Average Void Research Articles

Two-phase flow properties for air sheets injected from rectangular slots in high-velocity crossflow

This work presents an experimental contribution towards the understanding of the two-phase jet dynamics in high-speed transverse flows, a field that is essential to many industrial applications. Four slot injectors are the subject of this investigation, which examines air flows as high as 0.05 m3/s and cross flows as high as 6 m/s. Using an invasive optical probe, the study reveals the presence of an air cavity surrounded by a two-phase mixing zone. Analysis of the spatial void fraction reveals counter-rotating vortices that direct bubbles towards their centers while larger bubbles rise to the top of the vein and form an upper zone with a maximum void fraction. The study also looks at the dynamic evolution of air sheet characteristics, which reveals an increasing trend in average void fraction along the flow. The comparison of bubble characteristics of different slots and conditions provides valuable insights into the impact of environment pressure and slot geometry on void fraction and bubble size in two-phase flows. Finally, correlations based on dimensionless numbers generalize the information on bubble characteristics for a wide range of conditions, assisting in estimation and validation in numerical simulations.

Characteristics of Open-Graded Friction Course Macrotexture and Macrostructure and Its Effect on Skid Resistance under Rainfall.

An Open-Graded Friction Course (OGFC) presents a rough surface and a porous structure and provides skid resistance under wet conditions, differing from that of a dense graded mixture. This study explored the distribution of surface macrotexture with depth in OGFC. Using cross-sectional images and semantic image segmentation techniques, the internal structure, porosity, and void size distribution were analyzed to assess the effectiveness of rainfall drainage. Skid resistance was evaluated with a British Pendulum Tester, focusing on the influence of surface macrotexture and internal macrostructure, particularly with regard to contact depth. Results show that finer gradations increase surface roughness peaks, which are concentrated near the top surface. In contrast, coarser mixtures exhibit a greater effective contact depth and more peaks with higher curvature. Finer gradations also result in lower porosity, greater void dispersion, and smaller average void diameters. During heavy rainfall, OGFC-13 exhibits the highest friction coefficient due to its effective contact, surface roughness, and internal voids, which facilitate water expulsion. This research provides insights into the skid resistance mechanism of OGFC in wet conditions and offers practical guidance for selecting the optimal gradation.

Uroflowmetry: nomograms in healthy young Pakistani men.

To create a nomogram based on urine volume and flow of males without lower urinary tract symptoms. The prospective, cross-sectional study was conducted at the Department of Urological Surgery and Transplantation, Jinnah Postgraduate Medical Centre, Karachi, from November 1, 2020, to October 31, 2022, and comprised healthy young males without lower urinary tract dysfunction who were recruited from the hospital as well as a large textile mill. They were asked to void on their normal desire. Uroflowmetry was done to determine maximum flow rate, average flow rate, and void volume values. A best-fit regression model was used to formulate uroflowmetry nomogram using average and maximum urine flow rate over voided volume. The sample size was calculated using PASS 2020 Power Analysis and Sample Size Software (2020). NCSS, LLC. Kaysville, Utah, USA. The database was developed on NCSS 2020 Statistical Software (2020). NCSS, LLC. Kaysville, Utah, USA for the data analysis. Of the 468 male subjects enrolled, data was analysed related to 432(92.3%). The mean age was 25.59±4.32 years. Mean maximum flow rate, average flow rate and void volume were 25.28±8.70mL/s, 14.77±4.79mL/s and 405.48±163.86mL, respectively. The association of age was noted with maximum flow rate (r=0.1435, p=0.004), average flow rate (r=0.1135, p=0.004) and void volume (r=0.0619, p=0.004). The best-fitted model for maximum and average flow rate was subsequently developed which was statistically significant (p<0.05). The nomograms developed could reliably predict the maximal flow rate in young Pakistani men.

