Abstract
The natural circulation loop (NCL) is a highly reliable and noise-free heat transfer device due to the absence of moving components. Working fluid used in the natural circulation loop plays an important role in enhancing the heat transfer capability of the loop. This experimental study investigates the subcritical and supercritical heat transfer performance of a natural circulation loop (NCL) with CO2 as the working fluid. Operating pressures and temperatures are varied in such a way that the loop fluid should remain in the specified state (subcooled liquid, two-phase, superheated vapor, supercritical). Water and methanol are used as external fluids in cold and hot heat exchangers for temperatures above zero and below zero (in °C) respectively, depending on operating temperature. For loop fluids, the performance of CO2 is compared with water for above zero and with brine solution for the subzero case. Further, the impact of loop operating pressure (35–90 bar) on the performance of the system is also studied. For hot heat exchanger inlet temperature (5 to 70 °C) and cold heat exchanger inlet temperature (−18 to 32 °C), it was observed that the maximum heat transfer rates in the case of subcritical vapor, subcritical liquid, two-phase and supercritical CO2 based systems are 400%, 500%, 900%, and 800% higher than the water/brine-based system respectively.
Highlights
The heat transfer loops are classified as forced circulation loop (FCL) and the natural circulation loop (NCL)
Pressure drop and temperature distribution of single phase and two-phase CO2 based NCL compared with water/brine based natural circulation loop at same operating temperatures
The heat transfer rate of CO2 based NCL is compared with widely used loop fluid i.e., water and brine
Summary
The heat transfer loops (secondary loops) are classified as forced circulation loop (FCL) and the natural circulation loop (NCL). In an NCL, the heat sink is situated at a higher elevation than the heat source This establishes a density gradient in the system, due to which, lighter (warmer) fluid rises up and heavier (cooler) fluid moves down. Aqueous solutions are generally either salt based or alcohol based products These are having one or more non-favorable effects like corrosiveness, toxicity, high pH value, etc. CO2 has gained popularity as a loop fluid in NCL due to its excellent thermophysical properties and environment benignity (no ozone depletion potential and negligible global warming potential) and has been employed for various applications such as solar thermal collector[20], heat pump[21], geothermal system[22], etc. Suitability of CO2 as a loop fluid has been studied by Kiran Kumar et al.[23] for NCL, and by Yadav et al.[24] for forced circulation loop
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