The effect of characteristic of heating surface on nucleate boiling heat transfer is well reported via many previous results. However, until recently, the study of surface influences on nucleate ...boiling was complicated by coupled surface factors; wettability and surface roughness. This study focuses on nucleate pool boiling under different wetting conditions, in the absence of microscale roughness, which is coupled with wetting phenomena. Heterogeneous boiling occurs on hydrophilic (54°) and hydrophobic (123°) surfaces, even without microstructures that can trap water vapor. In the very low heat flux regime, hydrophobic surfaces offer better nucleate boiling heat transfer than hydrophilic surfaces. Moreover, a heterogeneous wettability surface composed of hydrophobic dots on a hydrophilic surface provides better nucleate boiling heat transfer than a homogeneous wettability surface (hydrophilic or hydrophobic). Analysis of bubble generation and departure was carried out via a high-speed visualization technique in order to understand these experimental results. Based on the bubble analysis, it was inferred that changes in wettability could lead to totally different boiling performance when microstructures are absent. Also, the number of hydrophobic dots and the pitch distance between dots were key parameters for explaining boiling performance under heterogeneous wetting conditions.
Although sodium-cooled fast reactors (SFRs) can have inherently enhanced safety features, there is a concern regarding unexpected events arising from a sodium-water reaction (SWR) in a Rankine cycle ...steam generator. Innovative concepts of integrated steam generators (ISGs), including an intermediate heat exchanger, have been suggested for eliminating an SWR by employing an intermediate fluid between liquid sodium and steam/water system. In this study, we propose an ISG designed with a novel tube configuration: a serpentine tube-type integrated steam generator (S-ISG). Furthermore, we developed a dedicated one-dimensional thermal-hydraulic analysis code for the specific design evaluation of the S-ISGs. The evaluation results are reported with reasonable explanations and compared with the results obtained using a conventional CFD tool to analyze local thermal-hydraulic characteristics that cannot be shown in the one-dimensional analyses. The proposed S-ISG has an inherent advantage for modularization as well as prevention of SWR; therefore, it is characterized by its ability to improve operational efficiency even in the case of repair, despite the low space efficiency. The results demonstrate that the component developed in this study can serve as an option to maintain cost efficiency while enhancing the public acceptability of an SFR.
Flow-boiling canopy wick capillary-viscous limit Kim, Tong Kyun; Ferreira, Julio; Jo, HangJin ...
International journal of heat and mass transfer,
December 2021, 2021-12-00, 20211201, Letnik:
181
Journal Article
Recenzirano
•The fabricated-tested flow-boiling canopy wick shows significant enhancement in the dryout limit and thermal conductance, compared to the plain surface.•The bimodal, bilayer evaporator wick is ...capable of high maximum capillary pressure, and yet the capillary-viscous limit dominates over the levee-enhanced hydrodynamic limit.•Good agreement is found between CFD simulations/predictions and experiments.
It was predicted that the perforated, leveed flow-boiling canopy wick (FBCW) can significantly increase the hydrodynamic instability limit of the flow boiling 1,2. The perforations represent and modulate the vapor columns in the boiling crises hydrodynamic stability theory. The lower capillary-viscous limit, governed by the evaporator wick maximum capillary pressure and the viscous drag within the three-dimensional composite wick, may control the actual dryout performance. Saturated-water flow-boiling (one atm) experiment results are presented employing an FBCW, created from sintered-copper particles, with and without the hydrodynamic-stabilizing levees. The bimodal bilayer evaporator wick allows for sufficiently large maximum capillary pressure and permeability, compared to the monolayer wick. The experimental results indicate that the levees can increase the critical heat flux (CHF) over that without levees, whereas the existing maximum capillary pressure controls disallow achieving this larger hydrodynamic heat flux limit. This capillary-viscous dryout limit is predicted by employing the minimum surface energy principles, network models, and computational fluid dynamics (CFD), in good agreement with experiments. By employing both CFD and flow visualizations, good agreement is also established between the predicted and observed two-phase hydrodynamics above the canopy wick. They indicate that although the local dryout commences downstream; however, dryout is experimentally observed when a critical fraction of the liquid supply tracks dry downstream. The measured large thermal conductance (heat transfer coefficient) is beneficial for the predictions, verifying the vapor-occupied space between the canopy and evaporator wicks.
