CHF Values Research Articles

Numerical analysis of subcooled flow boiling and critical heat flux under variable wall heat flux

This paper presents a numerical analysis of subcooled flow boiling in a vertical pipe through CFD simulation. The study is aimed to investigate the effect of variable heat flux profiles on CHF. To achieve this, a two-phase Eulerian-Eulerian model coupled with the RPI and CHF boiling models is employed for numerical solutions. The variable heat flux profiles examined include linear-increasing, exponential-increasing, and three sinusoidal-shaped profiles. The results highlight that when variable heat flux profiles are utilized, the RPI model lacks reliability, and the CHF boiling model proves to be a more suitable choice. Among the profiles studied, the sinusoidal-shaped profile with a peak in the middle of the tube exhibits the highest CHF value, while the lowest CHF value corresponds to the exponential-increasing heat flux profile. In sinusoidal profiles, as the number of thermal peaks increases, the CHF value decreases, its location shifts towards the pipe's end, and augments the risk of approaching CHF. By illuminating the intricacies of subcooled flow boiling under variable heat flux, this study provides valuable insights for the development of more efficient and reliable heat transfer designs.

Effect of Surface Wettability on Nanoparticle Deposition during Pool Boiling on Laser-Textured Copper Surfaces

Prior studies have evidenced the potential for enhancing boiling heat transfer through modifications of surface or fluid properties. The deployment of nanofluids in pool boiling systems is challenging due to the deposition of nanoparticles on structured surfaces, which may result in performance deterioration. This study addresses the use of TiO2–water nanofluids (mass concentrations of 0.001 wt.% and 0.1 wt.%) in pool boiling heat transfer and concurrent mitigation of nanoparticle deposition on superhydrophobic laser-textured copper surfaces. Samples, modified through nanosecond laser texturing, were subjected to boiling in an as-prepared superhydrophilic (SHPI) state and in a superhydrophobic state (SHPO) following hydrophobization with a self-assembled monolayer of fluorinated silane. The boiling performance assessment involved five consecutive boiling curve runs under saturated conditions at atmospheric pressure. Results on superhydrophilic surfaces reveal that the use of nanofluids always led to a deterioration of the heat transfer coefficient (up to 90%) compared to pure water due to high nanoparticle deposition. The latter was largely mitigated on superhydrophobic surfaces, yet their performance was still inferior to that of the same surface in water. On the other hand, CHF values of 1209 kW m−2 and 1462 kW m−2 were recorded at 0.1 wt.% concentration on both superhydrophobic and superhydrophilic surfaces, respectively, representing a slight enhancement of 16% and 27% compared to the results obtained on their counterparts investigated in water.

The effect of the radial heat loss, upper reservoir temperature, and corrugated wall channel on the flooding critical heat flux

This study measured the flooding critical heat flux (flooding CHF) for two vertical annular channels with different channel gaps. The experiments were conducted at atmospheric pressure and with no flow. The effects of the upper reservoir temperature within 30–80 °C and radial heat loss have been studied. It was found that the upper reservoir temperature has a small effect on the flooding CHF values, and it varied in the range of up to 10% compared to the saturated conditions. On the other hand, a significant effect of the radial heat loss was observed on the measured flooding CHF values (up to 100%). The flooding CHF experimental results were compared with numerous empirical correlations from the literature, some initially based on air-water counter-current flow experiments (flooding experiments with no heating). It was found that based on the onset flooding velocity experiments, the flooding CHF results can be successfully predicted. The effect of the wall corrugation on the flooding CHF was also examined in the simple air-water counter-current flow setup. This study shows that the air flooding velocity in a corrugated channel is higher than in a smooth channel with the same cross-section geometry and dimensions. Therefore, using a smooth channel flooding CHF value for a corrugated one guarantees a conservative result.

