Micro-Patterned Substrates With Nano-Scale Elements for Pool Boiling

2011 ◽  
Author(s):  
Chad N. Hunter ◽  
Nicholas R. Glavin ◽  
Chris Muratore ◽  
Timothy S. Fisher ◽  
John G. Jones ◽  
...  
Keyword(s):  
Heat Flux ◽  
Chemical Process ◽  
Heat Fluxes ◽  
Test Surface ◽  
Nano Scale ◽  
Boiling Process ◽  
Simple Chemical ◽  
Post Test ◽  
Lower Heat ◽  
Layer Composition

The critical heat flux values of copper substrates were increased from 87 to 125 W/cm2 by using a simple chemical process resulting in growth of micro and nano-scale copper structures on the surface. Pre- and post-test surface analysis revealed that the morphology of the micro and nano-scale features of these copper structures changed during the boiling process accompanied by a change in oxide layer composition. Boiling performance of the micro and nano-structured samples was repeatable when testing at lower heat fluxes.

Effects of Gap Geometry and Gravity on Boiling Around a Constrained Bubble in 2-Propanol/Water Mixtures

2006 ◽  
Vol 129 (2) ◽  
pp. 114-123
Author(s):  
Chen-li Sun ◽  
Van P. Carey
Keyword(s):  
Heat Flux ◽  
Heated Surface ◽  
Molar Fraction ◽  
Nucleate Boiling ◽  
Confined Space ◽  
Cold Surface ◽  
Boiling Regime ◽  
Wall Superheat ◽  
Boiling Process ◽  
Lower Heat

In this study, boiling experiments were conducted with 2-propanol/water mixtures in confined gap geometry under various levels of gravity. The temperature field created within the parallel plate gap resulted in evaporation over the portion of the vapor-liquid interface of the bubble near the heated surface, and condensation near the cold surface. Full boiling curves were obtained and two boiling regimes—nucleate boiling and pseudofilm boiling—and the transition condition, the critical heat flux (CHF), were identified. The observations indicated that the presence of the gap geometry pushed the nucleate boiling regime to a lower superheated temperature range, resulting in correspondingly lower heat flux. With further increases of wall superheat, the vapor generated by the boiling process was trapped in the gap to blanket the heated surface. This caused premature occurrence of CHF conditions and deterioration of heat transfer in the pseudo-film boiling regime. The influence of the confined space was particularly significant when greater Marangoni forces were present under reduced gravity conditions. The CHF value of x (molar fraction)=0.025, which corresponded to weaker Marangoni forces, was found to be greater than that of x=0.015 with a 6.4mm gap.

Effect of Acoustic Excitation on R134a/Al2O3 Nanolubricant Mixture Boiling on a Reentrant Cavity Surface

2015 ◽  
Vol 137 (11) ◽  
Author(s):  
M. A. Kedzierski ◽  
S. E. Fick
Keyword(s):  
Heat Flux ◽  
Volume Fraction ◽  
Power Level ◽  
Pool Boiling ◽  
Heat Fluxes ◽  
Cavity Surface ◽  
Test Surface ◽  
Acoustic Excitation ◽  
Al2o3 Nanoparticles ◽  
Maximum Enhancement

This paper quantifies the influence of acoustic excitation of Al2O3 nanoparticles on the pool-boiling performance of R134a/polyolester mixtures on a commercial (Turbo-BII-HP) boiling surface. A nanolubricant with 10 nm diameter Al2O3 nanoparticles at a 5.1% volume fraction in the base polyolester lubricant was mixed with R134a at a 1% mass fraction. The study showed that high-frequency ultrasound at 1 MHz can improve R134a/nanolubricant boiling on a reentrant cavity surface by as much as 44%. This maximum enhancement occurred for an applied power level to the fluid of approximately 6 W and a heat flux of approximately 6.9 kW/m2. Applied power levels larger and smaller than 6 W resulted in smaller boiling heat transfer enhancements. In total, five different applied power levels were studied: 0 W, 4 W, 6 W, 8 W, and 12 W. The largest and smallest enhancement averaged over the tested heat flux range were approximately 12% and 2% for the applied power levels of 6 W and 4 W, respectively. In situ insonation at 1 MHz resulted in an improved dispersion of the nanolubricant on the test surface. An existing pool-boiling model for refrigerant/nanolubricant mixtures was modified to include the effect of acoustic excitation. For heat fluxes greater than 25 kW m−2, the model was within 4.5% of the measured heat flux ratios for mixtures, and the average agreement between measurements and predictions was approximately 1% for all power levels.

