## 4 Results and Discussion
### Cavitation Characteristics
#### 4.2.3 Thermal Effect  
During the cavitation process, the gas generated by cavitation extracts heat from the fluid around the cavitation area, resulting in a specific temperature gradient between the liquid temperature in the surrounding area and the interface of the cavitation area. The decrease in liquid temperature in the surrounding area reduces its corresponding saturation pressure, further suppressing the phase transition process. This process of suppressing phase transition caused by the temperature drop of phase transition is called the cavitation thermal effect. The cavitation thermal effect varies for different working fluids. Table 3 compares the thermophysical properties of water and refrigerant. Due to the enormous difference in thermophysical properties, the influence of the thermal effect of the working fluid on cavitation is also very different. The cavitation thermal effect can be ignored for the water cavitation process at 288 K. The thermal effect has a more substantial influence on the cavitation process for the refrigerant than water, which also complicates the cavitation mechanism of the refrigerant. To further investigate the impact of thermal effects on the cavitation process within the prototype, the numerical simulation modifies the saturation pressure of the refrigerant from a non-isothermal model, represented by a polynomial function of temperature, to an isothermal model. Figure 21 shows that the thermal effects of the R134a refrigerant have significant impacts on the local distribution and flow characteristics of cavitation clouds. When ignoring the thermal effects, cavitation clouds generated at the inducer and impeller leading edges propagate downstream along the suction surface of the blades as the cavitation number decreases. The length and thickness of the clouds gradually increase, and there \begin{table}
\begin{tabular}{c c c} \hline \hline
**Working Fluid** & **Water** & **R134a Refrigerant** \\ \hline Temperature & 288 K & 288 K \\ Saturation pressure & 1.69 kPa & 486.03 kPa \\ Liquid-gas density ratio & 78519.3 & 52.6 \\ Heat of vapor & 2465.7 kJ/kg & 186.7 kJ/kg \\ Liquid thermal conductivity & 588.45 mW/(m-K) & 85.51 mW/(m-K) \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Comparison of Thermal Properties Between Water and Refrigerant. Figure 20: (**a**) Bubble volume fractions and (**b**) temperature distributions under different cavitation numbers. is a clear distinction between the gas and liquid phases of R134a refrigerant within the cavitation region. The gaseous refrigerant nearly fills the area. However, in the case of the non-isothermal model, the gas phase volume fraction of refrigerant within the cavitation region is mostly below 50%, exhibiting a mist-like appearance. The cavitation clouds within the inducer passages appear thicker and shorter by the influence of rotational effects. Additionally, the cavitation clouds are more prone to fill the entire flow passage, leading to blockage. Figure 22 illustrates the impact of thermal effects on the variation of fluid temperature during the cavitation process in the prototype. It can be observed that the simulations using the isothermal model significantly overestimate the temperature drop during cavitation, reaching as high as 30 K. In contrast, when the non-isothermal model is employed, the temperature drop in the core region of the cavitation cloud is only around 2 K.  
Figure 21: The influence of thermal effect on bubble volume fractions under different cavitation. Figure 22: The influence of thermal effect on temperature distributions under different cavitation. Figure 23 depicts the distribution of the refrigerant bubble volume fraction, which varies from the temperature in the inducer-installed pump. As the temperature increases, the thermal effects of R134a refrigerant intensify, rapidly suppressing cavitation clouds' development.