Impingement and film cooling techniques are emerging areas in the turbine component cooling technology. Vast amount of information on this subject is available in the literature and mist assisted turbine blade cooling has been attempted by many researchers since last decade. This article presents a comprehensive review focusing on the mist assisted impingement and film cooling of turbine blades. Common to all the mist assisted works, the addition of water mist with the regular air/steam coolant is beneficial for the cooling phenomena which was observed from the lower surface temperature for the mist cases. Larger mist concentration in the coolant mixture and smaller droplets are advantageous as that results in higher evaporation and better cooling. Better cooling performance is observed with uniform droplet size distribution compared to that for the non-uniform droplet size distribution. During mist assisted impingement cooling, the effects of parameters such as mist loading fraction (f), droplet diameter (dp), geometry of the impinging surface, Reynolds number (Rep), heat flux on the surface, number of holes, number of rows of holes, confinement (nozzle to plate ratio) working conditions (elevated and laboratory) etc. on the cooling performance are discussed in detail. Similarly, comprehensive analysis of effects of mist characteristics (mist concentration, droplet diameter, mist temperature), coolant characteristics (blowing ratio, temperature, relative humidity), geometrical characteristics (curved, flat, ribbed surfaces, presence of deposition) etc. on the film cooling performance are featured in this article. Information of this review will be useful as a reference for researchers and engineers interested in working with energy efficient gas turbine component cooling technology.

Purpose The purpose of this study is to explore the mist-air film cooling performance on a three-dimensional (3-D) flat plate. In mist-air film cooling technique, a small amount of water droplets is injected along with the coolant air. The objective is to study the influence of shape of the coolant hole and operating conditions on the cooling effectiveness. Design/methodology/approach In this study, 3-D numerical simulations are performed. To simulate the mist-air film cooling over a flat plate, air is considered as a continuous phase and mist is considered as a discrete phase. Turbulence in the flow is accounted using Reynolds averaged Navier–Stokes equation and is modeled using k–e model with enhanced wall treatment. Findings The results of this study show that, for cylindrical coolant hole, coolant with 5% mist concentration is not effective for mainstream temperatures above 600 K, whereas for fan-shaped hole, even 2% mist concentration has shown significant impact on cooling effectiveness for temperatures up to 1,000 K. For given mist-air coolant flow conditions, different trend in effectiveness is observed for cylindrical and fan-shaped coolant hole with respect to main stream temperature. Research limitations/implications This study is limited to a flat plate geometry with single coolant hole. Practical implications The motivation of this study comes from the requirement of high efficiency cooling techniques for cooling of gas turbine blades. This study aims to study the performance of mist-air film cooling at different geometric and operating conditions. Originality/value The originality of this study lies in studying the effect of parameters such as mist concentration, droplet size and blowing ratio on cooling performance, particularly at high mainstream temperatures. In addition, a systematic performance comparison is presented between the cylindrical and fan-shaped cooling hole geometries.

In the last few decades, alternate and renewable energy harvesting techniques, such as Concentrated Solar Power (CSP), have attracted attention as a unique source of electricity. However, the intermittent nature of solar energy results in discontinuous electricity generation. Thus, the integration of a Latent Heat based Thermal Energy Storage (LHTES) system will help in managing this issue. This numerical study explores the heat storage and discharge abilities of Phase Change Material (PCM) to design an efficient energy storage system. In this study, a 2D novel geometrical model is introduced to enhance the performance of PCM based heat exchanger module. Finite Volume Method (FVM) has been used to simulate the complex phase change problem via ANSYS Fluent solver. The double pipe heat exchanger with concentric cylinders is considered as the base case and multiple design modifications with augmented heat transfer areas have been provided. Comparison of different configurations by varying geometrical parameters with base case is carried out. The improvement in the rate of phase change is observed as the base case tube is replaced by semi-circular tubes in the system. The reduction in sensible heat storage is large during initial stages of charging, a reduction is observed as 46%, 43.5% and 48.9% respectively for all three cases. The time taken to achieve a maximum storage and discharge effectiveness of 82%, 85% respectively is less for novel configurations as compared to base case. It is observed that both novel configurations can be useful to store and release faster which can be used in meeting the energy demand on time.

