Single-phase Coolant Research Articles

Extremely high heat flux dissipation and hotspots removal with nature-inspired single-phase microchannel heat sink designs

A novel pyramid thermal dissipation unity with conduction–convection coupling thermal dissipation and nature-inspired hotspot removal channel heat sinks is proposed to degrade and dissipate ultra-high heat flux with the order of 103 W cm−2 by using single-phase coolant. A 3D conjugate thermal and flow numerical model is used to validate the feasibility of the proposed ultra-high heat flux dissipation method. The pyramid thermal dissipation unit consists of a spiral tube embedded with etched lotus leaf vein or snowflake-shaped channels. This configuration enables the dissipation of heat fluxes ranging from 1000 to 1500 W cm−2 while maintaining safe operating temperatures for the chip using a single-phase coolant. By utilizing high thermal conductivity materials, the proposed dissipation unit achieves chip working temperatures of 89.89 °C for 1500 W cm−2 and 66.58 °C for 1000 W cm−2. Furthermore, the new design allows for a reduction in the minimum required thermal conductivity for materials to 700 W m−1 K−1 at a pump power of 7.05 W, while still operating within the chip's safe temperature limit of ≤ 120 °C. This expanded range of thermal conductivity values provides more options for selecting suitable materials to fabricate the dissipation unit.

Тепловая математическая модель двухфазного контура с механическим насосом и тепловым гидроаккумулятором

Growing heat release in spacecraft accompanied by simultaneous increase in its amount set the task of developing thermal control systems using the two-phase boiling coolant. It accumulates heat in the form of latent vaporization heat making it possible to transfer much larger amount of heat per the coolant unit mass flow rate than at using a single-phase coolant. In addition, introduction of heat transfer at boiling allows maintaining the object temperature in almost the entire circuit close to the boiling temperature of the selected coolant. All heat transfer processes that occur, when the substance aggregation state changes, are much more intensive than with the conventional convective heat transfer; therefore, the mass of heat exchangers, fittings and control elements of the two-phase circuit would be significantly lower than their mass in a single-phase coolant circuit. Capillary or mechanical pumps should pump the coolant in two-phase systems to ensure the thermal regime. At high power, it is more advantageous to use the two-phase boiling coolant with a mechanical pump. Creation of thermal control systems based on the two-phase circuit should be preceded by elaboration of an adequate mathematical model of the two-phase boiling coolant. Mathematical model is proposed that could be used to analyze operation of the two-phase boiling coolant and calculate hydrodynamic, heat and mass transfer processes.

RECENT PROGRESS ON HEAT TRANSFER PERFORMANCE AND INFLUENCING FACTORS OF DIFFERENT MICROCHANNEL HEAT SINKS

The microchannel heat sink (MCHS) is an efficient thermal management technology widely used in various fields, including electronic equipment, automobiles, and aerospace. In this paper, the recent advances in cross-sectional shape, coolant type, flow channel shape, flow pattern, and application scenarios of the MCHS are systematically reviewed. The liquid film thickness in circular microchannels is the smallest, followed by rectangle, trapezoid, and triangle sections. Conversely, the pressure drop experienced exhibits an inverse relationship with the liquid film thickness. Comparatively, the heat transfer performance of the liquid phase surpasses that of the gas phase, and the two-phase coolant consistently outperforms the single-phase coolant. The study also investigates the impact of flow direction and shape on heat transfer performance. It is found that the implementation of wavy, fractal, and cavity structures enhances heat transfer performance at the expense of increased fluid motion variability, resulting in a loss of pressure drop. Additionally, this paper discusses the occurrence of laminar and turbulent flow phenomena within MCHSs and summarizes their respective influences on heat dissipation performance. On the basis of the aforementioned findings, four key applications of MCHSs are emphasized, accompanied by recommendations for their present utilization and future development. Future research endeavors will concentrate on striking a balance between altering the shape and material characteristics of MCHSs to optimize heat transfer performance while developing novel theoretical models continuously.

