The turbine inlet temperature of modern gas turbine engines has been increased to achieve higher thermal efficiency. However, the increased inlet temperature can result in material failure of the turbine system due to the higher heat transfer and induced thermal stresses. In order to overcome the potential problem from the high temperature environment and prevent failure of turbine components, effective blade cooling is essential. Blade cooling is performed by film cooling at the external surface of the turbine blade and also by internal forced-convection cooling which uses winding flow passages inside the turbine blade. Internal cooling passages that are mostly modeled as square or rectangular channels with various aspect ratios. Several methods are used to increase the heat transfer coefficient inside the channels. The passive method of heat transfer enhancement is based on two main strategies: disturbing the thermal boundary layer and using bulk fluid mixing. Destruction and restarting of the boundary layer in presence of roughness elements causes an increase in heat transfer by producing a boundary layer that is thinner on average than the uninterrupted boundary layer. Vortices in the flow can enhance heat transfer through bulk fluid mixing which reduces temperature gradients in the core flow concentrating thermal gradients in the near wall region. Such mixing can be effected using ribs, vertex generators, dimples, or surface bumps. But the increase in heat transfer is also accompanied by increase in pressure drop.

Rib-turbulators are often cast on both walls of the internal cooling passages that are mostly modelled as square or rectangular channels with various aspect ratios. Cooling enhancements induced by the ribs are fairly straightforward for stationary airfoils which are due to the additional secondary flow from the ribs, but become much more complicated in rotating blade channels due to the effects of rotation, coolant velocity, and buoyancy which may result in strong secondary flows and even local flow reversal. V.S. Hans et al. [1] conducted experimental investigations of heat and fluid flow in a rectangular duct with multiple V-ribs on one broad wall subjected to uniform heat flux and found an increase Nusselt number and friction factor. They also developed the correlations for Nusselt number and friction factor in terms of roughness geometry and flow parameters V. Sri Harsha et al. [2] studied the effect of rib height to the hydraulic diameter ratio on the local heat transfer distributions in a double wall ribbed square channel with 900 continuous attached and 60° V-broken ribs .The effect of detachment of ribs in case of broken ribs on the heat transfer characteristics is also presented. They observed that the heat transfer augmentations in the channel with 900 continuous

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the pressure drop across the test section. The enhancements caused by 60° V-broken ribs are higher than those of 90° continuous attached ribs and also result in lower pressure drops. Pongjet Promvonge [3] conducted experiments to assess turbulent forced convection heat transfer and friction loss behaviours for air flow through a channel fitted with multiple 60° V-baffle turbulator. The experimental results show that the V-baffle provides the drastic increase in Nusselt number, friction factor and thermal enhancement factor values over the smooth wall channel due to better flow mixing from the formation of secondary flows induced by vortex flows generated by the V-baffle. Substantial increase in Nusselt number and friction factor values are found for the rise in blockage ratio and for the decrease in pitch ratio values Teerapat Chompookham et al. [4] study the effect of combined wedge ribs and winglet type vortex generators (WVGs) on heat transfer and friction loss behaviours for turbulent air flow through a constant heat flux channel. Wedge ribs (right angle ribs) create reverse flow in the channel whereas winglet type vortex generators (WVGs) generate longitudinal vortex flows through the tested section. The presence of the combined ribs and the WVGs shows the significant increase in heat transfer rate and friction loss over the smooth channel. In conjunction with the WVGs, the in-line wedge pointing downstream provides the highest increase in both the heat transfer rate and the friction factor. Sukhmeet Singh et al.[5] in this investigation, thermo-hydraulic performance of rectangular ducts roughened with a new configuration of V-down rib having a small symmetrical gap equal to rib height created at the centre of both legs of V of continuous V-down rib. The symmetrical gap equal to rib height in both legs of V results in substantial improvement in the thermo-hydraulic performance parameter based on equal pumping power.. S. C. Lau, et al. [6] examined turbulent heat transfer and friction characteristics of fully developed flow of air in a square channel in which two opposite walls are roughened with aligned arrays of V-shaped ribs. The angles of-attack of the V-shaped rib arrays are 45°, 60°, 90°(same as 90° full ribs) 120°, and 135°. From this investigation, gives the 60° V shaped ribs with p/e = 10 have the highest ribbed wall heat transfer and smooth wall heat transfer for a given air flow rate, and the highest channel heat transfer per unit pumping power. Lanjewar et al (7) experimentally investigated on heat transfer and friction factor characteristics of a rectangular duct roughened with W shaped ribs arranged at an inclination with respect to the flow direction and found that thermo hydraulic performance of W –down arrangement with angle of attack of flow as 600 gives the best thermo hydraulic performance.

