Sub-atmospheric Environment Research Articles

Numerical investigation on boiling heat transfer in a rectangular channel under sub-atmospheric environment

Water evaporators are often used as heat sinks in spacecraft. The aim of this study is to investigate the heat transfer characteristics and the spatial and temporal characteristics of bubble formation in an airborne water evaporator at an absolute pressure of 7 KPa. In order to achieve this, a rectangular single-channel model based on the structure of the water evaporator is established. Since the temperature change of the glycol in the water evaporator is not obvious when it flows through the rectangular single-channel, the inner wall surface is set to be 333.15 K in order to reduce the computational cost in the simulation. In this paper, a VOF method is adopted to numerically investigate the heat transfer characteristics of the boiling in the upper pool of the rectangular channel under a low-pressure(7 kPa) environment. Good agreement was obtained by comparing the numerical results with the experimental data with the error within 6 %. The numerical results show that bubbles rise along the wall, combine with nearby small bubbles, grow and attach to the wall, and slide along the wall until they are discharged from the top of the wall. The perturbations generated by the bubbles cause the heat transfer coefficient and wall temperature to fluctuate, with the maximum fluctuation of the wall temperature being around 5 K. In addition, the effects of different rectangular channel structure sizes (0.5 mm, 1.5 mm, and 2.5 mm) on the boiling heat transfer and bubble characteristics were investigated. A channel size of 0.5 mm leads to severe bubble coalescence, which increases the wall temperature and deteriorates the heat transfer performance. When the channel spacing is increased, bubble coalescence is improved, but a channel spacing of 2.5 mm results in less bubble turbulence and weaker heat transfer performance.

Effect of Pressure on Burning and Soot Characteristics of RP-3 Kerosene Droplets under Sub-Atmospheric Pressure.

The burning and soot characteristics of RP-3 kerosene droplets under sub-atmospheric pressure were experimentally investigated in a pressure chamber. The droplet size during combustion was continuously recorded using a high-speed camera, and the burning rate based on the d 2-law was determined. The flame temperature was calculated from ICCD camera spectral data using two-color pyrometry, and the carbon soot volume fraction was measured by the calibrated laser-induced incandescence (LII) technique. Soot particles were also sampled using a thermophoretic deposition probe and characterized using a transmission electron microscope (TEM) for particle size and morphology. The results showed that the droplet burning rate increased monotonically with increasing pressure under sub-atmospheric pressure, and the flame temperature slightly decreased with increasing pressure. Sub-atmospheric pressure environment significantly inhibits the formation of soot particle clusters during the ignition of droplets. The average soot volume fraction in the flame increases approximately with increasing pressure at 0.2-1.0 bar with a power of 2.044 ± 0.066. As the pressure decreases from 1.0 to 0.2 bar, the average soot volume fraction decreases significantly from 11.801 to 0.437 ppm. This is mainly due to the fact that the sub-atmospheric environment not only inhibits the collision growth of soot particles but also promotes the oxidation process of soot particles. The collected soot particles reveal a significant reduction in particle size under sub-atmospheric pressure with average primary soot particles of 48.23, 40.06, 27.096, and 18.718 nm at 1.0, 0.6, 0.4, and 0.3 bar pressures, respectively. The change in the number density of carbon soot particles is not significant, which reveals that the change in the diameter of soot particles under sub-atmospheric pressure plays a key role in the change in the volume fraction of soot.

Experimental and Numerical Investigation on Heat Transfer Performance of Water Evaporators with Different Channels and Fin Structures in a Sub-Atmosphere Environment

A water evaporator is a phase change heat exchanger, as a heat sink, especially for high-speed flight vehicles. It is composed of internal channels for convective cooling and external fins for boiling. In this paper, six water evaporators, Hex0–Hex5, are designed with five different internal channels and two different external fins. The VOF simulation models are built to investigate their boiling heat transfer performances in a sub-atmospheric environment. Experimental studies were carried out to verify the accuracy of the numerical model of water evaporator. The simulation results show that the structures of the internal channel and external fin both have an impact on the heat transfer performance. For the internal channels, their height change has much more obvious effect than their structural continuity. For the external fins, the increase of the fin bulge structure helps to improve the heat transfer performance. For the aircraft, the optimal design structure of water evaporator is Hex4 and its heat transfer performance can be improved by 13.31% compared with Hex0.

