High Heat Flux Values Research Articles

Nucleate pool boiling heat transfer characteristics of R600a with CuO nanoparticles

The present research work investigates the effect of CuO nanoparticles on the nucleate boiling heat transfer characteristics of Isobutane (R600a) refrigerant. All the pool boiling experiments are carried with both pure and nano-refrigerants of 0.01, 0.025, 0.05 and 0.1 percentage by volume. The heat flux is varied from 2 kW.m-2 to 20 kW.m-2 at a regular interval of 2 kW.m-2. The heat transfer coefficient values for the pool boiling condition of R600a refrigerant are calculated experimentally, which are less deviating from the established theoretical correlations. The added CuO nanoparticles significantly influenced the nucleate boiling heat transfer coefficient of R600a refrigerant at higher heat flux values. The experiment results reveal that the thermophoretic mobility of nanoparticles play a major role in nanofluids heat transport. In the present work, CuO nanoparticles addition in R600a is optimized and is justified based on gravity and agglomeration effect.

Two-phase cooling system with controlled pulsations

One of the promising ways of removing large heat fluxes from the surface of heat-stressed elements of electronic devices is the use of evaporating thin layer of liquid film, moving under the action of the gas flow in a flat channel. In this work, a prototype of evaporative cooling system for high heat flux removal with forced circulation of liquid and gas coolants with controlled pulsation, capable to remove heat flux of up to 1,5 kW/cm2 and higher was presented. For the first time the regime with controlled pulsation is used. Due to pulsations, it is possible to achieve high values of critical heat flux due to a brief increase in the flow rate of the liquid, which allows to "wash off" large dry spots and prevent the occurrence of zones of flow and drying.

Gardon gauge heat flux sensor verification by new working facility

In the present paper, a novel working standard for metrological verification of Gardon gauge (Gg) is presented. The authors propose to couple the radiative and convective thermal sources in order to achieve high values of thermal flux. This is particularly important for those laboratories where high heat flux values have to be measured, such as on ground test facilities for the simulation of spacecraft re-entry. These test facilities, frequently, have the need to verify their Gardon gauges through working standards, before each running test. Unfortunately, the common Black Body temperature does not allow to reach the desired heat flux values to verify these kinds of probe. The proposed working standard, instead, allows to achieve heat flux values higher than those reachable with only the radiative flux emitted by the Black Body, of at least one order of magnitude. The experimental tests have shown high repeatability of the measurements; this proves the feasibility of this kind of verification technique.In addition, the authors have assembled a further working standard to evaluate the reproducibility of the proposed technique. In fact, the low values of compatibility index (less than 1) indicate that the proposed technique has also a satisfactory reproducibility.

Role of inter-nanowire distance in metal nanowires on pool boiling heat transfer characteristics

Energy management in data centres is crucial where a maximum portion of energy is spent on thermal management and electronics cooling systems. It becomes very crucial when it comes to immersion cooling techniques (pool boiling mechanism) using dielectric fluids. Role of metal nanowires (Cu and Ag) with different inter-nanowire distance values were analysed for their pool boiling performance. Templates with different inter-pore distances (260 ± 20 nm, 320 ± 20 nm and 360 ± 20 nm) and diameter of 200 nm were used to deposit copper nanowires (CuNW) and silver nanowires (AgNW) over the copper substrate using electrodeposition technique. Electrodeposition conditions like voltage and time were optimised to obtain nanowires of near constant height and different surface density coverage. To investigate the role of these metal nanowires on pool boiling characteristics, a dedicated pool boiling experimental facility was fabricated and experiments were carried out using FC-72 as working fluid. As a result of experimental investigation, boiling incipience superheat was observed to reduce which is very important for electronics systems. Also, both the critical heat flux (CHF) and heat transfer coefficient (h) were found to be increased as compared to the bare copper surface. Increasing the distance between nanowires, decreases the number of nanowires per unit area and thus the surface density coverage. This increases the cavity density and cavity size of micron-scale cavities in favour of pool boiling enhancement. This also decreases the resistance to fluid flow at high heat flux values which delays the surface dryout and critical heat flux.

