Heat Flux Intensity Research Articles

Numerical study of heat transfer during nucleate pool boiling

In this paper three-dimensional numerical simulation of the atmospheric saturated pool boiling was performed. The applied modelling and numerical methods enable full representation of the two-phase mixture behaviour on the heating surface with the inclusion of the swell level prediction. The three-dimensional investigation presented here was performed in order to take into account a convective heat transfer on the heated surface, as well as spatial effects of the vapour generation and a two phase flow such as phase dispersion within the two-phase mixture. The results are presented for a short period of time after the initiation of the heat supply and vapor generation on the heating surface. The replenishment of the heating surface with water and partial surface wetting for lower heat fluxes is shown. The influence of the density of nucleation sites and the bubble residence time on the wall on the pool boiling dynamics is discussed. Also, the influence of the heat flux intensity on the pool boiling dynamics is investigated. The applied numerical and modelling method showed robustness by allowing stable calculations for wide ranges of applied modelling boiling parameters.

Numerical investigation of nucleate pool boiling heat transfer

Multidimensional numerical simulation of the atmospheric saturated pool boiling is performed. The applied modelling and numerical methods enable a full representation of the liquid and vapour two-phase mixture behaviour on the heated surface, with included prediction of the swell level and heated wall temperature field. In this way the integral behaviour of nucleate pool boiling is simulated. The micro conditions of bubble generation at the heated wall surface are modelled by the bubble nucleation site density, the liquid wetting contact angle and the bubble grow time. The bubble nucleation sites are randomly located within zones of equal size, where the number of zones equals the nucleation site density. The conjugate heat transfer from the heated wall to the liquid is taken into account in wetted heated wall areas around bubble nucleation sites. The boiling curve relation between the heat flux and the heated wall surface temperature in excess of the saturation temperature is predicted for the pool boiling conditions reported in the literature and a good agreement is achieved with experimentally measured data. The influence of the nucleation site density on the boiling curve characteristic is confirmed. In addition, the influence of the heat flux intensity on the spatial effects of vapour generation and two-phase flow are shown, such as the increase of the swell level position and the reduced wetting of the heated wall surface by the heat flux increase.

Sensitivity of a regional climate model to land surface parameterization schemes for East Asian summer monsoon simulation

Land surface processes play an important role in the East Asian Summer Monsoon (EASM) system. Parameterization schemes of land surface processes may cause uncertainties in regional climate model (RCM) studies for the EASM. In this paper, we investigate the sensitivity of a RCM to land surface parameterization (LSP) schemes for long-term simulation of the EASM. The Weather Research and Forecasting (WRF) Model coupled with four different LSP schemes (Noah-MP, CLM4, Pleim–Xiu and SSiB), hereafter referred to as Sim-Noah, Sim-CLM, Sim-PX and Sim-SSiB respectively, have been applied for 22-summer EASM simulations. The 22-summer averaged spatial distributions and strengths of downscaled large-scale circulation, 2-m temperature and precipitation are comprehensively compared with ERA-Interim reanalysis and dense station observations in China. Results show that the downscaling ability of RCM for the EASM is sensitive to LSP schemes. Furthermore, this study confirms that RCM does add more information to the EASM compared to reanalysis that imposes the lateral boundary conditions (LBC) because it provides 2-m temperature and precipitation that are with higher resolution and more realistic compared to LBC. For 2-m temperature and monsoon precipitation, Sim-PX and Sim-SSiB simulations are more consistent with observation than simulations of Sim-Noah and Sim-CLM. To further explore the physical and dynamic mechanisms behind the RCM sensitivity to LSP schemes, differences in the surface energy budget between simulations of Ens-Noah-CLM (ensemble mean averaging Sim-Noah and Sim-CLM) and Ens-PX-SSiB (ensemble mean averaging Sim-PX and Sim-SSiB) are investigated and their subsequent impacts on the atmospheric circulation are analyzed. It is found that the intensity of simulated sensible heat flux over Asian continent in Ens-Noah-CLM is stronger than that in Ens-PX-SSiB, which induces a higher tropospheric temperature in Ens-Noah-CLM than in Ens-PX-SSiB over land. The adaptive modulation of geopotential height gradients affects wind field (through geostrophic balance) simulation especially at lower levels, which subsequently affects the simulation of large-scale circulation, 2-m temperature and monsoon precipitation as well as RCM’s downscaling ability.

