Quasi-steady Stage Research Articles

A new calculation method of viscoplastic heat production generated by plastic flow of friction stir welding process

Friction stir welding (FSW) is a solid phase welding method. The weld forming and welding quality are related to the heat production generated by plastic flow during FSW process. Therefore, it is of great importance to measure the viscoplastic heat production during FSW process. In this paper, firstly, taking the welding system during quasi-steady state welding stage as a rotary viscosity measuring meter, the relationship between viscous moment and viscosity is deduced. Secondly, the analysis model for plastic flow around welding pin in FSW process is established by applying the principle of fluid dynamics. The relationship between the temperature gradient on the surface of welding pin, the rotation rate of the FSW machine and viscous torque is obtained. Lastly, a new calculation method for viscoplastic heat production in FSW process is proposed with the help of numerical simulation. To verify the validity of the new calculation method, several groups of friction stir welding experiments are carried on. • The welding system during quasi-steady state welding stage is taken as a rotary viscosity measuring meter. • The analysis model for plastic flow around welding pin in FSW process is established. • A calculation method only related to rotation rate for viscoplastic heat is proposed.

Injection characteristics and fuel-air mixing process of ammonia jets in a constant volume vessel

The direct-injection of gaseous ammonia is a possible way to fuel engines to meet the scenario of zero-carbon emission. In this study, for the first time, the injection characteristics and fuel–air mixing process of ammonia jets are investigated. A Schlieren system is employed to characterize the macroscopic behavior of ammonia jets including the tip penetration and jet angle. Besides, the fuel concentrations of ammonia jets are quantitatively measured by LIBS to investigate the fuel–air mixing processes. According to the time evolutions of jet tip penetration, the three-stage behavior, namely the t, t0.5 and (t-τ)0.25 dependence corresponding to the early stage of injection, quasi-steady stage and after two times of injection duration respectively, are proposed and the mechanisms are discussed in detail. The effects of both injection and ambient pressures on the tip penetration and jet angle, as well as the fuel concentration distributions are investigated. The increased injection pressure leads to increases in the tip penetration and fuel concentration but decreases in the jet angle. The effects of ambient pressure are opposite, indicating that the ammonia jet development is governed by the injection-to-ambient pressure ratio. The comparisons between ammonia and methane jets are conducted. The results show they have quite similar tip penetrations and jet angles, as well as the nearly equivalent fuel mole fractions at the jet axis. However, the equivalence ratios in ammonia jets are significantly lower than those in methane jets, leading to the distinct flammable mixture distributions in ammonia and methane jets.

Modeling of the entire processes of diesel spray tip penetration including the start- and end-of-injection transients

The start-of-injection (SOI) and end-of-injection (EOI) transients are important for sprays with multiple-injection strategy. Owing to the change of needle position and sac pressure, the spray behaviors in the transient processes are significantly different from those in the quasi-steady stage. In this study, considering the sac pressurization processes during the SOI transients and effects of ''entrainment wave'' after the EOI, a theoretical zero-dimensional model for the entire development processes of the spray tip penetration (Stip) is summarized. Then, a high-speed CMOS camera and constant-volume chamber are employed to study the spray characteristics and verify the model. The model and experimental results clarify four key time points in the entire development processes of Stip: the sac pressurization time (tp), breakup time (tb), injection duration (ti) and two injection durations (2ti). Five stages can be divided according to these four time points: the acceleration stage (Stip proportion to t1.5), transition stage 1 (t1 or t0.75), quasi-steady stage (t0.5), transition stage 2 (t0.5) and decelerating stage ((t-ti) 0.25). Moreover, the newly developed model is compared with the Wakuri model, the Hiroyasu & Arai model, and the Naber & Siebers model. The results show that all the models can well predict Stip during the quasi-steady stage and transition stage 2 with the optimized model constants. While the other models overpredict Stip in the acceleration and decelerating stages, the newly developed model still works very well.

