Sub-atmospheric Pressure Conditions Research Articles

Characterizing burning behaviour of convection-controlled pool fires at sub-atmospheric pressure by stagnation theory

Pool fires play a critical role in examining the burning behaviour of diffusion flames and are sensitive to changes in scale and environmental conditions. Understanding the evolution of pool fires under low-pressure conditions, common in plateau or flight scenarios, is essential for both fire safety and energy utilization. This study investigates the effects of pressure on pool fires through a series of small-scale experiments carried out in a pressure chamber that provides stable conditions ranging from 95 kPa to 40 kPa. Pool scales between 7 cm and 16 cm were examined, a range dominated by convective heat transfer. Methanol and heptane were chosen to discern fuel-specific variations in burning behaviour under decreased pressure. The findings demonstrate an increase in the axial flame temperature with reducing pressure, while radiative output exhibits an inverse relationship with pressure. Additionally, convective heat transfer toward surroundings appears to rise with decreasing pressure. A modified stagnation theory solution is proposed to model the mass burning rate of convection-controlled pool fires under low pressure, which shows satisfactory agreement with the experimental data. This research could potentially contribute to the comprehension of pool fires and the safety of energy in sub-atmospheric pressure conditions in the future.

Effect of mixed wettability surfaces on flow boiling heat transfer at subatmospheric pressures

Subatmospheric flow boiling heat transfer is a promising method for electronics cooling due to lower saturation temperatures. However, pressure is a crucial parameter that affects surface tension and vapor density. In this study, the effect of surface mixed wettability configuration on bubble dynamics and flow boiling was investigated under atmospheric and subatmospheric pressure conditions. Superhydrophilic, superhydrophobic, and mixed-wettability surfaces were prepared and tested at various heat fluxes and three system pressures of 48 kPa, 68 kPa, and 101 kPa. The channel dimensions were 50 mm × 15 mm, and the channel had a depth of 1 mm. The results showed that biphilic surfaces enhanced the performance up to 28% compared to superhydrophilic surfaces at high heat fluxes for subatmospheric boiling. Flow visualization efforts reveal that mixed-wettability surfaces improve heat transfer by extending the efficient slug regime to higher heat fluxes by preventing dried spot formation. These surfaces benefit from high density nucleation sites at low and medium heat fluxes, resulting in a noticeable performance improvement compared to the superhydrophilic surface. The obtained experimental data in this study will be helpful for the development of thermal-fluid systems operating under subatmospheric conditions.

Two-dimensional simulation of emptying manoeuvres in water pipelines with admitted air

ABSTRACT This study examines the impact of sub-atmospheric pressures in water pipelines during emptying manoeuvres with air admitted. Previous research has looked at this issue but has not studied it in detail. This research presents a two-dimensional model using the OpenFOAM software to analyse different emptying manoeuvres in a single pipeline with entrapped air. The results show the sensitivity of the ball valve opening percentage, which show that absolute pressure drop can reduce to 23% for each 5% of ball valve opening percentage. The influence of the size of the entrapped air pocket and different air-admission orifices was also analysed. The numerical model showed that the selection of the percentage and times of opening drainage valves in pipelines with air-admission orifices is crucial in controlling sub-atmospheric pressure conditions. Finally, this study demonstrates the ability of the two-dimensional model to show the sensitivity of hydraulic drainage parameters in pipelines with entrapped air.

Influence of subatmospheric pressure on bubble evolution on the TiO2 photoelectrode surface.

The increase of reaction resistance caused by bubble nucleation and long-time growth on the surface of the photoelectrode is an important factor that leads to the low efficiency of photoelectrochemical water splitting. In this study, we adopted an electrochemical workstation synchronous with a high-speed microscopic camera system to achieve in situ observation of oxygen bubble behavior on the surface of TiO2 and to study the internal relationship between the geometric parameters of oxygen bubbles and photocurrent fluctuations under different pressures and laser powers. The results indicate that with the decrease of pressure, the photocurrent decreases gradually and the bubble departure diameter increases gradually. In addition, the nucleation waiting stage and the growth stage of bubbles are both shortened. However, the difference between the average photocurrents corresponding to the moment of bubble nucleation and the stable growth stage hardly changes with the pressure. The production rate of gas mass reaches a peak near 80 kPa. In addition, a force balance model suitable for different pressures is constructed. It is found that as the pressure decreases from 97 kPa to 40 kPa, the proportion of the thermal Marangoni force in the Marangoni force decreases from 29.4% to 21.3%, while the proportion of the concentration Marangoni force increases from 70.6% to 78.7%, indicating that the concentration Marangoni force is the main factor affecting the bubble departure diameter under subatmospheric pressure conditions.