Influence of smooth heater size on critical heat flux and heat transfer coefficient of saturated pool boiling heat transfer

In order to study the influence of smooth heater size on the performance of pool boiling heat transfer, saturated boiling experiments of FC-72 were conducted on eight sized smooth silicon chips (the test surfaces are denoted as S5 ∼ S30, and their dimensionless dimensions are in the range of Lh/Lc = 7 ∼ 42). The bubble dynamics during boiling were observed with a high-speed camera. The results indicated that in natural convection regime, the heat transfer coefficient decreases with increasing heater size at a given heat flux. Besides, the larger surfaces enter into the nucleate boiling regime earlier. In a fully developed nucleate boiling regime, S5 surface displays the optimal heat transfer performance. The reality is that no vapor columns are produced even in the high heat flux range due to its small bubble departure diameter and high bubble departure frequency. For S8 ∼ S15 surfaces, the wall temperatures increase continuously with the increase of heat flux, while S20 ∼ S30 surfaces present a trend of first keeping constant and then increasing. The active nucleation site densities on S15 ∼ S30 surfaces follow the law of Na = 0.45q2 in the whole nucleate boiling regime. By contrast, for S5 ∼ S12 surfaces, the nucleation site densities abide by this law only in a fully developed nucleate boiling regime. In addition, the departure diameter and departure frequency of bubbles increase with the increase of heat flux for all surfaces. It is observed that with the increase of smooth heater size, CHF presents a changing trend of first increasing (in range of Lh ≤ λD, λD is the most dangerous Taylor wavelength), then decreasing (in range of λD < Lh ≤ 3λD) and finally leveling off (in range of Lh > 3λD). Accordingly, the CHF value maximizes at the heater size Lh = λD. In the segment of decreased CHF, the decline rate of CHF in range of λD < Lh ≤ 2λD is larger than that in range of 2λD < Lh ≤ 3λD. Meanwhile, the influencing mechanisms of heater size on CHF were explained by consideration into bubble patterns and behavior.

Labor market costs for long-term family caregivers: the situation of caregivers of persons with spinal cord injury in Switzerland

BackgroundFamily members are key in the provision of care to persons facing disability. To undertake the role as caregivers, they face many costs, being the setback in the labor market one of the most relevant.MethodsWe analyze comprehensive data from long-term family caregivers of persons with spinal cord injury (SCI) in Switzerland. Using information about their working situation before and after becoming caregivers, we estimated the reduction in working hours and the associated income loss.ResultsOn average, family caregivers reduced their working hours by about 23% (8.4 h per week), which has a monetary value of CHF 970 per month (EUR 845). Women, older caregivers, and less educated caregivers have a much higher opportunity cost in the labor market: CHF 995 (EUR 867), CHF 1,070 (EUR 932), and CHF 1,137 (EUR 990) respectively. In contrast, family members who care for a person that works have a much lower impact on their working status, CHF 651 (EUR 567). Interestingly, the reduction in their working time is only a third of the extra work they face as caregivers.ConclusionHealth and social systems rely on the unpaid work of family caregivers. To guarantee their long-term involvement, family caregivers need to be recognized for their work and potentially compensated. Without family caregivers, it is very unlikely societies can cope with the increasing need for care, as professional services are limited and expensive.

Parametric experimental trends, interfacial behavior, correlation assessment, and interfacial lift-off model predictions of critical heat flux for microgravity flow boiling with subcooled inlet conditions – Experiments onboard the International Space Station

This study examines the critical heat flux data obtained during the “Flow Boiling and Condensation Experiment (FBCE)” which was launched to the International Space Station (ISS) in August 2021. The overarching goals of FBCE are to obtain flow boiling and flow condensation data in high quality, long-duration microgravity, and investigate gravitational effects on two-phase flow physics. The first stage of FBCE completed features the Flow Boiling Module (FBM), which collected flow boiling heat transfer data and flow visualization images in a highly accurate and steady microgravity environment from February 2022 until July 2022. Flow boiling experiments were performed with subcooled inlet of n-Perfluorohexane in a rectangular channel of dimensions 114.6-mm heated length, 2.5-mm heated width, and 5.0-mm height with either one or two, opposite, heated walls. The long-duration microgravity database encompasses a broad range of operating conditions: mass velocity of 199 – 3200 kg/m2s, inlet subcooling of 2.6 – 45.6°C, and inlet pressure of 124.8 – 176.7 kPa. Image sequences leading up to and at CHF are presented to both illustrate the physical mechanism triggering CHF and explain the experimental trends observed in microgravity. Experimental results show CHF is strongly dependent on mass velocity and higher degrees of inlet subcooling, but less dependent on inlet pressure in the tested ranges. Examination of the relationships and parametric trends between dimensionless groups governing CHF reinforces conventional trends and reveals, for subcooled CHF, a dependence of Boiling number at CHF on outlet thermodynamic equilibrium quality. Comparison of the new microgravity CHF data with Earth-gravity CHF data reveals, for single-sided heating, CHF in microgravity is degraded up to ∼38% at low mass velocity, with diminishing differences as mass velocity is increased, and for double-sided heating, less significant differences in CHF between the two gravitational environments. Experimental data are compared to predictions of various flow boiling CHF correlations which previously demonstrated their merit, and the most suitable one for the entire database is recommended. Flow visualization reveals a wavy vapor layer with wetting fronts described in the Interfacial Lift-off Model, which is used to predict CHF values with good accuracy.

