Norman Chigier

The atomization and burning of liquid fuel sprays

Publisher Summary Atomization of the liquid fuel is carried out either by injecting the fuel through small orifices at high pressure or by mixing the fuel with high pressure air or gas. The most effective atomization is achieved when thin liquid sheets are formed that subsequently become unstable and then break up to form ligaments and large drops, which then break down further into small droplets. This chapter describes the twin-fluid and pressure jet atomizers. It examines the most recent experimental information that shows fundamental differences between liquid spray combustion and single droplet combustion. On the basis of this evidence, physical models are developed and an idealized spray flame is postulated. High-speed photography of droplets in sprays provides a direct measure of droplets in flight and obviates the introduction of probes into the spray. When a high-powered light source from a spark is focused on a small area within the spray, the shadow photographs of the droplets can be obtained, provided the spark duration is short, so as to freeze the droplet in flight.

Air-blast atomization of non-Newtonian liquids

Air-blast atomization of viscous non-Newtonian liquids was carried out using a co-axial twin-fluid atomizer. Both Newtonian and non-Newtonian liquids were investigated with particular emphasis on the non-Newtonian rheological characteristics. Shear thinning, extension thinning and extension thickening fluids were investigated. Non-Newtonian shear viscosities were measured over five decades of shear rates γ for 12 solutions of polymeric materials. By using the die-swell technique, the first normal stress difference N1 was determined for all solutions. The contraction flow technique was also used for measurement of the extensional viscosity. It was found that viscoelastic liquids are much more difficult to atomize than viscoinelastic liquids. The normal stresses developed in viscoelastic materials are much higher than their associated shear stresses. Consequently, the development of the large normal stresses appears to be the most important rheological mechanism that inhibits breakup. The accuracy of the wave-mechanism-based models in predicting droplet sizes after breakup of inelastic non-Newtonian liquids has also been demonstrated. The atomized drop sizes were expressed in terms of three dimensionless groups, the liquid/air mass ratio (M1MA), the Weber number (We) and the Ohnesorge number (Z) in simple forms whose exponents and coefficients were determined by least-squares fit to the experimental data. The exponents of the power dependences of the wave-mechanism-based simple models were found to be comparable to their counterparts reported in the literature for air-blast atomization of Newtonian liquids with viscosities up to 1 Pa s. For shear thinning viscoinelastic materials it was found that the atomization quality is closely related to the apparent viscosity of the fluid in the limit of infinite shear rates (ηx). The functional dependence of the Sauter mean diameter (SMD) on ηx is: SMD ∞ η∞0.42.

Disintegration of liquid sheets

The development, stability, and disintegration of liquid sheets issuing from a two‐dimensional air‐assisted nozzle have been studied. Detailed measurements of mean drop size (SMD) and velocity have been made using a phase Doppler particle analyzer. Without air flow the liquid sheet converges toward the axis as a result of surface tension forces. There is a linear increase in convergence length with increases in liquid flow rate. With air flow a quasi‐two‐dimensional expanding spray is formed. It is shown that the air flow is responsible for the formation of large, ordered, and small, chaotic ‘‘cell’’ structures. These structures are bounded by large diameter ligaments containing thin membranes inside. The ligaments are the origin of the large droplets in the spray and the membranes contribute to the formation of the smaller droplets. The air flow causes small variations in sheet thickness to develop into major disturbances with the result that disruption starts before the formation of the main breakup reg...

Dynamic behavior of liquid sheets

An experiment was conducted to study the aerodynamic instability of liquid sheets issuing from a two‐dimensional air‐assisted nozzle. Detailed measurements of the frequency of oscillation of the liquid sheet have been made. The measured vibrational frequencies were then correlated with the resulting spray angle. It was shown that the liquid sheet oscillations are dynamically similar to that of hard spring systems. For each air pressure, three distinct modes of breakup are distinguished. At low liquid flow rates, the sinusoidal mode of breakup is dominant. At intermediate liquid flow rates, both the sinusoidal and the dilational modes are superimposed on the liquid sheet. With a further increase in liquid flow rate, the liquid sheet oscillations mainly become of the dilational type. It was also shown that the effect of introducing air in the nozzle is similar to the effect of inducing forced vibrations on the nozzle jaws. Thus, for each air flow rate, there is a specific vibration frequency for the nozzle....

