Effect Of Gas Expansion Research Articles

Experimental and numerical investigation on dynamic behaviors of the concrete wall in underground coal mine with hydraulic blasting demolition

In order to investigate the dynamic response and failure modes of concrete wall under hydraulic blasting load in underground tunnels, the fracture mechanism and zoning characteristics of hydraulic blasting of concrete wall was analyzed theoretically. Stress field, displacement field and energy variation laws of concrete wall has been simulated based on the finite element dynamic program LS/dyna. Meanwhile, different experimental conditions on the dynamite, reserved hole parameters such as demolition parameters are designed studied according to the blasting fracturing principle. The effects of stress wave, high pressure gas expansion effect in the dismantling process, application of line of least resistance principle and directional blasting effect control principle are analyzed. Results reveal the FSI effect between concrete wall and the water-dynamite-pore wall and the relationship between parameters of the structure and the dynamite dynamite, the concrete dynamic compressive strength and dynamic tensile strength. Numerical and experimental methods as well as conclusions will provide basis for the blasting demolition of brick concrete wall under similar conditions.

Pulsating behaviors of flame spread across n-butanol fuel surface

Pulsating behaviors of flame spread over n-butanol fuel are investigated with various initial temperatures under quasi-quiescent condition. The flame pulsating is an alternate process of diffusion combustion (crawling phase) and premixed combustion (jumping phase). Non-dimensional pulsating velocity Vfmax-VfminVf varies directly proportionally with the square root of non-dimensional initial temperature (THb-T0)/THb. The fundamental mechanisms of flame pulsation are physically interpreted based on heat and mass transfer in both liquid- and gas-phase flows. In liquid phase, a thermal vortex that is driven by the combined effects of Marangoni force and buoyancy force is formed ahead of the flame tip to enhance fuel evaporation rate. In gas phase, the existence of a gas-phase recirculation cell is indispensable for flame pulsation. Counter-current buoyancy dominates hot gas expansion effect, which produces a gas-phase recirculation cell ahead of the flame tip in pulsating flame spread regime. Otherwise, the uniform flame spread regime is achieved. The critical temperature between the uniform and pulsating flame spread regimes of n-butanol is predicted to be 27.9°C, which is acceptably consistent with the experimental value of 30.2°C. Finally, the coupling process between flame tip and subsurface convection flow front is achieved. Three steps are included in the coupling process of both movements: (a) flame crawling and subsurface convection flow crawling, (b) flame jumping and subsurface convection flow crawling, and (c) flame crawling and subsurface convection flow jumping.

Analysis of combustion noise of a turbulent premixed slot jet flame

The sound generation due to the interaction between the turbulent flow field and a premixed slot flame is numerically studied by a hybrid analysis. The impact of varying gas expansion ratios and Markstein lengths is investigated. The acoustic flame response function and the acoustic source terms determine the overall flame response to heat release fluctuations. The location where sound is emitted is computed by the spatial and spectral acoustic source term distribution and a spectral flame front response study. The acoustic flame response of the slot jets is similar to that of round jets. From the current results and data from the literature it is suspected that as long as the flame undergoes a turbulent flame–flow interaction a typical acoustic pressure and heat release trend leads to an acoustic flame response trend of αSt2 in the low frequency region (St ≲ 20) and a plateau region βSt0 for St ≳ 20. In the energy containing region (αSt2), the acoustic sound is produced by local heat release fluctuations determined by the spectral flame front response at several mean flame front locations. The acoustic source terms depend on the gas expansion and shear layer effect induced by the flame front kinematics. At the flame base, flame pocket generation is enhanced at decreasing gas expansion ratio due to the increased vortex–flame interaction. At the flame tip, flame pocket generation is increased at high temperature ratios due to the hydrodynamic instability effect. The flame front kinematics affects the acoustic source terms by the flame–flow interaction and leads to a lower magnitude of flame response for decreasing gas expansion ratios.

