Heat Flux Reduction Research Articles

Power exhaust and core-divertor compatibility of the baffled snowflake divertor in TCV

A baffled snowflake minus low-field side (SF-LFS) is geometrically-optimised in tokamak à configuration variable, increasing divertor neutral pressure, to evaluate the roles of divertor closure (comparing with an unbaffled SF-LFS) and magnetic geometry (comparing with a baffled single null (SN)) in power exhaust and core-divertor compatibility. Ohmically-heated L-mode discharges in deuterium, with a line-averaged core density of approximately m−3, are seeded with nitrogen to approach detached conditions. Baffles in the SF-LFS configuration are found to reduce the peak outer target heat flux by up to , without significantly affecting the location of the inter-null radiation region or the core-divertor compatibility. When compared to the baffled SN, the baffled SF-LFS exhibits a reduction in the outer target heat flux by up to and the ability to balance the strike-point distribution of heat flux. These benefits are less significant with N2 seeding, with similar peak target quantities (such as heat flux, electron temperature and ion flux) and divertor radiated power. Despite a radiating region located farther from the confined plasma for the SF-LFS than the baffled SN, no change in core confinement is observed. Core effective charge even indicates an increase in core impurity penetration for the SF-LFS. These experiments constitute a good reference for detailed model validations and extrapolations, exploring important physics such as core impurity shielding and the dependence of divertor cross-field transport on magnetic geometry.

The Effect of the Welding Technology on the Thermal Performance of Welded Finned Tubes Used in Heat Exchangers

Due to the use of welded finned tubes in heat exchangers, gas blocks are characterized by very good electric energy production performance, and their reliability can reach 98%. Finned tubes used in heat exchangers are usually welded. Such tubes should be characterised by the following properties: high thermal performance, good resistance to high-temperature corrosion in a flue gas atmosphere, and appropriate joint strength and hardness. The most popular technology for welding finned tubes is MAG. MAG-welded joints are characterised by considerable spattering, weld discontinuities (up to 60%), and non-axial fin alignment. Such irregularities result in a considerably reduced heat flux due to the heat transfer resistance occurring in the welded joint. It has been shown that with weld discontinuities of 20%, the thermal performance of welded finned tubes reduces considerably. This paper proposes the possibility of welding finned tubes with a laser. The thermal properties of laser-welded finned tubes were compared with those of MAG-welded tubes. It was found that the thermal performance of welded finned tubes was three times higher than that of plain tubes. However, the thermal performance was not found to be affected by the welding technology. The correct heat flow at the level of 95% occurs in the tube/fin joint even with joint penetration of 0.01 mm, however, the absence of metallic continuity in a joint results in a drastically reduced thermal performance (by 50%) and the overheating of fins, which affects their durability.

Cooling effect analysis on para-ortho hydrogen conversion coupled in vapor-cooled shield

The conversion process from parahydrogen to orthohydrogen accompanies an endothermic effect. Embedment of a para-ortho hydrogen converter into the thermal insulation could enhance the thermal protection of a liquid hydrogen storage tank. A physical model was proposed to simulate the heat transfer behavior of the insulation structure that integrates a polyurethane foam, a blanket of multilayer insulation, a vapor-cooled shield, and a para-ortho hydrogen converter. The effect of the para-ortho conversion process was considered. The model was validated by experimental data and then used to investigate how the para-ortho hydrogen conversion influences the temperature distribution inside the composite insulation. It was found that a single converter improves the cooling performance most effectively if it is placed at the middle length of the venting pipe mounted on the vapor-cooled shield. Either incorporating more converters or extending the length of the vapor-cooled shield pipes brings limited further improvement. The optimum position of the vapor-cooled shield inside the multilayer insulation moves towards the cold boundary in the presence of para-ortho conversion, compared to conventional vapor-cooled shield and multilayer insulation structures. A net heat flux reduction of over 10% could be achieved when the para-ortho conversion is located at the optimal position inside the vapor-cooled shield.

