Baki M. Cetegen

Entrainment in Fire Plumes

A new technique for measurement of mass flow rates in buoyant fire plumes is described. The characteristics of 10 - 200 k W methane diffusion flames stabilized on porous-bed-burners of 0.10 - 0.50 m dia. are described. A transition in the dependence of flame height on heat input and burner size was observed when the flame height was about four times the burner diameter. The mass flow rates in the buoyant plumes produced by the fires were measured for a range of elevations starting just below the time-averaged top of the flame and extending to six times this flame height. The mass flow rates in this region of the plume were correlated by the use of a simple plume model. Atmospheric and forced disturbances in the air being entrained increased the entrainment rate of the plume.

TOWARDS DURABLE THERMAL BARRIER COATINGS WITH NOVEL MICROSTRUCTURES DEPOSITED BY SOLUTION- PRECURSOR PLASMA SPRAY

The feasibility of a new processing method—solution precursor plasma spray (SPPS)—for the deposition of ZrO2-based thermal barrier coatings (TBCs) with novel structures has been demonstrated. These desirable structures in the new TBCs appear to be responsible for their improved thermal cycling life relative to conventional plasma-sprayed TBCs. Preliminary results from experiments aimed at understanding the SPPS deposition mechanisms suggest that nanometer-scale particles form in the plasma flame, followed by their deposition by sintering onto the substrate in the intense heat of the plasma flame. The SPPS method, which offers some unique advantages over the conventional plasma-spray process, is generic in nature and can be potentially used to deposit a wide variety of ceramic coatings for diverse applications.

Experiments on the periodic instability of buoyant plumes and pool fires

Abstract An experimental study of buoyant propane diffusion flames was undertaken to identify the mechanism responsible for the periodic oscillations near the source of these flames. This phenomenon, often referred to as “puffing” in the literature, exhibits itself as quasi-periodic oscillations of the diffusion flame front near the axisymmetric source of a fire with formation of large scale flaming vortical structures. Experimental diagnostics primarily involved flow visualization, velocity, and pressure measurements under a variety of experimental conditions. First, the behavior of buoyant, noncombusting plumes originating from 0.10- and 0.30-m-diameter sources was investigated with either isothermal helium or high temperature combustion products as the buoyant fluid. It was found that the helium plumes exhibited puffing at both scales with puffing frequencies similar to the flames scaling with D −1 2 . The plumes of hot combustion products on the 0.30-m-diameter burner were highly turbulent and puffing was very weak compared with the flames that were generated upon ignition of the latter hot gas plume of vitiated combustion products. Second, effects of heat release were studied by dilution of fuel with a noncombustible gas. It was found that puffing persisted as long as a diffusion flame was sustained by the fuel stream, although its intensity diminished with increasing dilution. Third, effects of disturbances both internal and external to the flame were studied. These experiments strongly suggest that there exists a coupling of the flame front motion near the burner surface with the downstream development of large-scale flaming structures. Based on the reported experiments, a puffing mechanism is suggested. The puffing mechanism involves (1) acceleration of buoyant plume gas in stagnant surroundings resulting in formation of a toroidal vortical structure within one burner diameter above its surface, (2) the decaying influence of the toroidal vortex on the flame surface near the burner lip as it convects upward, and (3) the accumulation of buoyant gas inside the flame envelope and its buoyant acceleration to form the next vortical structure. The scaling of puffing frequency with the burner diameter is connected to the convection speed of toroidal vortices within one diameter height above the burner surface as it was shown with the use of a simple kinematic model.

Visible structure of buoyant diffusion flames

Natural gas diffusion flames stabilized on 0.10, 0.19 and 0.50 m. diameter porous bed burners have been studied for heat release rates ranging from 10 to 200 kW. Flame heights were measured from video tape recordings and by eye averaged techniques. The dependence of flame height on a dimensionless heat addition parameter shows a transition for values of the parameter around unity. For flames taller than three burner diameters, the initial diameter of the fire does not affect the length of the flame whereas for short flames, initial geometry becomes important. Another prominent feature of these flames is the presence of large scale axisymmetric structures which are formed close to the burner surface with more or less regular frequency and which rise through the flame region. These structures are responsible for the fluctuations of the flame top and strongly influence the geometry of the flame.

Mechanisms of ceramic coating deposition in solution-precursor plasma spray

The solution-precursor plasma spray (SPPS) method is a new process for depositing thick ceramic coatings, where solution feedstock (liquid) is injected into a plasma. This versatile method has several advantages over the conventional plasma spray method, and it can be used to deposit nanostructured, porous coatings of a wide variety of oxide and non-oxide ceramics for a myriad of possible applications. In an effort to understand the SPPS deposition process, key diagnostic and characterization experiments were performed on SPPS coatings in the Y 2 O 3 -stabilized ZrO 2 (YSZ) system. The results from these experiments show that there are multiple pathways to SPPS coating formation. The atomized precursor droplets undergo rapid evaporation and breakup in the plasma. This is followed by precipitation, gelation, pyrolysis, and sintering. The different types of particles reach the substrate and are bonded to the substrate or the coating by sintering in the heat of the plasma. The precursor also reaches the substrate or the coating. This precursor pyrolyzes in situ on the substrate, either after it reaches a “cold” substrate or upon contact on a “hot” substrate and helps bond the particles. The coating microstructure evolves during SPPS deposition as the coating temperature reaches approximately 770 °C.