Coupling discharge and gas dynamics in streamer-less spark formation in supercritical N2

A two-dimensional cylindrically symmetric model is developed to study the streamer-less spark formation in a short gap on the timescale of ion motion. It incorporates the coupling between the electric discharge and the gas through the heat generated by the discharge and the consecutive gas expansion. The model is employed to study electrical breakdown in supercritical N2. We present the simulation results of gas heating by the electrical discharge and the effect of gas expansion on the electrical discharge.

Hydrodynamic instability and shear layer effects in turbulent premixed combustion

A turbulent premixed plane jet flame is analyzed by large-eddy simulations. The analysis shows that the flame front wrinkling is strongly influenced by the shear layer effect when the gas expansion effects are small leading to larger flame front amplitudes at the flame base than at high gas expansion ratios. However, the hydrodynamic instability effect induces a continuously increasing flame front amplitude which yields an enhanced flame pocket generation at the flame tip. Both phenomena influence the magnitude of the turbulent burning area and burning area rate response through the flame front deflections which are determined by the contribution coefficient. This coefficient represents the mutual interaction between the flame and the flow. At low gas expansion ratios, the total heat release rate spectra of the turbulent flame are wider in terms of dominant modes at Strouhal numbers which are linked to the mean flame height oscillations. Thus, at low gas expansion ratios, the vortex-flame interaction is less damped by the flame in the sense that vortices can perturb the flame front stronger. The total heat release rate trend of St−2.2 previously found for a round jet flame is also determined for the current slot jet at realistic gas expansion ratios indicating a general tendency to transfer energy from large to small flame structures. At high gas expansion ratios, an increasing Markstein length leads to an energy transfer between neighboring dominant modes in the low frequency range 1 < St < 10 and the burning area rate response becomes more important for the total heat release rate spectra of the turbulent slot flames which agrees with recent findings for a laminar premixed plane flame.

Nonlinear analysis of an acoustically excited laminar premixed flame

The nonlinear effects of the fully coupled two-way interactions between a disturbance field defined by an amplitude variation of the inflow velocity distribution and a laminar premixed flame incorporating gas expansion effects are investigated by numerically solving the conservation equations of a compressible fluid. Results of the higher harmonics of the flame front perturbation for two burnt to unburnt temperature ratios show how the nonlinear transfer of flame response is affected by the hydrodynamic instability and the shear layer effect. The flame response at the fundamental forcing frequency, i.e., the flame-describing function can be well predicted by the reduced-order model (ROM) for both investigated temperature ratios at low Markstein length values, where the balance between the flame stretch effects and the hydrodynamic instability is insignificant. However, at high Markstein lengths the balance between the flame stretch and the hydrodynamic instability influences the flame response characteristics at all excitation amplitudes. The nonlinear study of the flame front perturbation indicates that differences in the higher harmonic content of the overall nonlinear flame response between the detailed computation and the ROM predictions are located at the flame front just before the flame pockets separate. Thus, the shear layer and the hydrodynamic instability effect determine the nonlinear character of the flame response through the nonlinearity introduced at the flame base, the location of flame pocket separation, and the annihilation process at the flame tip especially at high Markstein length values. In addition, the flame base movement response is compared to a recent developed reduced-order flame base model to understand the findings with respect to the assumption of flame attachment at the burner lips.

Hydrodynamic instability and shear layer effect on the response of an acoustically excited laminar premixed flame

Combustion instabilities can cause serious problems which limit the operating envelope of low-emission lean premixed combustion systems. Predicting the onset of combustion instability requires a description of the unsteady heat release driving the instability, i.e., the heat release response transfer function of the system. This study focuses on the analysis of fully coupled two-way interactions between a disturbance field and a laminar premixed flame that incorporates gas expansion effects by solving the conservation equations of a compressible fluid. Results of the minimum and maximum flame front deflections are presented to underline the impact of the hydrodynamic instability on the flame and the shear layer effect on the initial flame front wrinkling which is increased at decreasing gas expansion. These phenomena influence the magnitude of the burning area and burning area rate response of the flame at lower frequency excitation more drastically than reduced-order model (ROM) predictions even for low temperature ratios. It is shown that the general trend of the flame response magnitudes can be well captured at higher frequency excitation, where stretch effects are dominant. The phase response is influenced by the DL mechanism, which cannot be captured by the ROM, and by the resulting discrepancy in the flame pocket formation and annihilation process at the flame tip.

