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The heat transfer characteristic of supercritical water is one of the crucial issues in SuperCritical Water-Cooled Reactors (SCWRs). The efficiency and safety of the SCWR system are largely dependent on the local heat transfer performance. This paper establishes the numerical model for supercritical water in a long vertical circular loop (inside diameter = 10 mm) and analyzes the flow and heat transfer mechanism during the transition process from subcritical to supercritical states under various heat fluxes (uniform and nonuniform). The results reveal that the difference in thermophysical properties between the boundary layer and the core region is the main reason for the heat transfer behavior, especially during the transition from subcritical to supercritical and liquidlike to gaslike. The flow structure on the buffer layer is a dominating factor for heat transfer deterioration. The cases under variable nonuniform heat fluxes have a higher heat transfer coefficient compared with uniform heat fluxes. But, this will cause large changes of the parameter locally. The dominating factors of heat transfer deterioration under these conditions are also identified. [ABSTRACT FROM AUTHOR]

Summary: Supercritical heat transfer loops are widely used in large‐scale thermal equipment, which development is restricted by the thermal‐hydraulic characteristics, especially the safety and thermal efficiency under high‐heat fluxes. This study is focused on numerical simulation for a 4‐m test section of the loop and further analyzes the flow transitions and heat transfer mechanisms from subcritical to supercritical. The heat transfer characteristics of supercritical water in the vertical loop were simulated for conditions: P = 24 MPa, Tin = 320°C, 340°C, mass flux (G) = 1000 kg/m2s, 1500 kg/m2s, and heat fluxes varied from 100 to 1200 kW/m2. The heat transfer transitions are analyzed from the perspective of local parameters that affect the status of both core flow and boundary layers. The empirical correlation and the error backpropagation (BP) neural network prediction model are proposed and compared with experimental and numerical results. The results show that transitions of both boundary layer and core flow from subcritical to supercritical contribute to the heat transfer behaviors: normal heat transfer, heat transfer deterioration, and heat transfer enhancement. The zero‐velocity gradient (∂u/∂r = 0) on the M‐shaped velocity curve causes the turbulent shear stress near the wall to be close to zero, which leads to the suppression of the turbulence. At the same time, the thickness of the thermal boundary layer reaches the maximum, resulting in a large thermal resistance, which affects the heat transfer from the wall to the core area. [ABSTRACT FROM AUTHOR]

Special flame merging behaviour and complex combustion characteristics can be formed under the coupling effect of tangential flow and heat transfer of multiple pool fires (MPFs) in a shaft. In this study, quadruple pool fires (DPFs) evolutionary experiments in shaft were carried out, and the influence of pooled spacing and side slit width of the shaft on flame merging behaviour were analyzed. The primary influencing mechanism of merged flame evolution was expounded from heat transfer and thermal feedback between fire sources. The shape of conical merged fire whirls that can completely fill the gap between the oil pools with flame is confirmed, and its cone angle is determined by the position of outer edge of oil pools. In the formation of merged fire whirls, the critical conditions are that the distance between fire sources is less than three times the pool diameter and the width of side slits is 1/4 of the side length of the shaft, respectively. Compared with the single pool fire whirls with the same liquid surface area, the thermal flow field of DPFs in shaft is relatively unfavorable to the formation of fire whirls. The formation time of merged fire whirls is nearly five times that of the single pool fire whirls, and the flame height can be reduced by nearly half. The evolution process of the DPFs that can form merged fire whirls exhibits the multi-stage morphological characteristics over time and all of these stages possess different heat transfer mechanisms. Before forming the merged fire whirls, intermittent merging among pool fires is a necessary stage. And the duration of this stage is relatively less affected by the width of slit on the side of shaft. The transverse force of asymmetric tangential flow on the flame is difficult to be completely overcomed by merged fire whirls. Because of the inclination of merged flame, the overall vertical height is unsuitable as a direct characterization parameter of combustion intensity. [ABSTRACT FROM AUTHOR]

