UNDERSTANDING THE ROLE OF HEAT RECIRCULATION IN ENHANCING THE SPEED OF PREMIXED LAMINAR FLAMES IN A PARALLEL PLATE MICRO-COMBUSTOR

This dissertation investigates the role of heat recirculation in enhancing the flame speeds of laminar flames stabilized in a parallel plate reactor by: 1) developing analytical models that account for conjugate heat transfer with the wall and 2) making measurements of temperature profiles in a simulated microcombustor using non-intrusive FTIR spectroscopy from which heat recirculation is inferred. The analytical models have varying degrees of complexity. A simple heat transfer model simulates the flame by incorporating a concentrated heat release function along with constant temperature wall model. The next level model accommodates conjugate heat transfer with the wall along with a built in heat loss model to the environment. The heat transfer models identify the thermal design parameters influencing the temperature profiles and the Nusselt number. The conjugate heat transfer model is coupled with a species transport equation to develop a 2-D model that predicts the flame speed as an eigenvalue of the problem. The flame speed model shows that there are three design parameters (wall thermal conductivity ratio ( &kappa ), wall thickness ratio ( &tau ) and external heat loss parameter ( NuE )) that influence the flame speed. Finally, it is shown that all these three parameters really control the total heat recirculation which is a single valued function of the flame speed and independent of the velocity profile (Plug or Poiseuille flow). On the experimental side, a previously developed non-intrusive diagnostic technique based on FTIR spectroscopy of CO2 absorbance is improved by identifying the various limitations (interferences from other species, temperature profile fitting, ... etc) and suggesting improvements to each limitation to make measurements in a silicon walled, simulated microcombustor. Methane/Air and Propane/Air flames were studied for different equivalence ratios and burning velocities. From the temperature profiles it can be seen that increasing the flame speed pushes the flames further up the channel and increases the combustors inner gas and outer wall temperatures (measured using IR thermography). The temperature profiles measured are used to make a 2-D heat recirculation map for the burner as a function of the equivalence ratio and burning velocity. The experimental results are compared to the analytical models predictions which show a linear trend between flame speed and heat recirculation

Heat Transfer Analysis for Improved In-situ Infrared Tempertaure Diagnostics in Microcombustors

This thesis investigates heat transfer processes occurring in microcombustors. First, a simple 2D model is developed for predicting temperature profiles in a premixed laminar flame propagating between two parallel plates. The model is used to generate a correlation for the variation in Nusselt number with downstream distance which is useful for numerical simulations. It also shows that the temperature profile across the channel is well approximated using either 2nd or 4th order polynomials. Second, the functional form of the gas temperature profile is used to demonstrate a new diagnostic technique for making non-intrusive measurements of gas temperature and wall heat fluxes. The technique is applied in a silicon-walled microcombustor (5 cm x 2 mm x 5.5 cm). The gas temperature and wall heat flux measurements are combined with measurements of the wall temperature distribution to develop a complete picture of heat transfer in the microcombustor. The results show that thermal feedback from the post-flame to the pre-flame via the structure is the dominant heat transfer path in microcombustors

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

This 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 by means of an analytical model. The heat losses from the lateral surface of the PCM tank are modeled using Newton’s law of cooling by prescribing a heat-loss coefficient on the lateral surfaces. The results show that at high heat losses, low thermal efficiencies are realized. Correspondingly larger solar concentrations are required to fully melt the PCM tank. At low heat losses, such as can be expected when using thermal insulation on the lateral surfaces, approximately 40% thermal efficiency can be realized. The results also demonstrate that a high absorber area:length of PCM tank squared (SR) ratio enables the system to have a high thermal efficiency. For a high-SR, low-heat-loss design case, having a high taper ratio, high area ratio between absorber area and inlet hole area, and small PCM tank length all achieve higher thermal efficiencies. It is expected that these SISTPV systems will be designed at steadystate to be fully molten in order to maximize thermal energy storage via the latent heat of the PCM. The analytical model developed here can be used to predict the design conditions under which the PCM tank will be fully molte

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 CH/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, 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, and eventually show flame stabilisation at a high inflow velocity condition of 0.8 m s 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

