On the Gas Entrapment and Nucleation Site Density During Pool Boiling of Saturated Water

1993 ◽  
Vol 115 (3) ◽  
pp. 670-679 ◽  
Author(s):  
C. H. Wang ◽  
V. K. Dhir
Keyword(s):  
Contact Angle ◽  
Nucleation Site ◽  
Contact Angles ◽  
Heating Process ◽  
Heater Surface ◽  
Site Density ◽  
Uniform Temperature Field ◽  
The Stability ◽  
Free Energy Analysis ◽  
Nucleation Site Density

A model to describe the effect of wettability on nucleation site density is presented. First, from Helmholtz free energy analysis, a criterion for the entrapment condition in a uniform temperature field is developed. Second, the stability condition of preexisting gas/vapor nuclei during the heating process and the minimum superheat required to initiate nucleation are determined. The prediction of the entrapment condition and the incipient temperature are consistent with the experimental observations made on surfaces having naturally existing cavities. Third, a naturally formed cavity on a heater surface is modeled as a spherical cavity. The cumulative active nucleation site density for a specified contact angle is expressed in terms of the cumulative density of cavities existing on the surface as Na = Pas · Nas where Nas is the heater surface cumulative cavity density with cavity mouth angles less than a specified value and Pas is a function of contact angle and cavity mouth angle. The model successfully predicts active site densities for different contact angles.

Onset of Nucleate Boiling and Active Nucleation Site Density During Subcooled Flow Boiling

2002 ◽  
Vol 124 (4) ◽  
pp. 717-728 ◽  
Author(s):  
Nilanjana Basu ◽  
Gopinath R. Warrier ◽  
Vijay K. Dhir
Keyword(s):  
Heat Flux ◽  
Contact Angle ◽  
Flow Boiling ◽  
Nucleate Boiling ◽  
Nucleation Site ◽  
Subcooled Flow Boiling ◽  
Site Density ◽  
Wall Superheat ◽  
Subcooled Flow ◽  
Nucleation Site Density

The partitioning of the heat flux supplied at the wall is one of the key issues that needs to be resolved if one is to model subcooled flow boiling accurately. The first step in studying wall heat flux partitioning is to account for the various heat transfer mechanisms involved and to know the location at which the onset of nucleate boiling (ONB) occurs. Active nucleation site density data is required to account for the energy carried away by the bubbles departing from the wall. Subcooled flow boiling experiments were conducted using a flat plate copper surface and a nine-rod (zircalloy-4) bundle. The location of ONB during the experiments was determined from visual observations as well as from the thermocouple output. From the data obtained it is found that the heat flux and wall superheat required for inception are dependent on flow rate, liquid subcooling, and contact angle. The existing correlations for ONB underpredict the wall superheat at ONB in most cases. A correlation for predicting the wall superheat and wall heat flux at ONB has been developed from the data obtained in this study and that reported in the literature. Experimental data are within ±30 percent of that predicted from the correlation. Active nucleation site density was determined by manually counting the individual sites in pictures obtained using a CCD camera. Correlations for nucleation site density, which are independent of flow rate and liquid subcooling, but dependent on contact angle have been developed for two ranges of wall superheat—one below 15°C and another above 15°C.

Towards an Explanation of the Mechanism of Boiling Critical Heat Flux Enhancement in Nanofluids

2007 ◽  
Author(s):  
Jacopo Buongiorno ◽  
Lin-Wen Hu ◽  
In Cheol Bang
Keyword(s):  
Heat Flux ◽  
Contact Angle ◽  
Critical Heat Flux ◽  
Hot Spot ◽  
Heated Surface ◽  
Nucleation Site ◽  
Nanoparticle Layer ◽  
Site Density ◽  
Static Contact ◽  
Nucleation Site Density

Nanofluids exhibit a very significant enhancement of the boiling Critical Heat Flux (CHF) at low nanoparticle concentrations. This paper reviews the nanofluid boiling database in a quest for the CHF enhancement mechanism. Briefly, buildup of a nanoparticle layer on the heated surface occurs upon boiling of nanofluids. This layer changes the surface roughness and the nucleation site density and, remarkably, can improve the surface wettability, as shown by a reduction of the static contact angle on the nanofluid-boiled surfaces. Significant differences are also observed in the dynamic behavior of the hot spot at CHF.

