Experimental evaluation of the onset of subcooled flow boiling at high liquid velocity and subcooling

The knowledge of the onset of subcooled boiling in water forced convective flow at high liquid velocity and subcooling is of importance in thermal hydraulic studies of high heat flux components in fusion reactors. The present paper reports the results of an experimental research on the onset of subcooled boiling in water forced convective flow. From the measurement of the pressure drop along the heated test channel (D = 8 mm, L = 100 mm) it is possible to evaluate the heat flux at which the subcooled boiling occurs. As far as the coolant is in single-phase flow, it is possible to simply calculate its pressure drop, also taking into account the temperature effect on the friction factor, for the temperature variation along the heated channel and in the cross section (due to the steep thermal gradients). The classical corrections available in the literature have been used. Once bubble formation is established, the additional, significant contribution of the bubbles presence to the pressure drop leads to a deviation of the experimental pressure drop curve from the single-phase theoretical line. This latter identifies the onset of subcooled boiling. A comparison of the experimental heat flux at the subcooled boiling incipience with that provided by the major correlations available in the literature is given. A further comparison with measurements performed with an accelerometer device is provided in the paper. The accelerometer detects the additional noise due to bubbles formation close to the heated wall and subsequent collapse in the subcooled bulk of the liquid. The results given by the accelerometer and those obtained with the present evaluation method are in close agreement.