title: On the dynamics of heat transfer and combusting flows in porous media
Creators: name: Habib, Rabeeah
Publisher: name: University of Glasgow
Year: 2022

description: Dynamics of heat transfer and combusting flows has attracted increased attention in porous media in recent years. A growing number of technologies require prediction of unsteady forced convection in porous media when the inlet flow is unsteady. Also, in practical combustion systems fluctuations in the fuel flow rate can occur and result in flame destabilisation, in particular in lean and ultra-lean modes of operation. This is due to heat transfer being dominant in combusting flows in porous media. To address these challenges a joint numerical and experimental approach is adopted. A numerical study of heat convection response of a reticulated porous medium to the harmonic and ramp disturbances in the inlet flow is investigated taking a porescale approach. The developed model consists of ten cylindrical obstacles aligned in a staggered arrangement with set isothermal boundary conditions. A few types of fluids, along with different values of porosity and Reynolds number, are considered. Assuming laminar flow, the system is first modulated by sine waves superimposed on the inlet flow velocity, and the spatio-temporal responses of the flow and temperature fields are calculated. The results are then utilised to assess the linearity of the thermal response represented by the Nusselt number on the obstacles. In general, it is found that for low Reynolds numbers, the dynamics of heat convection can be predicted decently by taking a transfer function approach. However, the dynamical relations between the inlet flow fluctuations as the input and those of Nusselt number as the output, can be non-linear. Second, the thermal system is subject to a ramp disturbance superimposed on the entrance flow temperature/velocity. A response lag ratio (RLR) is defined to further characterise the transient response of the system. The results reveal that an increase in amplitude increases the RLR and interestingly, the Reynolds number has almost negligible effects upon RLR. An experimental investigation is undertaken to examine the response of ultra-lean flames, stabilised in a porous burner, to the fluctuations imposed on the fuel flow rate. The employed porous burner includes layers of silicon carbide porous foam placed inside a quartz tube. The burner is equipped with a series of axially arranged thermocouples and is imaged by a digital camera. The fuel streams are measured and controlled separately by programmable mass flow controllers, which impose sinusoidal fluctuations with variable amplitude and frequency on the steady flow. To replicate realistic fluctuations in the fuel flow rate, the period of oscillations is chosen to be in the order of minutes. The flame embedded in porous media is imaged while the fuel flow is modulated. First, methane and blends of methane and carbon dioxide (mimicking biogas) are mixed with air and then fed to the burner at equivalence ratios below 0.3. Amplitude of the flame oscillations for methane is found to be higher than that for biogas. Further, it is observed that exposure of the burner to the fuel fluctuations for a long time (180s) eventually results in flame destabilisation. In a separate set of experiments, the hydrogen and methane blends are premixed with air at equivalence ratios below 0.275 and fed to the porous burner. It is found that fuel mixtures are noted to be rather insensitive to hydrogen flow fluctuation with a modulation amplitude below 30% of the steady flow. This study reveals the strong effects of unsteady heat transfer in porous media upon the fluctuations in flame position.