José Graña-Otero. Assistant Professor

Fundamental studies on boiling: vapor entrapment in microcavities.

Boiling is a complex phenomenon encountered in a broad range of applications, from industrial processes to everyday appliances. It usually occurs in two steps: first an initial nucleus of vapor must originate in the liquid; and second, once an initial vapor nucleus is available, it will grow if the degree of superheat in the liquid phase is large enough. Thus, the nucleation of the vapor phase is a key process in the boiling dynamics. Among the several known routes leading to new vapor nuclei, the most accessible is the entrapment of gas/vapor pockets in cavities or defects on the walls of containers as liquid flows over them. However, the entrapment dynamics is not yet well understood. Most of the analyses either deal with a purely static evolution, or are based on thermodynamic approaches, overlooking the dynamic nature of the process. This project addresses this important aspect of the boiling dynamics, namely the dynamical effects on gas entrapment in microcavities, which are essential in determining the initial size of the entrapped vapor nucleus, as well as its subsequent evolution. For that, we solve the Navier-Stokes equations of motion for the free surface flow of a liquid flooding a microscopic cavity on the surface of an otherwise smooth wall.

Students

Elisabeth Morris

Trace Kuberaki

Sam Landoch

Software

Computational resources
We use in-house Boundary Elements Methods developed in C. Extensive use is made of mathematical libraries such as the linear algebra package LAPACK, the suite of nonlinear and differential equations solvers SUNDIALS, and GNU and PETSs scientific libraries.

Quantitative chemiluminescence and flame structure in unsteady combustion

In the quest for low pollutant emissions and highly efficient combustion systems, modern combustors technologies, such as Lean Premixed (LP) or Lean Premixed Prevaporized (LPP) ones, search to operate in very dilute and lean regimes, close to, or even beyond, the extinction limits of fuels. It is well known that these near limit conditions are prone to intrinsic flame instabilities, which very easily couple with system-induced instabilities, rendering these configurations extremely unstable and difficult to control. Among the many factors that contribute to these difficulties is the lack of understanding of the intrinsic dynamics of unstable flames in these limit conditions. The aim of the project is to understand the dynamics of these unstable, near-limit flames. In particular, we want to analyze how the well-known structure of steady flames changes when the flame is subjected to unsteady conditions, such as those found in LP and LPP combustors. We consider detailed, realistic kinetic models, as well as classical, steady reduced schemes in order to determine the range of validity of these last as the ratio between the characteristic time of unsteady effects to the transit time through the flame decreases below unity.

Students

Siamak Mahmoudi

Linda Maria Hirt, Visiting Scholar

Software

Computational resources
We use in-house finite differences codes developed in C to simulate unsteady, planar and axisymmetric premixed and diffusion fames with detailed chemistry. EGLib and CANTERA libraries are used to evaluate reactions kinetics and multicomponent thermodynamic and transport properties. As well we make extensive use of mathematical libraries such as the linear algebra package LAPACK, the suite of nonlinear and differential equations solvers SUNDIALS, and GNU and PETSs scientific.

Collaborators

Miguel Gomez-Lopez, Universidad Politécnica de Madrid

Publications

Grants