Heat Flux Limit Research Articles

Ultrahigh Flux Thin Film Boiling Heat Transfer Through Nanoporous Membranes.

Phase change heat transfer is fundamentally important for thermal energy conversion and management, such as in electronics with power density over 1 kW/cm2. The critical heat flux (CHF) of phase change heat transfer, either evaporation or boiling, is limited by vapor flux from the liquid-vapor interface, known as the upper limit of heat flux. This limit could in theory be greater than 1 kW/cm2 on a planar surface, but its experimental realization has remained elusive. Here, we utilized nanoporous membranes to realize a new "thin film boiling" regime that resulted in an unprecedentedly high CHF of over 1.2 kW/cm2 on a planar surface, which is within a factor of 4 of the theoretical limit, and can be increased to a higher value if mechanical strength of the membranes can be improved (demonstrated with 1.85 kW/cm2 CHF in this work). The liquid supply is achieved through a simple nanoporous membrane that supports the liquid film where its thickness automatically decreases as heat flux increases. The thin film configuration reduces the conductive thermal resistance, leads to high frequency bubble departure, and provides separate liquid-vapor pathways, therefore significantly enhances the heat transfer. Our work provides a new nanostructuring approach to achieve ultrahigh heat flux in phase change heat transfer and will benefit both theoretical understanding and application in thermal management of high power devices of boiling heat transfer.

Pool-boiling enhancement using multilevel modulated wick

Main challenges in saturated pool-boiling heat transfer are limited Critical Heat Flux (CHF) and Heat Transfer Coefficient (HTC), caused by the counterflow of liquid and vapor over the heated surface. Using multilevel modulated wicks, i.e., monolayer, columnar, and mushroom post wicks, we control the liquid and vapor flow for efficient phase separation, thereby improving the CHF and HTC. The wicks are fabricated using multi-step sintering process using 200 μm copper particles, and the pool boiling uses n-pentane at ambient pressure. The monolayer wicks without and with the mushroom post structure provide 20% and 87% CHF enhancements, respectively, compared to the plain surface. It is found that the CHF enhancement of the mushroom wick is attributed to its pitch distance, 3.5 mm, which effectively reduce the hydrodynamic instability (Rayleigh-Taylor) wavelength. The further reduction of the pitch distance, 1 mm, results in the 250% CHF improvement in agreement with the theory. The columnar and mushroom posts with monolayer increase HTC by tenfold, compared to the plain surface, due to the reduced conduction path through the thin monolayer wick under the controlled vapor region using the columnar and mushroom post wick (vapor chamber-like environment in pool boiling). The multilevel wick design provides fundamental insights into simultaneous CHF and HTC enhancements with potential use in advanced thermal management systems.

Multivariable Closed Control Loop Methodology for Heliostat Aiming Manipulation in Solar Central Receiver Systems

Maintaining receiver’s thermal stresses and corrosion below the material limits are issues that need careful attention in solar thermal towers. Both depend on heliostats’ aiming points over the central receiver and available direct solar radiation at any instant. Since this technology relies on an unavoidable time-changing resource, aiming points need to be properly manipulated to avoid excessive hot spots. This paper proposes a new aiming point strategy based on a multivariable model predictive control (MPC) approach. It shows an alternative approach by introducing an agent-based group behavior over heliostats’ subsets, which makes possible either concentrating or dispersing solar radiation as required by the MPC algorithm. Simulated results indicate that it is feasible to develop a closed-loop control procedure that distributes solar irradiance over the central receiver according to the predefined heat flux limits. The performance of the proposed approach is also compared with the results found in the available literature that uses a different methodology.

带热流密度限制的推力可控导弹三维制导律

The traditional three-dimensional guidance law generally divides three-dimensional space into vertical plane and horizontal plane respectively, which may lead to the lack of coupling information. It also doesn't take heat flux of infrared seeker and constraint of terminal velocity into account. To solve this problem, a guidance law based on the missile of adjustable thrust was proposed in the instantaneous rotary line-of-sight plane of rotary line-of-sight frame. The acceleration instruction in the direction of perpendicular to line-of-sight was designed to satisfy the criterion of motion camouflage strategy. Meanwhile, an overload instruction in the direction of line-of-sight was built with variable weighted change rate of heat flux limit and ideal overload which can satisfy the constraint of terminal velocity. Simulation results demonstrate that the designed guidance law has a good performance with the heat flux limit of infrared seeker, constraint of terminal velocity and available axial overload limit when attacking the high maneuvering target and initial misalignments of infrared seeker exists. The rate of line-of-sight is under 3 ()/s, so it has certain engineering application value.

