Axial droplet velocities

Fig. 9.13 Radial profile of the average axial droplet velocity at two different cross-sections for various injection conditions [57], (a) Injection pressure 25 bar, Fo,pvp= 10%. (b) Injection pressure 30 bar, Fo,pvp= 10%. (c) Injection pressure 16 bar, Fo,pvp= 20%. (d) Injection pressure 25 bar, Fo,pvp= 20%...

In order to provide further evidence on the results discussed above, the gas flow now is fully coupled to the spray equations. Figure 9.14 shows a comparison of the experimental and numerical profiles of the axial droplet velocity for the PVP-water spray with 10 % PVP mass fraction and an injection pressure of 25 bar at the cross-section of 120 mm after the nozzle exit. The figure shows both the numerical simulations without and with coupling of the DQMOM to the gas phase Eqs. (9.27)-(9.30). It can be observed that the results are greatly improved by resolving the gas phase. However, at the periphery of the spray the computed axial droplet velocity differs stiU about 3 m/s from the experimental results. This is explained by inconsistent boundary conditions the simulations correspond to a confined jet while the experiments represent a free jet. Moreover, at the spray periphery, the experimental error is larger than in the spray center. 

A co-swirl distribution in the burner increases both the droplet size and the axial mean velocity at all positions in the spray, including the longitudinal axis through the spray centerline and all radial positions downstream of the burner exit. This effect becomes more pronounced as the axial distance from the burner exit increases. The combustion airflow distribution in the burner has also been shown to have a significant effect on the size and velocity distribution of droplets in the spray flames. Since it is possible to change the droplet size, velocity, and number-density distribution in the spray flame using these techniques, it becomes possible to control the flame characteristics for any set of operating conditions of the combustor. 

From the 2D axisymmetric simulation results the primary droplets in the region above z = 30 mm are collected. Figure 18.11 shows the droplet velocity components (i.e. axial component U, radial component Uy, and tangential component against the droplet size at a melt mass flow rate 186 kg/h. It can be found that the radial velocity component generally plays a more important role in droplet motion than the tangential component, which implies that the tangential momentum of the... 

In the spray, the rms value of the droplet velocity in the axial direction is twice as high as the fluctuating value of the radial velocity component (cf. [56]). The anisotropic fluctuations are supporting the physical mechanism of spray jet fragmentation and clustering by periodic acceleration and deceleration of the gas flow. The difference of the axial and radial fluctuations is decreased with larger distance from the nozzle imtil it is on the same level further downstream. This corresponds to the point where gas vortices can be found on the spray axis. 

The mean and rms velocities of the droplets are differing significantly for the different droplet sizes (Fig. 19.20a, b). The anisotropic character of the gas phase fluctuations is visible for both droplet sizes, but the droplets show lower velocity fluctuation intensities in the axial and radial direction for the intermediate droplets. The velocity profiles for the 30 pm droplet case are very similar to the profiles from the polydisperse simulation case. Despite the relatively high air-to-liquid ratio of the considered spray flow (ALR = 0.62), the impact of the dispersed phase on the mean gas flow in the core of the spray is low, indicated by similar profiles of the radial and axial rms velocity profiles of the gas phase for 10 and 30 pm droplets. 

By analyzing the correlation of the radial and axial droplet and gas velocity components on the spray axis for 10 pm droplets correlation values of 0.9 (x = 200 mm) and 0.95 (x = 300 mm) can be found (Fig. 19.25). Here, the droplets do not fully follow the continuous phase like tracers. Thirty micrometer droplets follow the flow less, because of their higher inertia. The initial correlation value for the radial velocities are lower (0.6), but the value atx = 300 is the same (0.87) as the small droplets at x = 200 mm. The basic clustering pattern is not changed in this area of the spray flow. 

Figure 5 shows the variation of the droplet mean axial velocity at the same axial location. The primary feature of this velocity profile is that the maximum velocity peaks at the centerline. The velocity magnitude and direction in the center region tend to be related to the hquid swid strength and axial distance. A reverse (recirculation) flow with negative velocity is possible if the swid is intense. Under such conditions, the maximum velocity tends to shift away from the centerline. 

