Ananthanarayanan Veeraragavan

In Situ Species and Temperature Measurements in a Millimeter-Scale Combustor

An FTIR-based spectroscopic technique is described that exploits silicons transmissivity in the IR to make nonintrusive measurements of species concentration and temperature profiles in microcombustors. Species concentration is determined from the integrated absorbance (Beers law), whereas gas temperature is determined by fitting a narrow-band spectral model (EM2C) to CO2 absorption spectra. The technique is demonstrated in a millimeter-scale combustor burning a lean (Φ = 0.86) CH4–air mixture. The results show that accuracies of ±0.25 e-3 mol/L and ±100°C with spatial resolution ∼1 mm are possible. Heat fluxes to the wall are also estimated and thermal losses are found to be very high (∼90%).

Use of the method of manufactured solutions for the verification of conjugate heat transfer solvers

This paper demonstrates the use of the method of manufactured solutions to verify the implementation of tightly coupled conjugate heat transfer for fluid-solid solvers. The interface conditions in the prescribed manufactured solutions were implemented to mimic real effects such as no-slip, and temperature/heat flux match between the solid and fluid domains. The newly developed solid heat transfer solver was verified in standalone mode using this prescribed manufactured solution and was found to have no apparent coding errors. Our pre-existing in-house compressible fluid solver (Eilmer) was used to demonstrate the conjugate heat transfer implementation. Both the fluid and solid solvers showed an expected spatial order of convergence of 2.0 in the standalone mode. The coupled conjugate heat transfer mode also showed no coding errors and demonstrated that the spatial order of convergence was again 2.0. The one-sided spatial discretisation utilised to enforce the tight coupling for the interface conditions were effectively equivalent to a central difference. Hence, the overall spatial order of the error convergence for the entire domain, including the interface, was 2.0. The method prescribed in this work can be extended for verification of other conjugate heat transfer solvers, in particular for compressible flow scenarios where analytical solutions may not be readily available.

Heat Transfer in Mini∕Microchannels With Combustion: A Simple Analysis for Application in Nonintrusive IR Diagnostics

An analytical solution for the temperature distribution in 2D laminar reacting flow between closely spaced parallel plates is derived as part of a larger effort to develop a nonintrusive technique for measuring gas temperature distributions in millimeter and submillimeter scale channel flows. The results show that the exact solution, a Fourier series, which is a function of the Peclet number, is approximated by second and fourth order polynomial fits to an R value of almost unity for both fits. The slopes of the temperature near the wall (heat fluxes) are captured to within 20% of the exact solution using a second order polynomial and to within 2% of the exact solution using a fourth order polynomial. The fits are used in a nonintrusive Fourier transform infrared spectroscopy technique and enable one to infer the temperature distribution along an absorbing gas column from the measured absorption spectrum. The technique is demonstrated in a silicon-walled microcombustor.

Two-dimensional analytical model of heat transfer for flames in channels

A two-dimensional model for heat transfer in reacting channel flow with a constant wall temperature is developed along with an analytical solution that relates the temperature field in the channel to the flow Peclet number. The solution is derived from first principles by modeling the flame as a volumetric heat source and by applying jump conditions across the flame for plug and Hagen-Poiseuille velocity profiles and is validated via comparison with more detailed computational fluid dynamics solutions. The analytical solution provides a computationally efficient tool for exploring the effects of varying channel height and gas velocity on the temperature distribution in a channel in which a flame is stabilized. The results show that the Peclet number is the principal parameter controlling the temperature distribution in the channel. It is also found that although the Nusselt number is independent of the Peclet number (or velocity) in the postflame region, it can change by nearly 3 ord ers o f magnitude in the preflame region over the range of Peclet numbers (or velocities) expected in microcombustors. This has important implications for quasi-onedimensional numerical modeling of micro/mesoscale combustion, in which it is usual to select a single Nusselt value from the heat transfer literature.Acorrelation to facilitate incorporation of the streamwise Nusselt number variation is provided.

Modeling of heat losses from a PCM storage tank for solar thermophotovoltaic systems

AbstractThis work explores the influence of lateral heat losses from a phase change material (PCM) storage tank on the performance of a storage integrated solar thermophotovoltaic (SISTPV) system b...

