The present study investigates the heat transfer performance of W-shaped ribs in a rectangular channel with typical geometries and flow conditions for a combustor liner cooling passage. In order to assess the Reynolds number dependence on heat transfer enhancement by the ribs for the combustor cooling passage, experiments were conducted with channel Reynolds number ranging from 40,000 to 550,000. The ribs were located on one side of the channel and the rib height-to-hydraulic diameter ratio (e/Dh) was 0.006 to 0.014, which simulate the combustor liner cooling configurations. Rib pitch-to-height ratio (P/e) was 10. Rib-roughened copper plates with constant temperature were used to measure the averaged heat transfer coefficients. Measured results show that the heat transfer enhancements of about 3 were obtained over that of a flat plate at high Reynolds numbers for all cases. The slope of heat transfer coefficient becomes constant with increasing Reynolds number because of the laminar-turbulent transition around the ribs, which is considered to occur at Reynolds number based on rib height of about 1,000. Pressure loss measurements showed that the friction coefficients are constantly 3–4.5 times higher than those of a flat plate for a fully turbulent flow such as a combustor cooling passage. Pressure loss by ribs seems not to have a significant impact to the overall combustor performance. Numerical calculations were conducted additionally for all test cases. Predicted amount of heat released from the ribs contributes about 40% of overall heat release even for low ribs. Heat transfer on the rib surface is essential in the evaluation of the rib-roughened cooling passage.

The present study investigates the heat transfer performance of W-shaped ribs in a rectangular channel with typical geometries and flow conditions for a combustor liner cooling passage. In order to assess the Reynolds number dependence on heat transfer enhancement by the ribs for the combustor cooling passage, experiments were conducted with channel Reynolds number ranging from 40,000 to 550,000. The ribs were located on one side of the channel and the rib height-to-hydraulic diameter ratio (e/ D),) was 0.006-0.014, which simulate the combustor liner cooling conflgurations. Rib pitch-to-height ratio (Pie) was 10. Rib-roughened copper plates with constant temperature were used to measure the averaged heat transfer coefficients. Measured results show that the heat transfer enhancements of about 3 were obtained over that of a flat plate at high Reynolds numbers for all cases. The slope of heat transfer coefficient becomes constant with increasing Reynolds number because of the laminar-turbulent transition around the ribs, which is considered to occur at Reynolds number based on rib height of about 1000. Pressure loss measurements showed that the friction coefficients are constantly 3-4.5 times higher than those of a flat plate for a fully turbulent flow such as a combustor cooling passage. Pressure loss by ribs seems not to have a significant impact to the overall combustor performance. Numerical calculations were conducted additionally for all test cases. Predicted amount of heat released from the ribs contributes about 40% of the overall heat release even for low ribs. Heat tran.sfer on the rib surface is essential in the evaluation of the rib-roughened cooling passage.

Flow and heat transfer of lattice cooling channel are investigated numerically. Firstly, simulations are performed for two channels to reproduce the experimental results reported in open literatures. Based on the literatures, sub-channels consisting lattice network are designed with aspect ratio of near unity and crossing angle of 45 degrees. Predicted heat transfer patterns of primary surfaces have agreed qualitatively and quantitatively well with the experimental results. Cooling air turns mainly through turning at the end of each sub-channel. After impinging the sidewall, strong acceleration occurs at the entrance of the opposite sub-channel, which enhances local heat transfer. Based on the above discussions, the present study also compares heat transfer coefficient of all surfaces (rib + primary) surrounding the sub-channel. The highest local heat transfer coefficient is found at rib surfaces. Predicted flow pattern indicates that a longitudinal vortex is formed in parallel to the sub-channel after impinging the sidewall, and that transient flow from one to another side of the sub-channels keeps the core of the vortex. This transient flow substantially contributes to the heat transfer enhancement at the upper edge of a rib surface, and more than half of total heat flux transfers through the rib. It follows that, in designing lattice cooling channel, rib surfaces should also be treated as heat transfer surface. Moreover, the effect of sub-channel (or rib) inclination angle on flow and heat transfer is examined. Rib inclination angle strongly affects translation flow between the lower and upper sub-channels and impingement at the sidewall. Further experimental investigation is expected in the near future.

Heat transfer characteristics of a cooling channel with densely arranged, angled rib turbulators were investigated experimentally and numerically. The dense arrangement of the ribs is one of the potential candidates to improve heat transfer performance because of its surface area enlargement effect. The square test channel consisted of six square ribs, which were placed on one side. The ribs were arranged with a rib height to channel hydraulic diameter ratio (e/Dh) of 0.13, an angle of attack to the mainstream of 60deg, and rib pitch-to-height ratios (P/e) of 3, 6 and 10. Local heat transfer distribution on all surfaces of the rib and the floor surface between the ribs were measured by the naphthalene sublimation method. Channel Reynolds number ranges from 30,000 to 70,000. Measured data showed that the P/e of 3 provided the largest total heat transfer. It was found that 60% of heat flux was transferred through the rib surface. Numerical simulations using a Reynolds-Averaged Navier-Stokes (RANS) method and a Large Eddy Simulation (LES) were carried out for the above test cases. The RANS underestimated the experimental heat transfer data by 40–50% for all rib surfaces with close rib arrangement. On the other hand, time-averaged heat transfer distribution obtained by LES showed better agreement with experimental data. Moreover, the LES predicted the periodic large vortex structure ranging over several rib pitches. Further investigation is expected on the periodic secondary flow and the application of LES to the prediction of heat transfer in the near future.

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