Heat Transfer Characteristics of a Pipeline for CO2 Transport with Water as Surrounding Substance

AbstractThe heat transfer characteristics of pipelines for transport of CO2 is crucial for the events following a depressurization or a crack formation, involving rapid cooling. In this work, we present and analyze recent experiments from an experimental facility tailored to investigate these phenomena. With stagnant water as the surrounding substance, we quantify the contribution to the heat transfer from the surroundings, the insulation and the CO2 boiling inside the pipeline, for a large set of operating conditions. We discuss whether empirical expressions in the literature can describe the outer heat transfer coefficient and analyze the experimental results in detail using computational fluid dynamical simulations. The work gives insight into and quantifies the heat transfer characteristics of a CO2-pipeline. In particular, the outer heat transfer coefficient was between 80 and 210W/m2K, the thermal conductivity of the insulation was well described by a linear temperature relation and the mean value of the overall heat transfer coefficient was 44.7W/m2K. The work lays the foundation for future work on this subject, which will involve other surrounding substances such as clay and gravel as well as the forming of ice

A combined fluid-dynamic and thermodynamic model to predict the onset of rapid phase transitions in LNG spills

Transport of liquefied natural gas (LNG) by ship occurs globally on a massive scale. The large temperature difference between LNG and water means LNG will boil violently if spilled onto water. This may cause a physical explosion known as rapid phase transition (RPT). Since RPT results from a complex interplay between physical phenomena on several scales, the risk of its occurrence is difficult to estimate. In this work, we present a combined fluid-dynamic and thermodynamic model to predict the onset of delayed RPT. On the basis of the full coupled model, we derive analytical solutions for the location and time of delayed RPT in an axisymmetric steady-state spill of LNG onto water. These equations are shown to be accurate when compared to simulation results for a range of relevant parameters. The relative discrepancy between the analytic solutions and predictions from the full coupled model is within 2% for the RPT position and within 8% for the time of RPT. This provides a simple procedure to quantify the risk of occurrence for delayed RPT for LNG on water. Due to its modular formulation, the full coupled model can straightforwardly be extended to study RPT in other systems.Comment: 22 pages, 11 figure

Predicting triggering and consequence of delayed LNG RPT

Predicting triggering and consequence of delayed LNG RPTacceptedVersio

Optical Properties of truncated and coated spheroidal Nanoparticles on a Substrate

In nanoparticle research it is common to perform optical measurements on particle films during deposition, to help understand the growth process. GranFilm is a software under development which can calculate the optical properties of an array of truncated nanoparticles supported on a substrate. The theory behind these calculations is based on the work of Bedeaux and Vlieger. One feature which was missing from the software until now was the ability to do such simulations on the case of truncated spheroidal nanoparticles with an arbitrary number of coatings of different materials. In the beginning of this work, the equations needed to perform these simulations are derived, and then reduced to previously derived special cases for verification. The new equations are then implemented into GranFilm, and the new code is put through numerical tests. Finally, the new functionality is tested with the help of experimental data from an oxidation process of a silver nanoparticle film. The qualitative evolution of the optical properties of the film is reproduced quite successfully, but some issues remain

