## 1 Introduction  
Liquid entrainment/carryover is a common phenomenon for the shell-and-tube flooded or falling film evaporators of a water-cooled centrifugal chiller and has an important impact on the performance and safety of the chiller and compressor. Liquid entrainment reduces the amount of liquid that could otherwise be evaporated for refrigeration and reduces the cooling capacity. At the same time, it vaporizes in the compressor and consumes compressor power; therefore, it is detrimental to the chiller performance. In severe cases, the entrained liquid can erode and damage the impeller. In the boiling tube bundle of the flooded evaporator, liquid droplets are generated by the burst of bubblesnear the liquid surface and the shearing effect of the vapor flow inside the tube bundle on the liquid ligament. When the liquid level is higher than the bundle, a liquid layer may exist above the bundle and the droplets may also be produced by the wave action of this layer. In addition, if the pressure difference through the outlet of the evaporator exceeds the suppression effect of gravity and surface tension, droplets may also be generated at the vapor-liquid interface under the evaporator outlet. After the liquid droplets are generated, they enter the headspace above the bundle, the larger liquid droplets fall back to the liquid surface under the gravity effect and the smaller liquid droplets are entrained by the vapor flow into the compressor to form liquid entrainment, as shown in Figure 1. The figure depicts the scenario where the liquid level is just above the tube bundle. For clarity, the bubbles under the liquid level are not shown. To leave enough space for vapor-liquid separation, the height of the tube bundle in an actual flooded evaporator is generally designed to be 50-60% of the shell diameter. According to the different flow structure and liquid carryover mechanism, there are three types of liquid entrainment. The first type is liquid carryover by gas flow in straight tubes [1; 2; 3; 4] and the second type is carryover through T-branches [5; 6; 7], the third category is the liquid entrainment in a liquid pool [8; 9; 10]. Liquid entrainment in straight tubes is caused by the shearing effect of the turbulent gas flow on the annular liquid film, while entrainment in the T-branches is caused by the destabilization of the gas-liquid interface under the pressure difference between the main and branch tubes and entrainment in the pool is due to the boiling and bubbling gas flow from the liquid pool. The liquid entrainment problem in straight tubes [1; 2; 3; 4] and T-branches [5; 6; 7] has received great attention with a large amount of experimental data having been accumulated and well-developed physical or semi-empirical models having been developed. However, there are relatively few studies on pool entrainment. [PERSON] and [PERSON] [8] proposed a systematic pool entrainment model and classified pool entrainment into three regions, which are near-surface, momentum controlled and deposition-controlled regions. In each region, a specific correlation model is proposed to calculate the droplet entrainment factor. Due to the differences in the flow patterns in the pool droplet generation mechanisms, the momentum-controlled region is further divided into three sub-regions based on different levels of gas fluxes, but the exact form of entrainment correlation is not given when the gas flux is high. [PERSON] et al. [9] experimentally studied the pool entrainment with side exit in the near-surface region. [PERSON] et al. [10] studied pool entrainment with and without side exit in the momentum-controlled region and proposed a new correlation with side exit in the high gas flux region. [PERSON] and [PERSON] [11] studied pool entrainment under small air flow rate and proposed an entrainment correlation at low air flow rate, with analysis on the effect of outlet location on the entrainment factor. All the above studies of pool entrainment are for air-water or steam-water systems.