Page 86 - Defrosting for Air Source Heat Pump

78 Defrosting for Air Source Heat Pump

Meanwhile, the heat transfer in Control Volume j can be expressed as [24],

T r, j T ICW, j
q j ¼ A t (4.13)
R r
where T ICW is the temperature at the interface between the coil surface and the water
layer and T r, j is the average temperature of the refrigerant in Circuit j. The relationship
between T ICW and T w, j in this stage was [22]:

T ICW, j + T tp
T w, j ¼ ð j ¼ 1 3Þ (4.14)
2
As the heat transfer resistances of the tubes and fins were neglected (Assumption (ii)),
the thermal resistance of the refrigerant, R r , was evaluated by an empirical experimen-
tal correlation for the refrigerant-side mean heat transfer coefficient, h TPM [24]:

1
R r ¼ (4.15)
h TPM
During defrosting, two different heat transfer regions existed in the refrigerant side of
the outdoor coil, namely (1) a superheated region and (2) a two-phase region. In the
superheated region, the convective heat transfer coefficient of the refrigerant, h r, sh ,
was evaluated by the standard Dittus-Boelter correlation [25]:

λ L
0:8 0:3 sh 4
h r,sh ¼ 0:023Re sh Pr sh , for 10, Re sh 10 ,Pr sh ¼ 0:7 160 (4.16)
d i d i
where λ sh is the thermal conductivity of the refrigerant in the superheated region, d i the
inner diameter of the refrigerant tube, and L the tube length of a refrigerant circuit.
The convective heat transfer coefficient of the refrigerant in the two-phase region,
h r, tp , was evaluated using the liquid refrigerant heat transfer coefficient, h r, L , which
was evaluated by [27]:
" 0:04 #
3:8x 0:76 ð 1 xÞ
0:8
h r,tp ¼ h r,L ð 1 xÞ + 0:38 (4.17)
p re
λ
0:8 0:3 L
h r,L ¼ 0:023Re L Pr L (4.18)
d i
where x is the thermodynamic vapor quality, λ L thermal conductivity of the liquid
refrigerant, and P re the reduced pressure, determined by [27]

P
P re ¼ (4.19)
P c