Sub-atmospheric Conditions Research Articles

Experimental study of combustion efficiency and outlet temperature distribution characteristics of a combined evaporative flameholder

In ramjet engines and afterburners, combined stabilizers are employed to achieve effective flame stabilization. This paper designs a combined evaporative flame stabilizer, aimed at enhancing combustion performance and flame propagation under sub-atmospheric conditions. Combustion efficiency and outlet temperature distribution were experimentally studied under varying inlet Mach numbers, temperatures, pressures, and equivalence ratios. Results indicate that combustion efficiency increases with temperature from 600 K to 800 K, initially increases and then decreases with Mach number from 0.11 to 0.19, and decreases with pressure from 0.04 to 0.101 MPa. At an inlet temperature of 600 K, combustion efficiency drops from 55.84 % to 19.36 % as the fuel–air ratio (FAR) increases from 0.008 to 0.044, and then rises to 24.74 % at a FAR of 0.055. Similar outlet temperature distribution characteristics were observed under different inlet conditions. As the Mach number increases, the peak position of the outlet temperature gradually moves from the center to the lower wall. This study provides design guidance and engineering value for improving the combustion performance of flameholders in afterburners or ramjet engine combustion chambers.

Experimental investigation of thermal characteristics on a new loop pipe model for passive cooling system

The heat pipes have the potential to be used as a passive cooling system (PCS) in thermal installations including nuclear facilities. In that context, this experimental study is correlated with the research on the new type of Loop Heat Pipe (LHP) as the PCS. This work aimed to study the characteristics of LHP with capillary pipe wick. The experiments involved varying the pool water temperature as evaporator heat load at 35, 45, 55, and 65 °C, filling ratio charged at 20, 40, 60, 80, and 100 % of LHP evaporator volume, and the flow air velocity to the fins in the condenser at 0, 1, 1.5, 2, and 2.5 m/s. The LHP was operated in sub-atmospheric conditions with an initial pressure of 2666.4 Pa. The results show that LHP has a stable two-phase natural circulation under all variations of heat loads, filling ratios, and air velocities tested. The results also show that display excellent thermal performance with the lowest thermal resistance of 0.0524 ± 0.0004 °C/W operating at the highest water temperature source, the highest filling ratio, and the highest air velocity. It is concluded that the LHP model with capillary pipe wick has similar thermal characteristics to other types of heat pipes. This wick function effectively in preventing vapor flow from the evaporator toward the condenser, and otherwise it can facilitate as condensate path from the condenser to evaporator. The stable two-phase natural circulation stability analysis showed that the LHP with capillary wick has very good thermal performance as heat absorber and heat releaser.

Comparison of Cu+, Ag+, and Au+ Ions as Ionization Agents of Volatile Organic Compounds at Subatmospheric Pressure.

Ionization of volatile organic compounds (VOCs) by coinage metal ions (Cu+, Ag+, and Au+) generated by laser desorption and ionization (LDI) of a metal nanolayer in subatmospheric conditions is explored. The study was performed in a commercial subatmospheric dual MALDI/ESI ion source. Five compounds representing different VOC classes were chosen for a detailed study of the metal ionization mechanism: ethanol, acetone, acetic acid, xylene, and cyclohexane. In the gas phase, ion molecular complexes of all three metal ions were formed, typically with two ligand molecules. The successful detection of the metal complexes with VOCs strongly depended on the applied voltages across the ion source, minimizing the in-source fragmentation. The employed orbital trap with ultrahigh resolving power and sub-parts-per-million mass accuracy allowed unambiguous identification of the formed complexes based on their molecular formulas. The detection limits of the studied compounds in the gas were in the range 0.1-1.4 nmol/L. Compared to Cu+ and Ag+ ions, Au+ ions exhibited the highest reactivity, often complicating spectra by side products of reactions. On the other hand, they also allowed detecting saturated hydrocarbons, which did not produce any signals with Ag+ and Cu+.

