## 3 Results and Discussion  
The liquid entrainment of the evaporator is related to the cooling capacity, chiller operating conditions and the liquid level in the evaporator. Under fixed cooling capacity and operating conditions, with the increase of the refrigerant charge, the liquid level in the evaporator rises and the number of tubes participating in boiling heat transfer gradually increases, which results in the evaporation temperature gradually increasing and the heat transfer temperature difference between cold water and refrigerant gradually decreasing, i.e., the total heat transfer coefficient (HTC), \(K\), of the evaporator gradually increases. This process continues until the bundle is fully immersed in liquid refrigerant, at this time the evaporative temperature increases to the highest and the total HTC reaches the maximum. Therefore, under fixed cooling capacity and operating conditions, \(K\) of the evaporator can be used to indirectly reflect the change of the evaporator liquid level with the charge, as shown in Figure 6. The total HTC of the evaporator is calculated by the following Equation (22). \[K=\frac{Q_{e}}{A_{e}\Delta t} \tag{22}\]  
where \(A_{e}\) is the total heat transfer area of the evaporator based on the envelope diameter of the enhanced tube and \(\Delta t\) is the logarithmic mean temperature difference between the refrigerant and cold water, calculated by Equation (23). \[\Delta t=\frac{t_{evi}-t_{ewo}}{\ln\left(\frac{t_{evi}-t_{e}}{t_{ewo}-t_{e}} \right)} \tag{23}\]  
It can be seen from Figure 6 that under fixed operating conditions for each cooling capacity, \(K\) gradually increases with the increase of charge. At 600 and 700 RT, when the refrigerant charge is about 581 kg, \(K\) reaches the maximum value. The amount of refrigerant continues to increase subsequently; however, there is little variation of \(K\). At 800 RT, when the refrigerant charge is 596 kg, \(K\) reaches the maximum, indicating that as the cooling capacity increases, more refrigerant is needed so that the evaporator tube bundle can be fully immersed. It can be seen from Figure 6 that \(K\) increases by about 12%, 12% and 20% respectively during the charging process at 600, 700 and 800 RT. Figure 7 shows the variation of entrainment factor with charge under 700 RT AHRI conditions, where \(E_{fg}\)-1 and \(E_{fg}\)-2 represent \(E_{fg}\) calculated by Equation (14) (Method 1) and Equation (15) (Method 2), respectively. It can be seen that at this cooling capacity, the variation of \(E_{fg}\) with charge conforms to a quadratic curve: with the increase of charge, \(E_{fg}\) gradually increases and the growth rate gradually accelerates. This is because, on the one  
Figure 6: Evaporator total HTC as a function of charge at each cooling capacity. hand, with the increase of charge, the liquid level in the evaporator rises. Under the shear entrainment of the vapor flow inside the tube bundle, liquid droplets can enter a higher position in the headspace above the bundle with a certain initial velocity, making it easier to be drawn into the compressor to form the liquid carryover. On the other hand, when the liquid level exceeds the bundle, a layer of liquid will be formed on the bundle, which reduces the height of the space above the bundle and the wave action of the liquid layer will entrain more large droplets into the headspace, further increasing the amount of liquid entrainment [13]. The reason for the close calculation results of the two methods is that the thermal balance at each experimental point is within 1% as mentioned before. Figure 7 also shows the uncertainty of each experimental point. It can be seen from the figure that the uncertainty of \(E_{fg}\) remains essentially unchanged as charge increases and the average uncertainties of the two methods are 0.22% and 0.20%, respectively. Because \(E_{fg}\) is small at low charge, the relative uncertainty at point 1 is large, reaching about 50%.