Buoyant Thermals Research Articles

Structure and Flame Length of Fully-Modulated, Turbulent Diffusion Flames

The turbulent flame structure and flame length of fully-modulated diffusion flames was examined over a range of pulsing frequencies, injection flow rates, and duty-cycles. An injection system employing an electronically-controlled solenoid value was used to discharge puffs of unhealed natural gas and ethylene fuel into still air at one atmosphere pressure. Video imaging of the luminosity from the sooting regions of the flame revealed two distinct types of flame structure. For small injected volumes and short injection times, compact, puff-like structures with a short flame length were generated. More elongated “cigar-shaped” structures, with a longer flame length closer to that of steady-state flames, resulted from longer injection times and larger injected volumes. An injection parameter which characterizes the transition from puff-like to cigar-shaped flame behavior is presented. For puffs, an increase in duty-cycle generally led to an increase in flame length. This increase was less for cigar-shaped flames. The downstream location of the puff-like flame structures increased roughly with time to the 1/2 power, in agreement with buoyant thermals and starting jets.

Thermal compression waves. II: Mass adjustment and vertical transfer of total energy

AbstractA fully compressible model is used to simulate the mass adjustment that occurs in response to a prescribed heat source. Results illustrate the role that thermal compression waves have in this process. The vertical mass transport associated with compression waves decreases rapidly with height. Most of the mass transport occurs in the horizontal, with the vertical structure of the disturbance similar to that of a Lamb wave. The vertical transfer of total energy in a thermally driven mixed layer is also examined. It is shown that the upward transport of total energy is accomplished by a compression effect rather than by the exchange of warm and cold air by buoyant thermals. Model results are analysed to determine budgets of total energy, mass and entropy. It is demonstrated that buoyant thermals are predominantly responsible for a transfer of entropy, rather than total energy. In the light of these results the notion of ‘heat transport’ in a fluid is discussed.

The role of buoyant thermals in salt gradient solar ponds and in convection more generally

A salt gradient solar pond is a large thermo-haline double-diffusive system of depth up to 3 m, having a lower convecting or storage zone with about 20% salt by weight, and a non-convecting uniformgradient zone above providing insulation. Heating by solar radiation is partly within the liquid and partly at the base. The principal fluid mechanics problems are growth of the lower convecting zone, emergence and growth of an upper convecting zone and the possible breakdown of the gradient zone into a number of convecting zones simultaneously. It is shown that the well accepted model of double-diffusive convection for ‘strong’ heating from below a stable salinity gradient does not apply at the very modest level of solar radiation, more especially when part (or even all) of the heat is generated within the fluid. New models are developed in which buoyant thermals play causal roles (as, indeed, they do in the case of ‘strong’ heating). Thermals from all three kinds of boundaries are investigated, and it is found that although there is a limited quantity of fluid in a thermal, connection is retained with the source during its lifetime. Thermals from the solid base are of the well known axisymmetric kind. Those from the boundary with air, and from liquid of a different density, the so-called ‘free’ boundaries, are shown to be two-dimensional and Gaussian in profile. Thermal ranges are found to be well defined and increase according to environmental (water) temperature, with a common form of range equation. The equation is calibrated experimentally for each type of thermal. For a given environmental temperature, axisymmetric thermals have by far the longest range whilst Gaussian thermals from a boundary with air have the shortest. If thermals do not reach the opposite boundary there is local heating or cooling ; if they do, there is boundary erosion and zone growth, that is, penetrative convection. Each convecting zone has two sets of thermals, upwards and downwards, which are the principal means of heat transfer. This is illustrated for the particular case of a solar pond. Finally, it is shown that the onset of ‘turbulence’ at Rayleigh numbers in the region of 5 × 10 4 to 10 5 marks the transition from laminar Bénard-Rayleigh heat flow to quantised flow by thermals.

Frobenius Analysis of Higher Order Equations: Incipient Buoyant Thermal Convection

In this article the solution procedure is formulated when the Frobenius method is applied to the equations of motion for incipient buoyant thermal convection in vertical cylindrical geometries. The indicial equation associated with the resulting sixth-order linear ordinary differential equation has four roots, two of which are repeated and all of which differ by the same integer. The resulting solution suggests a decomposition of the sixth-order equation into three second-order equations. The solution and decomposition procedure is then expanded to equations of nth order.

