Jay P. Gore

A Review of Heat Transfer Issues in Hydrogen Storage Technologies

Significant heat transfer issues associated with four alternative hydrogen storage methods are identified and discussed, with particular emphasis on technologies for vehicle applications. For compressed hydrogen storage, efficient heat transfer during compression and intercooling decreases compression work. In addition, enhanced heat transfer inside the tank during the fueling process can minimize additional compression work. For liquid hydrogen storage, improved thermal insulation of cryogenic tanks can significantly reduce energy loss caused by liquid boil-off. For storage systems using metal hydrides, enhanced heat transfer is essential because of the low effective thermal conductivity of particle beds. Enhanced heat transfer is also necessary to ensure that both hydriding and dehydriding processes achieve completion and to prevent hydride bed meltdown. For hydrogen storage in the form of chemical hydrides, innovative vehicle cooling design will be needed to enable their acceptance.

NOx emission and major species concentrations in partially premixed laminar methane/air co-flow jet flames☆

Abstract Measurements of visible flame heights, global radiative heat loss fractions, distributions of mole fractions of stable gas species, and pollutant emission indices in laminar partially premixed flames burning various fuel-rich mixtures of CH4 and air in an overventilating co-flow of air are reported. Mole fractions of CO2, CO, H2, O2, N2, CH4, C2H4 and C2H2 were measured, using sampling and gas chromatography, at several radial locations at three different heights above the fuel tube for a fixed fuel flow rate and six different fuel tube equivalence ratios. Mole fractions of H2O were inferred from the dry based measurements. With increasing levels of partial premixing following effects are observed: (1) the visible flame height decreases and the overall flame color changes from yellow to blue; (2) the radiative heat loss fraction first decreases and then reaches a constant value; (3) the mole fractions of CO decrease and those of CO2 and H2O increase in the lean parts of the flame; (4) mole fractions of C2H2 decrease and those of C2H4 first increase and then decrease in the rich parts of the flame; (5) mole fractions of CO and H2 first decrease slightly and then increase in the rich parts of the flame; and (6) the O2 mole fractions at the point of negligible CH4 mole fraction decrease. Measurements of emission indices for NO, NOx, CO and HC show that, for a fixed fuel flow rate and overall equivalence ratio, an optimum level of partial premixing exists.

Heat Feedback to the Fuel Surface in Pool Fires

Abstract A series of measurements designed to investigate the heat feedback in pool fires burning liquid fuels are reported. Such measurements are essential for the development and validation of detailed models which predict the burning rate of liquid hydrocarbons and solid polymers. The radial variation of the local radiative and local net heat flux incident on the surface of 0.30 m diameter pool fires were measured. A water-cooled, nitrogen purged, narrow view-angle gauge was developed to measure the radiative flux incident on the fuel surface. Measurements of the mass burning rate in a burner composed of annular rings was used to estimate the local heat feedback. A number of different fuels were studied, yielding flames with a wide range of heat release rates and luminosities. Consideration of the heat balance for a control volume enclosing the liquid PPOI indicated that radiation was an important component of the heat feedback for non-luminous fires and a dominant component in luminous fires.

Structure and spectral radiation properties of turbulent ethylene/ air diffusion flames

An experimental and theoretical study of the structure and radiation properties of round, turbulent, luminous ethylene/air diffusion flames is described. Measurements included mean and fluctuating velocities, mean concentrations of major gas species, soot volume fractions, monochromatic absorption (632.8 nm), spectral emission (1000–5500 nm), and total radiative heat flux distributions. Flame structure was predicted using a Favre-averaged k-∈-g turbulence model and the laminar flamelet approximation. Spectral radiation intensities were predicted using a narrow-band model—both ignoring (using mean properties) and considering (using a stochastic method) turbulence/radiation interactions. Total radiative heat fluxes were found by summing spectral intensities over wavelength and paths through the flame. The comparison between predicted and measured flame structure was reasonably good. Differences between mean-property and stochastic radiation emission predictions were significant (50–300 percent) suggesting strong turbulence/radiation interactions in luminous flames. However, both radiation analyses represented trends due to changes in position and burner flow rate reasonably well. This suggests that soot volume fractions may approximate universal functions of mixture fraction in turbulent flames, even though this correspondence is only crudely observed in laminar flames due to hydrodynamic complications of soot particle motion.

A study of the structure of submerged reaction zone in porous ceramic radiant burners

The first measurements of temperature and species distributions within the submerged reaction zone stabilized inside radiant burners made of reticulated ceramic matrices are reported. The radiant burners and the flames stabilized within them are characterized in terms of stability limits, radiation efficiencies, and global pollutant emission indices. In contrast to adiabatic flame behavior, a tendency of the burners to flash back with increasing firing rates was experimentally observed. This is in qualitative agreement with the predictions reported in the literature [8]. However, for the present equivalence ratio, the firing rate at flashback is approximately five times higher than typical estimates given in Ref. 8. The temperature and species profiles show that the reaction zone is very broad, and that the tendency to flashback results from higher preheating of the unburnt mixture. Quenching at the ceramic matrix strand surfaces was observed from local temperature and species data. The NOX emission indices of these burners are extremely low (0.1–0.35 g/kg). The CO and HC emission indices are slightly higher due to the quenching of reactions near the solid surfaces (CO: 0.1–3.6 g/kg, HC: 0.1–1.2 g/kg).

