Increase In Heat Flux Research Articles

Thermoelastic analysis of variable thickness truncated conical shell subjected to thermomechanical load with internal heat generation using perturbation technique

In this article, the behavior of the truncated conical shell subjected to thermomechanical loading in their inner and outer layers with internal heat generation source is investigated. The displacement field obeys kinematic of the first order shear deformation theory and the first-order temperature theory is used. Two-dimensional temperature analysis has been performed along the thickness and axis of the shell, which can be defined under various loading and thermomechanical boundary conditions. The set of governing equations is a system of differential equations with variable coefficients which are solved by using the analytical matched asymptotic expansion of the perturbations technique. The mentioned solution has little computational cost, therefore can be used well in parametric studies for optimization. It was shown, axial displacement is somehow independent of the radial axis and its maximum value occurs near the upper boundary. The conical shell has expanded in the radial direction. Also the maximum temperature happens approximately in the middle of the length of the conical shell. The parametric study showed, with the increase heat flux in the outer layer thereby expanding the area of heat application which would let more heat in than out. Also it was found, controlling the lifetime of the structure is the result of directing the cone angle. The results obtained from the analytical solution were compared with the Finite Element Method (FEM) analysis and the results of similar related articles, only to show a good agreement.

Mechanism of island dolostones formation in the Cretaceous calcitic ocean: Insights from the Mid-Pacific Guyots

Dolomite is common in carbonate successions. There is an ongoing debate about the origin of ancient massive dolostones. Recent studies suggest widespread syndepositional and early-burial dolomitisation by normal and near-normal seawater, in the rock record, than previously recognised. Seawater dolomitisation is ubiquitous in Cenozoic islands but Cretaceous counterparts are rare, except for Resolution and Allison guyots, Mid-Pacific Mountains. We explored their dolomitisation conditions and the implications on our understanding of Cretaceous normal seawater syndepositional and near-surface dolomitisation. Integrating geochemical and borehole data reveals temporal overlap between 87Sr/86Sr ages of dolomites, and K/Ar and 40Ar/39Ar ages of the basement. Dolomites near basaltic basement exhibit 42-times Fe-enrichment compared to normal seawater dolomites. Host-rock thermal modelling suggested that δ18Odol temperatures are ∼18–29 °C higher than concurrent temperatures. Seismic interpretation and seismic facies forward modelling demonstrate a spatial overlap between dolomites and magmatic sills in both guyots. Accordingly, magmatically driven hydrothermal dolomitisation is proposed, which promoted dolomitisation by providing high heat flow, increasing seawater circulation, and overcoming limited dolomitisation potential of Cretaceous seawater. The H2O–CO2–SO2–HCl-rich magmatic-hydrothermal acidic fluids would mix with seawater causing slight modification by leaching Mg and Fe from basaltic basement. Consequently, magmatically active Cretaceous islands had greater potential for seawater dolomitisation than counterparts with ceased magmatism, due to increased heat flux and slight increase in Mg:Ca of dolomitising fluids. Our findings imply that deep-seated island dolostones don't only form by deep cold seawater below calcite saturation depth. Instead, magmatism appears crucial in this study and potentially other locations (e.g. Xisha Islands). The absence of dolostones in other Cretaceous islands supports previous models of reduced effectiveness of massive dolomitisation by Cretaceous normal seawater, questioning interpretations of syndepositional and near-surface dolomitisation. Thus, caution is advised when interpreting these dolomites by considering additional factors (dolomite geometry, geological settings, and dolomitisation age), as they may share characteristics with syndepositional and near-surface normal seawater dolomites.

