Experimental and Theoretical Analyses of Adiabatic Two-phase Flows in Horizontal Feed Pipes

The majority of technical separation processes for fluid mixtures utilize the principle of rectification. If a two-phase mixture is fed into the column, possibly undesirable flow morphologies or severe droplet carry-over may occur, which detrimentally affect separation efficiency and equipment integrity. Currently, the two-phase flow behavior in feed pipes is hardly predicable and mostly based on empirical or heuristic methods, which do not properly account for a broad range of possible fluid properties and plant dimensions. As a consequence, costly safety margins are applied. Feed pipes to separation columns are often characterized by horizontal inlet nozzles, small length-to-diameter ratios and complex routing, involving elbows or bends. The pipe lengths are too short to enable the two-phase flow to fully develop, which thus, enters the column with unknown flow morphology. Since developing flows have rarely been studied, today’s engineering practice relies on existing predictive methods for fully developed two-phase flows. Graphical methods can hardly represent gradual transitions between flow regimes. Analytical models provide only simplified flow representations of the two-phase flow that have not yet been qualified for developing pipe flow. In this work, a comprehensive experimental database of horizontal water-air flows in two test sections with nominal pipe diameters of D = 50 mm and D = 200 mm and feed pipe lengths in the range 10 < L/D < 75 was established. This way, the data cover developing pipe flows with entrance lengths typical for two-phase feeds of separation columns and more developed flows that are comparable with the extensively studied reference system water-air. A particular focus was put on the effect of pipe bends on the flow morphology up- and downstream. The flow morphology was captured using imaging wire-mesh sensors. A 4D fuzzy algorithm was applied to objectively identify the flow two-phase morphologies. Based on their fuzzy representation, the flow morphologies were classified and a novel 2D visualization technique is proposed to discuss the flow development along the feed pipes. Undesired flow morphologies (intermittent flow and entrainment) during the operation of two-phase feeds are hardly predictable by conventional design tools. The inception of intermittent flows was analyzed using the experimental data. Consequently, the inception criteria based on the required liquid levels for fully developed intermittent flows were adapted for short entrance lengths. The characteristic dynamics of flow morphologies that are known to cause the onset of entrainment were analyzed. Based on wave frequencies, a predictive criterion for the susceptibility of wavy flows for the onset of entrainment is introduced and applied to straight feed pipes and horizontal 90° bends. Among the dozens available, 66 reduced-order models for the prediction of the void fraction were tested for straight feed pipes and horizontal 90° pipe bends. Thereof, the ones most suitable for variable operating conditions and pipe geometries were identified and adapted. Complementary 3D simulations were performed to verify the applicability of numerical codes (VoF, AIAD) for flows with free interfaces. The flow morphologies were successfully reproduced at macroscopic scale, however, the simulation results rank behind reduced-order models considering their quantitative predicting capabilities.:Abstract II Kurzfassung IV Acknowledgement VI Nomenclature VIII Table of Contents XIII 1 Introduction 1 1.1 Thermal separation in view of the 21st century 1 1.2 Engineering and design of rectification plants 2 1.3 Outline of the thesis 4 2 State of the art 5 2.1 Two-phase feeds in thermal separation 5 2.1.1 Feed condition as adjustable parameter 5 2.1.2 Thermohydraulic