Self-similarity, momentum scaling and Reynolds stress in non-premixed turbulent spray flames

Abstract

An experimental study was conducted in a turbulent spray flame in which droplets were produced ultrasonically at low velocity relative to the host gas. In this fashion, injector-specific effects on the two-phase flow were minimized and a scenario generally characteristic of the far field of practical spray systems could be simulated. Close to the burner exit, the spray flame appeared as a dense column of drops burning with an envelope flame. Further downstream, it opened up slowly in the radial direction and developed a turbulent ‘brush’ appearance. Measurements of the size, velocity and concentration of the droplets, and of gas-phase velocity and temperature were made by combining a Phase-Doppler interferometric technique with Stokes/anti-Stokes Raman thermometry. The experimental data were used to derive scaling and self-similarity for the Reynolds-averaged continuity and momentum equations using suitable transformations.Results showed three distinct regions, on the basis of the behaviour of the gas axial velocity in the spray flame. In the lower part of the flame, the gas momentum increased because of vaporization. In the intermediate region of the spray flame, the axial velocity decayed along the centreline as an inverse power of the distance from the virtual origin, with exponents smaller than unity. In the upper part of the spray flame, the flow field recovered the axial velocity decay that is typical of incompressible jets, namely as an inverse of the axial distance. Self-similar behaviour held for the axial velocity throughout the intermediate region. The vapour source term in the gas continuity equation scaled approximately as an inverse power of axial distance, and exhibited self-similarity throughout the spray flame. As a result, a simple model of the Reynolds stress term could be formulated, in which two competing contributions appear: one, that is due to turbulent transport, tends to increase the value of the velocity correlation; another, that is due to the vaporization term, tends to reduce the value of the velocity correlation and can be construed as a vaporization-induced tendency towards relaminarization. The first term is modelled by a classic gradient-transport approach yielding an empirical mixing length relating the velocity correlation to the average velocity gradient. Model and experiments are found to be in good agreement, especially sufficiently far from the injector, where one-way coupling between the two phases holds.

LES of a methanol spray flame with a stochastic sub-grid model

“…With increasing the axial distance, the difference in the velocity is minimal regardless of the droplet diameter. The computed droplet size-velocity correlation is found to be very close to measurements [19]. …”

“…Figure 2 presents a comparison of the simulated droplet number density and D 10 against measurements just above the burner exit (z/D = 0.1). The computed number density exhibits under-predictions but falls within the range of experimental errors which are as large as 30% [19] while there is an excellent agreement for the average droplet size. Figure 3 shows a comparison between the simulated and measured pdf of drop size at z/D = 0.1 and z/D = 3.5 along the centreline of the flame.…”

“…Furthermore, the mean temperature from the LES tends to decrease towards the shear layer of the flame and reach the atmospheric temperature. This decreasing trend is not observed experimentally because the technique used therein cannot measure gas temperatures with accuracy below about 600 K [19].…”

Structure of laminar coflow spray flames at different pressures

“…This would create new difficulties at high pressure and would require large combustion chambers [10]. Ultrasonic atomizers [3,11] are also not appropriate since the pressure waves generated during the atomization process cause strong disturbances to the flame at pressures above 1 atm.…”

Small Electrocyclone Performance

“…Probably the best approach to reduce harmful emissions is either to improve on the combustion efficiencies [2,3] or increase the cleaning efficiencies of particle removal devices. Two of the most effective particulate gas cleaning devices are the electrostatic precipitator [4,5] and the cyclone separator [6±8] and, very generally, ESPs are most suited to removing small particles (< 10 lm) and cyclones larger particles (> 10 lm).…”