Displacement and temperature discontinuity boundary integral equation and boundary element method for analysis of cracks in three-dimensional isotropic thermoelastic media

• The fundamental solutions for unit point displacement and temperature discontinuities are derived. • The displacement and temperature discontinuity boundary integral equation method is developed. • The displacement and temperature discontinuity boundary element method is proposed. • The stress and heat flux intensity factors are derived in terms of displacement and temperature discontinuities. The displacement and temperature discontinuity boundary integral equation and boundary element method are developed for the analysis of cracks in three-dimensional isotropic thermoelastic media. The fundamental solutions for unit point displacement and temperature discontinuities are derived, and the displacement and temperature discontinuity boundary integral equations are obtained for an arbitrarily shaped planar crack. The displacement and temperature discontinuity boundary integral equation method is developed to analyze singularities of near-crack border fields, and the stress and heat flux intensity factors are derived in terms of displacement and temperature discontinuities across crack faces. The fundamental solutions for a constant triangular element are obtained by integrating the fundamental solutions for unit point displacement and temperature discontinuities. On the basis of these solutions, the displacement and temperature discontinuity boundary element method is proposed. As an application, elliptical cracks are analyzed to validate the developed method.

Changes in the boundaries of the permafrost layer and the methane hydrate stability zone on the Eurasian Arctic shelf, 1950–2100

By using the model for subsea sediments (SSs) (Institute of Atmospheric Physics, Russian Academy of Sciences, IAP RAS) and the general circulation model in the Arctic Ocean–North Atlantic (GCM AO-NA) (Institute of Computational Mathematics and Mathematical Geophysics, Siberian Branch, Russian Academy of Sciences, ICMMG SB RAS), the response of the parameters of the permafrost layer and the methane hydrate stability zone (MHSZ) to external impacts in dependence on the parameters of the problem is considered: the degree of the geothermal heat flux intensity G at the lower (bottom) boundary of the computation domain of the permafrost layer of subsea sediments and the depth Z of this boundary.

Imaging the deep source of the Rotorua and Waimangu geothermal fields, Taupo Volcanic Zone, New Zealand

Magnetotelluric data were recorded in a 45×10km band crossing the Rotorua and Waimangu geothermal fields in the northern part of the Taupo Volcanic Zone in the central North Island of New Zealand. 3-D inverse modelling of these data show that beneath the low resistivity areas marking the near surface geothermal fields, localised electrically conductive zones are present in the crust below about 2.5 and 3.5km depth at Rotorua and Waimangu, respectively. At increasing depth these conductive zones broaden and appear to merge with a larger conductive zone at 8km depth situated between the geothermal systems. At Rotorua the top of the conductive zone is situated directly beneath the area of greatest surface heat and gas discharge. At Waimangu the uppermost part of the deeper conductive zone is situated beneath the western part of Lake Rotomahana, also an area of intense surface thermal activity and high heat flux. The localised conductive zones are interpreted to be high temperature (quasi-magmatic) fluids rising from a broader zone of partial melt at deeper levels.

Overall thermal conductivity of a fiber reinforced composite with partially debonded inhomogeneities

A homogenization problem for conductivity of a composite containing infinite fibers of circular cross-section with interface arc cracks is solved with the account of the orientation distribution of cracks. Solution for a single inhomogeneity with debonded zone is based on the known result on heat flux intensity factor and utilizes the connection between this quantity and resistivity contribution tensor. Maxwell's scheme is used for the homogenization procedure. The results are compared with exact numerical solution for the case when all the fibers are debonded and all the interfacial cracks have the same length and orientation. The application of the proposed approach is illustrated on the example of the recovery of information on damage accumulated in the process of hydrothermal aging.

The effect of transient temporal pulse shape on surface temperature and tungsten damage

The plasma-facing components (PFCs) in future fusion devices such as ITER will receive intense transient heat fluxes from plasma instabilities, such as edge localized modes (ELMs) and disruptions, which will limit material lifetime. The energy density and transient pulse duration are typically used to characterize the PFC damage threshold. However, these parameters are not sufficient to define a damage threshold, because different transient pulse shapes with the same energy density and same pulse duration produce different peak surface temperatures (and thus stresses, which ultimately determine material damage). The surface temperature and damage of tungsten (the material to be used for the ITER divertor target armor) in the form of surface roughening and melting are investigated using various temporal pulse shapes from an Nd:YAG laser in the PISCES-B facility. The heat flux factor is examined and shown to be an inadequate parameter to characterize the temperature rise except for square temporal pulse shapes. For ELM-like heat pulses, the long tail in the temporal shape results in a lower peak surface temperature and less damage compared to a symmetric triangle pulse with equal energy density.

