Toshi Kubota

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.

Applications of Shock-Induced Mixing to Supersonic Combustion

Families of two-dimensional, unsteady shock-induced vortical flows are simulated numerically. The flows consist of one or more regions of light gas, surrounded by heavy gas, being overtaken by a normal shock wave. The interaction of the density gradient at each light/heavy interface with the pressure gradient from the shock wave generates vorticity. This causes the light gas regions to roll up into one or more counter-rotating vortex pairs, which stir and mix the light and heavy gases. The mixing is characterized by an asymptotic stretching rate. The effects of shock strength, light/heavy gas density ratio, and geometry on the mixing are investigated. These two-dimensional, unsteady flows are analogous to three-dimensional, steady flows that may be used in SCRAMJET combustors demanding rapid and efficient mixing of fuel and oxidizer. For such applications, 1) the fuel injectors should be elongated in the direction of the shock; 2) multiple smaller injectors are preferable to a single larger injector; 3) injectors should be arranged in groups of closely spaced pairs, rather than uniformly; and 4) multiple shock waves should be utilized, if possible.

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.

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.

Finite disturbance effect on the stability of a laminar incompressible wake behind a flat plate

An integral method is used to investigate the interaction between a two-dimensional, single frequency finite amplitude disturbance in a laminar, incompressible wake behind a flat plate at zero incidence. The mean flow is assumed to be a non-parallel flow characterized by a few shape parameters. Distribution of the fluctuation across the wake is obtained as functions of those mean flow parameters by solving the inviscid Rayleigh equation using the local mean flow. The variations of the fluctuation amplitude and of the shape parameters for the mean flow are then obtained by solving a set of ordinary differential equations derived from the momentum and energy integral equations. The interaction between the mean flow and the fluctuation through Reynolds stresses plays an important role in the present formulation, and the theoretical results show good agreement with the measurements of Sato & Kuriki (1961).

An experiment on the adiabatic compressible turbulent boundary layer in adverse and favourable pressure gradients

A wind-tunnel model was developed to study the two-dimensional turbulent boundary layer in adverse and favourable pressure gradients with out the effects of streamwise surface curvature. Experiments were performed at Mach 4 with an adiabatic wall, and mean flow measurements within the boundary layer were obtained. The data, when viewed in the velocity transformation suggested by Van Driest, show good general agreement with the composite boundary-layer profile developed for the low-speed turbulent boundary layer. Moreover, the pressure gradient parameter suggested by Alber & Coats was found to correlate the data with low-speed results.

A model for characterization of a vortex pair formed by shock passage over a light-gas inhomogeneity

This work investigates the two-dimensional flow of a shock wave over a circular light-gas inhomogeneity in a channel with finite width. The pressure gradient from the shock wave interacts with the density gradient at the edge of the inhomogeneity to deposit vorticity around the perimeter, and the structure rolls up into a pair of counter-rotating vortices. The aim of this study is to develop an understanding of the scaling laws for the flow field produced by this interaction at times long after the passage of the shock across the inhomogeneity. Numerical simulations are performed for various initial conditions and the results are used to guide the development of relatively simple algebraic models that characterize the dynamics of the vortex pair, and that allow extrapolation of the numerical results to conditions more nearly of interest in practical situations. The models are not derived directly from the equations of motion but depend on these equations and on intuition guided by the numerical results. Agreement between simulations and models is generally good except for a vortex-spacing model which is less satisfactory. A practical application of this shock-induced vortical flow is rapid and efficient mixing of fuel and oxidizer in a SCRAMJET combustion chamber. One possible injector design uses the interaction of an oblique shock wave with a jet of light fuel to generate vorticity which stirs and mixes the two fluids and lifts the burning jet away from the combustor wall. Marble proposed an analogy between this three-dimensional steady flow and the two-dimensional unsteady problem of the present investigation. Comparison is made between closely corresponding three-dimensional steady and two-dimensional unsteady flows, and a mathematical description of Marbles analogy is proposed.

The effect of walls on a spatially growing supersonic shear layer

The inviscid instability, with respect to supersonic disturbances of a spatially growing plane mixing layer inside parallel flow guide walls, is investigated using linear stability analysis. For supersonic convective Mach numbers, it is found that the maximum amplification rates of the shear layers approach an asymptotic value and that this maximum amplification rate increases to its maximum value and decreases again as the distance between the walls decreases continuously. Contour plots of the pressure perturbation fields indicate that there are waves propagating outward from the shear layer along the Mach angle, and that the walls provide a feedback mechanism between the growing shear layer and this compression/expansion wave system. The streak lines of the flow confirm that the spreading rate of the shear layer is unusually small for supersonic disturbances.

Instability of inviscid, compressible free shear layers

The linear spatial instability of inviscid, compressible laminar mixing of two parallel streams, comprised of the same gas, has been investigated with respect to two-dimensional wave disturbances. The effects of the velocity ratio, temperature ratio and the temperature profile across the shear layer have been examined. a nearly universal dependence of the normalized maximum amplification rate on the convective Mach number is found, with the normalized maximum amplification rate decreasing significantly with increasing convective Mach number in the subsonic region. These results are in accord with those of recent growth-rate experiments in compressible turbulent free shear layers and other similar recent calculations.

Experimental Investigation of Laminar Near Wakes Behind 20 Wedges M=6

An experimental investigation at Mm = 6 has been conducted to determine mean-flow properties in near wakes behind several 20° included- angle wedges at zero angle of attack. One cold-wall (H = 0.3 in., TW/TQ = 0.19) and two adiabatic-wall (H = 0.15 in., H = 0.3 in.) configurations were tested. Freestream Reynolds numbers were varied from 0.5 X 10 5 to 2 X 105 per in. for each model. Flowfield mappings and flow-property profiles were obtained in the base region for the wedge of 0.3-in. base height with and without cooling by combining Pitot- pressure data with total temperature and mass flux results from hot-wire measurements. The variation of total pressure along streamlines was initially negligible during the shearlayer turning process. Downstream boundaries of these isentropic turns corresponded to viscous-layer edges that were positioned in the outer portions of the shear layers, indicating that wake shocks originated from within viscous regions of the shear layer.