Global Heat Flux Research Articles

Rayleigh-Bénard Convection In Molten Salt At Elevated Temperatures

The velocity field of turbulent Rayleigh-Bénard convection is analyzed by means of 2D2C particle image velocimetry (PIV). In order to investigate the influence of thermal radiation to the global heat flux hot molten salt with temperatures between 160 °C and 250 °C is used as working fluid. We report on the construction of the convection cell, the application of the PIV measurement technique and show first results of the velocity fields at two different Rayleigh numbers Ra = 1.7 x 10^7 and Ra = 1.3 x 10^8 .

Influence of PCMs on thermal performance of wet fire protective clothing

The inclusion of phase change materials (PCMs) in firefighting protective clothing (FPC) has been previously shown to be beneficial during the exposure stage, but detrimental in the post-exposure, mainly due to PCM latent heat release towards the skin, after the exposure. It is unclear whether that is also the case for when the FPC is wet. Hence, in this study, a one-dimensional numerical approach is used to study the effect of PCM parameters (PCM mass, melting temperature) and global heat flux, on the thermal performance of a wet fire protective clothing. It is concluded that under wet conditions, the PCM amount, and its phase change temperature have a significant effect on thermal performance, depending on the heat exposure scenario considered. Great benefits are observed in introducing PCMs to prevent water condensation near the skin, but at the cost of greater PCM re-solidification skin damage.

Multimodel ensemble estimation of Landsat-like global terrestrial latent heat flux using a generalized deep CNN-LSTM integration algorithm

Accurate estimates of high-spatial-resolution global terrestrial latent heat flux (LE) from Landsat data are crucial for many basic and applied research. Yet current Landsat-derived LE products were developed using single algorithm with large uncertainties and discrepancies. Here we proposed a convolutional neural network-long short-term memory (CNN-LSTM)-based integrated LE (CNN-LSTM-ILE) framework that integrates five Landsat-derived physical LE algorithms, topography-related variables (elevation, slope and aspect) and eddy covariance (EC) observations to estimate 30-m global terrestrial LE. CNN-LSTM-ILE not only conserves good performance of LE estimation from pure deep learning (DL) algorithm, but partially inherits physical mechanism of the individual physical algorithms for improving the generalization of the integration algorithms for extreme cases. CNN-LSTM is an algorithm that combines two deep learning structures (CNN and LSTM) to effectively utilize the spatial and temporal information contained in the forcing inputs. The data were collected from 190 globally distributed EC observations from 2000 to 2015 and were provided by FLUXNET. The cross-validation results indicated that the CNN-LSTM integration algorithm improved the LE estimates by reducing the root mean square error (RMSE) of 5–8 W/m2 and increasing Kling-Gupta efficiency (KGE) of 0.05–0.16 when compared with the individual LE algorithms and the results of three other machine learning integration algorithms (multiple linear regression, MLR; random forest, RF; and deep neural networks, DNN). The CNN-LSTM integration algorithm had highest KGE (0.81) and R2 (0.66) compared to ground-measured and was applied to generate the Landsat-like regional and global terrestrial LE. An innovation of our strategy is that the CNN-LSTM-ILE model integrates pixel proximity effects and daily LE variations to enhance the accuracy of 16-day LE estimations. This approach can produce a more reliable Landsat-like global terrestrial LE product to improve the representativeness of heterogeneous regions for monitoring hydrological variables.

Unifying heat transport model for the transition between buoyancy-dominated and Lorentz-force-dominated regimes in quasistatic magnetoconvection

In magnetoconvection, the flow of an electromagnetically conductive fluid is driven by a combination of buoyancy forces, which create the fluid motion due to thermal expansion and contraction, and Lorentz forces, which distort the convective flow structure in the presence of a magnetic field. The differences in the global flow structures in the buoyancy-dominated and Lorentz-force-dominated regimes lead to different heat transport properties in these regimes, reflected in distinct dimensionless scaling relations of the global heat flux (Nusselt number $Nu$ ) versus the strength of buoyancy (Rayleigh number $Ra$ ) and electromagnetic forces (Hartmann number $Ha$ ). Here, we propose a theoretical model for the transition between these two regimes for the case of a static vertical magnetic field applied across a convective fluid layer confined between two isothermal, a lower warmer and an upper colder, horizontal surfaces. The model suggests that the scaling exponents $\gamma$ in the buoyancy-dominated regime, $Nu\sim Ra ^\gamma$ , and $\xi$ in the Lorentz-force-dominated regime, $Nu\sim (Ha^{-2}Ra)^\xi$ , are related as $\xi =\gamma /(1-2\gamma )$ , and the onset of the transition scales with $Ha^{-1/\gamma }Ra$ . These theoretical results are supported by our direct numerical simulations for $10\leq Ha\leq 2000$ , Prandtl number $Pr=0.025$ and $Ra$ up to $10^9$ and data from the literature.