Pool boiling performance on the textured hemi-wicking surfaces fabricated by nanosecond laser ablation

Today the problem of the boiling improvement is directly related to the heating surface modification at the micro- and nanoscale. As was shown by different authors the hemi-wicking surfaces demonstrate extremely high enhancement of heat transfer (HTC) and critical heat flux density (CHF) during boiling. In this paper, the laser ablation technique was used to fabricate textured hemi-wicking silicon surfaces. This surface modification technique is one of the most promising and discussed techniques today due to its high accuracy, implementation ease and high stability of the fabricated surfaces. The effect of the laser type on the silicon surface properties and boiling improvement was studied in detail. The texturing was performed using lasers with different wavelength - infrared (1064 nm) and visible (532 nm), but with the same number of pulses per unit area and laser spot diameter. The experiments showed that the water pool boiling performance differs significantly depending on the type of laser treatment. In particular, while the usage of infrared laser results in the HTC enhancement up to 78% compared to the untreated surface, the visible laser-textured surface in contrast shows the deterioration of the heat transfer rate up to 40%. At the same time, the visible laser-textured surface demonstrates the maximum CHF value of 1806 kW/m2, which is more than 2 times higher than for the untreated surface. The comparison with the models showed, that such an enhancement is explained by the highest capillary wicking of this surface. Based on the analysis of departure diameters, nucleation frequencies and the conditions for the vapor bubbles formation the observed difference in the heat transfer rate during boiling was revealed. Also using the high-speed thermography the dynamics of reversible and irreversible dry spots was studied during boiling on laser-textured hemi-wicking surfaces for the first time.

Saturated pool boiling heat transfer enhancement of R245fa based on the surface covered by sintered copper powder with and without nanostructure

The pool boiling characteristics on the enhanced surfaces with sintered copper powder coatings with and without nano-treatment were experimentally investigated. Three kinds of enchanted surfaces formed by sintered porous layers with copper powder particles and their further nano-treated surfaces were adopted as the test surfaces. The thermal oxidation process was used to form nanostructures with super hydrophilic characteristic on the surface of sintered copper particles. Refrigerant R245fa was selected as the test fluid due to the potential the application in electronic cooling field. The saturation temperature of boiling experiments was maintained about 20 °C. The result showed that the boiling heat transfer performance was effectively improved by coating sintered copper powder porous layer on the plain copper surface, and the maximum heat transfer coefficient and CHF value enhancement could reach 151% and 160% of the plain one, respectively. The enhanced surfaces no matter with or without nano-treatment could trigger the nucleation process at the wall superheats lower than 12 °C rather than the value as high as 30 °C for plain copper surface. After nano-treatment, the heat transfer enhancement for sintered porous surfaces was further improved at medium and low heat flux, where the bubble nucleation or detachment remained robust. The CHF peaked as 140% of untreated surface, while the heat transfer coefficient increased to 108% at most.