Development and application of a miniature mass spectrometer with continuous sub-atmospheric pressure interface and integrated ionization source

For the miniature mass spectrometer (MS) with a continuous atmospheric pressure interface (CAPI), the gas in the multi-stage chambers directly affects the performance of the instrument. In this study, a sealed ionization chamber is designed to couple with a conventional mini CAPI-MS. In this configuration, the gas environment in the first ionization chamber can be flexibly changed to regulate the gas conditions throughout the entire instrument. By studying the effect of gas pressure on the performance of the instrument, we found that the instrument shows some unique advantages when the first ionization chamber is under sub-atmospheric pressure (SAP) conditions, such as reducing the load of the vacuum pump by 40%, achieving pump-free injection for gas and liquid samples, and improving the resolution by a factor of 2 without loss of detection sensitivity. Therefore, we propose a new integrated interface called continuous sub-atmospheric pressure interface (CSAPI) for building a miniature ion trap mass spectrometer. The CSAPI specially integrates sample introduction, gas/ions interface, and ionizations, including electrospray ionization (ESI) and secondary electrospray ionization (SESI), making this system more convenient for non-professional handlers to rapidly identify or monitor target analytes in gaseous- and solution-phase samples. We also use this system to study gas composition to further improve performance, being able to achieve a 5-fold sensitivity and 2-fold resolution improvement. At last, some custom applications of the current CSAPI-MS platform are explored and demonstrated, including real-time monitoring of chemical reactions in solution and long-distance sampling and analysis of dried Chinese herbs. In conclusion, this study provides a new approach to constructing a complete, versatile and practical miniature MS instrument.

Study of Combustion Characteristics of Magnesium/Sodium Nitrate Pyrotechnics Under Sub-Atmospheric Pressure

ABSTRACT In order to study the combustion characteristics and reaction mechanism of luminescent pyrotechnics under sub-atmospheric pressure conditions, low-pressure combustion tests and thermal analysis were performed for magnesium/sodium nitrate pyrotechnics with zero oxygen equilibrium in the pressure range of 1 kPa~101 kPa. The results reveal that when the pressure drops, the flame height rises, and the flame transitions from a steady-state convergent to a turbulent evanescent flame. Reduced oxygen concentration at low pressure reduces the flame’s high temperature response zone, as well as burning speed and luminous intensity. Furthermore, both the light intensity and the burning rate comply with Vier’s law. The weight loss increased from 16.3% to 36.7% when the pressure was reduced from atmospheric pressure to 1 kPa, the enthalpy change of reaction decreased from 6.75 kJ/g to 2.45 kJ/g, and the characteristic peak of magnesium oxide at 500 nm in the flame spectrum disappeared at 1 kPa, indicating that the reduction of pressure would affect the degree of reaction of pyrotechnics, leading to the decrease of magnesium oxide content and the decrease of heat release. These scientific findings provide useful insights into the low-pressure combustion behavior of pyrotechnics and support the growing demands for military and civilian applications.

Ignition, lean blowout, and flame propagation in a combustor using flameholder with a trapped vortex cavity

In this study, experiments are carried out to investigate a combustor based on the trapped vortex flame stabilization concept. The combustor involves a pilot trapped vortex cavity and a radial V-shaped flameholder. Multiple tests are conducted to measure the ignition and lean blowout performance (LBO) under a wide range, especially under sub-atmospheric pressures. High-speed photography methods are applied to capture the ignition process. The results showed that in sub-atmospheric pressure conditions, the ignition and LBO performance, flame area, and flame radial penetration length are all experiencing a significant decrease, while increasing inlet Mach number and temperature have both positive and negative impacts. Different periods of the ignition process are sorted by post-processing the images; during the ignition, the pilot flame rotates with the main vortex and propagates along the cavity; the flameholder entrains the flame into the mainstream zone. The study would establish a fundamental overview of the performance of this structure under sub-atmospheric pressure, further promoting the optimization and application of the cavity-based flame stabilization conception.