Modeling and Simulation of Temperature Field in High Speed Milling based on Weighted Residual Method

In the process of high speed milling of titanium alloy, the milling temperature change had an adverse effect on machining accuracy, machining surface integrity and machining efficiency. To deal with this problem, heat conduction mode, the heat convection between cutting tools and air medium, and heat flux intensity boundary conditions in actual milling process are fully considered. And according to general heat equation, the heat conduction model of transient temperature field is built. The differential form of temperature field model are converted into corresponding form of the integral functional equation by the weighted residual method. A certain accuracy method for calculating the approximate solution is presented. And the transient temperature response of space element in milling system is obtained under equal time intervals. Finally it is proved that the model is correct by experiment result, which laid a solid foundation for accurately analyzing milling temperature.

Simulation of Flux Distributions on the Foam Absorber with Solar Reactor for Thermo-chemical Two-step Water Splitting H2 Production Cycle by the 45 kWth KIER Solar Furnace

Abstract The flux distributions on the device in the solar reactor with the solar furnace were studied. This study aims to understand of characters of concentrated sun rays through KIER solar furnace and design of device shape for uniform heat distribution on the device surface. For the calculation of heat flux on the device with the KIER solar furnace, the optical modeling program Soltrace was used. At first, the KIER 45 kWth solar furnace and flat disk type device shape was simulated for understand of past experimental results. And then 3 cylinder shape device model and 1 conical shape device model was suggested and the heat flux intensity on the device was calculated. Finally, 5 models which is including flat disk type device shape, 3 cylinder shape, and 1 conical shape device models was calculated and compared. The results show that the concentrated sun rays from dish and heat flux intensity are has a directional characteristic concentrated to normal direction than perpendicular direction. The results will be applied to next solar demonstrations which are design of new solar reactor and new device shape.

An experimental assessment of the ignition of forest fuels by the thermal pulse generated by the Cretaceous–Palaeogene impact at Chicxulub

A large extraterrestrial body hit the Yucatán Peninsula at the end of the Cretaceous period. Models suggest that a substantial amount of thermal radiation was delivered to the Earth’s surface by the impact, leading to the suggestion that it was capable of igniting extensive wildfires and contributed to the end-Cretaceous extinctions. We have reproduced in the laboratory the most intense impact-induced heat fluxes estimated to have reached different points on the Earth’s surface using a fire propagation apparatus and investigated the ignition potential of forest fuels. The experiments indicate that dry litter can ignite, but live fuels typically do not, suggesting that any ignition caused by impact-induced thermal radiation would have been strongly regional dependent. The intense, but short-lived, pulse downrange and at proximal and intermediate distances from the impact is insufficient to ignite live fuel. However, the less intense but longer-lasting thermal pulse at distal locations may have ignited areas of live fuels. Because plants and ecosystems are generally resistant to single localized fire events, we conclude that any fires ignited by impact-induced thermal radiation cannot be directly responsible for plant extinctions, implying that heat stress is only part of the end-Cretaceous story.

Effects of impurity transport and melt layer motion to the tungsten wall erosion during anomaly events

Developing designs of future fusion devices, safety and soundness of the rector at anomaly events must be ensured. A computational approach is being taken by developing a homegrown integrated reactor simulation code and analyzing a loss-of-cooling-gas-puff accident (LCGA). This code currently includes simple plasma, edge, and wall models. In this study, models for the tungsten transport and melt layer motion was added and used for the analysis. It was found that this accident results significant erosion of the wall while impurities from the wall would contribute the radiation cooling for the intense heat flux. However, these effects strongly depend on an uncertain parameter of the tungsten transport as well as the tungsten melt layer motion. Thus, parametric survey for these uncertain quantities were taken and discussed.

Efficiency of Water Curtains

It is possible to use a water curtain to preclude a house fire spreading. Water curtains are designed pursuant to theoretical calculations. Experiments were prepared and realised with a goal to determine the radiant heat flux intensity reduction of fire passing through a water curtain in VŠB, TU – Ostrava.

