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.

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.

scholarly journals Predicting Critical Heat Flux With Multiphase CFD: 4 Years in the Making

2017 ◽  
Author(s):  
Emilio Baglietto ◽  
Etienne Demarly ◽  
Ravikishore Kommajosyula
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Critical Heat Flux ◽  
Bubble Dynamics ◽  
Flow Boiling ◽  
Heated Surface ◽  
Force Balance ◽  
Modeling Framework ◽  
Lift Off ◽  
Limiting Condition

Advancement in the experimental techniques have brought new insights into the microscale boiling phenomena, and provide the base for a new physical interpretation of flow boiling heat transfer. A new modeling framework in Computational Fluid Dynamics has been assembled at MIT, and aims at introducing all necessary mechanisms, and explicitly tracks: (1) the size and dynamics of the bubbles on the surface; (2) the amount of microlayer and dry area under each bubble; (3) the amount of surface area influenced by sliding bubbles; (4) the quenching of the boiling surface following a bubble departure and (5) the statistical bubble interaction on the surface. The preliminary assessment of the new framework is used to further extend the portability of the model through an improved formulation of the force balance models for bubble departure and lift-off. Starting from this improved representation at the wall, the work concentrates on the bubble dynamics and dry spot quantification on the heated surface, which governs the Critical Heat Flux (CHF) limit. A new proposition is brought forward, where Critical Heat Flux is a natural limiting condition for the heat flux partitioning on the boiling surface. The first principle based CHF is qualitatively demonstrated, and has the potential to deliver a radically new simulation technique to support the design of advanced heat transfer systems.

scholarly journals A Study of Critical Heat Flux During Flow Boiling in Microchannel Heat Sinks

2011 ◽  
Vol 134 (1) ◽  
Author(s):  
Tailian Chen ◽  
Suresh V. Garimella
Keyword(s):  
Heat Flux ◽  
Critical Heat Flux ◽  
Flow Rate ◽  
Heat Input ◽  
Heat Sink ◽  
Flow Boiling ◽  
Strong Dependence ◽  
Heat Fluxes ◽  
Abrupt Decrease ◽  
Parallel Microchannels

The cooling capacity of two-phase transport in microchannels is limited by the occurrence of critical heat flux (CHF). Due to the nature of the phenomenon, it is challenging to obtain reliable CHF data without causing damage to the device under test. In this work, the critical heat fluxes for flow boiling of FC-77 in a silicon thermal test die containing 60 parallel microchannels were measured at five total flow rates through the microchannels in the range of 20–80 ml/min. CHF is caused by dryout at the wall near the exit of the microchannels, which in turn is attributed to the flow reversal upstream of the microchannels. The bubbles pushed back into the inlet plenum agglomerate; the resulting flow blockage is a likely cause for the occurrence of CHF which is marked by an abrupt increase in wall temperature near the exit and an abrupt decrease in pressure drop across the microchannels. A database of 49 data points obtained from five experiments in four independent studies with water, R-113, and FC-77 as coolants was compiled and analyzed. It is found that the CHF has a strong dependence on the coolant, the flow rate, and the area upon which the heat flux definition is based. However, at a given flow rate, the critical heat input (total heat transfer rate to the coolant when CHF occurs) depends only on the coolant and has minimal dependence on the details of the microchannel heat sink (channel size, number of channels, substrate material, and base area). The critical heat input for flow boiling in multiple parallel microchannels follows a well-defined trend with the product of mass flow rate and latent heat of vaporization. A power-law correlation is proposed which offers a simple, yet accurate method for predicting the CHF. The thermodynamic exit quality at CHF is also analyzed and discussed to provide insights into the CHF phenomenon in a heat sink containing multiple parallel microchannels.

Critical Heat Flux Enhancement in Water-Based Nanofluid With Honeycomb Porous Plate on Large Heated Surface

2016 ◽  
Author(s):  
Suazlan Mt Aznam ◽  
Shoji Mori ◽  
Kunito Okuyama
Keyword(s):  
Heat Flux ◽  
Critical Heat Flux ◽  
Heated Surface ◽  
Porous Plate ◽  
High Heat ◽  
High Heat Flux ◽  
Heater Surface ◽  
Solid Structure ◽  
Water Based ◽  
Plain Surface