Time-resolved heat flux determination from fast-responding temperature-sensitive paint measurement in a shock tunnel using transient in-situ calibration

Measurement of the global heat flux using fast-responding temperature-sensitive paint (fast TSP) in a shock tunnel is crucial to understanding complex aerothermal effects, but its accuracy is limited by the simplified inverse algorithm as well as the uncertainty in the thermal properties and the thickness of the TSP. In the present work, an in-situ transient calibration method is proposed to determine important parameters in one-dimensional double-layer semi-infinite heat conduction for TSP measurement via heat flux gauges. A fast-converging iterative scheme based on the Levenberg–Marquardt algorithm is used to search for the optimal parameters that minimize the difference between the transient experimental and simulated temperature histories. Furthermore, the heat flux history is determined using the impulse method according to the calibrated parameters. A fast-TSP measurement of 34° ramp flow was conducted at 75 kHz in a Mach-12.1 shock tunnel to validate the proposed method. The results show that the heat flux histories obtained from the TSP and the heat flux gauge results are in good agreement throughout the test period of 11 ms. Moreover, the use of multiple gauges in the calibration further improves the overall consistency of the heat flux amplitude. The in-situ transient calibration method effectively improves the accuracy of the measurement of time-resolved heat flux with relatively low hardware and computational costs.

Global impacts of vegetation clumping on regulating land surface heat fluxes

The clumping index (CI) quantifies the non-random distribution of vegetation across space, which regulates the canopy radiative transfer processes and land surface carbon, water, and energy cycles. However, its impact on global surface energy budget, particularly sensible heat fluxes and surface temperature, is not well understood. Additionally, while there have been studies showing significant seasonal variations in CI, the impacts of these variations on surface energy fluxes remain unclear. In this study, we incorporated satellite-derived spatially and temporally explicit CI data into the Community Land Model version 5 (CLM5) to evaluate the effects of CI on global land energy fluxes. Our results showed that including CI increased the global mean sensible heat flux dissipated from ground by 3.9 W m−2 (∼18%), while decreasing the global mean vegetation sensible heat flux by 4.9 W m−2 (∼65%), resulting in a total sensible heat decrease of 1.0 W m−2 (∼3%). In contrast, CI increased the global mean latent heat flux by 0.8 W m−2 (∼2%), primarily due to increased evapotranspiration (up to 11 W m−2) in tropical regions. We also found considerable impacts of seasonal variations in CI, particularly on sensible heat fluxes from ground and vegetation in evergreen needleleaf forests and deciduous needleleaf forests. Using constant CI rather than considering seasonal variations resulted in significant overestimation and underestimation of the sensible heat fluxes from vegetation and ground, respectively, in boreal summer. These changes in surface energy fluxes caused by CI and its seasonal variations led to up to 1.7 and 0.5 K differences in simulated mean ground temperature. These findings highlight the importance of including CI and considering its seasonal variations in modeling land surface energy fluxes.

