Seok Min Choi

Effect of Unsteady Wakes on Local Heat Transer of 1st Stage Blade Endwall

We investigated the local heat and mass transfer on the endwall surface at different Strouhal numbers. The Strouhal number represents the interaction between the turbine stator and rotor. Thus, characteristics of flow and heat transfer on the endwall surface change for different Strouhal numbers. The experiment was performed in five-bladed linear cascade blades with moving cylindrical rods simulating unsteady wakes effects. The Reynolds number which was based on the blade cord length and inlet velocity was 100,000. The range of the Strouhal number varies from 0 to 0.22 to investigate the effect of unsteady wakes on the endwall surface. The local heat and mass transfer were measured on the endwall surface using the naphthalene sublimation method. The results showed that the local heat transfer characteristics changed depending on the Strouhal number. In the steady case, the occurrence of a horseshoe vortex, passage vortex and corner vortex caused non-uniform heat transfer on the endwall surface. However, in the unsteady case, the unsteady wake effect caused more uniform heat transfer on the endwall surface. As the Strouhal number increased, heat transfer increased and became more uniform compared with the steady case. Thus, an appropriate cooling system is necessary for stator and rotor endwalls in situations of real gas turbine operation.© 2015 ASME

Conjugate Heat Transfer on Full-Coverage Film Cooling With Array Impingement Jets

We report an investigation of the total cooling effectiveness of a film cooled surface with staggered array impingement jet cooling using infra-red thermography. Heat transfer experiments were carried out using three film cooled test plates of different thermal conductivities: stainless steel (with a thermal conductivity, k = 13.4 W/mK), Corian® (k = 1 W/mK), and polycarbonate (k = 0.2 W/mK). The effects of conduction through the test plates and convective heat transfer due to the arrayed impingement jets were analyzed. The inclination angle of the film cooling holes was 35° and that of the impingement jet holes was 90°. The film and impingement jet holes on each plate were arranged in a staggered pattern, and the film cooling holes and impingement jet holes were also positioned in a staggered pattern. The jet Reynolds number based on the hole diameter was Rejet = 3,000 and the equivalent blowing rate was M = 0.3. The ratio of the target surface height to the hole diameter was varied in the range 1 < H/d < 5. The diameter of both the film cooling holes and impingement jet holes was 5 mm. The total cooling effectiveness was investigated with and without the impingement jets. When the impingement jets were added to the internal cooling, the averaged total cooling effectiveness was enhanced about 8.4%. The stainless steel plate was found to exhibit better cooling performance with more uniform temperature distribution. The total cooling effectiveness was increased up to 0.87 in the stainless steel plate, and the maximum deviation of total cooling effectiveness in the stainless steel was reduced to 85% from that in polycarbonate plate along the lateral direction. The total cooling effectiveness was related to the Biot number of the film cooled plate, however, the effect of the H/d ratio was not significant.Copyright

Augmented Heat Transfer for Angled Rib With Intersecting Rib in Rectangular Channels of Different Aspect Ratios

This study was an experimental investigation of the effect of an intersecting rib on heat/mass transfer performance in rectangular channels with angled ribs and different aspect ratios. In a rib-roughened channel with angled ribs, heat/mass transfer performance deteriorates as the channel aspect ratio increases, since the vortices induced by angled ribs diminish with increasing aspect ratio. A longitudinal rib that bisects the angled ribs is suggested to overcome this disadvantage. The heat transfer performance of angled rib configurations with a 60° attack angle were tested with and without an intersecting rib using naphthalene sublimation method. The channel aspect ratio is varied from 1 to 4. When the intersecting rib was present, additional vortices were generated at every point of intersection with the angled ribs. Thus the heat/mass transfer performance was significantly enhanced for all channel aspect ratios when an intersecting rib was added to an ordinary angled rib configuration.Copyright

Effect of Secondary Flow Direction on Film Cooling Effectiveness

Key Words: Double-jet Film Cooling(이중분사 막냉각), Secondary Flow Channel(이차유로), Film CoolingEffectiveness(막냉각효율), Anti-Kidney Vortex(안티키드니와류), Numerical Analysis(수치해석)초록: 막냉각에관한많은연구들은주유동과이차유로가평행한형태로연구가이루어졌다. 하지만실제터빈블레이드에서이차유로의방향은일반적으로주유동의방향에수직한형태이다. 그래서본연구에서는이차유동의방향이이중분사막냉각의효율에미치는영향을수치해석을통해알아보고자한다. 분사율은1, 2이고횡방향분사각은22.5°이다. 분사율이1일때평행형상에서는안티키드니와류가잘형성되어막냉각효율이수직형상의경우보다더높다. 반면에분사율이2일때수직형상의막냉각효율은평행형상보다향상되었다. 많은유량의제트가서로반대방향으로분사되기때문에두형상모두막냉각효율이높게나타난다. 하지만안티키드니와류의영향은다른분사율보다상대적으로작다.Abstract: Several studies of film cooling were accomplished with a secondary flow channel parallel to themain flow. In real turbine blades, however, the direction of the secondary flow channel is generally normalto the main flow. Thus, this study performs a numerical analysis to investigate the effects of the direction ofsecondary flow on the effectiveness of double-jet film cooling. The blowing ratio is 1 and 2, and the lateralinjection angle is 22.5°. The parallel channel case creates a well-developed anti-kidney vortex with a blowingratio of 1, and the laterally averaged film cooling effectiveness of the parallel channel is enhanced comparedto the normal channel. The normal channel shows higher performance with a blowing ratio of 2. Both casesshow high film cooling effectiveness. These phenomena can be attributed to a high blowing ratio and flowrate rather than an anti-kidney vortex.