Modeling and Prediction of Two-Phase Refrigerant Flow Regimes and Heat Transfer Characteristics in Microgap Channels

2007 ◽  
Author(s):  
Avram Bar-Cohen ◽  
Emil Rahim
Keyword(s):  
Heat Transfer ◽  
Annular Flow ◽  
Two Phase Flow ◽  
Flow Regimes ◽  
Phase Flow ◽  
Transfer Coefficients ◽  
Two Phase ◽  
Channel Diameter ◽  
Dielectric Liquids ◽  
Transition Criteria

This keynote lecture will open with a brief review of the primary two-phase flow regimes and their impact on thermal transport phenomena in tubes and channels. The Taitel and Dukler flow regime mapping methodology will then be described and applied to the two-phase flow of refrigerants and dielectric liquids in microgap channels. The effects of channel diameter, as well as alternative transition criteria, on the prevailing flow regimes in microgaps will be explored along with available criteria for microchannel behavior. Available microgap data will then be shown to reflect the dominance of annular flow and to display a characteristic heat transfer coefficient curve in such configurations. It is found that the heat transfer coefficients in the low-quality annular flow segment of this locus can be predicted by available, microtube correlations, but that the moderate-quality transition to the axially-decreasing segment occurs at substantially.

Experimental Study on Heat Transfer of Single-Phase Flow and Boiling Two-Phase Flow in Vertical Narrow Annuli

2002 ◽  
Author(s):  
Suizheng Qiu ◽  
Minoru Takahashi ◽  
Guanghui Su ◽  
Dounan Jia
Keyword(s):  
Heat Transfer ◽  
Single Phase ◽  
Two Phase Flow ◽  
Annular Channel ◽  
Phase Flow ◽  
Single Phase Flow ◽  
Transfer Coefficients ◽  
Two Phase ◽  
Narrow Annuli ◽  
Narrow Gaps

Water single-phase and nucleate boiling heat transfer were experimentally investigated in vertical annuli with narrow gaps. The experimental data about water single-phase flow and boiling two-phase flow heat transfer in narrow annular channel were accumulated by two test sections with the narrow gaps of 1.0mm and 1.5mm. Empirical correlations to predict the heat transfer of the single-phase flow and boiling two-phase flow in the narrow annular channel were obtained, which were arranged in the forms of the Dittus-Boelter for heat transfer coefficients in a single-phase flow and the Jens-Lottes formula for a boiling two-phase flow in normal tubes, respectively. The mechanism of the difference between the normal channel and narrow annular channel were also explored. From experimental results, it was found that the turbulent heat transfer coefficients in narrow gaps are nearly the same to the normal channel in the experimental range, and the transition Reynolds number from a laminar flow to a turbulent flow in narrow annuli was much lower than that in normal channel, whereas the boiling heat transfer in narrow annular gap was greatly enhanced compared with the normal channel.

Thermoreflectance Wall Temperature Measurement in Annular Two-Phase Flow

2019 ◽  
Author(s):  
Jason Chan ◽  
Brian E. Fehring ◽  
Roman W. Morse ◽  
Kristofer M. Dressler ◽  
Gregory F. Nellis ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Wall Temperature ◽  
Annular Flow ◽  
High Heat ◽  
High Heat Flux ◽  
Phase Flow ◽  
Transfer Coefficients ◽  
Two Phase ◽  
Heat Transfer Coefficients ◽  
Dry Out

Abstract A thermoreflectance method to measure wall temperature in two-phase annular flow is described. In high heat flux conditions, momentary dry-out occurs as the liquid film vaporizes, resulting in dramatic decreases in heat transfer coefficient. Simultaneous liquid and vapor thermoreflectance measurements allow calculations of instantaneous and time-averaged heat transfer coefficients. Validation, calibration and uncertainty of the technique are discussed.

scholarly journals Two-dimensional heat transfer coefficients with simultaneous flow visualisations during two-phase flow boiling in a PDMS microchannel