In road tunnel fires, exhaust vent layout and smoke exhaust rate are two important parameters that affect the smoke exhaust performance of ceiling central smoke extraction. However, the synergistic effect between the two (cooperation of two factors) has not yet been quantitatively analysed. Reduced-scale (1:20) burning experiments were conducted in this study. Through variations in the vent area, vent width, and exhaust rate, the factors influencing the ventilation system efficiency V S E ′ and ventilation system output V S O ′ were analysed. It can be seen from the experimental results that for a constant vent width, the exhaust vent area has nearly no effect on V S E ′ and V S O ′. Furthermore, increasing the smoke exhaust rate can improve V S E ′ , but will reduce V S O ′ , whereas increasing the vent width can increase both V S E ′ and V S O ′. We also considered the effects of heat transfer at the tunnel wall and obtained a normalised V S E ′ estimation correlation, which includes the aforementioned dimensionless influencing factors and the factor of distance between the exhaust vent and fire source. After verifying the correlation, we demonstrated its application value as a significantly useful reference for central smoke extraction design. • Reduced-scale experiments were performed to study the central smoke extraction. • Ventilation system efficiency and ventilation system output were analysed. • Synergistic effect of exhaust vent layout and exhaust rate was studied. • A normalised ventilation system efficiency estimation correlation was obtained. [ABSTRACT FROM AUTHOR]

• Experimental platform established for sCO 2 mini-channel flow local visualization. • Quantitative procedures established for sCO 2 boundary in field info extraction. • Rapid density decreases and small t differences (0.01 K) under sCO 2 local heating. • Grouped nu trends found for the first time during local boundary visualization. This study is focused on measuring the transient boundary heat transfer in supercritical CO 2 mini-channel flows under local heat application. A pixelated-array masked phase-shifting interferometer system and quantitative data procedures have been established for sCO 2 boundary flow. The experiments have been conducted at a Reynolds number level of 104, spanning across both subcritical and supercritical regions (5.0 ∼ 7.9 MPa). High resolution local density and temperature have been extracted from the raw interferograms by developing a digital phase-shifting algorithm. The temporal and spatial evolutions of Nu show good agreement with classic correlations. The effects of differential pressures (10.87 ∼ 52.59 Pa) and heat fluxes (2014, 5500, 14,057 W/m2), along with their associated heat transfer behaviors, have been quantitatively examined under transient scenarios using the current method. The results show: (1) although the Re number is high, such small thermal perturbations can be clearly visualized in the entire window, which shows the capability of field extraction of the current method proposed for complicated supercritical boundary flows; (2) rapid density decrease (1.0 kg/m3) occurs locally, while small temperature differences (0.01 K) are observed, which also lead to local Nu variations under different operation conditions; (3) detailed convective patterns and time evolution of Nu curve groups identified for subcritical, near-critical and supercritical states. [ABSTRACT FROM AUTHOR]

• Evaluated coupled heat and moisture transfer in EAHEs in harmonic thermal condition. • Air temperature and moisture content exhibit phase shift and amplitude dampening. • Moisture transfer intensifies phase shift of outlet air enthalpy. • Moisture transfer attenuates fluctuation amplitude of outlet air enthalpy. The heat and moisture transfer process of an earth-to-air heat exchangers (EAHE) determines its ability to cool/heat an indoor environment. In humid climates, the moisture content of the EAHE inlet air is relatively high and exhibits a different fluctuation behavior than the air dry bulb temperature. In this study, a mathematical model was accordingly developed to evaluate the coupled heat and moisture transfer in an EAHE. Two prerequisites were considered by the model: the inlet was exposed to both harmonically fluctuating outdoor air temperature and moisture content, and the surface of the EAHE was directly formed by the surrounding material and always saturated due to water infiltration from this material. Based on the harmonic analytical method and a numerical approach, solutions for the amplitude dampening and phase-shifting of air temperature and moisture content were obtained. The model was validated using the results of well-regulated experiments, which indicated that phase-shifting and amplitude dampening of both air temperature and moisture content fluctuations occur in an EAHE. The coupled heat and moisture transfer between the humid air and the wet EAHE walls was found to intensify the phase shift but considerably attenuates the air enthalpy fluctuation amplitude. Then, application examples were analyzed corresponding to typical humid and dry cities-Chongqing and Beijing, respectively. The results showed the normalized fluctuation amplitude of the outlet air enthalpy for the Chongqing and Beijing environments decreased by 56.7 % and 61.1 %, respectively, indicating that moisture transfer considerably influences the air enthalpy fluctuation at the EAHE outlet. [ABSTRACT FROM AUTHOR]

