Heat Flux Transport Research Articles

The Correlative Approach of 3D Imaging Techniques, Simulation and Diffusion Experiments to Explore Transport Properties in Porous Otm Support Material

Solid state electrochemical energy devices such as batteries, fuel cells and oxygen transport membranes (OTMs) have the potential to revolutionize distributed low-carbon power supply. However, in order to accelerate their commercialization across a range of applications, a much improved understanding of the underlying material microstructure is required. In particular, the rate limiting step of an OTM membrane applied for syngas production is found in concentration losses caused by mass transport limitations. In this field, microstructural parameters, such as tortuosity, porosity and average pore diameter, play a vital role in characterising, comparing and evaluating porous materials. However, unlike the latter two parameters, tortuosity cannot be measured directly which is why different methods have been developed for this purpose. These differ considerably from each other in terms of calculation approach and data preparation techniques. Here, we utilise X-ray nano computed tomography (CT) techniques to reveal the complex microstructure and resolve geometric features affecting mass transport of tubular porous support layers of OTMs to examine the relationship between the pore structure and mass transport (Figure 1). This is achieved via image based tortuosity calculation methods: a fast marching computational technique and a Laplace solver using MATLAB is employed to examine the geometrical tortuosity, and StarCCM+ and COMSOL software packages are used to model the heat flux and effective transport parameters of the porous phase, respectively. In turn, the results are validated using diffusion cell experiments to evaluate the bulk transport properties of the same samples for different binary gas mixtures at different temperatures. Tortuosity is then extracted by applying equimolar and equimass diffusion models [1]. We conclude, that image based calculation methodologies arrive at lower tortuosity values in comparison to diffusion cell experiments (Figure 2). Even the widely used Bruggeman [2] and Maxwell relations prove to be unfit to estimate tortuosity values in the analysed microstructure. The observed difference may stem from Knudsen diffusion effects which are not accounted for in the entirety of tomography based modelling techniques. Secondly, tortuosity values extracted via diffusion cell experiments are a function of gas mixture, temperature and microstructural characteristics rather than a constant value. Consequently, care must be taken in applying purely continuum interpretations of diffusion processes in these complex porous media.

Distribution of heat flux by working fluid in loop heat pipe

The main topics of article are construction of loop heat pipe, thermal visualization of working fluid dynamics and research results interpretation. The work deals about heat flux transport by working fluid in loop heat pipe from evaporator to condenser evolution. The result of the work give us how the hydrodynamic and thermal processes which take place inside the loop of heat pipe affect on the overall heat transport by loop heat pipe at start-up and during operation.

Reactivation of Kamb Ice Stream tributaries triggers century‐scale reorganization of Siple Coast ice flow in West Antarctica

Ongoing, centennial-scale flow variability within the Ross ice streams of West Antarctica suggests that the present-day positive mass balance in this region may reverse in the future. Here we use a three-dimensional ice sheet model to simulate ice flow in this region over 250 years. The flow responds to changing basal properties, as a subglacial till layer interacts with water transported in an active subglacial hydrological system. We show that a persistent weak bed beneath the tributaries of the dormant Kamb Ice Stream is a source of internal ice flow instability, which reorganizes all ice streams in this region, leading to a reduced (positive) mass balance within decades and a net loss of ice within two centuries. This hitherto unaccounted for flow variability could raise sea level by 5 mm this century. Furthermore, better constraints on future sea level change from this region will require improved estimates of geothermal heat flux and subglacial water transport.

Thermal Analysis of Nanofluids on the Thin Film Evaporation of Meniscus

A mathematical model for predicting evaporation in the thin film region was developed and its analytical solutions were obtained for thin‐film thickness, the heat transport per unit length and the total heat flux transport in the thin‐film region. These analytical solutions show that the higher heat flux through the thin film region occurs due to the higher superheat. The maximum evaporative rate occurs when the effects of the increase in the temperature difference and in the thin film thickness on the heat flux q stay equal. A nanofluid, which is a colloidal mixture of nanoparticles (1 nm to 100 nm) and a base liquid (nanoparticle fluid suspensions), is employed as the working fluid. In a certain range, increasing the volume fraction of nanoparticles in the base fluid leads to decreasing the kinematic viscosity of the nanofluid and increasing the thermal conductivity, which influences the evaporation in the thin film region. The heat transfer rate per unit length and the total heat flux in the thin film region display various characteristics among the different type of nanofluids due to the differences of the kinematic viscosity and the thermal conductivity.