Spread and burning characteristics of continuous spill fires in a tunnel

Changing burning area of spill fire may cause chain effects and expand consequences in tunnels, which poses a greater challenge to personnel escape and rescue. Research on tunnel spill fire is needed to reveal its behavior and internal mechanisms. This paper carried out a series of continuous spill fires on fireproof glass both in tunnel environment (TE) and open environment (OE). The spread, burning, and heat transfer characteristics were compared between these two environments. The results showed that spread areas of TE spill fires in quasi-steady stage were smaller than those of OE. The burning rate enhancement coefficients ranged from 0.92 to 1.76 from small spill fires and larger ones in tunnel. With respect to penetrating heat radiation and heat conduction loss, TE spill fires were slightly larger than OE spill fires. And it was proved that the heat feedback is the main cause for the burning rate enhancement in tunnel. Then a semi-empirical model was proposed by coupling the spread, burning and heat transfer process. The model was proved to be quite accurate by validating with experimental data.

Flow morphology in bottom-propagating gravity currents over immersed obstacles

The interaction of bottom-propagating gravity currents with immersed obstacles in the path of the lock-exchange configuration was numerically investigated based on large eddy simulations. The three-dimensional Navier–Stokes solver was quantitatively employed to resolve the flow structure of gravity currents and their dynamics during impact. The integral measure of analysis comprises the front condition, the energy budget, the turbulent mixing, and the force response. Some flow parameters involved in momentum and energy fluctuations are the fractional depth of volume release, relative density difference, and obstacle dimensions. A particular focus in this study was on the scale effect of obstacles (W/D, the aspect ratio of a cross-sectional obstacle with side length W to height D) that affect the propagation of gravity currents. Depending on integral measures of the simulation, the flow morphology could be demarcated for the condition W/D = 2 with different flow regimes in accordance with the reattachment of the current as the plunged front overflows and separates from the obstacle. For W/D > 2, as the current impinges on the obstacle, the plunged current front overtops and travels on the obstacle surface and consequently causes the entrainment without intense mixing to form a circulation zone at the downstream of the obstacle. Accordingly, the predicted drag forces acting on the downstream surface are reduced by ∼25% for W/D ≥ 4 comparing to the case of W/D = 0.5, which is beneficial to the structural stability of the barrier in practical aspect. Notably, the integrated analysis of gravity currents provides insights into physical mechanisms by identifying distinct propagation stages during transitions, including the impact stage, transient stage, and quasi-steady stage.

Flame spread and combustion characteristics of two adjacent jatropha oil droplets

The present study investigated the flame spread and combustion characteristics of two jatropha oil droplets, droplet A and B. Specifically, the pyrolysis characteristics of jatropha oil were studied using thermogravimetric and differential scanning calorimetry (DSC) analysis. Liquid chromatography-mass spectrometry (LC-MS) was used to analyze the components in jatropha oil. Then, the flame spread from droplet A to droplet B and combustion dynamics of two droplets were studied in a constant volume chamber under atmospheric pressure and room temperature (300 K). The effects of normalized droplets spacing S/d0 (2.7–4.9) on flame spread were also studied. The results showed that the combustion of jatropha oil droplet containing three stages: initial expansion stage, quasi-steady combustion stage and micro-explosive combustion stage. Periodic puffing and micro-explosion occurred during micro-explosive combustion stage. This is due to the superheating of low boiling-point components produced by pyrolysis of long-chain fatty acids in jatropha oil. The duration of initial expansion stage increases significantly with the increase of droplet spacing. When the normalized droplet spacing reaches 4.9, the second droplet cannot ignite and reached flame spread limit. Interestingly, the flame spread velocity increase at first and then decrease with increase droplet spacing. This is because the diffusion flame is cooled by unburned droplet when the spacing is relatively small. However, when the spacing is relatively large, the flame spread velocity mainly depends on the duration of second droplet in first stage.

Flow field generated by a dielectric barrier discharge plasma actuator in quiescent air at initiation stage

A single Dielectric Barrier Discharge (DBD) plasma actuator driven by Alternating Current (AC) power, capable of inducing a starting vortex and a wall jet in quiescent air, is suited for low-Reynolds-number flow control. However, the starting vortex and the wall jet are usually observed after the plasma actuator has been operated for dozens of and hundreds of cycles of the voltage, respectively. The detail of the induced flow field at the initiation stage of the plasma actuator has rarely been addressed. At the initiation stage, a thin jet that provides the impetus for the entrainment of the induced flow at the beginning of the plasma actuation is first observed by using a high-accuracy phase-lock Schlieren technique and a high-speed Particle Image Velocimetry (PIV) system. This is the initial form of the momentum transfer from the plasma to the fluid. Then, an arched type jet is created by the plasma actuator. In addition, the whole development process of the induced flow field from the starting point of the thin jet to the quasi-steady stage of wall jet is presented for providing a comprehensive understanding of the plasma actuator and proposing a relevant enhancement of the numerical simulation model.

