Experimental investigation of the flow and heat transfer instabilities in n-decane at supercritical pressures in a vertical tube

The fuel may be used in hypersonic vehicles to cool various surfaces; however, supercritical pressure fuels have been found to have deteriorated heat transfer rates and instabilities for some flow conditions. The flow and heat transfer instabilities in supercritical pressure n-decane were investigated experimentally in this work, with pressure of 2.5 and 3.0 MPa and inlet temperature of 16-225 °C. The heat flux was slowly increased to observe the flow in different stages as well as to obtain the boundary lines for a stability map. Seven stages were observed with different stability features. The transition to turbulence was found to be the main reason for the instability for stage b with slightly irregular oscillations, while dramatic variations of the thermal properties caused Helmholtz oscillations with regular frequencies and large amplitudes. The heat transfer deterioration in conjunction with an instability with buoyancy due to the density variation was found to be the reason. Higher pressures, inlet mass flow rates or fluid temperatures or downward flow weakened the instabilities, so these should be used in engineering designs to reduce the heat transfer deterioration and instability. The stability map provides further support for the nonlinear dynamic theory explaining the oscillations.