Purpose This study aims to elucidate the role of curved walls in the presence of identical mass of porous bed with identical heating at a wall for two heating objectives: enhancement of heat transfer to fluid saturated porous beds and reduction of entropy production for thermal and flow irreversibilities. Design/methodology/approach Two heating configurations have been proposed: Case 1: isothermal heating at bottom straight wall with cold side curved walls and Case 2: isothermal heating at left straight wall with cold horizontal curved walls. Galerkin finite element method is used to obtain the streamfunctions and heatfunctions associated with local entropy generation terms. Findings The flow and thermal maps show significant variation from Case 1 to Case 2 arrangements. Case 1 configuration may be the optimal strategy as it offers larger heat transfer rates at larger values of Darcy number, Da m . However, Case 2 may be the optimal strategy as it provides moderate heat transfer rates involving savings on entropy production at larger values of Da m . On the other hand, at lower values of Da m (Da m ≤ 10 −3 ), Case 1 or 2 exhibits almost similar heat transfer rates, while Case 1 is preferred for savings of entropy production. Originality/value The concave wall is found to be effective to enhance heat transfer rates to promote convection, while convex wall exhibits reduction of entropy production rate. Comparison between Case 1 and Case 2 heating strategies enlightens efficient heating strategies involving concave or convex walls for various values of Da m .

Flow and heating characteristics associated with heat transfer rates vis-a-vis entropy generation based on flow and thermal irreversibilities are numerically studied for thermal convection in the porous bed confined within various configurations of triangular enclosures. The triangular configurations involve various vertex angles (phi = 45 degrees and 60 degrees) with three tilt positions (configurations 1-3) subjected to a linear heating pattern of the slanted walls. Numerical studies are carried out for various dimensionless parameters (Pr-m and Da(m)) at fixed Ra-m. Simulation results infer that the triangular configuration 3 [configuration 3: triangle is tilted perpendicularly from horizontal position (configuration 1 or 2)] involving phi - 60 degrees is the optimal configuration based on the larger rate of heat transfer and moderate entropy production at larger porosities (Da(m) >= 10(-3)) for buoyancy dominant scenarios (Ra-m = 10(6)) irrespective of Pr-m.

This paper deals with the experimental investigation of convection heating of air inside an experimental chamber similar to a room prototype. A solid-liquid phase change material (PCM) is used for space heating of the room. A PCM carrier made up of aluminum is placed at various locations in the experimental chamber. The PCM is heated in a water bath and the water in the bath can be heated via solar energy. A theoretical insight highlighting the usage of the current heating system involving the water bath and solar energy is also briefly discussed. The analysis is carried out for various temperatures representing mild winter temperatures in most of the regions in India. Temperature measurements were performed inside the air cavity as well as near the PCM carrier during the air heating process. Three different PCM carriers with different aspect ratios have been used to test the effect of different orientations on space heating. Further, the PCM carrier was kept at two different locations inside the chamber to observe the effect of location on space heating. A new dimensionless temperature has been defined to compare the effect on room air with each PCM configuration.

