Ratio Of Heat Flux Research Articles

Flow and heat transfer characteristics of natural convection in hydrothermal reactor with segmented heating

Hydrothermal reactor is very suitable to utilize low-grade heat especially concentrated solar energy. However, the heat flux generated by solar heating is usually non-uniform, which affects the natural convection inside the reactor. In this work, natural convection of near critical water in cylindrical hydrothermal reactor is investigated by numerical simulation. With constant total heat input into the cylinder, the sidewall is divided into two halves with different heat flux density. The effect of heating schemes and physical properties on flow structure, temperature distribution, fluid motion and wall heat transfer are analyzed. It is found that natural convection is dominated by a large vortex with high velocity when heat flux ratio is lower than 1, and is dominated by two independent weak vortices when heat flux ratio is higher than 1. In addition, the former case has more uniform vertical temperature distribution, stronger fluid flow and weaker wall heat transfer, while the latter case is in the opposite. When the temperature approaches the pseudocritical point, the flow structure remains qualitatively unchanged, but wall heat transfer is enhanced and fluid motion is generally weakened. The change of heat flux ratio significantly regulates fluid motion but slightly affects wall heat transfer. Besides, the regulating effect of heat flux ratio is weakened in near critical region.

New Insights in Turbulent Heat Transfer With Oil and Hybrid Nano-Oils, Subject to Discrete Heating, for Parabolic Trough Absorbers

Abstract The concentrated solar thermal systems, operating in the medium temperature range of 373–573 K, will be extremely useful for several industrial processes. However, the need for an in-depth understanding of the turbulent heat transfer in parabolic trough absorbers with pure and hybrid nano-oils, including the effect of buoyancy or gravity, is realized. This paper presents the Reynolds-averaged Navier–Stokes (RANS)-based turbulent heat transfer analyses in a 3D, long, straight for Reynolds number from 5000 to 20,000 and discrete heating conditions with different heat flux ratios such as 1, 5, 10, 20, 40, and 50 for pure oil and hybrid nano-oils having 1, 4, and 6% volume concentration of the nanoparticles. The major findings are, (a) gravity-induced anisotropy leads to high and low-speed fluid flows near the lower and upper walls, and temperature redistribution at a plane, which is beneficial, (b) the statistical axial-velocity deviates from the standard logarithmic law at a Reynolds number of 5000, and (c) the ratio of surface-area-averaged Nusselt number between the lower half and upper half of the tube is 4–12. Some important recommendations are (a) the effect of gravity must be included, (b) the local Richardson number may be used for improving the standard logarithmic law for the axial velocity, and (c) Nusselt number correlations are deduced for the upper half surface and lower half surfaces. The findings, albeit for limited parameters, will be useful for improving the heat transfer aspects in the parabolic trough absorber.

Experimental investigation of bubble dynamics and flow patterns during flow boiling in high aspect ratio microchannels with the effect of flow orientation

Multiphase flow and boiling phase-change within microchannels are of paramount importance to thermal management and electronics cooling, among others. The present experimental study investigated the bubble dynamics and flow patterns during flow boiling in a high aspect ratio microchannel at different channel orientations. Hydrofluoroether (HFE−7000) was used as the working fluid within a transparent glass microchannel with a width-to-height aspect ratio of 10 and a hydraulic diameter of 909 μm. A tantalum layer coating was applied to the channel, allowing visual observation to be conducted whilst imposing uniform heating along the channel. Under horizontal and vertical upward flow, the mass fluxes were varied between 14 kg m−2 s−1 and 42 kg m−2 s−1 for Reynolds numbers between 28.4 and 85.2, while the heat fluxes were set from 2.8 kW m−2 to 18.7 kW m−2. The results show that single bubble growth can be divided into three stages, namely: bubble-free growth, partially confined bubble growth, and fully confined growth. The effects of mass flux and heat flux on bubble behaviour and transition points between the defined stages are discussed. Equally important, the channel orientation influences the bubble behaviour in terms of bubble evolution, bubble shape, and bubble nose velocity, with no significant differences observed for the flow patterns when comparing horizontal and vertical upward cases. The comparison of the present work with previous flow pattern map results from the literature shows that earlier flow pattern transition models cannot accurately predict the observed flow patterns in the present configuration. Two types of instabilities are observed, which are associated with the dominant flow pattern during flow boiling and hence a function of the mass flux to heat flux ratio. The first type of instability, high amplitude and long period oscillation, is found during slug flow which is characterised by the presence of bubble rapid expansion along the channel. In addition, a second instability ensues when churn and annular flow create short period oscillations due to the high number of nucleation events, bubble coalescence, and the breakage of liquid slug or liquid film.

