M. Queiroz

Pulsed air jet impingement heat transfer

Abstract Impingement heat transfer from a pulsing jet was studied experimentally. The pulsing jet was generated using a valve similar to a ball valve in design, which was rotated at a known frequency. Temporal average heat transfer characteristics of the jet were quantified using a thermographic infrared imaging technique, while the temporal characteristics of the flow were measured using hot-wire anemometry. Jet Reynolds numbers (based on time-average flow rate) in the range 4000–40,000 were investigated for pulsing jet frequencies ranging from the steady jet condition (zero frequency) to approximately 200 Hz. The experimental results indicate that, for the configuration studied, the heat transfer is degraded at all frequencies, generally in the range 0–20% relative to that of the steady jet at the same time-average flow rate. This is in spite of the significant increase in turbulence intensity for the pulsing jet. The power spectral density function for the pulsing jet reveals a dominant peak at the pulsing frequency, with secondary peaks believed to be due to natural frequency of the fluid delivery tubing downstream of the valve. The degradation in heat transfer for the pulsing jet is believed to be due to relatively small turbulent fluctuations superimposed on the instantaneous periodic flow.

Local particle velocity, size, and concentration measurements in an industrial-scale pulverized coal-fired boiler

Abstract Parametric, in situ, particle velocity, size, and number density measurements have been made in a full-scale, coal-burning power plant using an optical diagnostic technique. Available ports in the boiler allowed measurement at three locations above the burner level. Variable test parameters included furnace load, excess air, and burner tilt, using a medium volatile bituminous coal. Higher particle velocities were observed when the boiler was operated at maximum capacity due to increased air and coal flows. Port-to-port velocity variations were attributed to the rotational nature of the mean flow, changes in gas density with changing gas temperature, and the interaction of the flow with the boiler nose. Measured particle number density profiles were characterized by high values in the small particle size class (

Temperature dissipation measurements in a lifted turbulent diffusion flame

Abstract Temperature dissipation measurements have been completed in a lifted turbulent non-premixed propane flame issuing from a converging nozzle at several axial stations and along the centerline. The radial and axial components of the temperature dissipation were measured directly. The assumption of lognor-mality is shown to be a good approximation of the temperature dissipation character in the core of the flame out to ro. the radius where the mean temperature is at a maximum. Significant deviations from the lognormal distribution are observed on the outer side of the shear layer. The radial and axial components of the dissipation are reasonably isotropic in the region close to the jets centerline. However, in direct contrast to the results found in nonreacting flows, detailed comparisons of the radial and axial temperalure dissipation profiles in the off-axis region of 0.1 < r/ ro < 1.3 indicate that there are definite anisotropic and non self-similar characteristics in the temperature dissipation....

ONSET OF DRY-WALL HEAT TRANSFER IN LOW-MASS-FLUX SPRAY COOLING

Abstract An experimental investigation was performed to study a low-mass-flux, spray nucleate boiling phenomenon termed dry wall during which all the impinging liquid is immediately vaporized upon contact with the heated surface. Measurements of wall heat flux and spray coolant mass flux were performed together with a characterisation of spray parameters (such as local droplet size and velocity), using a laser-based diagnostic technique. Two different atomizing nozzles were used and the effect of liquid subcooling on the transition was also studied. The transition to the dry-wail heat transfer regime was found to correlate well with the average global concentration of the spray obtained by the ratio of the spray mass flux to the average spray velocity. The experimental results showed that above a concentration of approximately 0.20 kg/m3, no evidence was seen of transition to dry wall. This concentration corresponding to transition was found to be independent of the two different nozzle types used in the ...

A parametric exploration of the dynamic behavior of flame propagation in planar sprays

Abstract An exploration of the dynamic nature of flame propagation in planar sprays at ambient conditions has been performed. The parametric influence of three different levels of fuel-vapor concentrations (corresponding to three liquid fuel initial temperatures of 43°C, 25°C, and 15°C), two droplet sizes (305 and 407 μm), three fuel compositions (hexane, iso-octane, and a 50-50 by weight mixture of these fuels), and two levels of gas-phase turbulence intensity (17% and 2.5%) on the average speed of flame propagation and its fluctuations have been investigated using sequential photographic information. It was found that higher fuel-vapor concentrations, smaller droplet sizes, and more volatile fuels caused the average flame speed to increase while reducing its fluctuations. The effect of higher gas-phase turbulence intensity also caused the average flame speed to increase, but increased its fluctuations. Some of the observed phenomena can be qualitatively explained through the ignition process in a spray. When the presence of fuel vapor in the gas phase becomes significant due to the evaporation of droplets before the flame front, a premixed-gas type of ignition process prevails. At this condition the variations of average flame speed and its fluctuations may be explained through the effect of gas-phase equivalence ratio on the premixed-gas average flame speed. When the presence of fuel vapor in the flame front is reduced at high turbulence, a relay type of ignition process occurs. In this latter case, the increase of average flame speed and of flame speed fluctuations may be interpreted from the enhanced turbulent diffusion of thermal energy and from the turbulent fluctuations caused by the eddy transport phenomenon in the flow field, respectively.

