Water Boiling Heat

Transfer in Vertical

Jacketed Pipe: A

CFD Model

Mohammad Amin Abdollahi

June 2015

Boiling Heat Transfer Heat transfer of boiling is an important

phenomenon which is widely used as a

key solution for critical applications, such

as:

BWR type light-water nuclear reactors

fixed-bed catalyst reactors coolant

Column Reboilers

Boiling Heat Transfer Many correlations have been developed

to predict and formulize the convective

heat transfer coefficient and so the heat

flux in vertical tube-side and shell-side

thermosyphon reboilers.

The correlation developed by Chen is one

of the most widely used methods for

calculating heat-transfer coefficients in

convective boiling.

Boiling Heat Transfer Assumption of Chens Correlation:

the convective and nucleate boiling

sections of heat transfer can be added to

each other, considering parameters which

import suppression effect of convection on

nucleate boiling.

Boiling Heat Transfer Chen used the DittusBoelter equation, a

predecessor of the SeiderTate equation for turbulent flow, as follows:

= 0.023(

)

0.80.4

Boiling Heat Transfer Chen used the ForsterZuber correlation

for the nucleate boiling heat-transfer

coefficient which is usually stated by:

= 0.00122

0.79,0.45

0.490.25

0.240.75

0.50.290.240.24

Vertical jacketed pipe model

A vertical jacketed pipe which uses

saturated water as coolant in annulus side

has been modeled.

The saturated water boils in constant

temperature by absorbing the heat from

the tube side while flowing through the

annulus around the tube.

Vertical jacketed pipe model

Vertical jacketed pipe model

Saturated Water/Steam Properties At 563 K

Liquid Phase Vapor Phase

Pressure (MPa) 7.455

Density (kg/m3) 732.19 39.05

Enthalpy (KJ/kg) 1289.2 2766.9

Cp (J/kg.K) 5.4897 5.5736

Viscosity (cP) 0.08971 0.019147

Thermal Conductivity (W/m2.K)

0.56521 0.064641

Surface tension (N/m)

0.01669 -

Vertical jacketed pipe model

A 3D full CFD model developed using

ANSYS-FLUENT and it was meshed with

4160 quadrilateral concurrent cells.

Calculation and Simulation Considering different values for vapor fraction

and by using Chens relevant heat transfer coefficient, the void fraction and equivalent

length of jacketed pipe were calculated.

By curve fitting for scatters of jacketed pipe

length against vapor fraction, an implicit

correlation is derived with a 5th degree

polynomial curve.

y = 37500x5 - 29394x4 + 6854.2x3 - 420.58x2 +

48.771x + 0.0217, (R2=1)

by this correlation, the relevant vapor quality

at the end of the pipe is observed as 6.36%.

Calculation and Simulation By curve fitting for scatters of heat transfer

coefficient against jacketed pipe length,

an implicit correlation was derived with a

3rd degree polynomial curve.

y = 643.75x3 - 3599.8x2 - 9575.9x + 93047,

(R2=1)

Below table indicates the new data

obtained by this correlation.

Jacketed pipe length

(m)

Heat transfer coefficient(W/m2

.0k)

Heat flux (W/m2)

Void fraction

0.1-2.7 92054-53620 920540-536200

0.0278-0.5558

Calculation and Simulation Modified data for Heat Flux by curve

fitting:

Calculation and Simulation Modified data for void fraction by curve

fitting:

Calculation and Simulation It is also necessary to calculate the critical

heat flux Using Mostinski correlation.

Assuming the critical pressure of 22.064 MPa,

the critical heat flux will be 3820000 W/m2.

It is obvious that the calculated heat flux is

less than 70% of critical heat flux in order to

keep the flow regime in bubbly flow.

CFD Simulation Results By comparing the results of the CFD

method with the ones of theoretical

correlations, it could be seen that there is a

good consistency between the results of

CFD and theoretical correlations

CFD Simulation Results Results of Heat Flux against pipe length:

CFD Simulation Results Results of Void Fraction against pipe length:

CFD Simulation Results While there is a good adaption between the

results of theoretical correlations and CFD

simulation, it could be referred also to other

data acquired from the CFD model, which

are not extractable from theoretical

correlations easily.

CFD Simulation Results Results of mass transfer rate against pipe length:

CFD Simulation Results Contour and Vectors of Velocity at Outlet of

jacketed pipe:

Conclusion The CFD model developed in this article,

presents reasonable results regarding

compatibility of general trends of heat flux and

void fraction with predictions of theoretical

correlations.

It is notable that theoretical correlations just

produce average figures irrespective of the

radial distance in annulus and flow velocity

curve generated from hydraulic effects.

Conclusion The capability of predicting hydraulic and

heat transfer behavior of the fluid in all

directions and in all sections of the geometry

of the problem makes this CFD method a

powerful tool for design and simulation.

Thanks for your kind attention!