The friction characteristics of low-frequency transitional pulsatile flows in narrow channel

Narrow or mini-channels are widely used in various devices of process and energy engineering including research-reactors, compact heat exchangers and fuel cells. Friction characteristics in laminar-turbulent transition range of low-frequency pulsatile flows in a narrow rectangular duct have been studied. The experiments were conducted under the conditions of time-averaged Reynolds number 575 < Reta < 5583, pulsatile amplitude 0.056 < Au < 0.988, and dimensionless frequency 0.52⩽ω′⩽2.34. The results indicate that: (1) the impact of flow fluctuation on friction characteristics is most significant in the transitional range. The pulsatile amplitude and dimensionless frequency have comprehensive impact on time-averaged friction characteristics. The ratio C (C=λta/λst) of time-averaged friction factor to steady-state friction factor, increases with the increasing pulsatile amplitude Au and dimensionless frequency ω′. A dimensionless acceleration was proposed to analyze the influence form and degree of pulsatile parameters. (2) The increasing of ω′ and Au cause a decrease in the pulsatile critical Reynolds number Reta,cr. In addition, a correlation is proposed to predict the pulsatile critical Reynolds number. Transitional pulsatile flow may undergo cross regime process, i.e., laminar, transition, turbulence and re-laminarization. The decrease of pulsatile critical Reynolds number caused by the increasing ω′ and Au, and the cross regime process was inferred to be responsible for the increase of time-averaged friction factor.