Improving thermal efficiency and stability of laser welding process for magnesium alloy by combining power modulation and subatmospheric pressure environment

The laser welding (LW) process of highly reflective materials presents low thermal efficiency and poor stability. To solve the problem, the effects of subatmospheric environment on LW process, technological parameters in subatmospheric environment on weld formation and welding with sinusoidal modulation of laser power on the stability of LW process in subatmospheric environment were explored. The AZ31 magnesium (Mg) alloy was used as the test materials. The test result revealed that the weld penetration in subatmospheric environment can increase by more than ten times compared with that under normal pressure. After the keyhole depth greatly rises, significantly periodic local bulge is observed on the backwall surface of the keyhole and the position of the bulge shifts along the direction of the keyhole depth. Eventually, the hump-shaped surface morphology of the welded seam is formed; moreover, the weld width in local zones in the lower part of the welded seam remarkably grows. During LW in subatmospheric environment, the weld penetration can be further greatly increased through power modulation. Besides, power modulation can inhibit the occurrence of bulges in local zones on the backwall of the keyhole during LW in subatmospheric environment, thus further curbing the significant growth of the weld widths of hump-shaped welding beads and local zones in the lower part of welded seams. Finally, the mechanism of synchronously improving the thermal efficiency and stability of LW process of highly reflective materials through power modulation in subatmospheric environment was illustrated. This was conducted according to theoretical analysis of recoil pressure and observation results of dynamic behaviors of laser induced plasma clouds and keyholes in the molten pool through high speed photography.

Influences of sub-atmospheric pressure on downward flame spread over typical insulation material with parallel glass curtain wall structure in underground buildings

Downward flame spread over insulation materials by restriction of parallel glass curtain wall is an important potential scenario in modern underground building fire accidents. Meanwhile, considering the continuous challenge of research efforts in low pressure fire behavior, experimental investigation was conducted for the individual and coupling effects of pressure and curtain wall spacing on flexible polyurethane (FPU) downward flame spreading characteristics. Three pressures (70/85/100 kPa) below or near atmospheric pressure, and six kinds of spacings (4.5/6.0/8.5/11.0/14.0 cm and non-curtain wall) were applied to give a phenomenological study on the complicated variations of pyrolysis front, melting and dripping process, burning rate, flame spread velocity, flame height and pulsation frequency. First, evolution of vertex angle in pyrolysis front was observed to be stable and insensitive to both ambient pressure and interlayer spacing. Also, three dripping formations of molten fuel were found, and enhanced by decreasing pressure and spacing. Second, comprehensive understandings of coupling effects on burning rate, spread velocity and various heat transfer components were performed and formulated with semi-quantitative correlations theoretically. Finally, nonmonotonic tendencies of spacing vs. flame height or pulsation frequency were interpreted based on classical fluid and fire dynamic theory. This work could be valuable for fire hazardous analysis of modern building under sub-atmospheric environment.

Performance modeling of pulse detonation engines using the method of characteristics

A quasi-one-dimensional method of characteristics (MOC) model is developed to evaluate the single-cycle gasdynamic flow field and associated propulsive performance of a fully- and partially-filled pulse detonation engine (PDE), and PDEs equipped with diverging nozzles. A detailed description of the PDE thrust chamber flow field is provided to highlight the dominant gasdynamic wave processes encountered in a general single-cycle operation. The MOC model is developed using a simplified unit process approach with an explicit inverse time marching algorithm in order to readily construct the complex thrust chamber flow field along a predefined grid. A grid dependency study is carried out to determine the appropriate grid resolution for the purposes of minimizing computational expense and maximizing numerical accuracy. Lastly, the MOC model is validated with existing numerical and experimental performance data for PDEs operating with oxyhydrogen and hydrocarbon mixtures in both atmospheric and sub-atmospheric environments. It is shown that the current MOC model has good agreement with existing numerical and experimental data for a variety of PDE configurations and operating conditions.

Momentum- and buoyancy-driven laminar methane diffusion flame shapes and radiation characteristics at sub-atmospheric pressures

Buoyancy and momentum are two major driving forces that affect the behavior of diffusion flames, which have not been fully interpreted in sub-atmospheric environments. In this work, a theoretical model based on the cylindrical flame shape is proposed incorporating with the flame width and height in order to predict the steady flame height with Richardson number. The theoretical equation of dimensionless non-flickering flame height y/C was deduced, and the value of y/C was shown to slightly increase for relatively small CRi and decrease significantly with increasing CRi. To verify this model, buoyancy (B)- and momentum (M)-driven methane laminar diffusion flames with the mass fuel flow rate in the range of 2.99–23.9×10−6kg/s were investigated at 0.45–1.00atm. The flow regimes are dominated by the secondary buoyancy acceleration and initial fuel axial velocity, respectively. Experimental results show that first, for steady flames y/C decreases with increasing CRi, which is consistent with the prediction of the model. The value of y/C decreases with increasing air pressure linearly at −0.046, −0.068, and −0.077 slopes for three different CRi levels. Second, radiant fraction of B-driven flames is generally bigger than that of M-driven ones due to longer soot residence time. The radiant fraction increases with increasing air pressure for both B- and M-driven flames at nearly p0.2. Third, for flame oscillation, Strouhal and fuel Froude numbers have the following relationship: St∝Fr−0.48, f∝uf,00.04/d0.52, i.e., B-driven flames flicker slower than M-driven ones. Considering the effect of air pressure, f∝p1/3−β (β≈0.30 for B-driven flames and β≈0.19 for M-driven ones), thereby indicating that flickering frequency increases with increasing air pressure, and the increasing rate of flickering frequency of M-driven flames is higher than that of B-driven ones.