Spray and dynamic heat transfer induced by 2-methylfuran transient spray impingement on piston surface

In the face of increasing air pollution and stricter emission requirements, it is very important to reduce particulate matter emissions in engine exhaust. The presence of the attached liquid film in gasoline direct injection engines produced by spray impingement is an important reason for the increase in engine particulate emissions. The dynamic heat transfer induced by the spray impingement is a key factor affecting the evaporation and existence time of liquid film. In this paper, a single-hole injector was used to carry out 2-methylfuran (MF) spray impingement experiments. The macroscopic morphological characteristics and the dynamic heat transfer of transient spray impingement under different conditions were systematically studied. Spray particle size distribution and spray impact force were also measured to assist in explaining the underlying mechanism responsible for the dynamic heat transfer. The results show that the surface heat flux increases with the rise of initial piston surface temperature. Increasing the injection pressure makes the droplets have a stronger impact force and smaller average diameter, which enhances the heat flux and also produces more bounce and splash of droplets on piston surface. Both the number of small-size droplets and impact force are reduced by the intense evaporation caused by increasing the injection temperature, resulting in the decrease of heat flux and surface temperature reduction. Increasing the distance decreases the droplet diameter and impacting force, as a result, the heat transfer intensity is weakened.

On the role of mode resonances in regulating zonal-flow-moderated plasma microturbulence

The onset of turbulent heat transport at a higher temperature gradient than the critical gradient of linear instability, known as the Dimits shift, is a recurring feature of nonlinear simulations for magnetically confined fusion plasmas. Resonance in the nonlinear coupling between the modes that dominate energy transfer can lead to suppression of turbulence and transport above the linear critical gradient. As an expression of this resonance, gyrokinetic simulations show a quasi-coherent interaction between streamers and sidebands coupled through the zonal flow within the Dimits regime. This mechanism is further confirmed by use of artificial complex frequencies which break the resonance. By incorporating corresponding saturation physics, the standard quasilinear model for rapid head flux prediction is improved, which can now predict reduced heat flux in the Dimits regime. In particular, the triplet correlation time, the lifetime of the nonlinear interaction, is shown to be well-approximated by combinations of linear eigenvalues, and yields good representations of the heat flux variation both in and above the Dimits regime. Thus, a reduced but predictive model for transport near the critical gradient of zonal-flow saturated turbulence now exists.

Study on hypersonic drag and heat reduction of a spiked blunt body with multiple flat aerodisks and rear opposing jets

The performance of a proposed drag and heat reduction scheme that combines multiple flat aerodisks and the rear opposing jet is numerically investigated using compressible Reynolds Averaged Navier-Stokes (RANS) equations coupled with the shear stress transport (SST) turbulence model. Results show that the introduction of the multiple flat aerodisks and the rear opposing jet push the reattachment shock wave away from the blunt body and reduce the intensity of reattachment shock wave as well. Compared with the configuration with only a spike and a conical aerodisk, the present configuration shows a reduction in peak pressure and peak heat flux of the blunt body surface by 70.21% and 84.84%, respectively. Furthermore, the impact of configuration parameters (i.e., jet pressure ratio, spike length, aerodisks diameter, number, and position) on the flow field, drag and heat reduction performance are also investigated. The results reveal that increasing the jet pressure ratio can enhance the performance of drag and heat reduction. Increasing the length of the spike can get a better drag reduction performance. Increasing the diameter of flat aerodisk Ⅲ significantly enhances the compression of hypersonic free stream to reduce the intensity of reattachment shock wave. The configuration with four flat aerodisks has better drag and heat reduction performance. Compared with the baseline configuration, the maximum reduction of peak pressure and peak heat load of the blunt body surface can be achieved by moving the flat aerodisk Ⅱ forward.