Heat transfer performance by forced convection of microencapsulated phase change material-latent functional thermal fluid flowing in a mini-channels heat sink

Conventional single-phase coolants have not been able to meet the high cooling requirements of electronic devices. Microencapsulated phase change material-latent functional thermal fluid (MPCM-LFTF), a solid–liquid two-phase cooling medium, has the dual advantages of active liquid cooling and passive phase change material cooling. However, it also increases the power consumption in the mini-channels, and the cooling efficiency of MPCM-LFTF in practical applications needs to be further studied. A new MPCM-LFTF was prepared by paraffin melamine resin MPCM and base fluid (absolute ethanol and deionized water) in this work. The heat transfer performance and hydrodynamic properties were investigated using the dimensionless wall temperature and friction factor, respectively. The efficiency factor was introduced to evaluate the cooling efficiency of MPCM-LFTF in mini-channels. The experimental results showed that the addition of MPCM increased the viscosity of MPCM-LFTF, which increased the friction factor. MPCM-LFTF with a mass fraction of 15 % had the best effect of temperature control and reduced the dimensionless wall temperature by 40.5 %. However, considering the characteristics of flow resistance and improved heat transfer, MPCM-LFTF with a mass fraction of 10 % had the highest cooling efficiency and the efficiency factor was 57.0 % higher than that of the base fluid.

Machine learning enabled condensation heat transfer measurement

Measuring condensation heat transfer and its associated heat transfer coefficient is not trivial. Rigorous measurements require careful experimental design and tradeoff studies to properly select sensor type, sample geometry and size, coolant fluid and flow rate, operating conditions, working fluid purity, purge methodology, and measurement protocol. Conventional tube-based condensation heat transfer measurements quantify the change in the enthalpy of a single-phase coolant flow via measurement of the inlet and outlet bulk coolant temperatures. The uncertainties associated with this classical and well-established experimental method are high. The high uncertainty stems from the high characteristic heat transfer coefficient or heat flux associated with the condensation process, making the thermal resistance on the external tube side typically on the same order of magnitude as the internal single-phase coolant convective heat transfer thermal resistance. Even when taking the utmost care and using extremely accurate sensors having low uncertainty, the relative uncertainties of heat flux and heat transfer coefficient can be in the range of 20% to 100%. Here, we take advantage of machine learning (ML) to develop an optical visualization method for dropwise condensation heat transfer characterization. Using state-of-the-art intelligent vision, we demonstrate a previously-unexplored method for characterizing the condensate droplet shedding frequency, droplet shedding size, and heat flux without the need for high-speed imaging. We verify our technique by conducting rigorous steam condensation measurements on Parylene C coated smooth copper tube samples having 500 nm, 1 μm, and 5 μm Parylene C thicknesses. We validate our ML predictions with data obtained simultaneously using the enthalpy-change method on a custom and well-established condensation chamber. In contrast to conventional heat transfer measurement methods, the uncertainty of our ML method is constant (∼10%) and does not vary with heat flux. We finally demonstrate the key advantage of our ML measurement technique on a custom-made tube having axially varying surface properties resulting in differing local heat transfer coefficient. Our ML heat transfer measurement method enables the high fidelity characterization of phase change heat flux, reduction in relative measurement uncertainty, resolution of local effects, and elimination of the need for temperature measurement across samples.

Heat transfer characteristics of thermal energy storage system using single and multi-phase cooled heat sinks: A review

• Passive and active cooling techniques for battery and electronic components cooling • Computational and experimental techniques for battery and electronic components cooling • Single phase coolant investigation • Geometry optimization concerning energy management • Flat plate, pin fin, and microchannel heat sink cooling investigation Heat sinks are considered as heat exchangers employed to cool high-temperature devices such as electronic components. They can significantly improve heat dissipation from the base surface. A wide range of heat sink geometries is categorized into three major types: flat-plate, pin-fin, and microchannel heat sinks. Over the past few decades, to keep up with the rate of electronics components heat flux, extensive examinations carried to enhance heat sinks thermal performance include various fabrication materials, single-phase coolants, geometry optimization, and designing complicated heat sink concepts. Heat sink optimization contributes a significant opportunity to improve thermal management and reduce energy consumption. Hence, developing and reviewing different heat sink research methodologies is fundamental. This paper reviews various research methodologies (using single-phase coolant) to aim for heat sink optimization and thermal performance enhancement in three geometry categories (flat-plate, pin-fin, and microchannel). The reviewed articles focused on experimental, numerical, and computational efforts on energy storage thermal managements utilizing single-phase coolant for flat-plate, pin-fin, and microchannel heat sinks design.