On the coupling between direct numerical simulation of nucleate boiling and a micro-region model at the contact line

During the past years, numerical simulations of Nucleate Boiling have attracted increasing attention. If significant advances have already been performed on the conception of accurate numerical schemes accounting for jump conditions at a non-isothermal interface, further studies are required on the modeling of the heat flux in the vicinity of the contact line. Especially, at very low scale (<500 nm) the formation of a micro-region can be assumed, since the observed apparent contact angle can be higher than the microscopic contact angle and depend on the wall superheat. This variation of the apparent contact angle can be related to a temperature discontinuity between the saturation temperature and the local wall temperature at the contact line inducing a micro-region heat flux and an evaporation mass flow rate. Whereas this micro-region acts at very low scale, existing theoretical models suggest a strong influence of the micro-region heat flux on the global heat flux, and so on the overall bubble growth phenomenon. However, the numerical description of this micro-region in the framework of Direct Numerical Simulation (DNS) of nucleate boiling is a major issue due to the large range of spatial scales involved in the whole process. In the present paper, we propose a coupling methodology between an existing micro-region theoretical model and a larger scale DNS solver for Nucleate Boiling in axisymmetric configurations. Several benchmarks are proposed to show the consistency and the accuracy of the proposed method and to highlight the quantitative effect of the micro-region at the bubble scale.

Effect of isothermal rough boundaries on the statistics of velocity and temperature fluctuations in turbulent Rayleigh–Bénard convection

Using direct numerical simulations, we investigate the effect of surface roughness on the statistics of fluctuations in a 2D rectangular cell of aspect ratio Γ = 2 with air as the working fluid. We consider roughly two decades of Rayleigh number, 108≤Ra≤4.64×109, with three roughness configurations of R1, R2, and R3 characterized by their maximum heights of 5%, 10%, and 20% of the cell height, respectively. We show that roughened cells trigger stronger fluctuations, which further gets augmented with increasing Ra. Vertical variations of velocity and temperature fluctuations show different trends. While the temperature fluctuation becomes homogeneous in the bulk, it exhibits strong inhomogeneous vertical velocity fluctuations. The comparison of global heat flux with smooth case shows a significant increment beyond Ra=2.15×108. Surface roughness impacts local heat flux through augmented plumes, which is qualitatively ascertained by instantaneous temperature field. Furthermore, probability distribution functions reveal no particular trend for the taller roughness configurations, though the magnitude is amplified. Through identification of plumes and background regions, we show their behavior as a function of Ra for different rough cases. Finally, we decompose the shear production into its three components (based on the nature of mechanical forces) to understand the energy interaction between the mean flow and fluctuating flow field.

Boundary-Layer Disruption and Heat-Transfer Enhancement in Convection Turbulence by Oscillating Deformations of Boundary.

In this Letter, we propose a novel strategy for significantly enhancing the heat transfer in convection turbulence. By introducing a boundary deformation of the standing-wave type, flow modulation can be realized when the amplitude is comparable or larger than the boundary-layer thickness. For a fixed moderate frequency, the entire fluid layer follows the boundary motion at small wave numbers, while only the near-wall regions are affected by the boundary deformation at large wave numbers. The heat-flux enhancement happens for the latter. For a fixed wave number and gradually increasing frequency, the vortical flows inside the wave valleys exhibit nonlinear transition and alter the distribution of boundary heat flux, and the global heat flux increases significantly at large enough frequencies. The current findings suggest that oscillating deformations of boundary can efficiently break the boundary layers, which serves as the bottleneck of global heat transfer, and open a new venue for modulating the convection turbulence.

Experimental Investigation of a High-Speed Turbine With Rainbow Rotor and Rim Seal Purge Flow

AbstractThe present paper addresses the experimental and numerical study of the unsteady flow established in a high-pressure turbine stage with rim seal purge. The HPT test section, operated at engine-relevant flow conditions in the high-speed turbine rig of the von Karman Institute, is heavily instrumented for high-resolution, high-bandwidth aerothermal measurements. A rainbow rotor setup allows the simultaneous testing of six different sectors, each hosting a specific tip and platform geometry optimized for enhanced aerodynamic performance. This paper focuses on the flow over the baseline sector, equipped with an axisymmetric hub platform and a squealer tip, with a purge flowrate matching 1.74% and 1% of the stage mass flow. The numerical study relies on Reynolds-averaged Navier–Stokes (RANS) computations with test-calibrated boundary conditions. Unsteady pressure and heat transfer measurements are performed at the rotor shroud. The maximum heat transfer is achieved along the front pressure side rim, whereas the squealer cavity generates a region of uniformly low static pressure and heat flux. The experimental adiabatic wall temperature is derived to quantify the thermal contribution to the global heat flux, demonstrating an increase in over-tip gas temperature up to 1.2 T01 above the pressure side rim. A Euler-based model is proposed to evaluate the temperature-driving work-exchange mechanism in the tip gap. The peak in casing heat transfer coefficient (750 W/m2K) is found at the tip leading edge. Time-resolved measurements of outlet total pressure, Mach number, and flow angle confirm the predicted phase and intensity of the tip leakage and upper passage vortex in the near-casing region. At 20% hr, the total pressure minimum is highly underpredicted by the RANS, indicating an inaccurate modeling of the interaction between rim seal purge and main flow. The measured impact of the purge flow variation is more significant than predicted by the RANS computations, with a negative offset of about 0.5% P01 in the lower 50% hr at lower purge flowrate.