Transient response of a thermoelectric generator to load steps under constant heat flux

Most waste heat recovery applications involve a heat source that provides a limited heat flux that can be converted into electricity by a thermoelectric generator (TEG). When a TEG is used under limited or constant heat flux conditions the temperature difference across the device cannot be considered constant and will change depending on the electrical current generated by the TEG. This phenomenon is induced by the Peltier effect, which works against power generation and deviates the optimum operating point from the commonly known maximum power point (MPP). This point, dictated by the maximum power transfer theorem, is achieved when the source equivalent series resistance and the load resistance are equal, in conditions of constant temperature difference. Hence maximum power point tracking (MPPT) algorithms that regulate the TEG at half of the instantaneous open-circuit voltage are optimized only for applications where the TEG operates under constant temperature difference but are not ideal for constant heat flux conditions. Hill climbing MPPT methods, e.g., perturb-and-observe (P&O) or incremental conductance (IC), can reach the MPP more accurately if the sampling time is extended to the thermal time constant of the system.This article presents an analysis of the transient electrical and thermal response of a TEG to a load change. This investigation results fundamental to the design of MPPT algorithms such P&O or IC for TEGs operating under constant heat flux. A step-up (boost) dc-dc converter controlled by P&O is used to demonstrate the effects of the sampling time over of the transient response and hence the tracking performance of the MPPT algorithm.

Effects of Cooling Fluid Flow Rate on the Critical Heat Flux and Flow Stability in the Plate Fuel Type 2 MW TRIGA Reactor

The conversion program of the 2 MW TRIGA reactor in Bandung consisted of the replacement of cylindrical fuel (produced by General Atomic) with plate fuel (produced by BATAN). The replacement led into the change of core cooling process from upward natural convection type to downward forced convection type, and resulted in different thermohydraulic safety criteria , such as critical heat flux (CHF) limit, boiling limit, and cooling fluid flow stability . In this paper, a thermohydraulic safety analysis of the converted TRIGA reactor is presented by considering the Dynamic Nucleate Boiling Ratio (DNBR) criterion, Onset Nucleate Boiling Ratio (ONBR) limit, and cooling fluid flow stability at various cooling fluid flow rate.The numerical a nalyses were performed using the HEATHYD program on the hottest channels of reactor core. Th e combination of heat transfer and fluid flow analysis w ere conducted for reactor operation at 2 MW with 20 fuel element bundle s and four control rod bundles . I ncoming fluid flow to the cooling channel was fixed at 44.5 °C temperature and 1.9970 bar pressure, and its flow rate was varied from 1.25 to 3 . 5 m 3 / h . By inputting the se values, as well as the total power of fuel elements per bundle , the wall temperature distribution of the plate fuel element, cooling fluid temperature distribution, and pressure losses in the channels were obtained for the analysis of CHF limit, boiling limit, and flow stability. It was shown that no boiling occurred for the cooling fluid flow rate range of 2.4 to 3.5 m 3 /h, and even at the cooling fluid flow rate of 1.25 m 3 /h where some bubbles occurred, the DNBR was higher than the critical limit (more than 23) while the flow stability criterion in some channels were slightly less than 1 (unstable). A t the cooling fluid flow rate of 1.4 m 3 /h , however, the flow became stable in all channel . The results showed that even though some bubbles start to occur, the plate-fuel- type 2 MW TRIGA reactor can safely operate in the terms of CHF limit and flow stability .

A New Paradigm for Understanding and Enhancing the Critical Heat Flux (CHF) Limit

Nearly a century of research on enhancing critical heat flux (CHF) has focused on altering the boiling surface properties such as its nucleation site density, wettability, wickability and heat transfer area. But, a mechanism to manipulate dynamics of the vapor and liquid interactions above the boiling surface as a means of enhancing CHF has not been proposed. Here, a new approach is implemented to limit the vapor phase lateral expansion over the heat transfer surface and actively control the surface wetted area fraction, known to decline monotonically with increasing heat flux. This new degree of freedom has enabled reaching unprecedented CHF levels and revealed new details about the physics of CHF. The impact of wickability, effective heat transfer area, and liquid pressure on CHF is precisely quantified. Test results show that, when rewetting is facilitated, the CHF increases linearly with the effective surface heat transfer area. A maximum CHF of 1.8 kW/cm2 was achieved on a copper structure with the highest surface area among all tested surfaces. A model developed based on the experimental data suggests that the thermal conductivity of the surface structures ultimately limits the CHF; and a maximum CHF of 7–8 kW/cm2 may be achieved using diamond surface structures.