Experimental study of the effects of isolated roughness elements on the stability and transition of a hypersonic boundary layer on a flat plate

Surface roughness elements on hypersonic vehicles can cause early boundary layer transition, increasing wall skin friction and heat flux and affecting aircraft range and thermal protection systems. Accurate prediction of the transition caused by these roughness elements is crucial for the design of hypersonic vehicles. In this work, wind tunnel experiments on isolated roughness-induced boundary layer transition at Ma = 6 are conducted. Infrared thermography and high-frequency pressure sensors are utilized to investigate the effects of different roughness element configurations (cylindrical, diamond, ramp) on the hypersonic boundary layer instability and transition. The experimental results show that all three roughness elements can effectively enhance the generation of second mode waves and promote boundary layer transition. Compared to smooth surfaces, they exhibit similar frequency band range, faster growth, and earlier saturation. Among them, the ramp roughness element most effectively triggers the boundary layer transition, with a relatively small heat flux increase. Furthermore, bispectral analysis illustrates that all three roughness elements undergo self-interactions that lead to spectral broadening, ultimately resulting in boundary layer transitions.

The Effect of Heat Flux on the Frequency of Bubble Appearance in a Boiling Pool

This research was conducted to determine the effect of heat flux on the frequency of bubbles appearing in boiling ponds. All fluid movement in pool boiling is caused by natural convection currents. The boiling pool consists of four areas of the pool boiling regime. The division of the four areas is based on the value of the heat flux and the difference between the surface temperature of the heater and the fluid. Using a two-phase heat transfer unit (H654 P.A. Hilton machine), The results showed that the power used greatly influences the boiling process, and besides that, the volume of water used also affects the duration of the boiling process. Based on tests using various power levels of 75 W, 110 W, 168 W, 237.5 W, and 290 W. The occurrence of bubbles will be faster and more numerous when using a lower volume of water and greater power. The heat transfer will be greater if a bubble appears, where latent heat plays a very important role. With mathematical analysis, an increase of 1 bubble per minute occurs for every increase in heat flux of 1.3 W/m2.

Femtosecond laser-treated copper sintering surface to enhance pool boiling heat transfer

Surface properties have a crucial effect on advancing boiling heat transfer (BHT). This study proposes a porous microgroove-structured surface fabricated via copper powder sintering and femtosecond laser processing to enhance pool boiling heat transfer. Copper powder sintering creates a porous layer, and the application of a femtosecond laser subsequently induces the formation of a microgroove structure atop the porous layer. Eight porous microgroove-structured surfaces were fabricated under different laser surface energy densities. Laser surface energy density was controlled by laser power and scanning speed. Pool boiling experiments were conducted for each sample, including smooth, porous, microgroove, and porous microgroove structures. Interestingly, a distinctive displacement was identified within the heat flux curve of porous microgroove-structured surfaces. The wall superheat decreased with increasing heat flux, causing the graph to shift to the left. To elucidate this phenomenon bubble visualization on the boiling surfaces was performed for each sample. It was observed that a porous microgroove-structured surface could effectively activate several bubble nucleation sites. Additionally, secondary pool boiling on the porous microgroove-structured surface was observed leading to the unique “hook back” shift in the boiling curve and the nonlinear increase of HTC. A pool boiling enhancement mechanism was proposed based on experimental data and bubble visualization. This study offers a pathway for advancing heat dissipation efficiency and understanding of surface engineering strategies.

Benign termination of runaway electron beams on ASDEX Upgrade and TCV

This paper discusses the development of a benign termination scenario for runaway electron (RE) beams on ASDEX Upgrade and TCV. A systematic study revealed that a low electron density (n e) companion plasma was required to achieve a large MHD instability, which expelled the confined REs over a large wetted area and allowed for the conversion of magnetic energy to radiation. Control of the companion plasma ne was achieved via neutral pressure regulation and was agnostic to material injection method. The neutral pressure required for recombination was found to be dependent on impurity species, quantity and RE current. On TCV, n e increased at neutral pressures above 1 Pa, indicating that higher collisionality between the REs and neutrals may lead to an upper pressure limit. The conversion of magnetic energy to radiated energy was measured on both machines and a decrease in efficiency was observed at high neutral pressure on TCV. The benign termination technique was able to prevent any significant increase in maximum heat flux on AUG from 200 to 600 kA of RE current, highlighting the ability of this approach to handle fully formed RE beams.