Lifetime-based phosphor thermometry for global heat flux measurement in a hypersonic rarefied tunnel under strong background radiation

Lifetime-based phosphor thermometry using Mg4FGeO6:Mn is successfully implemented on a standard hypervelocity ballistic model 2 (HB-2 model) and an HB-2 model modified with tail fin in a hypersonic rarefied tunnel at a Mach number of 10. A novel Reynolds decomposition correction method is proposed to remove the heating device-induced strong background radiation, which is characterized by complex spatial and temporal distributions. Three methods are compared for heat flux calculation: an insitu calibration method, the Cook–Felderman method, and a double-layer numerical method. The in situ calibration method exhibits the least relative error (9.47 %), outperforming the other two methods. The global heat flux distribution at a high spatial resolution (0.25 mm/pixel) reveals the fine flow features of the models. Particularly, the modified HB-2 model exhibits a complex flow, related to the shock-boundary layer interaction, shock–shock interaction, and significant 3D effects. The presented measurement technique and data processing method can enable long-duration advanced aerothermal testing in hypersonic tunnels involving a large temperature rise.

A new bootstrap technique to quantify uncertainty in estimates of ground surface temperature and ground heat flux histories from geothermal data

Abstract. Estimates of the past thermal state of the land surface are crucial to assess the magnitude of current anthropogenic climate change as well as to assess the ability of Earth System Models (ESMs) to forecast the evolution of the climate near the ground, which is not included in standard meteorological records. Subsurface temperature reacts to long-term changes in surface energy balance – from decadal to millennial time scales – thus constituting an important record of the dynamics of the climate system that contributes, with low-frequency information, to proxy-based paleoclimatic reconstructions. Broadly used techniques to retrieve past temperature and heat flux histories from subsurface temperature profiles based on a singular value decomposition (SVD) algorithm were able to provide robust global estimates for the last millennium, but the approaches used to derive the corresponding 95 % confidence interval were wrong from a statistical point of view in addition to being difficult to interpret. To alleviate the lack of a meaningful framework for estimating uncertainties in past temperature and heat flux histories at regional and global scales, we combine a new bootstrapping sampling strategy with the broadly used SVD algorithm and assess its performance against the original SVD technique and another technique based on generating perturbed parameter ensembles of inversions. The new bootstrap approach is able to reproduce the prescribed surface temperature series used to derive an artificial profile. Bootstrap results are also in agreement with the global mean surface temperature history and the global mean heat flux history retrieved in previous studies. Furthermore, the new bootstrap technique provides a meaningful uncertainty range for the inversion of large sets of subsurface temperature profiles. We suggest the use of this new approach particularly for aggregating results from a number of individual profiles, and to this end, we release the programs used to derive all inversions in this study as a suite of codes labeled CIBOR v1: Codes for Inverting BORholes, version 1.