optimization 8 2.1.3 Hydrodynamic conditioning 9 2.2 Hydrodynamics of two-phase feeds 11 2.2.1 Flow morphologies in feed pipes 11 2.2.2 Droplet entrainment 14 2.2.3 Flow regime maps 17 2.2.4 Consequences for two-phase feeds 19 2.3 Modelling of two-phase feeds 23 2.3.1 Basic definitions 23 2.3.2 Fundamentals of the two-fluid model 25 2.3.3 The interfacial level gradient 29 2.3.4 Analytical models 32 2.3.5 CFD simulations for commercial feed pipes 34 2.4 Objectives of this thesis 36 3 Experimental method and algorithms for flow characterization 37 3.1 Experimental setups 37 3.2 Wire-mesh sensors 40 3.3 Experimental procedure 42 3.4 Data processing 44 3.4.1 Fuzzy flow morphology classification 45 3.4.2 Power spectral density 48 3.5 Measurement uncertainty 49 4 Flow morphologies in different feed pipe geometries 53 4.1 Developing two-phase flow in straight pipes 53 4.2 Effect of pipe curvatures on the flow morphology 55 4.3 Morphology recovery 57 4.4 Conclusions 60 5 Prediction of undesirable flow morphologies in feed pipes 61 5.1 Initiation of intermittent flows 61 5.2 Onset of droplet entrainment 62 5.2.1 Vulnerable flow morphologies 62 5.2.2 Derivation of a criterion for onset of entrainment 64 5.2.3 Adjustment of the criterion for the investigated pipe geometries 67 5.3 Conclusions 70 6 Reduced-order modelling of two-phase feeds 71 6.1 Prediction of void fraction 71 6.2 Liquid levels 75 6.3 Conclusions 78 7 CFD modelling of two-phase feeds 79 7.1 Simulation setup 79 7.2 Multiphase models 82 7.3 Comparison with experimental data 83 7.3.1 Straight pipes 83 7.3.2 Horizontal 90° bends 85 7.4 Conclusions 88 8 Summary and recommendations for future work 89 8.1 Summary 89 8.2 Recommendations for future work 91 References 94 List of figures 113 List of tables 118 Appendix i Scientific publications and contributions xxxiii Eidesstattliche Erklärung xxxviiDie meisten technischen Verfahren zur Trennung von Flüssigkeitsgemischen beruhen auf dem Prinzip der Rektifikation. Wird ein Zweiphasengemisch in die Trennkolonne eingespeist, können unerwünschte Strömungsmorphologien oder ausgeprägte Tröpfchenverschleppung auftreten, welche sich nachteilig auf die Trennleistung und die Integrität einzelner Anlagenkomponenten auswirken. Derzeit lässt sich das Verhalten solcher Zweiphasenströmungen in Einspeiseleitungen kaum vorhersagen und basiert meist auf empirischen oder heuristischen Methoden, die ein breites Spektrum möglicher Stoffeigenschaften und Anlagendimensionen nicht angemessen berücksichtigen. Infolgedessen müssen kostspielige Sicherheitszuschläge angewendet werden. Einspeiseleitungen von Trennkolonnen sind häufig durch horizontale Eintrittsstutzen, ein geringes Länge-zu-Durchmesser-Verhältnis und eine komplexe Leitungsführung mit Bögen und anderen Normteilen gekennzeichnet. Typische Rohrlängen sind zu kurz, um eine vollständig entwickelte Zweiphasenströmung auszubilden, welche daher mit unbekannter Strömungs-morphologie in die Trennkolonne eintritt. Da derartige Strömungen jedoch bisher nur selten untersucht wurden, verlässt man sich gegenwärtig in der technischen Praxis auf bestehende Vorhersagemethoden für voll entwickelte Zweiphasenströmungen. Grafische Methoden können jedoch die allmählichen Übergänge zwischen Strömungsformen kaum darstellen. Analytische Modelle liefern nur vereinfachte Näherungswerte der Zweiphasenströmung, die noch nicht für sich entwickelnde Rohrströmung qualifiziert wird. In dieser Arbeit wurde eine umfangreiche experimentelle Datenbasis horizontaler Wasser-Luft-Strömungen in zwei Versuchsstrecken mit Rohrinnendurchmessern von D = 50 mm und D = 200 mm und Einlauflängen im Bereich 10 < L/D < 75 erstellt. Auf diese Weise decken die Daten sowohl sich entwickelnde Rohrströmungen mit typischen Einlauflängen für Einspeiseleitungen ab, als auch weiter (in axialer Richtung) entwickelte Strömungen, die mit dem umfangreich