A Computational Approach to Study Heat Transfer Enhancement in Film Boiling due to the Addition of Surfactants

2016 ◽  
Author(s):  
Kannan N. Premnath ◽  
Farzaneh Hajabdollahi ◽  
Samuel W. J. Welch
Keyword(s):  
Heat Transfer ◽  
Phase Change ◽  
Surfactant Concentration ◽  
Computational Approach ◽  
Film Boiling ◽  
Two Phase ◽  
Diverse Range ◽  
Study Heat Transfer ◽  
Interfacial Surfactant ◽  
Adsorption Desorption

Two-phase flows involving phase change are ubiquitous in a diverse range of scientific and technological applications. There has been great recent interest in the enhancement of boiling heat transfer processes by means of additives such as surfactants. Surfactants can influence boiling through convection currents in the bulk fluids as a result of changes in the surface tension caused by local surfactant concentration due their adsorption/desorption from the bulk regions. This can result in changes in bubble release patterns and higher heat transfer rates if such changes lead to higher rate of vapor formation. We intend to study this effect in the context of film boiling. Our computational approach augments the CLSVOF method with bulk energy and diffusion equations along with a phase change model and an interface surfactant model. The challenge here is to accurately calculate the tangential gradients of the interfacial surfactant concentration in the presence of discontinuous bulk concentration gradients near the interface. We discuss a simplified model in which the interfacial surfactant concentration is always in equilibrium with the changing bulk concentrations and then present validation results to assess the accuracy of this approach. Finally, initial studies of surfactant enhanced film boiling will be presented and interpreted.

RBHT Steam Cooling and Droplet Injection Pre-Test Analysis Using COBRA-TF

2001 ◽  
Author(s):  
C. Frepoli ◽  
A. J. Ireland ◽  
L. E. Hochreiter ◽  
F. B. Cheung
Keyword(s):  
Heat Transfer ◽  
Local Heat Transfer ◽  
Injection System ◽  
Test Facility ◽  
Film Boiling ◽  
Two Phase ◽  
Field Representation ◽  
Dispersed Flow ◽  
Spacer Grids ◽  
Flow Film Boiling

Abstract The droplet injection experiments to be performed in a 7 × 7 rod bundle heat transfer test facility are being simulated using an advanced thermal hydraulics computer code called COBRA-TF. A current version of the code, which provides a three-dimensional, two-fluid, three-field representation of the two-phase flow, is modified to facilitate the simulation of the droplet field produced by the injection system in the test facility. The liquid phase is split into a continuous liquid field and droplet field where a separate momentum and mass equation is solved for each field, with the effects of spacer grids being properly accounted for. Pre-test analyses using the modified COBRA-TF code have been conducted for different injection conditions. Results indicate that there are specific ranges of conditions that can be simulated within the facility constraints to provide for validation of the dispersed flow film boiling models. The numerical results also show important effects of the spacer grids on the local heat transfer in the dispersed flow film boiling regime.

Phase Change Process Inside a Small-radii Cylinder Subjected to Cyclic Convective Boundary Conditions: a Numerical Study

2021 ◽  
Author(s):  
Yousef Kanani ◽  
Avijit Karmakar ◽  
Sumanta Acharya
Keyword(s):  
Heat Transfer ◽  
Surface Temperature ◽  
Phase Change ◽  
Phase Change Materials ◽  
Numerical Study ◽  
Convective Boundary Condition ◽  
Freezing Process ◽  
Fourier Number ◽  
Two Phase ◽  
Convective Boundary

Abstract We numerically investigate the melting and solidi?cation behavior of phase change materials encapsulated in a small-radii cylinder subjected to a cyclic convective boundary condition (square wave). Initially, we explore the effect of the Stefan and Biot numbers on the non-dimensionalized time required (i.e. reference Fourier number Tref ) for a PCM initially held at Tcold to melt and reach the cross?ow temperature Thot. The increase in either Stefan or Biot number decreases Tref and can be predicted accurately using a correlation developed in this work. The variations of the PCM melt fraction, surface temperature, and heat transfer rate as a function of Fourier number are reported and analyzed for the above process. We further study the effect of the cyclic Fourier number on the periodic melting and freezing process. The melting or freezing front initiates at the outer periphery of the PCM and propagates towards the center. At higher frequencies, multiple two-phase interfaces are generated (propagating inward), and higher overall heat transfer is achieved as the surface temperature oscillates in the vicinity of the melting temperature, which increases the effective temperature difference driving the convective heat transfer.