Mini but Mighty: Oscillations in diffusion flames at sub-atmospheric conditions

The oscillatory behaviour of buoyant diffusion flames plays a vital role in determining how flames evolve. Although previous studies have established an inverse square root relationship between oscillatory frequency and pool diameter, f ∼ D-1/2, few have explored the synergistic effect of diameter and reduced pressure on the pulsation phenomenon. This study mainly investigates the characteristics of convection-controlled pool fires (D < 20 cm) at varying sub-atmospheric pressures, with a particular focus on the influence of eddy effects. Four burner sizes (7 cm, 10 cm, 13 cm, and 16 cm) are selected, and pressure levels are set at 95 kPa, 80 kPa, 60 kPa, and 40 kPa, respectively. A pressure chamber, maintaining a nearly constant pressure during combustion, is employed. The morphology of diffusion flames is recorded and subsequently analysed at different pressures. The study finds that flame shapes change from turbulent to laminar as pressure decreases, which is attributed to the smaller vortex size and corresponds to the varicose and sinuous models. Flame height and oscillatory frequency show a slight increase with decreasing pressure. A new correlation based on the dimensionless Richardson number and Strouhal number is proposed to correlate the oscillation and diameter at different pressures, St=0.58(1/Ri)0.54. This study provides interesting insights into the behaviour of buoyant diffusion flames under sub-atmospheric conditions, highlighting the transition from turbulent to laminar flame shapes, changes in flame height, and pulsation frequency trends. This research could potentially contribute to the comprehension of buoyant diffusion flames at sub-atmospheric conditions in future.

Influence of sub-atmospheric pressure on flame shape and sooting propensity in ethylene laminar coflow non-premixed flame

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.

Effect of different atmospheric and subatmospheric cooking techniques on physical and chemical qualitative properties of pork loin

In this paper, the effects of four cooking procedures were evaluated, two occurring in atmospheric (in ventilated and steam oven) and two in subatmospheric (vacuum and sous vide cooking) conditions on pork Longissimus lumborum. The main objective of the study was to compare and evaluate the physical and chemical characteristics. Samples were cooked in four independent trials namely Oven (O), Steaming (ST), Vacuum Cooking (VC) and Sous Vide (SV). The analyses included temperature, cooking effect, percentage weight loss, texture (cutting and double compression tests), colour (superficially and inside the sample), microstructure (optical microscopy) and fibres shortening analysis. To assess cooking effects on significant nutritional constituents, the fatty acid composition and the content of B vitamins were analysed. Volatile profiles of samples were also compared using solid-phase microextraction. SV cooking resulted in the less favourable meat texture, presenting the highest hardness and chewiness. Moreover, high hardness values measured on SV samples is also related to the high weight loss. The technique of oven cooking (O) demonstrated superior results in terms of mechanical properties, which are closely associated with the cooking values. Specifically, the cook value C0 was significantly higher in the case of oven cooking compared to SV, VC, and ST. Mild temperature conditions and cooking times of the four considered cooking techniques did not induce significant variations in the fatty acid composition and volatile profile. Conversely, SV and VC allowed the highest amount of vitamin B retention in cooked meat. This work suggests that some differences emerged on the effects due to sub-atmospheric and atmospheric cooking compared to traditional ones.

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.

Experimental and Numerical Evaluation of an HCCI Engine Fueled with Biogas for Power Generation under Sub-Atmospheric Conditions

Energy transition to renewable sources and more efficient technologies is needed for sustainable development. Although this transition is expected to take a longer time in developing countries, strategies that have been widely explored by the international academic community, such as advanced combustion modes and microgeneration, could be implemented more easily. However, the implementation of these well-known strategies in developing countries requires in-depth research because of the specific technical, environmental, social, and economic conditions. The present research relies on the use of biogas-fueled HCCI engines for power generation under sub-atmospheric conditions provided by high altitudes above sea level in Colombia. A small air-cooled commercial Diesel engine was modified to run in HCCI combustion mode by controlling the air–biogas mixture temperature using an electric heater at a high speed of 1800 revolutions per minute. An experimental setup was implemented to measure and control the most important experimental variables, such as engine speed, biogas flow rate, intake temperature, crank angle degree, intake pressure, NOx emissions, and in-cylinder pressure. High intake temperature requirements of around 320 ∘C were needed to achieve stable HCCI combustion; the maximum net indicated mean effective pressure (IMEPn) was around 1.5 bar, and the highest net indicated efficiency was close to 32%. Higher intake pressures and the addition of ozone to the intake mixture were numerically studied as ways to reduce the intake temperature requirements for stable HCCI combustion and improve engine performance. These strategies were studied using a one-zone model along with detailed chemical kinetics, and the model was adjusted using the experimental results. The simulation results showed that the addition of 500 ppm of ozone could reduce the intake temperature requirements by around 50 ∘C. The experimental and numerical results achieved in this research are important for the design and implementation of HCCI engines running biogas for microgeneration systems in developing countries which exhibit more difficult conditions for HCCI combustion implementation.

