Publication: "Two-Phase Flow and Boiling Heat Transfer Analysis of Macro and Mini-channels at Atmospheric and Subatmospheric System Pressure"
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Date
2024-02-29
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Indian Institute of Tehcnology, Jodhpur
Abstract
Phase change heat transfer processes find extensive application in heat extraction, harnessing both sensible and latent heat. Flow boiling, driven by rapid fluid vaporization, emerges as a highly effective method for high-flux heat transfer. Its versatile applications span refrigeration systems, thermal power plants, nuclear facilities, thermal desalination processes, and electronics cooling. The dominant forces governing flow boiling encompass surface tension, inertia, buoyancy, and viscous forces, crucial for maintaining thermal and momentum equilibrium. These forces are notably influenced by channel orientation, geometry, hydraulic diameter, and operating parameters. Importantly, the impact of these variables varies distinctly between the subcooled and saturated regions of flow boiling. Existing literature lacks comprehensive insights into the impact of orientation, geometry, diameter, and subatmospheric system pressure on flow boiling phenomena. This study delves into the impact of orientation on flow boiling heat transfer coefficients, employing both conventional straight tubes and helical coils. A spectrum of channel hydraulic diameters, spanning from 2 mm to 18 mm, is scrutinized to determine flow boiling heat transfer coefficients, two-phase pressure variations, and the dynamic behavior of wall temperature under two-phase flow conditions. Spatial and temporal variations in wall temperature are meticulously analyzed using thermal imaging technology. The experimental investigations encompass a wide range of operating conditions, including atmospheric and subatmospheric system pressure, with varying mass flux and heat flux values. The analysis includes an examination of the impact of Froude number and Boiling number. The study further investigates pressure drop and fluctuations within these channels under different operating pressure, and it offers a comparative analysis of existing correlations for flow boiling heat transfer coefficients and two-phase pressure drop. In the study, the flow boiling heat transfer coefficient in straight tube exhibits the highest values for horizontal flow, followed by vertical upward flow, and the lowest for vertical downward flow. Wall temperature displays radial asymmetry for Froude numbers below 0.22 due to gravitational effects but is symmetric for vertical up and downflow at all Froude numbers. Two-phase pressure fluctuations are more pronounced in horizontally oriented straight conventional tubes, and two-phase pressure drop is increased with higher mass flux and vapor quality in smaller diameter tubes at lower subatmospheric system pressure. Subcooled flow boiling heat transfer coefficient is higher at lower subatmospheric system pressure, with surface temperature showing radial symmetry under such conditions at all Froude numbers. For mini-channels, heat transfer stability is observed up to a boiling number of 1.76 × 10-4, but subcooled flow boiling heat transfer coefficient exhibits more fluctuations at higher Boiling numbers. Helical coils show variations in heat transfer distribution, critical heat flux, and pressure drop during flow boiling. Vertical-oriented helical coils display higher local and average heat transfer coefficients, with a higher burnout heat flux value compared to horizontal orientation. Two-phase pressure drop in helical coils is higher at subatmospheric system pressure and increases linearly with higher heat flux. This work contributes valuable insights into optimizing flow boiling processes in various geometries and operating conditions, advancing the understanding of heat transfer phenomena.
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"Kumar, Arvind (2019).Two-Phase Flow and Boiling Heat Transfer Analysis of Macro and Mini-channels at Atmospheric and Subatmospheric System Pressure (Doctor's thesis). Indian Institute of Tehcnology, Jodhpur"