# Water Flow Boiling in Micro/Mini Channels Using Volume of Fluid Model
## 1 Introduction  
The conventional heat dissipation systems relied on single-phase flow, based on natural and forced convection, to exchange heat. Single-phase heat transfer coefficients (HTC) were limited by power and system size requirements. Further technological advancement required light-weight and compact thermal systems. As a result, two-phase flow boiling (FB) systems, such as shell and tube heat exchangers, boilers, and helical coil steam generators, were designed. These two-phase systems were based on high HTC dominated by the nucleate boiling (NB) mechanism at low-volume fractions and convection currents generated by the mist flow regime at high-volume fractions. The higher heat transfer dictated by two-phase flow has numerous applications in the nuclear and aerospace industries. [PERSON] et al. [1] used the VOF model to investigate FB in water in a boiling water reactor (BWR). Various flow regimes, including nucleate boiling, transition boiling, flow boiling, and complete local dry-out, were simulated inside the circular rod assembly of BWR, and HTC was worked out at 30 bar. [PERSON] et al. [2]reviewed the literature on FB and condensation under reduced gravity to explore their viability for aerospace applications. In lieu of developing energy-efficient air/space vehicles, [PERSON] et al. [3] reviewed aerospace-oriented spray cooling--that employs boiling--and made recommendations for incorporating gas-atomized spray cooling. A study in [4] reviewed pool boiling (PB) and FB of dielectric fluids with an emphasis on mitigating the heat dissipation requirements of electrical equipment. The effect of surface enhancement on the HTP of dielectric fluid was also considered. [PERSON] et al. [5] elucidated FB of ammonium diinitramide using VOF coupled with the Lee phase change model to design electronic spacecraft thrusters. Researchers have also employed the VOF model to study FB of refrigerants to design efficient cooling systems. Using the VOF model along with the level set (LS) scheme, the authors in [6] defined a criterion for estimating bubble departure diameter during FB of refrigerant R314a. [PERSON] et al. [7] similarly investigated HTC and flow instabilities during FB of R314a using VOF. They concluded that a discrete heat source was better at suppressing the instability than a continuous heat source. The VOF was also used by [PERSON] et al. [8] to simulate a three-phase FB consisting of water liquid, water arising in gas from the liquid phase, and embedded nanoparticles such as aluminum oxide, and titanium oxide, separately. The addition of nanoparticles enhanced heat transfer. The helical coil heat exchangers had a compact design, ease of manufacture, and high heat transfer efficiency. All these advantages render helical coil heat exchangers promising candidates for the nuclear and power generation industries [9]. [PERSON] et al. [9] were the first to experimentally study FB in helical coil heat exchangers. It was reported that forced convection evaporation was the dominant heat transfer mechanism (HTM) at high vapor quality, while the NB mechanism was present at low vapor quality. The experimental data showed good agreement with FB correlation for straight tubes proposed by [PERSON] [10]. [PERSON] et al. [11] presented a correlation for FB in large-diameter vertical tubes. The correlation was suitable for straight vertical tubes in a system with a pressure lower than 7 MPa. Researches [9; 10] separately carried out FB experiments in helical coil tubes using system pressure over 3.5 MPa. At low mass flux and high heat flux, the helical tubes were covered by regions of vapor quality lower than 0.1, and NB was the main heat dissipation mechanism. Both researchers reported good agreement with the correlation for straight vertical tubes proposed by [12].