Preliminary Investigation of Boiling and Transpiration of Water in a Porous Media Cooled With an Air Flow

2004 ◽  
Author(s):  
Alexis Schubert ◽  
John Keffler ◽  
Alfonso Ortega
Keyword(s):  
Heat Flux ◽  
Porous Media ◽  
Porous Structure ◽  
Heated Surface ◽  
Preliminary Investigation ◽  
Nucleate Boiling ◽  
Heat Fluxes ◽  
Foam Sample ◽  
Porous Sample ◽  
Experimental Apparatus

This paper describes a study focused on heat and mass transfer through various porous media involving both boiling and transpiration. Heat was supplied to a porous structure immersed in water. Water was boiled at the base of the porous material and in some cases advected from the porous structure by air blown over its surface. The porous media was expected to provide higher heat fluxes than those attained during pool boiling by providing additional surface area and by increasing the number of nucleation sites. The behavior was studied from just below the boiling point and into the nucleate boiling regime. The experimental apparatus consisted of a 2.5 cm square jet impinging onto a 2.5 cm square porous sample. A total of four copper foam samples and one carbon graphite foam sample were tested. The foam sample was placed in contact with a 2.5 cm square heated surface. Water was supplied through the sides of the porous sample and was able to leave the system as a vapor through the top surface of the sample, where it was advected away. It was determined that the presence of an impinging jet had no noticeable effect on heat flux. Up to 60% enhancement in heat flux was observed, compared to boiling of the plain surface. Contact resistance was significant and mitigated the affects of sample thermal conductivity.

Comprehensive Comparisons Between Evaporation and Pool Boiling on Thin Micro Porous Coated Surfaces

2006 ◽  
Author(s):  
Chen Li ◽  
G. P. Peterson
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Porous Media ◽  
Heated Surface ◽  
Pool Boiling ◽  
Nucleate Boiling ◽  
Evaporation Heat ◽  
Evaporation Heat Transfer ◽  
Coated Surfaces ◽  
Dry Out

The evaporation and pool boiling on micro porous coated surfaces have been shown to provide among the highest heat transfer rates achievable from any type of surfaces. The heat transfer modes in these surfaces, present a number of interesting similarities and also, some fundamental differences, which are the result of the liquid supply methods to the heated surface. For the evaporation from porous coated surfaces, the liquid return to the heated surface is assisted by the capillary pressure at the liquid-vapor interface; while for pool boiling, gravity is the principal driving force that rewets the surface. In order to better understand the physical phenomena that governs the flow behavior of both the liquid and vapor phases, and the heat transfer process inside the porous media, comprehensive comparisons between these return mechanisms and their respective characteristics, and the performance and the critical heat flux (CHF) for each have been made, based on similar physical situations. These systematic comparisons illustrate that at a lower heat flux, the evaporation and pool boiling curves are almost identical due to the similar heat transfer modes, i.e., convection and nucleate boiling. While with further increases in heat flux, the heat transfer performance of the evaporation on micro porous media is generally superior to pool boiling on an identical surface. This shift is believed to be due to the fact that for evaporation on micro porous media, the heat transfer mode is dominated by the film evaporation, while in pool boiling, it is principally the result of fully developed nucleate boiling. It was also observed that the impact of the effective thermal conductivity of the porous coating on pool boiling performance is larger than for evaporation heat transfer on the identical micro porous coated surfaces. In general, the experimental data indicated that the CHF for evaporation heat transfer is much higher than for pool boiling on the same surfaces. The mechanism of CHF for evaporation on porous coated surfaces is believed to be the capillary limit; while for pool boiling the limit is the result of the hydrodynamic instabilities. This difference in mechanisms is clearly demonstrated by the experimental observations, where initially, the dry out process of the porous coated surfaces during evaporation is gradual, while for pool boiling; the entire surface reaches dry out in a very short time. In addition, the sensitivity of the CHF to the thickness of the porous coatings at a constant volumetric porosity and pore size, as well as the various optimal volumetric porosity of the CHF at a given thickness, are clearly the results of the differences induced by the various CHF mechanisms.

