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ABSTRACT
An optimal performance film cooling implies a high cooling effectiveness along with a wide surface cooling coverage. During the past decades, film cooling effectiveness has been well defined with a specific formula to evaluate the quality of a cooling system. However, despite numerous research on film cooling, there is still no explicit parameter to quantify how well a coolant film is spread over a hot surface. This work introduces a new coefficient namely cooling uniformity coefficient (CUC) to evaluate how well a coolant film is spread over a surface that is being cooled. Four different cases at two blowing ratios of 0.5 and 1.5 are studied. Three cases are ordinary single jets with different cross sections whereas the fourth one is a novel combined-triple-jet, sometimes called anti-vortex-holes, introduced in a previous work. The cross sections of all four cases were kept the same to have equal coolant mass flow rate. This article suggests that CUC is a necessary parameter in order to have a well performance film cooling system, in addition to the film cooling effectiveness. Indeed, CUC evaluates how well coolant films are spread over hot surfaces which is very important from thermal loads and thermal stress points of view.
Acknowledgments
I would like to express my special thanks of gratitude to my brother, Prof. Aliyar Javadi for his insightful technical discussions while preparing this manuscript. I would also like to thank my friend Dariush Azimi for his help with the preparation of this manuscript.
Nomenclature
| CRVP | = | counter rotating vortex pair |
| CTJ | = | combined triple jets |
| CUC | = | cooling uniformity coefficient |
| D | = | jet hydraulic diameter (12.7mm) |
| = | mass flow rate (kg/s) | |
| p | = | pressure (Pa) |
| Prl | = | laminar Prandtl number |
| Prt | = | turbulent Prandtl number |
| R | = | jet-to-cross flow velocity ratio = Vjet/Vcf, (0.5, 1.5) |
| Re | = | Reynolds number |
| T | = | temperature (K) |
| = | mean velocity components (m/s) | |
| Ui | = | instantaneous velocity components (m/s) |
| ui | = | fluctuative velocity components (m/s) |
| Vcf | = | cross flow velocity (11m/s) |
| Vjet | = | jet velocity (5.5m/s) |
| x,y,z | = | Cartesian coordinate systems in horizontal (streamwise), vertical, and lateral (spanwise) directions, respectively. |
| x/D, y/D, z/D | = | dimensionless lengths in x,y,z direction, respectively |
| y+ | = | wall unit distance |
| Greek symbols | ||
| δ | = | boundary layer thickness |
| η | = | film cooling effectiveness = |
| = | spanwise-avereged film cooling effectiveness | |
| κ | = | turbulent kinetics energy (m/s)2 |
| μ | = | viscosity coefficient (kg/ms) |
| μt | = | eddy viscosity coefficient (kg/ms) |
| ρ | = | density (kg/m3) |
| = | Reynolds stress tensor (kg/ms2) | |
| Subscripts | ||
| aw | = | adiabatic wall |
| cf | = | cross flow |
Additional information
Notes on contributors
Khodayar Javadi
Khodayar Javadi received his BSc and MSc in mechanical engineering from Tabriz University. Afterward, he received his PhD degree in Aerospace Engineering in 2007 from Sharif University of Technology (SUT). During 2007–2009, he collaborated with Max-Planck Institute for Marine Microbiology in Bremen on Flow Control in seabed. Then, he came back to his home country and continued his academic research at Aerospace Research Institute (Ministry of Science Research and Technology) on Life Support Systems Control. He has been holding Assistant Professor position at SUT since 2012. He is now the head of Flow Control and Heat Transfer Research Lab. Since 2007, when he started his collaboration with Max-Plank Institute, he has been collaborating with this institute during past years on different projects. He has several publications in international journals, international conferences, and books in the field of flow control and heat transfer.