Visualized-experimental assessment on a two-phase thermosyphon loop downcomer state with wide range of filling ratios

The two-phase thermosyphon loop (TPTL), which has an excellent heat transfer performance with simple, compact structure and pump-free advantages, can effectively solve the heat transfer problem under high heat flux. It is necessary to optimize the TPTL to operate in good conditions, especially the state of the downcomer in which the liquid head is one of the critical parameters to determine the driving force of the circulation. However, in the existing studies, the flow features under high filling ratio and high heat flux were ignored in the downcomer. Since a high filling ratio has been proved to have good performance at high heat flux. Herein, the flow characteristics of the TPTL downcomer were seen and quantified using an experimental visual setup under different filling ratios in this study. Two flow features under high heat flux, for the first time, are reported: 1. two-phase flow in the downcomer at high filling ratios. 2. liquid column height oscillation under a partially liquid-filled state. According to the results, the filling ratio significantly impacts the flow characteristics of the downcomer. The TPTL downcomer state, contrary to the traditional understanding, can be partially filled with liquid but not totally filled with fluid all the time, implying that the driving force may be less than the typical assumption. TPTL with a high filling ratio can also provide a maximum gravity pressure head driving force due to the full downcomer. In addition, the bubble pump, which influences by the heat flux, also accelerates the mass flow in the TPTL. Consequently, TPTLs with a high filling ratio showed a significant thermal performance. A partially filled with the liquid phenomenon is demonstrated in moderate filling ratios, and the height of the liquid column will change as the heat flux increases, which indicates the self-regulation of TPTL. When the filling ratio is low, the liquid column height is greatly influenced by the filling ratio. The high liquid column will also give the TPTL excellent thermal performance. The ideal state of a TPTL downcomer is the downcomer filled with two-phase liquid under a high void fraction, which is realized by a high filling ratio TPTL. This paper sheds some light on the downcomer state of TPTLs to optimize the TPTL thermal performance.

Microstructure evolution and ablation mechanisms of Csf/ZrB2-SiC composites at different heat fluxes under air plasma flame

In this study, short carbon fibre (Csf) reinforced ZrB2-SiC composites were fabricated via direct ink writing followed by precursor infiltration and pyrolysis. The ablation behaviour of the composites was investigated at heat fluxes of 4–6 MW/m2 under an air plasma flame, which revealed excellent ablation resistance with negative linear ablation rates of − 0.1 to − 0.5 µm/s. The results showed that ZrO2-SiO2 oxide layer was formed on the ablation surface after ablation, which provided effective protection to the internal materials. With an increase in heat flux, ZrO2 precipitated from the SiO2 melt first in the form of nano-tetragonal ZrO2 (t-ZrO2), owing to the vaporisation of SiO2. During cooling, the nano t-ZrO2 particles were retained, and some of them reacted with SiO2 to form ZrSiO4, thus improving the thermal stability of the SiO2-ZrO2 oxide layer. In contrast, t-ZrO2 agglomerated and grew owing to capillary force and finally transformed into monoclinic ZrO2 (m-ZrO2). Moreover, a higher heat flux led to the size of precipitation ZrO2 increases, meanwhile increasing the content of t-ZrO2 and ZrSiO4 in the oxide layer.

Steady and Unsteady Complex Heat Transfer in Optically Thick Medium During Film Boiling

Abstract The paper presents the results of a study of radiative-convective heat transfer at film boiling of a liquid on a vertical heated plate. Both a steady-state problem of heat transfer and a transient problem were considered. The latter describes the instantaneous (flash) boiling up of a liquid on a heated surface. The novelty of the present study is the use of the optically thick medium approximation in a mathematical model when studying the process of radiation-convective heat transfer in the film boiling regime. For the first time, radiation heat transfer was considered for an optically thick medium. An analytical solution of the steady-state problems is obtained for boundary conditions involving a constant wall temperature and a constant wall heat flux. The effect of radiation and the temperature difference between the wall and liquid on the temperature profiles in the vapor phase is shown. The effect of radiation becomes more pronounced with an increase in the temperature difference between the wall and the liquid. As a result of solving the transient problem, the variation in time of the temperature profile and the heat transfer coefficient in the vapor film were obtained. The effect of radiation (Stark number) on the heat transfer coefficients is elucidated. An increase in the radiative heat flux leads to an increase in the Nusselt number, as well as the time it takes for the heat transfer process to reach a steady-state regime.