scholarly journals Critical Ignition Conditions of Wood by Cylindrical Firebrands

2021 ◽  
Vol 7 ◽  
Author(s):  
Hamed Salehizadeh ◽  
Raquel S. P. Hakes ◽  
Michael J. Gollner
Keyword(s):  
Heat Flux ◽  
Oriented Strand Board ◽  
Thermal Conditions ◽  
Heat Fluxes ◽  
Critical Conditions ◽  
Small Scale ◽  
Test Surface ◽  
Transient Heating ◽  
Wind Speeds ◽  
Critical Ignition

This study investigated the thermal conditions preceding ignition of three dense woody fuels often found on structures by firebrands, a major cause of home ignition during wildland-urban interface (WUI) fires. Piles of smoldering cylindrical firebrands, fabricated from wooden dowels, were deposited either on a flat inert surface instrumented with temperature and heat flux sensors or on a target fuel (marine-grade plywood, oriented-strand board, or cedar shingles) to investigate critical conditions at ignition. The former provided thermal data to characterize the time before and at ignition, while the latter provided smoldering and flaming ignition times. Tests were conducted in a small-scale wind tunnel. Larger firebrand piles produced higher temperatures at the center of the pile, thought to be due to re-radiation within the pile. Ignition was found to be dependent on target fuel density; flaming ignition was additionally found to be dependent on wind speed. Higher wind speeds increased the rate of oxidation and led to higher temperatures and heat fluxes measured on the test surface. The heat flux at ignition was determined by combining results of inert and ignition tests, showing that ignition occurred while transient heating from the firebrand pile was increasing. Ultimately, critical ignition conditions from firebrand pile exposure are needed to design appropriate fire safety standards and WUI fire modeling.

Effects of Gap Geometry and Gravity on Boiling Around a Constrained Bubble in 2-Propanol/Water Mixtures

2003 ◽  
Author(s):  
Chen-Li Sun ◽  
Van P. Carey
Keyword(s):  
Heat Flux ◽  
Heated Surface ◽  
Nucleate Boiling ◽  
Film Boiling ◽  
Confined Space ◽  
Cold Surface ◽  
Boiling Regime ◽  
Wall Superheat ◽  
Boiling Process ◽  
Lower Heat

In this study, boiling experiments were conducted with 2-propanol/water mixtures in confined gap geometry under various levels of gravity. The temperature field created within the parallel plate gap resulted in evaporation over the portion of the vapor-liquid interface of the bubble near the heated surface, and condensation near the cold surface. Full boiling curves were obtained and two boiling regimes — nucleate boiling and pseudo film boiling, the transition condition, and the critical heat flux (CHF), were identified. The observations indicate that the presence of the gap geometry pushed the nucleate boiling regime to a lower superheated temperature range and resulted in correspondingly lower heat flux. With further increases of wall superheat, the vapor generated by the boiling process was trapped in the gap and blanketed the heated surface. This caused premature occurrence of CHF conditions and deterioration of heat transfer in the pseudo film boiling regime. The influence of the confined space was particularly significant when greater Marangoni forces were present at reduced gravity conditions. The value of the CHF for x = 0.025, which corresponded to weaker Marangoni forces, was found to be greater than that of x = 0.015 with a 6.35 mm gap.

THE MECHANISM OF RAISING AND QUANTIFICATION OF SPECIFIC HEAT FLUX AT BOILING OF NANOFLUIDS IN FREE CONVECTION CONDITIONS

2017 ◽  
pp. 25-34
Author(s):  
V.N. Moraru
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Heat Capacity ◽  
Specific Heat ◽  
Chemical Properties ◽  
Heating Surface ◽  
Physical Models ◽  
Superheated Liquid ◽  
Two Phase ◽  
Boiling Process

