B. Premachandran

Numerical Simulation of the Jet Impingement Cooling of a Circular Cylinder

A numerical investigation was carried out on circular jet impingement heat transfer from a constant temperature circular cylinder to understand the major parameters which influence the fluid flow and heat transfer characteristics. In this study, air was considered as the working fluid. The flow was considered to be three-dimensional, incompressible, and turbulent. To select a suitable turbulence model for the parametric study, numerical simulations were carried out with standard k-ϵ, standard k-ω, RNG k-ϵ, Realizable k-ϵ, and SST k-ω turbulence models for modeling Reynolds stress terms. Simulations were also carried out using four low Reynolds number models. The results obtained using these models were compared with the available experimental results of jet impingement heat transfer from circular cylinder. It was identified that the RNG k-ϵ model predicts heat transfer characteristics better compared to all other turbulence models considered in this study. Using this turbulence model, a parametric study was carried out for the Reynolds number (Re d ), defined based on the diameter of the nozzle ranging from 10,000 to 50,000. The ratio of distance between the nozzle exit and the cylinder surface to the diameter of the jet (h/d) was varied from 4 to 16 and the ratio of nozzle diameter to cylinder diameter (d/D) varied from 0.11 to 0.25. For a fixed Re d and d/D, the stagnation point Nusselt number increases as h/d decreases. The stagnation point Nusselt number decreases as d/D increases for a fixed value of Re d and h/d. The effects of change in h/d and d/D are significant only near the stagnation region.

Prediction of film cooling effectiveness over a flat plate from film heating studies

ABSTRACT Film cooling is widely used to protect surfaces exposed to gases at a high temperature in gas turbine engines. Film heating is the reverse of film cooling, where hot secondary fluid is injected onto the walls to protect against a relatively cold mainstream. In the literature, the latter has often been used as an experimental analogue of the former, since mainstream flow rates are substantially higher, and it is relatively simpler to heat the smaller stream of secondary fluid for experiments. In this paper, the results obtained from a numerical study of film cooling and film heating over a flat plate through single-slot injection are presented. Since the objective of the work is to evaluate the suitability of film heating as a proxy for film cooling, it was decided to keep computational simple, using two-dimensional simulations. The effect of a density ratio of injectant-to-mainstream in the range of 0.2–5 is studied numerically to cover film heating and film cooling. Numerical simulations were carried out for three blowing ratios, M = 1, 2, and 3 at a fixed mainstream Reynolds number of 1.5 × 105 for three injection angles, 30°, 45°, and 60°. Numerical simulations were also carried out for a wide range of momentum flux ratio for film heating and film cooling at an injection angle of 30°. The results show that film heating and film cooling are not equivalent, especially when the density ratio deviates from unity substantially. Based on numerical study, it appears possible to predict film cooling effectiveness from film heating effectiveness for a wide range of density ratios, even though the effectiveness values obtained in regard to film cooling and film heating differ significantly.

A Numerical Study on the 2D Film Cooling of a Flat Surface

In this article, the results obtained from a detailed numerical investigation of 2D film cooling over a flat plate through single-slot injection are presented. The effects of mainstream Reynolds number, blowing ratio, density ratio, and injection angle on the effectiveness of film cooling were investigated in the present work. Numerical simulations were carried over a wide range of density ratio ranging from 1.1 to 5 at two mainstream Reynolds numbers (8 × 104 and 1.5 × 105), three blowing ratios (ranging from 1 to 3), and six injection angles (ranging from 15° to 90°). The results show that at lower injection angles of 15°–45°, maximum film-cooling effectiveness occurs at a particular value of velocity ratio which is found to be independent of mainstream Reynolds number, blowing ratio, and density ratio. Based on a combined effect analysis of blowing ratio, density ratio, and injection angle, a relation was obtained for velocity ratio that gives an optimum film-cooling effectiveness.

Analysis of Turbulent Natural and Mixed Convection Flows Using the v2–f Model

The main objective of this work is to investigate the performance of the v2-f model for the predictions of turbulent natural and mixed convection flows. For this purpose, a finite volume based flow solver was developed for the collocated grid arrangement and the v2-f model was implemented for turbulence modeling. For natural convection flows, a tall cavity with the aspect ratio of 28.68 has been selected as a test case. Mixed convection in a square cavity, ascending flow in a vertical pipe with constant wall flux, and fully developed mixed convection in a vertical channel are considered as the test cases for buoyancy dominant mixed convection flows. To evaluate the performance of the v2-f model, results obtained from the present study have been compared with the existing data of experimental studies or Direct Numerical Simulations. Results obtained from the two-equation models, viz., the RNG k-epsilon, Realizable k-epsilon and SST k-omega models are also presented for comparison. Overall, the v2-f model could predict flow and heat transfer characteristics of natural and mixed convection flows very well.

Numerical investigation of the pool film boiling of water and R134a over a horizontal surface at near-critical conditions

ABSTRACT Saturated pool film boiling over a flat horizontal surface is investigated numerically for water and refrigerant R134a at near-critical conditions for wall superheats (ΔTSup) of 2 K, 5 K, 8 K, 10 K, 15 K, and 20 K. The flow is considered to be laminar and incompressible. The governing equations are solved using a finite volume method with a collocated grid arrangement. For capturing the interface in two-phase boiling flows, a Coupled Level Set and Volume of Fluid (CLSVOF) with a multidirectional advection algorithm is used. Both single-mode and multimode boiling models are used for the numerical investigation to understand the effect of computational domain sizes on flow and heat transfer characteristics. In the case of water, the evolution of interface morphology shows the formation of a discrete periodic bubble release cycle occurring at lower Jacob numbers, Jav ≤ 10.2(ΔTSup ≤ 8 K), and the generation of jets of stable vapor film columns occurs at higher Jav ≥ 12.7 (ΔTSup ≥ 10 K). In the case of R134a, for all the Jav values considered in this study (0.163 ≤ Jav ≤ 1.63), the formation of a discrete periodic bubble release is observed. The results show that multimode boiling model should be used to understand the flow characteristics better. The magnitude of average Nusselt number obtained from the multimode film boiling model is lower than that of the single-mode film boiling model. The Nusselt numbers obtained from the present numerical studies are also compared with the available semiempirical correlations.

Numerical investigation of film cooling on a 2D corrugated surface

ABSTRACT A detailed numerical study on the film cooling of a corrugated surface through a single slot has been presented in this paper. The effects of the blowing ratio, density ratio (DR), and injection angle on the film cooling of the corrugated surface are discussed. Numerical simulations are carried out over a wide range of DRs ranging from 0.2 to 5.0 at a fixed mainstream Reynolds number of 1.5 × 105, three blowing ratios of 1, 2, and 3, and five injection angles ranging from 30° to 90°. Results show that the velocity profile on a corrugated surface is strongly influenced by the injection of the secondary fluid. It is observed that the film cooling effectiveness of the corrugated surface increases monotonically with an increase in the blowing ratio. The density ratio and injection angle also have a strong influence on the film cooling.