Analysis of methane pyrolysis experiments at high pressure using available reactor models

Most batch reactor methane thermal decomposition (MTD) studies have focused on sub-atmospheric pressures. In this study, MTD experiments were performed between 892 K and 1292 K and 4 atm inside a quasi-isothermal quartz batch reactor at residence times ranging from 0 to 300 s. At a given temperature, elevated pressure resulted in higher methane conversion compared with the sub-atmospheric conditions. A zero-dimensional, closed, isochoric, homogeneous gas-phase model was used to compare numerical predictions from available reaction mechanisms in literature to experimental data. It was found that most models accurately predict conversion at low pressure and high temperature and at high pressure and high temperature, but did not accurately predict methane, hydrogen, and intermediate species mole fractions at 4 atm and either 892 K or 1093 K. A reaction path analysis showed the reactions responsible for the delay in hydrogen formation at elevated pressure are 2CH3(+M)⟺ C2H6, C2H6 ＋ CH3⟺ C2H5 ＋ CH4, C2H6 ＋ H ⟺ C2H5 ＋ H2 and C2H5(+M)⟺ C2H4 ＋ H, where M is the collision partner. A gas-phase model accounting for the solid carbon shows that the observed discrepancy is not due to the unavailability of the carbon formation model but to slower reaction kinetics, especially at lower temperatures. The experimental data presented can be used to obtain a methane pyrolysis reaction mechanism suitable for high pressure conditions.

Experimental and Numerical Investigations on Ignition and LBO Performances for Staged Combustor under Sub-Atmospheric Conditions

Experimental and Numerical Investigations on Ignition and LBO Performances for Staged Combustor under Sub-Atmospheric Conditions

Use of Sub-Atmospheric Pressure Storage to Improve the Quality and Shelf-Life of Marmande Tomatoes cv. Rojito.

In this study, the feasibility of storing Marmande tomatoes (Solanum lycopersicum, cv Rojito) under hypobaric conditions was evaluated. The fruits were sorted into four lots of 72 fruits each. One lot was considered as a control, and the fruits were kept in the open box, while the fruits of the rest of the three remaining lots were enclosed in airtight containers and subjected to 101, 75 and 50 Kpa, respectively. Control fruits and airtight containers were kept at room temperature, and every three days from the beginning of the experiment the following main quality parameters were analysed: ethylene production rate, firmness, colour, total solids content, ascorbic acid, total phenolics and pigments, as well as a sensory analysis carried out by panellists. The results show that sub-atmospheric storage led a reduction in ethylene production, which was associated with a delay in ripening. The differences in the evolution of pigments were very significant, while a large degradation of chlorophylls was observed in the control fruits and in those kept at 101 kPa, in the fruits kept at 75 kPa and 50 kPa the degradation was much slower. In relation to carotenoid pigments, it was observed that sub-atmospheric treatments delayed their appearance compared to control and 101 kPa fruits. In relation to other quality parameters, it was found that control fruit and fruit held at 101 kPa softened more rapidly than fruit under sub-atmospheric conditions, whose loss of firmness was more gradual with differences found only at 9 and 12 days of storage with respect to fruit firmness at harvest. The appearance of these fruits was evaluated with the same score as at the time of harvesting, during 9 of the 12 days of the experiment, then a positive effect of sub-atmospheric treatments was also found in the sensory analysis. The results suggest that sub-atmospheric storage could be a suitable method of increasing the shelf-life of fruits.

