Experimental Investigations on Single/Two Phase Carbon Dioxide Based Natural Circulation Loops

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 thermo-hydraulic performance of subcritical and supercritical CO2 based natural circulation loop (NCL). In the subcritical region, different phases of CO2 such as liquid, vapour and two phase are considered for the study. 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 and below 0 °C respectively, depending on the 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. Results are obtained for hot heat exchanger inlet temperature ranging from 5 to 70 °C and cold heat exchanger inlet temperature from -18 to 32 °C. It was observed that the maximum heat transfer rates in the case of subcritical vapor, subcritical liquid, twophase and supercritical CO2 based systems are 400%, 500%, 900%, and 800% higher than the water/brine-based system respectively. However, the instability associated with a regular change in fluid flow direction due to an imbalance between friction and buoyant forces is a major disadvantage of NCL. Tilting the entire loop by a certain angle is one of the method to reduce the instability of NCL, with an inherent penalty in heat transfer and pressure drop. Pressure drop and heat transfer performance of CO2 based NCL with end heat exchangers, operating under subcritical (vapour, liquid and two phase) and supercritical conditions for various tilt angles (0°, 30°, 45°) in XY and YZ planes, have been investigated. Results are obtained for different operating pressures (35-90 bar) and temperatures (-18 to 70 °C). Results show that the effect of tilting on heat transfer rate, pressure drop and temperature distribution in the case of supercritical, subcritical liquid, subcritical vapour and two phase CO2 based system shows a small variation on the performance of the loop. Hence, loop tilting could be a one of the solution to reduce the instability problem associated with NCL without compromising the performance of the loop.