Experiments are performed in an experimental set-up as depicted in Fig. 3.1. The experimental system measurement of the pressure across the test section, variable speed blower having the power of about 600watts no load speed of blower is 0-16000rpm air volume ratio is 3.5 m3/min weight of the blower is around 1.7 kg, An orifice meter, venture meter, U-tube manometer of is used for the measurement differential pressure head across the venture meter ,a differential manometer with a combination of water and benzyl alcohol (specific gravity = 1.046) as the monomeric fluids connected to the two pressure taps of the test section to measure the pressure drop across the test section, and gate valve used to regulate and control the compressed air flow rate, ribs are glued on the bottom surface of square duct by strong adhesives for roughening the duct.

Blower sucks the air; air enters into the test section through the valve which controls the flow of air through the test section. Using venture meter and simple U-tube manometer the flow rate of air is maintained to required value, when air flow through the test section there is friction between air and surfaces of the square channel. Due to this pressure drop takes place. This pressure drop across the test section is measured by using Micro Differential manometer. Same procedure is followed to obtained friction factor for different configurations of the ribs which are glued on the bottom surface of the square duct. But for rib roughened surface simple manometer can be used to measure the pressure drop across the test section.

Square- shaped ribs are mounted into the bottom surface of this channel to analyze the flow characteristics‘ in the square duct The sides of the channel are denoted as L, H and W and channel aspect ratio (AR) is W/H The geometrical parameters of ribs influencing the friction factor performance are shown in Fig.3.2. These parameters can be defined as Axial pitch (p): the axial distance between the

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configuration.

Ribs height (e): the normal distance between the top of the rib surface and the surface of the wall upon which it is glued is the rib height. Length of ribs (l): base length of ribs at which it glued upon surface of the wall. The non-dimensional parameters are; Rib height to channel hydraulic diameter ratio (e/Dh): this is the ratio of the height of the rib, measured above the surface upon which they are glued to the channel hydraulic diameter. Pitch to height ratio (p/e): It is the ratio of the axial distance between two identical points of the adjacent Ribs to the Rib height.

1) Flow rate through venture –meter: Qth = Cd --------(1) A1= Area of pipe; Ao=Area of venture. Cd=co-efficient of discharge. 2) Mass flow rate: m‘= ρQ ---------(2) ρ= density of air 3) The Reynolds number based on the channel hydraulic diameter is given by: Re = ------ (3) μ=absolute viscosity of air Pa s v=Average velocity of air through the duct. 4) A differential manometer connected to pressure taps measures the pressure drop flow using equation: px – py = 2hgρ2 ------ (4) 5) For rib roughened square channel the pressure drop across test section can be used by using the following equation. Δp=ρw g h ------- (5) ρw=density of water; g= gravitational constant h= manometer head difference 6) The friction factor was determined in terms of pressure drop across the test duct and the mass velocity of air using equation. ff = -------- (6) 7) The friction factor of the present study was normalized by the friction factor for flow in smooth duct is given by: Fft = 0.046Re-0.2 ------ (7)

4. RESULTS AND DISCUSSION

The results of present study are presented in terms of friction factor as a function of geometrical parameters of ribs. The friction factor (ff) based on Reynolds number.

Fig. 4.2 and 4.4 effect of pitch to height ratio (p/e) on friction factor (ff) for different Reynolds number with attached ribs for e/Dh=0.2, 0.3 and 0.4 in a square duct. Friction factor have been plotted as function of pitch to rib height ratio (P/e) for different values of Reynolds number.

