Supercritical water oxidation using hydrothermal flames at microscale as a potential solution for organic waste treatment in space applications – A practical demonstration and numerical study

Supercritical water oxidation (SCWO) with hydrothermal flames is well established for the treatment of aqueous organic waste as it not only overcomes the limitations of simple SCWO, such as precipitation of salts, but also exhibits many advantages over other waste treatment processes. Seeking these advantages, we propose to perform SCWO using hydrothermal flames in microfluidic reactors ) for aerospace applications to be used in deep space/ISS missions. The novelty and highlight of this work are successful demonstration of realizing microreactors (channel width 200 ), which can withstand pressure of 250 bar with temperature °C, thereby presenting the feasibility to realize this technology. We present the first evidence of SCWO/hydrothermal in a flow microreactor of sapphire, which is captured through optical visualization. This is followed by a numerical investigation to understand the underlying physics leading to the formation of hydrothermal flame and thus differentiate it from a simple SCWO reaction. The simulations are performed in a 2D domain in a co-flow configuration with equal inlet velocity of fuel and oxidizer at two different inlet temperatures (350 °C and 365 °C), just below the critical temperature of water using ethanol and oxygen, the former acting not only as a model organic matter but also fuel for the formation of hydrothermal flames. It is observed that due to microscale size of the system, hydrothermal flames are formed at low inlet velocities (< 30 mm/s), while reaction at higher ones are characterized as simple SCWO reaction. This upper limit of inlet velocity was found to increase with inlet temperature. Finally, some key characteristics of hydrothermal flames - ignition delay time, flame structure, shape, and local propagation speed are analyzed

Modélisation numérique des fluides fortement compressibles proches du point critique

A fluid, in addition to its liquid and gas phase, is known to exist in another phase, wherein the fluid inherits some properties of both the phases. Such a fluid is called a supercritical fluid and the conditions (pressure and temperature) beyond which the fluid exists in this state is called the critical point. One of the peculiar feature of the fluids near the critical point is that the various thermo-physical properties show a singular behavior, such as diverging compressibility, vanishing thermal diffusivity etc. The flow behavior near the critical point leads to intriguing flow features ascribed to the strong thermo-mechanical coupling whose in-depth investigation can be limited by experimental constraints especially during a continuous transition from supercritical to subcritical regime. The current work focuses on analyzing the flow behavior in near-critical fluids with prime focus on supercritical fluids. This is achieved by developing a mathematical and numerical model which is followed by the validation study and error analysis of the numerical scheme wherein unusual behavior of the Courant number is observed. Subsequently, the flow behavior of supercritical fluid is studied when simultaneously subjected to thermal quench and vibration, mainly Rayleigh-vibrational and parametric instabilities, their physical mechanism and various parameters affecting them. In addition, two captivating phenomena, firstly where the temperature of the fluid region drops below the imposed boundary condition and secondly, the see-saw motion of the thermal boundary layer are observed and physical explanations are provided. In order to investigate the flow dynamics in subcritical regime, phase-field modelling approach is explored for isothermal conditions. The model is examined for elementary test cases illustrating the feasibility to extend the model for a continuous transition from supercritical to subcritical regime.Un fluide porté à une température et pression supérieures à celles du point critique est communément appelé fluide supercritique. Ce fluide possède des propriétés particulièrement intéressantes à cheval entre celles des gaz et celle des liquides. En effet, la masse volumique d’un fluide supercritique est proche de celle d’un liquide tandis que sa viscosité est proche de celle d’un gaz. Une des caractéristiques particulières de ces fluides quand ils s’approchent du point critique est que plusieurs des propriétés thermo-physiques montrent un comportement singulier (compressibilité divergente, diffusivité thermique évanescente etc). Dans ce travail, un modèle mathématique basé sur les équations de Navier-Stokes couplées à celle de l’énergie est proposé afin d’étudier les écoulements de ces fluides très proches de leur point critique. La validation du modèle a été effectuée sur un problème de propagation d’onde acoustique dans l'eau. Nous avons ainsi observé que des solutions précises avec des schémas implicites pour des systèmes non linéaires sont possibles avec des nombres de Courant élevés. L’étude des écoulements dans des fluides supercritiques, lorsqu'ils sont assujettis à une trempe thermique et à une vibration simultanées ont montré que de telles conditions pouvaient conduire à la formation d’instabilités thermo-vibrationnelles, en particulier les instabilités de Rayleigh-vibrationnelles et paramétriques. Les simulations numériques nous ont permis de relever deux phénomènes particulièrement surprenants : (i) la température du fluide à l’intérieur du domaine devient inférieure à la trempe de température imposée à la frontière et (ii) une oscillation des doigts d’instabilité apparaît dans la couche limite thermique dans la direction de la vibration. Dans le cas des fluides sous le point critique (cas diphasique), le modèle compressible développé est couplé à un de champ de phase (“phase field”) dans les conditions isothermes. Des cas tests élémentaires ont été considérés avec succès. Une discussion est proposée afin d’étendre le modèle dans le cas d’une transition continue du régime supercritique au régime sous-critique et vice-versa

