Numerical studies on the thermal regimes of the horizontal tube falling film evaporation under varying feeder height

ABSTRACT

This paper discusses about the effect of feeder height and heat flux on the heat transfer characteristics of horizontal tube falling film evaporation in the thermal regimes. In order to investigate this, a two- dimensional CFD model was developed to perform simulation and results were compared and validated with published data available in the literature. Heat transfer co-efficients in the thermal regimes were determined from the CFD simulation and the results were recorded, analyzed and validated with the mathematical models available in the literature. The novelty of the current study is to predict the commencement of the fully developed thermal region over the tube from the simulation model under varying feeder height and heat flux. An effort was also made to measure the liquid film thickness around the tube from the CFD model in the thermal regimes. It is observed that angle of thermally developing region contracts and fully developed thermal region extends with the increase of the feeder height and heat flux. It is observed from the study that increase of heat flux by 10 kW/m² resulted in increase of heat transfer co-efficient value by 10–12% average in thermally developing region and 12–15% average in fully developed region. Thinnest liquid film thickness observed between 85 and 127°angle. Shifting of thinnest region of liquid film upward from the mid tube with the increase of the feeder height and heat flux is noted.

KEYWORDS:

• Feeder height
• thermally developing region
• fully developed thermal region
• mass transfer co-efficient
• CFD simulation
• thermal regimes
• falling film evaporation

Nomenclature:-	=
h - Heat transfer co-efficient (W/m2.K)	=
hStag – Heat transfer co-efficient at stagnation region (W/m2.K)	=
hImg – Heat transfer co-efficient at impingement region (W/m2.K)	=
hDev – Heat transfer co-efficient at thermally developing region (W/m2.K)	=
hFD – Heat transfer co-efficient at thermally fully developed region (W/m2.K)	=
Tw – Wall temperature (oC)	=
qDev- Heat flux at developing region (W/m2)	=
mv– Mass flow rate of vapor (kg/s)	=
hfg – Latent heat of evaporation (W/m2.K)	=
min – Mass flow rate of feed water (kg/s)	=
Cpl- Specific heat capacity of water (kJ/kg.K)	=
To – Brine discharge temperature (oC)	=
kl – Thermal conductivity of liquid (W/m.K)	=
qDev – Heat flux at developing region (W/m2)	=
ho – Film heat transfer co-efficient (W/m2.K)	=
Ts – Saturation temperature (oC)	=
L – Tube length (m)	=
A – Heat transfer area of the tubes (m2)	=
u- x-component of water velocity(m/s)	=
v- y-component of water velocity (m/s)	=
ml→v rate of mass transfer due to evaporation(kg/s/m3)	=
mv→l rate of mass transfer due condensation (kg/s/m3)	=
g- Acceleration due to gravity (m/s2)	=
d – Tube diameter (m)	=
C – Constant for tube diameter	=
Greek Symbols:-	=
Г- Liquid spray density (kg/m.s)	=
µl – Dynamic viscosity (N-s/m2)	=
ρf – Density of fluid (kg/m3)	=
ρg – Density of gas (kg/m3)	=
ν- Kinematic viscosity (m2/s)	=
αl- Thermal diffusivity (m2/s)	=
Ө – Circumferential angle of tube (o)	=
ӨStag – Stagnation region angle (o)	=
ӨImp – Impingement region angle (o)	=
ӨDev – Developing region angle (o)	=
ӨFD – Fully developed region angle (o)	=
ρ – density of fluid (kg/m3)	=
ν i- vapor phase direction	=
Vv -the vapor phase velocity(m/s)	=
α is vapor volume fraction	=
Abbreviations:-	=
Pr – Prandtl number	=
Re – Reynolds number	=
R – Radius of the tube	=
w – Width of jet (m)	=
X – local	=
HTC – Heat transfer co-efficient (W/m2.K)	=