Formulation of Two-Dimensional Transport Modeling in Tokamak Plasmas

A two-dimensional transport modeling applicable to a whole tokamak plasma is proposed. The model is derived from the multi-fluid equations and Maxwell’s equations and the moment approach of neoclassical transport is employed as fluid closures. The multi-fluid equations consist of the equations for particle density, momentum, energy and total heat flux transport for each plasma species. The expressions of the parallel viscosity and heat viscosity are extended in order to be applicable to both inside and outside of the last closed flux surface. It is confirmed that our neoclassical transport model is consistent with the ordinary flux-surface-averaged onedimensional neoclassical transport model. Our transport equations are coupled with the electromagnetic equations in order to describe the time evolution of tokamak plasmas. The procedure for coupling a transport solver based on our transport model with an equilibrium solver is also briefly described. c

Capillary evaporator development and qualification for loop heat pipes

In the present paper, design consideration, fabrication techniques and post-assembly qualification tests to guarantee reliable structure of the capillary evaporator for loop heat pipe (LHP) with high heat flux transport capability has been presented. Cylindrical evaporator with nickel as high performance capillary pump, copper as high conductivity containment material and water as superior heat transfer working fluid was designed to transport 500 W heat load up to distance of 250 mm. Nickel powder with average particle diameter of 12 μm was sintered to obtain wick with high porosity (>65%) and fine pore radius <7.2 μm. High volumetric shrinkage (4–50% @750–1000 °C respectively) of the sintered wick results in poor control over the sample dimensions which makes wick integration inside evaporator difficult. Special procedures were conducted to develop machined nickel wick with good dimensional control (±0.15 mm) and high-roughness surface morphology, which provide high evaporation area and high surface porosity. These machined wick were successfully installed inside the evaporator using different integration methods. Qualification tests to validate evaporator structural integrity has been discussed in detail. For the designed loop heat pipe prototype, the minimum value of 0.15 °C/W for total thermal resistance was observed at maximum heat load of 500 W. In summary, capillary evaporator fabrication techniques (including wick development) and structural integrity qualification tests developed in this investigation can be used to fabricate miniature to large-scale loop heat pipes with high heat flux management capabilities.

Thermal crosstalk in arrays of III-N-based Lasers

Abstract This paper presents a 3D comprehensive thermal-electrical self-consistent model of the continuous-wave (CW) operation of one-dimensional arrays of III-N-based laser diodes at room-temperature (RT). Their performance is mostly limited by thermal processes, in particular by thermal crosstalk between array emitters. Based on data collected from a range of secondary sources, the temperature dependence of the thermal and electrical conductivities of III-N materials used to manufacture nitride-based devices is shown to be a function of the thickness, aluminum mole fractions and Si- and Mg-doping levels of the nitride layers. The impact of substrate width and thickness on increasing the efficiency of heat-flux transport and reducing thermal crosstalk is investigated. As expected, the application of a top-mounted diamond heat spreader was found to have considerable influence on the thermal crosstalk between array emitters, enabling the RT CW operation of laser diode arrays with additional emitters.

Transport of radial heat flux and second sound in fusion plasmas

Simple flux-gradient relations that involve time delay and radial coupling are discussed. Such a formulation leads to a rather simple description of avalanches and may explain breaking of gyroBohm transport scaling. The generalization of the flux-gradient relation (i.e., constitutive relation), which involve both time delay and spatial coupling, is derived from drift-kinetic equation, leading to kinetic definitions of constitutive elements such as the flux of radial heat flux. This allows numerical simulations to compute these cubic quantities directly. The formulation introduced here can be viewed as an extension of turbulence spreading to include the effect of spreading of cross-phase as well as turbulence intensity, combined in such a way to give the flux. The link between turbulence spreading and entropy production is highlighted. An extension of this formulation to general quasi-linear theory for the distribution function in the phase space of radial position and parallel velocity is also discussed.