Multi-scale multi-mode nonlinear interaction in tokamak plasma turbulence with moderate small-scale shear flow

Effects of moderate small-scale shear flow, e.g., which may be created by the trapped electron mode, on electromagnetic (EM) ion-scale turbulence in tokamak plasmas are numerically investigated via a self-consistent Landau-fluid model. A modeling analysis is carried out in slab geometry to reveal the underlying mechanism of the multi-scale multi-mode nonlinear interaction. Results show that while a Kelvin–Helmholtz (KH) instability with long wavelengths may be excited by the shear flows to dominate the multi-scale EM fluctuation, shorter wavelength ion temperature gradient (ITG) modes experience multiple quasi-steady (QS) stages with enhanced fluctuation level through different driving and saturation mechanisms. One mechanism is the secondary ITG instability due to the decrease in flow stabilization modified by the zonal flow. Meanwhile, the other one is the modulational interaction between the EM ITG and KH modes through the nonlinear mode coupling. Moreover, the synergism of these two mechanisms may sustain the final QS state near the marginal KH instability threshold. Complex linear and nonlinear interactions among multiple modes and external flow, as well as self-generated zonal flow, result in a weak dependence of the final saturation level of the dominant EM ITG mode on the small-scale flow amplitude. The turbulent heat transport is visibly suppressed by weaker shear flow, but is almost not affected by stronger shear flows. The underlying mechanism is elaborated.

Unsteady natural convection of water adjacent to the sidewall of a differentially heated cavity with multiple fins

The unsteady thermal flow and heat transfer adjacent to the sidewall of a differentially heated cavity with single and multiple nonconductive fins are investigated utilizing the finite element method. The transition to the unsteady flow, as well as flow oscillations, is described for the water-filled cavity and Rayleigh numbers ranging from 1 × 107 to 1 × 1010, including the critical Rayleigh number. Characteristics of unsteady flows are investigated for early, transitional, and quasi-steady stages. For all cases, heat transfer from the finned sidewall to the interior fluid is quantified and compared with the cavity without the fin. Results show that the volumetric flow rate along the vertical axis in the middle of the cavity rises by increasing the number of fins (up to 13.41% for the cavity with two fins for Ra = 1.84 × 109) in transitional and quasi-steady stages. Moreover, the increment of fin number and Ra leads to a higher enhancement factor (ε) that shows the heat transfer enhancement or depression. For Ra = 1.84 × 109, the maximum ε for the cavity with one and three fins is calculated about 17.5% and 34% respectively.

Molecular dynamics simulation on evaporation of a suspending difluoromethane nanodroplet

Nanodroplet evaporation is a basic process widely existing in nature and industrial applications. Difluoromethane (CH2F2, also called “R32”) has attracted more and more attention in refrigeration, heat pump, Organic Rankine Cycle and other fields because of its excellent thermophysical characteristics. In this work, the evaporation process of R32 nanodroplet in large space is studied by means of molecular dynamics simulation. This work focuses on the dynamic evaporation characteristics of droplets under different conditions (droplet size, initial temperature, and ambient temperature) in the evaporation process, and quantitatively analyzes the effects of different parameters on the evaporation rate. The simulation results were compared with the results calculated from the diffusion-based model and the kinetic model. Based on the dynamic law of evaporation process, such as density, temperature field and velocity distribution, the calculation accuracy of each model is evaluated, the reasons for the deviation between model prediction and simulation results are also discussed. The results show that the increase of ambient temperature and initial saturation temperature can lead to a significant increase of droplet evaporation rate, and shorten the pre-heating time needed for droplet to reach quasi-steady evaporation stage. The increase of droplet size also increases pre-heating time, but has no significant effect on evaporation rate. For the evaporation scenario studied in this paper (Knudsen number (Kn) ~ 1), the existing modification methods of the kinetic model can enhance the prediction accuracy of the evaporation rate of R32 droplets in the quasi-steady evaporation stage. The prediction deviation of diffusion-based model applied directly is large, the accuracy of prediction can be significantly improved by correcting the thermal conductivity considering the scale effect.

