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Deionized water is replacing fluorinated liquids as the preferred choice for two-phase immersion cooling in data centers. Yet, insufficient bubble removal capability at low saturated pressure is a key challenge hindering the widespread application. To solve this issue, this study employs non-ionic surfactant (Tween 20) and asymmetric structures (expanding microchannel) to enhance the boiling performances of deionized water under sub-atmospheric pressure. The research examines the effects of pressure (8.8~38.5 kPa), surfactant concentration (0.1~0.5 mL/L), and heat flux density (10~180 W/cm2) on the boiling heat transfer characteristics and analyzes the mechanism of unusual temperature oscillations induced by surfactants. It was found that the trade-off between the sub-atmospheric pressure, surface tension coefficient, and reduced static contact angle results in pronounced intermittent boiling on the heated surface. Even with the addition of surfactants, the improvement in heat transfer requires demanding conditions. Boiling enhancement throughout all heat flux conditions was achieved when the surfactant concentration was higher than 0.2 mL/L for the expanding microchanneled surface. The heat transfer coefficient reached 6.89 W·cm−2·K−1 under 8.8 kPa, which was 45% higher than without the surfactant. Under the same heat flux and sub-atmospheric pressure, as the concentration increased from 0.1 to 0.5 mL/L, the amplitudes of temperature fluctuation of the plane surface and expanding microchanneled surface decreased from 10 K to 2 K and 18 K to 1 K, respectively. The onset of nucleate boiling and wall superheat of the expanding microchanneled surface gradually decreased with the increase in surfactant concentration, where the onset of nucleate boiling decreased by 10.54 K. When the heat flux is 160 W/cm2, the wall superheat is reduced by 12.8 K. [ABSTRACT FROM AUTHOR]

The special geography of the plateau area leads to a series of problems in boiler operation. In this study, the absorption coefficients and total emissivity of flue gas were determined under different air pressures during air combustion using the Line-By-Line (LBL) method based on the HITEMP2010 database (High-temperature molecular spectroscopic database). The effects of pressure, temperature, and molar fraction (H2O and CO2) on the radiative properties of flue gas were analyzed. An improved Weighted-Sum-of-Gray-Gases (WSGG) correlation, which relates the absorption coefficients to temperature and total pressure, was proposed. The results show that the reduced total pressure diminishes the total emissivity of flue gas. The maximum differences in total emissivity along the path lengths for the four working conditions with a pressure drop from 0.101 325 to 0.061 655 are 0.093 4, 0.084 5, 0.091 1, and 0.084 3, respectively. For a larger molar fraction, the effect of pressure on the total emissivity is greater for shorter path lengths but not for longer ones. Similarly, the higher temperature would reduce the total emissivity of flue gas. The maximum differences in total emissivity along the path lengths for the four working conditions with a temperature increase from 1 000 K to 2 500 K are 0.273 6, 0.270 5, 0.251 5, and 0.250 5, respectively. For a larger molar fraction, temperature has a greater effect on the total emissivity for shorter path lengths but not for longer ones. Furthermore, increasing the molar fraction enhances the total emissivity of flue gas. The maximum differences in total emissivity along the path lengths for the four working conditions with a molar fraction increase from 1 to 2 are 0.088 1, 0.100 4, 0.088 9, and 0.100 6, respectively. For a higher temperature or lower pressure, the effect of molar fraction on the total emissivity is smaller for shorter path lengths but greater for longer ones. The maximum relative error of the improved WSGG model for the total emissivity of flue gas under different working conditions is 3.67%. It is a significant reduction in the error compared to that of the existing WSGG model. Therefore, the improved WSGG model is more accurate for air combustion atmosphere and sub-atmospheric pressure.

Sub-atmospheric pressure environment and oscillation laser are both excellent ways to improve welded joint quality. However, the differences in their effects and mechanisms are unclear. Therefore, the differences between sub-atmospheric pressure and oscillating laser in weld formation, porosity suppression and grain size refinement were analyzed in this study. The results revealed that penetration depth of welded joint was increased to 6.09 mm under sub-atmospheric pressure environment while it was decreased to 3.3 mm under oscillation laser. Furthermore, reducing fluctuating vapor pressure and suppressing vortex formation were the main means to improve the keyhole stability and promote the escape of bubbles in a sub-atmospheric pressure environment. The improvement of the oscillating laser in these two aspects was attributed to enlarging keyhole area, shortening escape path and oscillation keyhole absorption. Additionally, the average grain size in the upper weld was reduced from 134.6 μm to 118.6 μm and 114.1 μm under the sub-atmospheric pressure environment and laser oscillation, respectively. The reduction of the temperature gradient led to former grain refinement while the increase of heterogeneous nucleation rate was responsible for latter grain refinement. Correspondingly, the mechanical properties of welded joints were improved. The study offered valuable insights for the application of laser welding with oscillation laser and sub-atmospheric pressure in industrial production.

The peak explosion pressure and maximum rate of pressure rise of explosions in confined spaces are key safety parameters to evaluate the hazard of processes running in closed vessels and to design enclosures able to withstand explosions or of their vents used as relief devices. These properties are examined in the present paper, for hydrogen-hydrocarbon-air mixtures at sub-atmospheric pressures, using propane and n -butane as test fuels and variable hydrogen ratios between 0 and 0.3. The experimental study was carried out in a sphere and a cylindrical chamber at room temperature and sub-atmospheric pressures (between 0.3 and 1.0 bar). On the basis of pressure recordings, the characteristic explosion indices, i.e., the maximum explosion pressure (p max), maximum rate of pressure rise ((dp/dt) max), deflagration index (K G), and time to peak pressure (θ max) were derived. Numerical simulations were used to determine the adiabatic explosion pressures and adiabatic flame temperatures of isochoric combustion. The effects of initial pressure (p 0), hydrogen concentration, and shape and size of the explosion vessel on explosion indices are discussed. For each hydrogen concentration of the blended fuels, linear correlations p max = f(p 0) and (dp/dt) max = f(p 0) were found. In addition, the heat loss to the walls during explosion propagation was estimated on the basis of the difference between the adiabatic and experimental explosion pressures. The amount of heat lost increases with the increase of hydrogen blending ratio and vessel asymmetry. • Hydrogen-blended propane-air and butane-air explosions were studied in two closed vessels. • The experiments were performed at various initial pressures between 0.3 and 1.0 bar. • As Hydrogen blending ratio increases, the explosion pressure, the maximum rate of pressure rise, and the severity factor increase. • The increase of hydrogen blending ratio results in the decrease of the explosion times. • The observed variations depend largely on the volume and shape of the explosion vessel. [ABSTRACT FROM AUTHOR]

Deionized water is replacing fluorinated liquids as the preferred choice for two-phase immersion cooling in data centers. Yet, insufficient bubble removal capability at low saturated pressure is a key challenge hindering the widespread application. To solve this issue, this study employs non-ionic surfactant (Tween 20) and asymmetric structures (expanding microchannel) to enhance the boiling performances of deionized water under sub-atmospheric pressure. The research examines the effects of pressure (8.8~38.5 kPa), surfactant concentration (0.1~0.5 mL/L), and heat flux density (10~180 W/cm2) on the boiling heat transfer characteristics and analyzes the mechanism of unusual temperature oscillations induced by surfactants. It was found that the trade-off between the sub-atmospheric pressure, surface tension coefficient, and reduced static contact angle results in pronounced intermittent boiling on the heated surface. Even with the addition of surfactants, the improvement in heat transfer requires demanding conditions. Boiling enhancement throughout all heat flux conditions was achieved when the surfactant concentration was higher than 0.2 mL/L for the expanding microchanneled surface. The heat transfer coefficient reached 6.89 W·cm−2·K−1 under 8.8 kPa, which was 45% higher than without the surfactant. Under the same heat flux and sub-atmospheric pressure, as the concentration increased from 0.1 to 0.5 mL/L, the amplitudes of temperature fluctuation of the plane surface and expanding microchanneled surface decreased from 10 K to 2 K and 18 K to 1 K, respectively. The onset of nucleate boiling and wall superheat of the expanding microchanneled surface gradually decreased with the increase in surfactant concentration, where the onset of nucleate boiling decreased by 10.54 K. When the heat flux is 160 W/cm2, the wall superheat is reduced by 12.8 K.

