## 4 Results and Discussion  
### Effect of the Circuit Number on the Evaporator and Condenser Performance  
The effect of the circuit number on the FTHX performance was investigated for the evaporator and the condenser, respectively. Since there were in all 24 heat transfer tubes in the FTHX, one, two, three, four and six elementary circuits were naturally considered for the simulation, as shown in Figure 3. The constraint parameters for both the evaporator and the condenser simulations are listed in Table 2 and were obtained in experimental tests for the four-circuit reversely fixed FTHX in both heating and cooling cycles of the ASHP. Figure 4 illustrates the variations of FTHX performance with the circuit number. The pressure drop decreased monotonously with the increase in the circuit number for both the evaporator and the condenser, since more circuits meant a lower velocity of the refrigerant. The variation in the capacity, however, differed between the two modes. The capacity of the evaporator increased first and then decreased, exhibiting the highest value of 2.61 kW with four circuits. The capacity of the condenser, by contrast, decreased monotonously from 4.76 kW to 3.21 kW as the circuit number increased from one to four. The variations of the refrigerant flowrate with the circuit number showed the same trend. Therefore, it was deduced that more circuits are preferable in the evaporator, while in the condenser fewer are better, which is consistent with the observations in Ref. [8]. In fact, the FTHX performance under 120% rated air flowrate was also studied and showed a similar trend as that with rated air, as also shown in Figure 4, which confirmed that more circuits were a better option for the evaporator, while fewer circuits were best for the condenser. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Tube** & **Experimental** & **Numerical** & **Deviation** \\ \hline
1 & 81.9 \({}^{\circ}\)C & 83.3 \({}^{\circ}\)C & +1.4 \({}^{\circ}\)C \\
12 & 84.0 \({}^{\circ}\)C & 83.3 \({}^{\circ}\)C & \(-\)0.7 \({}^{\circ}\)C \\
18 & 75.5 \({}^{\circ}\)C & 83.3 \({}^{\circ}\)C & +7.8 \({}^{\circ}\)C \\
19 & 78.5 \({}^{\circ}\)C & 83.3 \({}^{\circ}\)C & +4.8 \({}^{\circ}\)C \\
3 & 47.7 \({}^{\circ}\)C & 48.5 \({}^{\circ}\)C & +0.8 \({}^{\circ}\)C \\
10 & 47.8 \({}^{\circ}\)C & 48.5 \({}^{\circ}\)C & +0.7 \({}^{\circ}\)C \\
16 & 41.7 \({}^{\circ}\)C & 48.5 \({}^{\circ}\)C & +6.8 \({}^{\circ}\)C \\
21 & 47.6 \({}^{\circ}\)C & 48.5 \({}^{\circ}\)C & +0.9 \({}^{\circ}\)C \\
6 & 46.2 \({}^{\circ}\)C & 48.5 \({}^{\circ}\)C & +2.3 \({}^{\circ}\)C \\
7 & 43.7 \({}^{\circ}\)C & 47.1 \({}^{\circ}\)C & +3.4 \({}^{\circ}\)C \\
13 & 37.0 \({}^{\circ}\)C & 46.5 \({}^{\circ}\)C & +9.5 \({}^{\circ}\)C \\
24 & 47.0 \({}^{\circ}\)C & 45.6 \({}^{\circ}\)C & \(-\)1.4 \({}^{\circ}\)C \\ \hline Capacity & 3.77 kW & 3.85 kW & +2.1\% \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Numerical and experimental results of the condenser performance for the two FTHXs. Figure 4: Effect of the circuit number on FTHX performance. Figure 3: Elementary circuit options for the outdoor FTHX. The tube capacity is a product of heat transfer area, overall heat transfer coefficient and temperature difference. The tube capacity is read directly in the EVAP-COND 4.0 software. The temperature difference in this article was determined as the logarithmic mean value calculated from the refrigerant inlet, refrigerant outlet, air inlet and air outlet temperatures. Then, the overall heat transfer coefficient for each tube was obtained by dividing the tube capacity by its outer surface area and the temperature difference. Figure 5 shows the tube-by-tube distribution of the overall heat transfer and temperature differences in the FTHX, which corresponded strongly to the circuit division. In addition, for both the evaporator and the condenser, the overall heat transfer coefficient generally decreased, but the temperature difference increased as the circuit number increased.