Computational Study of Saturated Flow Boiling Within a Microchannel in the Slug Flow Regime

2015 ◽  
Vol 138 (2) ◽  
Author(s):  
Mirco Magnini ◽  
John R. Thome
Keyword(s):  
Heat Transfer ◽  
Slug Flow ◽  
Flow Boiling ◽  
Periodic Regime ◽  
Liquid Slug ◽  
Modeling Framework ◽  
Saturated Flow ◽  
Bubble Liquid ◽  
Continuous Stream ◽  
Multiple Bubbles

This paper presents a fundamental study of the flow dynamics and heat transfer induced by a slug flow under saturated flow boiling in a circular microchannel. Numerical simulations are carried out by utilizing the commercial CFD solver ansys fluent v. 14.5, with its built-in volume of fluid (VOF) method to advect the interface, which was improved here by implementing self-developed functions to model the phase change and the surface tension force. A continuous stream of bubbles is generated (by additional user-defined functions) by patching vapor bubbles at the channel upstream with a constant generation frequency. This modeling framework can capture the essential features of heat transfer in slug flows for a continuous stream of bubbles which are here investigated in detail, e.g., the mutual influence among the growing bubbles, the fluid mechanics in the liquid slug trapped between two consecutive bubbles, the effect of bubble acceleration on the thickness of the thin liquid film trapped against the channel wall and on other bubbles, and the transient growth of the heat transfer coefficient and then its periodic variation at the terminal steady-periodic regime, which is reached after the transit of a few bubble–liquid slug pairs. Furthermore, the results for a continuous stream of bubbles are found to be quite different than that of a single bubble, emphasizing the importance of modeling multiple bubbles to study this process. Finally, the outcomes of this analysis are utilized to advance a theoretical model for heat transfer in microchannel slug flow that best reproduces the present simulation data.

Use of Two-Phase CFD Simulations to Develop a Boiling Heat Transfer Prediction Method for Slug Flow Within Microchannels

2015 ◽  
Author(s):  
Mirco Magnini ◽  
John R. Thome
Keyword(s):  
Heat Transfer ◽  
Liquid Film ◽  
Slug Flow ◽  
Boiling Heat Transfer ◽  
Prediction Method ◽  
Liquid Slug ◽  
Modeling Framework ◽  
Cfd Simulations ◽  
Two Phase ◽  
Elongated Bubble

This work presents a new boiling heat transfer prediction method for slug flow within microchannels, which is developed and benchmarked against the results of two-phase CFD simulations. The proposed method adopts a two-zone decomposition of the flow for the sequential passage of a liquid slug and an evaporating elongated bubble. The heat transfer is modeled by assuming transient heat conduction across the liquid film surrounding an elongated bubble and sequential conduction/convection within the liquid slug. Embedded submodels for estimating important flow parameters, e.g. bubble velocity and liquid film thickness, are implemented as “building blocks”, thus making the entire modeling framework totally stand-alone. The CFD simulations are performed by utilizing ANSYS Fluent v. 14.5 and the interface between the vapor and liquid phases is captured by the built-in Volume Of Fluid algorithm. Improved schemes to compute the surface tension force and the phase change due to evaporation are implemented by means of self-developed functions. The comparison with the CFD results shows that the proposed method emulates well the bubble dynamics during evaporation, and predicts accurately the time-averaged heat transfer coefficients during the initial transient regime and the terminal steady-periodic stages of the flow.

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.

Numerical Simulation of Saturated Flow Boiling Heat Transfer of Ammonia/Water Mixture in Bubble Pumps for Absorption–Diffusion Refrigerators

2013 ◽  
Vol 6 (1) ◽  
Author(s):  
Soo W. Jo ◽  
S. A. Sherif ◽  
W. E. Lear
Keyword(s):  
Heat Transfer ◽  
Numerical Simulation ◽  
Numerical Model ◽  
Boiling Heat Transfer ◽  
Flow Boiling ◽  
Water Mixture ◽  
Saturated Flow ◽  
Ammonia Water ◽  
Flow Boiling Heat Transfer ◽  
Bubble Pump

This paper addresses a multidimensional numerical simulation of the saturated flow boiling heat transfer in bubble pumps of absorption–diffusion refrigeration cycles. The bubble pump with a shape of vertical tube is subjected to a uniform heat flux from the tube outer wall surface along the entire pump length. As the bubble pump wall is heated, a nonazeotropic mixture of saturated strong ammonia/water entering into the bubble pump transforms to ammonia vapor and diluted ammonia/water mixture. The weaker ammonia/water mixture is lifted by the buoyant force created by the ammonia vapor. The present multidimensional numerical simulation was performed using the two-fluid model with the equilibrium phase change model and the standard k-ε turbulence model. The numerical model designed for the present simulation was validated through a comparative study referring to available experimental data. The present numerical model was compared with the one-dimensional model to assess its applicability for numerical simulation of the saturated flow boiling heat transfer in bubble pumps. As a result, it is seen that the present numerical model predicts the performance of ammonia/water bubble pumps more realistically than the one-dimensional model. In addition, the effects of the bubble pump's geometrical dimension and heat input on the pump performance were investigated using the present numerical approach.

Sign in / Sign up

Export Citation Format

Share Document