The operation and performance of electrospray ionization (ESI) is affected by the surrounding environment. In this study, a compact sub-atmospheric pressure ESI (SAP-ESI) source was designed and fabricated using the 3D printing method. This source has a simple structure and is easy to operate, as the sample solution and auxiliary gas are continuously sucked into the source through the pressure difference. The compact and enclosed ionization chamber can reduce the fluctuation of the surrounding gas flow to ensure a remarkably stable (< 3%) electrospray. Moreover, the source can offer variable SAP conditions for ESI analysis. The yield of analyte ions increases with decreasing pressure, while the production of background ions is suppressed under these conditions. In the analysis of protein samples, SAP-ESI can increase the yield and charge state of ions, which may be due to the reduction of proton transfer between charged proteins and surrounding gas. The SAP-ESI source was then used to continuously monitor the extract aqueous solution of tea leaves, and to detect the carbendazim residues on the apple surface by coupling with the liquid extraction surface analysis technique. Experimental results demonstrate that the developed SAP-ESI is a stable, practical, and versatile ionization technique. [ABSTRACT FROM AUTHOR]

Gas-fired boilers, gas stoves, and wall-mounted gas boilers are the main consumers of gas fuel, but they generally encounter problems when operating at high altitudes, such as reduced thermal efficiency and increased pollutant emissions. Previous studies on gas combustion characteristics under sub-atmospheric pressure were mostly carried out in a large space, which is quite different from chamber combustion equipment. Therefore, it is insufficient to guide the design and operation optimization of plateau gas equipment. In this paper, experimentations were carried out to explore the characteristics of a methane diffusion flame under sub-atmospheric pressures. The mass flow rates of methane and air remain consistent under different pressure conditions. The centerline temperature ( T c ) distribution, flame appearance, smoke point, CO emission, and NOx emission under different pressures (ranging from 61.66 to 97.75 kPa) were examined under both fuel rich and lean conditions. The results show that T c at the rear and front of furnace variation with pressure is opposite under fuel-lean and -rich combustion. The T c at the front of furnace decreases with decreasing pressure, whereas T c at the rear of furnace increases with decreasing pressure. With decreasing pressure, flame length decreases under lean combustion, but increases under rich combustion. The smoke point fuel flow rate, flame length, and residence time increases with decreasing pressure, following the law of negative exponent. The CO emission decreases with decreasing pressure, which indicates that the reduced pressure makes methane combustion more complete. For NO emission, the reduced pressure results in an opposite tendency under fuel-lean and -rich combustion. With decreasing pressure, the NO emission decreases under fuel-lean combustion but increases under fuel-rich combustion. [ABSTRACT FROM AUTHOR]

To prevent the performance degradation and flameout of an afterburner/ramjet combustor under sub-atmospheric pressure, the flash-boiling spray by twin-orifice nozzle is attempted to incorporate with the pilot flameholder for reliable ignition and flame stability. Herein, the effects of flash boiling spray by twin-orifice nozzle on the ignition and flame-holding characteristics of evaporating V-gutter flameholder under sub-atmospheric pressure are investigated. According to the analysis of flow patterns and fuel concentrate distribution, the flame stabilization mechanisms of evaporating V-gutter flameholder under lean-state and rich-state are concluded. Compared with subcooled spray, the significant improvement of flash boiling spray on fuel atomization and evaporation under sub-atmospheric pressure can effectively broaden the maximum ignitable velocity of the evaporating V-gutter flameholder, and the lean ignition and blowout fuel-to-air ratios can be reduced by 68.2% and 71.3%, respectively. Furthermore, the flash-boiling spray can promote the establishment of initial flame kernel and accelerate the flame-front propagation, and make the flame stabilization mechanism transform from the rich-flame stabilization mechanism under subcooled spray to the lean-flame stabilization mechanism, resulting in a better performance of flame stability. The results provide a theoretical and technical basis for the practical application of flash-boiling spray in engineering afterburner/ramjet combustors under extreme conditions. [ABSTRACT FROM AUTHOR]

In order to effectively widen the high-altitude operating limits of the multi-swirl staged combustor, the ignition and lean blow-out (LBO) performances of the model combustor were experimentally acquired under the conditions of room temperature and sub-atmospheric pressure with the altitude ranging from 0 km to 12 km. Moreover, the isothermal flow fields inside a staged model combustor with different sub-atmospheric conditions were simulated. Experimental results show that the minimum ignition and LBO fuel-air ratio (FAR) increase rapidly with the increase of simulated altitude. In addition, as the relative pressure drop of injector increases from 1% to 3%, the ignition and LBO performances are gradually improved. Side visualization of the flame by high-speed camera shows that the time-averaged flames under stable combustion have a similar distribution pattern under different pressure drops. The luminous intensity is stratified into dim-bright-dim layers along axial direction. The flame near LBO shrinks to the outlet of pilot stage with a great reduction in luminous intensity. The numerical results reveal that with the decrease of air pressure, the air mass flow rate involved in atomization and combustion is significantly reduced, and the aerodynamic shear force of swirling air is weakened, which are adverse to atomization and fuel-air mixing for airblast atomizer and further lead to the deterioration of ignition and LBO performances. [ABSTRACT FROM AUTHOR]

Triangular type pyramid surfaces are made as a new type of heat transfer surface, and its sub-atmospheric pressure pool boiling performance is investigated experimentally. Pure water is cited as a working fluid with liquid height of 25 mm and saturation pressures of 15 kPa and 10 kPa. Triangular type pyramid surfaces have pyramid sizes of 2.5 mm, 5.0 mm and 7.5mm, respectively. Performance of the triangular type pyramid surfaces is compared with square type pyramid surfaces whose pyramids sizes are 2.5 mm and 5.0 mm. Heat transfer performance of the triangular type pyramid surface with size of 2.5 mm is the highest among them for both saturation pressures. Heat transfer performance for triangular type pyramid surfaces with real surface area basis is higher by about 1.5 times for size of 2.5 mm, and higher by about 1.2 times for size of 5.0 mm compared with the smooth surface at 15 kPa, and higher by about 1.7 times for size of 2.5 mm, and higher by about 1.3 times for size of 5.0 mm at 10 kPa. When comparing the heat transfer performance of triangular type pyramid surfaces with the square type pyramid surfaces, the triangular type pyramid surfaces have higher performance than the square type pyramid surfaces. Observation of bubbles generated from pyramid surfaces is also conducted for pyramid surfaces with size of 2.5 mm, as well as measurement of bubble growth rates and bubble release times. The results qualitatively show that triangular type pyramid surfaces have good bubble foaming characteristics than the square type pyramid surface. [ABSTRACT FROM AUTHOR]

• The flame probability fluctuation and flame stability are quantitatively related. • The vertical flame fluctuation parameter γ can reflect the flame stability in advance. • The flame trajectory equation is not affected by the proportion of secondary air. • The size and temperature variation of the flame recirculation zone are revealed. The burning problem in the plateau environment attracts more and more people's attention, and the plateau adaptability and flame stability of burner are the focus of recent research. The main purpose of this paper is to reveal the horizontal jet spray flame fluctuation characteristics and stability change rule under extremely low pressure (0.05 MPa) at a high altitude (about 5500 m) inhabited by human beings. Flame morphology and temperature properties were investigated in a low-pressure chamber with different secondary air ratios (23–53 %). It is found that the normalization of the flame trajectory equation is not affected by the ratio of secondary air. The temperature in the flame recirculation zone can be increased by 110 K as the proportion of secondary air increases to 45 %, but the maximum temperature is still lower than 1500 K. A continuous flame image processing method was implemented to define and analyze the flame fluctuation region. The results show that the dimensionless fluctuation parameter γ can predict the flame stability in advance than β. When the secondary air ratio is 40 %, the flame has the largest upward fluctuation; and when it exceeds 45 %, the flame stability becomes worse. This work is the first to obtain the results of the study on flame morphology and stability of horizontal jet spray flame distribution ratio under extremely low atmospheric pressure, which provides a theoretical scientific basis for the design of plateau burner, flame control, and flame stability improvement. [ABSTRACT FROM AUTHOR]