Critical Heat Flux Limits for a Heated Surface Impacted by a Stream of Liquid Droplets

1994 ◽  
Vol 116 (3) ◽  
pp. 679-685 ◽  
Author(s):  
P. J. Halvorson ◽  
R. J. Carson ◽  
S. M. Jeter ◽  
S. I. Abdel-Khalik
Keyword(s):  
Heat Flux ◽  
Critical Heat Flux ◽  
Heated Surface ◽  
Droplet Impact ◽  
Heat Fluxes ◽  
Liquid Droplets ◽  
Impact Frequency ◽  
Experimental Apparatus ◽  
Key Factor ◽  
Wetted Area

An experimental apparatus has been constructed to allow investigation of heat transfer from a horizontal, upward facing, heated surface impacted by streams of monodisperse water droplets of varying size and impact frequency. Droplet diameters between 2.3 and 3.8 mm were used, with drop frequencies varying from 2 to 15 droplets per second. The droplet impact velocity was 1.3 m/s. Critical heat flux, surface superheat, droplet size, and frequency were the primary measured data. Heat fluxes as high as 325 W/cm2 were achieved with wall superheats of only 24°C. The liquid film thickness produced upon droplet impact is shown to be a key factor in these experiments, and the importance of investigating the wetted area is highlighted. The effectiveness of droplet impact cooling using droplets with diameters on the order of millimeters is shown.

Hotspot Cooling Performance for Submerged Confined Two-Phase Jet Impingement Cooling

2021 ◽  
Author(s):  
Tanvir Ahmed Chowdhury ◽  
Shawn A. Putnam
Keyword(s):  
Heat Flux ◽  
Single Phase ◽  
Heated Surface ◽  
Jet Impingement ◽  
Nucleate Boiling ◽  
Heat Fluxes ◽  
Nozzle Outlet ◽  
Two Phase ◽  
Impingement Cooling ◽  
Jet Nozzle

Abstract Jet impingement can be particularly effective for removing high heat fluxes from local hotspots. Two-phase jet impingement cooling combines the advantage of both the nucleate boiling heat transfer with the single-phase sensible cooling. This study investigates two-phase submerged jet impingement cooling of local hotspots generated by a diode laser in a 100 nm thick Hafnium (Hf) thin-film on glass. The jet/nozzle diameter is ∼1.2 mm and the normal distance between the nozzle outlet and the heated surface is ∼3.2 mm. Novec 7100 is used as the coolant and the Reynolds numbers at the jet nozzle outlet range from 250 to 5000. The hotspot area is ∼ 0.06 mm2 and the applied hotspot-to-jet heat flux ranges from 20 W/cm2 to 220 W/cm2. This heat flux range facilitates studies of both the single-phase and two-phase heat transport mechanisms for heat fluxes up to critical heat flux (CHF). The temporal evolution of the temperature distribution of the laser heated surface is measured using infrared (IR) thermometry. This study also investigates the nucleate boiling regime as a function of the distance between the hotspot center and the jet stagnation point. For example, when the hotspot center and the jet are co-aligned (x/D = 0), the CHF is found to be ∼ 177 W/cm2 at Re ∼ 5000 with a corresponding heat transfer coefficient of ∼58 kW/m2.K. While the CHF is ∼ 130 W/cm2 at Re ∼ 5000 with a jet-to-hotspot offset of x/D ≈ 4.2.

Convective Nucleate Boiling on a Heated Surface Cooled by an Impinging, Planar Jet of Water

1992 ◽  
Vol 114 (1) ◽  
pp. 152-160 ◽  
Author(s):  
D. T. Vader ◽  
F. P. Incropera ◽  
R. Viskanta
Keyword(s):  
Boundary Layer ◽  
Heat Flux ◽  
Surface Temperature ◽  
Heated Surface ◽  
Leading Edge ◽  
Nucleate Boiling ◽  
Heat Fluxes ◽  
Stagnation Region ◽  
Stagnation Line ◽  
Boundary Layer Turbulence

Convective nucleate boiling has been studied on a flat, upward facing, constant heat flux surface cooled by a planar, impinging water jet. Surface temperature distributions are presented for jet velocities between 1.8 and 4.5 m/s, fluid temperatures of 30, 40, and 50°C, and heat fluxes between 0.25 and 2.5 MW/m2. Although the critical Reynolds number, Rex*,c, is independent of heat flux for q” < q”ONB, boiling incipience strongly affects the transition to a turbulent boundary layer. As the heat flux increases, vapor bubbles of 1 mm diameter first appear at the point of maximum surface temperature, which also marks the onset of boundary layer turbulence. The leading edge of these bubbles moves toward the stagnation line and Rex*,c decreases with further increases in heat flux. Acceleration in the stagnation region stabilizes the flow, however, so that boundary layer turbulence is restricted to x/wj ≳ 1.6. With increasing heat flux, vigorous nucleate boiling covers more of the heater and surface temperature variations decrease.

