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 co 2 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 co 2 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 co 2 based systems are 400%, 500%, 900%, and 800% higher than the water/brine-based system respectively.

Inhibition effect of oleic acid hydrazide (OAH), a green corrosion inhibitor on API X70 steel in an Indian oilfield produced water was investigated for the first time under flow condition. The test was carried out at a more critical location (90-degree pipe elbow) in a circulating loop system with the fixed flow velocity. This location is deliberately chosen because, at this location, catastrophic failure of a low alloy steel piping system usually takes place under highly corrosive turbulent flow with higher wall shear stress and flow velocity. The corrosion examination was carried out by varying the concentrations (0.05, 0.15, 0.30 g L −1 ) of the OAH inhibitor under flow condition. The maximum inhibitor efficiency is found 87.7% at 0.30 g L −1 concentration. The formation of protective film was confirmed by SEM, XPS, Raman spectroscopy and FTIR spectra.

The natural circulation loop (NCL) is widely used where the safe and economic heat transfer device is desired. However, the instability associated with the regular change in fluid flow behavior due to the imbalance between friction and buoyant forces is a major disadvantage. One of the erudite solutions to overcome this is to tilt the entire loop by a certain angle, with an inherent penalty in heat transfer and pressure drop. In the present study, experimental studies have been carried out on two-phase carbon dioxide (CO2) based NCL, which has gained popularity because of its compactness and higher heat transfer rate. Pressure drop and heat transfer performance of the loop for various tilt angles (0 deg, 30 deg, and 45 deg) in different planes (XY and YZ planes) have been investigated. Methanol is used as the external fluid in cold and hot heat exchangers in order to maintain low operating temperature in the loop. Results show that the tilting of the loop causes a marginal drop in the heat transfer rate of two-phase CO2 based NCL. Hence, tilting of the loop could be a solution to instability problem without conceding the performance of the loop.