The evaporation, autoignition and micro-explosion characteristics of RP-3 kerosene droplets under sub-atmospheric pressure (0.2–1.0 bar) and elevated temperature (473–1023 K) were experimentally investigated using high-speed camera technology. The results showed that the droplet evaporation rate increased monotonically with increasing temperature and pressure under 573–873 K and 0.2–1.0 bar. The decrease of temperature and pressure was obviously detrimental to the successful autoignition of droplets and increased the ignition delay time. Autoignitions at 0.2 bar were very difficult and required an ambient temperature of at least 973 K, which was about 150 K higher than the minimum ignition temperature at 1.0 bar. Sub-atmospheric pressure environment significantly inhibits the formation of soot particle clusters during the autoignition of droplet. Reducing pressure was also discovered to reduce the likelihood of micro-explosions at 673, 773 and 823 K but increase the bubble growth rate and droplet breakage intensity. Strong micro-explosions with droplet breakage time close to 1 ms were observed at 0.6 bar and 773/823 K, showing the characteristic of bubble inertia control growth. [ABSTRACT FROM AUTHOR]

Tank farms are commonly used for storing large quantities of liquid fuels. The design, construction, operation, and maintenance of a tank farm must be in accordance with standards/codes applicable to specific countries. Existing codes on the design of storage tank farms are largely based on tests performed at standard atmospheric pressure and associated thermal hazard analysis. Studies based on small-scale experiments, however, have shown that the burning behavior and associated radiation impact of liquid fuels could be substantially affected by the reduced pressure at high altitude. In this study, large-scale n-heptane pool fire experiments with different pool diameters were carried out at sub-atmospheric pressure to firstly investigate their burning characteristics. Subsequently, specific fire scenarios for the cases under both standard and sub-atmospheric conditions were selected for assessing and comparing their thermal hazards. The experimental results showed that the burning rate at sub-atmospheric pressure is reduced, whereas the flame height is increased. Correlations for both the burning rate and flame height including the pressure effect were proposed and validated against the experiment data in this and existing studies obtained at different pressure conditions. The flame radiative fraction was found to decrease exponentially with the pool diameter. The thermal hazard analysis showed that whilst the thermal hazard at sub-atmospheric pressure is lower than that at standard atmospheric pressure, this difference gradually reduces with increasing tank diameter. The present results not only are important in understanding the effects of pressure on the burning characteristics of large-scale pool fires but provide guidance on the applicability and limitations in the use of design codes developed for standard atmospheric pressure for plateau regions. [ABSTRACT FROM AUTHOR]

Abstract Background Acoustic cavitation plays an important role in the medical treatment using high-intensity focused ultrasound (HIFU), but unnecessarily strong cavitation also could deform the morphology and enlarge the size of lesions. It is known that the increase of ambient hydrostatic pressure (P stat) can control the acoustic cavitation. But the question of how the decrease of P stat and dissolved oxygen concentration (DOC) influence the strength of cavitation has not been thoroughly answered. In this study, we aimed to investigate the relationship among the P stat, DOC and the strength of cavitation. Methods Ex vivo bovine liver tissues were immersed in degassed water with different DOC of 1.0 mg/L, 1.5 mg/L and 2.0 mg/L. Ultrasound (US) of 1 MHz and the spatial and temporal average intensity (I sata) of 6500 W/cm2 was used to expose two groups of in vitro bovine livers for 2 s; one group was under atmospheric pressure (P stat = 1 bar) and the other was under sub-atmospheric pressure (P stat = 0.1 bar). Acoustic cavitation was detected by a passive cavitation detector (PCD) during the exposure process. Echo signals at the focal zone of HIFU were monitored by B-mode ultrasound imaging before and after exposure. The difference between two pressure groups was tested using paired sample t-test. The difference among different DOC groups was evaluated by one-way analysis of variance (ANOVA). Results The results demonstrated a significant difference of broadband acoustic emissions from the cavitation bubbles, echo signals on B-mode image, morphology of lesions under various conditions of ambient pressure and DOC. The lesion volume in tissue was increased with the increase of ambient pressure and DOC. Conclusion Cavitation could be suppressed through sub-atmospheric pressure and low DOC level in liver tissue, which could provide a method of controlling cavitation in HIFU treatment to avoid unpredictable lesions.

Gas-fired boilers, gas stoves, and wall-mounted gas boilers are the main consumers of gas fuel, but they generally encounter problems when operating at high altitudes, such as reduced thermal efficiency and increased pollutant emissions. Previous studies on gas combustion characteristics under sub-atmospheric pressure were mostly carried out in a large space, which is quite different from chamber combustion equipment. Therefore, it is insufficient to guide the design and operation optimization of plateau gas equipment. In this paper, experimentations were carried out to explore the characteristics of a methane diffusion flame under sub-atmospheric pressures. The mass flow rates of methane and air remain consistent under different pressure conditions. The centerline temperature (Tc) distribution, flame appearance, smoke point, CO emission, and NOx emission under different pressures (ranging from 61.66 to 97.75 kPa) were examined under both fuel rich and lean conditions. The results show that Tc at the rear and front of furnace variation with pressure is opposite under fuel-lean and -rich combustion. The Tc at the front of furnace decreases with decreasing pressure, whereas Tc at the rear of furnace increases with decreasing pressure. With decreasing pressure, flame length decreases under lean combustion, but increases under rich combustion. The smoke point fuel flow rate, flame length, and residence time increases with decreasing pressure, following the law of negative exponent. The CO emission decreases with decreasing pressure, which indicates that the reduced pressure makes methane combustion more complete. For NO emission, the reduced pressure results in an opposite tendency under fuel-lean and -rich combustion. With decreasing pressure, the NO emission decreases under fuel-lean combustion but increases under fuel-rich combustion.

• Full-scale fuselage on external large fires at low pressures. • Fuselage presence leads to more intense combustion. • Flame height influenced by fuel fire interaction with fuselage. • Fuselage in fuel pool fire highly susceptible to burning through. Aircraft external fuel fires present catastrophic risks. Comprehensive testing in high-altitude, sub-atmospheric pressure conditions remains notably scarce. In this study, multiple Jet-A fire experiments were conducted using a 4.2 m diameter full-scale aircraft fuselage at a high-altitude airport (3,163 m). Three fire scenarios were formulated based on the spatial relationship between the fuel pool fire and the aircraft fuselage. Key parameters, including burning rate, flame height, and radiation fraction, were meticulously analyzed. The results indicate that the fuselage significantly impacts fuel fire combustion. In the worst-case scenario, the radiant heat emitted by the fuselage can lead to a 30 % increase in the burning rate. The structure of the fuselage limits the increase in flame height. At the interaction point, the temperature reached 800 °C, surpassing the durability limits of the fuselage's aluminum alloy material. Isotherm plots were generated to depict the temperature variations of the fuselage under various scenarios. This research enhances understanding of external fuel pool fire hazards in sub-atmospheric pressure conditions and informs the development of aircraft rescue and firefighting (ARFF) strategies at high-altitude airports. [ABSTRACT FROM AUTHOR]