Long-term performance of a front-end capillary heat exchanger for a metro source heat pump system

In recent years, the number of subways has rapidly increased, as they can effectively alleviate urban traffic congestion. However, subways can also result in thermal pollution problems in underground spaces. The long-term operation of a subway causes thermal accumulation in the surrounding rock of a tunnel, severely affecting the safe and efficient operation of the subway. The metro source heat pump technology is an effective approach to address the foregoing problem. However, currently, the research on its systematic design methods remains inadequate. In particular, studies on the evolution law of heat transfer characteristics of its front-end heat exchangers during years of operation are limited.In this study, a numerical model of a front-end capillary heat exchanger (CHE) is formulated. The model is based on a practical subway source heat pump demonstration project located in a cold area in China. The annual heat transfer characteristics of the CHE under periodic boundary conditions are simulated and analysed. The results reveal that the temperature of the CHE and its vicinity fluctuates periodically and increases annually under typical design conditions. The fluctuation range of CHE wall temperature gradually decreases from 25.0 °C at 0 m from the CHE inlet to 9.6 °C at 10 m from the CHE inlet in the 10th year. A distinct ‘inertia’ phenomenon is observed when the temperature wave is transmitted to the rock surrounding the tunnel. With the increase in the surrounding rock depth from 0 to 25 m, the amplitude of temperature wave gradually decreases from 4.277 to 0.013 °C, and the reduction rate gradually declines from 24 % to 14 %. Meanwhile, the phase delay of temperature wave increased from 0° to 282.40°. Moreover, a critical heat transfer point exists along the CHE tube length, and the heat transfer law and efficiency before and after the critical point differ. Statistics show that the ratio of CHE average heat flux after the critical point to that before the critical point ranges from 1.11 % to 46.16 % in the heating season and from 30.19 % to 46.12 % in the cooling season, respectively. The recommended CHE tube length is determined to be 4.9–6.3 m based on the analysis results. This research can provide reference data and a theoretical basis for the design of front-end heat exchangers of subway source heat pump systems.

Experimental and numerical studies of interaction between liquid droplets with hot surfaces under different conditions

AbstractIn the present paper, experimental and numerical studies for the interaction studies between different liquid droplets and hot metal surfaces were carried out. The obtained results were interpreted using graphs and figures. Droplet impact, spreading, breakup and Leidenfrost phenomenon were observed with the help of a video camera at 100 fps. The interaction studies were done at different surface temperatures and different droplet‐impinging velocities. Three metals and three liquids were selected for the present study. Copper, aluminium and steel were selected as metals with varying roughness factors maintained at different temperatures. At the same time, in liquid entities, methanol, ethanol and kerosene were selected along with water in some portions. The selections of the metals were based on the thermal diffusivity property as it influences the thermal interaction between the droplet and the hot metal surface. The temperature range of the metal surface was kept from 150°C to 425°C, and the impinging velocity range of droplets was in the field of 2.4–4.9 m/s. For computational studies, the Ansys Fluent R3‐2019 version was used. Numerical analysis for the interaction process was carried out using the volume of fluid (VOF) method, solving Navier–Stokes and energy equations in a 2D‐geometry transient simulation. Parameters like surface temperature, heat flux and spreading ratio w.r.t. change in Weber number were investigated and presented in the present paper. Obtained results are useful for better understanding of droplet physics while interacting with hot surfaces which further helps in development of new methods for effective spray system designs. Results show that for a lower value of Weber number, the droplets spread less as compare to high Weber number. Also, spreading ratio of droplets depends only on the surface roughness.

