Pin-finned Tip Research Articles

An optimum profile of a special pin fin: full solutions

Purpose This paper aims to present an elegant exact solution in terms of elementary functions for a special pin fin without the classical length-of-arc approximation. Design/methodology/approach The temperature distribution along the fin and the surface function, both being the functions of a shape parameter, is inversely proportional to each other. The specialty of the spine is such that its shape and temperature profile are linked for a given Biot number. Findings Exact formulas for the pin fin tip temperature, pin fin base heat transfer rate, surface area of the spine and thermal fin efficiency are also given. Originality/value Without the traditional arc length assumption, the pin fin is shown to be an effective extended surface to remove the excessive heat from the hot surface it is pinned to. Optimum pin fin dimensions leading to the maximum base heat transfer rate are also worked out for a specified fin volume.

Heat transfer and flow structure in a rotating duct with detached pin fins

The realizable turbulence model was used to calculate flow and heat transfer characteristics in a rotating rectangular duct with detached pin fins in staggered arrangement. Transverse and longitudinal spacings of the pin fins were 2.5 D and 2.5 D, respectively. Reynolds number was 7,000 and Rotation number (Ro) varied from 0 to 1 while the clearance ranged from 0 to 0.5. Streamlines, vortex structure, wall shear stresses and Nusselt numbers were obtained. Flow phenomenon at the pin fin rear side and a leakage vortex near the pin fin tip were observed. High Nusselt numbers appeared on leading side. The leakage vortex imposed high energy losses in the mainflow. The flow and heat transfer characteristics were associated with Ro number and clearance. Middle values of Ro and clearance induced highest Nusselt numbers on leading side. However, high Ro and small clearance gave highest Nusselt numbers on trailing side.

Smooth and non-smooth optimal pin fin profiles beyond the Schmidt optimality assumption and “length-of-arc” approximation

• Schmidt simplifying criterion is not adopted. • The ‘length-of-arc’ approximation is not adopted. • Direct optimal control methods are used. • The pin optimal longitudinal profile consists of two regions. • Non-smooth optimal pin fin profiles are more effective than smooth profiles. A parametric model of the pin fin shape has been developed. The pin fin profile optimization is performed without adopting the “length-of-arc” approximation. The model is based on the accurate treatment of the arc length. The heat transfer coefficient h is not constant but variable along the length of the pin fin. The objective is to minimize the pin fin volume for given value of the transferred heat flux. Direct optimal control methods are used. The model is validated for a specific case compatible with the application of the Schmidt optimality assumption. Results are shown for pin fins with circular cross section. Both forced convection and natural convection have been considered. Generally, the optimal profile of the pin fin consists of two regions; in the region near the pin fin base, the thickness is constant or decreases almost linearly while in the region near the pin fin tip the thickness is constant or decreases, depending on the thermal load. Including (instead of omitting) in calculation the accurate description of the arc length and using variable (instead of constant) h may yield thinner and shorter pin fins. The main conclusion is that, by including in calculation the accurate treatment of the arc length, non-smooth pin fin profiles may be obtained. Such profiles consist of a series of steps and are more effective than the smooth profiles.

Entropy generation and optimization of laminar convective heat transfer and fluid flow in a microchannel with staggered arrays of pin fin structure with tip clearance

Entropy generation rate accounting for heat transfer and flow friction is presented for the flow in microchannel with staggered pin fin arrays, clearance existing between the pin fin tip and the shroud plate. Within the scope of the present work, entropy generation rate due to heat transfer is levels of magnitude higher than that from flow friction, which is nonetheless not negligible considering its scaling effect on the pumping power consumption, for all cases under investigation. For the pin fin structure with high aspect ratio (height-to-diameter), the effect of tip clearance was found more pronounced with a conductive nature competing with the convective. When this ratio is low, the convective effect dominates the variation of entropy generation by heat transfer and flow friction, the effect of the clearance gap favored by higher aspect ratio. The entropy minimization method is applied to seek for an optimal combination of all impact factors under investigation. The Pareto frontier along with its corresponding solution sets are obtained by Multiobjective Optimization Genetic Algorithm. The solution sets acquired for the scenario with high-aspect-ratio pin fin fall within the region of lower pin fin density where the trade off between the convective and the conductive effects is identified. On the other hand, dominated by the convective nature, the solution sets for cases with lower aspect ratio are in principle located at the upper bounds.

Comparisons of Heat Transfer Enhancement of an Internal Blade Tip with Metal or Insulating Pins

AbstractCooling methods are needed for turbine blade tips to ensure a long durability and safe operation. A common way to cool a tip is to use serpentine passages with 180-deg turn under the blade tip-cap taking advantage of the three-dimensional turning effect and impingement like flow. Improved internal convective cooling is therefore required to increase the blade tip lifetime. In the present study, augmented heat transfer of an internal blade tip with pin-fin arrays has been investigated numerically using a conjugate heat transfer method. The computational domain includes the fluid region and the solid pins as well as the tip regions. Turbulent convective heat transfer between the fluid and pins, and heat conduction within pins and tip are simultaneously computed. The main objective of the present study is to observe the effect of the pin material on heat transfer enhancement of the pin-finned tips. It is found that due to the combination of turning, impingement and pin-fin crossflow, the heat transfer coefficient of a pin-finned tip is a factor of 2.9 higher than that of a smooth tip at the cost of an increased pressure drop by less than 10%. The usage of metal pins can reduce the tip temperature effectively and thereby remove the heat load from the tip. Also, it is found that the tip heat transfer is enhanced even by using insulating pins having low thermal conductivity at low Reynolds numbers. The comparisons of overall performances are also included.