In high-altitude areas, suppressing the flicker of methane laminar diffusion flame and reducing carbon emissions simultaneously is a challenge. This paper proposes a combustion strategy involving sub-atmospheric pressure and ammonia mixing. Through high-speed imaging and schlieren techniques, the study explores the flame flicker patterns under mixing ratios of 0–60% ammonia at 0.5–1.0 atm, as well as the structural and dynamic characteristics of shear layers and vortices. The stable combustion mechanism of sub-atmospheric pressure and ammonia mixing is also analyzed. It was found that reducing the pressure reduced the flame Richardson number, eased the formation of vortices and weakened the interaction with the flame. The heat release rate of the flame decreases after mixing ammonia, which leads to a decrease in the density gradient of the shear layer and also slows down the formation of vortices. The characteristic values of flicker were quantified. Decreasing the pressure, the flicker frequency decreases. Increasing the ammonia mixing ratio, the flicker frequency increases and the oscillation amplitude shows a highly nonlinear decreasing trend. It is verified that the flicker frequency still satisfies the function form of the Strouhal-Froude number under sub-atmospheric pressure and ammonia mixed combustion. • Established a combustion chamber capable of simulating plateau areas' pressure. • Provided vortex characteristic information through high-speed Schlieren imaging. • Complete suppression of flame flicker observed at 0.5 atm/50% ammonia mixture. • Ammonia mixture can both reduce carbon emissions and suppress flame flicker. • Pressure reduction and ammonia mixture alter density gradients in the shear layer. [ABSTRACT FROM AUTHOR]

• Increasing pressure has enhancing effect on heat transfer of pooling boiling. • Li et al. [27] predict very close results to the present experimental work. • Microstructure surface can improve pool boiling heat transfer. • Proposed correlations for boiling are accurate considering surface and pressure. Compared with standard atmospheric pressures (std-ATM), there is a significant difference in heat transfer phenomenon of pool boiling at sub-atmospheric pressures(sub-ATM). The mechanism by which pressure affects the heat transfer capacity of pool boiling cooling remains unknown. In this work, the effect of pressures on pooling boiling heat transfer was investigated and microstructure surfaces were proposed to enhance the heat transfer capacity. The experimental results demonstrate that the decrease of pressure has a suppressing effect on the pool boiling heat transfer capacity. At sub-atmospheric pressures, microstructure surface can improve pool boiling heat transfer, and the improvement increases with the decrease of pressure. The dimensionless correlation for pool boiling considering pressure was fitted, with an average error of 13.1%. A dimensionless correlation for pool boiling considering microstructure surfaces and pressures was fitted, and an average error of 12.5% was verified. [ABSTRACT FROM AUTHOR]

Thermal runaway is a major safety concern for Lithium-ion batteries in manufacture, storage, and transport. Facing the frequent incidents in the air transport of massive batteries, more reliable fire prediction and protection strategies under low-pressures conditions are urgently needed. Herein, thermal runaway criticality of the open-circuit cylindrical battery piles (up to 9 cells with 30% SOC) under a hot boundary is investigated inside a novel low-pressure chamber (20–100 kPa). Characteristics battery temperatures for the safety venting and thermal runaway are measured to analyze the influences of pressure and cell number on battery failures. Results indicate that lowering the pressure could promote an earlier and stronger safety venting and weaken the intensity of the exothermic reactions inside cells, which is verified by the surface morphology of the electrodes. The overall fire risk is higher with higher pressure and larger battery-pile size, as indicated by the lower minimum boundary temperature for thermal runaway (255 °C~385 °C). Moreover, a simplified heat transfer model is established to explain the trend of thermal-runaway criteria and the influence of the low-pressure environment. This work delivers new insights into the effects of pressure and pile size on battery thermal runaway, which can help to improve the safe storage and transport of large-scale lithium-ion battery piles under varied pressure conditions. [ABSTRACT FROM AUTHOR]

Background: Acoustic cavitation plays an important role in the medical treatment using high-intensity focused ultrasound (HIFU), but unnecessarily strong cavitation also could deform the morphology and enlarge the size of lesions. It is known that the increase of ambient hydrostatic pressure (Pstat) can control the acoustic cavitation. But the question of how the decrease of Pstat and dissolved oxygen concentration (DOC) influence the strength of cavitation has not been thoroughly answered. In this study, we aimed to investigate the relationship among the Pstat, DOC and the strength of cavitation.Methods: Ex vivo bovine liver tissues were immersed in degassed water with different DOC of 1.0 mg/L, 1.5 mg/L and 2.0 mg/L. Ultrasound (US) of 1 MHz and the spatial and temporal average intensity (Isata) of 6500 W/cm2 was used to expose two groups of in vitro bovine livers for 2 s; one group was under atmospheric pressure (Pstat = 1 bar) and the other was under sub-atmospheric pressure (Pstat = 0.1 bar). Acoustic cavitation was detected by a passive cavitation detector (PCD) during the exposure process. Echo signals at the focal zone of HIFU were monitored by B-mode ultrasound imaging before and after exposure. The difference between two pressure groups was tested using paired sample t-test. The difference among different DOC groups was evaluated by one-way analysis of variance (ANOVA).Results: The results demonstrated a significant difference of broadband acoustic emissions from the cavitation bubbles, echo signals on B-mode image, morphology of lesions under various conditions of ambient pressure and DOC. The lesion volume in tissue was increased with the increase of ambient pressure and DOC.Conclusion: Cavitation could be suppressed through sub-atmospheric pressure and low DOC level in liver tissue, which could provide a method of controlling cavitation in HIFU treatment to avoid unpredictable lesions. [ABSTRACT FROM AUTHOR]

The evaporation, autoignition and micro-explosion characteristics of RP-3 kerosene droplets under sub-atmospheric pressure (0.2–1.0 bar) and elevated temperature (473–1023 K) were experimentally investigated using high-speed camera technology. The results showed that the droplet evaporation rate increased monotonically with increasing temperature and pressure under 573–873 K and 0.2–1.0 bar. The decrease of temperature and pressure was obviously detrimental to the successful autoignition of droplets and increased the ignition delay time. Autoignitions at 0.2 bar were very difficult and required an ambient temperature of at least 973 K, which was about 150 K higher than the minimum ignition temperature at 1.0 bar. Sub-atmospheric pressure environment significantly inhibits the formation of soot particle clusters during the autoignition of droplet. Reducing pressure was also discovered to reduce the likelihood of micro-explosions at 673, 773 and 823 K but increase the bubble growth rate and droplet breakage intensity. Strong micro-explosions with droplet breakage time close to 1 ms were observed at 0.6 bar and 773/823 K, showing the characteristic of bubble inertia control growth.

This work explored how subsonic horizontal spray flames behave under sub-atmospheric pressure and the mechanisms behind their combustion. For examine a horizontal spray flame with rated air volume flow and equivalence ratio, large eddy simulation (LES) and probability density function (PDF) were employed, based on the discrete phase (DPM) random trajectory model. The simulation results are in good agreement with the previous experiments. Through the micro-analysis of the burning time, burning path, and rate of the burn path of droplets under different atmospheric pressures, the combustion characteristics of the flame have been revealed; as well, a component change analysis was conducted to investigate the combustion mechanisms of the flame. Density distributions in buoyancy plumes can reveal the morphologic mechanisms in the corresponding flame regions. A detailed explanation of the varied mechanisms in horizontal spray flames in low-pressure environments is provided, in conjunction with the results of previous experiments on macroscopic flame morphology. There is also a proposed chain of influence across certain characteristics of horizontal spray flames under low pressure. This improves our understanding of mechanisms governing flame behavior in sub-atmospheric pressure environments by allowing us to theoretically design and regulate flames. [ABSTRACT FROM AUTHOR]

This study evaluates the vapor transportation by transmission pipelines during seawater desalination. This study seeks to reach a high rate of water transportation during desalination. Hence, the results obtained from this research are closer to reality than other analyses. Other benefits of this research include increasing efficiency, studying the element-to-element transmission, and considering flow as a compression case. The water desalination system comprises three parts of evaporation, transportation, and condensation. In the transportation part, equations of continuity, momentum, and energy are implemented, and the temperature of the vapor is calculated at the beginning of the condensation pipe. Other achievements of this study include the division of transportation lines to small elements and the implementation of vapor condensation in transportation lines. This study used pipelines with diameters of 1, 2, and 4 m to transmit vapor to Ramsar city and the heights of Takhte Soleiman, 16 km away from the city with the elevation of 2000 m. The results show that diameter, transportation length, and temperature differences are, respectively, the most influential factors on the efficiency of sub-atmospheric vapor transportation. The outcomes of this study were presented as the outflow of condensed water at the destination. Considering the margin of safety in calculations, it was scientifically proved that the results obtained in this study were approximately 10% more than results derived from other studies in the literature that are based on the incompressibility of fluids.