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From the above figures, for all values of e/Dh ,the friction factor attain maximum corresponding to pitch ratio (p/e) of 10 and on either side of this value, decrease in Friction factor has been observed. It is occur if relative roughness pitch (p/e) is less than about 8–10. In these figure results are compared and it was found that the value of friction factor for the value of p/e (p/e=8 and p/e=10) is 80% higher than the value of p/e (p/e=6).

Effect of Ribs height to channel hydraulic diameter ratio (e/Dh) at different Pitch to height ratio (p/e = 8) for attached rib configuration on the friction factor (ff) is shown in Fig. 4.5 and 4.8 respectively. By observing these figures experimental result reveals that the friction factor is increasing with increasing e/Dh.

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For all the Reynolds number, monotonically increase in friction factor has been observed as relative roughness height, (e/Dh) increases. It is due to the fact that as relative roughness height value increases; roughness geometry protrudes more turbulence, thereby, resulting in increase in friction factor.

The friction factor is found to increase with Reynold number for different values of (e/Dh) for attached ribs and remains fairly constant at higher Reynold number as shown in figure 4.9 to 4.11

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From the above figures it is observed that the friction factor ratio increased marginally for higher (e/Dh) ratios. The friction factor is found highest for an (e/Dh) ratio of 0.4. Therefore it can be said that though there was an increase in the heat transfer augmentation it was only at the cost of the pressure drop. Thus the present study on continuous attached ribs suggests that the friction factor of the continuous attached ribs with an (e/Dh) ratio of 0.4 is the high amongst the studied configurations. From the figures it is observed that small increased in the friction factor as a Reynolds number increases.

In this study an experimental study has been performed for attached rib configuration to obtain pressure loss or friction factor for an air flow in square channel. By insertion of ribs it has been found that the ribs break the laminar sub-layer and create local wall turbulence due to flow separation and reattachment between consecutive ribs, which reduce the thermal resistance and greatly enhance the heat transfer. From the experimental results, it can be concluded as follows: 1. The air flow in the square channel, For attached ribs the friction factor attain maximum corresponding to pitch ratio (P/e) value of 10. On either side of these values, decrease in friction factor has been observed. 2. The friction factor is increased with increase in Rib height to channel hydraulic diameter ratio (e/Dh). Therefore in this study (e/Dh=0.4) presented the higher friction factors for attached rib configuration, that is 30% higher friction factor ratio compared to e/Dh=0.2 for attached. It is due to increase in e/Dh increases the blockage of the flow passage and therefore the friction factor is increased in the channel. friction factor with increase in Reynolds number.

V.S. Hans, R.P. Saini, J.S. Saini ―Heat transfer and friction factor correlations for a solar air heater duct roughened artificially with multiple v-ribs‖ Solar Energy 84 (2010) 898–911. V. SriHarsha, S.V. Prabhu, R.P.Vedula ―Influence of rib height on the local heat transfer distribution and pressure drop in a square channel with 900continuous and 600 V-broken ribs‖ Applied Thermal Engineering 29 (2009) 2444–2459. Pongjet Promvonge, ―Heat transfer and pressure drop in a channel with multiple 60° V-baffles‖ International Communications in Heat and Mass Transfer 37 (2010) 835–840. Teerapat Chompookham, et al ―Heat transfer augmentation in a wedge-ribbed channel using winglet vortex generators‖ International Communications in Heat and Mass Transfer 37 (2010) 163–169 Sukhmeet Singh et al, ―Investigations on thermo hydraulic performance due to flowattack- angle in V-down rib with gap in a rectangular duct of solar air heater‖ Applied Energy 97 (2012) 907–912. S. C. Lau, R. T. Kukreja, R. D.Mcmillin ―Effects of V-shaped rib arrays on turbulent heat transfer and friction of fully developed flow in a square channel‖ International Journal of Heat and Mass Transfer (1991) Vol. 34,N o.7,pp. 1605-1616. A. Lanjewar, J.L. Bhagoria, R.M. Sarviya ―Experimental study of augmented heat transfer and friction in solar air heater with different orientations of W-Rib roughness‖ Experimental Thermal and Fluid Science35 (2011) 986–995

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