atw 2018-04v6

inforum

atw Vol. 63 (2018) | Issue 4 ı April

RESEARCH AND INNOVATION 256

with φ=3.0%, pressure drop increment

is about 56% higher compared

to that of pure water.

However, the typical nanoparticle

loading in PWR coolant should be

less than 1.0 vol %. At such lower

con centration, nanofluid properties

are almost similar to that of pure

water and the rise in viscosity as well

as pressure drop will be negligible too.

The present study also portrays that

pressure drop is approximately 20 %

at 1.5 vol. % of nanoparticle concentration

which can also be treated as

tolerable.

The convective heat transfer coefficient

at such low concentration of

nanofluid is yet to be improved due to

higher turbulence produced near the

grid spacers by the presence of nanoparticles

in the base fluid. Since it is

quite difficult to take into account

such effects in numerical simulation,

further experimental investigation is

required for quantification of heat

transfer increment aroused from the

presence of nanoparticles near the

spacer grids.

6 Proposed new

correction factor

Finally, a multiple regression analysis

is performed with numerical results to

propose a new correction factor, β for

the existing correlation of square

array subchannel with pure water as

suggested by Presser [27] so that Nu

for nanofluid coolant can be approximated

in such geometry. Based on

regression results, β can be expressed

as follows:

β = 1 + 0.0247ϕ 1.39 (24)

Nu for nanofluid can be calculated as

follows:

Nu nf = β*(Nu Presser ) Water (25)

The validity of above correlation is for

3×10 5 ≤ Re ≤ 6×10 5 ; 0.847 ≤ Pr ≤

1.011; 1.25 ≤ P/D ≤ 1.35 and 0.5% ≤

φ ≤ 3.0% in case of square array

subchannel.

7 Chemical and physical

stability of nanofluid

Albeit nnanofluid can readily boost

the heat transfer capability of PWR

coolant, there is still no satisfactory

explanation proposed regarding the

prevention of clustering in nanoparticle

suspensions. Agglomeration

in nanofluids containing oxide nanoparticles

can be reduced remarkably

by adjusting the pH to form electric

changes on particle surface so that

they repel each other [29]. However,

the typical values of pH should be

such that nanofluid itself becomes not

corrosive and it should be agreeable

with same allowable pH range of

nuclear reactor, since altering the

PWR coolant chemistry is not a viable

option. Besides, use of surfactants are

also not recommended since it may

undergo severe radiolysis inside the

reactor core during operation.

Hence, issues concerning chemical

and physical stabilities of nanofluid

has yet to be resolved prior to utilizing

nanofluid as a promising coolant in

PWRs to achieve both extended life

time of associated equipment and

higher thermal efficiency.

8 Conclusion

Thermo- and hydrodynamic characteristics

of water/alumina nanofluid

have been studied in a square array

subchannel featuring the pitch-todiameter

ratios of 1.25 and 1.35 under

the steady-state, incompressible,

single- phase turbulent flow condition.

Numerical results have been compared

against correlations in the

literature and the following conclusions

can be drawn.

• Convective heat transfer is increased

with increasing volume

concentration of water/alumina

nanofluid given the inlet Reynolds

number.

• The convective heat transfer increment

of nanofluid is obtained at

the expense of increased pressure

drop and hence, larger pumping

power is required. Therefore,

nano fluid as PWR coolant can be

only be implemented in reality if

the replacement of reactor coolant

pump is a feasible option compared

to higher power gained from

increased nanofluid heat transfer.

Acknowledgements

This work was supported by the

National Research Foundation of Korea

(NRF) grant funded by the Korean

Government (MSIP) under Grant No.

2008-0061900 and partly supported

by the Brain Korea 21 Plus under

Grant No. 21A20130012821.

Nomenclature

∆p Pressure Drop Pa

ρ Density kg/m 3

v Flow Velocity m/s

f Friction Factor -

L Length of Flow Channel m

le Entrance Length m

EI Entrance Length Number -

Dh Hydraulic Diameter m

μ Dynamic Viscosity N.s/m 2

Re Reynolds Number -

Nu Nusselt Number -

Pr Prandtl Number -

Pe Peclet Number -

h

Convective Heat Transfer

CoefficientW/m 2 .K

k Thermal Conductivity W/m.K

C p Specific Heat J/kg.K

T m Bulk Temperature of Fluid K

T w

Surface Temperature

of Heater Rod

P Rod Pitch m

D Rod Diameter m

Q Total Heat Input W

q” Heat Flux W/m 2

φ

ṁ Mass Flow Rate kg/sec

Subscript

nf

bf

P

Volume Concentration

of Nanoparticles %

Nanofluid

Basefluid

Particle

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Research and Innovation

Nanofluid Applied Thermo-hydro dynamic Performance Analysis of Square Array Subchannel Under PWR Condition ı Jubair Ahmed Shamim and Kune Yull Suh

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