**Acknowledgements**

hazards, and the substitution of R134a in clinical application is urgent. R1234yf with boiling point of 29.5°C may be a potential candidate for environment protection due to its low GWP (<1). In this chapter, the clinical potential of R1234yf substi-

*Variations of effective surface heat flux as a function of spray distance [39]. (a) effective surface heat flux with R134a and R1234yf and (b) effective surface heat flux before and after cooling enhancement with R1234yf.*

Using the maximum surface heat flux correlation obtained by experimental spray characteristics (droplet temperature, velocity, and diameter) and surface heat transfer performance (surface heat flux calculated by 2D filter solution), **Figure 11(a)** shows the variations of effective surface heat flux with different R134a and R1234yf as a function of spray distance. The effective surface heat flux (*qe*) was obtained by multiplying the maximum surface heat flux, and cooling concentration within the radius of 2 mm is the interested area [39]. The effective surface heat flux by using R134a and R1234yf increased firstly due to the droplet temperature reduction as spray distance increased. Then, *qe* reaches their maximum value and finally decreases slowly. The maximum *qe* of R134a and R1234yf is 262.1 and 225.8 kW/m<sup>2</sup> at the optimal spray distances of 25.6 and 25.1 mm (see **Figure 11(a)**), respectively. The substitution of R1234yf for R134a can produce remarkable reduction of global warming potential to <1. However, the cooling capacity should be enhanced for the clinical application in laser treatment, owing to the 13.8% reduction in effective

capacity is necessary for the implementation of R1234yf in clinical laser treatment

According to our experience, two simple ways are available to enhance the cooling capacity, i.e., changing the nozzle diameter and decreasing the back pressure by decreasing the boiling point of cryogens [40, 41]. As shown in **Figure 11(b)**, the enhancement of effective surface heat flux at different spray distances was remarkable. After reducing the nozzle diameter and decreasing the back pressure, *qe* increases rapidly due to violent evaporation, and the peak value of the effective

of R134a under 1 atm, which proves the potential of R1234yf in the clinic CSC for

Several algorithms including the SFS, TF, and Duhamel's theorem methods were analyzed and compared in predicting time-varying surface heat flux during CSC.

). Therefore, the enhancement of cooling

) is increased by 18.8%. This result is comparable with that

tution for R134a was investigated.

*Inverse Heat Conduction and Heat Exchangers*

heat flux (from 262.1 to 225.8 kW/m<sup>2</sup>

of PWS.

**Figure 11.**

heat flux (268.3 kW/m<sup>2</sup>

**5. Conclusions**

**138**

the laser treatment of PWS.

This work was supported by the National Natural Science Foundation of China (51727811) and Fundamental Research Funds for the Central Universities.
