Distributed Flames in Type Ia Supernovae

TL;DR

This paper develops a theoretical and computational framework for understanding turbulent flames in Type Ia supernovae, identifying regimes where flames are broadened by turbulence and proposing models for their speeds and widths.

Contribution

It introduces a new scaling theory for distributed flames in supernovae, supported by simulations, enabling simplified turbulent flame modeling for large-scale simulations.

Findings

Turbulent flame speed can be twice the turbulent intensity for Da>1.

Localized flame speed excursions can reach five times the turbulent intensity.

Scaling relations allow prediction of flame properties based on turbulence and nuclear timescales.

Abstract

In the distributed burning regime, turbulence disrupts the internal structure of the flame, and so the idea of laminar burning propagated by conduction is no longer valid. The nature of the burning depends on the turbulent Damkohler number (Da), which steadily declines from much greater than one to less that one as the density decreases to a few 10^6 g/cc. Scaling arguments predict that the turbulent flame speed s, normalized by the turbulent intensity u, follows s/u=Da^1/2 for Da<1. The flame in this regime is a single turbulently-broadened structure that moves at a steady speed, and has a width larger than the integral scale of the turbulence. The scaling is predicted to break down at Da=1, and the flame burns as a turbulently-broadened effective unity Lewis number flame. We refer to this kind of flame as a lambda-flame. The burning becomes a collection of lambda-flames spread over a region approximately the size of the integral scale. While the total burning rate continues to have a well-defined average, s_{T} ~ u, the burning is unsteady. We present a theoretical framework, supported by both 1D and 3D numerical simulations, for the burning in these two regimes. Our results indicate that the average value of s can actually be roughly twice u for Da>1, and that localized excursions to as much as five times u can occur. The lambda-flame speed and width can be predicted based on the turbulence in the star and the turbulent nuclear burning time scale of the fuel. We propose a practical method for measuring these based on the scaling relations and small-scale computationally-inexpensive simulations. This suggests that a simple turbulent flame model can be easily constructed suitable for large-scale distributed supernovae flames.