Performance of high mach number scramjets - Tunnel vs flight

While typically analysed through ground-based impulse facilities, scramjets experience significant heating loads in flight, raising engine wall temperatures and the fuel used to cool them beyond standard laboratory conditions. Hence, the present work numerically compares an access-to-space scramjet's performance at both these conditions. The Mach 12 Rectangular-to-Elliptical Shape-Transitioning scramjet flow path is examined via three-dimensional and chemically reacting Reynolds-averaged Navier-Stokes solutions. Flight operation is modelled through 800 K and 1800 K inlet and combustor walls respectively, while fuel is injected at both inlet- and combustor-based stations at 1000 K stagnation temperature. Room temperature walls and fuel plena model shock tunnel conditions. Mixing and combustion performance indicates that while flight conditions promote rapid mixing, high combustor temperatures inhibit the completion of reaction pathways, with reactant dissociation reducing chemical heat release by 16%. However, the heated walls in flight ensured 28% less energy was absorbed by the walls. While inlet fuel injection promotes robust burning of combustor-injected fuel, premature ignition upon the inlet in flight suggests these injectors should be moved further downstream. Coupled with counteracting differences in heat release and loss to the walls, the optimal engine design for flight may differ considerably from that which gives the best performance in the tunnel

On the influence of modelling choices on combustion in narrow channels

This paper examines the effect of modelling choices on the numerical simulation of premixed methane/air combustion in narrow channels. Knowledge on standard and well-accepted numerical methods in literature are collected in a cohesive document. The less well-established modelling choices have been thoroughly evaluated and discussed. A systematic method of computing the grid convergence index (GCI) has been presented for refining the computational grid. Two types of inflow boundary conditions have been tested and compared in terms of their wave-damping characteristics. The effect of different reaction schemes on simulation results have been examined and an appropriate mechanism (DRM-19) has been selected. Various types of ignition strategies to initiate the flame have been tested and compared. The transient ignition process which has not been discussed extensively in existing literature has been quantitatively described in this paper

On flame propagation in narrow channels with enhanced wall thermal conduction

The influence of orthotropic wall materials, which have enhanced thermal conductivity in the axial direction, on the flame speed is explored via an analytical model in a parallel plate microcombustor. The model accounts for 2D conjugate heat transfer (both in wall and gas) and fuel species transport in the micro-channel. The effects of heat loss, orthotropic wall thermal conductivities, and wall thickness on the flame speed are explored. The results indicate that as the axial thermal conductivity of the wall is increased, the allowable heat losses to the ambient by the burner also increased. Thicker walls showed increased benefit to the thermal conductivity tailoring than thinner wall designs; both in increased flame speeds as well as the ability to tolerate higher heat losses without extinction. Total heat recirculation is shown to be the primary parameter to control the flame speed

Advanced measurements and monitoring techniques

Measurement and monitoring techniques for both underground coal gasification (UCG) and underground coal fires have several commonalities. In this chapter, we will present several of these techniques that were initially developed for UCG but have subsequently found applications also in the detection of underground combustion pertaining to coal fires. While there is strong commonality between both phenomena, their fundamental nature is different: UCG is a controlled reaction with targeted outcomes while an underground (or subsurface) coal fire is usually an uncontrolled reaction where we would like to contain or put-out the reaction as much as possible. The presence of an underground coal fire is usually felt through changes in the ambient air quality (foul smells), smoke, or heat emanating from the ground. Typically, these symptoms only manifest themselves long after the fire has persisted and spread over a wide area. In extreme cases, the detection becomes a relatively moot point as smoke emanating from the ground or subsidence of the soil makes it evident that there is an underground coal fire. UCG reaction fronts and their behavior are harder to detect as they are typically deep underground and are not likely to have such visual indicators. Therefore, it is rather challenging to detect them and requires tools and diagnostics beyond the human sensory perception

Limitations and improvements to an FT-IR based non-intrusive diagnostic technique for making temperature measurements

A non-intrusive diagnostic technique based on FT-IR spectroscopy for making temperature measurements in small scale combustors and reacting flows has been developed in previous work. The technique is based on absorption by naturally occurring CO molecules which is a well known function of temperature. Subsequent work, however, has shown that using this technique to infer gas temperature along a line of sight is more challenging than was recognized previously. The multiplicative nature of the absorption process causes spatial information to be lost and the temperature profile returned by the least-squares fitting procedure applied to the absorption spectra can depend on the initial guess. Spatial information can be recovered if the functional form of the temperature profile is known. A short-term solution to the convergence problem is to develop methods for supplying better initial guesses. The better solution (future work) is to replace the least-squares procedure with one that enables one to enforce a fitting tolerance at each data point