Study on smoke blocking and thermal radiation attenuation by water curtain in tunnel fire

A 1:10 scale model tunnel with a length, height and width of 9 m, 0.6 m and 0.8 m, respectively, was set up in this paper. A water curtain system was installed in the model to investigate the effect of water curtain systems on smoke flow and heat propagation. A reduced-scale experimental and theoretical study was carried out by varying the heat release rate of the fire source, the water curtain pressure, and the number of water curtain rows. A series of tests were carried out for various setups to quantify each mechanism of interaction between the water mist and hot smoke, to propose a method for qualitatively analysing water curtain systems blocking the propagation of heat radiation and the flow of smoke from combustion, and to propose a method for predicting heat fluxes. The study found that the pressure of the water curtain, the number of rows, and the heat release rate of the fire source all had an effect on the smoke blocking effect of the water curtain system. This effect decreased as the heat release rate of the fire source increased and increased significantly with the pressure of the water curtain and the number of rows. The smoke blocking effect was quantified using conservation of momentum by establishing a dimensionless parameter R to represent the ratio of water curtain momentum to smoke momentum, as well as the ratio of heat flux before and after the water curtain to represent the smoke blocking capacity delta of the water curtain. The smoke blockage rate delta ranges between 40 and 75%, and the smoke blockage rate increases as the momentum R increases. Finally, in tunnel fires, a predictive model for the attenuation of heat radiation by water curtains has been developed, providing theoretical support for the quantitative study of the smoke and thermal blockage effects of water curtains, which is beneficial to the protection of human life in confined spaces.

Voltage-amplified heat rectification in SIS junctions

The control of thermal fluxes -- magnitude and direction, in mesoscale and nanoscale electronic circuits can be achieved by means of heat rectification using thermal diodes in two-terminal systems. The rectification coefficient $\mathcal{R}$, given by the ratio of forward and backward heat fluxes, varies with the design of the diode and the working conditions under which the system operates. A value of $\mathcal{R}\ll 1$ or $\mathcal{R}\gg 1$ is a signature of high heat rectification performance but current solutions allowing such ranges, necessitate rather complex designs. Here, we propose a simple solution: the use of a superconductor-insulator-superconductor (SIS) junction under an applied fast oscillating (THz range) voltage as the control of the heat flow direction and magnitude can be done by tuning the initial value of the superconducting phase. Our theoretical model based on the Green functions formalism and coherent transport theory, shows a possible sharp rise of the heat rectification coefficient with values up to $\mathcal{R} \approx 500$ beyond the adiabatic regime. The influence of quantum coherent effects on heat rectification in the SIS junction is highlighted.

Study on heat reduction and lift-to-drag ratio increase of two-dimensional wedge-shaped waverider blunt leading edges and high pressure capture wing[1] combined configuration

In this paper, the aerodynamic and aerothermal performance of the simplified two-dimensional wedge-shaped waverider bluntness leading edge and high pressure capture wing(HCW) combined configuration in a hypersonic continuous flow basin is numerically simulated by the Computational Fluid Dynamics (CFD) method, the flow field structure of the model is analyzed, and the law of anti-thermal rise is revealed. The main contents of the study include the surface heat flux and lift-drag ratio characteristics of the bluntness leading edge under different bluntness radii, the lift-to-drag ratio, and surface heat flux characteristics of the composite configuration of the bluntness leading edge and the high pressure capture wing under different bluntness radius. Within the scope of the study: 1. The larger the bluntness radius is, the more effective it is to reduce the heat flux of the head. When the bluntness radius is 2mm, the decrease is about 82.5%;2. The larger the bluntness radius is, the more serious the lift-drag ratio decreases. When the bluntness radius is 2mm, the lift-drag ratio decreases by about 72.4%;3. The combined configuration of the bluntness leading edge and the high pressure capture wing can effectively reduce the head heat flux under the premise of ensuring a certain lift-drag ratio. The optimal bluntness radius is 1mm, the head heat flux decreases by about 76% and the lift-drag ratio only decreases by 24%.