Temperature-field structure within atmospheric buoyant thermals

Using stationary rakes of hot-wire anemometer probes, a series of temperature measurements have been conducted within rising buoyant thermals created by detonation of large-diameter gas-filled balloons. These atmospheric detonation simulation tests were performed at the SANDIA Laboratories Aerial Cable Test Facility, Kirtland AFB, during the late fall of 1973. All measurements were performed after pressure equilibrium (t > 1 s) but prior to completion of the toroid formation process and within the time required (t > 4 s) for the fireball to rise one diameter.Based on an extensive processing of measured data for two detonation events, unique results describing the mean flow field and turbulent structure statistics for the early-time large-scale buoyant thermal have been determined. Final results include: mean-velocity and temperature data; temperature and temperature gradient intensities; space, auto- and ‘eddy-lifetime’ correlation scales; power-spectral densities; probability density distributions and approximate microscale data.The present temperature data illustrate that the early-time rising fireball is characterized by a highly intermittent irregular top or cap, a constant (high-) temperature core and a thermal wake exhibiting a linear decay in mean temperature. Mean convection velocities for the fireball wake were measured to be approximately twice as large as fireball cap velocities (13.1 m/s). Maximum fireball temperatures (1100 °C) were in good agreement with peak brightness-temperature data determined from an independent IR measurements experiment. Core leading-edge mean-temperature gradients compare favourably with predictions (10 °C/cm) whereas trailing-edge gradients (0.4 °C/cm) are smaller than predicted results (3 °C/cm).Normalized temperature intensities at the fireball top and within the thermal wake were substantial (≈ 0.6) in contrast with quiescent fireball core levels (0.1). Moving-frame autocorrelation times (‘eddy lifetimes’, 0.17 + 0.04 s) were approximately four times larger than measured autocorrelation times. This result illustrates that the early-time fireball eddy structure is highly persistent, a finding which tends to substantiate those radar models for large-scale fireballs which postulate relatively slow ‘smearing’ of early-time electron density gradients. Wake integral scales, determined from vertical space correlation data, were approximately 5% of the fireball mean radius. Probability density distributions for the fireball thermal wake were ‘spiky’ in nature, with some evidence of bimodal behaviour, and slightly skewed to the negative side of their normalized values. Normalized spectra data, in general, follow a − &5/3; falloff behaviour for several decades in spectral power. The noted large ‘spread’ in the inertial subrange is consistent with measured turbulence Reynolds numbers (2000) and with the large-scale fully developed turbulent structure for the GEST fireballs (ReD= 3 × 107). Wake microscale data, estimated from temperature and temperature-gradient intensity data and the isotropic turbulence assumption, varied from 2 to 4 ern and were in favourable agreement with predicted microscale magnitudes.

Interaction Between a Discrete Downdraft and a Rotating Environment

Laboratory experiments with negatively buoyant thermals in a rotating fluid show that vortex forming interactions occur over a considerable range of buoyancies and rotation rates. The interactions are not strongly dependent on the presence of a bounding surface for attachment of the vortex. Observed enhancement of downdraft velocity and suppression of entrainment by the interaction are in agreement with theory. It is suggested that a sudden downdraft in a rotating thunderstorm could create an upper level vortex potentially hazardous to aviation.

Buoyant Density and Thermal Denaturation Profiles of Schistosome DNA

Buoyant Density and Thermal Denaturation Profiles of Schistosome DNA

Temperature Field Structure in Strongly Heated Buoyant Thermals

Laboratory measurements on the temperature field structure in rising turbulent thermals are presented. The relatively small size (about 10 cm initial diameter), but strongly heated thermal bubbles are generated by pulsed arc discharges of about 103 J energy in otherwise undisturbed air. Temperature field measurements are made by stationary and rotating fine-wire thermometers which probe through the rising bubble. The thermal front is found to be sharp at the top and around the side of the bubble, but become diffused near the bottom, suggesting thermal wake formation. The large amplitude fluctuation, strong gradient, and rich harmonic content of the interior temperature field indicate that the mass entrainment process penetrates deeply into the bubble, and turbulent mixing proceeds at all depths. By invoking passive scalar mixing theories and assuming local equilibrium spectral transfer, the averaged turbulence kinetic energy dissipation rate within the bubble ε can be estimated from the diffusive cutoff wavenumber apparent in these preliminary experiments to be of the order of 0.03 (U3/D), where U and D are, respectively, the instantaneous rise velocity and mean diameter of the bubble.

Motion of an Isolated Buoyant Thermal

Using a simplified model, the governing equations of the motion of an isolated buoyant thermal are formulated. The general solutions which include both large and small density differences and both the initial accelerating motion and the final decelerating motion, are obtained. The asymptotic behavior for large time is given and compared with experimental results. The effect of various parameters, such as density difference, mass entrainment, and effective drag coefficient are also discussed.

Experiments on the penetration of an interface by buoyant thermals

Isolated masses of a dense aqueous solution (i.e. thermals) were released at the surface of a freshwater layer overlying one of salt water. While the whole of a thermal remained in the upper layer, the equations , where x is the distance from the interface to the culminating point and C2 is a constant. C2 was approximately equal to 3·5. For strong thermals, the distance travelled while the front was in the lower layer obeyed the equation z2 = k2t. The origins of z and t usually differed from those found in the upper layer, and generally k2 ≠ k1.