Soil Thermal Diffusivity of a Gleyic Solonetz Soil Estimated by Different Methods in the Brazilian Pantanal

The soil temperature is an important microclimatic factor due to the interactions between soil and plant, and the energy exchange with the atmosphere. The soil energy exchange is affected by the incident solar radiation, type of coverage and mainly by the soil thermal properties. Among the soil thermal properties, the soil thermal diffusivity is highlighted because it affects the soil temperature profile and soil heat flux transport and distribution. Thus, the aim of this study was to evaluate different estimates of soil thermal diffusivity of a Gleyic Solonetz soil in the Brazilian Pantanal. The soil thermal diffusivity was determined by the amplitude, logarithmic, arctangent and the phase methods between 0.01 and 0.03 m, 0.01 and 0.07 m and 0.01 and 0.15 m depth. The soil thermal diffusivity estimated by the four methods showed significant differences and varied over the study period as a function of volumetric soil water content. The soil thermal diffusivity estimated by logarithmic methodshowed better performance at different depths, followed by the method of phase.

Ocean mixed layer processes in the Pacific Decadal Oscillation in coupled general circulation models

It is investigated how the Pacific Decadal Oscillation (PDO) is simulated differently among various coupled general circulation models (CGCMs), and how it is related to the heat budget of the simulated ocean mixed layer, which includes the surface heat flux and ocean heat transport. For this purpose the dataset of the climate of the 20th Century experiment (20C3M) from nine CGCMs reported to IPCC’s AR4 are used, while the MRI and MIROC models are examined in detail. Detailed analyses of these two CGCMs reveal that the PDO is mainly affected by ocean heat transport rather than surface heat flux, in particular in the MRI model which has a larger contribution of ocean heat transport to the heat budget. It is found that the ocean heat transport due to Ekman advection versus geostrophic advection contributes differently to the PDO in the western and central North Pacific. Specifically, the strength of PDO tends to be larger for CGCMs with a larger ocean heat transport in the region.

The East Asian upper-tropospheric jet streams and associated transient eddy activities simulated by a climate system model BCC_CSM1.1

In this paper, major features of the upper-tropospheric jet streams simulated by a coupled Climate System Model BCC_CSM1.1 are evaluated through comparison with the NCEP/NCAR reanalysis. The jet streams consist of the East Asian subtropical jet (EASJ) and the East Asian polar-front jet (EAPJ). Associated stationary wave and synoptic-scale transient eddy activities (STEA) are also examined. The results show that the climatological positions of the westerly jet streams are well captured by BCC_CSM1.1, but with slight intensity biases. Statistics from the 6-h model outputs reveal that the jet core number (JCN) of ESPJ is significantly underestimated. Examination of the simulated seasonal evolution of the westerly jet stream indicates that the model has produced a westward movement of the EASJ core in May, one month earlier than that in the reanalysis. Analysis of stationary wave activities shows that the overestimated meridional wind component may have caused considerable enhancement of meridional momentum and heat transport. The stationary Rossby wave represented by the wave activity flux at the southern flank of the Tibetan Plateau is favorable to the growth of asymmetric zonal wind and the multiple-center pattern of JCN. Unlike the stationary wave heat flux transport, the model tends to systematically generate weaker transient heat flux over East Asia. Further analysis of STEA exhibits a general consistent pattern between the simulation and the reanalysis, while the intensity of the northern STEA branch associated with the EAPJ is greatly reduced. The deficiencies of eddy momentum and heat flux transport and accompanied eddy forcing may contribute to the biases of the simulated upper-tropospheric jet streams, suggesting the potential importance of midlatitude internal atmospheric dynamics in shaping the tropospheric general circulation, which is not yet fully and accurately resolved in the current BCC_CSM1.1.