Phase-Change Heat Transfer of Ethanol-Water Mixtures: Towards Development of a Distributed Hydrogen Generator

2013 ◽  
Author(s):  
Manoj Kumar Moharana ◽  
Rohan M. Nemade ◽  
Sameer Khandekar
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Phase Change ◽  
Transfer Coefficient ◽  
Single Phase ◽  
Flow Boiling ◽  
Phase Region ◽  
Design Flow ◽  
Two Phase ◽  
Vapor Region

Hydrogen fuel from renewable bio-ethanol is a potentially strong contender as an energy carrier. Its distributed production by steam reforming of ethanol on microscale platforms is an efficient upcoming method. Such systems require (a) a pre-heater for liquid to vapor conversion of ethanol water mixtures (b) a gas-phase catalytic reactor. We focus on the fundamental experimental heat transfer studies (pool and flow boiling of ethanol-water mixtures) required for the primary pre-heater boiler design. Flow boiling results (in a 256 μm square channel) clearly show the influence of mixture composition. Heat transfer coefficient remains almost constant in the single-phase region and rapidly increases as the two-phase region starts. On further increasing the wall superheat, heat transfer starts to decrease. At higher applied heat flux, the channel is subjected to axial back conduction from the single-phase vapor region to the two-phase liquid-vapor region, thus raising local wall temperatures. Simultaneously, to gain understanding of phase-change mechanisms in binary mixtures and to generate data for the modeling of flow boiling process, pool-boiling of ethanol-water mixtures has also been initiated. After benchmarking the setup against pure fluids, variation of heat transfer coefficient, bubble growth, contact angles, are compared at different operating conditions. Results show strong degradation in heat transfer in mixtures, which increases with operating temperature.

scholarly journals ELETRIC FIELD ENHANCEMENT AND DETERIORATION OF BOILING HEAT TRANSFER AND CRITICAL HEAT FLUX IN DIELECTRIC FLUIDS

2001 ◽  
Vol 1 (1) ◽  
pp. 32
Author(s):  
P. M. Carrica ◽  
V. Masson
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Critical Heat Flux ◽  
Electric Fields ◽  
Boiling Heat Transfer ◽  
Nucleate Boiling ◽  
Film Boiling ◽  
Two Phase ◽  
Boiling Regime ◽  
Dielectric Fluids

We present the results of an experimental study of the effects of externally imposed electric fields on boiling heat transfer and critical heat flux (CHF) in dielectric fluids. The study comprises the analysis of geometries that, under the effects of electric fields, cause the bubbles either to be pushed toward the heater or away from it. A local phase detection probe was used to measure the void fraction and the interfacial impact rate near the heater. It was found that the critical heat flux can be either augmented or reduced with the application of an electric field, depending on the direction of . In addition, the heat transfer can be slightly enhanced or degraded depending on the heat flux. The study of the two-phase flow in nucleate boiling, only for the case of favorable dielectrophoretic forces, reveals that the application of an electric field reduces the bubble detection time and increases the detachment frequency. It also shows that the two-phase flow characteristics of the second film boiling regime resemble more a nucleate boiling regime than a film boiling regime.

Analysis on Heat Transfer and Phase Change for Two-Phase Boiling Flow in Quenching Process

2019 ◽  
Author(s):  
Lu Wang ◽  
Nobuyuki Oshima ◽  
Sangwon Kim
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Phase Change ◽  
Wall Temperature ◽  
Computational Domain ◽  
Heating Wall ◽  
Two Phase ◽  
Phase Form ◽  
Quenching Process ◽  
Boiling Flow

Abstract A series of numerical simulations using “interThermalPhaseChangeFOAM” solver with improved VOF multiphase flow model in OpenFOAM were conducted to investigate the heat transfer and phase change characteristics for liquid-vapor boiling flow in quenching process. The computational domain is a cuboid with the heating wall at the bottom for both the variable and fixed wall temperature cases. The results for the variable wall temperature case with the heating wall temperature Twall = 150K show that the boiling phenomenon can be divided into the vapor film stage, the boiling stage and the convection stage. Then the fixed wall temperature cases with Twall = 110K, 120K and 140K are analyzed. It is found that 140K case is the most stable one, in which bubble formation is regular such as the bubble at the corner, resulting in the steady variation of heat flux. 120K case is the most unstable one, since the liquid phase and gas phase form the cross-interface shape and maintain this for a long time, leading to the fluctuations in heat flux. Finally, the influence of computational sizes on predicting the properties of boiling phenomenon is investigated. Although the variations of heat flux are not exactly same, the whole tendency is similar.

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