Conditions for the occurrence of notable edge waves due to atmospheric disturbances

It has been observed and theoretically proved that edge waves can be generated by a moving atmospheric pressure disturbance near the coast. This study aims to clarify the necessary conditions for the occurrence of significant edge waves associating with moving atmospheric pressure disturbances. The complex physical problem is simplified as a pressure disturbance moving along a coast with a constant beach slope. The dominant factors include the spatial scale and translational speed of disturbance, the beach slope, as well as the transient response of water surface. The effects of each factor are discussed based on numerical modeling. It is found that the quasi-steady wave pattern is controlled by two non-dimensional parameters, the ratio of the disturbance spatial scale to the wave length of fundamental edge wave and the ratio of the disturbance spatial scale to the distance from the center of disturbance to shoreline. The duration of atmospheric disturbance or the growth time of edge wave is also found to play an important role in the occurrence of notable edge waves. Starting from a calm sea, the growth of edge wave can be divided into a fast growth stage and a quasi-steady stage. The duration of the fast growth stage is affected by the spatial scale and the beach slope. When the fundamental mode of edge wave is dominant, the appearance of a notable edge wave is affected by the translational speed and the beach slope.

Experimental study on evaporation characteristics of single and multiple fuel droplets

In this work, evaporation experiments of multiple droplets are carried out in a stagnant hot atmospheric environment (573, 673 and 773 K) using high-speed backlit image technique. Three fuel droplets with nearly same initial diameter are suspended at intersections of two 0.1 mm quartz fibers. The normalized droplet spacing (s/d0) of three droplets is 2.25. The results show that the evaporation process of single, edge and central fuel droplet containing three stage: initial heating, unsteady evaporation and quasi-steady evaporation stage. Classical d2 law is still suitable for edge and central droplet at quasi-steady evaporation stage. The third stage of edge and central droplet accounts for more than 60% of droplet lifetime at low temperatures and about 50% at high temperatures. The evaporation rate constant of edge and central droplet increases and droplet lifetime decreases with increasing ambient temperature. The evaporation time of edge and central droplet at first and third stage is higher than single droplet, but lower than single droplet in the second stage. More importantly, the evaporation interactions between droplets is significant at low temperature. Compared with single droplet, the lifetime of central droplet is increased by 31.8%, 18.6% and 25.9%, respectively.

Computational investigation on a self-propelled pufferfish driven by multiple fins

Based on experimental measurements of a live fish, a numerical model is established for a pufferfish driven by the locomotion of its multiple flexible fins. The self-propelled motion of the fish is investigated under unsteady swimming conditions, including the accelerating and subsequent quasi-steady stages, to analyse the motion mechanisms of a real fish model with multiple flexible fins. Present numerical approach combines Computational Fluid Dynamics (CFD) and Multi-Body Dynamics (MBD), which allows for the analysis of the interaction between fluid and a fish with multiple fins. Benefitting from the available experimental data of the live fish, the deformation of each fin surface can be prescribed, and we name this condition as flexible in this paper. To elucidate the impact of flexible fins on the propulsion performance of fish swimming, simulations are also performed for rigid fins with the same kinematic motion profiles imposed on the leading edge of fin surfaces. With the developed tool, unsteady hydrodynamic forces, vortex interaction between body and fins associated with MPF swimming mode are numerically predicted. The obtained results highlight the effect of flexibility of fins on thrust generation and efficiency improvement for fish undergoing free swimming.