Prediction of Turbulent Heat Transfer in Rotating and Nonrotating Channels With Wall Suction and Blowing

This paper reports on the prediction of heat transfer in a fully developed turbulent flow in a straight rotating channel with blowing and suction through opposite walls. The channel is rotated about its spanwise axis; a mode of rotation that amplifies the turbulent activity on one wall and suppresses it on the opposite wall leading to reverse transition at high rotation rates. The present predictions are based on the solution of the Reynolds-averaged forms of the governing equations using a second-order accurate finite-volume formulation. The effects of turbulence on momentum transport were accounted for by using a differential Reynolds-stress transport closure. A number of alternative formulations for the difficult fluctuating pressure–strain correlations term were assessed. These included a high turbulence Reynolds-number formulation that required a “wall-function” to bridge the near-wall region as well as three alternative low Reynolds-number formulations that permitted integration through the viscous sublayer, directly to the walls. The models were assessed by comparisons with experimental data for flows in channels at Reynolds-numbers spanning the range of laminar, transitional, and turbulent regimes. The turbulent heat fluxes were modeled via two very different approaches: one involved the solution of a modeled differential transport equation for each of the three heat-flux components, while in the other, the heat fluxes were obtained from an explicit algebraic model derived from tensor representation theory. The results for rotating channels with wall suction and blowing show that the algebraic model, when properly extended to incorporate the effects of rotation, yields results that are essentially identically to those obtained with the far more complex and computationally intensive heat-flux transport closure. This outcome argues in favor of incorporation of the algebraic model in industry-standard turbomachinery codes.

An Investigation of the Initial Development of the Double-ITCZ Warm SST Biases in the CCSM

Abstract This paper investigates the initial development of the double ITCZ in the Community Climate System Model version 3 (CCSM3) in the central Pacific. Starting from a resting initial condition of the ocean in January, the model developed a warm bias of sea surface temperature (SST) in the central Pacific from 5°S to 10°S in the first three months. This initial bias is caused by excessive surface shortwave radiation that is also present in the stand-alone atmospheric model. The initial bias is further amplified by biases in both surface latent heat flux and horizontal heat transport in the upper ocean. These biases are caused by the responses of surface winds to SST bias and the thermocline structure to surface wind curls. This study also showed that the warming biases in surface solar radiation and latent heat fluxes are seasonally offset by cooling biases from reduced solar radiation after the austral summer due to cloud responses and in the austral fall due to enhanced evaporation when the maximum SST is closest to the equator. The warming biases from the dynamic heat transport by ocean currents however stay throughout all seasons once they are developed, which are eventually balanced by enhanced energy exchange and penetration of solar radiation below the mixed layer. It was also shown that the equatorial cold tongue develops after the warm biases in the south-central Pacific, and the overestimation of surface shortwave radiation recurs in the austral summer in each year. The results provide a case study on the physical processes leading to the development of the double ITCZ. Applicability of the results in other models is discussed.

Modeling seasonal and diurnal pCO 2 variations in the northern South China Sea

This paper describes the simulated temporal variation of surface seawater CO 2 partial pressure ( pCO 2) in the northern South China Sea. We produced the simulations with a one-dimensional (1-D) coupled physical–biogeochemical model that had high-frequency, time-dependent atmospheric forcing and that were validated with field observations. We also examined the associated processes that modulate seawater pCO 2 at different time scales, from diurnal to seasonal, using a series of process-oriented experiments. At seasonal time scales, we revealed that the sea–air CO 2 exchange was a primary process that modulated surface pCO 2 and exceeded the role of sea surface temperature (SST) even though the phase of the pCO 2 variation generally followed the strong seasonal cycle of SST. This was because sea–air CO 2 exchange is a slow process and has an accumulative effect on surface water pCO 2 due to the buffering effect of the carbonate system once CO 2 has dissolved in the seawater, which leads to a long equilibration time of CO 2 between the atmosphere and seawater. The mixing effect on pCO 2 induced by total alkalinity and dissolved inorganic carbon variations was, generally, positively correlated with the seasonal evolution of wind speed. Biological processes were the smallest contributors to pCO 2 variations at the seasonal scale because of the oligotrophic characteristic of the region. At diurnal time scales, the dominant pCO 2 controlling factor was mainly associated with the local physical and biological conditions. Temperature and wind-induced vertical mixing played major roles in pCO 2 when the winter heat flux and upward transport of low temperature and high pCO 2 in deep water were intensified. Phytoplankton blooms generally occur after a period of strong wind, as a result, biological metabolism becomes the most important pCO 2 regulator when the surface chlorophyll-a reached its highest level. Unlike that in the seasonal scale, the effect of sea–air CO 2 exchange was minor at diurnal time scales due to the long equilibration time of CO 2 between the atmosphere and seawater. We also found that the frequency of the model driving force was important in reproducing the sea surface pCO 2. The high frequency forcing was important in controlling the pCO 2 variation through the feedback effect to the corresponding physical and biogeochemical responses.