Noise requirements of the cryogenic shielding for next generation cryocooled gravitational wave observatories: Newtonian noise

We explore the vibration isolation requirements imposed by Newtonian noise on the cryogenic shielding for next-generation gravitational-wave observatories relying on radiative cooling. Two sources of Newtonian noise from the shield arrays are analyzed: the tidal coupling from the motion of a shield segment to its nearby test mass's displacement, and the effect of density fluctuations due to heterogeneous boiling of cryogenic liquids in the vicinity of the test masses. It was determined that the outer shields require no additional vibration isolation from ground motion to mitigate the Newtonian noise coupling to levels compatible with the LIGO Voyager design. Additionally, it was determined that the use of boiling nitrogen as the heat sink for the cryogenic arrays is unlikely to create enough Newtonian noise to compromise the detector performance for either Voyager or Cosmic Explorer phase 2. However, the inherent periodicity of the nucleation cycle might acoustically excite structural modes of the cryogenic array which could contaminate the signals from the interferometer through other means. This last effect could be circumvented by using a single-phase coolant to absorb the heat from the cryogenic shields.

Experimental determination of the pressure drop and temperature fields in the working section of the fuel assembly model with microfuel

The paper describes an experimental stand and a working area on a model of fuel assembly with microfuels cooled by a single-phase coolant flow at high pressures. The experiments were carried out in a wide range of operating parameters of the coolant coolant pressure p = (2 ÷ 7) MPa; mass flow rate of the coolant Gwater = (0.05 ÷ 0.5) kg/s. The experiments were carried out on the working area of the body, which is made of high-temperature ceramics. The results of pressure losses and temperature distribution of balls along the length and radius of the filling are obtained. It is shown that the hydraulic resistance coefficient obtained on the basis of experimental data is in good agreement with the Bogoyavlensky formula. Significant temperature stratification is observed along the radius of working section.

Experimental study on heat exchange in a compact porous aluminium foam heat exchanger

In this paper the results of experimental study on hydrodynamics and heat transfer during a single-phase coolant flow in a heat exchanger made of porous foam aluminum are presented. The study was carried out on a specially designed and manufactured experimental setup provided with measuring equipment. The setup is capable of testing in a wide range of thermal and hydraulic characteristics and recording the measurement results automatically. The results allowed verification of previously obtained analytical solutions. This made it possible to use them to develop universal methods for calculating specifications of new porous compact heat exchangers used for cooling heat-stressed elements in various devices. The results obtained in this experimental study confirmed the possibility of using porous compact heat exchangers in thermal protection systems with a specific heat release of up to 100 W/cm2.

Surface treatment of an applied novel all-diamond microchannel heat sink for heat transfer performance enhancement

