## 3 Results and Discussion
### Discussion of CHF Correlations  
In terms of the CHF performance, the S2, S3, and S4 surfaces exhibited slightly lower CHF values of \(\sim\)24.09 W/cm\({}^{2}\), \(\sim\)23.32 W/cm\({}^{2}\), and \(\sim\)22.44 W/cm\({}^{2}\), respectively, compared with that of the S1 surface (\(\sim\)25.08 W/cm\({}^{2}\)). This suggested that the higher parallel CA and higher surface roughness of the microgrooved surfaces may cause slightly lower CHF values, primarily owing to the over-intensified interaction of bubbles on these surfaces. Figure 14 illustrates that on the S2-S4 surfaces, large vapor slug formed at a heat flux of \(\sim\)21.59 W/cm\({}^{2}\), impeding the heat transfer from the test surface to the working fluid. Thus, slightly lower CHFs were achieved compared with that of the plain surface (S1).  
Figure 14: High-speed images of bubble dynamics on S1–S4 surfaces in different heat flux regimes.  
A comparison between the CHF values obtained in the present study and those predicted using existing models is presented in Figure 16. One notable model proposed by [PERSON] [59], as shown in Equation (13), is based on hydrodynamic instability theory as follows:  
\[q^{\prime\prime}{}_{CHF}=Kh_{fg}\Big{[}\sigma g\rho_{v}{}^{2}(\rho_{l}-\rho_{v}) \Big{]}^{\frac{1}{4}} \tag{13}\]  
where \(K\) is the dimensionless CHF ratio, which equals 0.131 in [PERSON]'s model; \(h_{fg}\) is the latent heat; \(\sigma\) is the surface tension of the working fluid; \(g\) is the gravitational acceleration constant; and \(\rho_{v}\) and \(\rho_{l}\) are the density of the working fluid in the vapor and the liquid phases, respectively. However, this model solely accounts for the effect of pool hydrodynamics during boiling and neglects the influence of test surface characteristics. As expressed in Equation (14), [PERSON] [60] developed an extended model based on [PERSON]'s and incorporated the effect of the surface wettability as follows:  
\[q^{\prime\prime}{}_{CHF}=\rho_{v}h_{fg}\Big{(}\frac{1+\cos\theta_{rec}}{16} \Big{)}\Big{[}\frac{2}{\pi}+\frac{\pi}{4}(1+\cos\theta_{rec})\cos\omega\Big{]} ^{1/2}\Big{[}\frac{\sigma g(\rho_{l}-\rho_{v})}{\rho_{v}{}^{2}}\Big{]}^{1/4} \tag{14}\]  
where \(\theta_{rec}\) is the receding CA, and \(\omega\) is the incline angle, which equals 0 on a horizontal surface. In this correlation, the CHF values increased with increasing surface wettability. As depicted in Figure 16, deviations between our CHF results obtained for microgrooved structures and those obtained using [PERSON]'s prediction model emerged owing to  
Figure 15: Analysis of bubble evolution cycle on S1–S4 surfaces at a heat flux of \(\sim\)1.79 W/cm\({}^{2}\). (The red circles represent as indications to the specific bubbles for bubble analysis).  
insufficient consideration of the effects of the surface morphology. Thus, a modified correlation combining both the surface wettability and roughness factor was introduced by [PERSON] [61]. [PERSON]'s correlation is presented in Equation (15) as follows:  
\[q^{{}^{\prime\prime}}{}_{CHF}=\rho_{v}h_{fg}\left(\frac{1+\cos\theta}{16}\right) \left[\frac{2(1+\alpha)}{\pi(1+\cos\theta)}+\frac{\pi}{4}(1+\cos\theta)\cos \omega\right]^{1/2}\left[\frac{\sigma g(\rho_{l}-\rho_{v})}{\rho_{v}{}^{2}} \right]^{1/4} \tag{15}\]  
where \(\theta\) is the apparent CA; \(r\) is the roughness factor, which is defined as the ratio of the real wetted area to the projected base area; and \(\alpha\) equals \(r\cos\theta_{rec}\). As a result, in Figure 16, our CHF values match well with those theoretically predicted by [PERSON] owing to the consideration of the effects of the surface wettability and roughness factor induced by the microgrooved structure. As a result, the CHF values of the microgrooved surfaces can be accurately predicted within a 10% error, using [PERSON]'s prediction model.