HELIOSEISMIC RESPONSE TO THE X2.2 SOLAR FLARE OF 2011 FEBRUARY 15

The X2.2-class solar flare of February 15, 2011, produced a powerful sunquake event, representing a helioseismic response to the flare impact in the solar photosphere, which was observed with the HMI instrument on the Solar Dynamics Observatory (SDO). The impulsively excited acoustic waves formed a compact wavepacket traveling through the solar interior and appearing on the surface as expanding wave ripples. The initial flare impacts were observed in the form of compact and rapid variations of the Doppler velocity, line-of-sight magnetic field and continuum intensity. These variations formed a typical two-ribbon flare structure, and are believed to be associated with thermal and hydrodynamic effects of high-energy particles heating the lower atmosphere. The analysis of the SDO/HMI and X-ray data from the Ramaty High Energy Solar Spectroscopic Imager (RHESSI) shows that the helioseismic waves were initiated by the photospheric impact in the early impulsive phase, observed prior to the hard X-ray (50-100 keV) impulse, and were probably associated with atmospheric heating by relatively low-energy electrons (~6-50 keV) and heat flux transport. The impact caused a short motion in the sunspot penumbra prior to the appearance of the helioseismic wave. It is found that the helioseismic wave front traveling through a sunspot had a lower amplitude and was significantly delayed relative to the front traveling outside the spot. These observations open new perspectives for studying the flare photospheric impacts and for using the flare-excited waves for sunspot seismology.

The roles of surface heat flux and ocean heat transport convergence in determining Atlantic Ocean temperature variability

The temperature variability of the Atlantic Ocean is investigated using an eddy-permitting (1/4°) global ocean model (ORCA-025) forced with historical surface meteorological fields from 1958 to 2001. The simulation of volume-averaged temperature and the vertical structure of the zonally averaged temperature trends are compared with those from observations. In regions with a high number of observations, in particular above a depth of 500 m and between 22° N and 65° N, the model simulation and the dataset are in good agreement. The relative contribution of variability in ocean heat transport (OHT) convergence and net surface heat flux to changes in ocean heat content is investigated with a focus on three regions: the subpolar and subtropical gyres and the tropics. The surface heat flux plays a relatively minor role in year-to-year changes in the subpolar and subtropical regions, but in the tropical North Atlantic, its role is of similar significance to the ocean heat transport convergence. The strongest signal during the study period is a cooling of the subpolar gyre between 1970 and 1990, which subsequently reversed as the mid-latitude OHT convergence transitioned from an anomalously weak to an anomalously strong state. We also explore whether model OHT anomalies can be linked to surface flux anomalies through a Hovmöller analysis of the Atlantic sector. At low latitudes, increased ocean heat gain coincides with anomalously strong northward transport, whereas at mid-high latitudes, reduced ocean heat loss is associated with anomalously weak heat transport.