Electrical resistivity of liquid Fe to 12\u2009GPa: Implications for heat flow in cores of terrestrial bodies

Electrical and thermal transport properties of liquid Fe under high pressure have important implications for the dynamics and thermal evolution of planetary cores and the geodynamo. However, electrical resistivity (ρ) and thermal conductivity (k) of liquid Fe at high pressure still remain contentious properties. To date, only two experimental investigations of ρ of liquid Fe in the pressure region below 7 GPa are reported in literature. Here we report the results of measurements of ρ for solid and liquid Fe (inversely proportional to k through the Wiedemann-Franz law) at pressures from 3 to 12 GPa, using a large multi-anvil press. We show that ρ of liquid Fe decreases as a function of pressure up to the δ-γ-liquid triple point at ~5.2 GPa, and subsequently remains invariant from 6 to 12 GPa, which is consistent with an earlier study on liquid Ni. Our results demonstrate an important effect of solid phase on the structure and properties of liquid Fe. Our values of ρ for solid and liquid Fe are used to calculate k in Mercury’s solid inner core and along the adiabat in the liquid outer cores of Moon, Ganymede, Mercury and Mars. Our robust values of thermal conductivity place the focus on uncertainties in thermal expansion as the cause of variation in values of core conducted heat. Except for Mercury, our adiabatic heat flux values in these terrestrial cores validate the use of similar values used in several previous studies. Our high values of core adiabatic heat flux in Mercury would provide a stabilizing effect on, and lead to an increase in thickness of, the thermally stratified layer at the top of the core.

Capabilities of high y+ wall approaches in predicting heat transfer to supercritical fluids in rod bundle geometries

The present paper summarises the results of the simulation of heat transfer to supercritical water in rod-bundle geometries by a CFD code, using wall function models. Two different sets of experimental data were considered, concerning both relatively high and low mass flux conditions and inlet temperature spanning from low to near-critical values.In past analyses, the unsuitability of low-Reynolds number turbulence models was observed in predicting heat transfer in rod bundles, when both high mass and heat flux values were imposed; in fact, large overestimations of wall temperatures were reported in such conditions. This motivated to try simpler models, such as the wall function approach, in order to investigate if, though expectedly not very accurate, they could at least reproduce the experimental data at an acceptable level.As reported in the present paper, the selected models, which adopt a “high y+” wall treatment (indicating wall functions in the STAR-CCM+ code), seem reasonably able at reproducing the general observed experimental trends. The present understanding of the phenomena and the available modelling techniques unfortunately do not allow obtaining better results when the wall or fluid temperature approach the pseudocritical value. However, the comparison with results obtained by low-Reynolds models shows that, at least when facing operating conditions similar to the ones considered in the present work, a downgrading of the adopted modelling techniques may be beneficial and allows obtaining reasonable results. Analyses were also performed considering conditions far from the pseudo-critical temperature, in which low-Reynolds models had provided good performance, reporting that the wall function approach seems effective also in these cases for obtaining first guess results.

Two-phase convection in Ganymede’s high-pressure ice layer — Implications for its geological evolution

Ganymede, the largest moon in the solar system, has a fully differentiated interior with a layer of high-pressure (HP) ice between its deep ocean and silicate mantle. In this paper, we study the dynamics of this layer using a numerical model of two-phase ice–water mixture in two-dimensional Cartesian geometry. While focusing on the generation of water at the silicate/HP ice interface and its upward migration towards the ocean, we investigate the effect of bottom heat flux, the layer thickness, and the HP ice viscosity and permeability. Our results suggest that melt can be generated at the silicate/HP ice interface for small layer thickness ( ≲ 200 km) and high values of heat flux ( ≳ 20 mW m−2) and viscosity ( ≳ 1015 Pa s). Once generated, the water is transported through the layer by the upwelling plumes. Depending on the vigor of convection, it stays liquid or it may freeze before melting again as the plume reaches the temperate (partially molten) layer at the boundary with the ocean. The thickness of this layer as well as the amount of melt that is extracted from it is controlled by the permeability of the HP ice. This process constitutes a means of transporting volatiles and salts that might have dissolved into the melt present at the silicate/HP ice interface. As the moon cools down, the HP ice layer becomes less permeable because the heat flux from the silicates decreases and the HP ice layer thickens.