The results of our work and a number of foreign studies indicate that the sharp increase in the heat transfer parameters (specific heat flux q and heat transfer coefficient _) at the boiling of nanofluids as compared to the base liquid (water) is due not only and not so much to the increase of the thermal conductivity of the nanofluids, but an intensification of the boiling process caused by a change in the state of the heating surface, its topological and chemical properties (porosity, roughness, wettability). The latter leads to a change in the internal characteristics of the boiling process and the average temperature of the superheated liquid layer. This circumstance makes it possible, on the basis of physical models of the liquids boiling and taking into account the parameters of the surface state (temperature, pressure) and properties of the coolant (the density and heat capacity of the liquid, the specific heat of vaporization and the heat capacity of the vapor), and also the internal characteristics of the boiling of liquids, to calculate the value of specific heat flux q. In this paper, the difference in the mechanisms of heat transfer during the boiling of single-phase (water) and two-phase nanofluids has been studied and a quantitative estimate of the q values for the boiling of the nanofluid is carried out based on the internal characteristics of the boiling process. The satisfactory agreement of the calculated values with the experimental data is a confirmation that the key factor in the growth of the heat transfer intensity at the boiling of nanofluids is indeed a change in the nature and microrelief of the heating surface. Bibl. 20, Fig. 9, Tab. 2.

scholarly journals Assessing IDDES-Based Wall-Modeled Large-Eddy Simulation (WMLES) for Separated Flows with Heat Transfer

Fluids ◽  
2021 ◽  
Vol 6 (7) ◽  
pp. 246
Author(s):  
Rozie Zangeneh
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Large Eddy Simulation ◽  
Skin Friction ◽  
Heat Fluxes ◽  
Separated Flows ◽  
Detached Eddy Simulation ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Isothermal Wall

The Wall-modeled Large-eddy Simulation (WMLES) methods are commonly accompanied with an underprediction of the skin friction and a deviation of the velocity profile. The widely-used Improved Delayed Detached Eddy Simulation (IDDES) method is suggested to improve the prediction of the mean skin friction when it acts as WMLES, as claimed by the original authors. However, the model tested only on flow configurations with no heat transfer. This study takes a systematic approach to assess the performance of the IDDES model for separated flows with heat transfer. Separated flows on an isothermal wall and walls with mild and intense heat fluxes are considered. For the case of the wall with heat flux, the skin friction and Stanton number are underpredicted by the IDDES model however, the underprediction is less significant for the isothermal wall case. The simulations of the cases with intense wall heat transfer reveal an interesting dependence on the heat flux level supplied; as the heat flux increases, the IDDES model declines to predict the accurate skin friction.

scholarly journals Drift kinetic theory of alpha transport by tokamak perturbations

2021 ◽  
Vol 87 (2) ◽  
Author(s):  
Elizabeth A. Tolman ◽  
Peter J. Catto
Keyword(s):  
Heat Flux ◽  
Kinetic Theory ◽  
Alpha Particle ◽  
Heat Fluxes ◽  
Alpha Particles ◽  
Alpha Power ◽  
Mode Amplitude ◽  
Effective Collision Frequency ◽  
Perturbation Frequency ◽  
Particle Confinement

Upcoming tokamak experiments fuelled with deuterium and tritium are expected to have large alpha particle populations. Such experiments motivate new attention to the theory of alpha particle confinement and transport. A key topic is the interaction of alpha particles with perturbations to the tokamak fields, including those from ripple and magnetohydrodynamic modes like Alfvén eigenmodes. These perturbations can transport alphas, leading to changed localization of alpha heating, loss of alpha power and damage to device walls. Alpha interaction with these perturbations is often studied with single-particle theory. In contrast, we derive a drift kinetic theory to calculate the alpha heat flux resulting from arbitrary perturbation frequency and periodicity (provided these can be studied drift kinetically). Novel features of the theory include the retention of a large effective collision frequency resulting from the resonant alpha collisional boundary layer, correlated interactions over many poloidal transits and finite orbit effects. Heat fluxes are considered for the example cases of ripple and the toroidal Alfvén eigenmode (TAE). The ripple heat flux is small. The TAE heat flux is significant and scales with the square of the perturbation amplitude, allowing the derivation of constraints on mode amplitude for avoidance of significant alpha depletion. A simple saturation condition suggests that TAEs in one upcoming experiment will not cause significant alpha transport via the mechanisms in this theory. However, saturation above the level suggested by the simple condition, but within numerical and experimental experience, which could be accompanied by the onset of stochasticity, could cause significant transport.

scholarly journals Air–Sea Interaction in the Central Mediterranean Sea: Assessment of Reanalysis and Satellite Observations