untersuchten Referenzsystem Wasser-Luft vergleichbar sind. Die Auswirkung von Rohrbögen auf die Strömungsmorphologie stromauf- und stromabwärts wurde gezielt untersucht. Die Strömungsmorphologie wurde mit bildgebenden Gittersensoren erfasst. Ein 4D-Fuzzy-Algorithmus wurde zur objektiven Identifizierung der Strömungsmorphologien eingesetzt. Auf Grundlage dieser Fuzzy-Darstellung der Strömung wurden die Strömungsmorphologien klassifiziert, und es wurde eine neuartige 2D-Visualisierungstechnik entworfen, mit der die Strömungsentwicklung entlang der Einspeiseleitungen diskutiert wurde. Unerwünschte Strömungsmorphologien (intermittierende Strömung und Tropfenmitriss) während des Betriebs zweiphasiger Einspeisungen sind mit herkömmlichen Auslegungswerkzeugen kaum vorherzusagen. Das Einsetzen intermittierender Strömungen wurde auf Grundlage der experimentellen Daten analysiert. Daraufhin wurden existierende Kriterien, basierend auf den notwendigen Mindestfüllständen, für das Einsetzen intermittierender Strömungen in Abhängigkeit von den untersuchten Einlauflängen angepasst. Die charakteristische Dynamik von Strömungsmorphologien, die Tropfenmittriss hervorrufen, wurde analysiert. Voraussagemethoden zur Vorhersage der Anfälligkeit welliger Strömungen für das Auftreten von Tropfenmitriss wurden auf der Grundlage von Wellenfrequenzen entwickelt und für gerade Einspeiserohre und horizontale 90°-Bögen angewandt. Von den zahlreichen verfügbaren Modellen zur Vorhersage des Gasanteils wurden 66 Modelle reduzierter Ordnung für gerade Einspeiseleitungen und horizontale 90°-Rohrbögen getestet. Davon wurden die für variable Betriebsbedingungen und Rohrgeometrien am besten geeigneten Modelle ermittelt und angepasst. Komplementäre 3D-Simulationen wurden durchgeführt, um die Anwendbarkeit numerischer Codes (VoF, AIAD) für Strömungen mit freien Grenzflächen zu bestätigen. Die Strömungsmorphologien wurden im makroskopischen Maßstab erfolgreich reproduziert, die Simulationsergebnisse bleiben jedoch hinsichtlich ihrer quantitativen Vorhersagekraft hinter den Modellen reduzierter Ordnung zurück.:Abstract II Kurzfassung IV Acknowledgement VI Nomenclature VIII Table of Contents XIII 1 Introduction 1 1.1 Thermal separation in view of the 21st century 1 1.2 Engineering and design of rectification plants 2 1.3 Outline of the thesis 4 2 State of the art 5 2.1 Two-phase feeds in thermal separation 5 2.1.1 Feed condition as adjustable parameter 5 2.1.2 Thermohydraulic optimization 8 2.1.3 Hydrodynamic conditioning 9 2.2 Hydrodynamics of two-phase feeds 11 2.2.1 Flow morphologies in feed pipes 11 2.2.2 Droplet entrainment 14 2.2.3 Flow regime maps 17 2.2.4 Consequences for two-phase feeds 19 2.3 Modelling of two-phase feeds 23 2.3.1 Basic definitions 23 2.3.2 Fundamentals of the two-fluid model 25 2.3.3 The interfacial level gradient 29 2.3.4 Analytical models 32 2.3.5 CFD simulations for commercial feed pipes 34 2.4 Objectives of this thesis 36 3 Experimental method and algorithms for flow characterization 37 3.1 Experimental setups 37 3.2 Wire-mesh sensors 40 3.3 Experimental procedure 42 3.4 Data processing 44 3.4.1 Fuzzy flow morphology classification 45 3.4.2 Power spectral density 48 3.5 Measurement uncertainty 49 4 Flow morphologies in different feed pipe geometries 53 4.1 Developing two-phase flow in straight pipes 53 4.2 Effect of pipe curvatures on the flow morphology 55 4.3 Morphology recovery 57 4.4 Conclusions 60 5 Prediction of undesirable flow morphologies in feed pipes 61 5.1 Initiation of intermittent flows 61 5.2 Onset of droplet entrainment 62 5.2.1 Vulnerable flow morphologies 62 5.2.2 Derivation of a criterion for onset of entrainment 64 5.2.3 Adjustment of the criterion for the investigated pipe geometries 67 5.3 Conclusions 70 6 Reduced-order modelling of two-phase feeds 71 6.1 Prediction of void fraction 71 6.2 Liquid levels 75 6.3 Conclusions 78 7 CFD modelling of two-phase feeds 79 7.1 Simulation setup 79 7.2 Multiphase models 82 7.3 Comparison with experimental data 83 7.3.1 Straight pipes 83 7.3.2 Horizontal 90° bends 85 7.4 Conclusions 88 8 Summary and recommendations for future work 89 8.1 Summary 89 8.2 Recommendations for future work 91 References 94 List of figures 113 List of tables 118 Appendix i Scientific publications and contributions xxxiii Eidesstattliche Erklärung xxxvi