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.

Simulation of Nucleate Boiling With Interfacial Temperature Gradients and Sharp Interface

2018 ◽  
Author(s):  
Isaac Perez-Raya ◽  
Satish G. Kandlikar
Keyword(s):  
Heat Transfer ◽  
Contact Line ◽  
Growth Rates ◽  
Bubble Growth ◽  
Heated Surface ◽  
Nucleate Boiling ◽  
Bubble Nucleation ◽  
Heat Fluxes ◽  
Temperature Gradients ◽  
Interfacial Temperature

Effective heat transfer techniques benefit the development of nuclear and fossil fuel powered steam generators, high power electronic devices, and industrial refrigeration systems. Boiling dissipates large heat fluxes while keeping a low and a constant surface temperature. However, studies of the fluid behavior surrounding the bubble and the heat transfer near the contact-line are scare due to difficulties of flow visualization, chaotic conditions, and small length scales. The preset study shows the simulation of bubble growth over a heated surface from conception to departure. The computation of mass transfer with interfacial temperature gradients leads to proper bubble growth rates. Models to include the interface sharpness uncover the dynamic and thermal interaction between the interface and the fluid. Results indicate that the nucleation of a bubble (in water at 1 atm with 6.2 K wall superheat) has an influence region of 2Db (where Db is the departure bubble diameter). In addition, results reveal a thin thermal film near the interface that increases the heat transfer at the contact-line region. Numerical bubble growth rates compare well with experimental data on single bubble nucleation.

On the Mechanism of Pool Boiling Critical Heat Flux Enhancement in Nanofluids

2010 ◽  
Vol 132 (6) ◽  
Author(s):  
Hyungdae Kim ◽  
Ho Seon Ahn ◽  
Moo Hwan Kim
Keyword(s):  
Heat Flux ◽  
Critical Heat Flux ◽  
Heated Surface ◽  
Pure Water ◽  
Pool Boiling ◽  
Nucleate Boiling ◽  
Titania Nanoparticles ◽  
Nanoparticle Layer ◽  
Wall Superheat ◽  
Dry Spot

The pool boiling characteristics of water-based nanofluids with alumina and titania nanoparticles of 0.01 vol % were investigated on a thermally heated disk heater at saturated temperature and atmospheric pressure. The results confirmed the findings of previous studies that nanofluids can significantly enhance the critical heat flux (CHF), resulting in a large increase in the wall superheat. It was found that some nanoparticles deposit on the heater surface during nucleate boiling, and the surface modification due to the deposition results in the same magnitude of CHF enhancement in pure water as for nanofluids. Subsequent to the boiling experiments, the interfacial properties of the heater surfaces were examined using dynamic wetting of an evaporating water droplet. As the surface temperature increased, the evaporating meniscus on the clean surface suddenly receded toward the liquid due to the evaporation recoil force on the liquid-vapor interface, but the nanoparticle-fouled surface exhibited stable wetting of the liquid meniscus even at a remarkably higher wall superheat. The heat flux gain attainable due to the improved wetting of the evaporating meniscus on the fouled surface showed good agreement with the CHF enhancement during nanofluid boiling. It is supposed that the nanoparticle layer increases the stability of the evaporating microlayer underneath a bubble growing on a heated surface and thus the irreversible growth of a hot/dry spot is inhibited even at a high wall superheat, resulting in the CHF enhancement observed when boiling nanofluids.

Determination of Dimensionality of Pool Boiling on a Thin Horizontal Disk Under Subcooled Conditions Using Thin-Film Thermocouples

2008 ◽  
Author(s):  
Vijaykumar Sathyamurthi ◽  
Debjyoti Banerjee
Keyword(s):  
Thin Film ◽  
Heat Flux ◽  
Correlation Dimension ◽  
Pool Boiling ◽  
Nucleate Boiling ◽  
Constant Heat Flux ◽  
Film Boiling ◽  
Steady State Condition ◽  
Experimental Apparatus ◽  
Thin Film Thermocouples