A new heat flux model for the Antarctic Peninsula incorporating spatially variable upper crustal radiogenic heat production

AbstractA new method for modeling heat flux shows that the upper crust contributes up to 70% of the Antarctic Peninsula's subglacial heat flux and that heat flux values are more variable at smaller spatial resolutions than geophysical methods can resolve. Results indicate a higher heat flux on the east and south of the Peninsula (mean 81 mW m−2) where silicic rocks predominate, than on the west and north (mean 67 mW m−2) where volcanic arc and quartzose sediments are dominant. While the data supports the contribution of heat‐producing element‐enriched granitic rocks to high heat flux values, sedimentary rocks can be of comparative importance dependent on their provenance and petrography. Models of subglacial heat flux must utilize a heterogeneous upper crust with variable radioactive heat production if they are to accurately predict basal conditions of the ice sheet. Our new methodology and data set facilitate improved numerical model simulations of ice sheet dynamics.

The marine atmospheric boundary layer under strong wind conditions: Organized turbulence structure and flux estimates by airborne measurements

AbstractDuring winter, cold air outbreaks take place in the northwestern Mediterranean sea. They are characterized by local strong winds (Mistral and Tramontane) which transport cold and dry continental air across a warmer sea. In such conditions, high values of surface sensible and latent heat flux are observed, which favor deep oceanic convection. The HyMeX/ASICS‐MED field campaign was devoted to the study of these processes. Airborne measurements, gathered in the Gulf of Lion during the winter of 2013, allowed for the exploration of the mean and turbulent structure of the marine atmospheric boundary layer (MABL). A spectral analysis based on an analytical model was conducted on 181 straight and level runs. Profiles of characteristic length scales and sharpness parameter of the vertical wind spectrum revealed larger eddies along the mean wind direction associated with an organization of the turbulence field into longitudinal rolls. These were highlighted by boundary layer cloud bands on high‐resolution satellite images. A one‐dimensional description of the vertical exchanges is then a tricky issue. Since the knowledge of the flux profile throughout the entire MABL is essential for the estimation of air‐sea exchanges, a correction of eddy covariance turbulent fluxes was developed taking into account the systematic and random errors due to sampling and data processing. This allowed the improvement of surface fluxes estimates, computed from the extrapolation of the stacked levels. A comparison between those surface fluxes and bulk fluxes computed at a moored buoy revealed considerable differences, mainly regarding the latent heat flux under strong wind conditions.

Spatially advancing turbulent flow and heat transfer in a curved channel at friction-velocity-based Reynolds number 550

The flow and heat transfer in a curved channel were numerically analyzed for their spatial advancement under the influence of streamline curvature. The simulation was made for the curved channel of the radius ratio 0.92. The curved channel was provided with the fully developed velocity and thermal profile in a straight channel at the mean velocity Reynolds numbers up to 20,400. The number of grid points was more than 3.00×108 for the case of the highest Reynolds number. The local Nusselt number indicated that the heat transfer was enhanced on the outer wall of the curved channel and such enhancement was notable over the downstream area of ϕ≧80°. This heat transfer enhancement came from the large-scale organized vortex which intensified the lateral convection of the secondary stream and caused high values of the turbulent heat flux. The pre-multiplied spectrum was used to analyze the spatial advancement of the large-scale organized vortex. The spectrum indicated that the large-scale vortex was produced from the initial disturbance in the straight channel and the vortex grew and expanded to nearly the channel full width. It was shown that the advancement of the vortex was almost independent from the Reynolds numbers.

Metallic TIM Testing and Selection for IC, Power, and RF Semiconductors

Metallic TIM Testing and Selection for IC, Power, and RF Semiconductors

Metallic TIM Testing and Selection for Harsh Environment Applications for GaN RF Semiconductors