High-efficiency spray cooling of rough surfaces with gas-assist atomization

Spray impingement heat transfer experiments were conducted to assess the role of surface roughness on heat transfer performance. Surfaces covered with arrays of micro-pillars with diameters in the range 0.8–50 μm were fabricated using standard lithography technique and etching processes. A recently developed air-assist nozzle was used to produce fine droplets of de-ionized (DI) water. Experimental results obtained indicate that the heat transfer performance is better for length scales in the 5–10 μm range than for sub-micron scales. Heat transfer coefficients in the two-phase evaporative regime were calculated by subtracting the sensible heat. Accounting for the enhancement in surface area allows for assessment of the effects of capillary wicking and thin film flow. Surfaces with bigger pillar size and larger spacing (∼20μm) exhibits greater heat transfer coefficient values due to higher permeability for wicking flow. The effect of nozzle/target spacing, porosity and liquid/air flow rates were also studied. At the highest liquid flow rate of 120 ml/min, a CHF value of 969 W/cm2 was observed, which is among the highest values documented in the literature, and suggests the potential for further enhancement at higher flow rates.

Critical heat flux and bubble behaviour study on differently oriented flat plates during pool boiling

An experimental study is brought off to analyse the effect of flat plate length, width, orientation, and pool liquid temperature on CHF under atmospheric conditions during pool boiling. The bubble nucleation and departure for the aforementioned parametric and operational conditions are discussed. Based on bubble behavior, various pool boiling regimes under uniform heat flux conditions are investigated. The pool is maintained at 25℃ and saturation temperature corresponding to ambient pressure. The occurrence of burnout on the heater plates is analysed on the basis of bubble behaviour. It is analysed that prior to the occurrence of CHF a liquid film enclosed with vapour leads to initiate the dry patch. It is observed that with an increase in pool temperature, the magnitude of CHF reduces. It has been estimated that there is a decrement of around 62 % in the magnitude of CHF in saturated conditions as compared with subcooled conditions. The vertical orientation of the plates results in a higher CHF magnitude. In the subcooled pool, the CHF in vertical orientation ranges between 6.197 MW/m2 to 2.909 MW/m2. Whereas in the saturated pool the CHF magnitude in vertical orientation ranges between 2.652 MW/m2 to 0.998 MW/m2. As the length of the heater plate increases, the CHF value reduces. The minimum magnitude of CHF is obtained as 0.898 MW/m2 for downward facing heater of 300 mm in length at the saturated pool. Higher the width lesser the CHF. The obtained experimental results are compared with available correlations to access its viability. An empirical correlation is suggested, implicating the effect of heater length, width, orientation, and pool temperature for the pool boiling CHF on flat plates.

Nanosecond Laser-Textured Copper Surfaces Hydrophobized with Self-Assembled Monolayers for Enhanced Pool Boiling Heat Transfer.

Increased cooling requirements of many compact systems involving high heat fluxes demand the development of high-performance cooling techniques including immersion cooling utilizing pool boiling. This study presents the functionalization of copper surfaces to create interfaces for enhanced pool boiling heat transfer. Three types of surface structures including a crosshatch pattern, shallow channels and deep channels were developed using nanosecond laser texturing to modify the surface micro- and nanomorphology. Each type of surface structure was tested in the as-prepared superhydrophilic state and superhydrophobic state following hydrophobization, achieved through the application of a nanoscale self-assembled monolayer of a fluorinated silane. Boiling performance evaluation was conducted through three consecutive runs under saturated conditions at atmospheric pressure utilizing water as the coolant. All functionalized surfaces exhibited enhanced boiling heat transfer performance in comparison with an untreated reference. The highest critical heat flux of 1697 kW m-2 was achieved on the hydrophobized surface with shallow channels. The highest heat transfer coefficient of 291.4 kW m-2 K-1 was recorded on the hydrophobized surface with deep channels at CHF incipience, which represents a 775% enhancement over the highest values recorded on the untreated reference. Surface microstructure was identified as the key reason for enhanced heat transfer parameters. Despite large differences in surface wettability, hydrophobized surfaces exhibited comparable (or even higher) CHF values in comparison with their hydrophilic counterparts, which are traditionally considered as more favorable for achieving high CHF values. A significant reduction in bubble departure diameter was observed on the hydrophobized surface with deep channels and is attributed to effective vapor entrapment, which is pointed out as a major contributing reason behind the observed extreme boiling heat transfer performance.

