# A Review of Pool-Boiling Processes Based on Bubble-Dynamics Parameters
## 6 Boiling-Heat-Transfer Coefficient h and Heat Flux q
### Heat Flux \(q\)  
Models for the calculation of heat flux have also been developed by many researchers. Many researchers attempting to describe the boiling have proposed using mechanistic prediction and these descriptions are highly relevant to the various parameters of the bubble. Due to the perturbations caused by bubble generation and detachment, the boiling system can usually be divided into different zones for the calculation of heat transfer: the natural convection zone without bubble generation, the bubble zone, and the peripheral zone affected by the bubbles. Based on above assumption, [PERSON] and [PERSON] [93] proposed a total heat flux consisting of the natural convective and transient heat transfer due to thermal boundary reconstruction. [PERSON] and [PERSON] [128] modified the [PERSON] and [PERSON] [93] model and the effect of heating characteristics was given extra thought. They developed a total heat flux that includes transient conduction and natural convection. Then [PERSON] and [PERSON] [126] refined the [PERSON] and [PERSON] [128] thermal mechanism by summarizing the heat flux of evaporation from the micro-fluid layer at the bottom of the bubble and expressing it in terms of various bubble-dynamics parameters and liquid properties. [PERSON] and [PERSON] [129], on the other hand, expressed the total heat flux in three parts, i.e., natural convection, forced convection, and vaporization. [PERSON] and [PERSON] [130] modeled heat transfer in pool boiling according to the fractal distribution of nucleation locations on the boiling surface. In the computational model proposed by [PERSON] and [PERSON] [128], they divided the boiling heat flux density \(q_{total}\) into phase-change heat flux \(q_{b}\), micro-fluid layer evaporation heat flux \(q_{me}\) and heat flux density \(q_{nc}\), and validated the model with the help of experimental data from [PERSON] and [PERSON] at different contact angles [35]. The experimental data at different contact angles were verified and good agreement was obtained, and the value of the total nucleation boiling heat flux is expressed in Equation (58). \[q_{total}=q_{b}+q_{me}+q_{nc} \tag{58}\][PERSON] et al. [131] further analyzed the theoretical research proposed by [PERSON] and [PERSON] [93]. Then the heat transfer was further described, combining the transient conduction heat flow \(q_{tc}\) due to bubble-thermal-boundary reconstruction and the evaporation heat flux \(q_{me}\) of the liquid micro-fluid layer. The weighted average distribution of bubble growth time and bubble waiting time was adopted for the two kinds of heat flux, and their new descriptive model is shown in Equation (59). \[q_{total}=\big{(}q_{me}\times t_{g}+q_{tc}\times t_{w}\big{)}f+q_{nc} \tag{59}\]  
[PERSON] et al. [132] recommended the refined model for pool boiling heat transfer by considering the effect of sliding bubbles on the vertical heated wall. Four different heat transfer mechanisms were proposed for the calculation of heat flux, i.e., latent heat of microlayer evaporation, transient conduction of thermal boundary reconstruction, natural convection, and vertical heated surface sliding-bubble heat transfer, and the total heat flux was calculated as Equation (60), where the suffix of subscript \(s\) denotes the sliding bubbles and the superscript \(m\) denotes the modified model. \[q_{total}=(q_{me}^{m}+q_{tc}^{m})+(q_{mes}^{m}+q_{tcs}^{m})+q_{nc} \tag{60}\]  
[PERSON] et al. [30] conducted pool boiling experiments on microchannels of different heights and arrangements with Novec-649 at atmospheric pressure, visualization images of vapor bubbles were made, and the diameters of bubble departure and the frequency of departure from the surface were measured. Based on the model of transient conduction proposed by [PERSON] and [PERSON] [128] and the model of natural convection proposed by [PERSON] and [PERSON] [93], the total heat flux can be determined ultimately from the relationship from Equations (61)-(64), where \(\beta\) is the thermal expansion coefficient.