Fire propagation and burning characteristics of upward flame spread over flexible polyurethane (FPU) foam board were investigated under coupling effects of pressure and orientation. As a further comparative research of our previous work, three pressures (70, 85 and 100 kPa) and four fuel surface inclination angles (0°, 30°, 60°, 90°) were applied, respectively, as before, to study the variation of typical parameters including flame spread rate (FSR), burning rate, heat transfer components, flame length, etc. First, a phenomenological interpretation was taken to show the special spreading process with melting flow combustion and flash burning observed. Second, an overall theoretical analysis was proposed to reveal the individual or coupling effects of pressure and inclined burning surface on spreading behavior. A semi-quantitative correlation was developed and formulated to show the tendency of FSR as a function of pressure, inclination and other burning parameters, which was validated by data in paper. Meanwhile, comparison of detailed differences between upward and downward spread was conducted to give a full insight on FPU fire development. At last, comprehensive discussions of coupling effects on variation of spreading characteristics and heat transfer mechanisms were performed based on fire dynamics. [ABSTRACT FROM AUTHOR]

The occurrence of sub-atmospheric pressure in the drainage of pipelines containing an air pocket has been known as a major cause of several serious problems. Accordingly, some system malfunction and pipe buckling events have been reported in the literature. This case has been studied experimentally and numerically in the current research considering objectives for a better understanding of: (i) the emptying process, (ii) the main parameters influencing the drainage, and (iii) the air-water interface deformation. Also, this research demonstrates the ability of a computational fluid dynamic (CFD) model in the simulation of this event. The effects of the air pocket size, the percentage and the time of valve opening on the pressure variation have been studied. Results show the pipeline drainage mostly occurs due to backflow air intrusion. The worst case scenario is associated with a fast valve opening when a tiny air pocket exists in the pipeline. [ABSTRACT FROM AUTHOR]

The burning state of a plateau environment is attracting more and more attention. In this paper, in order to have a deeper scientific understanding of diesel spray combustion and the characteristics of a flame under different spray cone angles in a plateau environment, experiments were carried out in a low pressure chamber. The flame morphology was recorded by a high speed video instrument, and the temperature change was recorded by a thermal imager and thermocouples. The MATLAB programming was used to process the video image of the flame, and the probability of its binarization was calculated. The results indicate that the flame becomes longer and wider under different pressures with the same spray angle. The variation is more pronounced at a smaller spray taper angle. The flame uplifted height characteristic is mainly negatively related to the atmospheric pressure. According to the normalized flame temperature and the dimensionless horizontal projection, the length can be divided into three regions. In the region of buoyancy flame, the dimensionless temperature varies with sub-atmospheric pressure more than with normal pressure. In addition, under different spray cone angle conditions, the law of variation in the normalized flame temperature under sub-atmospheric pressure is exactly opposite to that under normal pressure. This study is of great significance to the scientific research on flames in a low pressure environment, and the design of different fuel nozzles for application in a plateau environment. [ABSTRACT FROM AUTHOR]

• Startup characteristics under various outdoor ambient temperatures were studied. • Sub-atmospheric suction pressures occurred during some cold startups. • Lower discharge temperature of R290 caused the condensation of R290 in compressor. • Influences of indoor fan control strategy of tested air conditioner were analysed. Propane (R290) has been considered as one of the alternatives for air conditioner system because of its zero ODP, low GWP and favourable thermo-physical properties. In this paper, experiments were conducted to investigate the dynamic characteristics and to evaluate the safety issues during the startups of a R290 room air conditioner under various outdoor ambient temperatures. Results indicate that the sub-atmospheric suction pressures occurred and lasted during the cold startup in heating mode under an extreme lower ambient temperature, which were potential hazards due to the high flammability of R290. Moreover, if air conditioner started with a smaller EEV opening and the compressor reached higher frequency much earlier, minimum suction pressures drastically declined. As soon as air conditioner started, almost all of liquid R290 stored in outdoor heat exchanger was drawn into accumulator in tens of seconds and some R290 accumulated in the cold compressor shell within a few minutes, resulting in the "starved" outdoor heat exchanger and "enriched" compressor. Additionally, the superheated R290 at EEV inlet greatly influenced the refrigerant supply to low pressure side, which retarded the increase of suction pressure. [ABSTRACT FROM AUTHOR]

Based on the principles of mass conservation, momentum, and energy, an improved version of the Desalination pipeline called Improved Desalination Pipeline System (IDPS) has been studied in the present paper. The main features of IDPS are the high transfer rate and the high raw water production rate. IDPS works with the help of thermal gradient generated due to the temperature difference between the seaside/destination. Heat injection has also been used to increase the efficiency of IDPS. In the condensation part, the heat pipe concept is adopted to facilitate the vapor-liquid process. The main portion of heat injected to the IDPS is recovered by generating electrical power using water flow due to available water head. To show the efficiency of IDPS, a case study is performed from Bandar Abbas (seaside) to Mount GNU (the destination) located30 km away with 2347 m height. The results show that IDPS does not necessarily require large diameter pipes for greater efficiencies. The energy consumption for distillation and transferring1 m3 of water is about 6.6 Kwh. In addition to the distilled transferred water, a significant amount of water is distilled on the site, which is about 25% of the transferred water. [ABSTRACT FROM AUTHOR]

Sooting flames have been a longstanding research topic and an extensive literature has been developed at both atmospheric and high pressure. In contrast, studies of sooting flames at subatmospheric pressures are relatively scarce. As pressure decreases buoyancy, and consequently buoyancy-driven convective flow, decreases as well. So one could expect characteristic residence times to be longer. To assess the intuitive finding, steady coflow non-premixed ethylene/air flames were established at different pressure conditions, ranging from 0.2 to 1 bar. The configuration was documented by both numerical and experimental works. By the Modulated Absorption Emission (MAE) technique, fields of temperature, soot volume fraction, and dispersion exponent as a measure of soot maturity were extracted. Extending the MAE setup from 2 to 4 spectral ranges allows a more accurate evaluation of the dispersion exponent together with the temperature calibration factor. Numerical simulations were conducted using the CoFlame code, giving access to the flow topology and the governing characteristic times. According to numerical simulations, with increasing pressure, while the buoyancy-driven convective flow does increase, the flow velocities do decrease. It seems consistent with experimental results, finding higher maturity, expected with higher residence time, when increasing pressure. In addition to the important database produced for flames under sub-atmospheric conditions, this paper also couples originally experimental and numerical results, leading to (i) the reconstruction of the synthetic signals that a camera would capture, and (ii) the tracking of the quantities of interest experienced along the streamlines. Most of the global trends are well-captured by CoFlame, i.e. decreasing pressure leads to the decrease of soot volume fraction, maturity, and flame height, together with the increase in temperature. Meanwhile, significant discrepancies can be noticed, i.e. the numerical simulations overestimate the soot volume fraction, especially for the lower pressure levels, together with an underestimated flame temperature leading to an overestimation of the flame height. [ABSTRACT FROM AUTHOR]

Studying sub-atmospheric pressure patterns in emptying pipeline systems is crucial because these processes could cause collapses depending on the installation conditions (the underground pipe covering height, type, fill, and pipeline stiffness class). Pipeline studies have focused more on filling than on emptying processes. This study presents an analysis of the following variables: air pocket pressure, water velocity, and water column length during the emptying of an irregular pipeline without an air valve by two-dimensional computational fluid dynamics (2D CFD) model simulation using the software OpenFOAM. The mathematical model predicts the experimental values of the study variables. Water velocity vectors are also analysed within the experimental facility, assessing the sensitivity of the drain valve to different openings and changes in water column length during the hydraulic phenomenon.