Subcooled pool boiling experiments are conducted at subcooling levels of 10 °C on a thin silicon disk (∼ 400μm thickn, 3 – inch diameter) with in situ micro-machined K-type thin film thermocouples (TFT) using a perfluorocarbon liquid refrigerant (PF-5060) with a boiling point of 56 °C. The experimental apparatus is of constant heat flux type. Surface temperature (from TFT) and heat flux data is obtained at each steady state condition to generate the pool boiling curve. The time – delay embedding technique is used to re-construct higher dimensional vectors with the optimal delay being estimated from the first minimum of mutual information. The correlation dimension measure is then estimated from the delay re – constructed phase space vectors. In this preliminary study correlation dimension measures are seen to vary from ∼ 12 in nucleate boiling, to ∼ 7 – 9 near Critical Heat Flux (CHF) condition, and ∼ 7 – 8 in film boiling. The results suggest that the attractors underlying thermal transport mechanisms in nucleate boiling are affected by a greater number of parameters than that at CHF. The dimension of the attractor is reduced further in the film boiling regime.

Experimental and Numerical Study of the Effects of Asymmetric Micro Ratchets on Pool Boiling Performance

2013 ◽  
Author(s):  
Lance Brumfield ◽  
Sunggook Park
Keyword(s):  
High Performance ◽  
Numerical Study ◽  
Heated Surface ◽  
Pool Boiling ◽  
Nucleate Boiling ◽  
Industrial Applications ◽  
Heat Fluxes ◽  
Working Fluid ◽  
High Heat Flux ◽  
Central Processing

Nucleate boiling is an attractive method for achieving high heat flux at low superheat temperatures. It is frequently used for industrial applications such as heat exchangers and is being considered to cool advanced central processing units (CPU) which produce heat fluxes on the order of 1 MW/m2 and are becoming increasingly less efficient to cool via forced conduction of air. The issues with implementing nucleate boiling as a cooling mechanism lies in the difficulty of quantifying the complex and numerous mechanisms which control the process. A comprehensive nucleate boiling model has yet to be formulated and will be required in order to safely and reliably cool high performance electronics. Spatially periodic systems with localized asymmetric surface structures (ratchets) can induce directed transport of matter (liquid/particles) in the absence of net force. It was hypothesized that ratchets may enhance pool boiling heat transfer by aiding in the removal of vapor which forms on the heated surface. Therefore, experiments on pool boiling using asymmetric micro ratchets of various geometries, with FC-72 as the working fluid, were investigated. Additionally, various numerical pool boiling simulations were performed using FLUENT to better understand the underlying physical principles behind pool boiling.

Pool Boiling in Saturated and Subcooled FC-72 Dielectric Fluid From a Porous Graphite Surface

2004 ◽  
Author(s):  
Mohamed S. El-Genk ◽  
Jack L. Parker
Keyword(s):  
Heat Flux ◽  
Pool Boiling ◽  
Nucleate Boiling ◽  
Heat Fluxes ◽  
Porous Coating ◽  
Dielectric Fluid ◽  
Subcooled Boiling ◽  
Porous Graphite ◽  
Rate Of Increase ◽  
Boiling Incipience

Experiments are conducted that investigated pool boiling of FC-72 liquid at saturation and 10, 20, and 30 K subcooling on porous graphite and smooth copper surfaces measuring 10 × 10 mm. The nucleate boiling heat flux, Critical Heat Flux (CHF), and surface superheats at boiling incipience are compared. Theses heat fluxes are also compared with those of other investigators for smooth copper and silicon, etched SiO2, surfaces and micro-porous coating. No temperature excursion at boiling incipience on the porous graphite that occurred at a surface superheats of &lt; 1.0 K. Conversely, the temperature excursions of 24.0 K and 12.4–17.8 K are measured at incipient boiling in saturation and subcooled boiling on copper. Nucleate boiling heat fluxes on porous graphite are significantly higher and corresponding surface superheats are much smaller than on copper. CHF on porous graphite (27.3, 39.6, 49.0, and 57.1 W/cm2 in saturation and 10 K, 20 K, and 30 K subcooled boiling, respectively) are 61.5%–207% higher than those on copper (16.9, 19.5, 23.6, and 28.0 W/cm2, respectively). The surface superheats at CHF on the porous graphite of 11.5 K in saturation and 17–20 K in subcooled boiling are significantly lower that those on copper (25 K and 26–28 K, respectively). In addition, the rate of increase of CHF on porous graphite with increased subcooling is ~ 125% higher than that on copper.

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