Review: The flow and heat transfer investigation inside tapered and straight two pass channels with rib turbulators

This paper is a review of the number of experimental and CFD experiments performed with rib turbulators about the heat transfer and the at two pass channel. In respect to achieve higher thermal efficiency of gas turbines, efforts are made to raise the inlet temperature. The secondary fluxes resulting from the rib and U-shaped curvature play an important role enhancing heat transfer in the two pass ribbed channels. The ribs on the internal surface of cooling passage will increase the strength of heat transfer. This paper deals with the effect of tapered and straight two-pass channels with the influence of the variable rib cross-section on flow and heat transfer enhancement


Introduction
Modern gas turbines operate at a very high entry temperature that may reach 1370 degrees Celsius, which is crossing the highest permissible temperature, and the gas turbine blade may reach. In circulating the air at the inner passages, cooling is achieved. These 1internal passageways are detached by a 180° bend .1The switch triggered secondary1f1ow is an more important to inner turbine1b1ade coo1ing. Various turn design have several the f1ow fie1ds in turn zone of the mu1ti1pass duct. The Properties of the flow varies about the turn due to secondary flow, isolation f1ow, f1ow Collision f1ow and turn f1ow therefore play a significant role at the improvement of the pressure and the heat transfer for inner cooling of the turbine blade. The modern design of gas turbine requires 1ower pressure1drops1across1its total duration and improved heat1transfer improvements. Most of the experiments focused in the inland ducts of the gas turbine blade on fixed crosssectional areas from entry to rotation. To reduce heat packing, gas turbine blades are usually tapered from hub to tip. These channels occur inside the blades of the high performance turbine to supply the outer surface of the rotor, which is subjected to a high temperature gas flow, with effective cooling. Many variables affect the degree of1heat distribution improvement in the ribbed chanals: • Orientation of rib • Spacing. of rib.

Straight channel 2.1. Effects1 of rib 1orientation on heat transfer1enhancement
The combined effects at the f1ow and heat transfer through two1pass ducts with ribs for rib directions (N type, p type) and rib angles (30° -75°). The transfer of additional secondary flows through the downstream passage from the weakening of the energy loss and local flow secondary of the upstream flow secondary is primary factor for improving local heat transfer [1]. Gao et al. study the pairing of nine distinct rib orientations and the 180° bend in the U1shaped passage of the ANSYS CFX industrial CFD at Re = 30000 on total friction loss and forced convection Qualitative findings demonstrate that the Nusselt ratio and the average loss of pressure in the downstream passage are heavily influenced by the upstream geometry [2]. Mochizuki and Murata numerically investigated studied the influence of channe1 rotation and rib orientation on heat transfer in a squre two pass channe1 at180° sharp turn . The heat 1transfer in and after bend was improved in the stationary state, and the sharp-bend induced flow field dominated it. In both the arrangement of the rib and the rotation of the tube, the friction factor was more sensitive than trans-heat [3].1In the revolving1two pass1square1channe1, the influence of different 45° rib angle turbulator arrangement on the Nu number ratio was studied with rotated number up to 0.11 , and with two channe1orientations with respect to axis of rotated ( = 90 , 135deg). At an angle (+45° or -45°) to the main stream, five separate structures of rib turbu1ators are mounted on trai1ing and 1eading surfaces [4]. Tao and Zhao studied the inf1uence of five ang1es of the rib (45°, 60°, 90°, 120° and 135°) of heat transfer properties in the U-shaped stationary ducts has been experimentally tested. The findings revealed that in a 60° ribbed channe1, followed by 45°, 90° and 135°, the optimum Sherwood number ratio was identified ; the rib ang1e and inclination of having major flow on local overall heat1 transfer [5].