Experiments on the oscillatory behavior of buoyant plumes of helium and helium‐air mixtures

Experiments on the oscillatory behavior of axisymmetric buoyant plumes of helium and helium‐air mixtures are reported for a range of nozzle diameters (3.6 cm<d<20 cm), source velocities, and plume densities. Measurements include pulsation frequencies as determined from total pressure fluctuations along the plume centerline in addition to the phase resolved laser Doppler velocity measurements. These nonreacting buoyant plumes are found to exhibit periodic oscillations of plume boundaries which subsequently evolve into toroidal vortices within one‐half diameter above the nozzle exit. These oscillations and vortices are similar to those observed in pool fires, although their frequency scaling is somewhat different. The frequency relationship is well represented by the expression S=0.8Ri0.381, where the Strouhal number is S=fd/V0 and the Richardson number is defined as Ri1=[(ρ∞−ρp)gd]/ρ∞V20. Parameters f, V0, ρ are frequency, source velocity, and density and subscripts p and ∞ refer to the plume fluid and amb...

Entrainment in the Near and Far Field of Fire Plumes

This paper describes entrainment measurements made in fire plumes with a new technique. Measurements were in plumes rising from natural gas diffusion flames stabilized on 0.10, 0.19 and 0.50 m diameter burners and the heat release rates ranged from 10 to 200 kW. The heights examined ranged from elevations starting very close to the burner surface to distances about five times the average flame heights. Experiments indicate the presence of three regions: a region close to the burner surface where plume entrainment rates are independent of the fuel flow (or heat release) rates; a far field region above the flame top, where a simple point source model correlates the data reasonably well; and an intermediate region where entrainment appears to be similar to that of a turbulent plume.

Experiments on stability and oscillatory behavior of planar buoyant plumes

Experiments on the onset of buoyant instabilities leading to periodic formation of vortical structures in planar buoyant plumes of helium and helium/air mixtures injected into quiescent air are reported for a range of nozzle widths (w=20–70 mm), plume fluid densities (pure helium to that approaching air), and velocities at the nozzle exit. First, the plume parameters corresponding to the onset of the oscillatory instability were experimentally determined by varying the nozzle exit velocity for different nozzle widths and plume fluid densities in two different nozzle configurations. These configurations corresponded to a freestanding rectangular nozzle and a rectangular nozzle surrounded by a flat plate in the plane of the nozzle exit. The observed plume behavior in the near field was characterized as nonoscillatory, transitional, or pulsatile. The onset of pulsations in the near field of these buoyant plumes (within a height of two nozzle widths) was best correlated in terms of the plume source Reynolds n...

Behavior of naturally unstable and periodically forced axisymmetric buoyant plumes of helium and helium–air mixtures

Effects of forcing on the behavior of naturally unstable buoyant plumes of helium and helium/air mixtures are investigated experimentally. Axisymmetric buoyant plumes originating from a 10 cm diam nozzle are perturbed in a periodic manner by an upstream loudspeaker introducing a sinusoidal streamwise velocity oscillation with a peak-to-peak velocity magnitude of about 3% of the mean velocity at the nozzle exit. The experimental conditions in this study corresponded to Richardson number, Ri=[(ρ∞−ρp)gd]/ρ∞V02, of 42. Video images of the naturally unstable plumes as well as the forced plumes were analyzed to study the features of the resulting flow oscillations and the vortical structures. It was found that the plume responds readily to the imposed oscillations with the toroidal vortices forming at the forcing frequency. Plume images indicate a more complicated and turbulent state of the flow within the large-scale toroidal vortices as they convect downstream. These vortical structures have lateral dimension...

Spectroscopic temperature measurements in direct current arc plasma jets used in thermal spray processing of materials

An experimental study was conducted to determine the plasma temperature field and its parametric variation with respect to plasma operating conditions using emission spectroscopy. The focus of our study was the direct current (DC) arc plasma systems used in thermal spray processing of ceramic materials. A commercial plasma system (Metco 9M series) was operated with mixtures of argon and hydrogen in the power input range from 12 to 36 kW. Temperature measurements were based on the detection of emission line intensities from Ar-I neutral species. Spatially resolved measurements were obtained of the plasma temperatures in axisymmetric plasma jets using Abel deconvolution. The variation of plasma axial and radial temperature distributions was measured as a function of the plasma input power, the total gas flow rate, and the binary gas composition of argon and hydrogen. Time-averaged plasma gas temperatures were found to increase with increasing plasma input power, increasing hydrogen content of the plasma gas, and decreasing total gas flow rate. Plasma temperatures decrease progressively with increasing distance from the nozzle exit. The peak temperatures near the nozzle exit are in the range of 12,500 to 14,000 K. The radial temperature profiles show an approximately self-similar decay in the near field of these plasma jets. It was also determined from time resolved intensity measurements that there are significant fluctuations in the argon emission intensity with increasing hydrogen fraction in the mixture. These fluctuations with a typical frequency of 5.2 kHz are attributed to the arc root instabilities observed before. Finally, the measured plasma temperature field is empirically correlated in terms of radial and axial coordinates, plasma electrical input power, plasma efficiency, and gas composition. These temperature data can be used to validate numerical simulations as well as in choosing locations where different materials can be introduced into the plasma jets. This is particularly important for “nanostructured” materials, which loose their structure upon melting as a result of being exposed to high plasma temperatures.