Self-accelerating flames in long narrow open channels

In this work we extend our earlier asymptotic one-dimensional analysis of flame propagation in long narrow channels open at both ends to two-dimensional flames. The analysis follows two tracks; a multi-scale asymptotic study and a full numerical study of the unsteady propagation. We show that during the early stages of propagation the flame accelerates at a nearly constant rate, independent of the channel height. In sufficiently narrow channels, the flame retains a constant acceleration until it reaches the end of the channel, consistent with our earlier work. In wider channels, however, the flame beyond a certain distance begins to accelerate at a nearly-exponential rate, reaching exceedingly large speeds at the end of the channel. The flame self-acceleration arises from the combined effects of gas expansion and lateral confinement. The gas expansion that results from the heat released by the chemical reactions produces a continuous flow of burned gas directed towards the ignition end of the channel. Due to the frictional forces at the walls and, since the pressure at both ends is maintained constant, the gas motion that develops in the burned gas sets a pressure gradient that drives the fresh unburned gas towards the other end of the channel. Stretching out to reach additional fuel, the flame extends towards the fresh mixture propagating faster. And because of lateral confinement, the gas expansion induces large straining on the elongated flame surface that further increases its propagation speed. The asymptotic approximation properly predicts the initial propagation stage, the location within the channel where the sudden acceleration begins and the early stages of the self-accelerating process. The full numerical study confirms and extends the asymptotic results, showing that in long but finite channels premixed flames could self-accelerate reaching velocities that are ten-to-twenty times larger than the laminar flame speed.

Flow around individual Taylor bubbles rising in a vertical column with water: Effect of gas expansion

This paper presents an experimental study to evaluate the effect of bubble expansion on the dynamics of an isolated Taylor bubble rising in a vertical column of water by inspection of the liquid flow around the bubble front. An increase in bubble volume results from gas decompression due to hydrostatic pressure drop as the bubble ascends through the liquid. Two reduced pressures (33.3 and 20.0kN/m2) and atmospheric pressure were maintained at the free liquid surface to provide a wide range of expansion rates.PIV coupled with a Shadowgraphy Technique was used to simultaneously obtain the flow field ahead the bubble and a well-defined gas–liquid interface.A very good correlation was found between predicted bubble volume expansion rates considering ideal gas behavior and the upward net volumetric liquid flow rate obtained by integration of the velocity profiles. Above the bubble nose, the liquid velocity at the centerline decreases rapidly from the value of the Taylor bubble velocity at the tip of the bubble. For distances greater than 0.5D, the liquid flow is undisturbed, with a significant axial velocity component for high bubble volume expansion rates due to the displacement of the liquid promoted by gas expansion. The Reynolds number calculated for the liquid flow was less than 2100 for all conditions used. While this suggests a laminar flow regime in the liquid, coefficient C from Nicklin’s equation obtained from data was equal to 1.4. The shapes of the velocity profiles obtained at the undisturbed liquid flow show that for high expansion rates they resemble the power-law velocity profile for turbulent flow. The bubble volume expansion rate modifies continuously during the Taylor bubble rise and a well-defined liquid velocity profile is never attained.