Characterization of Fe-Cr alloys irradiated by neutrons at intermediate temperature

A series of Fe-Cr alloys, with 3 at.% - 18 at.% Cr, was irradiated in the Advanced Test Reactor (ATR) at the Idaho National Labs (INL), USA, up to a dose of ∼6.7 dpa at a temperature of ∼456 °C. Transmission electron microscopy (TEM) samples were extracted using a focused ion beam (FIB) instrument, and the resulting microstructural defects, such as voids, dislocation loops, network dislocations, Cr rich precipitates, etc., were characterized using a TEM. It was found that the size and number density of these defects varied widely over the different alloys with varying Cr content. As expected, there were no Cr rich precipitates in samples with Cr up to 9 %, and they started appearing only in samples with 12 at.% Cr and above. The particle size decreased from about 15 nm at 12 % Cr to 8 nm at 18 % Cr, while the number density increased from ∼7e20 /m3 to 6e22 /m3 for the same Cr contents. Grain boundary segregation of Cr, along with a precipitate free zone, was observed in the cases where a boundary was present in the sample. Large voids (>1–2 nm) were almost invisible in the Fe-3at.%Cr sample, while the average void size remained almost constant between 15 and 18 nm for samples with 6–15 at.% Cr and increased slightly at 18 at.% Cr to ∼22 nm. Fe-3 %Cr showed a high density of small voids(<2 nm), estimated to be about 1e23/m3. The number density of large voids increased from ∼0 at 3 % Cr to a peak of ∼6.4e20 /m3 at 12 % Cr, then decreased to about 1.1e20 /m3 at 18 % Cr. The dislocation loops, which appeared in linear arrays in the Fe-9 %Cr sample, were analysed in detail using both invisibility criteria and image simulations using the Oxford University TEMACI software, and it was found that they are most likely to be ½ 〈111〉 type loops on {100} planes. These loops seem shaped like sections of helices in some places and are most likely formed by loops near screw dislocations climbing into a helix and then collapsing into a loop array. Similar dislocation analysis was performed in the other samples as well wherever feasible, and it was found that there is a mixture of ½ 〈111〉 and [100] dislocation loops.

Systematic Evaluation of Adhesion and Fracture Toughness in Multi-Material Fused Deposition Material Extrusion.

Material extrusion (MEX) additive manufacturing has successfully fabricated assembly-free structures composed of different materials processed in the same manufacturing cycle. Materials with different mechanical properties can be employed for the fabrication of bio-inspired structures (i.e., stiff materials connected to soft materials), which are appealing for many fields, such as bio-medical and soft robotics. In the present paper, process parameters and 3D printing strategies are presented to improve the interfacial adhesion between carbon fiber-reinforced nylon (CFPA) and thermoplastic polyurethane (TPU), which are extruded in the same manufacturing cycle using a multi-material MEX setup. To achieve our goal, a double cantilever beam (DCB) test was used to evaluate the mode I fracture toughness. The results show that the application of a heating gun (assembled near the nozzle) provides a statistically significant increase in mean fracture toughness energy from 12.3 kJ/m2 to 33.4 kJ/m2. The underlying mechanism driving this finding was further investigated by quantifying porosity at the multi-material interface using an X-ray computed tomography (CT) system, in addition to quantifying thermal history. The results show that using both bead ironing and the hot air gun during the printing process leads to a reduction of 24% in the average void volume fraction. The findings from the DCB test and X-ray CT analysis agree well with the polymer healing theory, in which an increased thermal history led to an increased fracture toughness at the multi-material interface. Moreover, this study considers the thermal history of each printed layer to correlate the measured debonding energy with results obtained using the reptation theory.

Clinical efficacy and safety analysis of type A botulinum toxin in the treatment of adolescents with refractory overactive bladder.