Simulation of Toroidal Rotation Effect on Heat Flux Transport in the Edge Plasma of Small Size Divertor Tokamak

The effect of toroidal rotation on heat flux transport in the edge plasma of small size divertor was simulated by B2SOLP0.5.2D transport code. The main results of simulation shows that, the following: (1) the radial heat flux is strongly influenced by toroidal rotation. (2) The amplification of conduction part of radial heat flux imposes nonresilient profile of ion temperature, under which the effect of toroidal rotation on ion temperature profile is strong. (3) The ion distribution and its gradients are lower for counter-injection neutral beam than for co-injection neutral beam. (4) Reversal of toroidal rotation during using neutral beam injection result in reverses of radial electric field and E × B drift velocity. (5) The toroidal rotation strong influence on the ion temperature scale length of the ion temperature gradient (ITG). (6) Switch on and off all drifts leads to higher change in the ion density distribution in edge plasma of small size divertor tokamak when the unbalance neutral beam injection are considered (7) the comparison between radial heat flux at different momentum input shows that, the radial ion heat flux with larger ion temperature scale length in the case of co-injection neutral beam is larger than the ion heat flux with smaller ion temperature scale length in the case of counter-injection neutral beam.

Vertical heat flux and lateral mass transport in nonlinear internal waves

Comprehensive observations of velocity, density, and turbulent dissipation permit quantification of the nonlinear internal wave (NLIW) contribution to vertical heat flux and lateral mass transport over New Jersey's shelf. The effect of NLIWs on the shelf heat budget was significant. On average, heat flux in NLIWs was 10 times larger than background at the pycnocline depth. NLIWs were present at midshelf &lt;10% of the time, yet we estimate that they contributed roughly one−half the heat flux across the pycnocline during the observation period, which was characterized by weak to moderate winds. Lateral transport distances due to the leading 3 waves in NLIW packets were typically (100 m) but ranged several kilometers. The month‐averaged daily onshore transport (per unit alongshelf dimension) by NLIWs is estimated as 0.3 m2s−1. This is comparable to a weak downwelling wind, but sustained over an entire month.

Mixed Layer Temperature Response to the Southern Annular Mode: Mechanisms and Model Representation

Abstract Previous studies have shown that simulated sea surface temperature (SST) responses to the southern annular mode (SAM) in phase 3 of the Coupled Model Intercomparison Project (CMIP3) climate models compare poorly to the observed response. The reasons behind these model inaccuracies are explored. The ocean mixed layer heat budget is examined in four of the CMIP3 models and by using observations–reanalyses. The SST response to the SAM is predominantly driven by sensible and latent heat flux and Ekman heat transport anomalies. The radiative heat fluxes play a lesser but nonnegligible role. Errors in the simulated SST responses are traced back to deficiencies in the atmospheric response to the SAM. The models exaggerate the surface wind response to the SAM leading to large unrealistic Ekman transport anomalies. During the positive phase of the SAM, this results in excessive simulated cooling in the 40°–65°S latitudes. Problems with the simulated wind stress responses, which relate partly to errors in the simulated winds themselves and partly to the transfer coefficients used in the models, are a key cause of the errors in the SST response. In the central Pacific sector (90°–150°W), errors arise because the simulated SAM is too zonally symmetric. Substantial errors in the net shortwave radiation are also found, resulting from a poor representation of the changes in cloud cover associated with the SAM. The problems in the simulated SST responses shown by this study are comparable to deficiencies previously identified in the CMIP3 multimodel mean. Therefore, it is likely that the deficiencies identified here are common to other climate models.

D211 金属多孔質体を用いた高熱流束除去デバイスの工学的課題と展望(多孔質中の熱・物質輸送現象III)

Key issues to enable heat removal exceeding 10MW/m^2 of heat flux are evaluated by summarizing experimental and numerical data ever obtained. Porous media focused on are bronze particle-sintered compacts. The experiments clarify that the heat transfer characteristics strongly depend on the level of the heat flux input and suggest that the heat transport by capillary effect works effectively under several MW/m^2. However, under the conditions of over the maximum heat flux transport by the capillary effect, the permeability for the vapor discharge becomes the most important factor to make it possible to remove the extremely high heat flux. In order to evaluate the effect of porous material, the two-phase flow characteristics are simulated by the two-phase mixture model. The results show that applying higher thermal-conductivity matrix leads to delay of onset of the two-phase formation and to much higher cooling performance even at higher liquid saturation.