Faster-than-real-time forecast for the ceiling jet head propagation is crucial for evacuation guidance and rescue operation in the early stages of tunnel fires. The ceiling jet head propagation is driven by buoyancy and thus significantly dominated by the heat transfer process in the head region. Simplified models have been established but proved challenging in forecasting complex dynamic processes. The cumulative deviations induced by the model limitations and the input parameter uncertainties lead to distorted forecast during the ceiling jet dynamic propagation process, thereby constraining the applicability of such models in practical scenarios. In this study, an assimilation method, the ensemble Kalman filter (EnKF), was combined with a simple theoretical model to forecast the ceiling jet propagation. Using the EnKF to assimilate the monitored local data, the key input parameters such as mass flow rate m ˙ and the temperature of the ceiling jet head T s are corrected in real time, thus alleviating model prediction deviation. Meanwhile, a heat loss rate correction coefficient λ α , introduced to correct the heat loss rate during the ceiling jet propagation process, is dynamically estimated as well. This enables the model to characterize the heat transfer process more accurately, consequently ensuring faster-than-real-time forecasts of the subsequent propagation stage. Finally, the forecasting effectiveness of the proposed combined method was examined in a 1:6 reduced-size experimental tunnel fire scenario and full-scale simulated tunnel fire scenarios with various fire source locations, tunnel geometries, and suddenly changing HRR. In response to the demand of minimizing both the type and quantity of sensors in practical applications, a factor analysis was also performed to examine the robustness of this method. The results exhibited good adaptability to different fire scenarios with very limited monitored data. • Data assimilation combined with simple model for tunnel ceiling jet spread. • The combined method enabled real-time correction of heat transfer calculation. • Data assimilation enabled the dynamic regulation of model inputs. • The effectiveness is verified under diverse tunnel fire scenarios. • Considerable forecast leading time and small errors achieved. [ABSTRACT FROM AUTHOR]

Experiments were conducted to investigate the heat transfer and flow characteristics of the vertical upward smooth and rifled tubes from subcritical to supercritical pressure. The distributions of wall temperature and heat transfer coefficient (HTC) were obtained, and the HTC correlations and friction resistance coefficient correlations were fitted with experimental data. In addition, the influences of heat flux and type of tube on heat transfer performance were analyzed. The research shows that heat flux has different influences on the heat transfer characteristics under different pressures. The increase in heat flux improves the heat transfer characteristics in the nucleate boiling region, yet it leads to the advance in heat transfer deterioration. However, for supercritical water, the increase in heat flux reduces the heat transfer ability. In addition, using the rifled tube not only improves the heat transfer performance, but also inhibits the occurrence of heat transfer deterioration. The fitted correlations have great predictive ability for the heat transfer coefficient and friction resistance coefficient, and the average relative fitting errors are limited to 20%. [ABSTRACT FROM AUTHOR]

A numerical study is conducted to investigate heat transfer characteristics of supercritical water flowing upward in an inclined smooth tube under uniform heating conditions. In this study, model verification was performed under the conditions of low heat flux and high heat flux to ensure the accuracy of experimental data. Inner wall temperature and heat flux distribute non-uniformly on the circumference due to the influence of natural convection. The increase of heat flux enhances the non-uniform circumferential distribution, while the increase of mass flux reduces the non-uniform circumferential distribution. Heat transfer of upward vertical and inclined flows is compared under low and high heat flux conditions. Radial distributions of fluid temperature, density, specific heat capacity, velocity, and turbulent kinetic energy are presented and analyzed. The analysis shows that the increase of heat flux reduces radial turbulence and heat-absorbing capacity of the boundary fluid, while increases radial temperature and density gradient. Three dimensionless buoyancy parameters are used to evaluate the buoyancy effect of inclined flow. [ABSTRACT FROM AUTHOR]