Effect of non-condensable gas in steam condensation at sub-atmospheric pressure condition

In the International Thermonuclear Experimental Reactor (ITER), a postulated Loss Of Coolant Accident (LOCA) in the Vacuum Vessel (VV) has to be managed with a pressure suppression system working at sub-atmospheric pressure. The operating conditions considerably differ from those experienced in the fission nuclear power plants such as BWR, since the ITER Tokamak works at very low pressure conditions and can withstand a maximum pressure of 0.15 MPa. For this reason, the pressure value must not exceed 10 kPa for a water temperature of 30 °C inside the Vapour Suppression Tanks (VSTs) that are the fundamental components of the Vacuum Vessel Pressure Suppression System (VVPSS). During a LOCA some non-condensable gases (mainly hydrogen and oxygen gases due to the water radiolysis or thermolysis) may be mixed in the steam and this could impair the condensation efficiency. In order to investigate the effects of non-condensable gas on DCC, we conducted a research program funded by ITER Organization at the laboratory of the University of Pisa: we designed and built a small-scale experimental rig to study the steam Direct Contact Condensation (DCC) with the presence of non-condensable gas and simulate the behaviour of a VST. Since DCC can occur with different characteristics, we ran 12 closed mode tests exploring all condensation regimes injecting a certain mass of air with the steam discharged in the subcooled water. The tests started at the saturation pressures corresponding to water temperatures ranging from 40 °C to 80 °C and ended when the free space volume reached the atmospheric pressure. From the analysis of the data acquired during the tests we observed that the condensation efficiency remained higher than 95%. We observed that despite this, the presence of a certain quantity of non-condensable gas has negative aspects on condensation: the condensation regime never reaches stability (the regimes quickly shift towards instability). Furthermore, the presence of air triggers a turbulence of the flow which interferes with the transfer of heat from the steam to the water. Not only the introduction of air into the flow increases linearly the pressure above the water head but its high temperature further contributes to the pressure increase. The air mixed with the flow also forms eddies that can trap the steam and transport it out of the water, preventing it from condensing. Apart from condensation, the most noticeable problem is the rapid increase in pressure inside the condensation tank.

Heat transfer intensity at water boiling on the surface of a capillary structure under sub-atmospheric pressure

This paper considers the effect of structural parameters and saturation pressure on the intensity of heat transfer from boiling on porous structures made of copper metal fibers. The study involved changing the structural and geometric characteristics of porous samples and saturation pressure. The study regime parameters were chosen based on the conditions of operation of steam chambers, namely the horizontal orientation of the work area, the capillary transport of the heat carrier to the work area. It was determined that reducing saturation pressure from 0.1 MPa to 0.012 MPa leads to a reduction in heat transfer by 15‒20 % depending on the parameters of porous structures. This pattern has been explained in this paper by the increased detachable diameters of steam bubbles that thus overlap part of the capillary structure's vaporization area, which leads to a decrease in the values of the discharged heat flux at the same temperature gradient values. The influence of values of the porosity and diameters of fibers, which the samples of a capillary structure were made from, was ambiguous. The parameter chosen for generalizing the data obtained was an effective diameter of the samples' pores, which is a more general characteristic. The generalization of the experimental data has demonstrated that the efficiency of heat transfer increases with an increase in the effective diameter of pores in the examined range from 20 to 90 µm. Estimation dependences have been built to determine the intensity of heat transfer under sub-atmospheric pressures for metal-fibrous porous structures at a deviation of up to ±30 %. It turned out that the resulting dependences could be used to determine the intensity of heat transfer by the examined powder structures under the sub-atmospheric pressure conditions. Applying these dependences would make it easier to design thermal stabilization systems based on steam chambers.

Spark ignition and stability limits of spray kerosene flames under subatmospheric pressure conditions

Flame stability limits and ignition delay time of the flameholder are experimentally studied in this work at the pressure of 0.03 – 0.06 MPa, Mach number of 0.1 – 0.3, and temperature of 320 K – 800 K. The results indicate that the reduction in equivalence ratios of lean ignition and blowout can be achieved by the increase in pressure or Mach number. Temperature rise will first increase and then decrease the lean ignition equivalence ratio. While increasing lean blowout equivalence ratio was observed with increasing temperature. The envelope of the stable combustion between the pressure and Mach number is measured, and the lowest temperature of successful ignition is 300 K, acquired at Mach 0.1 and 0.06 MPa. Besides, the correlation between the equivalence ratios of flame stability and inlet parameters, such as pressure, Mach number, temperature, is developed. The ignition process of the flameholder is imaged, and the ignition delay time decreases with increased pressure or temperature. The ignition delay time is shown first to decrease and then increase with the increase in Mach number.

