Abstract
While there has been some recognition regarding the impact of thermal boundary conditions (adiabatic versus isothermal) on premixed flame propagation mechanisms in micro-channels (hydraulic diameters <10 mm), their impact in macro-channels has often been overlooked due to small surface-area-to-volume ratios of the propagating combustion wave. Further, the impact of radiative losses has also been neglected due to its anticipated insignificance based on scaling analysis and the high computational cost associated with resolving it’s spatial, temporal, directional, and wavelength dependencies. However, when channel conditions promote flame acceleration and deflagration-to-detonation transitions (DDT), large pressures are encountered in the vicinity of the combustion wave, thereby increasing the magnitude of radiative losses which in turn can impact the strength and velocity of the combustion wave. This is demonstrated for the first time through simulations of lean (equivalence ratio: 0.5) hydrogen-air mixtures in a macro-channel (hydraulic diameter: 174 mm) with obstacles (Blockage ratio: 0.51). By employing Planck mean absorption coefficients in conjunction with the P-1 radiation model, radiative losses are shown to affect the run-up distances to DDT in a long channel (length: 11.878 m). As anticipated, the differences in run-up distances resulting from radiative losses only increased with system pressure.
Highlights
In order to gain public acceptance of hydrogen as an energy carrier requires addressing key safety issues related to its production, storage and application
The position of the combustion wave at 6.4 × 10−2 s at a system pressure of 0.1 atm relative to the open end of the tube is shown in Figure 3a, and is represented by the gas temperature contours
If L represents the thickness of the combustion wave, we find that kL is on the order of 10−3 for the 0.1 atm case and of the order 10−2 for the 3 atm case
Summary
In order to gain public acceptance of hydrogen as an energy carrier requires addressing key safety issues related to its production, storage and application. Serious explosion accidents involving hydrogen have been attributed to massive release rates of hydrogen into a congested space with obstacles and due to accidental introduction of air into high-pressure storage vessels [1]. High-fidelity numerical simulations can yield valuable insights into deflagration and detonation scenarios involving hydrogen-air mixtures. While considerable care has gone into the selection of appropriate turbulence and gas-phase chemistry modeling methodologies in these scenarios, the impacts of radiative heat losses have often been overlooked or ignored. These may either be attributed to the fact that a propagating deflagration/detonation wave may be considered to be optically thin and, radiative losses are anticipated to be minimal. There is a significant computational overhead associated with modeling the spatial, temporal, directional, and wavelength dependencies of radiative transport
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