A study of propagation of spherically expanding flames at low pressure using direct measurements and numerical simulations

The outwardly propagating spherical flame (OPF) method is widely used to measure the laminar burning velocity (LBV). However, there are still numerous challenges to perform unambiguous measurements for extreme conditions, mainly for high and low (sub-atmospheric) pressures. In this work, the density weighted displacement speeds relative to burned (Sd,b˜) and fresh gases (Sd,u˜) are considered to report LBV for a large range of sub-atmospheric pressure (from 1 to 0.3 bar), equivalence ratio and fuels (methane and n-decane). These stretched flame speeds are obtained from experiments by using advanced optical diagnostics and the spherical flame propagation is simulated by the code A-SURF with different kinetic schemes to perform a direct comparison with the experimental results. It is shown that Sd,b˜ is significantly affected by the pressure decrease and becomes strongly non-linear with flame stretch. This is explained by the finite flame thickness structure and qualitatively modeled by the finite thickness expression (FTE) model used for extrapolation at zero stretch. However, assumptions (equilibrium and static burned gases) made to report Sd,b˜ are no longer valid and leads to strong under-prediction of LBV from extrapolation especially at low pressures. The shape of Sd,u˜ is less impacted by the pressure decreases and a linear behavior with flame stretch is globally found even for strong sub-atmospheric conditions. However, the temperature of the fresh gas isotherm where Sd,u˜ is extracted is not rigorously equal to the fresh gas’s temperature, and a correction factor is required to get accurate LBV. Therefore, a direct comparison of Sd,u˜ instead of extrapolated LBV between experimental and numerical data seems necessary to validate kinetic schemes at low pressures. A brief discussion is conducted on the validation of FFCM-1 and GRI3.0 for methane and Dryer mechanisms for n-decane at low pressures.

Experimental study of the correlation for turbulent burning velocity at subatmospheric pressure

Turbulent burning velocity is one of the most relevant parameters to characterize the premixed turbulent flames. Different correlation has been proposed to estimate this parameter. However, most of them have been obtained using experimental data at atmospheric pressure or higher. The present study is focused on obtaining a correlation for the turbulent burning velocity using data at sub-atmospheric pressure. The turbulent burning velocity was experimentally calculated using the burner method, where turbulent premix flames are generated in a Bunsen burner. Stoichiometric and lean conditions were evaluated at a pressure of 0.85 atm and 0.98 atm, whereas the turbulence intensity was varied for each condition. Perforated plates and a hot-wire anemometer were used to generate and measure the turbulence intensity. Schlieren images were used to obtain the average angle of the flame and calculate the turbulent burning velocity. Experiments and theory show that the turbulent deflagration rate decrease as pressure decrease. The turbulent deflagration speed decreased by up to 16 % at 0.85 atm concerning atmospheric conditions for the same turbulence intensity, discharge velocity, and ambient temperature, according to the experimental results. The comparison among the experimental results at sub-atmospheric conditions and the correlations reported in the literature exposes prediction issues because most of them are fitted using data at atmospheric conditions. A general correlation is raised between turbulent burning velocity (ST), laminar burning velocity (SL) and turbulence intensity (u’) proposed from the experimental data. This correlation has the form For sub-atmospheric and atmospheric conditions, the coefficients were determined

Experimental Analysis of Steam Condensation Heat Transfer Coefficient in Water Pool at Sub-Atmospheric Pressure

The experimental analysis of steam direct contact condensation in a water pool at sub-atmospheric condition was carried out in relevant configurations for the International Thermonuclear Experimental Reactor (ITER) Vacuum Vessel Pressure Suppression System, during a postulated Ingress of Coolant Event category IV accidental scenario. This transient accident was experimentally simulated in a reduced scale (1:22) facility at University of Pisa, adopting a defined scaling law. The steam jet plumes were video recorded and image analysis was performed at stable and interfacial oscillation condensation regimes, providing steam plume average length and interfacial area. The average heat transfer coefficient was therefore computed and correlated to the condensation driving potential. Also, the steam jet length divided by orifice diameter (L/D) was characterised as function of driving potential. Empirical correlations available in literature at atmospheric conditions were modified for evaluating vacuum conditions peculiarities. The remarkable higher dependence of heat transfer coefficient (HTC) to driving potential parameter, at vacuum conditions, was highlighted by the driving potential exponent equal to 1.2 instead of 0.04 at ambient pressure. The obtained L/D and HTCcorrelations are able to predict with ± 15% and ± 30% of error the corresponding experimental data, respectively.

