier frequency and the modulating frequency for the second case have been studied. These measurements revealed a significant dependence of mean centerline velocity and turbulence intensity to excitation signal, frequency, and duty cycle. The statistical flow properties, such as mean centerline velocity decay and turbulence intensity distribution of pulsed jets are quantified and compared together over a wide range of pulsing parameters. This investigation aids in designing and optimizing flow control actuators in aerodynamics and combustion chambers.

Investigation on the Gas Jet Flow Performance Confined in Round Pipe

In this study, the round jet flow behavior arose by the confinement was investigated experimentally and numerically. The confinement characteristics based on multi-scale characterizations in the atmospheric gas jet flow, which generated from a circular symmetrical subsonic nozzle and flowed into a confined round pipe, was used to studied. The studied region near the nozzle possess really short axial length (within 12d), providing initial conditions and boundaries that affected the flow behaviors in this region. A wide range of inlet velocities (0.98 m/s~84.72 m/s) and confined space sizes (1~20) were involved for simulated and quantitative discussions. The velocity profile evolution was recorded by simulations and characterized by a laser Doppler anemometer (LDA) with a resolution of 0.01 m/s. The confinement characteristics were systematically presented to elucidate the performance under the studied confined conditions with different metrics, such as centerline velocity decay (VR), entrainment rate (MR), pressure coefficient (Cp) and length of recirculation region (LR). The results indicated that the recirculation fluid action mainly contributed to the promoted initial velocity profile evolution with the introduction of the confined space. Theoretically, the confined space could reduce the maximum absolute entrainment mass flux, initiating a peak at the center of the recirculation region, which was controlled by the inlet velocity and the confined space size collectively. The remarkable effect of confined space size further contributes to the confinement characteristics of gas jet flow, representing a shortened flow distance despite similar process as that by reducing the diameter ratio (dR). In some cases, because of the miniaturization of confined space size, the dispersion of initial velocity profile evolution was not significant displayed throughout the variable inlet velocity range, especially when the dR was less than 2. This study gives a new insight in the performance of jet flow confined in round pipe, and such knowledge will be helpful to provide great potential and reference to applied fluid mechanics.

Velocity distribution of wall-attached jets in slotted-inlet ventilated rooms

The wall-attached jets for ventilation in spaces have been drawn considerable interest for their benefit of high ventilation efficiency and thermal sensation. In this study, the velocity distribution of a wall-attached jet under isothermal conditions in slotted-inlet ventilated spaces is investigated by experimental and numerical approaches. The wall-attached jet issues air from a slot inlet that flows down along the surface of a vertical wall, impinging on the floor attached to the vertical wall and spreading over the floor surface. The results show that the flow field of a wall-attached jet can be divided into three regions: vertical attachment region (region I), horizontal air reservoir region (region II), and jet impingement region. The correlation equations for the centerline velocity decay and velocity profiles are obtained. For the fully developed jet in region I and II, the dimensionless velocity profiles can be expressed as u0/um(y*)~(y*/b)γ and u0/um(x)~(x/b + Kh)γ, respectively, where γ is a constant related to the momentum decay rate. Over the whole range of jet flow conditions in this study, the value of γ is 1.11. Moreover, a unified exponential expression for the cross-sectional velocity profiles is proposed for both regions I and II. For the jet impingement region, the flow is complex and involves separation, reattachment, vortical flow, and pressure gradients. Large-scale vortices exist in the corner, and the vortices diminish as the jet inlet velocity increases. The current study can provide a theoretical basis for further engineering design applications of wall-attached jets for attachment ventilation.

Wake of transversely rotating and translating sphere in quiescent water at low Reynolds number

