Effect of rib orientation on thermal and fluid-flow features in a two-pass parallelogram channel with abrupt entrance

- The combined effects of rib angle and abrupt entrance on secondary flow and heat transfer are investigated.
- Detailed Nu distributions over two ribbed endwalls of stationary two-pass parallelogram channels are analyzed.
- f coefficients are measured with TPF performances comparatively examined.
- The Reynolds number effects on heat transfer performance are disclosed.

Detailed secondary flow patterns, Nusselt number (Nu) distributions, Fanning friction factor (f), and thermal performance factor (TPF) are compared between three different rib orientations (45-deg, −45-deg, and 90-deg) in a stationary two-pass parallelogram channel with an asymmetrically and suddenly contracted inlet condition. Velocity and temperature measurements are performed with Particle Image Velocimetry (PIV) and Infrared Thermometry (IT). Temperature and pressure drop measurements are conducted under Reynolds number (Re) ranging from 5000 to 20,000, while the velocity measurements are carried out at fixed Re = 10,000. A cross-sectional equal length, 45.5 mm, of adjacent sides is selected and two pairs of opposite angles are 45-deg and 135-deg. The rib-height to channel-height ratio and rib pitch-to-height ratio are 0.1 and 10, respectively. It is found that the combined effect of inlet condition and rib orientation extends to whole parallelogram channel rather than the first passage only. Among the rib orientations investigated, the 45-deg ribs strengthen the effects of parallelogram slant sidewalls and entrance and further widen the Nu difference between top and bottom walls. In contrast, the other two rib configurations, 90-deg ribs especially, weaken these effects and narrow the corresponding differences. Based on the ability to elevate the surface-averaged Nu0‾/Nu∞ ratio, the orders (45-deg, −45-deg, 90-deg) and (90-deg, 45-deg, −45-deg) respectively for present top and bottom walls are significantly distinct from those in square and rectangular channels. The variations of channel-averaged Nu0‾/Nu∞ and f0‾/f∞ with the associated TPF values against Re are plotted and compared with previous results obtained from literature. Overall speaking, three tested angled ribs all provide the TPF values above than unity; however, only the TPF values in 45-deg case are always higher than those in corresponding smooth parallelogram channel (ASI case). Divided by the TPF values in ASI case, the normalized TPF values are respectively 1.09-1.04, 1.07-0.87, and 0.83-0.99 for 45-deg, 90-deg, −45-deg ribbed cases. It is thus important to take the entrance effect into account when applying the angled ribs in a two-pass parallelogram channel.