Heat and fluid flow trajectories are visualized during natural convection subjected to linear heating of the inclined walls involving two representative inclination angles (phi = 45 degrees and phi = 60 degrees) for different configurations (configurations 1 - 3) of the porous isosceles triangles. Heat transport to the core and cold section of inclined walls due to linear heating is studied using heatlines via tuning various dimensionless parameters Pr-m, Da(m) at fixed Ra-m. Local heat transfer rates are found by evaluating Nu at the inclined walls. Overall, configurations 2 and 3 are recommended based on larger magnitudes of local heat transfer rates at middle regime of the walls, irrespective of phi and Pr-m at Da(m) = 10(-2):

Film cooling is a well known technique used to cool the gas turbine blades from hot combustion gases.In this study, a more prominent technique called mist assisted film cooling is numerically investigated.Water droplets are injected in to the cooling air to enhance the cooling performance.The cooling performance is investigated downstream the film cooling hole in a straight channel.The coolant jet (dry air) is supplied via the film cooling hole along with water droplets injected at same velocity and temperature.The computational simulations are performed using the commercial software ANSYS FLUENT 17.2.As the concentration difference exists between the droplet surface and bulk air, the evaporation takes place and before reaching to the mainstream, the water droplets vaporize to form water vapors.Thus, the temperature of the coolant further reaches a lesser temperature.This phenomenon does not occur when the coolant is pure dry air and this part is addressed for the first time in this study.Results showed that the overall cooling effectiveness is always larger for the mist-air case compared to that for the pure dry air case.At a specific mass flow rate and specific mist percentage, the optimum angle of inlet coolant is found to be 35 degrees.Also, at a specific mass flow rate and specific mist percentage, increase in the droplet diameter results in the decrease of the cooling effectiveness.It is also found that increasing the number of holes to two, increases the cooling effectiveness.

The computational study of natural convection within enclosures with curved (concave/convex) walls (curved side walls or top and bottom walls) is carried out via entropy generation analysis. Two heating strategies are considered such as: (a) differential and (b) Rayleigh-Benard heating. The numerical simulation has been carried out for air at Prandtl numbers, Pr = 0.7 for various Rayleigh numbers (103 ≤ Ra ≤ 105) with different wall curvatures. A comparative study of the heating strategies is carried out in detail for a number of test cases with different wall curvatures. Finally, the optimal situation is recommended based on the less entropy production and and high heat transfer rate.

The inlet air temperature of turbines are significantly high which may result in the damage of the blade material. As a consequence, it is required to cool the turbine blades and a number of cooling techniques are introduced in the past. The cooling technique involving evaporation of water droplets is the motivation and focus of this work. When the water droplets are injected along with the dry air, the existing concentration difference between the water droplets and dry air results in the evaporation leading in the drop of the coolant temperature. This phenomena does not occur when the coolant is dry air and this part is addressed for the first time in this study. The cooling performance is investigated on wall of straight channel in the presence of a film cooling hole. Dry air along with water droplets are supplied through the film cooling hole and the computations are carried out for different droplet diameters and mist concentrations. Results showed that, the overall cooling effectiveness is always larger for the mist-air case compared to that for the dry air case. Also, the cooling effectiveness increases with the percentage of mist in the mist-air system for a specific droplet diameter. At a specific mass flow rate and specific mist percentage, the increase in droplet diameter results in the decrease of cooling effectiveness.

In the mist assisted film cooling, water droplets are injected along with the coolant air. The concentration difference between the water droplets and the coolant air results in the evaporation of the former leading to drop in temperature and enhancement of the cooling efficiency. In the present work, a new definition for mist-air film cooling effectiveness is proposed and the performance of mist assisted film cooling is investigated on the wall of straight channel in the presence of a film cooling hole. Detailed two dimensional computational studies are carried out for various droplet diameter, relative humidity of air and mist concentrations. Results show that the film cooling effectiveness is always larger for the case of mist-air compared to that of the dry air. Further, the film cooling effectiveness increases with the percentage of mist in the mist-air system. At a specified mass flow rate of dry air and various quantities of mist, the increase in droplet diameter results in the decrease of cooling effectiveness.The cooling effectiveness increases with the relative humidity of air at all percentages of mist.