Enhanced boiling heat transfer on binary surfaces

A novel idea for the improvement of boiling heat transfer is that of a binary surface – where a non-boiling liquid coats sub-surface irregularities and aids in heat transfer to the primary working fluid. The threefold goal of this effort was to: (i) prepare durable, low-cost, scalable binary surfaces for boiling heat transfer enhancement, (ii) conduct pool boiling experiments on these surfaces, and (iii) derive the physical mechanisms perceived as responsible for the observed boiling enhancements. Accordingly, robust binary surfaces were prepared on copper using a facile, scalable bulk micro-manufacturing approach. These surfaces consist of numerous micro-/nano-cavities filled by a non-boiling liquid creating puddles around solid islands. Boiling experiments were carried out using a dielectric liquid, PF-5060, as the primary working fluid (the boiling liquid). It was observed that, compared to a smooth/plain copper surface, a binary copper surface, with water as the NBL, was able to simultaneously enhance the maximum heat flux limit by ∼2.2 times and the average heat transfer coefficient by ∼7.5 times. A maximum heat flux of 35.06W/cm2 was recorded, which is higher than the enhancements reported so far in literature for the pool boiling of PF-5060 on any enhanced surface. It was further found that decreasing the contact angle of the non-boiling liquid on the binary surface enhances both the heat transfer coefficient and the maximum heat flux limit.

Flowing liquid lithium plasma-facing components – Physics, technology and system analysis of the LiMIT system

The use of low atomic number liquid metals has been shown to have the potential to solve many of the prevalent problems like erosion and radiation losses associated with the interaction of fusion plasma with the plasma facing component (PFC) structures in tokamaks. Since the first evidence of lithium increasing plasma performance in TFTR [1], the benefits of using lithium in fusion environments have been seen in many devices, including CDX-U [2], NSTX [3], LTX [4], and DIII-D [5]. While both fast flow and slow flow concepts have been studied with regards to liquid lithium first wall alternatives, this report will focus on efforts placed on fast flow research and will mainly focus on advancements in the LiMIT device that help to eliminate concerns over the broad use of liquid lithium. Due to the promising TFTR results along with results obtained at the University of Illinois at Urbana-Champaign [6], suitably designed trench structures holding liquid lithium could be an appropriate fast flow candidate for PFC modules in future fusion devices. There are four potential shortcomings of this approach: (1) Droplet ejection, (2) Wetting control, (3) Tritium retention, and (4) Limited heat flux handling. Droplet ejection is discussed in a companion publication [7], while this paper addresses the topics of wetting control and heat flux handling. Limitations in wetting and prevention of lithium creep (i.e. getting and keeping the lithium only where it should be) have been solved by laser-texturing the base material with extreme short laser pulses (pico – femto second) of high power (several 10s of W). Micro- and nano-structuring results indicate that the textured substrates displayed significant change in their wetting properties, increasing the temperature needed to wet from 310 °C to 390 °C. Lastly, initial designs for the Lithium Metal Infused Trenches (LiMIT) [6] showed dryout above 3 MW/m2, but new designs of the trench shaping show potential to be able to handle up to 10MW/m2. Dryout is accompanied by lithium evaporation which is shown to mitigate the incident heat flux, which may be viewed as beneficial [8]. The advances shown here will increase the viability of the LiMIT system in large-scale testing, and allow for extensive design iteration to begin tackling the large powers and heat fluxes present in reactor-relevant systems.

Dynamics of a Leidenfrost Droplet Modulated by Electrowetting

Leidenfrost phenomenon is closely related to film boiling and the critical heat flux (CHF) limit of boiling heat transfer. Understanding the mechanisms of Leidenfrost phenomenon and devising effective ways to suppress it is of great interest to the heat transfer enhancement community. In this work, a synchronized high-speed optical imaging and infrared (IR) thermography approach was employed to investigate the dynamics of a Leidenfrost droplet under the influence of electrowetting (EW). The Leidenfrost droplet was produced by dispensing a water drop on a Teflon-coated silicon wafer maintained at a wall temperature of Twall = 200 °C. Both direct-current (DC) and alternating-current (AC) electric fields were applied to induce EW effect to suppress the Leidenfrost state. The interfacial instabilities of the Leidenfrost droplet were observed, and the instantaneous temperature and heat flux distributions on the heating surface were measured. The results suggest that the electrical forces destabilize the liquid-vapor interface and cause the vapor film that insulates the heating surface from the droplet to collapse. Re-establishment of the liquid-solid contact helps to drastically improve the heat transfer, as evidenced by the reduced surface temperature and the enhanced heat flux.