Study of molten pool dynamics and porosity formation mechanism in full penetration fiber laser welding of Al-alloy

To study molten pool dynamics, root hump formation, periodic keyhole behavior in full penetration laser welding (FPLW) of aluminum alloy, a numerical simulation was carried out, in which volume of fluid (VOF) method and ray-tracing algorithm were adopted. A varied metallic vapor shear stress model was considered. Meanwhile a series of welding experiments on specimens being composed of aluminum alloy and quartz glass were conducted to analyze the porosity formation process. It was found that the over-heated sagging with low surface tension stagnated at the bottom side and solidified to form root hump. The dominant backward heat convection at the lower part can well explain the extreme spreading of molten pool at bottom surface. Periodic behavior of the keyhole was identified during quasi-steady stage. The porosity can be effectively suppressed in FPLW. Both the porosity ratio and average porosity number were reduced obviously. The main mechanism of porosity formation in FPLW is the collapse of the rear keyhole wall, mainly caused by bulges on the front keyhole wall. Bubble coalescence is responsible for large porosity size and coalescence efficiency depends on bubble size difference.

Inner and outer electron diffusion region of antiparallel collisionless reconnection: Density dependence

We study inflow density dependence of substructures within electron diffusion region (EDR) of collisionless symmetric magnetic reconnection. We perform a set of 2.5D particle-in-cell simulations which start from a Harris current layer with a uniform background density nb. A scan of nb ranging from 0.02 n0 to 2 n0 of the peak current layer density (n0) is studied keeping other plasma parameters the same. Various quantities measuring reconnection rate, EDR spatial scales, and characteristic velocities are introduced. We analyze EDR properties during quasisteady stage when the EDR length measures saturate. Consistent with past kinetic simulations, electrons are heated parallel to the B field in the inflow region. The presence of the strong parallel anisotropy acts twofold: (1) electron pressure anisotropy drift gets important at the EDR upstream edge in addition to the E×B drift speed and (2) the pressure anisotropy term −∇·P(e)/(ne) modifies the force balance there. We find that the width of the EDR demagnetization region and EDR current are proportional to the electron inertial length ∼de and ∼denb0.22, respectively. Magnetic reconnection is fast with a rate of ∼0.1 but depends weakly on density as ∼nb−1/8. Such reconnection rate proxies as EDR geometrical aspect or the inflow-to-outflow electron velocity ratio are shown to have different density trends, making electric field the only reliable measure of the reconnection rate.

Research on passive control of cloud cavitation based on a bionic fin-fin structure

Purpose The purpose of this paper is to use the NACA 0015 symmetric hydrofoil as the research subject and control cloud cavitation on hydrofoils. Design/methodology/approach Based on observed distribution of caudal fin spines on fish, a bionic structure of fin-like spines is arranged on the hydrofoil suction surface, which maintains the cavitation in a quasi-steady state stage by eliminating the cyclic shedding process of cloud cavitation. Based on the modified shear stress transport k-ω turbulence model and the Zwart–Gerber–Belamri cavitation model, this paper compares and analyzes the NACA 0015 hydrofoil and the bionic NACA 0015 hydrofoil under condition of an angle of attack of 8° and a cavitation number of 0.8. Findings The results show that the average drag of the hydrofoil is reduced but the lift is decreased, and the lift-drag ratio is increased after arranging the bionic structure. The bionic structure can effectively reduce the turbulent kinetic energy and make the flow more stable; it also can effectively control the hydrofoil surface side-entrant jet and the vortex shedding process of the near wall region. Originality/value Based on the above conclusions, the bionic structure of fin-like spines can achieve a significant passive control in the hydrofoil cloud cavitation process.

Enhancing the flow and heat transfer in a convective cavity using symmetrical and adiabatic twin fins

The enhancement of natural convection flow and the associated heat transfer in a differentially heated cavity with two horizontal adiabatic fins attached to each sidewall are numerically investigated at different Rayleigh numbers and with various fin positions in this study. The numerical approach is validated by both representative scales and experimental shadowgraph results. A typical double-plume flow regime is identified at both the early and fully-developed stages. The interaction of the two plumes is investigated in detail in this paper. With the increasing of the Rayleigh number, the separation frequency of the plume above both fins increases. It is also revealed that the separation frequency of the upstream plume at the quasi-steady stage does not only depend on the characteristics of the upstream flow, but also greatly depends on the position of the downstream fin. The frequency varies non-monotonically as the downstream fin moving toward the cavity ceiling, suggesting complex nonlinear characteristics. Compared to the scenario without any fin, the flow rate across the cavity is significantly boosted, which is increased by up to 136.2% at the early stage and 124.8% at the fully-developed stage. The heat transfer rate across the cavity having symmetrical fins on sidewalls is enhanced by up to 12.7% at a Rayleigh number of 3.68 × 109.