The necessity of equipping with an emergency water supply system in a supercritical circulating fluidized bed (CFB) boiler has long been a controversial topic in the industry. In this study, the heating surface of a 660 MW supercritical CFB boiler at the Pingshuo Power Station of China Coal Group is taken as the research object. And a boiler electricity failure experiment was accomplished at the power plant, and the variations of thermal parameters during electricity failure were obtained from power plant distributed control system. By analyzing the physical process of the heating surfaces during electricity failure, a mathematical model of heat transfer to the heating surface was established and a calculation program in Fortran language was developed. In this study, the effect of different flow rates and operating times of the supply pumps on the safety of the heating surfaces were then investigated. The numerical simulation results show that an emergency supply pump with a flow rate of 250 t/h, activated within 120 s, ensures the safety of the heating surface. The results of this study have important implications for the selection of emergency supply pumps and the determination of flow rates and pressures for supercritical CFB boilers with external beds. [ABSTRACT FROM AUTHOR]

Heat transfer to supercritical water in a smooth tube with an inclined angle of 25° was investigated experimentally and numerically. Experimental results show that the increase of mass flux improves the heat transfer. As the pressure increases, influence of physical property variation to the heat transfer becomes gentle. Thereby, the increase of pressure reduces the heat transfer under an enhanced condition. Dimensionless buoyancy parameters Gr q */ Gr qth and G r ‾ / R e 2.7 are compared on the basis of experimental results. The evaluated result shows that Gr q */ Gr qth is applicable to the inclined tube flow. Base on the supercritical data, a new heat transfer correlation is proposed. Numerical analysis was completed with the shear stress transport k - ω model. Corresponding results show that the influence of variable mass flux and pressure on supercritical heat transfer is realized by changing radial distributing characteristics of turbulence and physical property. The increase of mass flux improves the radial turbulence and heat-absorbing capacity of the boundary fluid, so the supercritical heat transfer is enhanced. At a low mass flux condition where heat transfer reduction occurs, the increase of pressure improves the heat transfer. At a high mass flux condition where heat transfer enhancement occurs, the increase of pressure reduces the heat transfer. Influence of natural convection caused by the buoyancy leads to the occurrence of heat transfer disparity between top and bottom sides of the inclined tube. The increase of mass flux weakens the influence of natural convection and improves local heat transfer at the top side. At the low mass flux condition, the increase of pressure conducts the same effect with the increase of mass flux. At the high mass flux condition, the increase of pressure reduces the local heat transfer at the top side. • Buoyancy parameter Gr q */ Gr qth is applicable to the inclined tube flow. • The increase of pressure improves heat transfer reduction at low mass flux. • The increase of pressure reduces heat transfer enhancement at high mass flux. [ABSTRACT FROM AUTHOR]

The boundary heat transfer dynamics of supercritical carbon dioxide (CO 2) is vital to its application in solar heater, extraction, refrigeration, heat pumps, power generations, nuclear systems and many other occasions. In this study, an experiment system using pixelated-array masked phase-shifting interferometer has been established for the visualization of temperature of supercritical CO 2. The laser diagnostic process has been operated with a carefully designed visualization chamber to give clear transient results of the density and temperature fields for the first time. Those field information has been used to calculate the heat transfer characteristic parameters of the supercritical CO 2 fluid in different boundary conditions. The current quantitative visualization analysis by the utilization of pixelated methods is shown capable of giving transient analysis of detailed field evolutions. The results show that (1) small temperature differences are generated in supercritical boundary due to the typical large thermal capacity; (2) supercritical convection boundary is with much quicker thermal quenching than subcritical states, while the subcritical states boundary parameter show localized characteristics while supercritical cases show fluctuation phenomena; (3) the boundary fluctuations heat transfer field exist for supercritical states and the fluctuations affect the overall heat transfer behaviour and the field evolution in supercritical states. [ABSTRACT FROM AUTHOR]