The unsteady wake of a rotating and translating sphere is experimentally investigated at Reynolds numbers (Re) ranging from 115 to 550 for two rotational speeds. Non-dimensional rotational speed, $$\Omega^{ * }$$ , is defined as the ratio of the maximum azimuthal speed of the sphere and the speed of translation. Motion of the sphere is generated using a specially designed mechanism in a channel, and its wake has been visualized using fluorescent dye. Unlike the steady flow past a stationary sphere where the limiting Reynolds number is 270, the flow becomes unsteady when sufficient rotation rate is introduced at much lower Reynolds number ( $${\text{Re}} = 115,\Omega^{ * } = 0.375$$ ). The transition from one-sided to double-sided hairpin vortex occurs at one critical Re for a given rotational speed, and it switches back to one-sided hairpin vortex shedding at another critical Re. Particle image velocimetry results demonstrate the vorticity distribution in the wake and the variation of the wake width with axial distance. At very low Re, velocity profiles in the wake are nearly self-similar. The strength of the vortex rings weakens in the far wake. The decay of centerline velocity in axial direction confirms the dissipation of the vorticity in the far wake. In this unsteady flow, the time-averaged value of the separation angle is higher than that of the steady flow on the advancing side due to enhanced adverse pressure gradient. The Strouhal number calculated from the dye visualization in this range of $$\Omega^{ * }$$ and Re varies from 0.175 to 0.25, a clear increasing trend both with increasing Re and $$\Omega^{ * }$$ . The coefficient of drag evaluated at the center plane remains constant at 0.94 for the present investigation up to $${\text{Re}} = 550$$ . The wavelength of the unstable hairpin vortices is found to scale inversely with the translational velocity of the sphere.

Effects of Hydrogen-Enrichment on Flame-Holding of Natural Gas Jet Flames in Crossflow at Elevated Temperature and Pressure

The effect of hydrogen (mathrm {H}_{mathrm {2}}) enrichment on the flame-holding characteristics of two natural gas jet flames in crossflow is investigated here, experimentally. The flame and flowfield measurements are analyzed using simultaneously acquired high-speed (10 kHz) stereoscopic particle image velocimetry, planar laser-induced fluorescence of the hydroxyl radical, and OH* chemiluminescence. The flames, enriched with 20% and 40% mathrm {H}_{mathrm {2}}, by volume, are studied at conditions typical of the mixing duct of a modern gas turbine engine; specifically in confinement, at 10 bars, and with a crossflow preheat of 530 K. Consistent with previous findings, the 40% mathrm {H}_{mathrm {2}} flame was found to be stabilized on the windward and leeward side of the jet, while the 20% mathrm {H}_{mathrm {2}} flame was stabilized only on the leeward side. Analysis of mean and instantaneous velocity fields showed no major differences in the trajectories and principal compressive strain fields of the two flames. The presence of the windward stabilized flame in the 40% mathrm {H}_{mathrm {2}} case was, however, found to decrease the centerline velocity decay and greatly reduce or eliminate large scale vortices along the windward shear layer. The difference in the flame-holding here was attributed to the difference in the extinction strain rate from the addition of hydrogen, which would impact the local and global extinction of the flame along the high shear windward region of the flame.

Experimental investigation of the influence of Reynolds number and buoyancy on the flow development of a plane jet in the transitional regime

Heated horizontal plane jets find wide applications in engineering appliances such as air curtains and discharge of industrial effluents. In the present study, experimental investigations are conducted on a heated horizontal plane jet with the Reynolds numbers in the transitional regime, using a hotwire anemometer. In the far to very far-field (20 < x/d < 100) centreline velocity decay and jet spread increases faster with the decrease of Reynolds number. This is because, with the increase of Reynolds number, the turbulent kinetic energy is distributed on a broadband of scales. As a result, larger scales, which are responsible for increased entrainment, get weaker. The shifting of the centre plane generally occurs in the far region for low Reynolds number jets. A comparison with the result of an isothermal jet at similar Reynolds numbers from the literature at identical conditions shows that the turbulence intensity is decreased due to heating. Centreline velocity decays slowly and half-width increases marginally for a heated jet when compared with an isothermal jet. The effect of heating is prominent for low Re jets. Spectral development shows a delayed transition due to heating. Probability density function plots reveal lack of equilibrium and presence of large-scale eddies in the flow field.

Comparative studies on isothermal attachment ventilation based on vertical walls, square and circular columns

Ventilation and air distribution methods play an important role in indoor air quality and thermal environments. Effective airflow distribution within indoor environments has always been a great cause of concern. Attachment ventilation has drawn considerable interest since it is more energy efficient and space saving than traditional mixing or displacement ventilation methods. As a newly developed and applied attachment ventilation method, vertical wall-based attachment ventilation (WAV) was first proposed in the 1990’s by the authors. Later, the concepts of square column-based attachment ventilation (SAV) and circular column-based attachment ventilation (CAV) were also presented to meet the needs of more industrial applications. In this paper, three attachment ventilation methods— i.e., WAV, SAV and CAV are compared based on experimental studies and numerical simulations. For the vertical attachment region, semi-theoretical equations characterizing the dimensionless centerline velocity of WAV, SAV and CAV are proposed. The velocity profiles of all three types of attachment ventilation methods are similar. Of the three attachment ventilation methods, for the same distance y* along the attached wall measured from the slot inlet, the dimensionless centerline velocity of WAV is the largest, while those of SAV and CAV remain almost identical. In the horizontal air reservoir region (air lake zone), the semi-theoretical equations to predict dimensionless centerline velocity also are presented. The results show that for the same distance x normal to the wall/column, the dimensionless centerline velocity of WAV is the largest, followed by SAV and CAV. Additionally, unified semi-theoretical equations are presented for predicting the jet centerline velocity decay of attachment ventilation, which is helpful for the design of air distribution systems in heating, ventilation and air conditioning (HVAC) engineering applications.