Effect of the rib height and the rib Spacing on Heat 1Transfer Enhancement
Experimentally examines the effect of rib spacing in both non-rotating .and rotating rectangular cooling ducts on pressure penalty 1 heat transfer improvement and the overall thermal efficiency. The leading and trailing surfaces, 45° angled ribs are mounted in the 1:2 rectangular channels [6] .The rib pitch varies, 10, 7.5, 5, and3 rib pitch to-height (P/e) ratios are considered. The influence of rib spacing in pressure penalty and heat transfer in stationary and the rotary U shaped channels has been experimentally studied. The findings showed the rib spacing of p/e = 3 achieved greatest heat transfer improvement, while .p/e = 5 performed the maximum pressure loss [7]. The influence of the spacing of rib and height of rib on heat flow in the 1:4 tube is studied. The three variants in the rib spacing 1(p/e=2.5,5,10) with the ratio 0.078 e/D .Rib structure with e/D ratio=0.156 and a P/e ratio =10 was considered to investigate the effect of the rib height [8]. Experimental analysis was studied the influence rib spacing on heat transfer and friction in a revolving two1pass square tube. On the pressure side, the rib pitch-to-height (p/e) ratio ranges from 3.8 to 14.4 in the positive rotational direction, while the suction side holds a constant value of 10.The findings show that the rib spacing influence is more pronounced than the second passage in the first radially outward flow passage [9]. In a revolving two passage channe1 .(aspect ratio. 2:1), the spacing of rib influence on heat transfer was tested at an orientation angle of 135°. At a 45° flow angle, parallel ribs were added to the trailing and the leading of the revolving channe1 .Rib1pitch to rib1height ratios tested 1(5, 7, 5 and 10) [10].   5 mist cooling improvement is found to be around 1.8 times. In air/mist offers the greater enhancement of heat flow over the air with trapezoidal ribs (38) [12]. Ali et1al. studied the effect of the change in the trapezoidal with theang1e (5°, 10°, 15° and 20°)1 trapezoida11rib with decrease in the f1ow path height on the flow, and its consequent influence on the rise in heat transfer at the Re numbers( 9400, 27,120, 44,600 and 61,480 )used LCT and PIV techniques .The area of the secondary recirculation bubble has been found decrease at higher Re and higher trapezoidal angles, contributing the elimination of the hot spot directly downstream of the rib [13]. Numerical analysis records fluid flow and heat transfer properties in a cooling canal with separate crescent ribs mounted at one wall. In order to optimize thermal efficiency of cooling tube, three types of ribs are considered, i.e. a straight rib, a crescent rib concave to a stream-wise direction, a crescent rib convex to a stream-wise direction to a stream-wise direction [14]. The influence of differing pentagonal (5 ° to 20 °) angle and the rib p.The result shows that the best thermal efficiency is the case with crescent ribs concave pitch to height (6-12) ratio on the friction factor characteristics and local heat transfer were analyzed inside the rectangular channels roughened by the one main wall with pentagonal ribs. LCT was used to calculate the distribution of surface temperature and finally to show the local HTC at distinct Re (9400-58,850) over the ribbed surface .A major increase in the heat transfer augmentation directly behind the pentagonal ribs has been discover at higher Reynolds number and pentagonal angle, contributing to prevention of the hot spots [15]. Sixteen rib structures with varying cross-sectional shape were numerically analyzed and the bootshaped rib turbulators were found to have the best of the heat transfer output with the pressure drop equal to that square rib .The researchers have stated that the front surface slope of the rib is a crucial factor in deciding the achievement of the heat transfer because it influenced directly the size of the recirculate region [16]. Lei Wang and Bengt Sunde´n study the ribs of trapezoidal shape with reducing height there flow direction have been shown to provide the largest improvement in friction factor and heat transfer between the triangle, square and trapezoidal ribs with rising height in flow direction and decreasing height ,and perhaps helpful in preventing creation of hot-spots [17]. . In with detail turbulent kinetic energy (TKE) and turbulent statistics budget for same the permeable Shorter reattachment duration for the maximum stresses of Reynolds, TKE output and pressure transport was observed among all the tested configurations for incline split-slit rib [19].The ability of PIV and LCT method was used to investigate influence of differ the trapezoidal angle of a continuous slit trapezoidal-rib (0°, 5°, 10°, 15°, and 20°). on the flow pattern and its corresponding effect onthe heat transfer improvement at the set of Reynolds 61,480.The trapezoidal angle variance has been shown to efficiently regulate the small scale corner vortices behind the firm rib, thereby helping to prevent hot spots [20]. The thermal efficiency of the permeable shape of ribs placed onthe bottom surface of the square two-pass duct was analyzed and show the split.-slit design demonstrated the significant improvement in the heat transfer relative to solid and slit rib without any proportionate rise in the penalty pressure 2018 [21].Five different geometric models, include rectangular slit ribs, "(V-shaped)" slits , (anti -shaped) slits , "(broken V-shaped)" slits , and "(broken anti V shaped)" slits ribs, have been examined. Results obtained show that due to the higher turbulence strength, the ducts with the broken-slits provide greater heat transfer efficiency [22]. In the rectangular channel with the solid, converging. slit and alternating solid slit ribs placed transversely at a bottom wall, the heat transfer and frictional characteristics were analysed . As predicted, the converging slit rib greatly improved the rate of heat transfer in downstream locality and prevented creation hot spot local [23] . Liu et al .study with the inclined angle of the holes varying from 0° to 45°, two pairs of perforated ribs are used and the crosssections are, respectively. The total average number of Nusselt (Nu) for all inclined cases is significantly higher than for straight cases .The change ratio is around 1.85 percent -4.94 percent . For the inclined hole situations, the average Nu in the half section toward the inclined path is expanded [24] .Experimental analysis the detailed of aerothermal properties in the duct with the set of the solid and the permeable pentagonal-ribs placed on bottom wall with the parallel and the inclined slit. Open area ratio set through studies is (0.125%, 12%, and 25 %), respectively .The result shows that the flow from the inclined-slit pentagonal rib has a major effect on magnitude of the stream velocity ,statistics of fluctuation and the vorticity [25].    Figure 9. Rib configurations [25]. Figure 10. Schematic diagram of geometry [22].