Performance evaluation of a solar and vacuum assisted multi-stage humidification-dehumidification desalination system

This study compares the freshwater productivity of a three-stage vacuum humidification-dehumidification (VHDH) desalination system to a single-stage VHDH system. Experimental study on a multi-stage humidification in sub-atmospheric pressure condition confirmed its superiority over previous multi-stage humidification-dehumidification processes. The variation of the desalination rate was measured experimentally using the three-stage vacuum humidification and heating modes. The results showed that the maximum desalination rate of the unit was 1.8 kg.h−1 per 1 m2 of solar water heater area with minimum specific power consumption (SPC) of 0.1 kW h.kg−1 when the maximum solar intensity was 1106 W.m-2 in July 2020. This investigation indicated that the system had high freshwater productivity and low SPC compared to a single-stage VHDH system. The system performance was improved when the operation was further optimized using the response surface methodology (RSM). The optimal values of saline water to air mass flow rate ratio (sw/a) and humidifier pressure (PH) of 1.77 and 33 kPa led to the maximum freshwater productivity. In all operating conditions, through converting the single-stage humidification to two-stage and three-stage humidification processes, the humidity ratio was enhanced by about 51 % and 19 %, respectively. The optimal freshwater productivity could be realized only when multi-stage vacuum humidification was utilized at isothermal heating mode in all stages and the cost was $0.002 per liter.

Experiments to understand crystallization of levitated high temperature silicate melt droplets under low vacuum conditions

We report experiments on crystallization of highly undercooled forsterite melt droplets under atmospheric and sub-atmospheric pressure conditions. Experiments have been conducted under non-contact conditions using the principles of aero-dynamic levitation. Real time dynamics of solidification, along with the transient evolution of surface textures, have been recorded using high speed camera for three cooling rates. These images have been matched with the time-tagged temperature data to understand the effect of pressure conditions and cooling rates on the crystallization dynamics. Compared to normal pressure, relatively higher levels of undercooling could be achieved under sub-atmospheric conditions. Results showed a strong dependence of surface textures on pressure conditions. For any externally employed cooling rate, relatively small length scale morphological textures were observed under sub-atmospheric conditions, in comparison to those achieved under ambient conditions. The observed trends have been explained on the basis of influence of pressure conditions on recalescence phenomenon and the rate at which latent heat of crystallization gets dissipated from the volume of the molten droplet. Sub-atmospheric experiments have also been performed to reproduce one of the classical chondrule textures, namely the rim + dendrite double structure. Possible formation conditions of this double structure have been discussed vis-à-vis those reported in the limited literature. To the best of our knowledge, the reported study is one of the first attempts to reproduce chondrules-like textures from highly undercooled forsterite melt droplets under sub-atmospheric non-contact conditions.

Simplified Mathematical Model for Computing Draining Operations in Pipelines of Undulating Profiles with Vacuum Air Valves

The draining operation involves the presence of entrapped air pockets, which are expanded during the phenomenon occurrence generating drops of sub-atmospheric pressure pulses. Vacuum air valves should inject enough air to prevent sub-atmospheric pressure conditions. Recently, this phenomenon has been studied by the authors with an inertial model, obtaining a complex formulation based on a system composed by algebraic-differential equations. This research simplifies this complex formulation by neglecting the inertial term, thus the Bernoulli’s equation can be used. Results show how the inertial model and the simplified mathematical model provide similar results of the evolution of main hydraulic and thermodynamic variables. The simplified mathematical model is also verified using experimental tests of air pocket pressure, water velocity, and position of the water column.