Kinetic study of the effect of sub-atmospheric conditions on the laminar burning velocity of high C2H6 content natural gas mixtures

The laminar burning velocity (S L) was measured at sub-atmospheric pressure (0.84 atm) and an environmental temperature of 295 ± 2 K for two high C2H6 content fuel mixtures, 75% CH4 – 25% C2H6 (mixture M1), and 50% CH4 – 50% C2H6 (mixture M2), as well as the pure constituent fuels. The equivalence ratios for the experiments ranged between 0.8 and 1.4. Numerical calculations predicting S L were performed using 3 detailed reaction mechanisms, finding GRI-Mech 3.0 to achieve the best agreement at the pressure conditions evaluated. The pre-exponential factor of reaction H + O2 = O + OH (R38) was modified in order to improve the numerical results at sub-atmospheric conditions. Kinetic analysis by means of the defined reaction factor () was carried out to identify the mechanism for S L changes at sub-atmospheric conditions. According to the experimental results, S L increased by 15.9% and 26.3% for mixtures M1 and M2, respectively, at 0.84 atm as compared to 1.0 atm. The reaction pathways elaborated employing F R indicate that the increase in S L at sub-atmospheric conditions is caused by increased CH3 radical production by reaction C2H5 + H = 2CH3 (R159), which increases the formation of H radical through reactions O + CH3 = H + CH2O (R10) and O + CH3 = H + H2 + CO (R284). The recombination reactions associated with the production of CH4 and C2H6 also contribute to S L increases at sub-atmospheric conditions.

Enhanced pool boiling heat transfer by metallic nanoporous surfaces under low pressure

The nano-structure enhanced pooling boiling at standard atmosphere has been fully studied in literature but it is rarely investigated under low pressure. However, the boiling behaviors and bubble dynamics of nano-structures highly depend on the working pressure. This study investigated the enhanced pool boiling (under low pressure) of a nanoporous copper surface (NCS) fabricated by the hot-dip galvanizing & dealloying. The boiling curves and bubble characteristics of the NCS under low-pressure boiling conditions (25 kPa and 65 kPa) were evaluated by a test set up with high-speed visualization. To validate the present experimental plain-wall HTC under low pressure, three predicted models (Labuntsov, modified Rohsenow, and Stephan-Abdelsalam) were applied to obtain the theoretical HTCs for comparison. Besides, the optimization of the galvanization time has been conducted. Experimental results showed that the NCS significantly improved the pool boiling heat transfer and mitigated the negative influence of sub-atmospheric conditions. The enhanced mechanisms are the augmented nucleate site density, decreased bubble departure diameter, and increased bubbles generation frequency induced by the promoted hydrophilicity and rough morphology of the NCS. Among the three analyzed samples, the NCS-500 °C-3 min surface yielded the lowest wall superheat at the onset of boiling (ONB) and the highest heat transfer coefficient (HTC). In addition, the nanoporous surface leads to higher HTC enhancement upon the plain surface at a lower pressure. Specifically, the HTC enhancement percentage was increased from 71.6% (at 65 kPa) to 139% (at 25 kPa) as the pressure decreased.

Laminar Burning Speeds and Flammability Limits of CH4/O2 Mixtures With Varying N2 Dilution at Sub-Atmospheric Conditions

ABSTRACT This paper presents experimentally determined laminar burning speeds () of premixed methane/oxygen/nitrogen mixtures at sub-atmospheric conditions obtained by using the constant-pressure spherical flame method. The goal of the conducted experiments was to investigate the effects of nitrogen dilution and pressure reduction on the laminar flame speed and flammability limits of methane/oxygen mixtures. Nitrogen content was varied between 3.1 and 79.3 volume percent. at a target pressure of 0.5 bar was investigated. Actual pressure levels ranged between 0.506 and 0.568 bar. Experiments at 1 bar were conducted as a benchmark case of the setup compared to results from literature. The laminar flame speed is found to increase with decreasing nitrogen content and decreasing pressure. The obtained results show good agreement with simulated data using the several chemical reaction mechanisms within 4.8% for near stochiometric mixtures. The upper flammability limit (UFL) is shown to be pressure-sensitive while the lower flammability limit (LFL) remains stationary for both pressure levels compared to reference data at 1 bar. Furthermore, the LFL is shown to stay at 6% vol methane content, independent of nitrogen dilution ratio.

