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The present study aims to propose innovative louver-type turbulators to enhance the heat transfer rate in three major ways, i.e. core flow disturbance, jet impingement, and extended heat transfer surface. These louvers are installed in the twin-pass square channel with a hydraulic diameter ( D H ) of 45.5 mm and a fully developed inlet condition. Three parameters are examined to find out the optimal design, including the pitch ratio ( Pi / D H = 1, 2, 3, 4, and ∞), the number of slat per half louver (1 ≤ N s ≤ 4), and Reynolds number (5000 ≤ Re ≤ 20000). Particle Image Velocimetry (PIV) and Infrared Thermometry (IT) are respectively employed to measure the detailed velocity maps and wall temperature distributions. With acquired Nusselt number ( Nu ) ratio, the pressure measurements are also performed to estimate the Fanning friction factor ( f ) and further evaluate the thermal performance factor (TPF). The results show that both Nu ‾ / Nu ∞ and f ¯ / f ∞ ratios rise with descending Pi / D H and ascending N s under the present test conditions. Among all the tested cases, the case with Pi / D H = 1 and N s = 4 provides the highest Nu ‾ / Nu ∞ , almost twice the value of smooth reference; nevertheless, it suffers from high f ¯ / f ∞ penalty. It is also found that the TPF level is a relatively weak function of Pi / D H . The new finding is that there exists a critical slat number of N s = 3 above which the TPF value is a weak function of N s . In contrast, below the critical N s the TPF value increases with decreasing N s . From the viewpoint of heat transfer enhancement, one could apply the louvered channel as a heat exchanger with small Pi / D H and large N s . The boundary layer disturbance, on the other hand, is more cost-effective than core flow disturbance as a mechanism to augment heat transfer from the viewpoint of thermal performance. [ABSTRACT FROM AUTHOR]

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 Nu 0 ‾ / 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 Nu 0 ‾ / Nu ∞ and f 0 ‾ / 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. [ABSTRACT FROM AUTHOR]

PIV and Infrared Thermometry measurements are respectively conducted to study the effects of entrance geometry and Reynolds number ( Re ) on the detailed flow fields and local temperature distributions in a stationary two-pass smooth parallelogram channel with 180-deg sharp turn. Two entrance geometries, including a fully developed inlet condition (FDI) as well as an asymmetrically and suddenly contracted inlet condition (ASI), are investigated. The smooth parallelogram channel has equal adjacent sides of 45.5 mm in length and two pairs of adjacent angles are 45-deg and 135-deg. Local ( Nu 0 ) and regionally averaged ( Nu ‾ 0 ) Nusselt numbers over entire top and bottom walls along the first and second passages and through the bend region with the associated pressure drop are examined under Re ranging from 5000 to 20,000. Moreover, cross-sectional secondary-flow patterns as well as the near-wall streamwise mean velocity components and turbulent kinetic energy are analyzed to correlate the relationship between flow characteristics and heat transfer distributions at Re = 10,000. The most distinct finding of the present study is that the asymmetric thermal and fluid flow features on the top and bottom wall side, in contrast to symmetric ones in the corresponding square and rectangular channels. The affected top and bottom wall Nu 0 distributions of ASI respectively extends downstream to the mid-turn and the middle of second pass. Compared with FDI, ASI elevates the Nu ‾ 0 about 65.3–70.1%, 14.2–13.7%, and 23.9–14.0% in the first passage, turn region, and second passage, respectively, under constant flow rate condition. Thermal performance factors of the ASI are about 43.5% and 53.7% higher than the FDI at Re = 5000 and 20,000, respectively. Moreover, the correlations of Nu 0 and fanning friction factor ( f 0 ) with Re are obtained and further compared with those of the corresponding square channels available from the literature. [ABSTRACT FROM AUTHOR]

Infrared thermography and pressure measurements are presented of the detailed local Nusselt number ( Nu 0 ) distributions over top-endwall and the associated pressure drop coefficients ( f 0 ) of a stationary two-pass 90-deg ribbed parallelogram channel. The parallelogram cross-section has equal adjacent sides of 45.5 mm in length (aspect ratio = 1) and channel hydraulic diameter of 32.17 mm. The rib pitch-to-height ratio, rib width-to-height ratio and rib-height to channel-height ratio are 10, 1 and 0.1, respectively. It is aimed at exploring the effects of Reynolds number, Re = 5000–20,000, on Nu 0 and regionally averaged ( Nu ‾ 0 ) Nusselt numbers along the ribbed inlet/outlet legs and over the bend. To gain insight into the heat transfer augmentation mechanisms, the PIV measurements were also performed for correlating the fluid flow and heat transfer characteristics in the two legs and inside the bend at Re = 10,000 in terms of the streamwise main flow and cross-sectional secondary flow, turbulent kinetic energy as well as spanwise and streamwise Nu 0 and Nu ‾ 0 . It is found that the flow dynamic mechanisms responsible for the rib-top and mid-rib heat transfer enhancement are different for the inlet and outlet passes. The presences of a skewed high Nu 0 streak between the last inlet-leg rib and the bend as well as two high Nu 0 zones inside the bend are the new found features lacking in the corresponding two-pass 90-deg ribbed square channel. In addition, simple correlations of Nu 0 and f 0 with Re are acquired. Thermal performance factors are about 66% and 28% higher than the previous reported smooth-walled counterpart at Re = 5000 and 20,000, respectively. They are further compared with those of the corresponding square channels available from the literature. [ABSTRACT FROM AUTHOR]