Effect of aspect ratio on the flow and heat transfer characteristics of mist/steam in rectangular ribbed channels

ABSTRACT

The effects of Reynolds number from 10,000 to 80,000, mist mass ratios from 1 to 6%, and droplet sizes from 5 to 20 µm on flow and heat transfer behaviors of mist/steam in rectangular channels with various aspect ratios of 1/4, 1/2, 1/1, 2/1, and the rib angle of 60° are numerically studied in this paper. Additionally, secondary flow distribution in the four ribbed channels and its effect on heat transfer are analyzed in detail. The 3D steady Reynolds-averaged Navier–Stokes equations with a SST k-ω turbulent model are solved by using ANSYS CFX. The CFD model has been verified by the experimental data for steam-only case with a good agreement. The results indicate that similar secondary flow pattern can be observed in the four ribbed channel except for the size of main secondary flow; the heat transfer augmentation of mist/steam raises as Reynolds number and mist mass ratio increase; a peak value of average Nu is obtained in the case of 15 µm mist among all the sizes of droplets. The friction coefficient decays with increase of Reynolds number and mist mass ratio but is insensitive to droplet sizes. The case of AR = 1/2 obtains the best thermal performance in mist/steam cooling channels.

Nomenclature

AR	=	aspect ratio of ribbed channel, W/H.
D	=	hydraulic diameter of channel, mm.
e	=	Rib eight, mm.
f	=	dimensionless friction coefficient.
fo	=	friction coefficient for fully developed region in a smooth tube.
H	=	height of channel, mm.
h	=	heat transfer coefficient, W/(m2K).
ΔL	=	length of channel, mm.
Le	=	extended entrance section, mm.
Lh	=	length of heated section, mm.
Lo	=	extended outlet section, mm.
Nu	=	Nusselt number.
Nuo	=	Nusselt number for fully developed region in a smooth tube.
p	=	Rib pitch, mm.
ΔP	=	pressure loss of channel, Pa.
Pr	=	Prandtl number.
q	=	heat flux, W/m2.
Re	=	Reynolds number.
Tw	=	temperature of the ribbed side wall, K.
Tb	=	bulk temperature at x-direction, K.
u	=	inlet velocity of channel, m/s.
W	=	width of channel, mm.
x	=	x-coordinate, m.
y	=	y-coordinate, m.
z	=	z-coordinate, m.
y+	=	dimensionless distance.
μ	=	fluid dynamic viscosity, m2/s.
α	=	Rib angle, °.
λ	=	thermal conductivity, W/m/K.
η	=	.
ρ	=	density of steam, Kg/m3.

Nomenclature

AR	=	aspect ratio of ribbed channel, W/H.
D	=	hydraulic diameter of channel, mm.
e	=	Rib eight, mm.
f	=	dimensionless friction coefficient.
fo	=	friction coefficient for fully developed region in a smooth tube.
H	=	height of channel, mm.
h	=	heat transfer coefficient, W/(m2K).
ΔL	=	length of channel, mm.
Le	=	extended entrance section, mm.
Lh	=	length of heated section, mm.
Lo	=	extended outlet section, mm.
Nu	=	Nusselt number.
Nuo	=	Nusselt number for fully developed region in a smooth tube.
p	=	Rib pitch, mm.
ΔP	=	pressure loss of channel, Pa.
Pr	=	Prandtl number.
q	=	heat flux, W/m2.
Re	=	Reynolds number.
Tw	=	temperature of the ribbed side wall, K.
Tb	=	bulk temperature at x-direction, K.
u	=	inlet velocity of channel, m/s.
W	=	width of channel, mm.
x	=	x-coordinate, m.
y	=	y-coordinate, m.
z	=	z-coordinate, m.
y+	=	dimensionless distance.
μ	=	fluid dynamic viscosity, m2/s.
α	=	Rib angle, °.
λ	=	thermal conductivity, W/m/K.
η	=	.
ρ	=	density of steam, Kg/m3.

Additional information

Funding

This work is financially supported by the Innovative Research Team in University of Ministry of Education of China (IRT1280).