Effect of gas expansion on the front shape of a Taylor bubble: an experimental contribution

An experimental study where an individual Taylor bubble rises through water with different bubble volume expansion rates is presented with the (front) bubble shape determination as main objective. A combination of two techniques, Particle Image Velocimetry (PIV) and Pulsed Shadowgraphy (PS), was used to collect images for further treatment in order to characterize the liquid flow pattern in front of the bubble and the bubble shape. Processing the images acquired with pulsed illumination from behind the bubble it was possible to define with precision the bubble shape at different stages when it was expanding. The operation conditions used allowed a wide range of volume expansion rates (0 to 28.5 × 10 -6 m 3 /s) with a significant effect on the Taylor bubble velocity; increases in bubble velocity up to 21% were observed relatively to constant volume system condition. Nevertheless, it seems that the front shape of Taylor bubbles does not change significantly with the upward liquid flow rates induced by gas expansion, at least for the volume expansion rates used in the experiments.

Experimental investigation of two-stage active magnetic regenerative refrigerator operating between 77 K and 20 K

This paper presents an experimental investigation on a magnetic refrigerator operating between 77K and 20K. Since the magneto-caloric effect of typical magnetic refrigerant is concentrated in a narrow temperature region, four different rare-earth intermetallic compounds (GdNi2, Dy0.85Er0.15Al2, Dy0.5Dr0.5Al2, and Gd0.1Dy0.9Ni2) are utilized according to their favorable temperature regions. The magnetic refrigerator is composed of two stages each of that is made of two different porous active magnetic regenerators. Helium gas as the heat transfer medium in the magnetic refrigerator is shuttled similar to a GM type pulse tube refrigerator so that the overall cooling capacity is enhanced by gas expansion effect when both gas and magnetic expansions are properly coordinated. The approximate peak magnetic field of 4T is provided for the internal region of the regenerators by the AC superconducting magnet which can fast ramp up to the maximum field for 3s. The fabricated magnetic refrigerator reached the lowest no-load temperature of 24K and the whole temperature span was approximately 56K rather than 57K. The cooling performance of the system is sensitive according to the load match between the first and the second stages in conjunction with helium mass flow rate distribution.

Modeling of Gas Thermal Effect Based on Energy Equipartition Principle

In the present article, a gas thermal effect is modeled based on the energy equipartition principle. Two new independent state equations for an ideal gas are developed that provide a new way to analyze thermal effects in gas lubrication. Furthermore, the energy equation is derived for gas lubrication and the analysis of thermal effects is carried out on a gas spiral thrust bearing and a gas hydrostatic journal bearing. The results show that gas temperature increases significantly in the lubricated region at high speed for both the thrust and hydrostatic journal bearings, and the thermal effect positively influences the load capacity of the thrust bearing. The gas expansion effect makes the gas temperature decrease in the hydrostatic journal bearing, and the gas temperature decreases with an increase in the inlet pressure.

Effect of CH4–Air Ratios on Gas Explosion Flame Microstructure and Propagation Behaviors

To reveal the inner mechanism of gas explosion dynamic behavior affected by gas equivalent concentration, a high speed Schlieren image system and flow field measurement technology was applied to record the gas explosion flame propagation and flame structure transition. The results show that a flame front structure transition occurs, followed by a flame accelerating propagation process. The laminar to turbulence transition was the essential cause of the flame structure changes. The laminar flame propagation behavior was influenced mainly by gas expansion and fore-compressive wave effect, while the turbulent flame speed mostly depended on turbulence intensity, which also played an important role in peak value of the explosive pressure and flame speed. On the condition that the laminar-turbulent transition was easier to form, the conclusion was drawn that, the lowest CH4 concentration for maximum overpressure can be obtained, which was the essential reason why the ideal explosive concentration differs under different test conditions.