Investigation of drag and heat flux reduction induced by a novel combinational spike-aerodisk and channel concept for hypersonic blunt body

For the drag and heat flux reduction of hypersonic blunt body, a novel combinational spike-aerodisk and channel concept has been proposed. The high pressure air behind the bow shock flows into the channel at the head of the aerodisk, and then sprays out through the lateral jet in the middle of the spike. The Reynolds-averaged Navier-Stokes (RANS) equations coupled with the shear stress transport (SST) k - ω turbulence model have been employed to simulate the flow field. The numerical results reveal that compared with the spike-aerodisk, with the application of the channel, the lateral jet pushes the separated shock wave away from the spike. The range of the recirculation zone between the shear layer and the spike is increased, and the intensity of the reattached shock wave is significantly weakened, so the drag and heat flux reduction properties of the combinational configuration are significantly improved. The influences of the channel convergent half angle and lateral jet location on the flow field, drag and heat flux reduction properties are investigated thoroughly. When the channel convergent half angle is appropriate and the location of the lateral jet is close to the middle of the spike, the drag and heat flux reduction effect becomes better. Besides, with the increase of the Mach number and the decrease of the flying height, the drag and heat flux reduction performance become more significant. In the research range, the combinational configuration in which the convergent half angle is 60° and the lateral jet is located in the middle of the spike has a better overall effect of drag and heat flux reduction, and the effect reaches the best when the Mach number is 6 and the flying height is 20 km. Compared with the spike-aerodisk, the total drag coefficient of the configuration is reduced by 12.95%, and the peak value of Stanton number along the blunt body surface is reduced by 35.19%.

AEROTHERMODYNAMICS OF COMBINED SPIKE AND COUNTERFLOW JET TECHNIQUE FOR REACTING HYPERSONIC FLOWS

Computational investigation is carried out to estimate the drag force and surface heating load for hypersonic reacting flows. An in-house viscous nonequilibrium finite volume-based reacting gas solver has been utilized. This solver is capable of investigating 11 chemical elementary reactions and temperature-dependent specific properties to reveal the effect of lower as well as higher freestream stagnation enthalpy conditions. Initially, the calorically perfect-gas and real-gas model-based simulations are carried out to understand the real-gas effects in the presence of a metallic spike. Computed surface pressure and heat flux are compared for the freestream stagnation enthalpy of 2 MJ/kg. The real-gas model predicts a 5% higher drag and 57.21 kW/m<sup>2</sup> higher peak heat flux compared to the perfect-gas model. However, lower enthalpy conditions predict almost the same drag force for any spike length. Further, a counterflowing jet is installed at the root of the spike, and flow field alterations are studied for this proposed integrated configuration. The root jet further pushes the conical shock in the upstream direction and provides an extra-large recirculation zone. Here, the possibility of drag and surface heat flux reduction is very much evident due to the decrease in surface pressure and presence of low-temperature jet gas in the vicinity of the object. Various freestream stagnation enthalpies, as well as the jet pressures, are considered to investigate the performance alterations by the combination technique. It is observed that the drag and surface heat load reduction efficiency of the combined configuration decreases with an increase in the freestream stagnation enthalpy. Moreover, it increases when increasing the root jet pressure for given enthalpy conditions. Hence, instead of attaching a long spike at the stagnation region of a blunt-shaped object, the use of a short spike and low-pressure root jet is recommended for a better reduction in drag and surface heat load.

Film cooling comparison of shaped holes among the pressure surface, the suction surface and the leading edge of turbine vane

The present study employed commercial computational fluid dynamics software ANSYS 2019R3 to explore the adiabatic film cooling effectiveness and the net heat flux reduction (NHFR) for the comparison of the five selected shaped holes and conventional cylindrical holes between the pressure surface, the suction surface and the leading edge. Amo4ng the shape parameters of shaped holes, the lateral divergence angle (β) and the forward divergence angle (δ) were fixed as 12° and 7° in all shaped holes structures, respectively. The others varied with different regions of the turbine vane. Results showed that different holes fit different positions of vanes. On the suction surface, laidback holes performed the worst net heat flux reduction in most blowing ratios conditions, which indicated the forward divergence angle was not conducive to the flow field and heat transfer characteristics on the suction surface. Whereas, the lateral divergence angle was beneficial to the film cooling and heat transfer characteristics. Laidback fan-shaped holes performed the best adiabatic film cooling effectiveness, but once simultaneously thinking about the heat transfer, fan-shaped holes performed better in net heat flux reduction due to less vortices at holes exit. On the leading edge, the divergence angle towards the upper wall (the lateral divergence angle of spanwise expansion holes) of vanes was not conducive to steady flow. And conical holes performed best, which indicated that coolant from holes with axial divergence angles (the lateral divergence angle in axial direction of conical holes) under the influence of mainstream impact could perform higher film cooling effectiveness and more stable flow fields. On the pressure surface, holes had a lateral divergence angle in the direction of vane height, which was conducive to increasing the coolant coverage area and improving the ability to attach to the pressure surface. Additionally, the laidback hole case was observed the lowest net heat flux reduction when the blowing ratio was less than 2, which revealed that holes that expanded only in flow direction was not conducive to film cooling and heat transfer characteristics.