Summary An important determinant of device reliability is operating temperature control. Maintaining a semiconductor device at or below the maximum rated junction temperature (Tj) is accomplished through careful thermal management design and selection of well-performing thermal interface materials (TIM) that minimize efficiency losses in packaging and between the semiconductor device package and a heat sink or liquid cold plate. Thermal management design is increasingly important in harsh operating environments, especially where higher operating temperatures are specified. Minimizing thermal resistances through each semiconductor package material stack and at the external case or package surface of the device are important aspects of maintaining operating temperatures within specified maximum values. In addition, certain semiconductor devices require an electrical path from the semiconductor case to an external component. Maximizing electrical performance of gallium nitride (GaN) RF semiconductors is critical to system performance, as a primary example. The on-going transition within RF and microwave systems from silicon to GaN devices has increased the need for thermal interface materials which offer both improved thermal performance and electrical conductivity. Additionally, GaN semiconductor die are typically smaller in footprint and, even with equivalent power dissipation values, therefore may operate with higher heat flux values that require greater attention to proper thermal solution design. To address such needs, recently-developed forms of metallic TIM preforms are available for integrated circuits, power semiconductors, and RF devices. Understanding how these materials may be tested and selected for specific application requirements is the subject of this discussion.

Radiation effects on natural convection in a vertical channel with an auxiliary plate

In this experimental study the effects of unheated auxiliary plate positions along the centerline of a vertical channel on combined natural convective in air and radiative heat transfer are investigated. The vertical channel is symmetrically heated and a uniform heat flux along the principal walls of the channel is assumed. Comparisons between experimental data carried out from different positions of the auxiliary plate and two surfaces emissivity values are given in terms of heated wall temperatures and radiative heat transfer. Average Nusselt numbers by means of experimental data are evaluated. Results are given for different thermal and geometrical dimensionless parameters. Results show that the lowest maximum channel wall temperatures are attained for the auxiliary plate placed at the channel exit for high channel Rayleigh numbers and heat flux values. Correlations for average Nusselt numbers, global average Nusselt numbers and dimensionless maximum wall temperatures in terms of channel Rayleigh numbers and auxiliary plate-to-channel height ratio, h/L, are evaluated in the ranges: 20 < Ra'< 1.5 × 105, 22 <Ra′∗ < 1.7 × 105 and 0 ≤ h/L ≤ 0.5 for inlet and outlet positions and walls emissivity equal to 0.9. The proposed correlations are in good agreement with correlation for simple channel.

Results for high heat-flux flow realizations in innovative operations of milli-meter scale condensers and boilers

This paper defines the nature of high heat-flux innovative boiler and condenser operations that keeps these devices functional for many critically needed thermal management applications where gravitational force is negligible (naturally or made to be so) relative to other forces (shear, pressure, etc.). These applications include heat removal from narrow spaces to address electronic cooling needs (i.e. mm-scale boilers and condensers with μm-scale flows), gravity insensitive aircraft operations, and zero-gravity applications. Further, the paper presents fundamental condensing and boiling flow results that describe the physics that needs to be utilized to achieve significantly enhanced heat-flux values within the context of these applications.Reported experimental results demonstrate: (i) the ability to replace ineffective and problematic liquid–vapor configurations (plug-slug, etc.) with annular flows where a flowing thin liquid film covers the entire heat-exchange surface, (ii) the ability to use pulsation-induced interfacial wave phenomena in conjunction with sub-micron thin film phenomena (such as contact line physics) to achieve very high heat-flux during boiling and condensation, and (iii) the ability to desensitize these flows that would otherwise be highly sensitive to inadvertent but ever present flow fluctuations (due to sensitive coupling between vapor and liquid motions, as well as sensitivity of these micro-meter scale thick liquid films to minuscule transverse vibrations of the heat-exchange surface) when flow channel hydraulic diameters reduce to milli-meter or sub milli-meter scales.Innovative realizations of shear-driven boiling and condensing flows have been investigated within horizontal channels (with condensation or boiling on the bottom, horizontal surface). Results from boiling and condensing flows of FC-72 fluid in horizontal rectangular cross-section (2 or 6mm gap height and 15 or 24mm width, respectively) ducts of 1m length are presented. Utilizing a controlled presence of pulsatile mass flow rates, significant enhancements in heat-transfer rates are obtained for these innovative devices at a location within the device length – these enhancements (relative to non-pulsatile conditions) are sometimes >860% for condensing flows and >190% for boiling flows. The paper reports representative time-varying heat-flux values at this location in support of the understanding that this phenomena can potentially be used (in future experiments) to significantly enhance average heat-flux value over the entire length of an innovative boiler/condenser. The reported phenomena arises from superposing relatively fast time-scale (2.5–30Hz) flow rate pulsations on otherwise steady-in-the-mean flows to beneficially change certain flow variable averages – over the longer time-scale (<0.01Hz) of practical interest – relative to their values obtained in the absence of externally imposed pulsations. Such externally imposed pulsations may cause standing waves to form on the thin film interface. The observed asymmetric reduction in the mean film thickness (associated with high heat-flux values) is likely due to “stickiness” of wave-troughs resulting from interaction of phenomena associated with several length scales (namely nano-scale, micro-scale, and continuum length scales) and two different time scales – short (less than 0.5s) and long (greater than 1min). That is, associated micro-scale flows at wave-troughs interact with and, possibly, de-stabilize the local adsorbed layer (whose thickness may be <200nm and is exposed to phenomena such as disjoining pressures) on the wetting heat-exchange surface.