Performance Limits of Oscillating Heat Pipes: Theory and Validation

Oscillating heat pipes have been the subject of significant research since 1990, but to allow their entry into broad application, it is important to understand their operational limitations. This paper presents analytical and experimental investigations of the various performance limits of oscillating heat pipes. The Bond number limit, vapor inertia limit, heat flux limit, viscous limit, sonic limit, and limitations on heated length are discussed in detail and developed with analytical tools for prediction. These limits are then tested against experimental studies. It is found that the Bond number coefficient can be as high as 2.4–2.74, rather than the typically reported 1.8–2.0. The established vapor inertia, heat flux, viscous, and sonic limits show good agreement with published experimental data sets, as well as controlled experiments run specifically to test these limits. Finally, it was determined that longer evaporator lengths result in a reduction of overall heat transport capability. An analytical approach is presented to predict this performance loss, showing good agreement with experimental results.

A new scaling for divertor detachment

The ITER design, and future reactor designs, depend on divertor ‘detachment,’ whether partial, pronounced or complete, to limit heat flux to plasma-facing components and to limit surface erosion due to sputtering. It would be valuable to have a measure of the difficulty of achieving detachment as a function of machine parameters, such as input power, magnetic field, major radius, etc. Frequently the parallel heat flux, estimated typically as proportional to Psep/R or PsepB/R, is used as a proxy for this difficulty. Here we argue that impurity cooling is dependent on the upstream density, which itself must be limited by a Greenwald-like scaling. Taking this into account self-consistently, we find the impurity fraction required for detachment scales dominantly as power divided by poloidal magnetic field. The absence of any explicit scaling with machine size is concerning, as Psep surely must increase greatly for an economic fusion system, while increases in the poloidal field strength are limited by coil technology and plasma physics. This result should be challenged by comparison with 2D divertor codes and with measurements on existing experiments. Nonetheless, it suggests that higher magnetic field, stronger shaping, double-null operation, ‘advanced’ divertor configurations, as well as alternate means to handle heat flux such as metallic liquid and/or vapor targets merit greater attention.

The DEMO wall load challenge

For several reasons the challenge to keep the loads to the first wall within engineering limits is substantially higher in DEMO compared to ITER. Therefore the pre-conceptual design development for DEMO that is currently ongoing in Europe needs to be based on load estimates that are derived employing the most recent plasma edge physics knowledge.An initial assessment of the static wall heat load limit in DEMO infers that the steady state peak heat flux limit on the majority of the DEMO first wall should not be assumed to be higher than 1.0 MW m−2. This compares to an average wall heat load of 0.29 MW m−2 for the design assuming a perfect homogeneous distribution. The main part of this publication concentrates on the development of first DEMO estimates for charged particle, radiation, fast particle (all static) and disruption heat loads. Employing an initial engineering wall design with clear optimization potential in combination with parameters for the flat-top phase (x-point configuration), loads up to 7 MW m−2 (penalty factor for tolerances etc not applied) have been calculated. Assuming a fraction of power radiated from the x-point region between 1/5 and 1/3, peaks of the total power flux density due to radiation of 0.6–0.8 MW m−2 are found in the outer baffle region.This first review of wall loads, and the associated limits in DEMO clearly underlines a significant challenge that necessitates substantial engineering efforts as well as a considerable consolidation of the associated physics basis.

Evaluating the impact of green roof evapotranspiration on annual building energy performance