Diamond with superior thermal conductivity would effectively unlock the heat dissipation problem, while the all-diamond microchannel heat sink (AD-MCHS) is immature. In the present work, surface modification of an applied novel AD-MCHS for heat transfer enhancement associate with single-phase (deionized water) coolant was elucidated, for the first time, on an ultra-thick free-standing chemical vapor deposition (CVD) diamond plate. The heat transfer performance of as-processed and post-treated AD-MCHS by surface oxygen-introduction were systematically studied under three heat flux levels, i.e., 40 W/cm2, 80 W/cm2 and 120 W/cm2, respectively. The results demonstrated that the heat transfer performance of the AD-MCHS after acid post-treatment (which was a preferable manner comparing with oxygen plasma and H2O2 bath) was prominently enhanced: 20–50% improvement of heat transfer coefficient (to the maximum of 11917 W/m2·K), 14–28% reduction of thermal resistance and minimum thermal resistance of 0.28 °C/W as well as maximum temperature drop of 11.49 °C for heat source surface. These were the results of hydrophilic diamond surface associated with stronger surface interaction with water molecules, resulting from the reconstructed surface polar carbon-oxygen components, removal of surface graphitic phases as well as the accompanying temperate rising of surface roughness. And the heat transfer acting a more important factor would be enhanced at the expense of acceptable steadily increasing pressure drop (about 1 kPa) and negligible extra of merely <5% with the rising of Reynolds number.

ОПРЕДЕЛЕНИЕ КОЭФФИЦИЕНТА КОНВЕКТИВНОГО ТЕПЛОПЕРЕНОСА В ГИДРОАККУМУЛЯТОРЕ С ТЕПЛОВЫМ РЕГУЛИРОВАНИЕМ ДЛЯ УСЛОВИЙ НЕВЕСОМОСТИ

In the modern world with the increasing power of spacecraft heat dissipation, the need has arisen to use efficient heat removal systems with two-phase heat transfer contours. Their advantages are determined by the fact that they can carry a much larger amount of heat per unit of consumption than when using a single-phase coolant. The energy consumption of the pump for pumping the coolant is insignificant, and the use of heat exchange during boiling allows you to maintain the temperature of objects almost the entire length of the circuit close to the saturation temperature. All heat transfer processes occurring with a change in the state of aggregation of a substance (boiling, condensation) occur much more intensively than during convective heat exchange in a single-phase liquid. A feature of this system is the change in mass of the coolant in the circuit when changing the operating modes of the two-phase heat transfer system. To regulate the amount of coolant in the circuit, as well as to maintain a predetermined pressure (boiling point), a hydraulic accumulator with thermal regulation (HCA) is used. The actual processes of heat and mass transfer in HCA are non-equilibrium, which complicates the calculation of the thermal control system. This paper describes the concept of a nonequilibrium mathematical model for calculating heat and mass transfer processes in HCA. It is shown that the nonequilibrium of processes can be taken into account by the convective heat transfer coefficient “k” in the mathematical model of HCA: k = 1 corresponds to the absence of convection; k &gt; 100 corresponds to an equilibrium process. Based on the analysis of the flight space experiment for heating HCA, a prediction was made of the value of the convective heat transfer coefficient k under zero-gravity conditions. To assess the influence of the “k” value on the process of regulating the two-phase heat transfer contour under weightless conditions, it is recommended to use the values k = 30 ± 15. In ground-based experiments at high-intensity convection in HCA, processes are much more equilibrium than in zero gravity. It is concluded that the equilibrium process (high values of k) can be considered as more conservative concerning the regulatory process in weightlessness.

МАЛОИНЕРЦИОННЫЙ ТЕПЛООБМЕННИК АММИАК-АНТИФРИЗ ДЛЯ ИССЛЕДОВАНИЯ ПЕРЕХОДНЫХ ПРОЦЕССОВ В ЭЛЕМЕНТАХ ДВУХФАЗНОЙ СИСТЕМЫ ТЕРМОРЕГУЛИРОВАНИЯ СПУТНИКА