•We conducted CHF experiments on heterogeneous wetting surfaces.•The heterogeneous wetting surface was composed of a hydrophilic substrate with hydrophobic dots.•The area ratio of hydrophobic dots to ...heating surface had a dominant effect on CHF.•We analyzed bubble dynamics on hydrophobic dots to explain the trend of CHF.•We could propose optimized conditions of hydrophobic patterned surfaces for boiling.
We used a heating surface composed of a hydrophilic substrate with hydrophobic dots to characterize the effect of spatially-different surface characteristics on critical heat flux (CHF) and nucleate boiling. To ascertain important surface factors that control CHF and boiling on heterogeneous wetting surfaces, we adjusted the hydrophobic dot diameter and the relative pitch between adjacent dots. Based on the dynamics of bubbles on hydrophobic dots, we analyze the trend of CHF on differently-fabricated heterogeneous wetting surfaces. CHFs on heterogeneous wetting surfaces were strongly dependent on ratio R of the area covered by hydrophobic dots to the heated area, but independent on the diameter of hydrophobic dots and the pitch distance. The improvement of boiling heat transfer (BHT) varied according to the conditions, and appeared to be related to the diameter, pitch distance and the number of hydrophobic dots, but the effect of R on BHT was negligible. Based on this study, we propose optimized conditions of a hydrophobic patterned surface. To sustain high CHF of a hydrophilic surface and high BHT of a hydrophobic surface, numerous micron-size hydrophobic dots should be fabricated with small R.
Biphilic surfaces exhibit outstanding condensation efficiency compared to surfaces having homogeneous wettability. Especially, hydrophilic patterns on a superhydrophobic substrate significantly ...promote the coalescence-induced jumping of condensed droplets by increasing the nucleation rate of condensation, thus enhancing the condensation efficiency drastically. However, the application of biphilic surfaces in practical industries remains challenging because controlling the size and spacing of the hydrophilic spots on large and complex surfaces is difficult. In this study, we have achieved heterogeneous wettability using the evaporation–crystallization method, which can be applied to various surfaces as required by industries. The crystals generated using the evaporation–crystallization process drastically increased the number density of condensed droplets on a superhydrophobic surface (SHS), so the developed biphilic surface increased the cumulative volume of jumping droplets by up to 63% compared to that on a conventional superhydrophobic surface. Furthermore, the condensation dynamics on the biphilic surface were analyzed with the classical nucleation theory and the Ohnesorge number. The analysis results indicated that the generated hydrophilic crystals can reduce the nucleation energy barrier and decrease the available excessive surface energy of coalesced droplets on the biphilic surface; this implies that the size distribution of the crystals determines the condensation dynamics. In sum, this study not only introduced an effective surface tailoring approach for enhancing condensation but also provided insights into the design of optimum biphilic surfaces for various conditions, creating new opportunities to widen the applicability of biphilic surfaces in practical industries that exploit condensation.
Blood viscosity may affect the mechanisms of stroke and early neurological deterioration (END). We aimed to investigate the relationship between blood viscosity, stroke mechanisms, and END in ...patients with middle cerebral artery (MCA) infarction. Patients with symptomatic MCA atherosclerosis (≥ 50% stenosis) were recruited. Blood viscosity was compared across patients with different mechanisms of symptomatic MCA disease: in situ thrombo-occlusion (sMCA-IST), artery-to-artery embolism (sMCA-AAE), and local branch occlusion (sMCA-LBO). END was defined as four points increase in the National Institutes of Health Stroke Scale score from baseline during the first week. The association between blood viscosity and END was also evaluated. A total of 360 patients (76 with sMCA-IST, 216 with sMCA-AAE, and 68 with sMCA-LBO) were investigated. Blood viscosity was highest in patients with sMCA-IST, followed by sMCA-AAE and sMCA-LBO (P < 0.001). Blood viscosity was associated with END in patients with MCA disease. Low shear viscosity was associated with END in patients with sMCA- LBO (adjusted odds ratio, aOR 1.524; 95% confidence interval, CI 1.035-2.246), sMCA- IST (aOR 1.365; 95% CI 1.013-1.839), and sMCA- AAE (aOR 1.285; 95% CI 1.010-1.634). Blood viscosity was related to END in patients with stroke caused by MCA disease.
