Effects of turbulence and secondary flows on subcooled flow boiling

Experiments are conducted on the influence of turbulence and longitudinal vortices on subcooled flow boiling in a vertical, rectangular channel. Different flow inserts are used to create turbulence and vortices in the channel. Studied boiling regimes range from the onset of nucleate boiling over the critical heat flux up to fully developed film boiling. A wide range of measuring techniques is applied: time averaged particle image velocimetry (PIV) is used in cold flows for the evaluation of the effects the inserts have on the flow, high speed PIV and photography are used to determine the effects on the fluid and vapor movement in boiling experiments. Digital Holographic Interferometry is used for the evaluation of temperature distributions in the boiling flow. Furthermore, optical microprobes are used to obtain pointwise measurements in areas inaccessible to the imaging techniques. The experiments show that the flow inserts can have considerable impact on the heat fluxes and the distribution of vapor and temperature along the channel. All used inserts lead to an increase in critical heat flux, which is more pronounced for stronger turbulence and higher flow rates and fluid subcoolings. The measuring techniques reveal both a better transport of vapor from the heater surface as well as an increase in mixing in the liquid phase with flow inserts.