Experimental Study of Critical Heat Flux and Heat Transfer Coefficient Enhancements in Pool Boiling Heat Transfer With Nanostructure Modified Active Nucleation Site and Contact Angle

2012 ◽  
Author(s):  
Eric Nolan ◽  
Russell Rioux ◽  
Calvin Hong Li
Keyword(s):  
Heat Transfer ◽  
Experimental Study ◽  
Heat Flux ◽  
Contact Angle ◽  
Heat Transfer Coefficient ◽  
Boiling Heat Transfer ◽  
Nucleation Site ◽  
Copper Surfaces ◽  
Site Density ◽  
Nucleation Site Density

An experimental study of nanostructure modified nucleation site density and contact angle that significantly enhances the Heat Transfer Coefficient (HTC) and the Critical Heat Flux (CHF) in pool boiling heat transfer of water on copper surfaces has been conducted. The nanostructures on copper surfaces have been created by an electrodeposition technique. It has been found that the nanostructured copper surfaces show an increase in CHF of up to 142% and an increase in HTC of 33% over that of a mirror-finished plain copper surface. Calculations for nucleation site density and active nucleation site diameter reveal a direct correlation between these factors and the HTC, as well as the CHF. More interestingly, a contact angle study on the tested surfaces shows that there is a connection between the contact angle reduction and CHF enhancement, which confirms the contact angle mechanism of CHF with experimental evidence.

A NEW ANALYTICAL MODEL FOR HEAT TRANSFER IN POOL BOILING

2010 ◽  
Vol 24 (12) ◽  
pp. 1229-1236 ◽  
Author(s):  
BOQI XIAO
Keyword(s):  
Heat Transfer ◽  
Pool Boiling ◽  
Statistical Treatment ◽  
Nucleation Site ◽  
Contact Angles ◽  
Site Density ◽  
Wall Superheat ◽  
Proposed Model ◽  
Nucleation Site Density ◽  
Good Agreement

In this paper, dependence of active nucleation site density on boiling surfaces are developed. For pool boiling heat transfer, a mathematical model is derived based on statistical treatment using the probability density function of the cavity mouth radius and existing correlation for active nucleation site density, the volume of single bubble at departure, the bubble departure diameter and the bubble departure frequency. The proposed model is expressed as a function of wall superheat, the contact angle, maximum and minimum active cavities, and physical properties of fluid. It is shown that the wall heat flux can be determined by the consideration of the variation of the cavity mouth radius. A good agreement between the proposed model predictions and experimental data is found for different contact angles. It also turns out that the present model explains well the mechanism on how wettability affects the pool boiling.

scholarly journals CNN-based visual analysis to study local boiling characteristics

2021 ◽  
Vol 2119 (1) ◽  
pp. 012068
Author(s):  
A N Chernyavskiy ◽  
I P Malakhov
Keyword(s):  
Network Architecture ◽  
High Speed ◽  
Visual Analysis ◽  
Nucleation Site ◽  
High Speed Video ◽  
Site Density ◽  
Segmentation Approach ◽  
Nucleation Site Density ◽  
Instance Segmentation

Abstract Visual analysis allows an estimate of different local boiling characteristics including bubble growth rate, departure diameters and frequencies of nucleation, nucleation site density and evolution of bubbles and dry spots in time. At the same time, visual determination of the presented characteristics in case of big amounts of data requires the development of the appropriate software which will allow not only determination of bubble location, but also an estimate of their sizes based on high-speed video. The presented problem can be solved by using the instance segmentation approach based on a convolutional neural network. In the presented work Mask R-CNN network architecture was used for estimation of the local boiling characteristics.

