Flow resistance of low-frequency pulsatile turbulent flow in mini-channels

Abstract

Plate-type nuclear reactor fuel is currently getting increasing attentions as it features excellent heat transfer ability and compact structure. To gain an insight into the influences of channel size and pulsatile parameters on flow resistance characteristics, steady and pulsatile turbulent experimental investigation were performed. Three channels with varying height were used. The covered ranges of time-averaged Re Reta=(0.5–2) ×104, dimensionless frequency sqrtω’= 0.3–3.2, and pulsatile velocity amplitude Av= 0.04–0.93. A normalized parameter, friction factor ratio C =λta/λta, was proposed to denote the effects of flow fluctuation on time-averaged friction factor, where λta and λta are time-averaged and steady friction factor. The results show that in channel I (40 × 2 mm2), the time-averaged friction factor is remarkably larger than the steady values in the ranges of 1.5<sqrtω’<3.2.. The friction factor ratio C increases with the increasing sqrtω’ and Av, decreases with the increasing Reta. Further analysis indicated the friction factor ratio C is a function of dimensionless acceleration α*=4Avω′2/Reta. As the channel height increasing, the time-averaged friction factor decreases gradually to the same values as that of steady flows in the present ranges of sqrtω’. Finally, correlations to predict time-averaged friction factor were proposed.The effect of channel size is inferred to associate with the fractions of turbulent Stokes layer thickness to half-height of channels. In mini-channels, the turbulence Stokes layer is almost filled the entire flow area. As channel height increasing, the turbulence, which generated in near wall layer, cannot timely propagates into pipe core region before decaying-which cause the impact of superimposed unsteadiness fades rapidly. The effect of dimensionless frequency is interpreted as the response time of fluid to the rapid changes of pulsatile pressure gradient.