Validation study of a turbulence radiation interaction model: Weak, intermediate and strong TRI in jet flames

Abstract A TRI methodology developed previously for hydrogen jet flames is applied to carbon monoxide and methane jet flames. The paper demonstrates that the TRI methodology can accurately predict the spectral intensity distribution and heat flux distribution across a range of jet flames. A β probability density function (PDF) together with Reynolds averaging of the instantaneous properties along rays traversing the flames is used to prescribe the instantaneous incident intensity. Unlike most other studies in this area the model is used to predict the heat flux distribution as well as the spectral intensity. The underlying reason why hydrogen jet flames exhibit strong TRI compared to carbon monoxide and methane is also considered and shown to be a complex phenomena deserving of more research. The paper demonstrates that for the data available the heat flux distribution tends to be more sensitive to TRI than the spectral intensity distribution. Another interesting finding is that TRI can reduce as well as enhance the spectral intensity.

Numerical modelling of thermal decomposition processes and associated damage in carbon fibre composites

Thermo-chemical degradation of carbon fibre composite (CFC) materials under intensive heat fluxes are modelled. The model couples together heat diffusion, polymer pyrolysis with associated gas production and convection through partially decomposed CFCs, and changes in transport properties of the material due to the damage. The model is verified by laser ablation experiments with controlled heat input. The numerical predictions indicate that the thermal gas transport has a minimal effect on the decomposition extent. On the other hand, the model shows that the internal gas pressure is large enough to cause fracture and delamination, and the damage extent may go far beyond the decomposition region as witnessed from experimental verification of the model.

Simulation of end-member scenarios of density-driven flow: prediction of dolostone geobody dimensions using COMSOL

Abstract Density-driven flows occur in nature and are initiated by density contrast. Salinity and temperature differences are remarkably effective in initiating density-driven flow, which may instigate convective flow. In turn, convective flow is seen by many authors as key for solute and mass transport, which are important processes controlling dolomitization. Two key dolomitization models rely on the application of density-driven flow concepts: (1) dolomitization by shallow or deep reflux of hypersaline brines in coastal ramp settings; and (2) fault-related dolomitization by the inflow of deep-seated fluids. These dolomitization scenarios are evaluated in this paper to infer plausible architecture, geobody dimensions and rock property distributions of dolomitic reservoirs. Since dolomitization processes may also alter porosity and permeability of the host lithology, changes in these properties are also discussed. This contribution focuses on the application of numerical models to assess the applicability of specific dolomitization models/mechanisms in different geological scenarios. We discuss end-member results of flow simulations using the numerical code COMSOL. This software has been chosen to model these processes as it provides a multiphysics framework for solving coupled systems, as in the case of brine reflux and geothermal convection. Our results provide a useful insight to the dynamic propagation of the limestone–dolomite front through fingering effects. The dolomite geobodies/patterns generated with the help of numerical simulation compare favourably with dolomite geobodies seen in outcrops. This study shows that during the dolomitization process the shape, the extent of propagation of the limestone–dolomite front and the rock properties (porosity and permeability) of the dolomitized reservoir are mainly influenced by the ‘contaminant’ source initiating dolomitization: that is, the magnesium-rich brine density in the case of brine reflux and the heat flux intensity in the case of geothermal reflux circulation.

On the mechanism of convective heat transfer enhancement in a turbulent flow of nanofluid investigated by DNS and analyses of POD and FSP

In this study, direct numerical simulations (DNS) are performed for turbulent nanofluid flows in order to investigate the mechanism of convective heat transfer enhancement induced by the addition of nanoparticles to the base fluid. In the energy equation of a turbulent nanofluid flow, a new thermal dispersion term taking the thermal dispersion effect of nanoparticles into account is introduced, which makes DNS of nanofluid turbulent flow with heat transfer available. The newly established thermal dispersion model is validated by comparing DNS results of heat transfer in a turbulent nanofluid flow with experimental data. Statistical analyses on fluctuating temperature field obtained by DNS show that the mean temperature profile is lowered and the temperature fluctuation intensity, the stream-wise and wall-normal turbulent heat fluxes are all depressed in the buffer layer of nanofluid turbulent flows as compared with the base fluid flow. Proper orthogonal decomposition (POD) analysis, which has been widely used in the investigations of coherent structures in velocity field of turbulent flows, is carried out to extract coherent structures in the temperature fields of turbulent nanofluid flows. POD analysis shows that the turbulent energy contributions from the first few eigenmodes are reduced by adding nanoparticles and the overall energy distribution becomes more uniform among coherent structures in the fluctuating temperature field. Finally, from the viewpoint of field synergy principle (FSP), the average intersection angles between the two vectors of velocity and temperature gradient are estimated for different cases. It indicates that FSP can also apply to explaining the mechanism of heat transfer enhancement in a turbulent nanofluid flow.