3D Heat Transfer Analysis of a Miniature Copper-Water Vapor Chamber with Wicked Pillars Array

A three-dimensional analysis of the heat and mass transfer phenomena inside a vapor chamber is essential for correctly understanding its thermal performance limitations and structural optimization. This paper presents a complete three-dimensional numerical analysis and comparative study of two different miniature vapor chambers designs with identical external geometry and dimensions but different internal structures: one having a wicked pillar array and the other one without the wicked pillars array. The distribution of the wicked pillar array in the vapor core was aligned. Detailed comparative experimental results are also reported, which were performed to verify the calculations from the numerical simulations. It was found that the numerical and experimental results agree quite well, especially at high heat flux values. It is also observed that the vapor chamber with wicked pillars had a better thermal performance than the conventional design, with a 5% decrease in terms of total thermal resistance due to the added extra channels that allow a better flow of the working fluid to the evaporator surface. An insight into how improving the thermal performance of a vapor chamber is provided through the detailed three-dimensional numerical simulations.

Thorny Devil Nanotextured Fibers: The Way to Cooling Rates on the Order of 1 kW/cm2

In the present work high-heat-flux surfaces, which should serve at temperatures of up to 200 °C, were covered by electrospun polymer nanofiber mats with thicknesses of about 30 μm. Then, four different metals were electroplated on separate polymer mats, namely, copper, silver, nickel, and gold. As a result, copper-plated nanofiber mats took on an appearance resembling that of a small Australian thorny devil lizard (i.e., they became very rough on the nanoscale) and acquired a high thermal diffusivity. Silver-plated nanofiber mats also became very rough because of the dendritelike and cactuslike nanostructures on their surfaces. However, nickel-plated nanofibers were only partially rough and their mats incorporated large domains of smooth nickel-plated fibers, and gold-plated nanofibers were practically smooth. Drop impacts on the hot surfaces coated with copper-plated and silver-plated nanofibers revealed tremendously high values of heat removal rates of up to 0.6 kW/cm(2). Such high values of heat flux are more than an order of magnitude higher that the currently available ones and probably can be increased even more using the same technique. They open some intriguing perspectives for the cooling of high-heat-flux microelectronics and optoelectronics and for further miniaturization of such devices, especially for such applications as UAVs and UGVs.

Soil Surface Energy and Water Budgets during a Monsoon Season in Korea

Abstract In this study, attention has been focused on the climatology of some variables linked to the turbulent exchanges of heat and water vapor in the surface layer during a summer monsoon in Korea. In particular, the turbulent fluxes of sensible and latent heat, the hydrologic budget, and the soil temperatures and moistures have been analyzed. At large scale, because the measurements of those data are not only fragmentary and exiguously available but also infeasible for the execution of climatologic analyses, the outputs of a land surface scheme have been used as surrogate of observations to analyze surface layer processes [this idea is based on the methodology Climatology of Parameters at the Surface (CLIPS)] in the Korean monsoonal climate. Analyses have been made for the summer of 2005. As a land surface scheme, the land surface process model (LSPM) developed at the University of Torino, Italy, has been employed, along with the data collected from 635 Korean meteorological stations. The LSPM predictions showed good agreement with selected observations of soil temperature. Major results show that, during the rainfall season, soil moisture in the first tenths of centimeters frequently exceeds the field capacity, whereas most of the rainfall is “lost” as surface runoff. Evapotranspiration is the dominant component of the energy budget, sometimes even exceeding net radiation, especially during the short periods between the precipitation events; in these periods, daily mean soil temperatures are about 28°C or even more. The Gyeonggi-do region, the metropolitan area surrounding Seoul, shows some particularities when compared with the neighboring regions: solar radiation and precipitations are lower, causing high values of sensible heat flux and soil temperatures, and lower values of latent heat flux and soil moistures.