2018 ◽  
Vol 130 ◽  
pp. 624-636 ◽  
Author(s):  
Sofia Korniliou ◽  
Coinneach Mackenzie-Dover ◽  
John R.E. Christy ◽  
Souad Harmand ◽  
Anthony J. Walton ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Two Phase Flow ◽  
Flow Boiling ◽  
Phase Flow ◽  
Two Dimensional ◽  
Transfer Coefficients ◽  
Two Phase ◽  
Heat Transfer Coefficients ◽  
Pdms Microchannel

Heat Transfer for Annular Flow in Microchannel Bends: A Free Energy Minimization Model for Square Channels

2007 ◽  
Author(s):  
Sara Beaini ◽  
Van P. Carey
Keyword(s):  
Heat Transfer ◽  
Free Energy ◽  
Annular Flow ◽  
Two Phase Flow ◽  
Phase Flow ◽  
Centrifugal Acceleration ◽  
Two Phase ◽  
The Mean ◽  
Flow Morphology ◽  
Liquid Vapor

For annular liquid-vapor two-phase flow in straight microchannels, effects of gravity are generally small compared to viscous and/or inertia forces. In serpentine evaporator or condenser passages with semicircular return bends, the bend radius may be so small that large centrifugal body forces are generated as the fluid flows through the bend region of the passage. This paper summarizes a model analysis based on the premise that flow morphology in the bend is dictated by radial acceleration forces and the thermodynamic Second Law requirement that the established two-phase flow morphology minimizes the free energy at the local temperature and pressure. An analytical model is derived relating the dependence of the free energy on vapor core geometry, and the geometry that minimizes free energy is determined numerically. This provides a prediction of the mean thickness of the liquid surrounding the vapor core, and the mean heat transfer coefficient for annular flow vaporization or condensation, as a function of flow parameters and physical properties. When this relation is cast in dimensionless form, the effect of centrifugal acceleration is quantified in terms of a Weber number (We) that represents the ratio of centrifugal body force to surface tension force. The analysis indicates that centrifugal acceleration acts to displace the vapor towards the inside of the curved passage and distort the liquid-vapor interface. Displacement occurs at any level of acceleration. Significant distortion is found to occur only for We > 1. The effects of these morphology changes on heat transfer are analyzed and the implications of these predictions for designing microchannel evaporators and condensers are explored.

Numerical Simulation of Flow Regime Transition From Slug to Wavy Flow in Helically Coiled Tubes

2016 ◽  
Author(s):  
Zhaoxu Li ◽  
Hongye Zhu
Keyword(s):  
Heat Transfer ◽  
Numerical Simulation ◽  
Centrifugal Force ◽  
Slug Flow ◽  
Two Phase Flow ◽  
Flow Regimes ◽  
Phase Flow ◽  
Pressurized Water Reactor ◽  
Two Phase ◽  
Pressurized Water

Two-phase flow in helically coiled tubes is becoming the interest of many investigators because of its importance in various applications, such as nuclear engineering, chemical engineering, refrigerating engineering and power engineering. Compared with U-type tubes used in pressurized water reactor (PWR), helically coiled tubes have advantages in size, heat transfer capacity, thermal stress toleration and two-phase stability. Accordingly the helically coiled tubes have been utilized in the steam generators of the next general reactors, such as gas-cooled reactor, fast breeder reactor and integrated pressurized water reactor. In helically coiled tubes the characteristics of momentum and heat transfer are distinct from those in straight tubes due to the presence of centrifugal force, especially for two-phase flow. Meanwhile, the transitions of flow regime, which is the crucial knowledge for the designers to determine the heat transfer rates and flow resistance, are also significantly affected by the centrifugal force. In this study, two-phase flow regimes in helically coiled tubes are investigated. Computational fluid dynamics (CFD), using fractional volume of fluid (VOF) model, is carried out to simulate wavy and slug flow regimes in helically coiled tubes. The corresponding experiment is also conducted to visualize these flow regimes at different superficial flow velocities. Numerical simulation results actually reflect the influence of centrifugal force on the two-phase flow and show a good agreement with the photographs captured from the experiment. Based on the simulations at different superficial flow velocities, the boundary between the slug and wavy flow regimes is predicted, in addition, compared with that in inclined tubes. The comparison indicates that centrifugal force could induce the appearance of wavy flows in advance and prompt the transition from slug flow to wavy flow.

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