Microstructure and Properties of Nickel Layers Electrodeposited under Reduced Pressure and Temperature Gradient Conditions

Abstract Our previous studies showed that at a considerably high plating speed up to 1 mm/h, the nickel electrodeposits with smooth surface, few deposition defects and some improved properties could be prepared under the conditions of an additive-free bath at a reduced pressure of 5 kPa and a temperature gradient of 35°C/65°C. This paper aims at further analyzing the effect of bath pressure and temperature gradient on the surface morphology, the microstructure and the properties of electrodeposited nickel coatings in a subatmospheric environment. Our findings indicate that the surface quality, the microstructure and the properties of the deposited nickel coatings are, to some extent, affected by the applied bath pressure and the temperature gradient; favorable nickel electrodeposits can be only achieved when the electrolyte in the vicinity of cathode surface is in a boiling state. The temperature gradient influences the texture characteristic of nickel deposits, but the bath pressure has few effects on the preferred orientation. Grain-fining, microhardness, and corrosion resistance of nickel deposits are improved with increasing of the temperature-gradient and/or the reducing of bath pressure. These variations are closely relevant with the boiling-driven mass transfer effect and the vacuum-degassing effect during the electrodeposition in a reduced pressure bath

Characteristic emission enhancement in the atmosphere with Rn trace using metal assisted LIBS

Several characteristic emission lines from the metal targets (Cu, Zn and Pb) were investigated in trace presence of radon gas in the atmospheric air, using Q-SW Nd:YAG laser induced plasma inside a control chamber. The emission lines of metal species are noticeably enhanced in (Rn+air), relative to those in the synthetic air alone. Similar spectra were also taken in various sub-atmospheric environments in order to determine the optimum pressure for enhancement. Solid-state nuclear track detectors were also employed to count the tracks due to alpha particles for the activity assessment.

The vacuum-assisted closure device as a bridge to sternal wound closure.

Sixteen patients were treated for sternal wound infections after undergoing cardiac procedures. Their management involved prompt surgical debridement and quantitative wound biopsies. At the time of the initial debridement, the Vacuum-Assisted Closure Device (V.A.C.) was placed in the open sternal wound. A subatmospheric environment was maintained by the device at a level of 75 to 150 mmHg. The V.A.C. sponge was changed every 2 to 3 days, and operative debridement was performed until quantitative biopsies showed resolution of infection or until systemic signs of sepsis had resolved. At this time the sternal wounds were closed with regional muscle flaps. Patients were excluded from the use of the device if the pleural cavity was entered during operative debridement. Fifteen of the 16 patients survived and went on to complete wound healing and discharge from the hospital (average length of stay, 16.7 days). One patient sustained a cardiac dysrhythmia during the muscle flap procedure and died. There were no complications related directly to the use of the V.A.C. It is the opinion of the authors that the V.A.C. offers several advantages over their traditional methods of treatment. They noted improvement in sternal wound stabilization during the perioperative period and a decreased need for paralysis and mechanical ventilation. Wound management was improved by avoiding the need to perform debridement or to make desiccating dressing changes to an open sternum. Moreover, they also think that this device may lessen the risk for ventricular rupture because of better control of the wound environment and markedly improved stabilization of the debrided sternal elements.

An experimental correlation of turbulent burning velocities for premixed turbulent methane-air combustion

This paper is devoted to the general correlation of turbulent burning velocities in terms of straining rates for premixed flames propagating in intense turbulence. This problem was investigated by the Leeds group led by Professor Bradley and many other researchers. We present here a new methodology based upon a downward propagating premixed CH4–air flame through a nearly isotropic turbulent flow field with a pair of specially designed ion probes for quantitative measurements of turbulent burning velocities. The improvements are that the flame propagation is not influenced by the ignition source and the unwanted turbulence from walls, effects of buoyancy and pressure rise due to burning are minimized, and a greater parameter range than hitherto is covered. The results show that both the turbulent burning velocity bending and the vitality of turbulent premixed flames are certain and surprising. Logarithmic plots of turbulent burning velocities S T/ S L – 1 against the turbulent intensities u '/ S L reveal a transition, where S L is the laminar burning velocity. Across the transition, the slope n changes from positive to negative when values of u '/ S L and/or Karlovitz number are greater than some critical values. This transition seems to correspond to the Klimov–Williams criterion that separates corrugated flamelets from distributed reaction zones. Interestingly, no global quenching of premixed turbulent flames is observed, even at u ' S L ≊ 40, a value significantly higher than in most previous measurements. At a fixed u '/ S L, values of the S T/ S L data vary with the equivalence ratio ϕ. This indicates that the common expression of the form S T/ S L = 1 + C ( u '/ S L) n cannot be applicable generally, because values of the constant C are different for different mixture compositions. It is found that all of the present data with different values of ϕ can be approximated by a simple expression, ( S T – S L)/ u ' ≊ 0.06 Da 0.59, where Da is the Damkohler number. Hence a better correlation of turbulent burning velocities in terms of straining rates for premixed turbulent (methane–air) combustion is proposed.