Technical progress entails the use of more powerful equipment on satellites. In connection with the growth of heat generation onboard the spacecraft, the task is to develop thermal control systems based on two-phase mechanically pumped fluid loop (2PMPFL). The advantage of such systems is the ability to transport a greater amount of heat, reduced to a unit of flow, than when using circuits with a single-phase coolant. The study of two-phase thermal control systems in terrestrial conditions is difficult because gravity affects the hydraulics and heat transfer of two-phase flows. Particularly difficult is the study of transients. This article presents the results of tests of a recuperative heat exchanger, which allows to study transient processes in 2PMPFL with high accuracy.It was designed and manufactured the heat exchanger of simple “tube in tube” type design. The thermal characteristics of the heat exchanger were determined on the experimental stand, which is a prototype of a closed-type 2PMPFL with ammonia coolant. Single-phase “liquid” modes, two-phase modes with low mass vapor content (up to 0.04), and single-phase transient modes were investigated. It has been experimentally determined that a heat exchanger under given conditions is capable of removing up to 1323 W of heat in a single-phase mode and up to 1641 W of heat - when operating in a two-phase mode. The data obtained in the course of the experiments allowed us to select the most appropriate known correlation for calculating the stationary characteristics of the heat exchanger with an error not exceeding 5%, which is a high indicator of accuracy for engineering calculations.The heat exchanger has low thermal inertia. The conclusion is relevant for the range of parameters: the ammonia temperature at the inlet is 24...60 ⁰C; antifreeze inlet temperature 5… 16 ⁰C; ammonia mass flow rate 8...17 g / s; mass flow rate of antifreeze 1...4 kg/min.Due to the low thermal inertia of the heat exchanger, it can be used to study transients with the rate of change of the coolant temperature at the inlet up to 1.85 K / min. You can use the stationary method of thermal calculation, i.e. calculate the transient process in the quasi-stationary approximation.

The study of the temperature field in pebble beds with volumetric heating and radial flow of coolant

The paper presents the results of an experimental and numerical study of hydrodynamics and heat transfer in a pebble bed with internal volumetric heat release. An experimental setup was designed to investigate the hydrodynamics and heat transfer at the radial flow of a single-phase coolant. The pebble bed was set from 2.0 mm diameter sphere made of AISI 420 steel. The internal heat release model in the pebble bed is provided by high-frequency induction heating, whereby the thermal power of the high-frequency installation is 20 kW. The experiments were carried out in the range of mass flow rates of 0.1 - 0.6 kg/s. The test section simulates the fuel assembly of a nuclear reactor with microfuel. Pebbles are placed between the inner and outer perforated covers. Radial coolant flow is achieved to minimize pressure losses. The inner cover is cone-shaped and is the dispensing manifold for the coolant. The conical shape of the cover provides a uniform flow rate of the cooling liquid along the length of the test section. The temperature distribution in the pebbles were recorded.

Measurement uncertainty and quenching phenomena in uniform heating rod bundle CHF tests

The uniform heated rod bundle Critical Heating Flux (CHF) tests have been often conducted as a part of mixing vane performance evaluation processes. Presently, several commercial CHF correlations were developed based on a large amount of rod bundle CHF data using axial uniformly heated rod bundles. With a uniformly heated test section, majority of CHF events are dominated by dry-out mechanism under high quality, high void fraction local conditions. Considering the inverse proportional relationship between the quality and CHF, for an axially uniformly heated test section, the CHF events should occur near the end of heating length (EOHL) where the void fraction/quality is the highest. However, the upstream shift of CHF location is often observed in the uniform heating tests, sometime as much as 250 mm or higher upstream from the EOHL.In the past, without any sufficient convincing proof, long dryout areas as well as “unknown” “mysteries” mixing effects from the mixing vane grid were often considered as the contributing factors for this type of upstream shift. Based on further and detailed examination, this paper offer analysis and evidence to demonstrate another most plausible explanation, exit quenching effect, which ultimately concludes that rod bundle CHF using uniform heated rods could have data with huge measurement uncertainty and as much as over 30% of non-conservatism due to heat lose and quenching effects at the exit of the heated length. The quenching effect at the exit of heater length is mainly due to reflood phenomena generated between the high void/low flow test section exit region and the relatively cold single phase coolant immediately above in the top plenum resulting from either excessive heat loss or operating transient condition.In this paper, system code and CFD modeling were utilized to simulate the quenching phenomena in both steady state and transient conditions. The significant void fraction drop near the exit of the test section due to quenching were clearly observed in these simulations and also evidenced by the various degrees of coloration of the heaters surface in the experiment. This exit reflood quenching phenomena may change the CHF location, suppress boiling, cool down the heated surface, and eventually lead to non-conservative high CHF values. Based on this analysis and experimental observation, it is concluded that the CHF correlations developed based on these type of data are potentially non-conservative in various degrees pending on local thermal-hydraulic conditions, which should be taken into consideration for their applications, especially for safety analysis in nuclear reactor.