An experimental investigation on the explosion characteristics of hydrogen-air mixtures diluted with inert gases (Ar, N 2 , CO 2) was carried out in a cylinder chamber at room temperature (298 K) sub-atmospheric pressures (40 kPa, 60 kPa, 80 kPa, 100 kPa). Based on the pressure history recording, some explosion parameters, e.g., maximum explosion pressure ( P m a x ), maximum rate of pressure rise ( (d p / d t) m a x ), deflagration index ( K G ) and combustion duration (θ) were derived. Effects of initial pressure ( P 0 ), equivalence ratio (φ) and dilution fraction (Z %) on these explosion indices were discussed. The results show that with the increase of initial pressure, the maximum explosion pressure, maximum rate of pressure increase and deflagration index increase monotonously while the combustion duration decrease. The diluent effect on these parameters is increased in the order of Ar, N 2 and CO 2. Linear correlations P m a x / P 0 = f (Z) , P m a x = f (P 0) and (d p / d t) m a x = f (P 0) were found. In addition, the heat loss to the walls during the explosion propagation was estimated on the basis of the difference between the adiabatic and experimental measured pressure. The energy loss increases with the increase of initial pressure and specific heat of the dilutions. • Effects of dilutions on explosion behaviors of hydrogen-air are studied at reduced pressure. • Explosion indices are derived from the experiment and analyzed. • Heat loss to walls during the explosion propagation is estimated. [ABSTRACT FROM AUTHOR]

The present study revealed experimentally flame spread behavior over high-conductivity copper (Cu) core electrical wire in sub-atmospheric pressures, and compares those with relative low-conductivity nickel-chrome (NiCr) core electrical wire, to quantify the evolutions of "wire-driven" and "flame-driven" heat transfer mechanisms in supporting flame spread with pressure decreasing. Polyethylene (PE) insulated Cu electrical wires with inner core diameter ( d c ) of 0.30 mm, 0.50 mm 0.80 mm and insulation thickness ( δ p ) of 0.15 mm, 0.20 mm and NiCr core electrical wires with inner core diameter ( d c ) of 0.30 mm, 0.50 mm and insulation thickness ( δ p ) of 0.15 mm were studied with ambient pressure ranged from 100 kPa to 40 kPa. Results showed that the blue flame height (or the "flame standoff distance" from the bottom edge of the high temperature yellow flame region to wire surface), and the flame base width both increased as the pressure decreased. The flame spread rate (FSR) of NiCr-core electrical wire just increased; however, that of Cu-core electrical wire first decreased then increased as the ambient pressure decreased. The critical pressure (P * ) for the turning of FSR variation of Cu-core wire with pressure was larger as the core diameter was smaller or the insulation was thicker. The "wire-driven" and "flame-driven" heat transfer mechanisms in controlling the flame spread behavior over electrical wires with metal core were well verified by the experimental results shown in the present study. The relative importance of "wire-driven" heat transfer mechanism decreased, while that of "flame-driven" increased, as the ambient pressure is reduced. The findings of the present study will be essential for simulative study of flame spread behavior over electrical wires concerning the core conductivity effect and the evolutions of controlling heat transfer mechanisms. • Flame spread behavior over electrical wire revealing core conductivity effect. • Flame spread behavior over electrical wire in sub-atmospheric pressures. • "Wire-driven" and "flame-driven" heat transfer mechanisms analyzed. • Controlling heat transfer mechanisms for flame spread over wires quantified. [ABSTRACT FROM AUTHOR]

The article presents the results of an experimental study of the dynamics of vaporization at water boiling at subatmospheric pressures up to 8.8 kPa. The usage of a transparent heater, high-speed video and IR thermography allowed analyzing the effect of pressure on the dynamics of vapor bubbles, the evolution of dry spots as well as the surface superheating corresponding to the onset of boiling. It is shown that the pressure reduction at a given heat flux density leads to a significant increase in the Jacob number at the activation of the nucleation site and the growth rate of vapor bubbles. It has been found that the curve of dry spots growth rate has a nonlinear dependence on the pressure in the range of 8.8-103 kPa. The minimum value of the growth rate of dry spots is observed at a pressure of 42 kPa. [ABSTRACT FROM AUTHOR]

Highlights • Subatmospheric pool boiling is experimentally studied with bubble dynamics. • Dynamic effect of subatmospheric boiling was analyzed. • A complex subatmospheric boiling regime of calcium chloride solution was observed. • A new bubble departure diameter correlation was proposed. Abstract The objective of this research is to compare the nucleate boiling characteristic of the calcium chloride aqueous solution with distilled water at sub-atmospheric pressure by analyzing the growing bubble dynamics in order to provide some data for optimizing the design of the dehumidification system. The experiments were carried out with water and calcium chloride solution on the upper surface of a polished stainless steel plate, under sub-atmospheric pressure ranging from 3.6 kPa to 22.0 kPa. An artificial nucleation site was created in the center of the plate to generate the isolated bubbles. A high-speed camera was used to capture the images of dynamic bubbles, and the relevant parameters on bubble dynamics were measured and calculated by frame-by-frame image treatment. Experimental results show that bubble diameter tends to increase with the pressure decrease which means the lower vapor density and stronger surface tension force at sub-atmospheric boiling. The influence of superheat and sub-cooling degree were also analyzed. Additionally, a complex boiling regime of calcium chloride solution with irregular bubble dynamic parameters was observed. Finally, bubble growth dynamics under sub-atmospheric were analyzed and the force balance equation were established. It is shown that the dynamic effect especially the inertial force dominated the growth stage under sub-atmospheric boiling. A new bubble departure diameter correlation within ±20% deviation was proposed. [ABSTRACT FROM AUTHOR]

Abstract This paper investigated the effects of changing ambient pressure (standard- and sub-atmospheric conditions) and crossflow air speed on the flame radiation fraction of a pool fire for a size range of pools where no data were available in the literature. Experiments were carried out using either heptane or acetone to produce relatively sooty flames for square pool fires with side dimensions from 10 to 70 cm. Steady cross airflows ranging from 0 to 2.8 m/s were created by wind tunnels in Hefei (ambient pressure: 100 kPa) and Lhasa (altitude: 3650 m; ambient pressure: 64 kPa), while the flame radiation was measured by wide angle radiometers. It was found that the flame radiation fraction decreased with increasing of cross flow air speed with a relatively larger decrease occurring at sub-atmospheric pressure compared to standard pressure conditions, and for smaller pools compared to larger pools. This evolution of flame radiation fraction with cross flow air speed was interpreted theoretically based on the change of soot volume fraction associated with a characteristic flow time (soot residence time) caused by the elongation of the flame. A general non-dimensional model was proposed to describe the decrease in flame radiation fraction in relation to cross flow air speed and pool size at both ambient pressures. This relationship showed that the ratio of flame radiation fraction in a cross airflow to that without cross airflow was a function of a modified Froude number that reflected the change in buoyancy by accounting for the ratio of ambient- to standard atmospheric pressure. Experimental data were shown to correlate well to this proposed function. [ABSTRACT FROM AUTHOR]

Abstract Burning characteristics (mass burning rate, natural convection boundary layer thickness, flame height and dark zone height) of laminar diffusion flames produced by a candle at sub-atmospheric pressures in the range of P ∞ = 50–100 kPa were experimentally studied in a reduced-pressure chamber; such data are not reported to date. Scaling analysis was performed to interpret the pressure dependence. The new experimental findings for candle flames in the sub-atmospheric pressures were well interpreted by the proposed scaling laws: (1) the mass burning rate was higher for a candle with larger wick length, and it increased with increasing ambient pressure, a stagnant layer B -number model based on natural convection boundary (flame boundary layer thickness) was developed to scale the mass burning rate of candle flames at various pressures; (2) the flame boundary layer thickness was wider in lower pressure and can be well represented by a natural convection boundary layer solution; (3) flame height was higher for a candle with larger wick length, meanwhile the ratio of flame height to burning rate was independent of pressure; (4) the flame dark zone height representing a soot formation length scale changes little with pressure, meanwhile its ratio to the total flame height is scaled with pressure by P ∞ − 1 / 2 / L w , e 3 / 4 (L w,e is effective wick length inside flame). This work provided new experimental data and scaling laws of candle flame behaviors in sub-atmospheric pressures, which provided information for future characterization and soot modeling for diffusion flames associated with melting and evaporation processes of solid fuels. [ABSTRACT FROM AUTHOR]