Infrared diagnostic technique for microscale combustors

A diagnostic technique is demonstrated for making in-situ measurements of the streamwise evolution of gas temperature and species concentration in a propane-fired silicon-walled microcombustor. The technique capitalizes on the transmissivity of silicon in the infrared by looking through the silicon walls to collect the absorption spectrum of the reacting gas flowing within. This is accomplished by inserting the micro-combustor in the optical path of a Fourier Transform Infrared Spectrometer. Gas temperature is computed from the absorption spectrum of COZ by fitting a narrow band spectral model, EM2C to the spectrum collected by the FTIR. Concentration of CO2 and CH4 are inferred from overall absorbance. The technique has the advantage of being truly nonintrusive as the combustor does not have to be modified in any way to accommodate either instrumentation or optical access. The spatial resolution demonstrated so far is about 1mm in the flow direction but is expected to become smaller as the technique is improved.

Improving scramjet performance through flow field manipulation

In airframe-integrated scramjets, nonuniform compression fields combine with thick boundary layers developed over the vehicle forebody to deliver density stratified flow to the combustor. Additiona...

Experimental investigation of influence of heat recirculation on temperature distribution and burning velocity in a simulated micro-burner

The effect of heat recirculation on the temperature distribution and burning velocity for premixed CH4-Air flames stabilized in micro-channels is investigated experimentally using a silicon-walled micro-burner. Measurements of the temperature profile in the gas are made at different axial locations in the micro-burner using a non-intrusive infrared absorption technique. These are used to compute the spatial distribution of the heat flux from the gas to the combustor structure. The heat flux measurements are combined with measurements of the outer surface temperature distribution made using an infrared camera and a model for heat loss to the environment to determine the amount of heat recirculation that occurs between the post and pre-flame regions. The results show that increasing heat recirculation increases burning velocity. These results are consistent with the predictions of classical models for flame propagation and more recent analytical and numerical models for the micro-combustion process.

Modeling and simulation of fuel-oxidizer mixing in micropower systems

This paper estimates the range of Reynolds numbers and diffusive mixing lengths associated with fuel-oxidizer mixing in micropower systems and then develops analytical and numerical models to explore how mixing performance varies with device size. Both axial and transverse diffusion of species are considered. The models show that Reynolds numbers associated with mixing in micropower systems fall in the laminar-transitional flow regime, where relatively little experimental data exists. They also indicate that fuel-oxidizer mixing lengths decrease with decreasing device size and that the relative importance of axial diffusion to fuel-oxidizer mixing on the microscale depends on the ratio of the diffusive to the convective velocity. At high flow velocities, the mixing length is proportional to the convective velocity and the physical dimensions of the device. At low flow velocities, diffusion dominates, and the mixing length is only proportional to the physical dimensions of the device. This transition in behavior is the result of axial diffusion, which becomes important at Re < 20. Overall, these results suggest that axial diffusion may impose additional limits on the degree to which a combustion-based micropower system can be miniaturized. Copyright

Numerical study of the effect of wall temperature profiles on the premixed methane–air flame dynamics in a narrow channel

Time-accurate simulations of premixed CH4/air flame in a narrow, heated channel are performed using the DRM-19 reaction mechanism. The effect of different wall temperature profiles on the flame dynamics is investigated for three different inflow velocity conditions. At a low inflow velocity of 0.2 m s−1, the flame shows instabilities in the form of spatial oscillations and even flame extinction. With the increase of the inflow velocity, flames are prone to showing more stability at a medium inflow velocity of 0.4 m s−1, and eventually show flame stabilisation at a high inflow velocity condition of 0.8 m s−1 for all the wall temperature profiles examined. The total chemical heat release rate and total gas–solid heat exchange rate are found to have a combined effect on the flame propagation speed that determines flame behaviours. Since the flame behaviours in terms of the oscillation frequency and amplitude for spatially oscillating flames, or the stream-wise stabilisation location for steady-state flames, are very sensitive to the chosen wall temperature profile, a “real” conjugate heat transfer model is recommended in order to capture all of the relevant combustion physics accurately.