Film boiling and rapid phase transition of liquefied natural gas

When liquefied natural gas (LNG) is spilled onto water there is a possibility that explosive rapid phase transition (RPT) events occur. According to experiments, these vapor explosions are highly unpredictable, with yields up to several kilograms of TNT equivalent. The leading theory of RPT claims that triggering occurs due to a sudden and rapid chain of events involving film-boiling collapse, liquid superheating, rapid nucleation and explosive expansion. Still, after over four decades of research on the topic, it appears that there is no reliable and accepted method for quantitative LNG RPT risk-assessment. The main goal of the present thesis is to remedy this issue through theoretical means. According to the leading theory of RPT, prediction of the triggering event necessitates modelling of two properties: the Leidenfrost temperature and the superheat limit temperature, both of which were investigated herein. The Leidenfrost temperature is by definition the surface temperature below which film-boiling collapse occurs. Therefore, it is necessary to understand film boiling and its stability. While much work has been done in the past on modelling the stability of thin liquid films with the long-wave approximation, these models are not directly applicable to film boiling. The equations describing a vapor film trapped between two dense phases of extremely different temperatures turn out to be different in subtle but important ways. In this project a new model for vapor-film dynamics has been developed within the long-wave approximation methodology. This model crucially involves a coupling to non-equilibrium evaporation models from kinetic theory, which allows for the inclusion of the thermocapillary effect at the evaporating interface. Based on stability analysis of this model, a novel and promising prediction method for the Leidenfrost temperature has been discovered. The method carries with it the surprising theoretical implication that film-boiling collapse occurs when the thermocapillary instability overpowers vapor-thrust stabilization. However, further experimental investigations of the Leidenfrost temperature is needed in order to draw strong conclusions regarding its validity. The superheat limit may be estimated within the framework of classical nucleation theory (CNT). These predictions have been compared with a wide array of relevant experimental data on hydrocarbons, both for pure fluids and binary mixtures. The performance of the CNT model was deemed satisfactory, and thus, no further efforts to improve superheat limit modelling have been made in this project. Finally, a framework for the prediction of RPT risk and consequence during LNG boil-off has been developed. This framework demonstrates how models for the Leidenfrost temperature and the superheat limit temperature as functions of LNG mixture composition may be combined with classical thermodynamics in order to predict when (and if) the conditions for RPT triggering may be met. Additionally, it has been shown how the predicted LNG composition at the time of triggering may be used to estimate the worst case explosive pressure and energy yield through the use of a simplified thermodynamic model. While quite idealized, this framework represents an important step towards practical risk assessment and mitigation for LNG rapid phase transition. The thesis concludes with a series of suggestions on how the framework may be further improved

The spinodal of single- and multi-component fluids and its role in the development of modern equations of state

The spinodal represents the limit of thermodynamic stability of a homogeneous fluid. In this work, we present a robust methodology to obtain the spinodal of multicomponent fluids described even with the most sophisticated equations of state (EoS) available. We elaborate how information about the spinodal and its uncertainty can contribute both in the development of modern EoS and to estimate their uncertainty in the metastable regions. Inequality constraints are presented that can be exploited in the fitting of modern EoS of single-component fluids to avoid inadmissible pseudo-stable states between the vapor and liquid spinodals. We find that even cubic EoS violate some of these constraints. Using a selection of EoS representative of modern applications, we compare vapor and liquid spinodal curves, superheat and supersaturation limits from classic nucleation theory (CNT), and available experimental data for the superheat limit. Computations are performed with pure species found in natural gas, binary mixtures herein, as well as a multi-component natural gas mixture in order to demonstrate the scalability of the approach. We demonstrate that there are large inconsistencies in predicted spinodals from a wide range of EoS such as cubic EoS, extended corresponding state EoS, SAFT and multiparameter EoS. The overall standard deviation in the prediction of the spinodal temperatures were 1.4 K and 2.7 K for single- and multi-component liquid-spinodals and 6.3 K and 26.9 K for single- and multi-component vapor spinodals. The relationship between the measurable limit of superheat, or supersaturation, and the theoretical concept of the spinodal is discussed. While nucleation rates from CNT can deviate orders of magnitude from experiments, we find that limit of superheat experiments agree within 1.0 K and 2.4 K with predictions from CNT for single- and multi-component fluids respectively. We demonstrate that a large part of the metastable domain of the phase diagram is currently unavailable to experiments, in particular for metastable vapor. Novel techniques, experimental or with computational simulations should be developed to characterize the thermodynamic properties in these regions, and to identify the thermodynamic states that define the spinodal

Comparison of kinetic theory evaporation models for liquid thin-films

We summarize the background and derivation of non-equilibrium evaporation models from kinetic theory, and demonstrate how they may be applied in the context of fluid mechanics and heat transfer problems. We find that the linearized Boltzmann-equation Moment Method is a good trade-off between complexity and accuracy for practical purposes, and that the use of non-equilibrium evaporation models in general has significant quantitative and qualitative impact on the results