Numerical simulations are performed to investigate the effect of wall-slip on natural convection inside a square cavity for Rayleigh number (Ra) = 105. The bottom wall is heated and top wall is maintained at a cold temperature keeping the other two vertical walls adiabatic. The analysis is conducted by varying the Slip factor (SF) from 0 (no-slip) to 1 (full-slip) in steps of 0.2. For each value of SF, the flow patterns are visualised using streamlines and heat transfer behaviour is examined using isotherms. Results are quantified using local Nusselt number (Nul ) and average Nusselt number (Nuavg ) along the hot wall. It is observed that for SF=0.8 and SF=1 configurations, the natural convection patterns and heat transfer characteristics undergo notable changes.
Numerical simulations are performed to deduce the effects of slip wall and orientation on entropy generation due to natural convection in a square cavity for Rayleigh number =100000 . The laterally insulated square cavity, heated at the bottom wall and cooled at the top wall, is subjected to various orientation angles and slip velocities characterised by Knudsen number. The two components of entropy generation i.e. entropy generation due to heat transfer and entropy generation due to fluid friction are separately investigated by varying the orientation from 0 degree to 120 degree and Knudsen number from 0 (no-slip) to 1.5. Evidences indicate that, for most of the cases considered, entropy generation due fluid friction dominates the one due to heat transfer. It is observed that the slip velocity on the isothermal walls has a strong influence on entropy generation due to heat transfer whereas, the variations in entropy generation due to fluid friction are closely connected to the change in the rate of shear strain. Interestingly, the presence of corner vortices and the secondary circulations near to the core of the cavity are also found to affect the variation in entropy generation. The existence of active zones of entropy generation due to heat transfer in the vicinity of isothermal walls and their elongation and migration while changing the orientation is another unique characteristic noticed in the present study. A new parameter called MVR (Maximum Velocity Ratio) is also proposed to highlight the variation in velocity components within the enclosure.
This study aims to evaluate adiabatic and conjugate effusion cooling effectiveness of combustion chamber liner plate of gas turbines. Validation of the adiabatic model was done by comparing computational fluid dynamics (CFD) result with the experimental results obtained using the subsonic cascade tunnel facility available at Heat Transfer Lab of Council of Scientific and Industrial Research-National Aerospace Laboratories (CSIR-NAL). Computational simulation of the conjugate model is validated against published numerical results. Numerical simulation for the adiabatic cooling effectiveness is carried out for a 1:3 scaled up flat plate test geometry, while the actual flat plate geometry is considered for the conjugate cooling effectiveness analysis. The test plate has 11 rows of cooling holes, and the holes are arranged in staggered manner with each row containing eight holes. For both adiabatic and conjugate cases, the same mainstream conditions are maintained with the inlet temperature of 329 K, velocity of 20 m/s, density ratio 1.3. The coolant to mainstream blowing ratios (BRs) are maintained at 0.4, 1.15, and 1.6. The coolant temperature is 253 K with the flow rates are according to the BRs. Cooling effectiveness is obtained by using CFD simulation with ANSYS fluent package. From the comparison of adiabatic and conjugate results, it is found that conjugate model is giving superior cooling protection than the adiabatic model and effusion cooling effectiveness increases with increase in BR. Investigations on comparison of angle of injection holes show that, 30 deg model give maximum effusion cooling effectiveness as compared to 45 deg and 60 deg models.
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