Numerical modelling of highly compressible near-critical fluids

Un fluide porté à une température et pression supérieures à celles du point critique est communément appelé fluide supercritique. Ce fluide possède des propriétés particulièrement intéressantes à cheval entre celles des gaz et celle des liquides. En effet, la masse volumique d’un fluide supercritique est proche de celle d’un liquide tandis que sa viscosité est proche de celle d’un gaz. Une des caractéristiques particulières de ces fluides quand ils s’approchent du point critique est que plusieurs des propriétés thermo-physiques montrent un comportement singulier (compressibilité divergente, diffusivité thermique évanescente etc). Dans ce travail, un modèle mathématique basé sur les équations de Navier-Stokes couplées à celle de l’énergie est proposé afin d’étudier les écoulements de ces fluides très proches de leur point critique. La validation du modèle a été effectuée sur un problème de propagation d’onde acoustique dans l'eau. Nous avons ainsi observé que des solutions précises avec des schémas implicites pour des systèmes non linéaires sont possibles avec des nombres de Courant élevés. L’étude des écoulements dans des fluides supercritiques, lorsqu'ils sont assujettis à une trempe thermique et à une vibration simultanées ont montré que de telles conditions pouvaient conduire à la formation d’instabilités thermo-vibrationnelles, en particulier les instabilités de Rayleigh-vibrationnelles et paramétriques. Les simulations numériques nous ont permis de relever deux phénomènes particulièrement surprenants : (i) la température du fluide à l’intérieur du domaine devient inférieure à la trempe de température imposée à la frontière et (ii) une oscillation des doigts d’instabilité apparaît dans la couche limite thermique dans la direction de la vibration. Dans le cas des fluides sous le point critique (cas diphasique), le modèle compressible développé est couplé à un de champ de phase (“phase field”) dans les conditions isothermes. Des cas tests élémentaires ont été considérés avec succès. Une discussion est proposée afin d’étendre le modèle dans le cas d’une transition continue du régime supercritique au régime sous-critique et vice-versa.A fluid, in addition to its liquid and gas phase, is known to exist in another phase, wherein the fluid inherits some properties of both the phases. Such a fluid is called a supercritical fluid and the conditions (pressure and temperature) beyond which the fluid exists in this state is called the critical point. One of the peculiar feature of the fluids near the critical point is that the various thermo-physical properties show a singular behavior, such as diverging compressibility, vanishing thermal diffusivity etc. The flow behavior near the critical point leads to intriguing flow features ascribed to the strong thermo-mechanical coupling whose in-depth investigation can be limited by experimental constraints especially during a continuous transition from supercritical to subcritical regime. The current work focuses on analyzing the flow behavior in near-critical fluids with prime focus on supercritical fluids. This is achieved by developing a mathematical and numerical model which is followed by the validation study and error analysis of the numerical scheme wherein unusual behavior of the Courant number is observed. Subsequently, the flow behavior of supercritical fluid is studied when simultaneously subjected to thermal quench and vibration, mainly Rayleigh-vibrational and parametric instabilities, their physical mechanism and various parameters affecting them. In addition, two captivating phenomena, firstly where the temperature of the fluid region drops below the imposed boundary condition and secondly, the see-saw motion of the thermal boundary layer are observed and physical explanations are provided. In order to investigate the flow dynamics in subcritical regime, phase-field modelling approach is explored for isothermal conditions. The model is examined for elementary test cases illustrating the feasibility to extend the model for a continuous transition from supercritical to subcritical regime