Flow boiling in a 1.1 mm tube with R134a: Experimental results and comparison with model

A detailed comparison of flow boiling heat transfer results in a stainless steel tube of 1.1 mm internal diameter with results of a three-zone flow model are presented in this paper. The working fluid is R134a. Other parameters were varied in the range: mass flux 100–600 kg/m 2 s; heat flux 16–150 kW/m 2 and pressure 6–12 bar. The experimental results demonstrate that the heat transfer coefficient increases with heat flux and system pressure, but does not change with vapour quality when the quality is less than about 50% for low heat and mass flux values. The effect of mass flux is observed to be insignificant. For vapour quality values greater than 50% and at high heat flux values, the heat transfer coefficient does not depend on heat flux and decreases with vapour quality. This could be caused by dryout. The three-zone evaporation model predicts the experimental results fairly well, especially at relatively low pressure. However, the dryout region observed at high quality is highly over-predicted by the model. The sensitivity of the performance of the model to the three optimised parameters (confined bubble frequency, initial film thickness and end film thickness) and some preliminary investigation relating the critical film thickness for dryout to measured tube roughness are also discussed.

Analysis of sheet-like thermal plumes in turbulent Rayleigh–Bénard convection

Sheet-like thermal plumes are investigated using time-dependent and three-dimensional flow fields obtained from direct numerical simulations and well-resolved large-eddy simulations of turbulent Rayleigh–Bénard convection in water (Prandtl number Pr=5.4) in a cylindrical container with the aspect ratio Γ=1 and for the Rayleigh numbers Ra=2×109 and 2×1010.To analyse quantitatively the physical properties of the sheet-like thermal plumes and the turbulent background and to obtain the temperature threshold which separates these two different flow regions, the temperature dependences of the conditionally averaged local heat flux, thermal dissipation rate and selected components of the velocity and vorticity fields are studied. It is shown that the sheet-like plumes are characterized by high values of the local heat flux and relatively large absolute values of the vertical components of the vorticity and velocity fields. The borders of these plumes are indicated by large values of the thermal dissipation rate and large absolute values of the horizontal vorticity components. In contrast to the sheet-like thermal plumes, the turbulent background is characterized by low values of the thermal dissipation rate, local heat flux and vertical vorticity component. The highest values of the local heat flux and the highest absolute values of the vertical vorticity component are found in the regions where the sheet-like plumes strike against each other. Fluid swirling at these places forms the stems of the mushroom-like thermal plumes which develop in the bulk of the Rayleigh–Bénard cell.Further, formulae to calculate the curvature, thickness and length of the plumes are introduced. Geometrical properties such as plume area, diameter, curvature, thickness and aspect ratio together with the physical properties of the sheet-like plumes such as temperature, heat flux, thermal dissipation rate, velocity and vorticity are investigated.

Seasonal patterns of heat loss in wild bottlenose dolphins (Tursiops truncatus)

This study investigated patterns of heat loss in bottlenose dolphins (Tursiops truncatus) resident to Sarasota Bay, FL, USA, where water temperatures vary seasonally from 11 to 33 degrees C. Simultaneous measurements of heat flux (HF) and skin surface temperature were collected at the body wall and appendages of dolphins during health-monitoring events in summer (June 2002-2004) and winter (February 2003-2005). Integument thickness was measured and whole body conductance (W/m(2) degrees C) was estimated using HF and colonic temperature measurements. Across seasons, HF values were similar at the appendages, but their distribution differed significantly at the flipper and fluke. In summer, these appendages displayed uniformly high values, while in winter they most frequently displayed very low HF values with a few high HF values. In winter, blubber thickness was significantly greater and estimated conductance significantly lower, than in summer. These results suggest that dolphins attempt to conserve heat in winter. In winter, though, HF values across the body wall were similar to (flank) or greater than (caudal keel) summer values. It is likely that higher winter HF values are due to the steep temperature gradient between the body core and colder winter water, which may limit the dolphin's ability to decrease heat loss across the body wall.