Experimental and theoretical study on liquid-vapor behavior characteristics near CHF

Liquid-vapor behavior characteristics near CHF conditions are investigated through flow visualization experiments and theoretical analysis. The detailed liquid-vapor behavior near CHF is captured by high-speed camera in this paper. With the increase of heat flux, the periodic vapor layer occurs due to numerous large bubble coalescence. A three-layer structure is observed close to CHF conditions, i.e., liquid film, vapor layer, and liquid core. The physical triggering mechanism of CHF is the complete evaporation at the liquid film wave trough position close to the end of the heating rod. Furthermore, based on the separated flow model and instability analysis, respective prediction models for essential liquid-vapor characteristics, including thickness of vapor layer, interfacial wavelength, and the maximum thickness of liquid film, are developed. The predicted results are in good prediction performance with MAE of 16.9 %, 12.8 %, and 26.1 % compared to experimental results, respectively. The relevant research in this paper provides an important foundation for further development of the mechanistic CHF model.

Numerical Investigations of the Thermal-Hydraulic Characteristics of Microchannel Heat Sinks Inspired by Leaf Veins

A microchannel heat sink (MCHS) is a potential solution for chip and battery thermal management. The new microchannel structure is beneficial for further improving the thermal-hydraulic performance of MCHSs. Inspired by leaf veins, six new channel structures were designed, and the effects of the channel structures (three parallel structures named PAR I, II, and III and three pinnate structures named PIN I, II, and III), channel depths (0.4, 0.8, and 1.6 mm), and heat fluxes (20, 50, and 80 kW/m2) were investigated via numerical simulation. The cooling medium was water, and the heating area was 40 × 40 mm2. Both PAR II and PIN III exhibit superior overall performance, characterized by the highest Nusselt number and the lowest heating wall temperature. Moreover, PIN III demonstrates the lowest standard deviation in heating wall temperature, while PAR II exhibits the lowest friction factor. The greater the channel depth is, the larger the solid–liquid contact area is, leading to a reduced wall temperature at the interface under identical conditions of inlet Reynolds number and heating wall heat flux. Consequently, an increase in the Nusselt number corresponds to an increase in the friction factor. The maximum value and standard deviation of the heating wall temperature increase with increasing heat flux, while the Nusselt number and friction factor remain unaffected. The overheating near the two right angles of the outlet should be carefully considered for an MCHS with a single inlet–outlet configuration.

Upstream Electrohydrodynamic Conduction Pumping for Flow Distribution Control of Parallel Microchannel Evaporators

Abstract Flow boiling in mini and microchannels has become an attractive option for many applications, such as compact and low charge heat exchangers. Microchannel heat exchangers, however, are more susceptible to maldistribution between parallel flow channels. When operating during uneven heat load conditions, the maldistribution becomes even more severe. Electrohydrodynamic (EHD) conduction pumping technology offers an innovative way to redistribute flow between parallel branches in a microchannel heat exchanger and is also being explored as a next generation mechanism of microgravity heat transport. In EHD conduction pumping, a strong electric field interacts with dissociated electrolytes in dielectric fluid to generate a net body force, and thus, a net flow, with no moving parts, no acoustical noise, lower power consumption, and the ability to operate in microgravity. An EHD conduction pump was designed, fabricated, and tested for upstream flow distribution control of a parallel microchannel evaporator in an opposing configuration. Flow redistribution capability was measured at system flowrates up to 6 ml/min. The EHD conduction pump was capable of completely blocking and reversing the flow in its branch. Recovery from near critical heat flux conditions up to a maximum heat flux of 77.5 W/cm2 was also demonstrated for the operating conditions and design of this study. This was achieved in the absence of enhanced surfaces. The working fluid is HFE 7100. The results show that EHD conduction is able to effectively control the flow distribution of the microchannel evaporator, however, its effectiveness decreases with increasing heat flux and flowrate.