Measuring the Temperature and Stress-Strain States of a Tube Sample under the Local Stochastic Temperature Pulsations

Provding a high level of durability of heat exchange equipment of water-cooled reactors under local stochastic temperature pulsations is an important scientific and technical problem for the nuclear power industry. Temperature pulsations produced by mixing non-isothermal coolant flows with high temperature gradient are most dangerous. This work is an experimental study of temperature and stress-strain state of a tube sample under local stochastic temperature pulsations caused by mixing of coolant flows.To solve the problems posed, aY-junction with «counter injection» was built, which was included in the thermal-hydraulic research facility. The design of theY-junction allows study of the thermal-hydraulic characteristics and durability of tube samples made of austenitic steel of 60 × 5 мм. Some tube samples had developed for measuring the temperature, stress-strain state of tube material and temperature field of coolant flow in mixing zone of single-phase coolants with different temperatures. Measuring tube samples were equipped with micro thermocouples and strain gauges.The experimental data of temperature pulsations, time-averaged temperature field in the coolant flow and on the outer surface of the sample were obtained, and statistical and spectral correlation characteristics of temperature pulsations were analyzed. According to results of measuring the relative strain, values of stresses were calculated.Devices and research techniques are developed. The combination of coolant flows parameters that provide thermal load of the metal surface at the highest level of stress intensity amplitude was obtained. The study results are used to verify the method for evaluating fatigue of reactor installations materials under stochastic temperature pulsations.

АНАЛІЗ ВПЛИВУ ОБСЯГУ ГІДРОАКУМУЛЯТОРУ НА ПРАЦЕЗДАТНІСТЬ ДВОФАЗНОГО КОНТУРУ ТЕПЛОПЕРЕНОСУ СИСТЕМИ ТЕРМОРЕГУЛЮВАННЯ КОСМІЧНОГО АПАРАТУ

The world trend in the development of space vehicles is the expansion of their functionality, which leads to an increase in the power consumption, most of which is allocated in the elements of spacecraft equipment in the form of heat. To remove heat from the equipment elements, transfer it to the heat sink subsystem with subsequent removal to outer space, and also to maintain the required temperature mode of the equipment operation, thermal control systems are used. The increase in the power-to-weight ratio and linear dimensions of new spacecraft in conditions of severe design and weight-and-size limitations leads to a complication and growth of the mass of the system of thermal control of space vehicles. At present, thermal control systems for space vehicles based on single-phase fluid heat transfer loops are used. For space vehicles with an energy consumption of more than 10 kW, thermal control systems based on two-phase heat transfer loops are the most promising. They have a number of advantages in comparison with single-phase thermal control systems: two-phase heat transfer loops can transfer much more heat per unit of flow; the use of heat transfer during boiling allows to maintain the temperature of objects practically on the whole extent of the circuit close to the saturation temperature; the mass of the thermal control system with a two-phase coolant is substantially less than with a single-phase coolant , and the energy consumption of the pump for pumping the coolant is negligible. In this paper, a two-phase heat transfer loop performances are analyzed. The process of increasing the thermal power up to the maximum under conditions of full filling of the accumulator is considered. The study was carried out on an experimental two-phase heat transfer loop with an ammonia. Transient processes associated with an increase in the thermal load from 73 % to 100 % are considered. The obtained data correlate well with the results of the calculation. Based on the results of the analysis, conclusions were made on the operability and stability of the spacecraft thermal control system under these conditions, and recommendations on the choice of the volume of the accumulator are given.