2021 ◽  
Vol 13 (11) ◽  
pp. 2188
Author(s):  
Salvatore Marullo ◽  
Jaime Pitarch ◽  
Marco Bellacicco ◽  
Alcide Giorgio di Sarra ◽  
Daniela Meloni ◽  
...  
Keyword(s):  
Heat Flux ◽  
Mediterranean Sea ◽  
Seasonal Cycle ◽  
Heat Fluxes ◽  
Meteorological Variables ◽  
Central Mediterranean ◽  
High Quality ◽  
Central Mediterranean Sea ◽  
In Situ Observations

Air–sea heat fluxes are essential climate variables, required for understanding air–sea interactions, local, regional and global climate, the hydrological cycle and atmospheric and oceanic circulation. In situ measurements of fluxes over the ocean are sparse and model reanalysis and satellite data can provide estimates at different scales. The accuracy of such estimates is therefore essential to obtain a reliable description of the occurring phenomena and changes. In this work, air–sea radiative fluxes derived from the SEVIRI sensor onboard the MSG satellite and from ERA5 reanalysis have been compared to direct high quality measurements performed over a complete annual cycle at the ENEA oceanographic observatory, near the island of Lampedusa in the Central Mediterranean Sea. Our analysis reveals that satellite derived products overestimate in situ direct observations of the downwelling short-wave (bias of 6.1 W/m2) and longwave (bias of 6.6 W/m2) irradiances. ERA5 reanalysis data show a negligible positive bias (+1.0 W/m2) for the shortwave irradiance and a large negative bias (−17 W/m2) for the longwave irradiance with respect to in situ observations. ERA5 meteorological variables, which are needed to calculate the air–sea heat flux using bulk formulae, have been compared with in situ measurements made at the oceanographic observatory. The two meteorological datasets show a very good agreement, with some underestimate of the wind speed by ERA5 for high wind conditions. We investigated the impact of different determinations of heat fluxes on the near surface sea temperature (1 m depth), as determined by calculations with a one-dimensional numerical model, the General Ocean Turbulence Model (GOTM). The sensitivity of the model to the different forcing was measured in terms of differences with respect to in situ temperature measurements made during the period under investigation. All simulations reproduced the true seasonal cycle and all high frequency variabilities. The best results on the overall seasonal cycle were obtained when using meteorological variables in the bulk formulae formulations used by the model itself. The derived overall annual net heat flux values were between +1.6 and 40.4 W/m2, depending on the used dataset. The large variability obtained with different datasets suggests that current determinations of the heat flux components and, in particular, of the longwave irradiance, need to be improved. The ENEA oceanographic observatory provides a complete, long-term, high resolution time series of high quality in situ observations. In the future, more similar sites worldwide will be needed for model and satellite validations and to improve the determination of the air–sea exchange and the understanding of related processes.

Inverse Heat Conduction Applied to the Measurement of Heat Fluxes on a Rotating Cylinder: Comparison Between an Analytical and a Numerical Technique

2008 ◽  
Vol 130 (8) ◽  
Author(s):  
Fabien Volle ◽  
Michel Gradeck ◽  
Denis Maillet ◽  
Arsène Kouachi ◽  
Michel Lebouché
Keyword(s):  
Heat Flux ◽  
Numerical Technique ◽  
Rotating Cylinder ◽  
Local Heat ◽  
Heat Fluxes ◽  
Inverse Heat Conduction ◽  
Analytical Technique ◽  
Inner Radius ◽  
Direct Model ◽  
Stable Estimation

A method using either a one-dimensional analytical or a two-dimensional numerical inverse technique is developed for measurement of local heat fluxes at the surface of a hot rotating cylinder submitted to the impingement of a subcooled water jet. The direct model calculates the temperature field inside the cylinder that is submitted to a given nonuniform and time dependent heat flux on its outer surface and to a uniform surface heat source on an inner radius. In order to validate the algorithms, simulated temperature measurements inside the cylinder are processed and used by the two inverse techniques to estimate the wall heat flux. As the problem is improperly posed, regularization methods have been introduced into the analytical and numerical inverse algorithms. The numerical results obtained using the analytical technique compare well with the results obtained using the numerical algorithm, showing a good stable estimation of the available test solutions. Furthermore, real experimental data are used for the estimation, and local boiling curves are plotted and discussed.

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