An Experimental Study of Boiling Heat Transfer During Quenching of Nanofluids With Carbon Nanotubes of Various Sizes

2016 ◽  
Author(s):  
Jia-Qi Li ◽  
Li-Wu Fan ◽  
Liang Zhang ◽  
Zi-Tao Yu
Keyword(s):  
Carbon Nanotubes ◽  
Heat Flux ◽  
Surface Roughness ◽  
Initial Temperature ◽  
Nucleation Site ◽  
Transient Temperature ◽  
Porous Layers ◽  
Site Density ◽  
Nucleation Site Density ◽  
Walled Cnts

Quenching experiments were performed with hot stainless steel spheres in a pool of water-based nanofluids in the presence of carbon nanotubes (CNTs) of various sizes. In order to explore the size effect, a test matrix was developed by choosing multi-walled CNTs with lengths from 1 μm to 5 μm and outer diameters from 30 nm to 60 nm. The concentration was fixed at 0.5% by mass for all types of CNTs. The initial temperature was 400 °C and the transient temperature variations at the center of the sphere were recorded as quenching curves. By establishing a lumped capacitance model, the transient surface heat flux variations were obtained as boiling curves. The original and boiled surfaces were both subjected to a series of characterizations to determine the changes in morphology, roughness, and wettability to identify the effects of CNT size on the surface properties of the formed deposition layers as well as to elucidate the mechanisms for regulation of the boiling and quenching behaviors. The results suggested that the critical heat flux (CHF) and the Leidenfrost point (LFP) are enhanced to various degrees due to the discrepancy in the size of the CNTs in nanofluids. It was shown that the CNTs deposited on the surfaces create various morphologies depending on their size. The CNTs with a length of 5 μm and a diameter of 60 nm exhibited the most significant effect on the boiling behaviors. In comparison to CNTs with a shorter length of 1 μm, the 5 μm long CNTs were much easier to form porous layers. The results of the contact angle and roughness tests showed that the porous layers tend to affect the surface roughness instead of surface wettability. The increases of the nucleation site density and surface roughness due to the presence of porous layers were identified as the primary cause for the modified boiling behaviors during quenching.

scholarly journals A spherical particle straddling a fluid/gas interface in an axisymmetric straining flow

1990 ◽  
Vol 217 ◽  
pp. 263-298 ◽  
Author(s):  
J. A. Stoos ◽  
L. G. Leal
Keyword(s):  
Contact Angle ◽  
Spherical Particle ◽  
Numerical Solutions ◽  
Contact Angles ◽  
Boundary Integral ◽  
Equilibrium Contact Angle ◽  
Equilibrium Configurations ◽  
Wide Range ◽  
The Stability ◽  
Neutrally Buoyant

Numerical solutions, obtained via the boundary-integral technique, are used to consider the effect of a linear axisymmetric straining flow on the existence of steady-state configurations in which a neutrally buoyant spherical particle straddles a gas–liquid interface. The problem is directly applicable to predictions of the stability of particle capture in flotation processes, and is also of interest in the context of contact angle and surface tension measurements. A primary goal of the present study is a determination of the critical capillary number, Cac, beyond which an initially captured particle is pulled from the interface by the flow, and the dependence of Cac on the equilibrium contact angle θc. We also present equilibrium configurations for a wide range of contact angles and subcritical capillary numbers.

Creation of Nano-Scaled Surface Structure for the Enhancement of Boiling Heat Transfer

2004 ◽  
Author(s):  
Sho Ngai ◽  
A. I. Leontiev ◽  
John R. Lloyd ◽  
S. P. Malyshenko
Keyword(s):  
Heat Transfer ◽  
Boiling Heat Transfer ◽  
Nucleation Site ◽  
Glancing Angle Deposition ◽  
Surface Structures ◽  
Transfer Enhancement ◽  
Site Density ◽  
Glancing Angle ◽  
Angle Deposition ◽  
Nucleation Site Density

The present research is an experimental investigation of nucleate pool boiling heat transfer enhancement on a surface with micro/nano-scaled surface structures. Glancing Angle Deposition (GLAD) was employed to fabricate porous surfaces in this study. The thin film microstructure consists of closely packed columns oriented in the plane of incidence formed due to a self-shadowing mechanism. Boiling heat transfer from the nano-structured surface was compared to that of a smooth reference surface and the commercial High Flux surface. The results of this study have shown that nano-structured films created by the GLAD process increase the nucleation site density as compared to the smooth surface. This research has opened up new areas in the field of heat transfer, which motivate new surface coating concepts to enhance the understanding of boiling heat transfer on nano-structured films.

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