The objective of this study was to assess the clinical effectiveness and safety of type A botulinum toxin in the treatment of refractory overactive bladder in adolescents. We conducted a retrospective analysis of 37 adolescent patients with refractory overactive bladder who were treated at the Urology Department of Hangzhou Third People's Hospital between January 2018 and August 2023. These patients received intravesical injections of type A botulinum toxin at a concentration of 10 U/mL, with an average of 20 injection points. We recorded changes in urination diaries and urodynamic parameters both before and 1 month after treatment. After 1 month of treatment, significant improvements were observed in several parameters, when compared to the pretreatment values. These included daytime frequency of urination (11.13 ± 6.45), average single void volume (173.24 ± 36.48) mL, nighttime frequency of urination (2.43 ± 0.31), urgency episodes (3.12 ± 0.27), initial bladder capacity (149.82 ± 41.34) mL, and maximum bladder capacity (340.25 ± 57.12) mL (all P < .001). After the first treatment, 5 patients had mild hematuria, 4 patients had urinary tract infection, and 1 patient had urinary retention, which was relieved after catheterization. No serious complications or adverse reactions were observed in other patients. The follow-up period ranged from 6 to 18 months, and the duration of efficacy varied from 2 to 8 months. Eight patients who initially had treatment failure achieved symptom relief after reinjection. In adolescents with refractory overactive bladder who do not respond well to conventional drug therapy, type A botulinum toxin can be administered safely and effectively. It significantly improves lower urinary tract symptoms and enhances the quality of life for these patients.

Low-cost method to reduce interlayer voids in material extrusion: in situ layer-by-layer solvent treatment

Material extrusion (MEX) is worldwide known as one of the most flexible additive manufacturing (AM) technologies for the fabrication of complex polymeric structures. However, the extremely high geometrical freedom has a price to pay: the presence of interlayer voids between consecutive extruded layers is the main backwards of MEX technology. Interlayer voids make 3D printed parts weaker (poor mechanical properties) compared to polymeric components fabricated by means of counterpart processes (i.e., injection molding). The present research work introduces a novel approach for the reduction of voids based on the layer-by-layer application of solvent vapor during the fabrication process, to smooth every single deposited layer. In this way, the new extruded layer has a greater area to bond with the previously extruded layer, resulting in an overall reduction of the porosity. The proposed approach is cost-effective, and it is based on the stop and go method enabled by MEX technologies: the fabrication process is paused after every layer, and when the solvent treatment is performed, the 3D printing process is resumed. The effectiveness of the layer-by-layer solvent vapor approach was evaluated, thereby resulting in a great reduction of the void density and average void area of 96%, and 79% respectively, and an increase of the wetting factor of 34%. Such findings pave the way for the exploitation of the proposed approach for the fabrication of complex structures with a reduced number of voids to be employed as structural components.