Thermodynamic design of cold storage-based alternate temperature systems

Alternate temperature systems are widely used in environmental tests in which the refrigeration device consumes the most energy. In this paper, we present a thermodynamic analysis of modified designs of an alternate temperature system. The modified designs are based on three methods: cold storage, independent air ducts and multi-source cooling. The cold storage method converts part-load operation to full-load operation and the cold storage-based designs decrease the work (or energy) consumption and the equivalent size of the refrigeration device simultaneously. The cold storage and the multi-source cooling improve the performance significantly, while the independent air ducts only have weak effect on the performance. When the refrigerant and the single-phase coolant (coming from the cold storage unit) cool the circulated air together, optimal area allocation between the evaporator and the coolant heat exchanger exists. We also explain the factors that affect the degree of improvement in product designs.

Determination of the Flow Rate of Coolant in Pipelines with Unstabilized Flow of Fluid in the Area Following a Bend

Questions related to the determination of the flow rate of coolant in the pipelines of electric plants are considered. Amethod of determining the flow of single-phase coolant for the case of an asymmetric unstabilized velocity profile that is present following a bend in a pipeline is proposed. Results from the use of the proposed method in novel experiments on the testing unit of the Elektrogorsk Scientific Research Center and in the steam pipelines of power unit No. 3 of the Balakovo nuclear power plant are presented.

Extending the reactivity initiated accident (RIA) fuel performance code SCANAIR for boiling water reactor (BWR) applications

In this paper, capabilities of the SCANAIR transient fuel performance code are evaluated and extended for boiling water reactor (BWR) fuel low temperature cladding failure predictions and high temperature thermal hydraulics modelling. The SCANAIR code, developed by the Institut de Radioprotection et de Sûreté Nucléaire (IRSN), is designed for modelling the behaviour of a single fuel rod during reactivity initiated accident (RIA) in a pressurized water reactor (PWR). In a previous study (Arffman et al., 2012), new BWR cladding material property correlations were developed and implemented into SCANAIR. Here, SCANAIR’s ability to predict BWR cladding failures due to pellet-cladding-mechanical interaction (PCMI) is evaluated by modelling the NSRR FK test series. SCANAIR is found to give correct predictions with reasonably good accuracy when applied to a larger dataset of several tests. As the standard thermal hydraulics model in SCANAIR is one-dimensional and able to model single phase coolant only, the simulation of a BWR RIA, the control rod drop accident, is not possible when the bulk boiling regime is reached. In this study, the code’s application field is broadened to consider bulk boiling in BWRs. In the chosen approach, SCANAIR is coupled with an external thermal hydraulics code. For that, VTT’s in-house general thermal hydraulics code GENFLO has been used. The first demonstration simulations show promising results.

Effects of operating conditions on flow and heat transfer characteristics of mist cooling in a square ribbed channel

Flow and heat transfer characteristics of mist/steam cooling and mist/air cooling in a square channel with 60o rib angle are numerically investigated for a wide range of operating parameters, such as Reynolds number ranging from 10000 to 60000, reference pressure from 0.1 MPa to 0.5 MPa and inlet temperature from 120 °C to 200 °C. Also, the heat transfer characteristics of mist cooling are compared with the corresponding cases of single-phase coolant such as steam and air. The 3D steady Reynolds-averaged Navier–Stokes equations with a standard k-ω turbulent model are solved by using commercial software ANSYS CFX. The CFD model has been validated by experimental data for steam-only case with a good agreement. In addition, distribution and evolution of secondary flow in the ribbed channel are analyzed by vortex core technology and their effects on heat transfer are investigated for these four coolants. The results show that the strength of longitudinal secondary flow has a significant influence on the Nusselt number (Nu) distribution on the ribbed surface. The Nusselt number distribution is periodical in stream-wise direction for steam and air cooling, whereas Nusselt number gradually increases for mist/steam and mist/air cooling. It is found that longer travelling distance of droplets in the ribbed channel result in a higher heat transfer enhancement of mist cooling. The heat transfer characteristics of mist cooling are insensitive to pressure, but inversely correlated with coolant inlet temperature compared with steam and air cooling for the tested parameter ranges.