Experimental study on pool boiling heat transfer characteristics of TiO2 nanofluids on a downward-facing surface

Boiling heat transfer is an effective method for equipment to dissipate heat, which has been widely used in power industry and electronic products, but it is limited as its CHF value. It is well known that the CHF value can be enhanced by adding nanoparticles in order to reach higher heat flux requirements of nuclear power plants, electric vehicles and other high-power equipment. This experiment discussed the effect of different concentrations of TiO2 nanofluid on downward-facing boiling heat transfer, the concentrations were 0.0001 wt%, 0.0005 wt%, 0.001 wt%, 0.005 wt% and 0.01 wt%. The law of bubble behaviors throughout the boiling process was also observed. It was found that TiO2 nanofluid has an enhanced effect on CHF, the enhancement effect of nanofluid on CHF changes with the change of concentration and the maximum enhancement can be about 33%. The change of contact angles and roughness of the heating surface was detected, combined with the SEM image, it could be concluded that the deposition of TiO2 nanoparticles which changes the structure of the heating surface is the main reason why the nanofluid enhances heat transfer.

Experimental study on saturation pool boiling heat transfer characteristics of R245fa on the surface covered by sintered copper powder

In this paper, the mechanism of pool boiling heat transfer enhancement on the surface covered with sintered copper powder porous layer has been investigated. The refrigerant R245fa with great application prospect in electronic cooling field was selected as the test fluid. According to the results, the boiling heat transfer performance of R245fa could be effectively enhanced by attaching the sintered copper powder porous layer to the plain copper surface. Even when the wall superheat was as low as 2.2 °C, the sintered copper powder surface with the particle size of 110 μm and thickness of 0.6 mm could still be efficiently triggered to boiling. The heat transfer coefficient and CHF were significantly improved up to 247.71% and 255.15% compared with that of plain copper surface, respectively. However, with the thickness of porous layer increased from 0.6 mm to 0.9 mm, the maximum heat transfer coefficient for the enhanced surface with particle size of 30 μm went up to 136.01%, while the CHF value decreased by 92.52%. For 65 μm and 110 μm surfaces, the change of porous layer thickness had little effect on the heat transfer coefficient in the initial boiling stage, but presented the contrary influence trend at high heat flux conditions.

Separate factors’ effects on CHF for flow boiling in an inclined rectangular channel under low mass flux

Flow boiling under natural circulation is a reliable method of safety strategy for a thermal dissipation system, especially in the nuclear power system, however, the CHF is the safety limit. Thus the CHF prediction is paramount important for such systems. In order to well understand CHF mechanism and develop the theoretical models, separate factors’ (heater surface orientation, mass flux and inlet quality) effects on CHF for flow boiling under low mass flux and low pressure are investigated both experimentally and theoretically in this study. A rectangular channel with the cross section of 17 mm × 10 mm is used as the flow channel and the main heater with stainless steel surface has the size of 17 mm × 17 mm. The flow boiling experiments were tested for deionized water under the atmosphere pressure at the outlet. The CHF values for flow boiling were obtained under various surface orientation of 15–90°(0° is completely downward), mass flux of 110–288 kg/m2s and inlet quality of 0.003–0.036. The separate factors’ effects on the CHF were analyzed. It is concluded that the CHF increases with the increase of the mass flux, inlet quality and heater surface orientation, however it is not linear. A CHF model based on the sublayer dry out model is proposed. The developed model well predicts the CHF variation trend with each separate parameters (orientation, mass flux and inlet quality). And more than 90% predicted results have the error within ±15%.

Surface wettability effect of plain and plasma-sprayed copper-coated surfaces on CHF enhancement during saturated pool boiling of FC-72