Urinary Bladder Dysfunction in a Novel Model of Neuroendocrine Stress

Chronic stress can lead to urinary bladder dysfunction and pain either through actions within the central nervous system or by mechanisms that are locally induced in the bladder. Prolonged upregulation of brain-derived neurotrophic factor (BDNF) in the paraventricular nucleus of the hypothalamus (PVN) has been shown to induce hypertension and chronically elevate activities of the sympathetic nervous system and hypothalamus-pituitary-adrenal axis. Taking advantage of this mechanism, we created a novel model of chronic neuroendocrine stress by subjecting male Sprague Dawley rats to bilateral PVN injections of AAV2 viral vectors expressing either BDNF or GFP (for control). We tested the hypothesis that model-induced chronic neuroendocrine stress would impair bladder function. Bladder activity was assessed 10 weeks post-injections by monitoring the number of urinary voids and average urinary void volume along with water intake over a 48-hour period. To further characterize changes in the urinary bladder, bladder weights were recorded, and in vitro bladder strip experiments were conducted 14 weeks after viral vector injections. BDNF overexpression in the hypothalamus led to significantly lower daily urine output even though water intake remained unaffected, suggesting that BDNF-treated animals showed a significantly higher non-urine-related fluid compared to GFP controls (daily urine output = GFP: 33.85±4.94, BDNF: 14.82±2.18, p &lt; 0.01. Interestingly, despite a lower total daily urine volume, BDNF-treated animals had a significantly higher number of urinary voids (GFP: 7.50±1.42, BDNF: 15.38±2.76; GFP vs. BDNF: p &lt; 0.001) and lower average void volume (GFP: 1.19±0.15, BDNF: 0.27±0.07; GFP vs. BDNF: p &lt; 0.01), thus emulating the overactive bladder phenotype, a common complication in humans with chronic stress. These findings were complemented by in vitro experiments where bladder strips from BDNF rats showed significantly higher contractility as tested with high K+ concentration and electric field stimulation. Moreover, Carbachol-induced stimulation of muscarinic receptors in BDNF bladder strips also trended towards higher contractility. However, urinary bladder weights were unaffected by BDNF overexpression. In summary, we demonstrated that prolonged hypothalamic BDNF overexpression, a model of chronic neuroendocrine stress, leads to significant alterations in bladder function such that BDNF-treated rats exhibited a higher number of urinary voids and lower average void volume compared to GFP controls, and bladder strip experiments suggest that enhanced smooth muscle contractility may contribute to this overactive bladder phenotype. These findings from our novel experimental model will help elucidate pathways through which neuroendocrine stress mechanisms contribute to the development of the overactive bladder syndrome. Funded by R01HL166464, R03AG072016 and R01DK125543. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

A numerical study on machine-learning-based ultrasound tomography of bubbly two-phase flows

Ultrasound tomography (UT) of bubbly two-phase flows using machine learning (ML) was investigated by performing two-dimensional ultrasound numerical simulations using a finite element method simulator. Studies on UT for two-phase flow measurements have been conducted only for some bubbles. However, in an actual bubbly flow, numerous bubbles are complexly distributed in the cross-section of the flow channel. This limitation of previous studies originates from the transmission characteristics of ultrasound waves through a medium. The transmission characteristics of ultrasound waves differ from those of other probe signals, such as radiation, electrical, and optical signals. This study evaluated the feasibility of combining UT with ML for predicting dense bubble distributions with up to 20 bubbles (cross-sectional average void fraction of approximately 0.29). We investigated the effects of the temporal length of the received waveform and the number of sensors to optimize the system on the prediction performance of the bubble distribution. The simultaneous driving of the installed sensors was simulated to reduce the measurement time for the entire cross-section and verify the method’s applicability. Thus, it was confirmed that UT using ML has sufficient prediction performance, even for a complex bubble distribution with many bubbles, and that the cross-sectional average void fraction can be predicted with high accuracy.

Spatio-temporal void fraction visualization in air-water two-phase flow regime transitions by combination of convolutional neural network and long short-term memory implemented into multiple current-voltage (MCV-CNN_LSTM)

Spatio-temporal void fraction in air-water two-phase flow regime transitions has been visualized by combination of convolutional neural network and long short-term memory implemented into multiple -current-voltage (MCV-CNN_LSTM). The MCV-ML is composed of two components, which are MCV-CNN_LSTM training and evaluation. In the first component, four steps are carried out as 1) true void fraction α measurement was conducted by wire-mesh sensor (WMS) as objective variables, 2) simulated MCV voltage U calculation by simulation of MCV measurement for explanatory variable, 3) CNN_LSTM training instance generation, and 4) CNN_LSTM model training and testing. In the second component, two additional steps are experimentally conducted which are 5) actual MCV voltage V measurement by MCV for input of trained ML, and 6) spatio-temporal void fraction α prediction. For the dataset generation, the experiment was conducted on air-water two phase flow regime transitions in vertical pipe with an inner diameter of 25 mm and a length of 350 mm from the elbow. The superficial velocity of liquid and gas phase are varied to present the flow regime transition of bubbly to slug flow. As a result, the estimated spatio-temporal void fraction αˆ by MCV-CNN_LSTM enables the visualization of detailed gas distribution in vertical upward two-phase flow. The spatial averaged instantaneous void fraction measured by WMS and estimated by MCV-CNN_LSTM show qualitative agreement in normalized cross correlation (INCC) of 0.364 on its temporal variation.

