TRANSFER: BASICS OF MASS AND HEAT TRANSFER - Biomass as a Sustainable Energy Source for the Future - page 117

350

340

320

300

0 5 10

Time (ms)

300

0 10 20 30 40 50

Time (ms)

100

50

0

0 10 20 30 40 50

Time (ms)

FIGURE 4.2 Droplet temperature and diameter versus time at a slip velocity between droplet
and gas, v s of 10 ms −1 . Initial droplet temperatures: 301 K (dark gray), 321 K (light gray), and
341 K (black). Initial droplet diameters: 100 μ m (solid lines) and 50 μ m (dashed) lines
(surrounding gas temperature T b = 800 K).

temperature quickly, which can then be used instead of the evolution equation of the
droplet temperature. When the carrier gas is hot and the boiling point of the liquid is
low, the wet-bulb temperature is close to the boiling temperature of the liquid.
The following aspects should be taken into account when considering application
of the simple model described in Example 4.2 to evaporation of a real biofuel:

1. The standard d 2 -law together with an infinite conductivity model for the droplet
temperature typically overestimates the evaporation rate, thus leading to shorter
droplet lifetimes. In the framework of the infinite conductivity model, some
corrections have been proposed in the literature. Several detailed studies,
described in Jenny et al. (2012), show the effects of convective heat transfer
on the Nusselt number and drag coefficient, and experiments suggest
lower Nusselt numbers for evaporating droplets compared to the classical
Ranz Marshall correlations. A recommended correlation mentioned by Turns

(2000) is

-

Re 2 Pr 3

Nu d =2+0 : 555 ð Eq : 4 : 46 Þ

−1

1
2

232 RePr 3 1+1 :