A novel non-destructive acoustic approach for investigating pool boiling phenomena

Analyzing boiling phenomena and performance within invisible boiling systems, where the interior cannot be observed, has traditionally been challenging. In this study, we propose a non-destructive approach using acoustic analysis method to figure out the boiling dynamics or performance, non-invasively. In pool boiling experiment, the acoustic emission (AE) sensor captured the emitted signals at the outside of the heater. Our analysis focused on two key aspects: single bubble characterization and multi-boiling phenomena. For single bubble, we observed that the AE signals originated from the pressure waves generated at the solid-vapor interface of the bubbles. Utilizing a finite difference method, the bubble radius was reverse calculated from the AE signals, enabling predictions up to a certain duration. Frequency analysis revealed a direct correlation between the signal frequency and the oscillation of the bubble radius. In multi-boiling phenomena, as heat flux increases, the number and intensity of AE signals rise, indicating more frequent and rapid bubble generation. A correlation was established between the AE signal counts in the 40 kHz range and the heat transfer coefficient (HTC). The AE count increased linearly with heat flux until it saturated near the critical heat flux. This correlation provided the non-destructive method to estimate the boiling HTC based on AE measurements. In conclusion, the findings offer insights into bubble dynamics, heat transfer characteristics, and the potential application for HTC estimation.

Molecular dynamics study of boiling heat transfer in nanochannels under capillary flow

The increasing heat flux of electronic devices poses significant challenges in thermal management. Passive heat dissipation technology based on capillary drive has been widely concerned because it does not require external driving force and stability. However, due to the small size of the internal capillary structure, typically in the micro and nano scales, and the significant scale effect, further research is needed to understand the mechanism of internal boiling heat transfer. This study systematically investigates the boiling process of liquid in nanochannels under capillary flow with varying wettability using molecular dynamics simulations. It is found that increasing the energy coefficient (c) causes a greater accumulation of atoms near the wall, thereby enhancing the efficiency of solid–liquid heat transfer. The bubble has a larger stable volume as c increases and will burst in the case of high c. As c increases, the region of high potential and kinetic energy expands more rapidly, reducing the time it takes for bubble nucleation and rupture. The behavior of bubbles is significantly influenced by capillary flow, which is also influenced by wettability. When c = 0.9, the flow velocity reaches its maximum before bubble nucleation occurs. It leads to a more homogeneous heat transfer, resulting in a higher location of bubble nucleation. Furthermore, the presence of capillary forces increases the average potential energy of surrounding liquid atoms, thereby enhancing the growth rate of the bubble. This study provides a theoretical basis for the optimal design of capillary-driven two-phase heat dissipation devices.

Subcooled flow boiling in ultrahigh-aspect-ratio microchannels for high heat flux cooling

In present work, the numerical simulation and experimental investigations were performed on the subcooled flow boiling behavior in ultrahigh-aspect-ratio microchannel heat sink (AR = 25). As the heat flux is low, the unique discotic bubble expands the condensation area while ensuring an adequate evaporation area in the channel. When the heat flux increases, the violently fluctuating liquid-vapor interface forms a throat shape that effectively reduces the cross-section of vapor flow. Furthermore, the flow pattern of annular flow at the channel bottom and nucleate bubble at the channel top realizes a combined heat transfer mechanism (nucleate boiling and liquid film evaporation occurs simultaneously). Moreover, the phenomenon of “droplet inside the bubble” caused by bubble coalescence also significantly enhances the thermal process. The fluctuation of local pressure drop in UARM acts gently, which effectively inhibits the boiling instability, and its average value can be reduced by 68.9%. The maximum CHF reaches 1258.1 W/cm2 under a mass flux of 410.2 kg/(m2·s) based on the experimental data, indicating a tremendous potential for the efficient heat dissipation.