Experiments and predictions for severe slugging of gas-liquid two-phase flows in a long-distance pipeline-riser system

Experiments concerning the flow regimes observed in gas-liquid two-phase flows in a long-distance pipeline-riser system were carried out and analyzed in this paper. The test loop with 0.075 m inner diameter consisted of a 314 m horizontal pipeline, followed by a 25 m downward-inclined pipe, and finally a 21.5 m riser pipe. An industrial-scale global flow regime map including 5 flow regimes was derived from differential pressure measurements in the riser and was used as a reference for improvements. Electrical capacitance volume tomography installed in the horizontal section was used to reconstruct the phase interface and calculate the average gas void fraction (GVF). An appropriate model for this system was selected by comparing the experimental GVF with different, widely used models. The occurrence of severe slugging flow (SS) can be predicted accurately using an improved model involving additional corrections that consider pipeline length, accurate GVF calculation, and gas density. This study supports the development of a suitable SS prediction model enabling safe design, operation, and control of practical systems.

Choriocapillaris flow features in children with myopic anisometropia

AimsTo examine differences between the eyes in choriocapillaris perfusion and choroidal thickness in children with myopic anisometropia.MethodsIn this observational and prospective study, 46 children with myopic anisometropia were enrolled. Choriocapillaris...

Effect of colloidal nano silica on the freeze-thaw resistance and air void system of portland cement concrete

The present study investigates the impact of colloidal nano silica (CNS) on the freeze-thaw durability of concrete, with a particular focus on understanding the fundamental mechanism via a systematic analysis of the air void system. Experimental results suggest that the incorporation of CNS increases the total air content of the sample while reducing the average void size. This phenomenon can be attributed to the remarkable capacity of CNS to stabilize air bubbles within the concrete matrix, by a heightened viscosity and lower surface tension of the fresh concrete mixture. The increase in air void chambers facilitates the release of internal stress during freezing expansion, thereby enhancing the freeze-thaw resistance of the concrete. Additionally, the inclusion of CNS results in the formation of a denser cement matrix that improves the water impermeability of the concrete and prevents the ingress of water and other potentially corrosive substances.

Area averaging of packed bed in continua conservation equations in axial flow

AbstractA novel averaging of the conservation equations for axial flow in packed beds is presented to express the momentum, continuity, and energy equations in terms of the cross‐sectional averages of concentration, temperature, and superficial velocity. The model integrates the radial fluctuations of the intrinsic variables into the partial differential equations of the model and presents a method for the design and analysis of catalytic reactors at a broad range of Reynolds numbers without the necessity of adjusting the operating conditions to minimize the impact of the radial profiles of velocity, concentration, and temperature. Since the control of the industrial reactors is dependent on the average values of the concentration and temperature, the model can be directly employed for design, optimization, and control processes with the added advantage of time and cost savings for both the numerical resolution and laboratory testing expenses. The model is limited to reaction systems with Péclet numbers of less than 700 with an average void fraction of for which the ratio of the bed length‐to‐particle diameter to particle Péclet number exceeds . The model was applied to the simulation of steam methane reforming (SMR). The results indicate that the average equations predict the responses of the SMR properly and thus the model could be reliably utilized for process design, operation, and control.