Effect of channel aspect ratio on heat transfer enhancement
The heat1 transfer and flow in the rotational U-shaped smooth ducts with aspect ratios (AR=0.25, 0.5 and 1) were numerically analyzed. They findings revealed that the rotating influence on the heat transfer (AR) aspect.-.ratio duct in narrow was more important [26].Wen-Lung Fu L et al. studied experimentally analyzes influence of buoyancy force and duct aspect-ratio W:H on the heat transfer with smooth walls and 45° ribbed walls in rectangular two-pass revolving channels. 4:1, 2:1, 1:1, 1:2, and 1:4 are the duct aspect ratios. The findings showed that, because it has the smallest pressure penalty, the 1:4 channel has stronger heat transfer [27]. The influences of ribs on heat transfer efficiency and cooling air flow properties are numerically tested under various working conditions in different aspect-ratios (AR) in U-shaped ducts. The wide of the duct (AR = 2:1) has a stronger increase factor than the narrow channel (AR = 1:2) and the heat transfer weight of ribs increases with an increase in the Reynolds number at inlet [28]. A experimental research based on the influence of the aspect-ratio =1:2, 1:1, and 1:6 on heat transfer of smooth and rib-turbulated rectangular ducts .They stated that of 1:2 aspect-ratio channels, there is a linear variance between the Re number and the normalized Nusselt number. Their analysis found that for the smooth and the ribbed channel with an aspect-ratio = 1:6, the maximum heat transfer was obtained [29]. Inside the smooth duct with different aspect.-.ratios at vary divider to tip wall lengths, flow and transition are numerically analyzed. The findings revealed that both pressure drop and heat transfer at the bend and outlet the passage were affected by divider-to-tip wall size [30].  Figure 11. Schematic1of U-shaped channel with1ribs [28].

Effect1of turn geometry on (turbulent fluid flow and heat1transfer)
In smooth U-shaped tube, the effect of bend geometry on the flow characteristics and heat1transfer has been studied. This research found that when the external wall of turn was closely linked without a rotund rim and the inner wall of turnwas semi-circular form, the best heat transfer efficiency was obtained [31]. The effect of turned vanes on1pressure loss1and heat transfer in two pass rectangle channel was numerically and experimentally studied. They found a decrease in pressure drop of 25 percent and could sustain the same degree of heat1transfer with the use of inner-turning vanes in bend area [32 ]. The effect of1the shape of the bend on overall heat1transfer efficiency was investigated and proposed that symmetrical bulb structure increased a thermal effectiveness agent by 41% [33]. In t he time-resolved particle picture velocimetry system is used in square cross-section Ushaped channels and different bend section configurations to study the properties of the internal flow area as show in fig (12). A variety of essential conclusions are derived from the data. The structure of the section of the turn has an evident effect on the f1ow fie1d properties for the main f1ow [34]. In two1pass,1square duct with1180°sharp turn as show in fig (13), empirical results for secondary flow patterns and heat transfer propagation are provided to explore the effects of the various turn design: straight ,rounded, and circular -.turn. The measured resu1ts show that the straight case has the strongest turn induced shift in heat1transfer at the turn [35].  Rounded1corner, and (c) Circu1ar1corner [35].

Tapered channels
The heat transfer influence of a tapered channel has been studied. The area of tapered ducts crosssection various frequently. The findings revealed that1the tapered1channe1with ribs gave greater gains in heat1transfer1than1straight ducts. However,1no research on exit mass f1ow1rate inside the1variab1e cross1section channe1 with1ribs has been published on1the Nusselt number distributions [36]. Liu and Feng investigated the heat effect within inclined ducts or ducts with b1eeding ho1es, researcher noted that the inc1ined channe1 or channe1 with b1eeding ho1es would provide a greater effect of1heat transfer in a way [37]. The pressure drop and heat transfer data were obtained by maintaining constant rib1height , side height to the mean diameter ratio equa11to (0.081), and the side slope height ratio equa1 to (6 17.8) for standard flow ribs. Heat transfer optimization was introduced on the basis of modified constant pumping capacity, constant pressure, and constant mass flow, drop, and performance of ribbed channels ranged from (0.8 to 0.6) times that of static air channel [38].

Figure 14.
Graphic of the straight and tapered1 two-pass channels and the distribution of ribs direction [36].

Conclusion
It is possible to derive some concluding conclusions from these experimental and numerical studies: • Rib mediated secondary flow has a1significant1effect on1the transfer of1heat to1the channel.1It is also important to refine the configuration of the rib to take advantage of the secondary flow caused by the rib. For eg, the rotation of the turbulator, such that it is not orthogonal to the mainstream flow, The heat transfer coefficients are greatly increased due to the angle of1rib1caused by secondaryvf1ow . The ideal space for ribs has been found to be about P/e1=10. However, there are major variations in the pressure11osses suffered in1the duct.1The 1:4 duct obtained the 1owest1pressure1penalty1thus, the thermal efficiency of the 1:4 duct is higher than the 1:2, 1:11and 2:11ducts1respectively. • The1researchers revealed that tapered duct with the ribs has more heat transfer benefits than straight duct.