Investigation of reacting fuel jets in hot vitiated crossflow

Near-wall reacting jets can occur in different engineering applications, such as rocket combustion chambers and in gas turbine combustors and turbines. This work presents results of experimental investigations of reacting gaseous fuel jets (hydrogen, methane, propane) injected normally into a hot oxygen-rich cross-flow using OH laser-induced fluorescence and OH* chemiluminescence diagnostics. RANS CFD simulations with EDC combustion model are performed for comparison. To determine the surface heat fluxes, the inverse heat conduction method was applied using measured wall temperature data as input. It was observed, that hydrogen jets ignite directly at the injection location and caused a higher surface heat flux than the hydrocarbon jets, where jet-like diffusion flames emerged. With the latter fuels, a significant ignition delay length was observed. Operation with methane fuel showed the least stability, least reactivity and the lowest surface heat flux. Increasing the jet momentum ratio led to a reduced ignition delay length due to enhanced jet-crossflow interaction, and to a reduction of surface heat flux as the reacting jet detached more from the surface. CFD results using the Eddy-Dissipation Concept (EDC) combustion model predicted ignition delay lengths and reaction zone shapes in qualitative agreement with the experiment but with a trend of overestimating combustion temperatures.

Gas turbine endwall film cooling and heat transfer characteristics with a multi-cavity slashface gap design

A novel multi-cavity slashface gap with different cavity obstruction modes, compared with a typical single-cavity slashface, is designed to arrange coolant flow on a gas turbine endwall. Solving the steady Reynolds averaged Navier–Stokes equations, the interaction condition between the coolant jet and cascade secondary vortices is studied. The corresponding film cooling and aero-thermal performances of the endwall are evaluated. The obtained results indicate that the dual-cavity slashface urges the coolant from the fore cavity to stagnate on the obstruction and to be redirected toward the spanwise direction. Consequently, the coolant downstream migration tendency near the obstruction is restricted, and the cooling effectiveness downstream of the obstruction increases. When the blowing ratio (M) is 0.5, the cavity obstruction is most effective when placed at 30%Ls and 50%Ls in redirecting the coolant to eject out more quickly. The endwall cooling effectiveness in the vicinity is increased from 0.2 to 0.4. The influence of the obstruction is the most moderate when M=1.0. The cooling effectiveness upstream of the obstruction is only increased by 0.06 in the DC-10% case. Another mainstream ingestion region appears downstream of the obstruction, and the lack of coolant decreases the average endwall cooling effectiveness by 0.04. At a high blowing ratio (M=1.5), the ingestion and ejection velocities at the slashface are the highest, and the obstruction can only influence the region of z/Cax < 0.4. After penetrating directly into the mainstream of the coolant, an uncovered region is formed on the endwall, and its average cooling effectiveness is increased by up to 0.08 with an increasing net heat flux reduction of 0.2 in the DC-30% case. This paper provides insights into elaborate purge flow control for turbine cooling designers.

Experimental and numerical research on film cooling characteristics on the compound angle hole