Numerical modelling of highly compressible near-critical fluids

Un fluide porté à une température et pression supérieures à celles du point critique est communément appelé fluide supercritique. Ce fluide possède des propriétés particulièrement intéressantes à cheval entre celles des gaz et celle des liquides. En effet, la masse volumique d’un fluide supercritique est proche de celle d’un liquide tandis que sa viscosité est proche de celle d’un gaz. Une des caractéristiques particulières de ces fluides quand ils s’approchent du point critique est que plusieurs des propriétés thermo-physiques montrent un comportement singulier (compressibilité divergente, diffusivité thermique évanescente etc). Dans ce travail, un modèle mathématique basé sur les équations de Navier-Stokes couplées à celle de l’énergie est proposé afin d’étudier les écoulements de ces fluides très proches de leur point critique. La validation du modèle a été effectuée sur un problème de propagation d’onde acoustique dans l'eau. Nous avons ainsi observé que des solutions précises avec des schémas implicites pour des systèmes non linéaires sont possibles avec des nombres de Courant élevés. L’étude des écoulements dans des fluides supercritiques, lorsqu'ils sont assujettis à une trempe thermique et à une vibration simultanées ont montré que de telles conditions pouvaient conduire à la formation d’instabilités thermo-vibrationnelles, en particulier les instabilités de Rayleigh-vibrationnelles et paramétriques. Les simulations numériques nous ont permis de relever deux phénomènes particulièrement surprenants : (i) la température du fluide à l’intérieur du domaine devient inférieure à la trempe de température imposée à la frontière et (ii) une oscillation des doigts d’instabilité apparaît dans la couche limite thermique dans la direction de la vibration. Dans le cas des fluides sous le point critique (cas diphasique), le modèle compressible développé est couplé à un de champ de phase (“phase field”) dans les conditions isothermes. Des cas tests élémentaires ont été considérés avec succès. Une discussion est proposée afin d’étendre le modèle dans le cas d’une transition continue du régime supercritique au régime sous-critique et vice-versa.A fluid, in addition to its liquid and gas phase, is known to exist in another phase, wherein the fluid inherits some properties of both the phases. Such a fluid is called a supercritical fluid and the conditions (pressure and temperature) beyond which the fluid exists in this state is called the critical point. One of the peculiar feature of the fluids near the critical point is that the various thermo-physical properties show a singular behavior, such as diverging compressibility, vanishing thermal diffusivity etc. The flow behavior near the critical point leads to intriguing flow features ascribed to the strong thermo-mechanical coupling whose in-depth investigation can be limited by experimental constraints especially during a continuous transition from supercritical to subcritical regime. The current work focuses on analyzing the flow behavior in near-critical fluids with prime focus on supercritical fluids. This is achieved by developing a mathematical and numerical model which is followed by the validation study and error analysis of the numerical scheme wherein unusual behavior of the Courant number is observed. Subsequently, the flow behavior of supercritical fluid is studied when simultaneously subjected to thermal quench and vibration, mainly Rayleigh-vibrational and parametric instabilities, their physical mechanism and various parameters affecting them. In addition, two captivating phenomena, firstly where the temperature of the fluid region drops below the imposed boundary condition and secondly, the see-saw motion of the thermal boundary layer are observed and physical explanations are provided. In order to investigate the flow dynamics in subcritical regime, phase-field modelling approach is explored for isothermal conditions. The model is examined for elementary test cases illustrating the feasibility to extend the model for a continuous transition from supercritical to subcritical regime

Blackout mitigation during space vehicle re-entry

Space vehicle re-entering the earth's atmosphere is surrounded by a layer of plasma, preventing radio communication from the vehicle, leading to a phenomena called ‘radio blackout’. In this study, the concept of guiding high power laser through plasma has been discussed for establishing communication with the space vehicle during re-entry. It is found that, with increase in relativistic electron mass the refractive index of the plasma medium increases, displaying properties similar to that of converging lens. It is also shown that the power of the laser, if maintained above a critical limit, will assist in focussing of waves. The self-focusing property of plasma is therefore explored for achieving this target and the theoretical proof has been provided

Use of Orthogonal Array to Study the Effect of Various Parameters on Liquid Metal based Microchannel Cooling

With increase in demand for efficient cooling technologies in electronic systems, use of microchannels with liquid metals as cooling medium has gained significant attention. To analyze the effect of various geometrical parameters on microchannel performance with such coolant, Taguchi Orthogonal Arrays are used in this study. To replicate the results for simulation, external noise is introduced in two parameters, channel width and depth. In addition, channel wall and base thickness along with type of material affecting the performance is also analyzed. Results show that channel width, height and substrate thickness at the base are the prime factors of concern. In addition, interactive effect of wall and base thickness is also found to affect the performance significantly and their low values for optimum performance is recommended

