The Effect of Heating Conditions on Convective Boiling Characteristics of Methanol/Water Mixtures in Microchannels at Low Reynolds Number

Abstract

In this study, flow boiling of methanol/water mixtures in a microchannel was investigated under the conditions of constant heat flux and constant wall temperature at low Reynolds number. The heating condition had a profound effect on regime maps of flow boiling in a microchannel. Under isothermal conditions, nucleation occurred when the wall was slightly superheated and annular flow was quickly established. The composition of the mixture played a minor role under isothermal conditions. On the other hand, bubbles could possibly emerge at subcooled temperatures under the constant heat flux condition, and the rupture of the liquid film in the annular flow lead to a unique phenomenon – sheet breakup. Because of the large deviation between the bubble point and dew point, sheet breakup spanned a very high superheat for mixtures with mole fractions of 0.07 and 0.3. As a result, the superheat associated with the transition to mist flow was elevated, and the critical heat flux (CHF) was increased. In contrast, annular flow barely persisted for mixtures with a mole fraction larger than 0.7. The low surface tension made it difficult to maintain the vapor-liquid interface and nucleation directly burst into sheet breakup or mist flow.