Maria Fernandino

Large eddy simulation of turbulent open duct flow using a lattice Boltzmann approach

Large eddy simulations of turbulent open duct flow are performed using the lattice Boltzmann method (LBM) in conjunction with the Smagorinsky sub-grid scale (SGS) model. A smaller value of the Smagorinsky constant than the usually used one in plain channel flow simulations is used. Results for the mean flow and turbulent fluctuations are compared to experimental data obtained in an open duct of similar dimensions. It is found that the LBM simulation results are in good qualitative agreement with the experiments.

Using Cahn-Hilliard mobility to simulate coalescence dynamics

In droplet-droplet collision processes, such as bouncing and coalescence, the following stages can be identified: droplet approach, film drainage, film rupture and the hydrodynamics of coalescence driven by capillary forces. The film rupture process represents a remaining challenge for numerical models intended to simulate the outcome of collisions. It has been proposed that the lattice Boltzmann framework has a mesoscale nature that is suitable for modeling of film rupture. The present work examines diffuse coalescence based on the Cahn-Hilliard free energy for non-uniform systems. No empirical coalescence criterion is needed as the diffuse mechanism is based on a thermodynamic description with its own time characteristics.

Simulation of a natural circulation loop using a least squares hp-adaptive solver

In this work the implementation of a high-order method for the simulation of a natural circulation loop is discussed. An adaptive method is developed in order to improve both accuracy and computational time in the resolution of nonlinear problems. Finally, several numerical simulations are discussed in relation to the development of this kind of high-order adaptive methods for unstable thermo-hydraulic systems.

Fractional step two-phase flow lattice Boltzmann model implementation

A hybrid method that combines an advanced lattice Boltzmann model with traditional finite difference techniques is developed. An operator splitting technique is applied to separate the numerical scheme into two subproblems. Each subproblem is solved with different techniques, namely lattice Boltzmann and finite differences. This method is applied for the first time to a two-phase flow simulation case. Major simplifications are obtained in the lattice Boltzmann implementation. Simulations are presented to show these improvements and to validate the technique with kinematic viscosity down to 10−4 in lattice Boltzmann units. This technique does not add instabilities, and in contrast improves the stability behavior compared to reducing the relaxation time.

Numerical Study of Pressure Drop Oscillations in Parallel Channels

Pressure drop oscillations have been modeled and analyzed in the present study. The purpose of the present study is to understand the behavior of the individual channels in a parallel arrangement under different conditions such as balanced and unbalanced heat loads and operation mass flux during pressure drop oscillations. The differential equations have been solved using the least squares method. The cases with two parallel channels with balanced and unbalanced heat loads have been discussed. The modes of oscillations found in both cases are in phase and the unstable region can be divided in two characteristic regions. The so called “Region 1”, where the oscillations themselves take place, and a second region, “Region 2”, dominated by stable maldistributed solutions.Copyright

Effect of Interfacial Waves on Turbulence Structure in Stratified Duct Flows

Stratified flows are encountered in many industrial applications. The determination of the flow characteristics is essential for the prediction of pressure drop and holdup in the system. The aim of this study is to gain insight into the interaction of a gas and a liquid phase flowing in a stratified regime, with especial focus on the effect of interfacial waves on the turbulence structure of the liquid phase. Measurements of mean velocities and turbulent intensities in the liquid phase of a stratified air-water duct flow are performed. Mean velocity profiles and turbulence structure are affected differently for different wave amplitudes. The effect of small amplitude waves is restricted to the near-interface region, resembling the effect of increasing shear rate on a flat interface. On the other hand, large amplitude waves modify the flow structure throughout the whole liquid depth. The mean velocity is greatly enhanced, resulting in a higher bulk velocity. Turbulent intensities are also significantly enhanced especially in the interface region. This big difference in flow structure is not observed after the appearance of the first waves but rather when a certain critical wave amplitude is triggered, indicating that the prediction of this critical wave type turns out to be more important than the determination of the transition from a smooth to a stratified wavy regime.