ABSTRACTThe performance of the building envelope predominantly determines the ultimate energy performance throughout the lifecycle of a building. A sustainable alternative to enhance roof performance while limiting heat flux through a roof is integrating passive techniques such as green roof. Particularly, green roof performance is sensitive to local climate. The main objective of this study was to evaluate the evapotranspiration effect of an extensive green roof on annual energy consumption of an office building in relation to the humid continental climate of Republic of Korea. The dynamic behavior of green roof and building energy performance were investigated through a parametric simulation method using green roof module in EnergyPlus coupled with jEPlus. Structural data of the reference building and ASHARE 90.1-2007 operational schedules were used as inputs for baseline building model while inputs for the green roof module were based on experimental data sets. Due to the influence of the humid conditions and local wind current on the evapotranspiration process, it was generally found that high leaf area index (LAI) reduced cooling energy demand and somewhat reduced heating energy demand as well; corresponding to the highest daily evapotranspiration fluxes of 4.79 mm day−1 in summer and 1.80 mm day−1 in winter. Increasing LAI from 20% to 100% cover increased evapotranspiration flux by 10.4% in summer and 80.2% in winter. Thus to minimize energy losses in winter, foliage cover must be carefully considered. Within limitations specified, the overall annual building energy consumption deceased by 90.9 GJ (3.7%).

Numerical study of mixed convection heat transfer of various fin arrangements in a horizontal channel

The mixed convection of a three-dimensional square duct with various arrangements of fins in both laminar and turbulent flow is numerically characterized and studied. This study focuses on the ability of fin arrangements to enhance a heat transfer while flow is incompressible and the fluid is air. In our models, the lower duct wall is defined with a constant heat flux condition while the two side walls and upper wall are insulated. The finite volume method with the SIMPLE (Semi Implicit Method for Pressure Linked Equations) algorithm is used for handling the pressure–velocity coupling. The numerical results are validated with experimental data and show good agreement. The computations confirm that fin arrangement significantly changes the temperature distribution and cooling performance of the fins. This study focuses on the new arrangements of the fins that are not presented in previous studies. These arrangements have 3 Models with two angles (30 and 45°). According to the results, heat transfer highly improves (approximately 40–50% in the laminar flow stream and 15–20% in turbulent flow in comparison to base plate heat transfer) as the simple fin arrangement is replaced by inclined fins. Moreover, the influence of limited heat flux on temperature distribution is extensively investigated. It is notable that the temperature of fins significantly reduces as the best arrangement of the fins is presented in the air flow stream. Therefore, an enhanced cooling zone occurs when the fin arrangement is adjusted according to the location of the source of the heat flux on the base plate.

Enhanced boiling heat transfer on plain and featured surfaces using acoustic actuation

Two-phase thermal management based on submerged boiling heat transfer has received considerable attention in recent years because of its potential to achieve high heat fluxes along with its relatively simple hardware and system-level coupling. However, this attractive heat transfer approach has been hampered by the critical heat flux (CHF) limit on the maximum heat transfer, which is caused by the dynamics of the vapor bubbles that form on the heated surface and the subsequent transition to film boiling that produces a large increase in the surface temperature. Recent work at Georgia Tech has exploited low-power ultrasonic acoustic forcing to enhance boiling heat transfer and increase the CHF limit by controlling the formation and evolution of the vapor bubbles and inhibiting the instabilities that lead to film boiling. These effects are investigated with plain and textured (surface-embedded microchannels) boiling heat transfer base surfaces. Even without acoustic actuation, the transfer of makeup fluid to the boiling sites in the presence of surface microchannels passively decreases the surface superheat and increases in the CHF by 218% compared to plain surfaces. Acoustic actuation has a profound effect on the onset and evolution of boiling by inducing interfacial forces that affect the bubbles’ contact line with the surface and lead to bubble detachment. Heat transfer measurements with acoustic actuation demonstrate a significant increase in the CHF as the acoustic field impedes the formation of large vapor columns and inhibits the instability that results in the collapse of these columns into a vapor film. The improvements in the CHF in stagnant bulk fluid exceed 66% for the plain surface (up to 183W/cm2), and 31% for the textured surface (up to 460W/cm2 with a 7°C reduction in surface superheat).

Study of reactor plant disturbed cooling condition modes caused by the VVER reactor secondary circuit