Study on aerodynamic force and aerothermal effect of two-dimensional wedge wave rider with different passivation shape of the head

With the rapid development of space technology, hypersonic vehicles flying at altitudes of 20-100 km near space are showing more and more important application value, and how to solve the serious aerodynamic heating problem caused by hypersonic vehicles in the process of flight is the focus of current research. In this paper, the simplified two-dimensional wedge-shaped waveride is treated with the passivation leading edge by the Computational Fluid Dynamics (CFD) method, and the aerodynamic and aerothermal properties of the hypersonic continuous flow basin are numerically simulated. The flow field structure of the model is analyzed, and the law of heat-resistant rise is revealed. The research contents mainly include the characteristics of body head heat flux and body lift-drag ratio under different passivation shapes. Within the research scope: 1. The larger the passivation radius, the more effective it is to reduce the head heat flux. When the passivation radius is 2 mm, the reduction is about 82.5%. 2. The larger the passivation radius, the more serious the lift-drag ratio reduction. When the passivation radius is 2mm, the maximum reduction is about 72.4%; 3. The four passivation leading edge modes have the best thermal protection effect when the passivation radius R is the same, and the lift-drag ratio of the Bezier curve passivation leading edge mode is the largest.

Control of the Air‐Sea Oxygen to Heat Flux Ratio During Deep Convection Events

AbstractEarth System Models project a decline of dissolved oxygen in the oceans due to climate warming. Observational studies suggest that the ratio of O2 inventory to ocean heat content is several fold larger than what can be explained by solubility alone, but the ratio remains poorly understood. In this work, models of different complexity are used to understand the factors controlling the air‐sea O2 flux to heat flux ratio (O2/heat flux ratio) during deep convection. Our theoretical analysis based on a one‐dimensional convective adjustment model indicates that the vertical stratification and distribution of oxygen before the convective mixing determines the upper bound for the O2/heat flux ratio. Two competing rates, the mean entrainment rate of deeper waters into the mixed layer and the rate of air‐sea gas exchange, determine how much the actual ratio departs from the upper bound. The theoretical predictions are tested against the outputs of a regional ocean model. The model sensitivity experiments broadly agree with the theoretical predictions. Our results suggest that the relative vertical gradients of temperature and oxygen at sites of deep water formation are an important local metric to quantify the marginal changes between years with high and lower heat loss.

Reducing the L-H transition power threshold in ITER-similar-shape DIII-D hydrogen plasmas

Recent dedicated DIII-D experiments in low-torque, ITER-similar-shape (ISS) hydrogen plasmas (at a plasma current I p ∼ 1.5 MA and ITER-similar edge safety factor q 95 ∼ 3.6) show that the L-H transition power threshold P LH can be reduced substantially (∼30%) with L-mode helium admixtures n He/n e ⩽ 25%. In the ensuing H-mode, helium ion fractions n He/n H remain below 25%. H-mode normalized pressure and confinement quality are only slightly affected by helium seeding, and Z eff ⩽ 2.15 (including helium and carbon content). The plasmas investigated here are electron-heat dominated, with temperatures T e(0)/T i(0) ⩾ 1 and edge heat flux ratio Q e/Q i(ρ = 0.95) ∼ 1.2–1.5. Without mitigation, P LH is higher by a factor of 2–3 in comparison to similar ISS deuterium plasmas. ISS hydrogen plasmas with lower plasma current I p ∼ 1 MA (increased edge safety factor q 95 ∼ 5.1) exhibit a substantially lower power threshold. This plasma current dependence, also observed previously on ASDEX-U and in JET, is not accounted for by the commonly used 2008 ITPA multi-machine threshold scaling, but could potentially allow H-mode access at marginal heating power during the initial plasma current ramp-up. Attempts to reduce P LH with low-field- and high-field-side hydrogen pellet injection, using 1.7 mm diameter pellets, have not demonstrated a robust threshold reduction, in contrast to successful earlier experiments with larger 2.7 mm pellets. Techniques for reducing P LH are very important for ITER, in particular for accessing H-mode in hydrogen plasmas during the Pre-Fusion Power Operation-1 (PFPO-1) campaign with marginal auxiliary heating power (20–30 MW of ECH).

Can evaporation from urban impervious surfaces be ignored?