Experimental investigation of flame spreading over pure methane hydrate in a laminar boundary layer

Flame spreading over pure methane hydrate in a laminar boundary layer is investigated experimentally. The free stream velocity (U∞) was set constant at 0.4m/s and the surface temperature of the hydrate at the ignition (Ts) was varied between −10 and −80°C. Hydrate particle sizes were smaller than 0.5mm. Two types of flame spreading were observed; “low speed flame spreading” and “high speed flame spreading”. The low speed flame spreading was observed at low temperature conditions (Ts=−80 to −60°C) and temperatures in which anomalous self-preservation took place (Ts=−30 to −10°C). In this case, the heat transfer from the leading flame edge to the hydrate surface plays a key role for flame spreading. The high speed flame spreading was observed when Ts=−50 and −40°C. At these temperatures, the dissociation of hydrate took place and the methane gas was released from the hydrate to form a thin mixed layer of methane and air with a high concentration gradient over the hydrate. The leading flame edge spread in this premixed gas at a spread speed much higher than laminar burning velocity, mainly due to the effect of burnt gas expansion.

Auto-ignition of methane–air mixtures flowing along an array of thin catalytic plates

In this paper, the heterogeneous ignition of a methane–air mixture flowing along an infinite array of catalytic parallel plates has been studied by inclusion of gas expansion effects and the finite heat conduction on the plates. The system of equations considers the full compressible Navier–Stokes equations coupled with the energy equations of the plates. The gas expansion effects which arise from temperature changes have been considered. The heterogeneous kinetics considers the adsorption and desorption reactions for both reactants. The limits of large and small longitudinal thermal conductance of the plate material are analyzed and the critical conditions for ignition are obtained in closed form. The governing equations are solved numerically using finite differences. The results show that ignition is more easily produced as the longitudinal wall thermal conductance increases, and the effects of the gas expansion on the catalytic ignition process are rather small due to the large value of the activation energy of the desorption reaction of adsorbed oxygen atoms.

The effect of gas expansion on edge flames stabilized in narrow channels

In this paper we numerically examine the stabilization of an edge flame in a confined mixing layer. Unlike most previous theoretical studies which have assumed, for convenience, that the density is constant and independent of temperature, the present work realistically accounts for density variations and their effect on the flow field. The focus is on the effect of lateral confinement on the flame structure and standoff distance, for both adiabatic conditions and in the presence of conductive losses to the channel walls. The inability of the reactants to diffuse outwards, as they would in an unlimited mixing layer, promotes mixing and as a result the premixed flame segment extends further in the transverse direction, stands farther away from the tip of the splitter plate, and the trailing diffusion flame is much shorter. In very narrow channels the resulting flame is a planar premixed flame that consumes all the supplied reactants. Heat losses cause a drop in temperature and as a result, the premixed flame segment is limited near the channel axis, with a shorter diffusion flame trailing behind.

The effect of spontaneous gas expansion and mobilization on the aqueous-phase concentrations above a dense non-aqueous phase liquid pool

The spontaneous expansion and mobilization of discontinuous gas above dense non-aqueous-phase liquid (DNAPL) pools can affect the aqueous-phase concentrations of the DNAPL constituents above the pool. The results of an intermediate-scale, two-dimensional flow cell experiment showed that the discontinuous gas flow produced by spontaneous expansion, driven by the partitioning of 1,1,1-TCA from the surface of a DNAPL pool, resulted in detectable aqueous-phase concentrations of 1,1,1-TCA well above the pool surface. In comparison to a conventional model for DNAPL pool dissolution in the absence of a discontinuous gas phase, these concentrations were greater than expected, and were present at greater than expected elevations. Additionally, this study showed that the discontinuous gas flow produced transient behavior in the aqueous-phase concentrations, where the elevated concentrations occurred as short-term, pulse-like events. These results suggest that the spontaneous expansion and mobilization of discontinuous gas in DNAPL source zones could lead to the misdiagnosis of source zone architecture using aqueous concentration data, and that the transient nature of the elevated concentrations could further complicate the difficult task of source zone characterization.

