Page 180 - Design and Operation of Heat Exchangers and their Networks

168 Design and operation of heat exchangers and their networks

The friction factor is given as
(
5
6:947 10 Re 0:5 Re 1:109 ,Re eq < 6000
Δp f ρ d h,b l eq
m
f ¼ ¼ 0:5 0:04557 (4.108)
2
2G L 31:21Re l Re eq , Re eq 6000
where
1
ρ ¼ _x=ρ +1 _xð½ v Þ=ρ Š (4.109)
l
m
Donowski and Kandlikar (2000) used Yan and Lin’s data (Yan and Lin,
1999) and developed an improved correlation for a heat transfer coefficient
of R134a for a chevron angle of 60degrees:
Nu sp ¼ 0:2875Re 0:78 Pr 1=3 (4.110)
0:3 2:8 0:003
Nu=Nu lo ¼ 2:312E cb Co + 667:3F fw E nb Bo ð 1 _xÞ (4.111)
where the enhancement factor for convective boiling contribution
E cb ¼0.512, the enhancement factor for nucleate boiling contribution
E nb ¼0.388, and the material-dependent parameter F fw ¼1 for stainless
steel and all fluids (Kandlikar, 1991). The subscript “lo” means that the
Nusselt number is based on the total flow as saturated liquid (liquid
only), which can be calculated from the corresponding single-phase
correlation.
Hsieh and Lin (2002) investigated the saturated flow boiling heat
transfer and pressure drop of R410A in a vertical plate heat exchanger
with a chevron angle of 60degrees (the same one used by Yan and Lin
(1999)). The boiling heat transfer coefficient was correlated as
0:5
Nu=Nu lo ¼ 88Bo (4.112)
with
Nu sp ¼ 0:2092Re 0:78 Pr 1=3 ð μ=μ Þ 0:14 (4.113)
w
q
Bo ¼ (4.114)
GΔh v
The correlation for the friction factor is proposed as

Δp f ρ d h,b 1:25
m
f ¼ ¼ 61,000Re eq (4.115)
2
2G L
Hsieh et al. (2002) further investigated the subcooled flow boiling heat
transfer characteristics of R-134a in the same plate heat exchanger