Two-phase flow boiling is the preferred method for efficient heat dissipation in electronic equipment, so the bubble dynamics and heat transfer characteristics of flow boiling in a single pentagonal rib channel are numerically analyzed by the volume of fluid (VOF) model. Three typical rib structures are obtained, whose block ratios are 0.4, 0.75, and 0.25, and the corresponding length-diameter ratios are 1.72, 0.59, and 12.70, respectively. The Reynolds number is 14,000 and 28,000. The heat flux is set from 50 to 150 kW/m2. The results show that the starting position of the bubble is at the cone and sides of the rib, and its moving direction is related to the configuration of the rib, the heat flux, and fluid velocity. Additionally, the bubbles are shedding at the corner of the rib, and the short rib does not appear to be the vortex shedding phenomenon. The wall temperature and temperature fluctuation increase with the increase of heat flux and with the decrease of fluid velocity. The heat transfer coefficient decreases with the increase of heat flux and with the decrease of fluid velocity. The short rib channel has a higher heat transfer coefficient, approximately 14% and 50% higher than that of the median and long rib channels, respectively. Compared with the single-phase flow, the main frequency of the lift coefficient increases with the increase of the fluid velocity while decreasing with the increase of heat flux. The main frequency of the median rib without harmonic signal keeps constant, and that for long rib decreases by 38%. The study of the instantaneous characteristics of bubble growth in pentagonal rib channels is instructive to the efficient heat dissipation in electronic equipment.

An experimental study is carried out to investigate the flow characteristics of the trailing edge of the horizontal vanes mounted at the branch entrance of a T-junction duct by means of particle image velocimetry (PIV). The measured region starts at the trailing edge of the vanes and ends at about 1.26D (hydraulic diameter) length at downstream of the branch duct. The velocity field is obtained across a number of vertical height planes (z/D = ±0.2, 0, and −0.4) under different flow conditions (cross velocity: uc = 30–50 m/s; velocity ratio: R = 0.08–0.18). The instantaneous flow results show that Kelvin-like vortices with counter-clockwise direction appear at the heights of z/D = ±0.2 and 0, and that a separation bubble is formed at the upper wall of the branch duct at the same heights, respectively. As for near wall z/D = −0.4, one large vortex is observed at the downstream channel, but the separation bubble vanishes as the branching Reynolds number is increased to 3.6 × 104. The time-average flow field is slightly different from that of instantaneous flow field. In addition, the vorticity distribution indicates that two significant vortex sheet layers with negative and positive values are found at the high velocity ratio or high cross velocity, and the normalized vorticity strength increases with increasing velocity ratio and decreases with increasing cross velocity except at z/D = −0.4.

The skin friction consumes some of the energy when a train is running, and the coherent structure plays an important role in the skin friction. In this paper, we focus on the coherent structure generated near the vent of a train. The intention is to investigate the effect of the vent on the generation of coherent structures. The ventilation system of a high-speed train is reasonably simplified as a T-junction duct with vertical blades. The velocity signal of the cross duct was measured in three different sections (upstream, mid-center and downstream), and then the coherent structure of the denoised signals was analyzed by continuous wavelet transform (CWT). The analysis indicates that the coherent structure frequencies become abundant and the energy peak decreases with the increase of the velocity ratio. As a result, we conclude that a higher velocity ratio is preferable to reduce the skin friction of the train. Besides, with the increase of velocity ratio, the dimensionless frequency St of the high-energy coherent structure does not change obviously and St = 3.09 × 10⁻⁴–4.51 × 10⁻⁴.