Urban evapotranspiration is an important component linking the urban hydrological cycle and energy balance. Previous studies on urban evapotranspiration (ET) mainly focused on evaporation from the soil, vegetation, and water surfaces. However, the urban Ei process was overlooked mainly due to its low amount. In this study, the urban Ei was determined by eddy correlation (EC) observations combined with a flux footprint model and urban land use information for two urban EC sites. The evaluation of flux data quality indicated that more than 76 % of observed flux data can be used for Ei analysis at the two stations during rainy periods. The daily urban Ei exhibited high intermittent temporal patterns. The Ei value was 0 during the non-rainy period and peaked on the following 1–2 days after the rainy event, then gradually decreased to 0. The average daily Ei accounted for 11–14 % of the total ET for the two EC stations during the rainy period. Further analysis indicated that urban Ei significantly altered the urban energy balance and turbulent transport processes. The urban Ei reduced sensible heat flux (H) and Bowen ratio (BR) during the rainfall period, thereby playing a vital role in mitigating urban heat land effect. Moreover, urban Ei improved the turbulent transport efficiency of latent heat flux (rwq) and restored turbulent transport similarity in an urban area. The temporal characteristics of urban Ei exhibited a large discrepancy among different impervious surface materials, which mainly depend on the water retention and thermodynamic properties of the impervious surface. Gravel had the best water retention capability and resulted in the highest spatially averaged Ei, while the concrete surface had the best thermal storage capability and resulted in the longest duration time of Ei. The spatial average of urban Ei and its duration time were positively correlated with the water retention and heat storage capabilities of impervious surface materials. This study gained insight into urban Ei, including its determination method, temporal characteristics, controlling factors, and impacts of urban Ei on the regional energy balance and turbulent transport process.

Measurements of multiple heat flux components at the divertor target by using surface eroding thermocouples (invited).

The Surface Eroding Thermocouple (SETC) is a robust diagnostic utilized in DIII-D to provide fast, edge-localized modes (ELMs) resolved heat flux measurements, in particular in geometric regions that are too shadowed for traditional infrared thermography. In order to further investigate the power dissipation in the divertor region, a combination of flush-mounted and recessed SETCs was developed to assess the effect on surface heating from non-charged particles at the divertor target. Utilizing the Divertor Materials Evaluation System sample exposure platform, the first demonstration of the feasibility of using this new method to distinguish between the heat flux from charged particles and that from neutrals and radiative heating was achieved. This paper details the process of using the combination of flush SETCs and recessed SETCs to measure the multiple heat flux components at the divertor target and further discusses how to determine two important ratios, α (ratio of heat flux from charged particles deposit on recessed SETC to that deposit on flush SETC) and β (ratio of heat flux from non-charged particles deposit on recessed SETC to that deposit on flush SETC), in the estimation of the heat flux from non-charged particle sources. Using a time dependent ratio α, it was found that ∼50% of the total incident heat flux is attributable to the non-charged particles in the fully detached open divertor in DIII-D. Finally, the new application of similar SETC diagnostics in the Small Angle Slot divertor with a V-like configuration and partial tungsten coated surface (SAS-VW) is also introduced.

Integrity of Newton’s cooling law based on thermal convection theory of heat transfer and entropy transfer

Although thermal convection is omnipresent in nature and technology and serves important purposes in various energy transport systems, whether convection can be viewed as an independent heat transfer means has long been argued The constant coefficient in the original version or convective heat transfer coefficient defined in the modern version of Newton’s cooling law quantifies the ratio of the surface heat flux to the temperature difference between a body surface and an adjacent fluid. However, none of the consistent analytical expressions for these two coefficients are present in Newton’s cooling law. The inherently complex relationship between these pending coefficients and convective heat flux vectors makes revealing the convective mechanism extremely difficult. Theoretical determination of these coefficients would bring new insights to thermal convection and direct applications to thermal management. Here we theoretically show consistent analytical expressions for the constant and convective heat transfer coefficients for various flows to make Newton’s cooling law a complete scientific law. For this purpose, a three-dimensional (3D) energy transfer theory of thermal convection is developed, and the convective heat flux vector, entropy flux vector and entropy generation rate inside the system are derived for both single-phase and phase-change flows. By recasting a control volume system into an equivalent control mass system and employing the first and second laws of thermodynamics, the fundamental advective heat transfer mode characterized by temperature differences and entropy changes is demonstrated. The physical implications underlying the 3D convective formulae are elucidated. Comparisons of the analytical results with laminar experiments and turbulent flow measurement benchmark data validate our theoretical findings. Our 3D heat and entropy transfer theory will broaden the research area of thermal convection processes and open up a new arena for the design and thermal management of convective heat transfer in single-phase and phase-change flows.