Experimental research was conducted for studying the effect of blowing ratio (M) as well as cross-flow Reynolds number (Rec) on distribution characteristics in film cooling effectiveness (η) and heat transfer coefficient (h) on compound angle hole configuration with applying transient liquid crystal technology. At the same time, the distribution characteristics of discharge coefficient (Cd) were measured by the flow resistance experiment. Furthermore, the net heat flux reduction (NHFR) is applied for assessing the film cooling performance. Numerical simulation was carried out to present the flow features for analyzing the film cooling mechanism. Results demonstrate that the increasing Rec presents a great influence on the measured Cd at the small M. The flow loss caused by the cross-flow presents the slight influence on the measured Cd at the large M. In addition, the measured Cd shows upward tendency with the increase in M and is almost no change until M increases to a certain extent. At small M, the measured η at the hole downstream area is higher and descends with the increase in Rec. At the large M, jets at the hole outlet almost separated from the surface. Thus, the measured η at the hole downstream area is lower and rises with the increase in Rec. The increase in M makes the measured h at the hole downstream area descend. The increase in Rec makes the measured h of the hole downstream area rise. The NHFR shows the decreasing tendency with the increasing M. Under the condition of M = 0.5, values of the laterally averaged NHFR are between 0 and 0.4, which represents the positive effect on film cooling function. At the condition of M = 2.0, values of laterally averaged NHFR are less than 0, involving negative effects. Under the condition of M = 1.0, most of the laterally averaged NHFR presents the positive values, and only a small part of the laterally averaged NHFR is the negative values. Due to the rising η and the descending h, the NHFR decreases in the increase in Rec under the condition of M = 0.5 and M = 2.0. The increase in Rec presents the slight influence on the NHFR under the condition of M = 1.0.

THERMAL BUCKLING AND BENDING ANALYSES OF CARBON FOAM BEAMS SANDWICHED BY COMPOSITE FACES UNDER AXIAL COMPRESSION

The bending and critical buckling loads of a sandwich beam structure subjected to thermal load and axial compression were simulated and temperature distribution across sandwich layers was investigated by finite element analysis and validated analytically. The sandwich structure was consisted of two face sheets and a core, carbon fiber and carbon foam were used as face sheet and core respectively for more efficient stiffness results. The analysis was repeated with different materials to reduce thermal strain and heat flux of sandwich beams. Applying both ends fixed as temperature boundary conditions, temperature induced stresses were observed, steady-state thermal analysis was performed, and conduction through sandwich layers along with their deformation nature were investigated based on the material properties of the combination of face sheets and core. The best material combination was found for the reduction of heat flux and thermal strain, and addition of aerogel material significantly reduced thermal stresses without adding weight to the sandwich structure.

Improved heat and particle flux mitigation in high core confinement, baffled, alternative divertor configurations in the TCV tokamak

Nitrogen seeded detachment has been achieved in the tokamak a configuration variable (TCV) in alternative divertor configurations (ADCs), namely X-divertor and X-point target, with and without baffles in H-mode plasmas with high core confinement. Both ADCs show a remarkable reduction in the inter-ELM particle and heat fluxes to the target compared to the standard divertor configuration. 95%–98% of the inter-ELM peak heat flux to the target is mitigated as a synergetic effect of ADCs, baffling, and nitrogen seeded detachment. The effect of divertor geometry and baffles on core-divertor compatibility is investigated in detail. The power balance in these experiments is also investigated to explore the physics behind the observed reduction in heat fluxes in the ADCs.

Aerodisk Effect on Hypersonic Boundary Layer Transition and Heat Transfer of HIFiRE-5 Vehicle

The substantial aerodynamic drag and severe aerothermal loads, which are closely related to boundary layer transition, challenge the design of hypersonic vehicles and could be relieved by active methods aimed at drag and heat flux reduction, such as aerodisk. However, the research of aerodisk effects on transitional flows is still not abundant. Based on the improved k-ω-γ transition model, this study investigates the influence of the aerodisk with various lengths on hypersonic boundary layer transition and surface heat flux distribution over HIFiRE-5 configuration under various angles of attack. Certain meaningful analysis and results are obtained: (i) The existence of aerodisk is found to directly trigger separation-induced transition, moving the transition onset near the centerline upstream and widening the transition region; (ii) The maximum wall heat flux could be effectively reduced by aerodisk up to 52.1% and the maximum surface pressure can even be reduced up to 80.4%. The transition shapes are identical, while the variety of growth rates of intermittency are non-monotonous with the increase in aerodisk length. The dilation of region with high heat flux boundary layer is regarded as an inevitable compromise to reducing maximum heat flux and maximum surface pressure. (iii) With the angle of attack rising, first, the transition is postponed and subsequently advanced on the windward surface, which is in contrast to the continuously extending transition region on the leeward surface. This numerical study aims to explore the effects of aerodisk on hypersonic boundary layer transition, enrich the study of hypersonic flow field characteristics and active thermal protection system considering realistic boundary layer transition, and provide references for the excogitation and utilization of hypersonic vehicle aerodisk.