Dynamics of fractional condensation of a substance on a probe for spectral analysis

The fractional separation of trace metals on a cold tungsten probe from salt matrix vapor, which interferes with the spectral analysis, is studied. The spatial structure of the vapor flows of sodium chloride, potassium sulfate, and indium atoms is visualized at characteristic wavelengths as they interact with the probe. The vapor flow rate and the probe orientation were varied. It is found that the smoke of the matrix does not prevent the deposition of the metal on the probe because of spatial separation of these fractions and that the detrimental effect of thermal gas expansion and other factors is eliminated. The sensitivity of the atomic absorption analysis of indium impurities in these salts is increased by an order of magnitude.

Physical model of the development of external signs of barotrauma in Pacific rockfish

Four species of Sebastes (Pacific rockfish) showed evidence of a wide array of internal injuries from capture-induced barotrauma, including liver and swimbladder damage, organ displace- ment related to esophageal eversion, and hemorrhage in the pericardium and abdominal cavity. However, clear evidence of swimbladder rupture was not observed in all fish with external signs of barotrauma. Injection of air through the body wall into the swimbladders of rockfish carcasses gen- erated all of the common external signs of barotrauma documented in wild-captured fish, suggesting that the physical effects of swimbladder gas expansion can create these gross external signs without embolism from dissolved blood gases. Dissections of injected black rockfish S. melanops carcasses showed that, typically, injected air escaped the swimbladder without obvious rupture, moving in an anterio-dorsal direction, generating bulges and air bubbles that were externally visible through the branchiostegal membrane. Injected air also collected dorsally to the esophagus, posterior to the pha- ryngeal teeth, causing the esophagus to roll outwards into the buccal cavity (esophageal eversion). Injected air also frequently traveled further forward, collecting medially to the eyeball, leading to exophthalmia, and then moved distally along the fascia, invading the corneal stroma from the edges, resulting in corneal emphysemas. Air injected into the swimbladders of quillback rockfish S. maliger carcasses generated similar eye effects, but also escaped through ruptures in the branchiostegal membrane and did not generate esophageal eversion, which is also infrequent in wild-caught speci- mens. These results demonstrate that the major external signs of barotrauma in Pacific rockfish can develop as result of escaping swimbladder gases following an internal 'path of least resistance'.

Effect of gas expansion on the velocity of individual Taylor bubbles rising in vertical columns with water: Experimental studies at atmospheric pressure and under vacuum

This study was designed to determine the effect of gas expansion on the velocity of Taylor bubbles rising individually in a vertical column of water. This experimental study was conducted at atmospheric pressure or under vacuum (33.3 and 20.0 kN m - 2 ) using three different acrylic columns with internal diameters of 0.022, 0.032, and 0.052 m, and more than 4.0 m high. A non-intrusive optical method was used to measure velocity and length of Taylor bubbles at five different locations along the columns. The operating conditions used correspond to inertial controlled regime. In experiments performed under vacuum, there is considerable gas expansion during the rise of Taylor bubbles, particularly when they approach the liquid free surface where the pressure drop (due to the hydrostatic pressure) is of the order of magnitude of the absolute pressure. The liquid ahead of the bubble is displaced upward by an amount proportional to the gas expansion resulting in increased bubble velocity. The calculated Reynolds number suggests a laminar regime in the liquid ahead of the bubble. However, the experimentally determined velocity coefficient C for each column was much smaller than 2, which would be expected for laminar flow. The value of C obtained ranges from 1.13 ± 0.09 , for the narrowest column, to 1.40 ± 0.24 , for the widest column. This suggests that a fully developed laminar flow in the liquid ahead of the bubble is never achieved due to continuous bubble expansion at a variable rate, regardless of column height. The velocity coefficient C can be used to calculate the contribution of liquid motion to bubble velocity. Subtracting this contribution from the measured bubble velocity defines a constant value which is nearly identical to the bubble rise velocity measured in the same column operated as a constant volume system (two ends closed) where gas expansion is absent.