Pool boiling, as an efficient phase-change heat transfer method, has attracted extensive attention in recent years. However, the bubble dynamics behavior (BDB) and heat transfer characteristics of pool boiling at sub-atmospheric pressure (sub-ATM) are significantly different from those at or above standard atmospheric pressure (std-ATM). This paper presents experimental results of BDB and heat transfer characteristics of pool boiling at sub-ATM in the range of 30–120 kPa. High-speed backlight imaging technique was adopted. The results show that with the decrease of boiling pressure, the diameter and near circularity coefficient of bubble departure increase, and the frequency and aspect ratio decrease. With the decline in pressure, the critical heat flux (CHF) of pool boiling on the surface of the electric heater decreases. Specifically, the CHF at 120 kPa is 23.2% higher than that at 40 kPa. At sub-ATM, the boiling heat transfer coefficient (HTC) grows with the increase of liquid height. However, the HTC hardly changes with liquid height at std-ATM and higher. [ABSTRACT FROM AUTHOR]

Fire propagation and burning characteristics of upward flame spread over flexible polyurethane (FPU) foam board were investigated under coupling effects of pressure and orientation. As a further comparative research of our previous work, three pressures (70, 85 and 100 kPa) and four fuel surface inclination angles (0°, 30°, 60°, 90°) were applied, respectively, as before, to study the variation of typical parameters including flame spread rate (FSR), burning rate, heat transfer components, flame length, etc. First, a phenomenological interpretation was taken to show the special spreading process with melting flow combustion and flash burning observed. Second, an overall theoretical analysis was proposed to reveal the individual or coupling effects of pressure and inclined burning surface on spreading behavior. A semi-quantitative correlation was developed and formulated to show the tendency of FSR as a function of pressure, inclination and other burning parameters, which was validated by data in paper. Meanwhile, comparison of detailed differences between upward and downward spread was conducted to give a full insight on FPU fire development. At last, comprehensive discussions of coupling effects on variation of spreading characteristics and heat transfer mechanisms were performed based on fire dynamics.

In recent years, water (R718) as a kind of natural refrigerant—which is environmentally-friendly, safe and cheap—has been reconsidered by scholars. The systems of using water as the refrigerant, such as water vapor compression refrigeration and heat pump systems run at sub-atmospheric pressure. So, the research on water boiling heat transfer at sub-atmospheric pressure has been an important issue. There are many research papers on the evaporation of water, but there is a lack of data on the characteristics at sub-atmospheric pressures, especially lower than 3 kPa (the saturation temperature is 24 °C). In this paper, the experimental research on water boiling heat transfer on a horizontal copper rod surface at 1.8–3.3 kPa is presented. Regression equations of the boiling heat transfer coefficient are obtained based on the experimental data, which are convenient for practical application.

The perceived advantages of laser powder bed fusion (PBF) at reduced pressure include a more stable melt pool and reduced porosity. In this study, high-speed imaging was used to investigate the interaction of the laser beam with the powder bed at sub-atmospheric pressures. At atmospheric pressure, the laser plume produces a flow in the ambient atmosphere that entrains particles toward the melt pool. As the pressure decreases, this hydrodynamic entrainment increases but eventually the expansion of the laser plume prevents the particles reaching the melt pool: profiles and cross-sections of the track reveal a drastic reduction in its cross-sectional area. As the pressure decreases further, into the molecular flow regime, particles are only repelled by the plume away from the melt pool. The regime between 1 bar and ∼50 mbar (the threshold pressure at which the penetration depth no longer increases) could provide a window for successful processing but might require a pre-sinter to maintain the integrity of the powder bed. Lower pressures would definitely require a pre-sinter, for which the additional processing time and increase in process complexity might be justified for porosity-critical applications. [ABSTRACT FROM AUTHOR]

Amino functionalized surfaces have been shown to improve cell adhesion, proliferation and viability. Herein, plasma polymerized cyclopropylamine (CPA) coatings were deposited at sub-atmospheric pressure with a dielectric barrier discharge (DBD) sustained in argon at different W/FM values (Yasuda parameter). The deposited coatings were then analysed with scanning electron microscopy, atomic force microscopy, Fourier transform infrared and X-ray photoelectron spectroscopy. Results reveal that changing the deposition time at a fixed W/FM value has no significant effect on the chemical composition of the deposits. It is found that the surface morphology strongly depends on W/FM values. The amino selectivity strongly decreases with increasing W/FM while the total nitrogen and oxygen concentration increases. [ABSTRACT FROM AUTHOR]

Surface wettability of a heating surface is one of the most important factors affecting boiling performance. While a biphilic surface (with juxtaposed hydrophilic and hydrophobic regions) is known as a promising technique to enhance water pool boiling at the atmospheric pressure, there is no research regarding its potential for sub-atmospheric applications. In the present study, we have investigated the characteristics of pool nucleate boiling on biphilic surfaces at sub-atmospheric pressures. Biphilic surfaces were made by applying Ni-TFEO (tetrafluoroethylene oligomer) electroplating (with a contact angle of about 140°) on a copper surface. The heat transfer performance of various biphilic surfaces (with different hydrophobic spot diameters and pitches) were measured in the pressure range from atmospheric to 6.9 kPa. At a pressure of 14.0 kPa, the wall superheat at the onset of nucleate boiling was reduced by 12 K on a biphilic surface compared with a mirror-finished copper surface. The experiment with three different biphilic patterns revealed that smaller pitch and diameter of the hydrophobic spots were favorable to heat transfer at 14.0 kPa. The enhancement of HTC over Kutateladze’s correlation reached 270%. A sharp transition from continuous to intermittent boiling, resulting in large deterioration of HTC, was observed on a biphilic surface at a much lower pressure than that on a copper surface. Boiling performance was less affected by the pressure level above the transition pressure. [ABSTRACT FROM AUTHOR]

Evaporators for adsorption heat pumps, chillers and storage devices mostly use water as refrigerant and work at sub-atmospheric pressures. However, there are hardly any applicable performance correlations or sizing guidelines for these rather unusual operating conditions. Within this study geometric and process-related impacts on non-stationary evaporation performance of copper tube-fin heat exchangers are investigated to start filling that gap. Cyclic condensation/evaporation measurements were performed in a thin film evaporation mode and in partially flooded operation with changing refrigerant filling level to cover different applications. For thin film evaporation a thermal resistance model was developed to quantify resistance contributions and identify performance-limiting factors. The presented measurement results reveal that evaporation performance in thin film operation is crucially governed by fluid side heat transfer (UA raise by 146% within tested Reynolds numbers) and wetting conditions while fin sheet thickness plays a marginal role (8% increase of UA). In partially flooded operation performance strongly depends on filling level. Apart from some refinement potential of the thin film model, the simulations reflect evaporation dynamics and effects of influencing factors fairly well which indicates that the employed model approach could be a suitable tool for an effective optimization of evaporator geometry and process parameters. [ABSTRACT FROM AUTHOR]