Structure and radiation properties of luminous turbulent acetylene/air diffusion flames

An experimental and theoretical study of the structure and radiation properties of luminous, round, turbulent acetylene/air diffusion flames is described. Measurements were made of mean and fluctuating velocities, mean concentrations, laser extinction (514 and 632.8 nm), spectral radiation intensities (1200--5500 nm), and radiative heat fluxes. The measurements were used to evaluate structure predictions based on the laminar flamelet concept, and radiation predictions based on a narrow-band model both ignoring and considering turbulence/radiation interactions. State relationships needed for the laminar flamelet concept were found from auxiliary measurements in laminar flames. Predictions were encouraging;however, quantitative accuracy was inferior to earlier findings for luminous flames. This is attributed to the large radiative heat loss fractions of acetylene/air flames (approaching 60 percent of the heat release rate);coupled structure and radiation analysis should be considered for improved results. The findings suggest significant turbulence/radiation interactions (increasing spectral intensities 40--100 percent from estimates based on mean properties);and that soot volume fractions may approximate universal fractions of mixture fraction in turbulent acetylene/air diffusion flames.

An evaluation of flame surface density models for turbulent premixed jet flames

Abstract We report a computational study of a turbulent premixed jet flame stabilized in a laminar coflow of a reactant mixture at the same stoichiometric ratio as the turbulent flame. The performance of four different flame surface density (FSD) models, earlier used by Duclos et al. [1] for studying flame propagation in constant density frozen turbulence, and that of the reaction progress variable approach of Bray and Moss (BM) is evaluated by comparison with existing experimental data. We also describe a sensitivity analysis of predictions to inlet turbulence quantities and inlet flame surface density. The predictions of the Mantel and Borghi (MB) model are observed to be highly sensitive to inlet turbulence quantities, while those of the coherent flame (CF) model and BM models are less sensitive. The mean velocity and temperature predictions are insensitive to the inlet FSD distribution because of the strong production and dissipation terms in the transport equation for FSD. The predictions of mean velocity and temperature profiles are compared with the experimental data of Chen et al. [2] . Among the FSD models, the CF and MB models gave good estimates of mean velocity and temperature. The Cant, Pope, and Bray (CPB) model overpredicted the mean temperatures slightly. The flame surface density model of Cheng and Diringer (CD) predicted too high temperatures. This is shown to be resulting from an overprediction of the generation of flame surface density. All the models predict the increase in flame brush thickness with distance from the injector exit, but the MB model provides the best quantitative estimates of the flame brush thickness.

TRANSIENT SCALAR PROPERTIES OF STRONGLY RADIATING JET FLAMES

Abstract An experimental and a theorietical study of transient radiation properties of strongly radiating turbulent diffusion flames is described. Simultaneous transient measurements of temperatures and volume fractions of soot obtained using a three wavelength emission-absorption probe are reported. Soot volume fractions inferred from absorption measurements are significantly higher than those inferred from emission data suggesting the presence oflarge quantities of relatively cold soot within the flames. The results show effects of progressive radiative cooling with distance from the injector exit. Reasonably good predictions of monochromatic intensities were obtained using a new bivariate stochastic analysis.

A study of pollutant emission characteristics of partially premixed turbulent jet flames

Abstract We measured emission indices for NO x , CO, and HC for turbulent partially premixed flames formed by injecting rich methane/air mixtures through a central burner tube into a co-flow of air. The operating conditions included a broad range of burner tube equivalence ratios (Φ B ) including the limiting cases of diffusion and stoichiometric premixed flames. At sufficiently high levels of partial premixing, a double flame structure consisting of a rich premixed inner flame and an outer diffusion flame is established similar to that previously observed in laminar flames. EINO x values remain approximately constant for 5 ≤ Φ B ≤ ∞ and then decrease slightly with decreasing Φ B at 3.5 ≤ Φ B ≤ 5. For 3.5 ≤ Φ B ≤ 1.5, EINO x decreases further with decreasing Φ B to a minimum at around Φ B = 1.5, followed by a sharp increase as Φ B approaches unity. The reduction in EINO x at Φ B = 1.5 as compared with that for the diffusion flame is approximately 25% and as compared with that for the stoichiometric premixed flame is approximately 35%. In addition, we measured temperature distributions and found that temperatures increase continuously with increasing partial premixing. We also estimated global residence times (τ 0 ) from flame length measurements and average velocities. The observed changes in temperatures and residence times are not sufficient to explain the observed changes in EINO x .

Experimental study of temperature and CH radical location in partially premixed CH4/air coflow flames

Abstract As part of an ongoing investigation of an exhaust NOx emission index minimum measured for partially premixed flames, radial temperature profiles and CH radical locations were measured in atmospheric-pressure, partially premixed, coflow, methane/air flames with fuel-side equivalence ratios of 1.6, 2.0, and 3.5, at three axial heights above the burner. The work was undertaken because of the importance of temperature and CH radical behavior in NO formation chemistry. Thin-filament pyrometry was found to be more appropriate than thermocouple thermometry for temperature measurements in partially premixed flames. Results demonstrated that the 1.6-equivalence-ratio flame exhibited classical double-flame structure, the 2.0-equivalence-ratio flame was a merged flame, and the 3.5-equivalence-ratio flame exhibited diffusion-flame structure. Signals from CH∗ chemiluminescence and CH laser-induced fluorescence provide evidence that, for the present measurement locations, double flames exhibit single CH peaks which can be associated with their premixed component flames. Double CH radical peaks, which were predicted to occur in low-strain-rate flames, were not found for the limited number of flame conditions and locations studied. In the near-burner region, the premixed and nonpremixed component flames of the ΦB = 1.6 double flame diverge radially with increasing downstream distance and merge together for larger values of ΦB.