ELECTRON HEAT FLUX IN THE SOLAR WIND: ARE WE OBSERVING THE COLLISIONAL LIMIT IN THE 1 AU DATA?

Using statistically significant data at 1 AU, it has recently been shown (Bale et al.) that in the solar wind, when the Knudsen number K {sub T} (the ratio between the electron mean free path and the electron temperature scale height) drops below about 0.3, the electron heat flux q intensity rapidly approaches the classical collisional Spitzer-Harm limit. Using a fully kinetic model including the effect of Coulomb collisions and the expansion of the solar wind with heliocentric distance, we observe that the heat flux strength does indeed approach the collisional value for Knudsen numbers smaller than about 0.3 in very good agreement with the observations. However, closer inspection of the heat flux properties, such as its variation with the heliocentric distance and its dependence on the plasma parameters, shows that for Knudsen numbers between 0.02 and 0.3 the heat flux is not conveniently described by the Spitzer-Harm formula. We conclude that even though observations at 1 AU seem to indicate that the electron heat flux intensity approaches the collisional limit when the Knudsen drops below ∼0.3, the collisional limit is not a generally valid closure for a Knudsen larger than 0.01. Moreover, the good agreement between the heat flux frommore » our model and the heat flux from solar wind measurements in the high-Knudsen number regime seems to indicate that the heat flux at 1 AU is not constrained by electromagnetic instabilities as both wave-particle and wave-wave interactions are neglected in our calculations.« less

Active radiative liquid lithium divertor concept

Developing a reactor compatible divertor has been identified as a particularly challenging technology problem for magnetic confinement fusion. Application of lithium (Li) in NSTX resulted in improved H-mode confinement, H-mode power threshold reduction, and reduction in the divertor peak heat flux while maintaining essentially Li-free core plasma operation even during H-modes. These promising Li results in NSTX and related modeling calculations motivated the radiative liquid lithium divertor (RLLD) concept [1]. In the RLLD, Li is evaporated from the liquid lithium (LL) coated divertor strike point surface due to the intense heat flux. The evaporated Li is readily ionized by the plasma due to its low ionization energy, and the poor Li particle confinement near the divertor plate enables ionized Li ions to radiate strongly, resulting in a significant reduction in the divertor heat flux. This radiative process has the desired effect of spreading the localized divertor heat load to the rest of the divertor chamber wall surfaces, facilitating divertor heat removal. The modeling results indicated that the Li radiation can be quite strong, so that only a small amount of Li (∼a few mol/s) is needed to significantly reduce the divertor peak heat flux for typical reactor parameters. In this paper, we examine an active version of the RLLD, which we term ARLLD, where LL is injected in the upstream region of divertor. We find that the ARLLD has similar effectiveness in reducing the divertor heat flux as the RLLD, again requiring only a few mol/s of LL to significantly reduce the divertor peak heat flux for a reactor. An advantage of the ARLLD is that one can inject LL proactively even in a feedback mode to insure the divertor peak heat flux remains below an acceptable level, providing the first line of defense against excessive divertor heat loads which could result in damage to divertor PFCs. Moreover, the low confinement property of the divertor (i.e., <1ms for Li particle confinement time) makes the ARLLD response fast enough to mitigate the effects of possible transient events such as large ELMs.

Controlling the contact line behavior by introduction of artificial perturbations in a nonisothermal liquid film

The effect of periodic artificial perturbations on a heated liquid film at small Reynolds numbers is studied experimentally using a high-speed infrared camera. It is shown that the development of thermocapillary perturbations leads to movement of rivulets and affects contact line that outlines dry spot in a vertically falling heated film. It has been established that at sufficiently intense heat fluxes the artificial perturbations exert a significant effect on the structure of the flow and on the repeated wetting of dry zones.