A system level optimization of on-chip thermoelectric cooling via Taguchi-Grey method

In this paper, a framework for the system level optimization of the thin-film thermoelectric cooler (TFTEC) in 3D electronic packaging is developed based on Taguchi-Grey method. The influences of the size effect, the geometric effect, the parasitic effect and the localized hotspot are comprehensively considered. An L25 (56) orthogonal array is employed to assess the influences of the leg height, the fill-ratio, the electrode height, the gap distance, the hotspot size and the hotspot heat flux on the passive and active cooling effects of the TFTEC. The results show that the electrode height, acting as the primary factor, contributes 45.5% and 45.3% on the passive cooling and active cooling, respectively. The contribution ratios of hotspot size and hotspot heat flux to passive cooling reach 21.4% and 14.7%, respectively. The contribution ratios of leg height and gap distance to active cooling reach 25.7% and 18.8%, respectively. The optimum design factors for maximizing the cooling effects are also approached by the Grey relational analysis. A passive cooling of 16.82 °C and a maximum active cooling of 12.29 °C are achieved, leading to a total localized cooling of 29.11 °C for a chip hotspot.

Land–Atmosphere Interactions during GRAINEX: Planetary Boundary Layer Evolution in the Presence of Irrigation

Abstract This paper analyzed observations from the Great Plains Irrigation Experiment (GRAINEX) to better understand land–atmosphere (L–A) interactions and PBL evolution. This study is focused on a day when the largest forcing on the boundary layer originated from the land surface/land use. To examine these impacts, we also applied the Weather Research and Forecasting (WRF) Model. Results from the observations show that compared to nonirrigated areas, air temperature, wind speed, and PBL height (PBLH) were lower while dewpoint temperature and latent heat flux were higher over irrigated areas. Findings suggest that entrainment layer drying and differences in energy partitioning over irrigated and nonirrigated areas played an important role in PBL evolution. In the final hours of the day, the PBL collapsed faster over nonirrigated areas compared to irrigated. The WRF Model simulations agree with these observations. They also show that the extent of irrigation [expressed as irrigation fraction (IF)] in an area impacts L–A response. Under ∼60% IF, the latent heat flux and mixing ratio reach their highest value while temperature and PBLH are at their lowest, and sensible heat flux is near its lowest value. Results are reversed for ∼2% IF. It is concluded that irrigation notably impacts L–A interactions and PBL evolution.

Competition between Rayleigh–Bénard and horizontal convection

We investigate the dynamics of a fluid layer subject to a bottom heat flux and a top monotonically increasing temperature profile driving horizontal convection (HC). We use direct numerical simulations and consider a large range of flux-based Rayleigh numbers $10^6 \leq Ra_F \leq 10^9$ and imposed top horizontal to bottom vertical heat flux ratios $0 \leq \varLambda \leq 1$ . The fluid domain is a closed two-dimensional box with aspect ratio $4\leq \varGamma \leq 16$ and we consider no-slip boundaries and adiabatic side walls. We demonstrate a regime transition from Rayleigh–Bénard (RB) convection to HC at $\varLambda \approx 10^{-2}$ , which is independent of $Ra_F$ and $\varGamma$ . At small $\varLambda$ , the flow is organised in multiple overturning cells with approximately unit aspect ratio, whereas at large $\varLambda$ a single cell is obtained. The RB-relevant Nusselt number scaling with $Ra_F$ and the HC-relevant Nusselt number scaling with the horizontal Rayleigh number $Ra_L=Ra_F\varLambda \varGamma ^4$ are in good agreement with previous results from classical RB convection and HC studies in the limit $\varLambda \ll 10^{-2}$ and $\varLambda \gg 10^{-2}$ , respectively. We demonstrate that the system is multi-stable near the transition $\varLambda \approx 10^{-2}$ , i.e. the exact number of cells not only depends on $\varLambda$ but also on the system's history. Our results suggest that subglacial lakes, which motivated this study, are likely to be dominated by RB convection, unless the slope of the ice–water interface, which controls the horizontal temperature gradient via the pressure-dependence of the freezing point, is greater than unity.

