Experimental investigations have shown that the enhancement in heat transfer coefficients for air flow in a channel roughened with angled ribs is on the average higher than that roughened with 90° ribs of the same geometry. Secondary flows generated by the angled ribs are believed to be responsible for these higher heat transfer coefficients. These secondary flows also create a spanwise variation in heat transfer coefficient on the roughened wall with high levels of heat transfer coefficient at one end of the rib and low levels at the other end. In an effort to basically double the area of high heat transfer coefficients, the angled rib is broken at the center to form a V-shape rib and tests are conducted to investigate the resulting heat transfer coefficients and friction factors. Three different square rib geometries, corresponding to blockage ratios of 0.083, 0.125 and 0.167, with a fixed pitch-to-height ratio of 10, mounted on two opposite walls of a square channel in a staggered configuration are tested in a stationary channel for 5000 < Re < 30000. Heat transfer coefficients, friction factors and thermal performances are compared with those of 90°, 45° and discrete angled ribs. The V-shape ribs are tested for both pointing upstream and downstream of the main flow. Test results show that: a) 90° ribs represent the lowest thermal performance, based on the same pumping power, and is essentially the same for the 2:1 change in blockage ratio, b) low blockage ratio (e/Dh =0.083) V-shape ribs pointing downstream produced the highest heat transfer enhancement and friction factors. Amongst all other geometries with blockage ratios of 0.125 and 0.167, 45° ribs showed the highest heat transfer enhancements with friction factors less than those of V-shape ribs, c) thermal performance of 45° ribs and the lowest blockage discrete ribs are among the highest of the geometries tested in this investigation, and, d) discrete angled ribs, although inferior to 45° and V-shape ribs, produce much higher heat transfer coefficients and lower friction factors compared to 90° ribs.
The results of an experimental investigation on the average surface heat transfer co-efficients under a perforated plate of multiple, square array, round impinging air jets are presented. Correlation of the heat transfer performance in a semi-enclosed environment is presented. The correlation includes the effects of the jet “spent air” flowing perpendicular to the jets; the effects of the jet diameter, jet spacing, and jet-to-surface distance. The data cover a range of jet diameter Reynolds number from 3 × 102 to 3 × 104, jet spacing from 3.1 to 12.5 dia, and plate-to-surface distance of 1.0 to 4.8 dia. The results are compared with previously reported investigations with reasonable agreement. Correlation is in the form NuD,x = φ1φ2ReDm(Zn/D)0.091Pr1/3 where φ1 and m are functions of the jet spacing parameter, Xn/D, and Reynolds number, and φ2 is the heat transfer coefficient degradation factor due to “spent air”. φ1, φ2 and m are presented in graphical form as a function of important dimensionless parameters.
Turbine blade cooling is imperative in advanced aircraft engines. The extremely hot gases that operate within the turbine section require turbine blades to be cooled by a complex cooling circuit. This cooling arrangement increases engine efficiency and ensures blade materials a longer creep life. One principle aspect of the circuit involves serpentine internal cooling passes throughout the core of the blade. Roughening the inside surfaces of these cooling passages with turbulence promoters provides enhanced heat transfer rates from the surface. The purpose of this investigation was to study the effect of rotation, aspect ratio, and turbulator roughness on heat transfer in these rib-roughened passages. The investigation was performed in an orthogonally rotating setup to simulate the actual rotation of the cooling passages. Single-pass channels, roughened on two opposite walls, with turbulators positioned at 45 deg angle to the flow, in a criss-cross arrangement, were studied throughout this experiment. The ribs were arranged such that their pitch-to-height ratio remained at a constant value of 10. An aspect ratio of unity was investigated under three different rib blockage ratios (turbulator height/channel hydraulic diameter) of 0.1333, 0.25, and 0.3333. A channel with an aspect ratio of 2 was also investigated for a blockage ratio of 0.25. Air was flown radially outward over a Reynolds number range of 15,000 to 50,000. The rotation number was varied from 0 to 0.3. Stationary and rotating cases of identical geometries were compared. Results indicated that rotational effects are more pronounced in turbulated passages of high aspect and low blockage ratios for which a steady increase in heat transfer coefficient is observed on the trailing side as rotation number increases while the heat transfer coefficient on the leading side shows a steady decrease with rotation number. However, the all-smooth-wall classical pattern of heat transfer coefficient variation on the leading and trailing sides is not followed for smaller aspect ratios and high blockage ratios when the relative artificial roughness is high.
The aero-thermal performance of a typical turbine blade three-pass turbulated cooling circuit geometry was investigated in a 10X plexiglas test model. The model closely duplicated the blade’s leading edge, midchord and trailing edge cooling passage geometries. Steady state heat transfer coefficient distributions along the blade pressure side wall (convex surface) of the cooling circuit passages were measured with a thin-foil heater and a liquid crystal temperature sensor assembly. The heat transfer experiments were conducted on rib-roughened channels with staggered turbulators along the convex and concave surfaces of the cooling passages. Mid-channel axial velocity and turbulence intensity measurements were taken by hot wire anemometry at each passage end of the three-pass cooling circuit to characterize and relate the local thermal performance to the turbulence intensity levels. The near-atmospheric experimental data are compared with results of a Computational Fluid Dynamics (CFD) analysis at the operating internal environment for a IX rotating model of the blade cooling circuit and other turbulator channel geometry heat transfer data investigations. The comparison between the measurements and analysis is encouraging. Differences with other heat transfer data appear reasonably understood and explainable.
Experimental investigations have shown that the enhancement in heat transfer coefficients for air flow in a channel roughened with angled ribs is on the average higher than that roughened with 90 deg ribs of the same geometry. Secondary flows generated by the angled ribs are believed to be responsible for these higher heat transfer coefficients. These secondary flows also create a spanwise variation in heat transfer coefficient on the roughened wall with high levels of heat transfer coefficient at one end of the rib and low levels at the other end. In an effort basically to double the area of high heat transfer coefficients, the angled rib is broken at the center to form a V-shaped rib, and tests are conducted to investigate the resulting heat transfer coefficients and friction factors. Three different square rib geometries, corresponding to blockage ratios of 0.083, 0.125, and 0.167, with a fixed pitch-to-height ratio of 10, mounted on two opposite walls of a square channel in a staggered configuration, are tested in a stationary channel for 5000 < Re < 30,000. Heat transfer coefficients, friction factors, and thermal performances are compared with those of 90 deg, 45 deg, and discrete angled ribs. The V-shaped ribs are tested for both pointing upstream and downstream of the main flow. Test results show that: (a) 90 deg ribs represent the lowest thermal performance, based on the same pumping power, and is essentially the same for the 2:1 change in blockage ratio, (b) low-blockage-ratio (e/Dh = 0.083) V-shaped ribs pointing downstream produced the highest heat transfer enhancement and friction factors. Among all other geometries with blockage ratios of 0.125 and 0.167, 45 deg ribs showed the highest heat transfer enhancements with friction factors less than those of V-shaped ribs, (c) thermal performance of 45 deg ribs and the lowest blockage discrete ribs are among the highest of the geometries tested in this investigation, and (d) discrete angled ribs, although inferior to 45 deg and V-shaped ribs, produce much higher heat transfer coefficients and lower friction factors compared to 90 deg ribs.
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