Discovery of active off-axis hydrothermal vents at 9° 54′N East Pacific Rise

Comprehensive knowledge of the distribution of active hydrothermal vent fields along midocean ridges is essential to understanding global chemical and heat fluxes and endemic faunal distributions. However, current knowledge is biased by a historical preference for on-axis surveys. A scarcity of high-resolution bathymetric surveys in off-axis regions limits vent identification, which implies that the number of vents may be underestimated. Here, we present the discovery of an active, high-temperature, off-axis hydrothermal field on a fast-spreading ridge. The vent field is located 750 m east of the East Pacific Rise axis and ∼7 km north of on-axis vents at 9° 50'N, which are situated in a 50- to 100-m-wide trough. This site is currently the largest vent field known on the East Pacific Rise between 9 and 10° N. Its proximity to a normal fault suggests that hydrothermal fluid pathways are tectonically controlled. Geochemical evidence reveals deep fluid circulation to depths only 160 m above the axial magma lens. Relative to on-axis vents at 9° 50'N, these off-axis fluids attain higher temperatures and pressures. This tectonically controlled vent field may therefore exhibit greater stability in fluid composition, in contrast to more dynamic, dike-controlled, on-axis vents. The location of this site indicates that high-temperature convective circulation cells extend to greater distances off axis than previously realized. Thorough high-resolution mapping is necessary to understand the distribution, frequency, and physical controls on active off-axis vent fields so that their contribution to global heat and chemical fluxes and role in metacommunity dynamics can be determined.

Formation of glacier tables caused by differential ice melting: field observation and modelling

Abstract. Glacier tables are structures frequently encountered on temperate glaciers. They consist of a rock supported by a narrow ice foot which forms through differential melting of the ice. In this article, we investigate their formation by following their dynamics on the Mer de Glace (the Alps, France). We report field measurements of four specific glacier tables over the course of several days, as well as snapshot measurements of a field of 80 tables performed on a given day. We develop a simple model accounting for the various mechanisms of the heat transfer on the glacier using local meteorological data, which displays a quantitative agreement with the field measurements. We show that the formation of glacier tables is controlled by the global heat flux received by the rocks, which causes the ice underneath to melt at a rate proportional to the one of the surrounding ice. Under large rocks the ice ablation rate is reduced compared to bare ice, leading to the formation of glacier tables. This thermal insulation effect is due to the warmer surface temperature of rocks compared to the ice, which affects the net long-wave and turbulent fluxes. While the short-wave radiation, which is the main source of heat, is slightly more absorbed by the rocks than the ice, it plays an indirect role in the insulation by inducing a thermal gradient across the rocks which warms them. Under a critical size, however, rocks can enhance ice melting and consequently sink into the ice surface. This happens when the insulation effect is too weak to compensate for a geometrical amplification effect: the external heat fluxes are received on a larger surface than the contact area with the ice. We identified the main parameters controlling the ability of a rock to form a glacier table: the rock thickness, its aspect ratio, and the ratio between the averaged turbulent and short-wave heat fluxes.

Off-centre gravity induces large-scale flow patterns in spherical Rayleigh–Bénard convection

We perform direct numerical simulations to study the effect of the gravity centre offset in spherical Rayleigh–Bénard convection. When the gravity centre is shifted towards the south, we find that the shift of the gravity centre has a pronounced influence on the flow structures. At low Rayleigh number $Ra$ , a steady-state large-scale meridional circulation induced by the baroclinic imbalance, created by the misalignment of the gravity potentials and isotherms, is formed. At high $Ra$ , an energetic jet is created on the northern side of the inner sphere that is directed towards the outer sphere. The large-scale circulation induces a strong co-latitudinal dependence in the local heat flux. Nevertheless, the global heat flux is not affected by the changes in the large-scale flow organization induced by the gravity centre offset.

Surface Divergent Eddy Heat Fluxes and Their Impacts on Mixed Layer Eddy‐Mean Flow Interactions

AbstractIn this study, the global surface divergent eddy heat flux (EHF) is estimated using remote sensing observations of sea surface height (SSH) and sea surface temperature (SST) over two decades (1993–2017). These results are used as a metric to assess model fidelity in a mesoscale eddy‐resolving version of the Community Earth System Model. The estimated EHFs show that the midlatitudes significantly contribute to the poleward transport of heat because of strong regional variability in SSH and SST. A Helmholtz decomposition is performed on the global EHFs to remove prominent nondynamic rotational wave‐like structures that appear in these fluxes. The dynamic divergent EHFs are responsible for driving cross‐frontal exchange and positive Baroclinic Conversion (BC) rates, suggesting a conversion of mean potential energy to eddy potential energy within the mixed layer. The results show that the model captures the same spatial patterns of EHFs and BC rates, but with relatively higher values in the midlatitudes than observations. The mixed layer geostrophic meridional eddy heat transport reaches maximums of 0.07 PW and 0.1 PW in the midlatitudes in the observations and climate model, respectively. The global integrated BC rate is 0.11 TW in the observations, which is ∼30% weaker than the climate model (0.16 TW). A cross‐spectral analysis further shows that the model has higher energy in low‐frequency bands for periods greater than 10 months in the northern hemisphere western boundary currents, but can capture the major spectral peaks in EHFs that are seen in observations.