Modeling of cryogenic hydrogen releases

Hydrogen is likely to be stored and transported as a liquid with high densities to have sufficient hydrogen on-site and for efficient distribution. Thus, cryogenic hydrogen jets and dispersion characteristics need to be studied to quantify the risks of accidental releases. The present study modeled cryogenic hydrogen jets with stagnation temperatures around 50 K released from round nozzles at stagnation pressures of 2–10 bar. The instantaneous flow fields were modeled using the Large Eddy Simulation (LES) turbulence model to accurately and efficiently capture the flow characteristics. The numerical models were validated with experimental data. The concentration and velocity distributions show that the centerline mass fraction and velocity decay at similar rates to the decays for room temperature jets. The radial distributions of the mass fraction, temperature and velocity are Gaussian and self-similar. Additionally, the numerical model can reproduce the shock structures near the nozzle for these underexpanded cryogenic hydrogen jets. The location of the first normal shock was predicted to be closer to the nozzle than for room temperature jets. The present work can be used to develop and revise safety codes and standards for liquid hydrogen facilities.

Effect of co-flow velocity ratio on evolution of poly-disperse particles in coaxial turbulent jets: A large-eddy simulation study

In the present work, the particle-laden coaxial turbulent jet flow is studied using large-eddy simulation (LES). An Eulerian–Lagrangian framework is used to study the interaction between the continuous phase (air) and the discrete phase (glass bead particles). The solver is validated, using single-phase and particle-laden simulations, with reference data from experiments. A good match is observed between the present results and the reference data, for centerline velocity decay and radial profiles of axial velocity. Simulations are performed for three co-flow velocity ratios of 0, 1, and 1.5. The results pertaining to particle characteristics are presented for three different particle size-classes. The effect of the co-flow velocity ratio on the particle size–velocity correlation and velocity statistics of both phases are studied with an emphasis on understanding the differences in the particle dispersion due to co-flow around the central jet. It is observed that the particle size–velocity correlation is negative in the potential core region, and it becomes positive as one moves downstream. For heavy particles, the axial distance required to attain the same velocity as that of air increases with an increase in the co-flow velocity ratio. The size-conditioned particle number density profiles along the axial and radial directions of coaxial jets showed some interesting trends that could be explained based on the particle Stokes number effect. Significant radial dispersion of particles is realized when the corresponding Stokes number (StL), defined based on large-scale turbulent eddies, is of the order of one. The axial evolution of the characteristic particle size exhibited non-monotonic trends for all co-flow ratios. Overall, the present work demonstrates potential application of LES for an in-depth study of dispersion of poly-disperse particles in turbulent coaxial jets.

Direct numerical simulations of turbulent viscoelastic jets

Direct numerical simulations (DNS) of spatially evolving turbulent planar jets of viscoelastic fluids described by the FENE-P model, such as those consisting of a Newtonian fluid solvent carrying long chain polymer molecules, are carried out in order to develop a theory for the far field of turbulent jets of viscoelastic fluids. New evolution relations for the jet shear-layer thickness , centreline velocity and maximum polymer stresses are derived and validated by the new DNS data, yielding , , and , respectively, where is the coordinate in the streamwise direction. It is shown that, compared with a classical (Newtonian) turbulent jet, the effect of the polymers is to reduce the spreading rate, centreline velocity decay, Reynolds stresses and viscous dissipation rate. The self-preserving character of the flow is analysed and it is shown that profiles of mean velocity, Reynolds stresses and polymer stresses are self-similar provided the proper scales are used in the normalisation of these quantities. A fundamental difference from the Newtonian jet in this regard is the necessity for two, instead of only one, different velocity and length scales to properly characterise the evolution of the turbulent flow. These extra velocity and length scales are directly related to a time scale associated with the characteristic fading memory property of viscoelastic fluids.