Heating and cooling of fluids play an important role in energy engineering, such as solar air heaters and the internal cooling of gas turbine blades. Heat transfer in the heat exchanger channels can be enhanced by ribs, which will also result in a significant decrease in coolant pressure. In our previous work, a novel bioinspired rib design with a rhombus-shaped pattern was proposed to enhance the channel cooling. The Nusselt numbers and flow characteristics are further compared to angled and parallel broken ribs in this work. It is interesting to find that the proposed channel with decreasing-height rhombus-shaped patterned ribs exhibits the best heat transfer performance. At the same time, the Nusselt number and friction factor are related to the Reynolds number in the studied range. The surface with rhombus-shaped patterned ribs shows greater thermal performance factors than that with angled ribs, while the surface with a decreasing-height rib arrangement has the highest thermal performance factor due to a better Coanda effect. This work confirms that the proposed ribs with decreasing height rhombus-shaped patterns are effective for enhancing the internal cooling of the channel.

Film cooling is a commonly used thermal protection technology for turbine airfoils. However, due to the counter -rotating vortex pair (CVP) generated by the interaction between the cooling jet and the mainstream, the coolant gradually breaks away from the wall, which is particularly evident at high blowing ratios. For the sake of improving the film cooling effectiveness (FCE) under high inlet temperature conditions, a novel film cooling structure (SerrTrench-VSS) that combines serrated trench film holes with shark skin-inspired V-shaped surface (VSS) is proposed. The RANS method and realizable k-epsilon model are used for the simulations. The flow charac-teristics, adiabatic FCE, and resistance loss are analyzed in detail when the blowing ratio (BR) is 0.5-1.5 and then compared with the traditional cylindrical film hole and transverse trench film hole. In addition, the impact of VSS height on the cooling performance is investigated and the best height of the structure is determined. The results show that the SerrTrench-VSS has better FCE. The serrated trench can destroy the CVP and improve the spanwise spreading ability of the coolant. Also, the VSS can converge the cooling jet downstream of the film hole toward the centerline, thereby improving its extension ability along the flow direction. In the studied range of blowing ratio, the SerrTrench-VSS has the best FCE, and the spanwise-averaged FCE increases by up to 10.50% when BR = 0.5. The empirical correlations between the globally-averaged FCE and the height ratio within the range of the present study are also given. For the serrated trench, the cooling jet has the best spanwise spreading and flow extension ability when the VSS height is 0.5D. The proposed novel film cooling structure (SerrTrench-VSS) can effectively improve the FCE and provide feasible means for the efficient cooling of a turbine blade under practical conditions.

Swirl cooling can provide effective protection for the turbine vane leading edge (LE). In this paper, a swirl cooling model for improving the turbine vane heat transfer is established. The model includes the high-temperature mainstream region, LE region, and swirl cooling region. The conjugate heat transfer (CHT) method is used to examine the influence of wall structures on swirl cooling. Then, the best surface structure in the studied range is selected to further analyze the impact of the coolant inlet mass flow. The results show that the circumferential micro-rib structure has a more excellent performance in both fluid flow and cooling performance. The hindering effect of the micro-ribs can effectively avoid the development of axial cross-flow, thus enhancing the heat transfer with a small friction loss increment and providing a lower surface temperature and more uniform temperature distribution. When the inlet mass flowrate improves, the thermal performance factor increases and the LE temperature decreases gradually. Under the same pumping power condition, the circumferential micro-ribs structure has higher heat transfer efficiency. This investigation can provide a new design for further improving the thermal performance of swirl cooling for turbine vanes.

Mist-assisted film cooling has exhibited great potential for efficient cooling of a turbine blade. Trench structures can significantly affect the film cooling performance of air, while their impact on the adhesion characteristics of the droplets and thus the mist-assisted film cooling performance is unclear. This work highlights the combination of these two aspects by exploring the improvement of the mist-assisted film cooling performance for trenched holes with shaped lips using 3-D computations. The results show that the upper lip structure plays a significant role in the adiabatic film cooling effectiveness, whereas the lower lip structure has little effect on it. It is found that the effect of upper lip structure on the adiabatic film cooling effectiveness increases with increasing blowing ratio. The Coanda effect makes the droplets more attached to the wall of the upper fillet lip structure and upper bevel lip structure. Meanwhile, the concentration of mist and the diameter of droplets are crucial for significantly improving the adiabatic film cooling effectiveness. In general, better adiabatic film cooling effectiveness values are observed for both 10 ?m diameter droplets at a specified concentration of mist and larger concentration of mist at a specified droplet size. This work proposes a novel and efficient means of enhancing the cooling performance using mist/air mixture and trenched holes with shaped lips.

Gas turbine blades can achieve better internal cooling performance with steam than air. A biomimetic structure based on sharkskin scales has demonstrated that the designed rhombus-patterned ribs can significantly improve the internal cooling performance with air. In this work, a channel with 60 degrees parallel ribs and two channels with rhombus-patterned ribs having different height arrangements (constant height and decreasing height) is established. The effect of Reynolds numbers on the flow and heat transfer is investigated. The results show that steam increases the averaged Nu by 16.59%-25.69% as compared to air. The constant height rhombus-patterned ribs cooled by steam show the best cooling performance, with also the highest flow resistance. The decreasing height rhombus-patterned ribbed channel with steam cooling has the best overall thermal performance, due to the weakened vortex intensity by decreasing rib height and the alleviation of poor heat transfer behind the ribs caused by the Coanda effect. Thus, steam cooling of the rhombus-patterned ribbed surface is proved to be capable of achieving excellent cooling performance. This work helps develop novel internal cooling structures inspired by bionic surfaces with the aim of heat transfer enhancement and drag reduction.