NUMERICAL ANALYSIS OF TRAPEZOIDAL SHAPE DOUBLE LAYER MICROCHANNEL HEAT SINK

With increasing demand for higher computational speed and emerging micro-systems, thermal management poses serious challenge for efficient cooling. Among these liquid cooling using microchannels has gained significant attention and has been extended to its double layer configuration which eliminates the drawback of significant temperature variations in single layer system. The double layer configuration has been primarily analyzed for rectangular ducts. In this study the performance of trapezoidal shape double layer microchannel heat sink is investigated and compared to rectangular double layer heat sink of same flow area. Four different possible configurations are analyzed and comparative study among respective counter and parallel configuration is performed followed by comparison among each configuration. The performance is evaluated on the basis of maximum temperature attained at the heated surface as well as minimum temperature variations. Finally the best performing configuration is compared with double layer rectangular heat sink. Analysis shows that among various trapezoidal configurations, the one with larger side face to face is most suitable. Further comparative study with rectangular system shows that performance of trapezoidal double layer heat sink is superior in both aspects, i.e. minimum thermal resistance as well as minimum temperature variations

Numerical Study on the Performance of Double Layer Microchannel with Liquid Gallium and Water

The performance of liquid gallium and water in the double layer microchannel has been analysed using three-dimensional conjugate heat transfer analysis. The effect of flow rate on the counter and parallel arrangement of each fluid is studied for three different lengths. Furthermore, cooling capability of liquid gallium and water is compared at the same length with flow rate and pumping power as governing parameters. The performance of fluid was judged on the basis of maximum temperature attained and minimal temperature variations at the heated region. Interesting results have been found showing the effect of specific heat on the type of arrangement for liquid gallium with similar observation for water for low Reynolds number and relatively longer length. Among liquid gallium and water, above certain pumping power use of liquid gallium is found to be favourable for a shorter length of the double layer microchannel. Furthermore, the range of flow rate and pumping power showing superior performance with water was found to increase with the length

Adaptive interface thickness based mobility—Phase-field method for incompressible fluids

The ubiquity of two-phase systems has rendered them a subject of prime importance especially from numerical perspectives. Among several methods described in the literature, which are generally classified as sharp or diffuse interface methods, phase-field (diffuse interface) method has been at the paramount of several recent investigations owing to its several advantages, primarily to handle complex topological changes. Though several advancements have been made in the subject, one of the important challenges with this approach using Cahn-Hilliard equation lies in the determination of an appropriate value of mobility. Despite certain propositions in the literature in terms of non-dimensional numbers (Peclet and Cahn numbers), ambiguity in the velocity scale to be chosen for evaluating mobility poses a challenge for their straightforward extension to real systems. In addition it renders the system to be dependent on numerical parameters. In the current work, we address this problem using a new approach to calculate the mobility parameter in terms of local equilibrium interface thickness which in itself is evaluated from the phase-field parameter. Consequently, this helps to make the model independent of numerical parameters making it more reliable. We test this approach for several canonical and complex cases, such as the rise of lighter bubble in a heavier medium, bubble coalescence, Rayleigh-Taylor instability. The results are compared with those in literature or obtained using level set method. An excellent agreement is observed illustrating the potential of this approach to make phase-field model more prominent

Vibration-induced thermal instabilities in supercritical fluids in the absence of gravity

Supercritical fluids (SCFs) are known to exhibit anomalous behavior in their thermophysical properties such as diverging compressibility and vanishing thermal diffusivity on approaching the critical point. This behavior leads to a strong thermomechanical coupling when SCFs are subjected to simultaneous thermal perturbation and mechanical vibration. The behavior of the thermal boundary layer leads to various interesting dynamics such as thermovibrational instabilities, which become particularly ostensive in the absence of gravity. In the present paper, two types of instabilities, Rayleigh-vibrational and parametric instabilities, have been numerically investigated under zero gravity in a two-dimensional configuration using a mathematical model wherein density is calculated directly from the continuity equation. A comparison of experimental observations with numerical simulations is also presented. The peculiarity of the model warrants the investigation of instabilities in a more stringent manner (in terms of higher quench percentage and closer proximity to the critical point), unlike the previous studies wherein the equation of state was linearized around the considered state for the calculation of density, resulting in a less precise analysis. In addition to providing an explanation of the physical causes of these instabilities, we analyze the effect of various parameters on the critical amplitude for the onset of these instabilities. Furthermore, various attributes such as wavelength of the instabilities, their behavior under various factors (quench percentage and acceleration), and the effect of cell size on the critical amplitude are also investigated. Finally, a three-dimensional stability plot is shown describing the type of instability (Rayleigh-vibrational or parametric or both) to be expected for the operating condition in terms of amplitude, frequency, and quench percentage for a given proximity to the critical point