|Abstract:||From thermodynamic point of view, first droplet of liquid during condensation appears at quality one and the last vapor disappears at quality zero. This is because thermodynamic assumes equilibrium and temperature to be uniform. Therefore, conventional modeling of condensation typically contains three regions: superheated (SH), two-phase (TP) and subcooled (SC) regions. However, since phase change requires temperature gradient, the first droplet in heat exchanger occurs as soon as the temperature of inner wall drops below saturation temperature even though the bulk enthalpy indicates the flow still in superheated region. Once the liquid appears, the heat transfer and pressure drop mechanism deviates from single phase analysis and switches into two-phase behaviors. According to this characterization, a fourth and fifth zone were proposed to be modeled in heat exchangers, classified as condensing superheated region (CSH) and condensing subcooled region (CSC). Previous experimental data has indicated the existence of these additional regions. Preliminary visualization directly confirmed the presence of liquid-film layer in SCH region. A correlation was made to calculate the heat transfer coefficient (HTC) in CSH region. However, flow characteristics in CSH region are almost never discussed, and little of the physics behind heat transfer mechanism in this region was explained. Therefore, the early-stage condensation flow regimes and void fractions predicted from existing flow maps and correlations are either missing or physically incorrect. As for heat transfer correlation in CSH region, despite it being accurate, the approach is highly empirical, and thus hard to be understood and generalized.
This study aims at better understanding the physics from flow characterization and heat transfer measurement, which can be later used for the development of a more physical model for void fraction, heat transfer and pressure drop. The paper presents flow visualization, liquid film thickness measurement and heat transfer results, showing the effect of mass flux and heat flux on the onset of condensation, film distribution, flow regime and HTC. The result of flow visualization reveals that the onset of condensation is always in SH region, as annular flow and the flow regime is strongly affected by mass flow but heat flux. Based on the flow visualization in CSH region, a new diabetic flow map is proposed to best represent the flow characters and predict flow regimes in CSH region. Film thickness measurement in CSH demonstrates that void fraction drops below one at the onset of condensation, and mass flux has greater effect on slip ratio than heat flux. Besides, having film distribution suggests an opportunity to more objectively determine the flow regime and develop a more physical model knowing more details about the two-phase structure inside the tube. Experimental HTC data shows that mass flux can affect HTC in TP region but not in CSH region, while heat flux can affect HTC in CSH region but not TP region. The counter intuitive results in CSH region gives rise to a new heat transfer coefficient based on the local heat transfer across liquid film, which is named as film heat transfer coefficient (HTC_f). The comparison between HTC and HTC_f illustrates the difference between two interpretations and shows that HTC_f is a better representative of heat transfer process in CSH region.