Experiments to compare the dynamics and thermal impact of single vapor bubble subjected to upward and downward flow boiling configurations

ABSTRACT

Experiments on subcooled flow boiling have been carried out to investigate the dynamical parameters and thermal impact of a growing vapor bubble under vertically upward and downward flow configurations with water as the fluid medium. Bubble growth and detachment phenomena have been visualized in real-time using high-speed rainbow schlieren technique. Schlieren-based imaging technique simultaneously maps both the bubble dynamics and thermal gradients field along the heater surface. Experimental data have been compared for a range of Reynolds number (Re = 3600–7200) at a constant subcooling level of ΔT_sub = 4°C. The thermal aspects have been identified in the form of real-time visualization of thermal boundary layer and its scavenging as the bubble detaches/lifts-off from the heater surface. Time variation of various bubble dynamic parameters have been determined from the quantitative processing of the schlieren images. Significant differences in the bubble trajectory after departure from the nucleation site were observed for downward and upward flow configurations. For Re ≤ 6000, comparatively higher evaporative heat flux dissipation was obtained for upward flow, while at Re ≥ 6000, the evaporative heat flux dissipation was found to be more or less similar for both upflow and downflow configurations.

KEYWORDS:

• Flow boiling
• upward and downward flow configuration
• single bubble dynamics
• thermal gradients
• gradients-based imaging

Nomenclature

d	=	distance (mm)
D	=	diameter (mm)
f	=	bubble release frequency (Hz)
h	=	latent heat (kJ/kgK)
H	=	hue (radians)
L	=	length of test section (mm)
n	=	number of nucleation sites
qʺ	=	heat flux (w/m2)
Re	=	Reynolds number
t	=	time (ms)
ΔT	=	temperature difference (°C)
U	=	bulk fluid velocity
V	=	volume of the bubble (mm3)
v	=	velocity (m/s)
x	=	direction normal to the heater surface (horizontal)
y	=	direction normal to the heater surface (vertical)

Subscripts

avg	=	average value
b	=	bubble
e	=	evaporative
eq	=	spherical equivalent
f	=	focal
fg	=	heat of vaporization
h	=	hydraulic
k	=	imaginary disk
lift-off	=	bubble lift-off
s	=	sliding
sub	=	subcooling

Acknowledgments

This work, toward its final stages, was partially supported by Department of Science and Technology (DST), India through the Grant ID RD/0119-DST0000-007. The authors acknowledge the support received from DST, India.

Additional information

Funding

This work was supported by the Department of Science and Technology, India [RD/0119-DST0000-007];