Natural convection within enclosures in the presence of isothermal curved walls involving (a) differential heating and (b) Rayleigh-Benard heating is considered for investigation. The two heating strategies are compared based on the distributions of fluid flow, heat flow and local or average heat transfer rates for various Da(m) (modified Darcy number) and Pr-m (modified Prandtl number) at the high Ra-m (modified Rayleigh number). Unidirectional circulation cells occur for differential heating whereas four (combination of clockwise and anticlockwise) circulation cells occur for Rayleigh-Benard heating. Consequently, unidirectional heatline cell occurs at the center for differential heating and four heatline circulation cells occur for Rayleigh-Benard heating specially at the high . Multiple heatline cells lead to larger thermal mixing for Rayleigh-Benard heating. The heat transfer rate is larger for differential heating case and the percentage enhancement of Nusselt number is calculated in terms of the gain in heat transfer rate for differential heating (E). The gain in heat transfer rate in terms of E involving concave and convex cavities (cases 1 and 2) is found to be the strong function of Da(m) and Pr-m at the high Ra-m.

Purpose This paper is aimed to study natural convection in enclosures with curved (concave and convex) side walls for porous media via the heatline-based heat flow visualization approach. Design/methodology/approach The numerical scheme involving the Galerkin finite element method is used to solve the governing equations for several Prandtl numbers (Prm) and Darcy numbers (Dam) at Rayleigh number, Ram = 106, involving various wall curvatures. Finite element method is advantageous for curved domain, as the biquadratic basis functions can be used for adaptive automated mesh generation. Findings Smooth end-to-end heatlines are seen at the low Dam involving all the cases. At the high Dam, the intense heatline cells are seen for the Cases 1-2 (concave) and Cases 1-3 (convex). Overall, the Case 1 (concave) offers the largest average Nusselt number ( Nur¯) at the low Dam for all Prm. At the high Dam, Nur¯ for the Case 1 (concave) is the largest involving the low Prm, whereas Nur¯ is the largest for Case 1 (convex) involving the high Prm. Practical implications Thermal management for flow systems involving curved surfaces which are encountered in various practical applications may be complicated. The results of the current work may be useful for the material processing, thermal storage and solar heating applications Originality/value The heatline approach accompanied by energy flux vectors is used for the first time for the efficient heat flow visualization during natural convection involving porous media in the curved walled enclosures involving various wall curvatures.

The present study deals with the finite element based numerical simulations of heat transfer and entropy generation rates during natural convection for fluid saturated porous media in enclosures involving curved walls (case 1: lower curvature and case 2: higher curvature) with various thermal boundary conditions. The differential heating (isothermally hot left wall and cold right wall and adiabatic horizontal walls) and Rayleigh-Benard heating (isothermally hot bottom wall and cold top wall involving adiabatic left and right walls) are considered. The locations and magnitudes of the entropy generation due to heat transfer (S-theta) and fluid friction (S-psi) are presented and discussed based on the spatial distributions of isotherms and streamlines, respectively. The magnitudes of local entropy generation (S-theta, S-psi), total entropy generation (S-total) and average heat transfer rates ((N-ur) over bar and (N-ut) over bar) are significantly lesser for the Rayleigh-Benard heating compared to the differential heating for all the cases involving all Da(m) and Pr-m. The Rayleigh-Benard heating is the optimal strategy for all Da(m), and Pr-m, involving both the concave cases except for 10(-3) <= Da(m) <= 10(-2), Pr-m = 10 and case 1 (concave) domain. The Rayleigh-Benard heating is also the optimal strategy compared to the differential heating involving the convex cases at 10(-5) <= Da(m) <= 10(-4) whereas the differential heating is the optimal heating strategy for Da(m) >= 10(-3) involving both Pr-m for the convex cases. (C) 2018 Elsevier Ltd. All rights reserved.

