Experimental investigation on flow boiling heat transfer characteristics of endothermic hydrocarbon fuel and its visualization at high-temperature conditions

Abstract

The endothermic hydrocarbon fuel is used as a coolant to realize active cooling for the high-heat-flux components of air-breathing hypersonic vehicles. Because of its complex composition and severe thermal boundary conditions, the unstable heat transfer and coking are usually the major problems. Flow boiling heat transfer of a kind of endothermic hydrocarbon fuel named EHF-204 and its visualization were experimentally investigated in mini-channels with 2.0 mm internal diameter at heat flux of 250 kW/m2 and 500 kW/m2. It is found that the heat transfer enhancement and heat transfer deterioration would occur obviously during the flow boiling heat transfer process at subcritical and near-critical pressures of 1.0 MPa and 2.5 MPa. After the fuel was completely vaporized, the heat transfer coefficient would slowly increase again with the increasing fuel temperature. The heat transfer enhancement and heat transfer deterioration process at subcritical temperatures were not existed at the supercritical pressure of 4.0 MPa. The flow patterns of bubbly flow, slug flow, and churn flow were clearly observed at the subcritical pressures. Flow pattern visualization proved that the constant wall temperature heat transfer processes at subcritical pressures were subcooled boiling and saturation boiling flow. The bigger bubbles were formed at lower pressures, which caused higher heat transfer coefficient. The bubbly flow, slug flow caused the heat transfer enhancement, and the appearance of churn flow was a feather that made the boiling heat transfer transition from heat transfer enhancement to heat transfer deterioration. Some interesting visualization phenomena were observed in this study including the twice vaporization during flow boiling.