In nuclear thermal-hydraulic system codes, most correlations used for vertical pipes, under downward two-phase flow, have been developed considering small pipes or pool systems. This suggests that ...there could be uncertainties in applying the correlations to accident scenarios involving large vertical pipes owing to the difference in the characteristics of two-phase flows, or flow conditions, between large and small pipes. In this study, we modified the Multi-dimensional Analysis of Reactor Safety KINS Standard (MARS-KS) code using correlations, such as the drift-flux model and two-phase multiplier, developed in a plant-scale air-inflow experiment conducted for a pipe of diameter 600 mm under downward two-phase flow. The results were then analyzed and compared with those based on previous correlations developed for small pipes and pool conditions. The modified code indicated a good estimation performance in two plant-scale experiments with large pipes. For the siphon-breaking experiment, the maximum errors in water flow for modified and original codes were 2.2% and 30.3%, respectively. For the air-inflow accident experiment, the original code could not predict the trend of frictional pressure gradient in two-phase flow as ⟨jg⟩/⟨j⟩ increased, while the modified MARS-KS code showed a good estimation performance of the gradient with maximum error of 3.5%.
In this study, we evaluate the thermal-hydraulic performance and economics of Printed Circuit Heat Exchanger (PCHE) according to the channel types and associated shape variables for the design of ...recuperators with Sodium-cooled Fast Reactors (SFRs). To perform the evaluations with variables such as the Reynolds number, channel types, tube diameter, and shape variables, a code for the heat exchanger is developed and verified through a comparison with experimental results. Based on the code, the volume and pressure drop are calculated, and an economic assessment is conducted. The zigzag type, which has bending angle of 80° and a tube diameter of 1.9 mm, is the most economical channel type in a SFR using CO2 as the working fluid. For a SFR using N2, we recommend the airfoil type with vertical and horizontal numbers of 1.6 and 1.1, respectively. The airfoil type is superior when the mass flow rate is large because the operating cost changes significantly. When the mass flow rate is small, volume is a more important design parameter, therefore, the zigzag type is suitable. In addition, we conduct a sensitivity analysis based on the production cost of the PCHE to identify changes in optimal channel types.
Stratified steam explosion energetics Jo, HangJin; Wang, Jun; Corradini, Michael
Nuclear engineering and technology,
02/2019, Letnik:
51, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Vapor explosions can be classified in terms of modes of contact between the hot molten fuel and the coolant, since different contact modes may affect fuel-coolant mixing and subsequent vapor ...explosion energetics. It is generally accepted that most vapor explosion phenomena fall into three different modes of contact; fuel pouring into coolant, coolant injection into fuel and stratified fuel-coolant layers. In this study, we review previous stratified steam explosion experiments as well as recent experiments performed at the KTH in Sweden. While experiments with prototypic reactor materials are minimal, we do note that generally the energetics is limited for the stratified mode of contact. When the fuel mass involved in a steam explosion in a stratified geometry is compared to a pool geometry based on geometrical aspects, one can conclude that there is a very limited set of conditions (when melt jet diameter is small) under which a steam explosion is more energetic in a stratified geometry. However, under these limited conditions the absolute energetic explosion output would still be small because the total fuel mass involved would be limited. Keywords: Steam explosion, Stratified configuration, Pool configuration, Fuel-coolant mixing
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