Serrated grooves enhance the film cooling performance of turbine blades through extended lateral diffusion when the air is mixed with mist. This work proposes a novel composite structure combining an upstream ramp and a serrated trench to fully exploit the cooling capability of the air/mist mixtures. The simulations show that the film coverage of the mixtures can be significantly improved because the composite structure has an advantage over the zigzag grooves without slopes, as the kidney vortices induced by the former is smaller than the latter. As a result, the amount of coolant entrained by the vortex pair is greater than that entrained by the mainstream. Also, the influence of the eddy current formed by the proposed cooling structure on the vaporization position of the droplet is analyzed for the first time, concluding that the composite structure can better utilize the latent heat of vaporization of the droplet. The benefits are clearly seen in the blow ratios studied and the improvement is more pronounced at high blow ratios. It is also found that the cooling effectiveness is not linearly related to the ramp angle, because it is obviously affected by the droplet size and concentration, and its high value can only be reached within a certain range of ramp angles. Even under high-temperature and high-pressure conditions, the composite structure can achieve ideal cooling effectiveness at the turbine blade endwall. This work explores the turbine blade endwall cooling structures, offering novel insights into designing endwall cooling structures in new directions.

Mist-assisted film cooling has exhibited great potential for efficient cooling of a turbine vane due to the vaporization of water droplets flowing with the air. To further improve the film coverage and cooling effectiveness, it is necessary to find a structure that allows the coolant flow to expand laterally and inhibits the separation of the coolant jet from the wall. Therefore, we propose in this work to study the influence of serrate structure on the cooling performance of trenched holes by examining the flow and heat transfer characteristics of the mist/air mixture. As water evaporation is involved in this process, the dry-bulb temperature may not be suitable for evaluating the film cooling performance. Therefore, a wet-bulb-temperature-based film cooling effectiveness (WFCE) is proposed. The results show that the included angle of serrate plays a significant role in WFCE. In addition, the area-average WFCE of the trenched holes increases first and then decreases as the blowing ratio increases. The concentration of mist and the diameter of droplets are found to be crucial for significantly affecting the WFCE. The relationship between the film cooling effectiveness and the contribution of mist is explicated by empirical correlation with the mist concentration and droplet diameter. The empirical correlation, especially the exponential increasing trend with the square root of mist concentration at a constant droplet diameter, provides the basis for further theoretical analysis of the mechanism of cooling effectiveness improvement. Under the practical condition, it is also proved that the serrated transverse trench structure and the addition of mist to the coolant air can improve significantly the film cooling performance for the turbine vane.

Swirl cooling with air/mist mixture coolant is a promising heat transfer method, for which the combined influence of swirl and mist should be clearly understood. In this work, numerical simulations on the swirl cooling performance of the leading edge of a turbine vane using mist/air mixture and air are implemented for comparative study. The Eulerian-Lagrangian particle tracking method is adopted to investigate the two phase cooling. The coolant air impacts tangentially the inner surface of the circular swirl chamber through two rectangular nozzles, and the mists with different concentrations (5%, 10%, and 20%) and droplet diameters (1 mu m, 2.5 mu m, 5 mu m, 7.5 mu m, and 10 mu m) are included to explore their effects on the cooling performance. The results demonstrate that the circumferentially-averaged Nusselt number, Nu(cir), increases with the increase of mist concentration, and this effect becomes prominent when the droplet diameter increases. For droplet diameter of 10 mu m and mist concentration of 20%, Nu(cir) can increase by 12.6% to 309.7% as compared to pure air. In addition, the empirical correlations of the global-averaged Nusselt number, and friction coefficient to the mist diameter and mist concentration are given for the first time. It is concluded that the thermal performance factor, which can be used to evaluate comprehensively the cooling characteristics, increases with the increase of mist concentration and droplet diameter. The empirical correlations and thermal performance evaluation provide the basis for further theoretical analysis of the cooling performance using air/mist mixture.

Serrate-type trenched cooling holes have shown good film cooling performance for the cooling of turbine vanes using coolant air. Mist-assisted film cooling has also exhibited great potential for efficient cooling of a turbine vane. The combination of them may create a substantial improvement of cooling performance, which is explored in this simulation. This work studies the influence of serrate structure on the cooling performance of trenched holes for the C3X turbine vane under practical conditions by examining the flow and heat transfer characteristics of the mist/air mixture. The results prove the concept proposed for significantly improving the cooling performance. At the mass flow ratio Rc = 0.25% (M = 0.5) and Rc = 0.50% (M = 1.0), replacing the trench edge by a double-curvature lip structure can further improve the film cooling effectiveness. The film cooling effectiveness of the vane is reduced when the angle of the serrated trench is too large and the vortex for the six trenched holes intensifies with increasing mass flow ratio. The comparison of streamlines distribution and vortex structures at different blowing ratios by the Q criterion for the trenched holes in the flat plate reveals why the serrated trench structure can improve the cooling effectiveness: it can generate anti-counter-rotating vortices, making it difficult for the coolant film to separate from the wall. Although it is worth of further study on this novel design of serrated trench with mist/air film cooling under engine-representative conditions, this work provide heuristic guidance for the cooling of turbine vanes.