This paper investigates the flame height and the fraction of stoichiometric air entrained ( n, the basic parameter in flame heights correlation) for rectangular turbulent jet fires at sub-atmospheric pressure, for which no data are available in the literature. Comprehensive experiments are carried out in a naturally sub-atmospheric pressure (Lhasa city, 64 kPa) by rectangular nozzles whose aspect ratio varied from 1:1 to 1:71. The flame heights are measured and correlated by the unified formula suggested by Quintiere and Grove which was obtained at standard atmospheric pressure, meanwhile the change of the important parameter, n or C f (reflecting air entrainment into flame and thus being basic in flame height correlation) in the sub-atmospheric pressure is addressed. It is found that: (a) the flame height in the sub-atmospheric pressure can be well approached by the unified formula suggested by Quintiere and Grove; however (b) the correlation parameter C f is larger (1.6 times that in standard pressure) or the value of n is smaller (73% of that in standard pressure) in the sub-atmospheric pressure than that in the standard pressure. The relative change of n at the sub-atmospheric pressure is then theoretically deduced from the change of entrainment strength ( C 1 ) and flame envelope along with pressure drop. The theoretically deduced ratio of n (or C f ) between these two pressures agrees with the experimentally correlated results. Then, by accounting for the changes of the correlation parameters, a proposed explicit model can generally predict the flame heights in both pressures. [ABSTRACT FROM AUTHOR]

This paper investigates the effect of cross flow of air on flame base drag in pool fires in sub-atmospheric pressure, where no data is available, by combining the effects of these two factors: sub-atmospheric pressure and cross flow. Experiments were carried out using heptane and acetone as fuels for 10–25 cm square pool fires where the cross flow was provided by a wind tunnel built in Lhasa city-Tibet (64 kPa) as well as correspondingly by a wind tunnel in Hefei city (100 kPa). Additional larger scale experiments were also conducted with 45 cm, 60 cm and 70 cm square pool fires in Hefei city. The evolution of flame base drag length as a function of cross flow air speed was quantified. The flame base drag length was found to be larger in the sub-atmospheric pressure than that in normal pressure, indicating higher risk of causing fire spread to nearby fuel at the ground level. Previous correlations based on a wind Froude number are not in agreement with each other. In addition, they do not include all the physics and they do not account for the effect of the pressure change on flame base drag length. A new theoretical approach is proposed accounting for the relation of wind force with the buoyancy of the flame and the interaction of wind with the total heat release rate from the fire. It follows that three independent dimensionless quantities are introduced: the (inverse) fire source Froude number owing to flame buoyancy, the total buoyancy–wind interaction number and the ratio of fuel vapor density near the fuel surface to the air density. This theory allows us to develop and validate a correlation for flame base drag length to account for pressure effect and for generalizing the existing relations for flame base drag. [ABSTRACT FROM AUTHOR]

To further reduce the combustion instability behavior in high-altitude areas, a synergistic suppression of buoyancy-driven methane laminar diffusion flame flicker was achieved by sub-atmospheric pressure combined with co-flow air, and the suppression mechanism was revealed through the schlieren technique. As the pressure decreases, the fluid density gradient within the shear layer decreases, slowing down the formation of vortices. When the co-flow air increases, the vortex is accelerated to rise until the vortex forms downstream of the flame and the flame flicker is completely suppressed. The suppression of flame flicker by sub-atmospheric pressure and co-flow air was nonlinear. The flicker frequency, oscillation amplitude, and flame mean height of flames were assessed for a pressure of 0.5–1.0 atm and co-flow air of 4–16 L/min. The sub-atmospheric pressure reduces the flicker frequency, which increases with increasing co-flow air. The reduction rate of oscillation amplitude at sub-atmospheric pressure varied among different co-flow air flow rates. The initial and that almost realize flame stabilization co-flow air flow rates are more effective than the intermediate flow rate in reducing the oscillation amplitude. The flame mean height decreases monotonically with decreasing pressure, but there is a small peak in the downward trend with increasing co-flow air. [Display omitted] • Sub-atmospheric pressure has a suppression effect on the flicker of methane flames. • Additional co-flow air can be added to completely suppress flame flicker. • The schlieren images of vortices reveal the suppression mechanism of flame flicker. • The suppression of flicker by sub-atmospheric pressure and co-flow air is non-linear. [ABSTRACT FROM AUTHOR]

Opposed flame spread of electric wire in sub-atmospheric pressure is studied experimentally. Thin-polyethylene (PE) coated nickel-chrome (NiCr) and iron (Fe) wire are used as test samples in this study. Total pressure is reduced from atmospheric (100 kPa) to sub-atmospheric (40 kPa) and range of employed external forced-flow speed is from 0 cm/s to 40 cm/s. Results show that the spread rate monotonically decreases, or stays nearly constant, as the forced-flow speed increases regardless of the material of the wire. Dependence of the spread rate on the opposed-flow speed appears differently depending on the material of the wire; with high-conductive material (Fe), decrement trend of the spread rate with wind is suppressed. Importantly, under the conditions considered in this study, the spread rate tends to increase as the pressure decreases regardless of the pressure and the material of the wire. Dependence of the spread rate on pressure is more pronounced with less-conductive (NiCr) wire, whereas less-pronounced with high-conductive (Fe) wire. Qualitative discussions are made to explain the observed spread trend and the importance of the presence of wire to characterize the flame spread of the wire is addressed.

• Pool boiling tests were conducted on surfaces with uniform and mixed wettability. • Superbiphilic surfaces offer enhancements at atmospheric and subatmospheric pressures. • The enhancement with superbiphilicity is more significant for atmospheric pressure (up to 98%). Surface wettability is one of the key parameters in the manipulation of the boiling phenomenon. Although there are a number of studies on the effect of surface wettability on boiling heat transfer, there are few research efforts to explain the boiling phenomenon on superbiphilic surfaces at sub-atmospheric pressures. In this study, pool boiling experiments were conducted to investigate the boiling heat transfer performance of surfaces with uniform (superhydrophobic and superhydrophilic) and mixed (superbiphilic) wettability. This study presents the results obtained from four different surfaces for both atmospheric (103.7 kPa) and sub-atmospheric (28.3 kPa) pressures and aims to provide an understanding of the wettability effect using saturated deionized water as the working fluid in the heat flux range of 7 - 290 kW/m2. The experimental results show that the superbiphilic surface (superhydrophobic spots with a pitch size of 3 mm and diameter of 0.7 mm) offers improvements in boiling heat transfer at both atmospheric and sub-atmospheric pressures up to 98% and 54%, respectively. Due to bubble coalescence being more likely to occur at sub-atmospheric pressure, the enhancement effect of superbiphilicity on boiling heat transfer is more significant for atmospheric pressure. [ABSTRACT FROM AUTHOR]

Aim: To assess the efficacy of topical negative pressure moist wound dressing as compared to conventional moist wound dressings in improving the healing process in chronic wounds and to prove that negative pressure dressings can be used as a much better treatment option in the management of chronic wounds. Materials and Methods: This is a prospective comparative study of data from 112 patients with chronic wounds, of which 56 patients underwent topical negative pressure dressings (17 diabetic, 10 pressure sores, nine ischemic, two varicose, 10 post-infective raw areas and eight traumatic - six had bone exposed, two orthopaedic prosthesis exposed). The remaining 56 patients underwent conventional moist dressings (20 diabetic, two ischemic, 15 pressure sores, three varicose, eight post-infective raw areas and eight traumatic - five had bone exposed, three orthopaedic prosthesis exposed). The results were compared after 10 days. The variables compared were, rate of granulation tissue formation as a percentage of ulcer area covered, skin graft take up as the percentage of ulcer surface area and duration of hospital stay. The variables were compared using Unpaired Student′s t test. A " P" value < 0.05 was considered significant. Results: Out of 56 patients who underwent topical negative pressure dressings, six (10.71%) were failures, due to failure in maintaining topical negative pressure due to defective sealing technique; these were included into the study group. After 10 days, the mean rate of granulation tissue formation was 71.43% of ulcer surface area. All these 56 cases underwent split-thickness skin grafting. The mean graft take-up was 79.29%. The mean hospital stay was 32.64 days. In the remaining 56 patients, the mean rate of granulation tissue formation was 52.85% of ulcer surface area. The mean graft take-up was only 60.45% of the total ulcer surface area. The mean hospital stay was 60.45 days. Conclusion: To conclude, topical negative pressure dressings help in faster healing of chronic wounds and better graft take-up and reduce hospital stay of these patients.