Study on kerosene spray combustion and self-extinguishing behaviors under limited ventilation

Kerosene spray combustion and self-extinguishing behaviors were experimentally studied in a ceiling vented compartment with various injecting pressures and ceiling vent sizes. The parameters including flame height, oxygen concentration and self-extinguishing time et al. were considered. The results show that the combustion process can be divided into three stages. e.g. growth stage, quasi-steady stage and self-extinguishing stage. Both injecting pressure and ceiling vent size have significant effects on the flame height. At the growth stage, the increase of injecting pressure or vent size results in a higher maximum flame height. However, when spray flame reaches quasi-steady stage, the flame height mainly depends on injecting pressure and the vent size effect can be almost negligible. The dimensionless average flame height at the quasi-steady stage was found to be well correlated with dimensionless heat release rate. Moreover, the self-extinguishing behavior was also investigated and both the oxygen concentration and self-extinguishing time are significantly influenced by injecting pressure and vent size. With the increase of injecting pressure, the self-extinguishing time is evidently shortened, while increasing the vent dimension has an opposite effect. Furthermore, the parameters Q̇VA and VQ̇A were proposed for evaluating the coupled effects of injecting pressure and vent size on oxygen concentration and self-extinguishing time, respectively. The corresponding correlations were also developed, and moreover the predicted value agree well with the experimental data.

Small scale experiment study on burning characteristics for in-situ burning of crude oil on open water

In order to improve the understanding of the burning behavior for the in-situ burning of the spilled crude oil on water, the influence of the oil pool diameter and oil layer thickness of pool fire on open water were studied in relation to the flame height, boilover, and burning efficiency. A series of experiments were conducted in a specially designed experimental setup which could simulate the boundary conditions of the actual ISB operation on water. Three quartz tubes with different diameters and three initial oil layer thicknesses were set as the initial conditions in this study. The results showed that the burning process of oil pool fire on open water could be divided into four stages which are growth stage, quasi-steady stage, decay stage and extinguishing stage. The average flame height of quasi-steady stage increased with oil pool diameter but was little influenced by the initial oil layer thickness. Boilover phenomena were observed in the experiment processes when the oil pool diameter was 150 mm, which showed much later boilover onset time, longer boilover duration and higher average flame height than that for the same size oil paool fire in steel vessel with water sublayer. The burning efficiencies increased with the initial oil layer thicknesses for a given oil pool diameter, which could be explained as the thermal insulation of oil layer. All of the burning efficiencies on experimental dimensions are below 65%, owing to the significant cooling effect of the large amount of water surrounding the burning system.

Dynamics of hollow cathode discharge using fluid models with and without metastable atom involvement in helium

The characteristics of hollow cathode discharge are simulated by two types of fluid models in helium, one with metastable atom involvement and another without. Discharge current, particle density, and ionization rate are simulated. Results show that metastable atoms exert an important influence on the steady state discharge and the temporal characteristics of this discharge. Discharge is maintained at a lower sustain voltage in the model with metastable atoms than in that without metastable atoms. At the same low sustain voltage, marked differences in quantitative and qualitative characteristics are observed between the two models. With increasing sustain voltage, the discharge parameters simulated by the two models become qualitatively similar, but their quantitative differences increase. The discharge current and electron density in the model with metastable atoms are significantly higher than that in the model without metastable atom involvement at the same sustain voltage. Furthermore, the discharge current and electron density gap simulated by the two models increase as the sustain voltage increases. The difference in characteristics between the two models originate from variation of the contribution of different ionization types to the generation of new electrons at different sustain voltages. With increasing sustain voltage, stepwise ionization originating from metastable atoms becomes increasingly crucial to the discharge. Results further show that the temporal characteristics of the two models are similar when the metastable atom density is low. However, in the model with metastable atoms, discharge current and particle density continue to rise until a steady discharge is obtained after a quasi-steady stage. The different temporal characteristics observed between the models appear to originate from the influence of metastable atoms. The percentage of stepwise ionization relative to the total ionization increases with time.