Accelerating Discovery of Polyimides with Intrinsic Microporosity for Membrane‐Based Gas Separation: Synergizing Physics‐Informed Performance Metrics and Active Learning

AbstractPolymer membranes are widely used for gas separation, addressing critical environmental and energy issues. Nevertheless, crafting high‐performance polymer membranes remains a challenging task, often relies on labor‐intensive trial‐and‐error experiments. Different from the traditional Edisonian approach, machine learning offers significant potential to expedite the design of new polymer membranes for gas separation, yet it faces a substantial hurdle due to limited data availability. To overcome this challenge, in this study two physics‐informed performance metrics are introduced, namely fractional free volume and average void size, to assess the separation performance of polymer membranes. By employing active learning and multi‐target screening, top‐performing polyimides with intrinsic microporosity are efficiently discovered from a pool of 155 610 candidates, through calculations of only 709 (0.45%) in the entire search space. As validated by molecular simulations, the top‐performing polyimides exhibit exceptional separation performance for CO2/N2, CO2/CH4, and O2/N2 mixtures, superior to existing polymers and surpassing the current upper bound. The utilization of physics‐informed performance metrics provides a novel strategy to advance the design and accelerate the discovery of new polymer membranes for gas separation and many other important applications.

Flow characteristics of dispersed two-phase flows in an 8 × 8 rod bundle

Two-phase flows in rod bundles or tube bundles appear in heat transfer systems. The bundle-average (or one-dimensional) void fraction is a critical parameter in overall system design, performance evaluation, and safety assessment. Accurate bundle-average void fraction prediction requires a precise interfacial momentum transfer term formulated using the distribution parameter and drift velocity, two essential drift-flux parameters in the drift-flux model. The currently utilized modeling approach is based on the distribution parameter and drift velocity determined together through drift-flux plots. This results in possible compensation errors between the distribution parameter and drift velocity in the model validation process. The independent validation of the constitutive equations for the distribution parameter and drift velocity has been challenging due to experimental difficulty in measuring detailed two-phase flow structures in rod bundles. The present study proposes an approximation methodology to obtain subchannel average two-phase flow parameters using local two-phase flow data measured at two local points: the subchannel center and minimum gap center in a rod bundle. The distributions of subchannel average two-phase flow parameters are invaluable in benchmarking subchannel analysis codes. Comprehensive subchannel average void fraction and gas velocity mappings are given to discuss the effects of gas and liquid velocities, pressure, spacer grid, and developing length on the two-phase flow characteristics in the rod bundle. The bundle-average distribution parameter values calculated by subchannel average two-phase flow parameters are used for independent validation of the constitutive equations of the distribution parameter. The bundle-average drift velocity values back-calculated using the distribution parameter values are also used for independent validation of the constitutive equation of the drift velocity.

Emulsion templating of simultaneously-synthesized interpenetrating polymers with degradable components for hierarchical porosities

Hierarchically porous polymers are of interest for a variety applications including adsorption, catalysis, scaffolds, energy storage, and controlled release. In this work, two simultaneous, mutually exclusive reactions within the external phases of water-in-oil high internal phase emulsions (HIPEs) were used to generate macroporous semi-interpenetrating polymer network (semi-IPN) polyHIPE monoliths. The styrenic copolymer of vinylbenzyl chloride (VBC) and divinylbenzene (DVB) was synthesized using free radical polymerization. The non-crosslinked poly(urethane urea) (PUU), based on a polycaprolactone (PCL) triol, was synthesized using step-growth polymerization. The open-cell, semi-IPN polyHIPEs had densities of ∼0.17 g/cm3, corresponding to porosities of ∼84 %. The semi-IPN polyHIPE with the higher P(VBC-co-DVB) content (75 wt %) exhibited a porous structure typical of polyHIPEs, with an average void diameter of ∼10 μm. The porous structure of the semi-IPN polyHIPE with the lower P(VBC-co-DVB) content (50 wt %) resembled a network of channels. Hierarchical porosities were generated by either etching the flexible PCL-based non-crosslinked PUU to generate mesoporosity or by hypercrosslinking the stiff styrenic copolymer framework to generate microporosity. Etching the semi-IPN polyHIPEs in 3 M NaOH removed most of the PUU. Hypercrosslinking generated microporosity and specific surface areas of 445 and 176 m2/g for 75 and 50 wt % P(VBC-co-DVB), respectively. In the semi-IPNs, the hypercrosslinking process also generated mesoporosity from the removal of some of the PUU.