Experimental exploration of the thermal structure of an array of burning droplet streams

Abstract The effect of turbulence intensity and fuel vapor pressure on the thermal structure in a linear array of burning droplet streams has been investigated using two levels of turbulence intensity and fuel vapor pressure. Particular attention has been focused on the relationship between the dynamic motion of the flame front and the thermal structure of the flame. Bare microthermocouples, digitally compensated for thermal inertia effects, were used to measure fluctuating gas-phase temperatures in this dilute spray flame with 407 μm nominal diameter hexane droplets. The flame was formed by nine vertical streams oriented in a plane and horizontally separated by a distance of 4 mm. Increasing the vapor pressure of the fuel caused higher flame temperatures at the average location of the premixed-gas flame. However, the essential features of the average and fluctuating temperature profiles as well as the probability density function (pdf) surfaces were unchanged over the range of vapor pressures investigated. The higher-turbulence intensity level promoted higher temperature fluctuations because the fluctuating combustion zone of the flame was widened. The dynamics of the drifting motion of the flame as well as the instantaneous temperature profiles across the flame were also influenced, causing the disappearance of the bimodal shape in the pdf surfaces.

Evaluation of a dimensionless group number to determine second-einstein temperatures in a heat capacity model for all coal ranks☆

Abstract A dimensionless group number is proposed to characterize the differences in chemical structures and physical properties between coal-like materials varying from lignites to anthracites, including graphite as a limiting case. This dimensionless number provides a simple and efficient correlation to determine second-Einstein temperatures (θ2) in a specific heat capacity (C) model for all coal ranks, using information derived directly from the chemical composition (proximate and ultimate analyses) and the calorifie value (H) of each substance. The nondimensional correlation has the form Rθ 2 /H = ƒ (FC) , where R is the gas constant for the heterogeneous material and FC is the amount of fixed carbon in the parent coal. Properties of 50 coal-like materials were used to obtain this functional dependence. It was found that a linear function provides a good fit of the experimental data. This dimensionless correlation allows calculations of the behavior of the specific heat capacities of the materials studied here with an average value of 3.55% for the mean deviation in relation to experimental curves in the important temperature range of 300–600 K. The applicability of Einstein theory of heat capacity is analyzed for the special case of coal-like materials, and a generalization of Merricks model for all coals of industrial interest is presented.

The Effect of Lateral Spacing on the Combustion Dynamics and Thermal Structure of an Array of Burning Droplet Streams

Abstract The effect of lateral stream separation distance on the dynamics of flame propagation and thermal structure of a simplified spray flame has been studied. The flame was made up of 300µm average diameter hexane droplets injected through ten droplet streams in a plane, horizontally separated by a distance varying from 1 to 6 mm. Sequential photography was used to document the flame front motion and micro-thermocouples were used to perform measurements of gas-phase temperatures. The reactive flow was characterized by an inlet pre-ignition zone, followed by a bluish partially-premixed flame which acted as the ignition source of the fuel streams. Further downstream, a pattern of yellowish diffusion flames surrounding individual streams or groups of them was established, depending on the lateral separation of the streams. As the lateral spacing of the streams was increased, the vertical region swept by the flame front increased due to an augmentation in the flame propagation unsteadiness associated with...

The effect of heat release on various statistical properties of a reacting shear layer

Abstract Three-dimensional direct numerical simulations were used to study the effect of heat release from a binary, single-step chemical reaction on the statistical properties of a temporally developing turbulent mixing layer. Various statistical moments, probability density functions, power spectral densities, and autocorrelations of a conserved scalar, and the velocity field are presented. Scalar-velocity and pressure-velocity correlations, and joint probability density functions, which are extremely difficult to measure experimentally, were also calculated from the simulations. Many features of the calculated statistics compare qualitatively well with results reported from related experimental studies. Significant changes in the vortex structure occur with moderate heat release, resulting in more diffuse vortices than in the isothermal simulation. Consequently, slower rotation rates of the coherent structures occur with moderate heat release. This effect has previously been shown to be caused by the baroclinic torques and thermal expansion in the mixing layer. The statistics in this study reflect these changes in the vortex structure due to moderate heat release.

The effect of monosized hexane droplets on the thermal structure of a lifted gaseous flame

Abstract Gas temperature measurements have been completed in a lifted propane flame with and without monosized droplets at two radial stations in the developing region of the jet. A general decrease in the average gas temperature was observed in the flame when the droplets were introduced, because of local evaporative cooling effects. Profiles of rms temperature with local maxima at either side of the average location of the reaction zone were observed for 5 in the gaseous flame. Further downstream along the centerline, this region disappeared because the jets fuel-rich central core ceased to exist. The droplets prolonged the axial region where these double-maxima rms temperature profiles existed, because of an extension of the flame core. A comparison of power spectral densities measured with and without the droplets suggests that the droplets substantially changed the flow field in the core region, to the point of enhancing a complex vortical structure weakly existent in the gaseous flame.