## 5 Analysis of Simulation Model and Experimental Results
### Simulation Analysis of the Impact of OCR on Compressor Performance  
#### 5.2.1 Experimental Results of OCR with Changing Conditions  
In refrigeration systems, the OCR needs to consider the heat transfer performance of the heat exchangers and the lubrication of the compressor. An excessively low oil circulation rate can lead to inadequate lubrication for the compressor, while too high an oil circulation rate can significantly reduce the heat transfer coefficient of the system's heat exchangers. Typically, the system oil circulation rate is maintained between 2% and 10%. By adjusting the charge amount, the oil content for most operation conditions is maintained between 2% and 10% to test the system and compressor performance. Figure 6 shows how the OCR varies with speed under different conditions. The figure indicates that a lower evaporation temperature results in a lower OCR. For instance, when changing from 10/50 \({}^{\circ}\)C to 0/50 \({}^{\circ}\)C and from 5/55 \({}^{\circ}\)C to 0/55 \({}^{\circ}\)C, each 10 \({}^{\circ}\)C decrease in evaporation temperature leads to about a 1% reduction in the OCR. As the evaporation temperature is lower, the viscosity of lubricating oil decreases. Under the same compressor speed, the oil mass flow rate becomes smaller, so the OCR also decreases. This trend is more notable at 3000 rpm. Changes from 10/45 \({}^{\circ}\)C to 0/45 \({}^{\circ}\)C show minimal shifts in an OCR below 3000 rpm, possibly influenced by testing deviations. The figure also reveals that a lower condensation temperature leads to a higher OCR. For example, changing from 0/50 \({}^{\circ}\)C to 0/45 \({}^{\circ}\)C and from 10/50 \({}^{\circ}\)C to 10/45 \({}^{\circ}\)C results in approximately a 2% increase in the OCR for every 5 \({}^{\circ}\)C drop in the condensation temperature. This is because when the condensation temperature decreases, the pressure ratio also decreases. Therefore, the mass flow rate increases under the same compressor speed. Then, the OCR rises rapidly. And, changes in the condensation temperature have a more pronounced effect on the OCR. This conclusion suggests that when the compressor operates under low evaporation and high condensation conditions, such as during winter heating, the system's OCR is at its minimum, where it is necessary to note the compressor's oil return in order to avoid insufficient lubricating oil due to a low OCR. Conversely, in high evaporation and low condensation conditions, such as in spring and autumn, the system's OCR is at its maximum, requiring attention to the heat transfer impact of the heat exchangers. Figure 5: Calculated data vs. test data for isentropic efficiency based on OCR \(\approx\) 0. Figure 7 shows that isentropic efficiency peaks at an optimal value as compressor speed increases, and this peak occurs sooner as the evaporation temperature rises. The curve for isentropic efficiency has a marked inflection point: when the pressure ratio is 2.41, the inflection point occurs at a speed of 2400 rpm, whereas at a pressure ratio of 4.02, it occurs at 5400 rpm. After these points, the curve flattens with minor changes. According to [PERSON] [13], isentropic efficiency decreases with an increasing pressure ratio. At low speeds, the low flow rate means less oil can be carried out from the compressor oil sump, leading to poor cooling and lubrication, hence a rapid decrease in isentropic efficiency. At high speeds, as the oil content is higher and lubrication is sufficient, friction losses in the compressor remain relatively constant, leading to consistent isentropic efficiency with minor influence from the pressure ratio. Figure 8 indicates that volumetric efficiency increases rapidly with the compressor speed before reaching the rated compressor speed and then levels off after surpassing it. As the compressor speed increases, the effect of the pressure ratio on volumetric efficiency diminishes. At a low pressure ratio of 2.41, volumetric efficiency reaches a maximum of up to 90% at a speed of 3600 rpm.