Numerical investigation of nucleate pool boiling outside a vertical tube under sub-atmospheric pressures

In this paper, nucleate pool boiling outside a vertical tube under sub-atmospheric pressure conditions has been numerically studied. The volume-of-fluid interface tracking technique is implemented on the basis of a two dimensional axis-symmetric Eulerian-Eulerian multiphase framework. The phase change model is applied into a computational fluid dynamics platform by user-defined functions. The numerical model results are compared with experimental data at both atmospheric and sub-atmospheric pressures, and good agreement is obtained. From the validation results, the logarithm of the mass transfer coefficient is linear to saturation temperature and wall heat flux, respectively. The simulations are performed under operating pressure of 1–10 kPa, heat flux from 22.4 to 77.7 kW/m2, and liquid subcooling of 0–20 K. The effects of tube length and liquid height on the boiling heat transfer have been studied. The two-phase flow patterns and bubble behaviors under different pressure conditions were analyzed and compared. Under lower pressures, because of the increase of liquid subcooling with the liquid height, boiling occurs at the top of the tube wall firstly, and then gradually develops downwards. The bubbles keep growing while they slide upwards along the wall and coalesce with the adjacent bubbles but the bubbles will not detach from the wall until leaving the top part of the tube. The bubble sliding velocity increases with the bubble size and tube height. From the computational results, the wall superheat increases with the decreasing operating pressure and the liquid subcooling under the same heat fluxes. The required heat flux for the onset of nucleate boiling is higher for the boiling outside a vertical tube than that for the boiling on a horizontal plate. Different from the sub-atmospheric pool boiling on a horizontal surface, the heat transfer over a vertical tube is not sensitive to the change of the liquid height. Furthermore, the averaged wall temperature is higher on the longer tubes due to the severe bubble coalescence.

Experimental study on effect of system pressure on two-phase flow pressure drop at sub-atmospheric conditions

The objective of the present work is to study two phase flow pressure drop in a helically coiled once through steam generator under sub-atmospheric conditions. Literature survey on two-phase flow pressure drop in a helical coil reveals that most of the research articles correspond to studies under high pressure and only few data are available for sub-atmospheric pressure conditions. In this study, an experimental investigation is carried out for a helically coiled tube of tube diameter 9.5 mm, coil diameter 282.7 mm and tube length 5330 mm. System is operated at sub-atmospheric pressure of 150 mbar and atmospheric pressure, respectively. Water and/or steam mass flow rate through the steam generator are taken at 36 kg/h, 54 kg/h and 72 kg/h. A 50 kW capacity DC power supply is used to supply required heat for steam generation. Experiment is carried out for power input ranging from 10 kW to 30 kW. The steam quality at helical coil exit is kept near to 0.5 and hence the power input is decided by amount of heat required to convert liquid water at inlet temperature to saturated vapour-liquid mixture with vapour quality of 0.5. The liquid to steam density ratio ranges from 7500 to 16000. In this study it is observed that the pressure drop at sub-atmospheric pressure is high as compared to atmospheric pressure.

Experimental analysis of steam condensation patterns within a pressure suppression system operating at sub-atmospheric pressure conditions

For some specific applications, the pressure suppression systems should be maintained at sub-atmospheric pressure, such as in fusion plants systems. This operation conditions differ considerably from those experienced in the suppression pools of BWR operating at atmospheric pressure.The steam condensation in water at sub-atmospheric conditions is not sufficiently known, if not at all; therefore its effectiveness has to be investigated experimentally. In fact the steam condensation at sub-atmospheric pressure requires a subcooling capacity 2.5 times higher than that at atmospheric pressure.In consideration of that, a low scale experimental facility was designed and built at the University of Pisa. More than 350 steam condensation tests were performed in broad influencing parameters range (e. g. pressure within the condensation tank, water temperature and steam mass flux) to study of effectiveness of the steam condensation. In doing that, different sparger configurations (with single or multiple holes) were considered to check possible interference of condensing steam jets and bubble coalescence.Results indicate a high efficiency of condensation for all the examined conditions. The 3D map of the condensation regimes demonstrate they are dependent only on the downstream exit pressure, the water temperature and the steam mass flux per hole.