Limited Enhancement of Subatmospheric Boiling on Treated Structured Surfaces With Biphilic Pattern

Abstract Boiling heat transfer suffers deteriorations under subatmospheric conditions, which can be attributed to a shortage of viable nucleation sites at declining pressures. In this work, the possibility of enhancing low-pressure saturated boiling of water using a combination of wettability patterning and structural modifications was experimentally explored. The copper test surface, comprised of an array of circular “dimples” (0.3 mm in depth, 0.5 mm in diameter, and 3.0 mm in pitch), was spray-coated by polytetrafluoroethylene (PTFE) coatings so as to form a matching biphilic pattern with the surface cavities. The resulting dimpled biphilic surface showed appreciable heat transfer enhancement—with a maximum 60% increase of the average heat transfer coefficient of nucleate boiling compared with a flat biphilic surface—down to about 9.5 kPa. Further lowering the pressure to 7.8 kPa, however, was found to lead to diminished performance gains. The visualization study of the bubble departure dynamics revealed signs of additional vapor trapping of the hydrophobic-coated cavities, which can induce uninterrupted bubble regeneration with zero waiting time and explain the qualified enhancement of subatmospheric boiling. Thanks to a potential secondary pinning of contact line inside the hydrophobic cavities, incomplete bubble detachment could prevail at somewhat lower pressures than was otherwise possible without the dimple structure, leaving behind significantly more vapor residues. However, the vapor-trapping capacity was found to decrease with pressure, which provided clues with regard to the reduced efficacy of the surface at even lower pressures.

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.

Full scale test facility for operability verification of tokamak pressure suppression system at sub-atmospheric conditions

The reliable and safe operation of tokamak fusion reactors is mainly based on radioactive contaminants confinement, achieved preserving the integrity of robust barriers, among which the Vacuum Vessel (VV) performs an essential role. The VV protection against postulated over pressurization events is guaranteed by the safety functions implemented in the Pressure Suppression System (PSS), in which steam from VV is discharged and Direct Contact Condensation (DCC) occurs at sub-atmospheric conditions. Aiming to verify the VVPSS performance of ITER machine (i.e. steam condensation efficiency at different DCC regimes with/without non-condensable), in steady state and transient conditions relevant for fusion reactor postulated events, and contribute to fill the knowledge gap regarding sub-atmospheric condensation, an extensive experimental research activity is ongoing with the Large and Full Scale Test Facility (LSTF and FSTF): designed, assembled, instrumented and commissioned at Laboratory B. Guerrini at DICI University of Pisa. This facility is mainly composed of a Steam Generator (up to 500 g/s, about 1.8 MW), three different steam lines for wide mass flowrate regulation, two air lines for mixed condensation characterization, Experimental Test Tank (92 m3, where steam condensation is investigated adopting two different spargers, scaled and full scale with 100 and 1000 holes, respectively), Auxiliary Tank (10 m3 for plant conditioning), Vacuum Pump, Chiller and Water Storage Tank (15 m3). Two Steam Storage Tanks (15 m3 each) allow to accumulate and provide the foreseen steam peak flowrate value (of 5 kg/s) in the postulated accident. FSTF is highly instrumented (81 thermo-resistances, 28 thermocouples, 35 pressure transmitters, 18 strain gauges and 4 vortex flow meters) and fully remotely controlled (64 on-off and control valves) by a dedicated Data Acquisition and Control System (DACS). Six video cameras and 4 led lights assure steam injection video-recording, for detailed characterization of condensation regimes. Preliminary results are shown for 50 g/s of steam injected through the scaled sparger into water pool at 30 °C with cover gas at 0.1 bar absolute, providing a condensation efficiency of about 100% and verifying instable chugging condensation regime.