In the current scenario, CHF study is essential for the safe operation of electronics equipment comprising a two-phase heat transfer process. Therefore, the present experimental investigation involves saturated pool boiling and CHF study of FC 72 over a plain stainless steel surface (SS) and microporous copper-coated SS surfaces under atmospheric conditions. Accordingly, three different plasma-sprayed copper-coated surfaces with coating thicknesses of 65 μm, 105 μm, and 145 μm prepared using micro copper particles of size 25–45 μm. The analysis of the results shows that with an increase in heat flux values, the boiling heat transfer coefficient increases over plain as well as coated surfaces. The plasma-spayed copper-coated surfaces with a coating thickness of 65 μm and 105 μm exhibit a higher boiling heat transfer coefficient as than the plain surface. On the other hand, a 145 μm thick coated surface resulted in a comparable boiling heat transfer coefficient with the plain SS surface. Among the three porous-coated surfaces, the boiling heat transfer coefficient decreases continuously from 65 μm to 145 μm of the coated surface. On the contrary, to the observed nucleate boiling behavior, all the porous-coated surfaces show a higher value of CHF than the plain surface, and the CHF value is found to increase continuously from 65 μm to 145 μm of the coated surfaces. The enhancement of CHF values was found to be 66.29%, 69.17%, and 77.75% for a coating thickness of 65 μm, 105 μm, and 145 μm, respectively, compared with the plain surface. The porous coating thickness of 65 μm shows a greater value of heat transfer coefficient than 105 μm and 145 μm whereas 145 μm exhibits a higher value of CHF as than 65 μm and 105 μm.

Alteration of pool boiling heat transfer on metallic surfaces by in situ oxidation

The critical heat flux during pool boiling has been investigated for a range of applications including electrical power generation and thermal management. Reported experimental CHF values during pool boiling of water on flat metallic surfaces, however, show a large discrepancy across studies. Here, we address this discrepancy in CHF values by accounting for oxidation of metallic surfaces during boiling. We studied the effect of in situ oxidation on flat Cu and Ni surfaces by changing the duration that samples were held in saturated water before conducting boiling experiments. The morphology and chemical composition of surfaces after the boiling experiments were analyzed by atomic force microscopy and X-ray photoelectron spectroscopy, respectively. Cu surfaces showed gradually increasing CHF values as the duration in saturated water increased, which could be attributed to the increase in roughness due to the formation of Cu2O nanostructures. Conversely, Ni surfaces showed relatively stable CHF and morphology as a nearly flat layer of NiO formed, with one exception: formation of a highly wetting hydroxide, Ni(OH)2, on a Ni coupon held in saturated water for 24 h resulted in a uniquely high CHF value, signifying the importance of surface chemistry in addition to morphology. The fundamental mechanisms resulting in the wide spread of CHF values on metallic surfaces elucidated in this work will lead to more accurate estimation of CHF as well as a deeper mechanistic understanding of CHF values on engineered surfaces.

The economic value of long-term family caregiving. The situation of caregivers of persons with spinal cord injury in Switzerland.

Health systems rely on the unpaid work of family caregivers. Nevertheless, demographic changes suggest a shortage of caregivers in the near future, which can constrain the long‐term care policy in many countries. In order to find ways to support family caregivers, a primary effort would be to estimate how much their work is worth. This paper estimates the economic value of long‐term family caregivers and how these costs would be shared by the health system, the social insurances and the cared‐for person in the absence of informal caregivers. We use data of 717 family caregivers of persons with spinal cord injury (SCI) in Switzerland. We implemented the proxy‐good method and estimated the market value of their work if performed by professional caregivers. Our results show that family caregivers in the sample spent an average of 27 hr per week caring for a relative for almost 12 years. This work, if undertaken by professional home care, has a market value of CHF 62,732 (EUR 56,455) per year. In the absence of family caregivers, these costs should be financed by the health insurances (47%), by the cared‐for person (24%) and by the social insurances (29%). It is in the best interest of the cared‐for person and of the healthcare and social systems to keep a sustained supply of family caregivers. One option is finding ways to recognise and compensate them for their work and make it less cumbersome.

Variations of nanoparticle layer properties during nucleate pool boiling

The nanoparticle layer detachment during nucleate pool boiling and its influences on heat transfer surface properties were explored experimentally. The material of the heat transfer surface was copper and the nanoparticle layer was formed on the heat transfer surface by nucleate boiling in the water-based TiO2 nanofluid. It was found that the detachment of the nanoparticle layer during nucleate boiling in pure water is significant. In the present experiment, more than half of nanoparticles deposited on the heated surface were detached before the CHF condition was reached. The thickness and roughness decreased accordingly. However, the wettability and wickability that are the influential parameters on the CHF value were maintained even after the occurrence of nanoparticle layer detachment and deteriorated only after the CHF condition was reached. It is therefore considered that the onset of CHF brings qualitative change to the capillary suction performance of the layer of nanoparticles. In exploring the effect of the nanoparticle layer properties on the nucleate boiling heat transfer, sufficient attention should be paid to the variation of the nanoparticle layer properties during nucleate boiling.