Heat removal through pool boiling is limited by the phenomena of critical heat flux (CHF). CHF enhancement is vitally important in order to satisfy demand for the cooling technology with high heat flux in In Vessel Retention (IVR). Various surface modifications of the boiling surface, e.g., integrated surface structures and coating of a micro-porous have been proven to effectively enhance the CHF in saturated pool boiling. We have been proposed a novel method of attaching a honeycomb structured porous plate on a considerably large heater surface. Previous results, by the authors in [1] reported that CHF has been enhanced experimentally up to more than approximately twice that of a plain surface (approximately 2.0 to 2.5 MW/m2) with a diameter of 30 mm heated surface. However, it is necessary to demonstrate the method together with infinite heater surface within laboratory scale. It is important that cooling techniques for IVR could be applicable to a large heated surface as well as remove high heat flux. Objective of this study is to investigate the CHF of honeycomb porous plate and metal solid structure in nanofluid boiling or water boiling on a large heated surface. Water-based nanofluid offers good wettability and capillarity. While metal solid structure is installed on honeycomb porous plate to increase the number of vapor jet. Experimental results of honeycomb porous plate and combination of honeycomb porous plate and metal solid structure in water-based nanofluid boiling shows that CHF is increased up to twice [2] and thrice, respectively compared to plain surface in water boiling. To the best of the author’s knowledge, the highest value (3.1 MW/m2) was obtained for a large heated surface having a diameter of 50 mm which is regarded as infinite heated surface. This demonstrates potential for general applicability to have more safety margin in IVR method.

Stability and Enhancement of Boiling in Microchannels

2011 ◽  
Author(s):  
A. E. Bergles
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Critical Heat Flux ◽  
High Heat ◽  
Heat Fluxes ◽  
Transfer Enhancement ◽  
The Past ◽  
The Stability ◽  
Saturated Boiling ◽  
Microelectronic Components

During the past 20 years, there has been intense worldwide interest in microchannel heat exchangers, particularly for cooling of microelectronic components. Saturated boiling of the coolant is usually indicated in order to accommodate high heat fluxes and to have uniformity of temperature. However, boiling is accompanied by several instabilities, the most severe of which can sharply limit the maximum, or critical, heat flux. These stability phenomena are reviewed, and recent studies will be discussed. Elevation of the critical heat flux will be discussed within the context of heat transfer enhancement. Means to improve the stability of boiling and the enhancement of boiling heat transfer, in general, are discussed.

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.

Critical Heat Flux of Butanol Aqueous Solution

2008 ◽  
Author(s):  
Shotaro Nishiguchi ◽  
Naoki Ono ◽  
Masahiro Shoji
Keyword(s):  
Heat Transfer ◽  
Aqueous Solution ◽  
Surface Tension ◽  
Heat Flux ◽  
Critical Heat Flux ◽  
Heated Surface ◽  
Pure Water ◽  
Liquid Temperature ◽  
Heat Transfer Characteristics ◽  
Heated Wire

Aqueous solutions of some alcohols such as butanol show peculiar temperature dependence of surface tension. Contrary to ordinary liquids or solutions, the surface tension increases with temperature at the range of high liquid temperature. So at the triple-phase point on a heated surface, the thermo-capillary force acts for the liquid to wet the heated surface, so the solutions are sometimes called as “self-wetting liquids”. Self-wetting liquids may prohibit the dry-out of a heated surface so that the heat transfer performance would be enhanced. For this reason, applications of self-wetting liquids to heat transfer devices such as heat pipes are actively studied in recent years. However, the heat transfer characteristics of boiling of self-wetting liquids are not fully understood. In the present research, a boiling experiment of butanol aqueous solution was performed on a heated fine wire in order to make clear the fundamental heat transfer characteristics. A heated wire configuration is easy to observe the phenomena and easy to address the fundamental issues of boiling. In the present experiment, nucleate boiling heat transfer were investigated with special attention to critical heat flux (CHF), by changing solution concentration and temperature. Bubbling aspects were observed by high-speed video camera. It is found from the experiment that CHF is generally enhanced 20 to 50% when compared to the case of pure water. It is also found that at a certain concentration and at a certain liquid temperature, peculiar boiling takes place where very small bubbles are emitted from the heated wire and CHF enhancement becomes very large from 2 to 3 times higher than CHF of pure water. The temperature when the peculiar boiling takes place is close to boiling temperature of the solution. These results suggest the possibility of application of aqueous solution to high-performance cooling devices utilizing micro-scaled channels because generating bubbles are small enough so that the pressure loss of the flow passage is small and heat transfer rate is very large.

Critical Heat Fluxes and Flow Patterns in High-Pressure Boiling Water Flows

1964 ◽  
Vol 86 (1) ◽  
pp. 12-22 ◽  
Author(s):  
F. E. Tippets
Keyword(s):  
Heat Flux ◽  
High Pressure ◽  
Critical Heat Flux ◽  
Rectangular Channel ◽  
Motion Pictures ◽  
Flow Patterns ◽  
Heat Fluxes ◽  
Boiling Water ◽  
Forced Flow ◽  
Flux Level

High-speed motion pictures (4300 pictures/sec) of boiling water flow patterns in conditions of forced flow at 1000 psia pressure in a vertical heated rectangular channel were taken over the range of mass velocities from 50 to 400 lb/sec-ft2, fluid states from bulk subcooled liquid flow to bulk boiling flow at 0.66 steam quality, and heat fluxes up to and including the critical heat flux level. Eighty critical heat flux determinations were made in the course of the experiment at 1000 psia in conditions of bulk boiling. The motion pictures provide photographic evidence of the general arrangement of the flow in conditions of bulk boiling at high pressure with heat fluxes near and including the critical heat flux level.

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