## 3 Results and Discussion
However, for the evaporator carryover problem of a centrifugal chiller, the meaningful work condition is the one with large cooling capacity and with tube bundle full immersed in liquid refrigerant, which corresponds to the experimental point of 581 kg charge in Figure 7 (see Figure 6 above), where the relative uncertainties of methods one and two are 24% and 22%, respectively (as shown in Table 2). Due to the relatively small experimental uncertainty of Method 2, only the results from Method 2 are shown later in this paper. Figure 8 shows the variation of the entrainment factor with charge under different cooling capacities and AHRI full load conditions. As a comparison, the results at 700 RT are also shown in this figure. It can be seen that the entrainment factor is small at 600 RT and increases smoothly with the increase of charge. However, the variation trend at 800 RT condition is similar to that at 700 RT, with the entrainment factor increasing slowly at the beginning and gradually accelerating later. Since the cooling capacity relates to the vapor superficial velocity \(j_{o}\) in the vertical direction and the refrigerant charge value relates to the space height \(h\) above the liquid surface, the relationship between the liquid entrainment factor \(E_{fg}\) and the charge amount under different cooling capacities in Figure 8 actually reflects the dependence of \(E_{fg}\) on \(j_{o}/h\). Similar to the pool entrainment in the momentum control region, the dependence of the entrainment factor on \(j_{o}/h\) varies with the vapor velocity (cooling capacity) [8]. Due to lower cooling capacity, the two-phase flow pattern in the tube bundle at 600 RT may be different from that at 700 and 800 RT and the initial velocity and diameter of droplets generated are different, resulting in different entrainment factor dependence with \(j_{o}/h\), that is, the relationship between \(E_{fg}\) and refrigerant charge is  
Figure 7: Liquid entrainment factor as a function of charge at 700 RT. different under different cooling capacity. Figure 8 also shows that the average uncertainties of \(E_{fg}\) at 600 and 800 RT are 0.21% and 0.18%, respectively. It can be seen from Figure 8 that under the full charge of the evaporator (point 5 on the 600 RT and 700 RT curves and point 6 on the 800 RT curve), the \(E_{fg}\) values at 600, 700 and 800 RT are 0.56%, 0.89% and 1.85%, respectively, and \(E_{fg}\) increases rapidly with the increase of cooling capacity. If it is required that the reasonable \(E_{fg}\) is no more than 1% when the unit is running, the refrigerant charge at 800 RT must be reduced to less than 565 kg, which is less than 596 kg of refrigerant required for the evaporator tube bundle to be fully immersed in liquid refrigerant at this working condition. This means that there are \"dry tubes\" in the evaporator when the unit is running, resulting in about 5% of the heat transfer performance loss at 800 RT, as shown in Figure 6. By using \(E_{fg}\) = 1% as the limit entrainment factor, the reasonable maximum cooling capacity of the evaporator limited by the liquid entrainment under AHRI conditions is between 700 and 800 RT. The variation of system COP is caused by the changes in system cooling capacity, system temperature lift (condensation temperature--evaporation temperature, LIFT) and entrainment factor \(E_{fg}\). As LIFT increases, COP decreases. Compared with the rated cooling capacity of the compressor in the experiment, the tested capacities of 600, 700 and 800 RT are all part loads. If the LIFT remains unchanged, the system COP increases with the increase in cooling capacity. Figure 9 shows the variation of evaporation temperature \(t_{e}\) and the temperature LIFT with charge at each cooling capacity. It can be seen from the figure that as the charge increases, the evaporation temperature at each cooling capacity increases gradually and then remains almost unchanged after the tube bundle is fully immersed in the refrigerant.