Theoretical Study on Cryogen Spray Cooling in Laser Treatment of Ota’s Nevus: Comparison and Optimization of R134a, R404A and R32 ◯ Jiameng Tian (School of Energy and Power Engineering, Jiangsu University, Zhenjiang 212013, PR China), Bin Chen (State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, PR China), Junfeng Wang (School of Energy and Power Engineering, Jiangsu University, Zhenjiang 212013, PR China), Zhifu Zhou, Dong Li (State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, PR China)