Thermal oscillation behavior profiling of supercritical methane in cooling rocket engines

Regenerative cooling is widely used in rocket engines as an efficient cooling method, and nowadays spacecraft shows faster flight speed and greater acceleration. Therefore, the fuel is subjected to large inertial force during flight acceleration or deceleration process. Hence, since it is novel, it is necessary to analyze the thermal performance with large acceleration. To reveal the effect of acceleration on the heat transfer of supercritical methane fuel in the cooling channel, the three-dimensional numerical simulation has been performed with structured mesh. Specifically, the effects of acceleration, mass flux, pressure and ratio of heat flux to mass flux on the flow and heat transfer in the cooling channel have been studied, and the buoyancy parameter (Bo*) and thermal acceleration factor (Kv) have been also employed to analyze the buoyancy effect and thermal acceleration. The results show that at the conditions of low supercritical pressure, low mass flux and large ratio of heat flux to mass flux, the buoyancy effect increases significantly when the acceleration increases from 0 g to 6 g. As a result, the heat transfer is deteriorated gradually, and the average heat transfer coefficient decreases from 3800 to 3000. On the other hand, when the acceleration and heat flux exceed a range, the unstable phenomenon of thermal oscillation will occur with the period of oscillation about 90 s. The thermophysical properties of the fluid near-wall will change rapidly when approaching the pseudo-critical region, which leads to the continuous change of the thickness of the viscous boundary layer.

Forced convection in a fully-filled bidisperse porous annular duct subject to asymmetric heat fluxes

• Heat transport in bidisperse porous annular duct is theoretically studied. • The bidispersivity triggers critical effective thermal conductivity ratio. • Nusselt number reaches the peak with a heat flux ratio of 0.175. • Wider range of high thermal performance appears by increasing the heat flux ratio. In the present work, convective heat transport through an annular duct fully filled with a bidisperse porous medium (BDPM) is analytically studied by using the two-velocity two-temperature model. The annulus is heated from inner and outer walls by asymmetric heat fluxes. Analytical solutions are deduced for the velocity and temperature distributions of f- and p-phases in the developed and laminar regime. The results show that the bidispersivity benefits the heat transfer enhancement in a lower range of effective thermal conductivity ratio compared to the monodisperse porous medium (MDPM). However, this advantage will vanish once the conductivity ratio exceeds its critical value. The BDPM annular duct having a peak average Nusselt number is found at the heat flux ratio equal to 0.175 under considered parameters.

Evaluation of the success of in-vessel corium retention using external reactor vessel cooling mechanism for a small PWR

Among the several study fields of severe accidents, In-Vessel Retention (IVR) via External Reactor Vessel Cooling (ERVC) referred to as IVR-ERVC is a critical subject being explored for various reactor designs. This study aims to investigate and assess the effectiveness of IVR of molten corium in a PWR utilizing the ERVC mechanism. A RELAP5/SCDAP model developed for the assessment of IVR-ERVC was applied to a small PWR after its successful validation. The boundary conditions applied to the model are evaluated through simulation of severe accidents of the small reactor (998 MWth). Thermal hydraulic studies of sensitive parameters have been carried out to determine the safety margin and effectiveness of the IVR-ERVC method under a variety of process conditions. The results indicate that a high mass flux is required to avoid boiling crises at low degrees of subcooling (around saturation conditions). A mass flux of 186.44 kg/m2-s or higher is required to provide a sufficient safety margin for inlet water with degree of subcooling of 8 K. The minimum Critical Heat Flux (CHF) is found to be 2.5 MW/m2 at a mass flux of 186.0 kg/m2-s for this case. Critical Heat Flux Ratio (CHFR) is found in the range of 6.0–17.0 with the cooling water flow rate of 20 kg/s (mass flux of 34.48 kg/m2-s) and 65 K subcooling, which provides a sufficient safety margin to avoid boiling crisis. It is concluded that the ERVC is an effective mechanism for the prevention of RPV rupture under active and passive conditions of the reference reactor.