Flow condensation heat transfer phenomenon and liquid film thickness in the Printed Circuit Heat Exchanger (PCHE) using a CO2 (Carbon dioxide) as a working fluid

Abstract

With the increase of the energy industry, the amount of CO2 generated continues to increase, which is accelerating environmental pollution. In considered of environmental pollution, various systems that reduce the volume by condensing CO2 have recently been proposed to store and utilize it. Previously, CO2 was condensed by heat exchanging with seawater after pressurization of CO2. However, this is less economical because high compressor work occurs. To solve this problem, CO2 condensing systems using LNG cold energy have been spotlight. LNG is a state that natural gas is liquefied to facilitate transportation and storage, but vaporization is generated in the vaporizer to reuse it. In this process, LNG generates energy loss by exchanging heat with seawater, and this energy is called LNG cold energy. Therefore, utilizing the discarded LNG cold energy has the advantage of being able to condense CO2 without pressurization system. Further studies are needed because these systems not only reduce energy loss, but also prevent environmental pollution. The CO2 condensing systems using the LNG cold energy can be applied to various applications such as CO2 trans-critical cycle, CO2 capture and storage systems, and CO2 purification system. The device commonly used in these systems is the condenser. Since the condenser uses LNG as the working fluid, it must to be able to operate at a very low temperature and high pressure. There are various heat exchanger types for the application of the condenser, but the suitable heat exchanger under this extreme condition is PCHE. Many researches on PCHE have been conducted to apply this heat exchanger to the Brayton cycle (High pressure and temperature conditions) in the early days. In other words, most studies are on the gas phase (Single-phase). In this regard, it is necessary to study the condensation phenomenon in PCHE in order to design the CO2 condenser using a LNG cold energy. Recently, several literatures have been presented to understand the condensation phenomenon in PCHE through simulation and experiment. However, studies on heat transfer and liquid film thickness in PCHE are still insufficient. Based on this, this paper conducted heat transfer experiments in the PCHE according to boundary conditions. In the experiment, the temperature and heat flux distribution were measured through optical fiber sensor. In this paper, to understand condensation phenomenon in PCHE, the analytical liquid film thickness model was developed through momentum equation and energy balance, and was verified with experimental result. Based on the liquid film thickness model and experimental results, the heat transfer coefficient analysis was conducted according to the each variable. The heat transfer coefficient increases as the mass flux increases because increase in mass flux makes liquid film thickness thinner due to increase in interfacial shear force. In the case of the saturation pressure, heat transfer coefficient increases as saturation pressure decreases. As the saturation pressure decreases, the density of the vapor decreases and the velocity increases, which causes an increase in shear force, making the film thinner. In addition, when the saturation pressure decreases, the increase in thermal conductivity of the liquid film increases the heat transfer coefficient, and the surface tension increases to spread the liquid film distribution evenly, thereby increasing the heat transfer coefficient. In terms of channel effect, most of the annular flow is formed without any change in flow patter in all channel diameters because the channel diameter is very small, and the impact on the gravity force is very small. Additionally, the smaller the channel size, the better the heat transfer coefficient. The small channel creates a lower Reynolds number under the same mass flux condition, increasing the shear friction factor. This result makes liquid film thinner. In addition, the distributions of liquid film thicknesses of water, R-22, and CO2 were calculated to analyze properties according to the working fluid. The liquid film thickness of water was the thinnest, and that of the R-22 was the thickest. It means the change in the liquid film distribution according to the physical properties of the working fluid. Since the density of water is relatively low, it has a high velocity under the same mass flus conditions, which increases the shear force. In the case of the effect of the surface force, the surface tension of water was the largest. A large surface tension means that the liquid film thickness spreads evenly in the radial direction, and the liquid film at the center point is formed thinner. Therefore, the characteristics of high surface tension and low density of water made the liquid film thickness thin. In order to analyze the heat transfer phenomenon of the liquid film, the correlation between the liquid film thickness and the heat transfer coefficient was presented. The developed correlation showed a big difference from the conduction dominant transport model presented in the Nusselt model, but the heat transfer of the liquid film was well predicted to be within about 30% from the models presented in convection dominant transport. The convection models suggest that the liquid film has a wavy-laminar flow condition, and most of the heat transfer is caused by convection. According to this result, it was found that most of the CO2 condensation in the PCHE is heat transferred by convection in the form of wavy-laminar flow. Through these experimental results and liquid film model, a heat transfer coefficient model applicable to a condenser was developed, and various correlations and comparisons were performed. The developed correlation showed a difference of more than 40% from the general heat transfer correlation in circular channel, and a difference of about 10% from the correlation from the experiment and simulation results in PCHE. In addition, PCHE design codes were developed to evaluate the condenser applicability. The optimal design diameter and Reynolds number (Diameter: 1mm, and Reynolds number: 10,000) were proposed through economic evaluation. Finally, it has been suggested that CO2 condensation through LNG cold energy is much more economical than the existing system that condenses CO2 through pressurization.