Recent increase in the observation-derived land evapotranspiration due to global warming

Estimates of change in global land evapotranspiration (ET) are necessary for understanding the terrestrial hydrological cycle under changing environments. However, large uncertainties still exist in our estimates, mostly related to the uncertainties in upscaling in situ observations to large scale under non-stationary surface conditions. Here, we use machine learning models, artificial neural network and random forest informed by ground observations and atmospheric boundary layer theory, to retrieve consistent global long-term latent heat flux (ET in energy units) and sensible heat flux over recent decades. This study demonstrates that recent global land ET has increased significantly and that the main driver for the increased ET is increasing temperature. Moreover, the results suggest that the increasing ET is mostly in humid regions such as the tropics. These observation-driven findings are consistent with the idea that ET would increase with climate warming. Our study has important implications in providing constraints for ET and in understanding terrestrial water cycles in changing environments.

Особенности формирования и эволюции полярного циклона в Баренцевом и Карском морях в декабре 2020 года

Based on the GDAS (Global Data Assimilation System) information, the conditions for the formation and evolution of a polar cyclone in the Barents Sea in December 2020 are studied. The cyclone in interesting because after entering the Kara Sea, the cyclone did not weaken as usual but continued to intensify. The formation of the cyclone occurred near the ice edge under conditions of the cold Arctic air outbreak. After entering the Kara Sea, the cyclone continued to deepen, which was facilitated by latent and sensible heat fluxes in the ice-free western part of the sea. According to weather station data, when the cyclone moved further over land, the wind direction changed from mainly southern to northern, and wind speed increased significantly. There also was a sharp decrease in air temperature by 7–10 °С for three hours. Keywords: polar cyclone, global data assimilation system, wind speed, latent and sensible heat fluxes

A Green’s function approach to heat-flux estimation from temperature-sensitive paint measurements

A method for estimating global heat flux from measurements of surface temperature using temperature-sensitive paint (TSP) in hypersonic wind tunnels is introduced. Exact solutions to the one-dimensional heat-conduction equation for two- and three-layer solids of finite depth are obtained using a Green’s function (GF) formulation. GFs describing both the temperature change anywhere within the solid and the mean temperature across the TSP form the basis of an inverse heat-conduction problem, which is then solved using regularized deconvolution to mitigate amplification of measurement noise. The disparity between apparent and true TSP temperatures, arising from the integrated emission intensity across the paint, is minimized by means of an iterative routine considering the statistics of a virtual temperature profile. Numerical simulations of heat conduction in various scenarios are then conducted to verify the validity of the method, while its effectiveness is assessed in the analysis of exemplary wind tunnel experiments.

A globally fragmented and mobile lithosphere on Venus

Venus has been thought to possess a globally continuous lithosphere, in contrast to the mosaic of mobile tectonic plates that characterizes Earth. However, the Venus surface has been extensively deformed, and convection of the underlying mantle, possibly acting in concert with a low-strength lower crust, has been suggested as a source of some surface horizontal strains. The extent of surface mobility on Venus driven by mantle convection, however, and the style and scale of its tectonic expression have been unclear. We report a globally distributed set of crustal blocks in the Venus lowlands that show evidence for having rotated and/or moved laterally relative to one another, akin to jostling pack ice. At least some of this deformation on Venus postdates the emplacement of the locally youngest plains materials. Lithospheric stresses calculated from interior viscous flow models consistent with long-wavelength gravity and topography are sufficient to drive brittle failure in the upper Venus crust in all areas where these blocks are present, confirming that interior convective motion can provide a mechanism for driving deformation at the surface. The limited but widespread lithospheric mobility of Venus, in marked contrast to the tectonic styles indicative of a static lithosphere on Mercury, the Moon, and Mars, may offer parallels to interior-surface coupling on the early Earth, when global heat flux was substantially higher, and the lithosphere generally thinner, than today.