Turbine nozzle endwall aero-thermal characteristics under combustor louver coolant with interface cavity

Targeting on combustor-turbine interface cooling system of combustor louver coolant and interface cavity, the endwall film cooling and heat transfer characteristics as well as the vane phantom cooling performance of a nozzle guide vane are investigated by solving the Reynolds-Averaged Navier-Stokes equations, and the secondary flow structure at endwall region are analyzed. The results show that most of the louver coolant leads to the core cooling region on the endwall after recirculating in the cavity with the cavity vortex, while a minority of the louver coolant escapes from the cavity vortex and forms the outer endwall cooling region limited by the horseshoe vortex. Under a certain mainstream flow condition, the endwall cooling performance and net heat flux reduction (NHFR) only depend on the mass flow ratio (MFR) of louver coolant instead of blowing ratio (BR). With increasing louver BR, the coolant mainly leads to the increase of the width and film cooling effectiveness of the core region at high MFR, but it leads to increasing cooling effectiveness of the outer region at low MFR. When the louver MFR increases from 0.25 % to 1.91 %, the averaged endwall cooling effectiveness at z/Cax = 0.1 is increased from 0.04 to 0.21. The NHFR of the core and outer cooling region reaches 0.3 and 0.1, respectively. The pressure and suction side horseshoe vortexes cause two protection failure regions, and its NHFR further decreases with increasing BR. At high MFR, the heat transfer in the low Nusselt number regions resulted by cavity vortex separation is further weakened with the increase of BR, and the region is enlarged toward two sides of the cascade. When MFR = 1.91 %, the Nusselt number of the high heat transfer region at z/Cax = 0.7 decreases from 2200 to 2000, and the averaged NHFR of fore part endwall reaches up to 0.2. This paper provides guidance for endwall cooling configuration design by taking the compound effect of the combustor-turbine interface cooling scheme into account.

Thermal transport modeling of laser-irradiated spheres

Thermal transport of uniformly laser-irradiated spheres of various materials is investigated computationally. One-dimensional simulations of low- to mid-Z materials (Be, Al, and Cu) are performed to evaluate the impact of nonlocal electron transport on experimental observables under laser intensities of relevance to direct-drive inertial confinement fusion. We compare thermal transport models of different levels of fidelity: flux-limited Spitzer–Harm diffusion, the Schurtz–Nicolai–Busquet (SNB) reduced-order nonlocal model, and a Fokker–Planck description. Spitzer–Harm diffusion with different flux-limiter factors are compared with different implementations of the SNB model in the HYDRA radiation hydrodynamics code. Under the conditions of interest, the peak heat flux in the thermal front with the SNB model shows good agreement with Fokker–Planck calculations, with the largest errors below 10% at 1015 W/cm2 laser intensity. From HYDRA-SNB simulations, two experimentally relevant effects are observed from nonlocal heat transport when compared to flux-limited Spitzer–Harm modeling: coronal temperatures are cooler due to reduced heat fluxes in the expanding plasma and (for mid-Z materials) x-ray emissions are enhanced due to preheating in the dense plasma.

Impacts of accelerating deployment of offshore windfarms on near-surface climate

The European Union has set the ambitious goal of becoming climate neutral by 2050, which has stimulated renewable energy production and accelerated the deployment of offshore wind energy in the North Sea. Here, a high-resolution regional climate model was used to investigate the impact on the sea surface climate of large-scale offshore wind farms that are proposed for the North Sea. The results show a significant reduction in the air-sea heat fluxes and a local, annual mean net cooling of the lower atmosphere in the wind farm areas down to more than 2.0 Wm−2, due to a decrease in 10 m wind speed and turbulent kinetic energy and an increase in low-level clouds. Mean surface winds decreased by approximately 1 ms−1 downstream of wind farms. Furthermore, an increase of approximately 5% in mean precipitation was found over the wind farm areas. At a seasonal timescale, these differences are higher during winter and autumn than in other seasons. Although the offshore wind farms reduce the heat transport from the ocean to the atmosphere in the region of large wind farms, the atmospheric layers below the hub height show an increase in temperature, which is on the order of up to 10% of the climate change signal at the end of the century, but it is much smaller than the interannual climate variability. In contrast, wind speed changes are larger than projected mean wind speed changes due to climate change. Our results suggest that the impacts of large clustered offshore wind farms should be considered in climate change impact studies. Moreover, the identified offshore windfarm impacts on the sea surface climate and the introduced spatial pattern in atmospheric conditions, in particular the modeled wind speed changes, suggest potential impacts on local ocean dynamics and the structure of the marine ecosystem. This should be considered in future scenarios for the North Sea marine environment and taken into account as a structuring influence in the offshore environment.