External function for GOTHIC code to estimate critical heat flux conditions for in-vessel retention assessment

GOTHIC is an integrated, general purpose thermal hydraulic software package for design, licensing, safety and operating analysis of Nuclear Power Plant containments, confinement buildings and system components. It bridges the gap between the lumped parameter codes frequently used for containment analysis (such as MELCOR, MAAP, COCOSYS, ASTEC codes) and Computational Fluid Dynamics codes. Within a single model, GOTHIC can include regions treated in conventional lumped parameter mode and regions with three-dimensional flows in complex geometries. The heat transfer correlations built into GOTHIC cover the portion of the boiling curve which spans single phase heat transfer up to pre-Critical Heat Flux (CHF) heat transfer. The implemented boiling curve is truncated to exclude post-CHF heat transfer as it has not been adequately verified and was considered by the developers to have little application in general containment analysis. As such, one area that the code is not currently qualified for is post-CHF heat transfer, which could occur for example in the case of In-Vessel Corium Retention, where cooling water enters in contact with the high temperature of the Reactor Pressure Vessel wall. The presented research focuses on creating an external subroutine that solves this limitation, enabling the GOTHIC code to account for CHF phenomena. The modeling of CHF would be very useful in order to enable the code to simulate the external Reactor Pressure Vessel (RPV) Cooling , as well as other types of severe accidents or analyses where post-CHF simulation is required. Several subroutine function switches were implemented in order to facilitate its usage for different types of heat structures and correlations. The subroutine determines the CHF values based on either: the 2006 Groeneveld Look-up Tables, Lookup Tables for Large Diameter Vertical Tubes, Look-up Tables for Large Diameter Horizontal Tubes, or the correlation used by the MELCOR code for critical heat flux situations. It shall be noted that the developed subroutine and its implementation were performed without the need to have access to the GOTHIC source code. In a previous paper, the GOTHIC code was used to perform a containment safety analysis for the Atucha-I NPP (CNA-I) for an in-vessel retention type of analysis. Highly conservative vapour generating boundary conditions were used in order to simulate the boiling between the cavity water and the RPV surface, and to bypass the GOTHIC limitation. The newly developed subroutine was used for the analysis of two postulated Atucha-I in-vessel retention scenarios, a Large Break Loss Of Coolant Accident (LBLOCA) and a Station Black-Out (SBO), with the simulation of heat transfer between RPV and cavity water. Specifically for in-vessel retention situations, a separate user selectable option for the subroutine was developed, in which the Critical Heat Flux is determined based on experiments performed at the ULPU facility from the University of California, Santa Barbara, USA, which were combined with the 2006 Groeneveld Look-up Tables primarily in order to have a pressure dependence. This was performed because for the Atucha-I analysis, the pressure was higher than in the ULPU facility.

Studies on the critical heat flux in fuel assembly using a modified two-phase multi-scale interface model—validation and sensitivity analysis

The flow regime transition with the multi-scale interface is an important mechanism in the CFD simulation of boiling flow, especially in the saturated boiling and even departure nuclear boiling (DNB), while the current model is still immature. In this paper, a modified two-phase multi-scale interface model, which considering the concentration and morphology of interfacial area and the interfacial transfer of momentum, heat and mass, was compiled into the Eulerian two-fluid model in Fluent code to investigate the characteristics of two-phase flow. The improved model was validated systematically by the adiabatic two-phase flow data for different interface morphologies and the boiling flow data with different void fraction. And then the overall ability of the models in predicting the CHF values in a simplified fuel assembly was evaluated. The results showed that most predicting deviations were less than ±20%. Finally, the sensitivity of the empirical values in the modified two-phase multi-scale interface model was analyzed. In summary, the results proved that the modified two-phase multi-scale interface model based on two-fluid model could be used to simulate boiling flow with flow regime transition and was able to predict CHF in the fuel assembly reasonably.