The least-squares spectral element method for phase-field models for isothermal fluid mixture

Abstract The phase-field approach has been regarded as a powerful method in numerically handling the interface dynamics in multiphase flow in several scientific and engineering applications. For an isothermal fluid mixture, the Navier–Stokes–Korteweg equation and the Navier–Stokes–Cahn–Hilliard equation have represented two major branches of the phase-field methods. We present a general discretization formulation for these two equations and conduct a comparison study of them. The formulation using a least-squares spectral element method is implemented by adopting a time-stepping procedure, a high-order continuity approximation and an element-by-element solver technique. To describe the same fluid mixtures by the isothermal Navier–Stokes–Korteweg and the Navier–Stokes–Cahn–Hilliard equations, we suggest a non-dimensionalization with the same dimensionless quantities. Numerical experiments are conducted to verify the spectral/ hp least-squares formulation for the isothermal Navier–Stokes–Korteweg model. Besides, the equilibrium state of the van der Waals fluid model is calculated both analytically and numerically. Through spontaneous decomposition example, the isothermal Navier–Stokes–Korteweg system and the Navier–Stokes–Cahn–Hilliard system are compared in terms of the equilibrium pressure and the energy minimizing process. As a general example, the coalescence of two liquid droplets is studied with our solver for the isothermal Navier–Stokes–Korteweg system. The minimum discretization levels for space and time are investigated and a parametric study on Weber number is carried out.

Experimental study of density wave oscillations in horizontal straight tube evaporator

Density wave oscillations (DWO) associated with refrigerant R134a in a uniformly heated horizontal in-tube boiling system were experimentally investigated. The experiment was performed in a range of inlet pressures P[5 – 12bar] and subcooled inlet temperature [8 – 24 C], maintaining constant heat fluxes q″[38kW/m2] and mass flux G[300kg/m2s]. The effect of the system parameters on the amplitude and period of the DWOs is discussed. In particular, it is observed that the period of the DWOs increases as the inlet pressure increases, but the slope changes as the flow regime at the outlet changes from annular-mist to mist flow.

Can the heat transfer coefficients for single-phase flow and for convective flow boiling be equivalent?

During the past few decades, heat transfer during convective flow boiling inside pipes has been widely studied with the goal of unveiling the physics of the process. Different heat transfer mechanisms have been suggested based on different assumptions. This fact has resulted in a large number of models including different dimensionless numbers and in some cases up to a dozen of adjusted parameters. Here, we show that the convective flow boiling heat transfer coefficient is equivalent to the one for single-phase flow when the influence of the vapour velocity is taken into account.

Can flow oscillations during flow boiling deteriorate the heat transfer coefficient

Two-phase flow instabilities have been identified as one of the impediments for achieving high heat flux in boiling systems due to their potential heat transfer deterioration. However, most of the fundamental characteristics of two-phase flow instabilities and the mechanisms leading to the heat transfer deterioration remain uncharted. In particular, up to what extent self-induced oscillations can deteriorate the heat transfer coefficient is not well understood. Here, we measure the flow boiling heat transfer coefficient under controlled oscillatory flow conditions. We show that flow oscillations can deteriorate the heat transfer coefficient significantly, but the deterioration depends on the amplitude and period of the oscillations. In particular, the deterioration is primarily a consequence of the dry-out at the wall that in turn increases the averaged wall temperature.Two-phase flow instabilities have been identified as one of the impediments for achieving high heat flux in boiling systems due to their potential heat transfer deterioration. However, most of the fundamental characteristics of two-phase flow instabilities and the mechanisms leading to the heat transfer deterioration remain uncharted. In particular, up to what extent self-induced oscillations can deteriorate the heat transfer coefficient is not well understood. Here, we measure the flow boiling heat transfer coefficient under controlled oscillatory flow conditions. We show that flow oscillations can deteriorate the heat transfer coefficient significantly, but the deterioration depends on the amplitude and period of the oscillations. In particular, the deterioration is primarily a consequence of the dry-out at the wall that in turn increases the averaged wall temperature.