Studies of methane and methane-air flames impinging on a cold plate

Jet-impingement heating is of interest because it offers, potentially, a method of increasing convectiveheat-transter rates. The impinging jet may be high-velocity combustion products or an actual flame. In the latter case, convection vivre may enhance the heat-transfer rate. The work reported here is concerned with the impingement of a methane-diffusion flame and a premixed methane-air flarne on a cold steel plate. The dominant feature of both flames was the presence of a cool central core of unreacted gas which impinged on the plate (nozzle-plate distance/burner diameter = 10–16). This caused the area of the plate around the stagnation point to be cooler than the maximum temperature which was achieved at some distance away. The heat flux through the plate followed a similar pattern. A nozzle-Reynoldsnumber increase extended the zone of influence of this cool central core. A special feature of the diffusion flame was the series of rings of “visible eddies” observed in the wall-jet region. Temperature measurements taken in the premixed flame showed that, as well as the cool core, there was also a small, very-high-temperature region whose temperatures approached the theoretical same temperature. The measurements were at too low a temperature to indicate whether convectior vivre enhanced the heat-transfer rate but the heat-transfer coefficient did not decrease with distance from the stagnation point as with nonreacting jets. The coefficient was bigher in the wall-jet region than in the impingement region due to the effect of the cool, central core.

Optical imaging of sprays

Review on the use of optical imaging, high speed cinematography and holography for detecting solid liquid or slurry particles over a wide range of particle diameters

Response characteristics of laser diffraction particle size analyzers - Optical sample volume extent and lens effects

The response characteristics of laser diffraction particle sizing instruments were studied theoretically and experimentally. In particular, the extent of optical sample volume and the effects of receiving lens properties were investigated in detail. The experimental work was performed with a Malvern Instruments Ltd. Model 2200 particle size analyzer using a calibration reticle containing a two-dimensional array of opaque circular disks on a glass substrate. The calibration slide simulated the forward-scattering characteristics of a Rosin-Rammler droplet size dis-tribution. The reticle was analyzed with collection lenses of 63 mm, 100 mm, and 300 mm focal lengths using scattering inversion software that determined best-fit Rosin-Rammler size distribution parameters. The Malvern 2200 data differed from the predicted response for the reticle by about 10%. The discrepancies are attributed to nonideal effects in the detector elements and the lenses. A set of calibration factors for the detector elements was determined here that corrected for the nonideal response of the instrument. The response of the instrument was also measured as a function of reticle position, and the results confirmed a theoretical optical sample volume model presented here.

Coaxial atomizer liquid intact lengths

Average intact lengths of round liquid jets generated by airblast coaxial atomizers were measured from over 1500 photographs. The intact lengths were studied over a jet Reynolds number range of 18 000 and Weber number range of 260. Results are presented for two different nozzle geometries. The intact lengths were found to be strongly dependent on Re and We numbers. An empirical equation was derived as a function of these parameters. A comparison of the intact lengths for round jets and flat sheets shows that round jets generate shorter intact lengths.

Laminarization of turbulent flames in rotating environments

The stability limits for transition from laminar to turbulent flow in free boundary layers can be substantially increased by imposing an external centrifugal force field on the boundary layer. For the case of an axial jet introduced into a rotating cylindrical flow, the centrifugal forces can have a stabilizing effect and impede transverse motion of fluid. The centrifugal forces act against the turbulent viscous forces with a consequent reduction in the entrainment of surrounding fluid into the boundary layer. For the case of burning jets, strong density gradients are set up which further influence the stability of the flow. When the density increases radially outward within a centrifugal force field, a stable radial density stratification is set up which again impedes turbulent mixing. The combination of these two stabilizing forces leads to the “laminarization” of flames and results in an increase in the length of the diffusion flames. Experiments were carried out in three systems. In the first, a rotating wire-mesh screen generated a free vortex with a central fuel-gas jet diffusion flame. The second system consisted of a vertical stationary cylindrical tube mounted on a variable swirl generator with a central burning gaseous fuel jet. The third system used was an isothermal model of the second system with a helium jet replacing the fuel jet. Measurements of temperature, gas concentration, velocity, and turbulence characteristics show that the imposition of a rotating flow field on a turbulent diffusion flame results in increase in flame length, reduction of the rate of spread of the flame, and laminarization of the boundaries of the flame. A modified Richardson number is proposed as a criterion for laminar-turbulent transition stability.

A study of vaporizing fuel sprays by laser techniques

Abstract The structures of turbulent kerosene sprays under cold conditions and under conditions of vaporization in hot surrounding air streams are investigated. A laser tomographic light scattering technique has been used for the mapping of internal spray structure. This technique is demonstrated to provide rapid, high-resolution measurements of droplet sizes, concentrations, and vaporization. The data give fundamental insight into the various processes occurring in turbulent spray vaporization and provide a data base for modeling work.