The Effects of Land Use and Land Cover Changes on the Land Surface Temperature Over Northeast China

To investigate the impacts of land use and land cover change (LULCC) on regional climate in Northeast China, the weather research and forecasting (WRF) model was run with the satellite land cover type, vegetation fraction (Fg), leaf area index, and albedo in 2001 and 2018, respectively, and the results were compared. The major LULCC during 2001–2018 were conversions from grasslands to croplands over the central regions of Northeast China and conversions from woody savannas to deciduous broadleaf forests in the northern and eastern portions of Northeast China. The changes in Fg and albedo are highly correlated with the daytime land surface temperature (Ts). From 2001 to 2018, the daytime Ts is decreased (increased) over the central regions of Northeast China by more than 1 K in summer (winter). There is also a widespread, about 1 K, cooling in the spring and autumn temperatures. The changes of the nighttime Ts and the daytime Ts from 2001 to 2018 are similar in summer, autumn, and winter, but the nighttime Ts changes are comparatively smaller. Analysis of the surface energy budget and the biophysical effects found that the cooling of the daytime Ts over the central regions of Northeast China in spring, summer, and autumn from 2001 to 2018 can be attributed to a decrease of absorbed shortwave radiation and an increase of latent heat flux corresponding to LULCC. This cooling is weakened by the decrease of the sensible heat flux. In contrast, the widespread warming trend of the daytime Ts in winter is mainly due to an increased net shortwave radiation, which offsets the cooling effects of surface fluxes. During the nighttime, the soil heat flux contributes the most to the changes of Ts.

PRESSURE EFFECT ON POOL BOILING HEAT TRANSFER CHARACTERISTICS OF HIGH-VOLATILE LIQUIDS WITH STRUCTURED SURFACE

The study of enhancement of the heat transfer during boiling and an increase of the critical heat fluxes (CHF) have a great importance in design of the modern and cost-effective heat transfer devices for thermal management of microelectronics, cryogenics and refrigeration, and the power electrical engineering industry. In this work we present the results of the experimental studies of the heat transfer and CHF during the pool boiling of refrigerants R113, RC318, and dielectric liquid HFE-7100 on a structured surface under saturation conditions in the range of reduced pressures 0.03-0.36. The effect the heat flux and pressure on the boiling heat transfer coefficient was studied. It is shown that the correlation of Yagov predicts with an accuracy of ± 35% the CHF for pool boiling of highly volatile liquids on structured surfaces in a wide range of reduced pressure.

Experimental study on flow boiling of HFE-7100 in rectangular parallel microchannel

With the rapid development of microelectronic technology, the integration and power of chip are increasing. Heat dissipation with high heat flux in limited space has become a bottleneck restricting the efficient and stable operation of the microelectronic devices. Flow boiling in microchannel heat sink is one of the most essential candidates for solving this problem. It has been shown that remarkable high heat transfer performance can be achieved through the liquid-to-vapor change process, which can dissipate a large amount of heat from a small area. In addition, dielectric fluorinated fluids, such as HFE-7100, HFE-7200, and FC-72, are especially suitable for cooling microelectronic devices, because of their excellent safety and environmental characteristics. However, dielectric fluorinated fluids have poorer thermophysical properties than water. Thus, the flow boiling heat transfer characteristics of dielectric fluorinated fluids can be different from those of water. In this work, flow boiling heat transfer and flow characteristics of HFE-7100 in a rectangular parallel microchannel are investigated. The tests are conducted at mass fluxes from 88.9 to 277.8 kg·m<sup>–2</sup>·s<sup>–1</sup>, inlet subcooling temperature from 20.5 to 35.5 ℃ and effective heat flux from 12 to 279 kW·m<sup>–2</sup> at nearly atmospheric pressure. The effects of mass flux, inlet subcooling temperature, effective heat flux and vapor quality are examined and analyzed. Additionally, flow visualization is also obtained to explain the heat transfer mechanism during the experiments. The results show that the boiling hysteresis is observed for HFE-7100 at low inlet subcooling temperature, and the increasing inlet subcooling temperature and mass flux can delay the onset of nucleate boiling. The increases of inlet subcooling temperature and mass flux can enhance the two-phase heat transfer coefficient. And the two-phase heat transfer coefficient is significantly dependent on the inlet subcooling temperature in the slug flow, while it is significantly dependent on the mass flux in the annular flow. The two-phase pressure drop increases drastically as the effective heat flux increases. And the two-phase pressure drops with different mass fluxes at constant vapor quality are obviously different between the slug flow and the annular flow. Furthermore, the experimental data are compared with four predicted values of the literature. It is found that the correlation of Lockhart has the best statistical agreement with an MAE of 19.6% and over 85% of points in the deviation bandwidth of ±30%. The results in this paper give valuable theoretical guidance for designing and optimizing heat dissipation equipment for microelectronic devices. By utilizing HFE-7100 as the coolant and microchannel heat sinks in flow boiling, it is possible to enhance the stability and reliability of the electronic devices. Additionally, the heat transfer performance associated with different heat fluxes can be improved by regulating the inlet subcooling and mass flow rate. Finally, the two-phase pressure drop correlation proposed by Lockhart can be employed to predict the pump power for heat dissipation equipment.