Understanding wind-driven melt of patchy snow cover

Abstract. The representation of snow processes in most large-scale hydrological and climate models is known to introduce considerable uncertainty into the predictions and projections of water availability. During the critical snowmelt period, the main challenge in snow modeling is that net radiation is spatially highly variable for a patchy snow cover, resulting in large horizontal differences in temperatures and heat fluxes. When a wind blows over such a system, these differences can drive advection of sensible and latent heat from the snow-free areas to the snow patches, potentially enhancing the melt rates at the leading edge and increasing the variability of subgrid melt rates. To get more insight into these processes, we examine the melt along the upwind and downwind edges of a 50 m long snow patch in the Finseelvi catchment, Norway, and try to explain the observed behavior with idealized simulations of heat fluxes and air movement over patchy snow. The melt of the snow patch was monitored from 11 June until 15 June 2019 by making use of height maps obtained through the photogrammetric structure-from-motion principle. A vertical melt of 23 ± 2.0 cm was observed at the upwind edge over the course of the field campaign, whereas the downwind edge melted only 3 ± 0.4 cm. When comparing this with meteorological measurements, we estimate the turbulent heat fluxes to be responsible for 60 % to 80 % of the upwind melt, of which a significant part is caused by the latent heat flux. The melt at the downwind edge approximately matches the melt occurring due to net radiation. To better understand the dominant processes, we represented this behavior in idealized direct numerical simulations, which are based on the measurements on a single snow patch by Harder et al. (2017) and resemble a flat, patchy snow cover with typical snow patch sizes of 15, 30, and 60 m. Using these simulations, we found that the reduction of the vertical temperature gradient over the snow patch was the main cause of the reductions in vertical sensible heat flux over distance from the leading edge, independent of the typical snow patch size. Moreover, we observed that the sensible heat fluxes at the leading edge and the decay over distance were independent of snow patch size as well, which resulted in a 15 % and 25 % reduction in average snowmelt for, respectively, a doubling and quadrupling of the typical snow patch size. These findings lay out pathways to include the effect of highly variable turbulent heat fluxes based on the typical snow patch size in large-scale hydrological and climate models to improve snowmelt modeling.

A generalized thermal deviation factor to evaluate the comprehensive stress of tubes under non-uniform heating

The heaters working with supercritical fluids normally suffer from high temperature, high pressure and non-uniform heating, which would lead to thermal fragile and stress failure in practical engineering. However, efficient and accurate methods to predict the comprehensive stress in non-uniformly heating tubes are missing. Besides, to reduce the comprehensive stress, a criterion is required to provide optimization guidance of non-uniformly heating tubes. In present study, the thermal and mechanical performances of three typical non-uniformly heating tubes are numerically investigated using the FVM-FEM method. A generalized thermal deviation factor (GTDF) is then proposed. The results showed that the GTDF can accurately predict the ratio of comprehensive stress to yield strength and GTDF > 1 indicates the plastic deformation, which is much more convenient and efficient than the conventional structural analysis. Based on the GTDF, two methods of reducing heat flux non-uniformity and enhancing tube-inside heat transfer are proposed to reduce the comprehensive stress. The results show that the unilaterally elliptic dimpled tube reduces the maximum comprehensive stress by 30.8% compared with the smooth tube and also prevents the stress concentration in the dimple zone, which can be employed to reduce the comprehensive stress in various tubes under non-uniformly heating conditions.