The bulk motion of fluid and diffusive transport within fluid are two processes during natural or forced convection. The complexity of the convective heat flow is realized since last few decades and the analysis of the heat flow as well as thermal characteristics gradually becomes cumbersome. Although earlier researchers studied convective heat flow via velocity profiles, streamlines and isotherms, these tools were not enough for the efficient visualization of the unique features of convection heat flow. An efficient tool, termed as ‘heatline’ (mathematically represented as heatfunction) was first proposed by Kimura and Bejan in 1983 for the heat flow visualization during convective heat flow. The aim of this article is to review existing works on ‘heatline’ involving various physical systems. The mathematical implications of heatfunctions based on derivations of governing equations and boundary conditions for heatfunctions are presented in detail. The non-homogeneous boundary conditions for heatfunctions arise due to hot or cold or adiabatic walls as well as the junction between the walls and these conditions vary with the location of the reference or datum of the heatfunction. The physics on the heat flow via ‘heatlines’ are found to be invariant with the locations of the reference value of the heatfunction. The heat flow visualization is analyzed for various test cases from simple one dimensional boundary layer problem to convection in two dimensional complex cavities. The detailed explanations of earlier works on ‘heatlines’ during one dimensional flow involving forced or natural convection with various applications are discussed. Further, applications of ‘heatlines’ during convective heat flow within enclosed cavities involving uniform or non uniform heating of walls, discrete heating or cooling, conjugate convection and mixed convection are discussed and ‘heatlines’ are found to be successful to demonstrate various complex heat flow paths and multiple heat flow circulation cells. Overall, the analysis of convective heat flow from simple to complicated geometries via ‘heatline’ is crucial for the visualization of the thermal transport, mixing and efficient thermal management.

The investigation of entropy generation is highly desirable for the optimization of the thermal systems to avoid larger energy wastage and ensure higher heat transfer rate. The numerical investigation of natural convection within enclosures with the concave and convex horizontal walls involving the Rayleigh-Benard heating is performed via entropy generation approach. The spatial distributions of the temperature (theta), fluid flow (psi), entropy generation due to heat transfer and fluid friction (S-theta and S-psi) are discussed extensively for various Rayleigh numbers and Prandtl numbers involving various wall curvatures. A number of complex patterns of spatial distributions of fluid flow and temperature for cavities with concave or convex isothermal walls (top and bottom) have been obtained. The zones of high entropy generation for temperature and fluid flow are detected within cavities with concave and convex horizontal walls. The optimal situation involves the high heat transfer rate with moderate or low entropy generation. Overall, case 3 (highly concave) is found to be optimal over cases 1 and 2 (concave) and cases 1-3 (convex) for all Pr and Ra.

The comprehensive analysis of natural convection within various enclosures with the concave and convex horizontal walls involving the classical Rayleigh-Benard heating (hot bottom wall, cold top wall and adiabatic side walls) is carried out via the entropy generation approach. Galerkin finite element method is employed to obtain the solutions in terms of the isotherms (theta), streamlines (Psi), entropy generation (S-theta, S-Psi, and S-total) and average heat transfer rates (Nu(t)) over bar for a selected range of Darcy numbers (10(-5) <= Da(m) <= 10(-2)), Prandtl numbers (Pr-m = 0.015 and 7.2) at Rayleigh number, Ra-m = 10(6) with various test cases with different wall curvatures (concave and convex). The magnitudes and spatial locations of local entropy generation due to heat transfer and fluid friction are studied for various thermal and geometrical parameters. The active zones with larger S-theta are found along near core region and middle portions of the curved walls with less and moderate concavities whereas, those are found only near the middle portions of the curved walls with the high concavity at the low Da(m), involving all Pr-m. In the convex domain, the active zones with larger S-theta are seen near the corner regions for the low Da(m) at all Pr-m. The zones prone to the entropy generation with the high S-theta are seen along the mid-horizontal axis of the cavity involving all the concave and convex domains at the high Da(m) and all Pr-m. Also, the left and right portions of the curved walls act as the active zones with the high S-theta for the concave domains at the high Da(m) and all Pr-m. The larger values of S-Psi are seen near all the solid walls (at all Da(m)) and at the interior zone (at the high Da(m)) for all Pr-m involving all concave and convex cases. Based on the comparison of all the concave and convex cases, the concave case with the high wall concavity may be preferred over the specific concave cases (with the less and moderate wall concavities) and all the convex cases due to the larger (Nu(t)) over bar and less S-total values for the entire range of Da(m) and Pr-m. (c) 2017 Elsevier Ltd. All rights reserved.