Investigation of vibrations caused by the steam condensation at sub-atmospheric condition in VVPSS

Fusion technology deployment passes through the design of safety systems aimed to protect the Vacuum Vessel (VV) from pressurizing accidents event such as LOVA (Loss Of Vacuum Accident) or the failure of the Tokamak Water Cooling System (TWCS) causing the LOCA (Loss Of Coolant Accident). One of important safety systems of the ITER plant is the Vacuum Vessel Pressure Suppression System (VVPSS), which is designed to protect the VV by the steam condensation at sub-atmospheric pressure conditions in Suppression Tanks.The aim of this study is to investigate vibrations phenomenon, originated during the steam-water direct contact condensation (DCC) at sub-atmospheric conditions, and determine any correlations between the steam jet dynamic and water temperature (TW), steam mass flux (GS) and downstream pressure (PW).According to the thermal hydraulic conditions characterising the DCC, vibrations may evolve because of the latent heat of the phase change (large amount of heat transmitted quickly to the water).In the paper also presents the numerical investigations performed by means of proper FEM code, the set-up procedure and the experimental activity carried out at Lab. B. Guerrini of DICI- University of Pisa. This latter allowed correlating the different steam condensation regimes (chugging, condensation oscillation, etc.) to the acceleration and force, to be in turn used to determine the strength capacity of the VVPSS.

Experimental investigation of steam condensation in water tank at sub-atmospheric pressure

Abstract The International Thermonuclear Experimental Reactor (ITER) Vacuum Vessel Pressure Suppression System (VVPSS) limits the Vacuum Vessel (VV) internal pressure, in case of loss of coolant (LOCA) or other pressurizing accidents from the in-vessel components, to 150 kPa (abs). This is key safety function because a large internal pressure could lead to a breach of the primary confinement barrier. Safety is ensured by discharging the steam evolved during the accident event to the VVPSS suppression tanks where it is condensed. Steam condensation occurs at sub-atmospheric pressure condition. Moreover, being this latter not standard for traditional nuclear systems, this investigation is quite new (not studied in detail before) and deals with an experimental investigation of the direct contact condensation at VVPSS prototypical thermal-hydraulic conditions. To the purpose, a small-scale experimental rig was properly designed and built at Lab. B. Guerrini of DICI-University of Pisa as well as different temperature, pressure and steam mass (flow rate per hole) conditions and sparger patterns have been investigated. The experimental test matrix is also presented in this study. The obtained results show high efficiency of condensation for all examined conditions. The main condensation regimes at sub-atmospheric pressure conditions were identified. In addition, a comparison was done between the condensation regimes experimentally determined and those available in the literature, which were obtained at atmospheric pressure. Finally, results demonstrated to be representative of the real configuration at ITER reactor.

Pressure Dependence of Rate Coefficients for Formation and Dissociation of Pentachlorodisilane and Related Chemical Activation Reactions

Four entrance channels to the pentachlorodisilane (Si2HCl5) potential energy well, namely, SiCl2 + SiHCl3, SiCl4 + SiHCl, Cl3SiSiCl + HCl, and SiCl3 + SiHCl2, were analyzed in detail through transition state theory, Rice–Ramsperger–Kassel–Marcus (RRKM) theory, and solution of the multichannel master equation. The stationary points in the potential energy surface were optimized, and their vibrational frequencies and rotational constants calculated at the (U)B3LYP/6–31+G(d,p) level of theory; the (U)CCSD(T)/aug-cc-pVTZ level was then used for accurate estimation of activation energies. The pressure and temperature dependence of the rate coefficients of the channels related to Si2HCl5 stabilization/dissociation was determined along a wide range of conditions, for the first time. All channels showed strong pressure dependence in the four cases, at least at low-to-moderate pressure conditions. Each entrance channel leads to the formation of different products under different conditions, and the mechanism was analyzed in detail. The results indicated that at atmospheric pressure the reactions are in the falloff region, and therefore do not support the adoption of high-pressure limit rate coefficients in reaction models designed for simulation of systems at atmospheric or subatmospheric pressure conditions.

Melting temperature of YBa2Cu3O7−x and GdBa2Cu3O7−x at subatmospheric partial pressure

The melting temperature, Tm, of YBa2Cu3O7−x and GdBa2Cu3O7−x superconductor ceramic compounds has been measured by differential thermal analysis (DTA) over a large oxygen partial pressure range of five orders of magnitude (2·10−5 < PO2 < 1 bar). It is shown that the DTA peak and endset temperatures are closer to Tm than the onset temperature. Correct measurement of Tm requires low heating rates and subatmospheric pressure conditions to avoid temperature shifts due to local oxygen overpressure and kinetic effects.