Fig. 5. Variation of droplet mean axial velocity in a typical hoUow-cone spray.

Droplet dispersion zone, in liquid atomization, 23 183-184 Droplet mean axial velocity, 23 189 variation of, 23 189... 

The variations of the mean droplet size and the droplet size distribution with axial distance in a spray generated by pressure swirl atomizers have been shown to be a function of ambient air pressure and velocity, liquid injection pressure, and initial mean droplet size and distribution 460]... 

It should be noted, however, that some of the tendencies described above may become invalid for very small droplets (for example, smaller than 10 pm under conditions in Ref. 156). Such small droplets may require a longer flight time to a given axial distance far from the atomizer due to the high deceleration, and their cooling rates may decrease as a result of the reduced relative velocity and temperature. In addition, the two-way coupling 576] may affect the momentum and heat transfer between atomization gas and droplets so that the droplet behavior may be different from that discussed above, particularly the radial distributions of droplet sizes and velocities. 

A two-color pyrometer has been used along with the phase-Doppler anemometer to simultaneously measure the local velocity and size of kerosene droplets and the temperature of burning soot mantle in a swirl burner.[648] The measurements were conducted within the flame brush that develops in the shear layer of a swirl-stabilized, gas-supported kerosene flame with a swirl number of about 0.19 and potential heat releases of 10.6 and 15.5 kW, respectively. The results showed that the maximum burning fraction of the droplets occurs adjacent to the region denoted as gas flame but the value ranges from 20 5 to 40 5% depending on the axial station, and decreases sharply across the shear layer. The flame mantle temperature was found to be independent of droplet diameter, which agrees with previous results in the literature. 

A two-component phase Doppler interferometer (PDI) was used to determine droplet size, velocity, and number density in spray flames. The data rates were determined according to the procedure discussed in [5]. Statistical properties of the spray at every measurement point were determined from 10,000 validated samples. In regions of the spray where the droplet number density was too small, a sampling time of several minutes was used to determine the spray statistical characteristics. Results were repeatable to within a 5% margin for mean droplet size and velocity. Measurements were carried out with the PDI from the spray centerline to the edge of the spray, in increments of 1.27 mm at an axial position (z) of 10 mm downstream from the nozzle, and increments of 2.54 mm at z = 15 mm, 20, 25, 30, 35, 40, 50, and 60 mm using steam, normal-temperature air, and preheated air as the atomization gas. 

The observed flame features indicated that changing the atomization gas (normal or preheated air) to steam has a dramatic effect on the entire spray characteristics, including the near-nozzle exit region. Results were obtained for the droplet Sauter mean diameter (D32), number density, and velocity as a function of the radial position (from the burner centerline) with steam as the atomization fluid, under burning conditions, and are shown in Figs. 16.3 and 16.4, respectively, at axial positions of z = 10 mm, 20, 30, 40, 50, and 60 mm downstream of the nozzle exit. Results are also included for preheated and normal air at z = 10 and 50 mm to determine the effect of enthalpy associated with the preheated air on fuel atomization in near and far regions of the nozzle exit. Smaller droplet sizes were obtained with steam than with both air cases, near to the nozzle exit at all radial positions see Fig. 16.3. Droplet mean size with steam at z = 10 mm on the central axis of the spray was found to be about 58 /xm as compared to 81 pm with preheated air and 96 pm with normal unheated air. Near the spray boundary the mean droplet sizes were 42, 53, and 73 pm for steam, preheated air, and normal air, respectively. The enthalpy associated with preheated air, therefore, provides smaller droplet sizes as compared to the normal (unheated) air case near the nozzle exit. Smallest droplet mean size (with steam) is attributed to decreased viscosity of the fuel and increased viscosity of the gas. 

Figure 16.5 Variation of droplet total velocity with radial and axial positions for different atomization gases 1 — steam 2 — preheated air 3 — normal air. (a) z 10 mm, (6) 20, (c) 30, (d) 40, (e) 50, and (/) 60 mm...

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