This paper analyzes the fluid and heat flow within entrapped porous triangular cavities involving various thermal boundary conditions (case 1: hot horizontal walls and cold inclined walls; case 2: cold horizontal walls and hot inclined walls). Finite element based numerical study has been performed for various values of Prandtl number (Pr-m = 0.026 and 7.2), Darcy number (Da(m) =10(-4) 10(-2)), Reynolds number (Re = 1 1000) at a high value of Grashof number (Gr = 10(5)). The upper cavity for the case 1 and the lower cavity for the case 2 show almost similar trends and vice versa based on the exact opposite thermal boundary conditions. Heat transfer is conduction dominant at Da(m) =10(-4) involving both Pr-m whereas, convection heat transfer dominates at Da(m) =10(-2) especially for Pr-m = 7.2. Although the motion of the horizontal walls significantly influences the fluid flow field within the cavities, due to the decoupling between the fluid and thermal fields at the low Pen, (Pe(m) = 0.026, 0.26 and 2.6), conduction dominant heat transfer occurs. At the high Da(m) and Pr-m = 7.2, a pair of symmetric streamline or heatline cells are seen for Re <= 1 whereas, a bigger primary streamline or heatline cell is accompanied by a secondary streamline or heatline cell for 1 <= Re <= 100. At 500 <= Re <= 1000, the single larger streamlines or heatline cells are seen especially for Pr-m = 7.2 involving Da(m) = 10(-2). At Pr-m = 0.026, the average heat transfer rates ((Nu(t)) over bar, (Nu(b)) over bar, (Nu(l)) over tilde, and (Nu(r)) over bar) are almost constant with Re and Da(m). At Pr-m = 7.2, the larger magnitudes of (Nu(r)) over bar and (Nu(l)) over tilde are observed for Re <= 10 compared to those for Re = 100 involving Da(m) >= 10(-3) whereas, the magnitudes of (Nu(r)) over bar are almost similar for all Re involving all Da(m) in the upper cavity (case 1) within Re <= 100. In the lower cavity (case 1), the magnitudes of (Nu(b)) over bar and (Nu(r)) over bar, are larger for Re = 100 whereas, the magnitudes of (Nu(l)) over bar are larger for Re <= 1 for almost entire range of Da. within Re <= 100 involving Pr-m = 7.2. The average Nusselt numbers are significantly larger at the high Re (Re = 500 and 1000) involving all Da(m) (except for the horizontal walls involving Da(m) <= 8 x 10(-4)) in the cases 1 and 2 at Pr-m = 7.2 (C) 2016 Elsevier Ltd. All rights reserved.

A comprehensive analysis based on the irreversibilities associated with the energy flow and entropy generation is highly essential for the optimization of thermal systems. Entropy generation during mixed convection process has been studied in entrapped triangular cavities for moving horizontal walls involving isothermally hot inclined walls and cold horizontal walls (case 1) or isothermally cold inclined walls and hot horizontal walls (case 2). Overall it is found that, Re = 100 may be preferred over Re → 0, Re = 1 and Re = 10 at Pr = 0.026 and 7.2, Gr = 103 − 105 within the cavities, irrespective of the cases. In addition to Re = 100, Re = 10 may be optimal for the upper cavity with case 1 and lower cavity with case 2 at Gr ≈ 105 (higher Gr regime) and Pr = 7.2 based on moderate heat transfer rates.