A combined immersed boundary/phase-field method for simulating two-phase pipe flows

The investigation of the flow in a pipe is a major issue for the pipeline capacity but also plays an important role for the control and prevention of phenomena that could damage the pipe, such as corrosion, erosion, and the potential formation of wax or their deposits. Therefore, the characterization of the flow patterns is also a major issue for the prediction of the distribution over the cross-section of the pipe, in order to understand any problems that may interrupt or shut down the operation of the production line. The main purpose of the present effort is to develop an appropriate numerical method for simulating two-phase pipe flows. Advanced Computational Fluid Dynamics (CFD) methods are employed as Navier-Stokes solver, while a Phase-Field method is used to simulate the interfacial region between the two fluids. A Ghost-Cell Immersed Boundary Method (GCIBM) was developed and implemented for the reconstruction of smooth rigid boundaries (pipe wall) based on the work of Tseng and Ferziger (2003). The method was also modified in order to incorporate appropriate boundary conditions for coupling the Phase-Field and Navier-Stokes solvers for two-phase pipe flows. Tseng and Ferziger (2003) used the GCIBM for turbulent single-phase flows; the present modified version comprises a continuation of the method for handling two-phase pipe flows. The computational model is capable of handling large density and viscosity ratios with good accuracy. The developed GCIBM algorithm was validated against analytical solutions for single and two-phase pipe flow, presenting very good agreement. The computational model was compared to available experimental data from the literature for single rising bubbles and bubble coalescence in vertical pipe also with good agreement. The numerical method was used to investigate the lateral wall effects of a 3-D single bubble in a viscous liquid for different pipe diameters and bubble flow regimes. The dynamics of 3-D Taylor bubbles was also examined in vertical pipes for different properties of fluids (e.g. air-water system) and dimensionless parameters relevant to the problem (e.g. ReB, Eo, Mo). The numerical results were compared with available experimental and numerical data from the literature, presenting good agreement.Open Acces

Parametric Study of Vertical Ground Loop Heat Exchangers for Ground Source Heat Pump Systems

We report on a numerical study conducted to investigate the effect of various parameters on the heat exchange inside a vertical ground loop heat exchanger (VGLHE) for a ground-source heat pump (GSHP) system. The simulations were conducted for three piping configurations of the ground loop which were U-Tube, Concentric pipes and Spiral. The results show a linear temperature rise along the pipe length for the U-Tube configuration. The Concentric pipes configuration shows two distinct linear trends for the temperature rise; a slow temperature rise during the downward flow through the inner pipe and a higher temperature rise during the upflow through the annulus. The Spiral configuration shows a steeper slope for the temperature rise in the spiral section and almost a flat slope for the temperature rise in the straight vertical section of the pipe. The research also examines a simulation case of integrating a VGLHE inside of a micro-pile foundation system