Spatio-temporal evolution laws of storage coefficient of coal mine underground reservoir and contact network of crushed rock.

Spatio-temporal evolution laws of storage coefficient and contact network of crushed rock are of great significance for the construction and utilization of underground reservoirs in coal mines. Based on discrete element method and irregular rigid block model, spatio-temporal evolution laws of storage coefficient and contact network of crushed rock under different overburden stresses are investigated and the following main conclusions are obtained: (1) The average storage coefficient and the storage coefficient at different vertical heights of the crushed rock packing system decrease exponentially as the overburden stress increases. When the overburden stress ranges from 0 to 20 MPa, the average storage coefficient decreases by 48.947%. (2) The average void radius and throat radius of water storage space decrease exponentially as the overburden stress increases. The increase in overburden stress leads to the transformation of large voids into smaller voids, causing a gradual decrease in void connectivity and a tendency towards irregular void shapes. (3) With the increase of overburden stress, the number of strong contacts in the packing system of crushed rock increases and gradually expands from the top to the bottom. The average contact force of crushed rock increases exponentially, while the coordination number increases linearly. (4) As the overburden stress increases, the majority of contact directions are concentrated within the ±30° range in the loading direction. This increase results in an enhancement of the anisotropy of the packing system structure of crushed rock.

Investigations of selected influencing process factors for transient liquid phase soldering (TLPS) as a die-attach method for automotive power modules

Increasing the operating temperatures in power electronic applications cause a decreasing lifetime of the die-attach interconnection. Transient liquid phase soldering (TLPS) is able to create a reliable and high temperature resistant connection layer via diffusion processes in the form of intermetallic phases with melting points up to &gt; 400°C. This study concentrates on a TLPS reflow-soldering process with a tin-copper-tin (Sn-Cu-Sn) layered preform. During the soldering process the influence of the selected process parameters for the Sn-Cu-Sn on the grown intermetallic phase interconnections between the preform and the components are investigated via the average void percentage and a process capability analysis (cpK). The application of mechanical pressure decreases the average void percentage below 10%. Due to the lower average void percentage the influence of the holding time, peak temperature and heating gradient are examined under low pressure. The biggest impact shows the heating gradient. With a slow heating gradient, the thermomechanical stress decreases by increasing the solubility and the average void percentage could be reduced to less than 2.7 %. In combination of the optimized selected process factors it is possible to reach a process capability value of 1.

A Semi-Empirical Correlation for the Onset of Density Wave Oscillations in a Helical Coil Steam Generator

NuScale Power has completed a series of full-length helical coil steam generator tests at SIET Laboratories in Piacenza, Italy. The test program included an investigation of the onset and evolution of density wave oscillations (DWOs). This paper describes the mechanisms leading to the onset of DWOs. A semi-empirical DWO stability correlation was developed from first principles using data from tests performed in the full-height (2015) TF-2 counterflow test facility. This research supports the concept that the onset of DWOs is caused by liquid bridging inside the boiling length of the tubes. A DWO onset correlation is obtained by performing a steady-state momentum balance on the liquid slug created by liquid bridging in annular flow. This yields a momentum balance equation in terms of a set of dimensionless superficial velocities, a corresponding dimensionless pressure drop term, and an average void fraction. Envelope theory is then used to obtain a critical void fraction, which is substituted into the dimensionless momentum balance to obtain a DWO onset correlation similar to the Wallis countercurrent flooding correlation. Predictions using the DWO stability correlation have been compared to DWO experiments from the (2022) TF-2, TF-1, and Polimi. The predictions for the onset and termination of DWOs show excellent agreement with the data.