Changes in boiling regime and heat transfer effectiveness during water droplet collision with a superheated wall

We experimentally investigated changes in boiling regime and heat transfer effectiveness when single droplets collided with superheated substrates at various initial temperatures. Saturated water droplets colliding at constant speeds were studied over a range of initial substrate temperatures that spanned all boiling regimes from nucleate through transition to film boiling. Space- and time-resolved infrared thermometry and convectional shadowgraph imaging were used to precisely derive local heat transfer distributions and detect characteristic boiling regimes. The results confirmed the existence of the typical regions of a boiling curve: nucleate, transition, and film. The transition boiling regime could be subdivided into three sub-regimes—bubbly, oscillating, and fingering—during which the heat transfer effectiveness (THE) varied in a non-linear manner, exhibiting local minima and maxima. To facilitate interpretation of non-linear variations in HTE during transition boiling, the basic physical parameters of local heat flux, contact area, and residence time were quantitatively measured using obtained thermometry images. In the transition boiling regime, HTE first rapidly decreased during bubbly boiling principally because of a substantial decrease in the droplet residence time. And THE increased during oscillating boiling because of contact area expansion and increased heat flux. Finally, THE decreased during fingering boiling due to substantial reduction in the area fraction of liquid contact. When liquid-solid contact was completely prohibited, stable film boiling was stable. Thus, the dynamic Leidenfrost temperature point could be detected based on heat transfer characteristics, rather than collision dynamics.

Direct numerical simulation of flow and heat transfer of supercritical water with different heat fluxes

The flow and heat transfer characteristics of supercritical water (SCW) are important considerations in the thermal-hydraulic design of the Supercritical Water-Cooled Reactor (SCWR). In the present study, the direct numerical simulation (DNS), developed with OpenFOAM, was utilized to investigate the effect of heat flux on the flow and heat transfer of SCW in a vertical circular pipe. The investigation involves detailed analyses of the wall temperature, velocity distribution, and turbulent statistics at various heat fluxes. It was found that buoyancy first impairs heat transfer, and then recovers heat transfer. This is attributed to the variation of buoyancy production and turbulence production, which are resulted from the buoyancy direct effect and indirect effect, respectively. As the heat flux increases, the recovery occurs earlier with higher magnitude. The vortex structures based on Q-criterion clearly illustrate the same process as well. Several heat-transfer correlations were assessed based on the DNS results. It indicates that Dittus and Boelter correlation (1930) cannot accurately predict the heat-transfer characteristics of SCW, while Chen and Fang correlation (2014) exhibits a higher level of predictive accuracy for Nusselt number. In addition, several turbulence models were evaluated against the DNS data. The results show significant deviations in predicting the heat transfer of SCW. The large discrepancy may be due to the inappropriate model coefficients, the failed predication of turbulence kinetic energy, as well as the constant turbulent Prandtl number commonly applied.