Based on the equations of energy, mass and momentum conservation for the three-dimensional unsteady flow of the two-phase mixture, simulations were made of reactor plant disturbed cooling condition modes caused by the VVER secondary circuit using computer thermohydraulic codes. The use of computer codes to investigate these modes makes it possible to analyze transient and some emergency processes, without involving the industrial testing method; this helps create a basis for solving the problems of reliability, operational safety, and efficiency of nuclear power plants.A modern nuclear reactor is a complex system which cannot be investigated or calculated by using only simple theoretical models. Thermohydraulic calculations are an essential part of most of the design and technological developments in the nuclear power industry. Since the NPP conditions exclude the traditional technical way to check and specify the results and conclusions of an aprioristic analysis based on industrial testing, in some instances, it is possible to examine and predict the parameters of thermohydraulic processes in the reactor circulation circuit involving computer simulations.The primary objective of this work is to calculate and investigate reactor disturbed plant cooling condition modes caused by the VVER secondary circuit in order to determine whether the calculated parameters conform to the acceptance criteria established in the regulatory documents.Based on the RELAP-5, TRAC, and TRACE software codes, reactor plant cooling condition malfunction modes caused by the VVER-1000 secondary circuit were simulated and investigated. Experimental data on the mode with the turbine-generator stop valve closing are presented. The obtained dependences made it possible to determine the maximum values of pressure and temperature in the circulation circuit as well as estimate the Minimum Critical Heat Flux Ratio (MCHFR). It has been found that, if any of the initial events occurs, safety systems are activated according to the set points; transient processes are stabilized in time; and the Critical Heat Flux (CHF) limit is provided. Therefore, in the event of emergency associated with the considered modes, the reactor plant safety will be ensured.

Effects of subcooling on downward facing boiling heat transfer with micro-porous coating formed by Cold Spray technique

External reactor vessel cooling (ERVC) is an effective strategy to achieve in-vessel retention (IVR) of core melt in the reactor pressure vessel (RPV) under severe accident conditions. Among the available strategies, micro-porous coating technique has been known to enhance the thermal margin of the RPV. In this study, a new and versatile micro-porous coating technique applicable to commercial size reactors known as “Cold Spray” has been developed to coat a hemispherical test vessel. Quenching boiling experiments at different degrees of subcooling (10°C, 5°C, 3°C, 1°C, and 0°C) were performed using bare and micro-porous coated vessels. Visual observations of the quenching process along with quantitative analyses of the boiling data were performed. It was found that the critical heat flux (CHF) limit varies significantly with the angular location at all subcooled conditions. Higher cooling rates and CHF limits were obtained with higher degrees of subcooling. A micro-porous coating formed by Cold Spray significantly improved the CHF limit compared to the bare vessel. In fact, nearly 90% enhancement was achieved using the Cold Spray coated vessel. CHF correlations for both bare and micro-porous coated vessel have been proposed capturing the effects of subcooling and angular variation along the outer surface of the hemispherical test vessels.

Fabrication and capillary characterization of micro-grooved wicks with reentrant cavity array

Micro-grooved wicks with reentrant cavity array (MGRAs) were fabricated with orthogonal Ploughing/Extrusion method for the application in the ultra-thin aluminum vapor chambers. Capillary rise tests using both ethanol and acetone as the working fluid were conducted by a novel IR-thermal imaging method. The experimental results indicated that the MGRAs yielded larger capillary height than the micro-grooved wicks (MGs) and the MGRA-1 exhibited the highest capillary rise rate of all samples studied. In addition, the permeability was characterized in the force flow tests. It was found that the MGRAs obtained comparable permeability to that for the MGs, revealing that they were able to enhance the capillary rise with little penalty of pressure drop. With regard to the capillary limited heat flux, the capillary parameter K·ΔPcap was employed to evaluate the performance of the wicks comprehensively. The comparison of K·ΔPcap between the prediction with/without the gravitational effect showed that the Washburn’s Law greatly underestimated the K·ΔPcap after the initial stage, so the gravitational effect should be taken into account when comparing the samples. In this case, K·ΔPcap determined in the ethanol and acetone tests were similar. Besides, the MGRAs yielded higher K·ΔPcap than that for the MGs, except for the MGRA-3/MG-3 couple, justifying the prominent performance of the MGRAs. Despite the fact that the MGRA-2 failed to obtain the largest capillary height, it showed the largest K·ΔPcap among all MGRAs due to the good balance between the capillary pressure and permeability. Thus, it may be the optimum choice of all MGRAs in this study.

Initiation of convection flows in the wall granular layer in the problem of boiling of subcooled coolant

A computational model of boiling of a subcooled liquid in the wall granular layer has been developed on the basis of new experimental data on initiation and evolution of convective flows during nonstationary heating of the wall. The influence of the thermophysical properties of the batch elements on the temperature distribution in the wall region has been studied. The characteristic features of initiation of microconvection in a model cell have been revealed, and the impact of microconvection on the conditions of vapor bubble nucleation depending on the initial subcooling and the extent of the delivered heat flux has been established. New experimental data on limiting heat fluxes that cause microconvection have been obtained with the help of a gradient heat flux sensor for different combinations of the properties of the liquid and the particles of the granular layer model.