Augmented Heat Transfer of an Internal Blade Tip by Full or Partial Arrays of Pin-Fins

Cooling methods are needed for the turbine blade tips to ensure a long durability and safe operation. A common way to cool the tip is to design serpentine passages with 180-deg turn under the blade tip-cap taking advantage of the three-dimensional turning effect and impingement like flow. Improved internal convective cooling is therefore required to increase the blade tip life. In the present study, augmented heat transfer of an internal blade tip by pin-fins has been investigated numerically. The computational models consist of a two-pass channel with 180-deg turn and an array of pin-fins mounted on the tip-cap, and a smooth-tip two-pass channel. The computational domain includes the fluid region and the solid pins as well as the tip regions. Turbulent convective heat transfer between the fluid and pins, and heat conduction within pins and tip are simultaneously computed. The main objective of the present study is to observe the effect of the full and partial pin-fins arrays on heat transfer enhancement of pin-finned tips. Results show that due to the combination of turning impingement and pin-fin crossflow, the maximum heat transfer coefficient of the full and partial pin-finned tip is a factor of 3.0 and 1.8, respectively, higher than that of a smooth tip. Disregarding the increased active heat transfer area, the tip with partial pin-fins array provides around 6% higher heat transfer enhancement than the tip with full pin-fins array. It is suggested that the use of partial pin-fins array is suitable for improving blade tip cooling when the added weight and thereby the increased stress on a blade are considered. (Less)

CONJUGATED ANALYSIS OF HEAT TRANSFER ENHANCEMENT OF AN INTERNAL BLADE TIP-WALL WITH PIN-FIN ARRAYS

To improve gas turbine performance, the operating temperature has been increased continuously. However, the heat transferred to the turbine blade is substantially increased as the turbine inlet temperature is increased. Cooling methods are, therefore, much needed for the turbine blades to ensure long durability and safe operation. The blade tip region is exposed to hot gas flow, and is difficult to cool. A common way to cool the tip is to use serpentine passages with 180-degree turns under the blade tip-cap, taking advantage of the three-dimensional (3D) turning effect and impingement. Increased internal convective cooling is, however, required to increase the blade tip life. In this paper, augmented heat transfer of a blade tip with internal pin fins has been investigated numerically using a conjugated heat transfer approach. The computational models consist of two-pass channels with 180-degree turns and pin-fin arrays mounted on the internal tip caps. The computational domain includes the fluid region and the solid pins as well as the solid tip regions. Turbulent convective heat transfer between the fluid and pins, and heat conduction within the pins and tip are simultaneously computed. The inlet Reynolds number ranges from 100,000 to 600,000. Details of the 3D fluid flow and heat transfer over the tip-walls are presented. A comparison of the overall performance of the models is presented. It was found that due to the combination of turning impingement and pin-fin crossflow, the heat transfer coefficient of the pin-finned tip is about three times higher than that of a smooth tip. This augmentation is achieved at the cost of a pressure drop penalty of 7%. With the conjugated heat transfer method, not only the simulated model is close to the experimental model, but also the distribution of the external tip heat transfer and pin-fin surface heat flux can be used in the thermal design of blade tips. (Less)

Computational Analysis of Pin-Fin Arrays Effects on Internal Heat Transfer Enhancement of a Blade Tip Wall

Cooling methods are strongly needed for the turbine blade tips to ensure a long durability and safe operation. Improving the internal convective cooling is therefore required to increase the blade tip life. A common way to cool the tip is to use serpentine passages with 180-deg turns under the blade tip cap. In this paper, enhanced heat transfer of a blade tip cap has been investigated numerically. The computational models consist of a two-pass channel with a 180-deg turn and various arrays of pin fins mounted on the tip cap, and a smooth two-pass channel. The inlet Reynolds number is ranging from 100,000 to 600,000. The computations are 3D, steady, incompressible, and nonrotating. Details of the 3D fluid flow and heat transfer over the tip walls are presented. The effects of pin-fin height, diameter, and pitches on the heat transfer enhancement on the blade tip walls are observed. The overall performances of ten models are compared and evaluated. It is found that due to the combination of turning, impingement, and pin-fin crossflow, the heat transfer coefficient of the pin-finned tip is a factor of 2.67 higher than that of a smooth tip. This augmentation is achieved at the expense of a penalty of pressure drop around 30%. Results show that the intensity of heat transfer enhancement depends upon pin-fin configuration and arrangement. It is suggested that pin fins could be used to enhance the blade tip heat transfer and cooling.

Enhanced Internal Heat Transfer on the Tip-Wall in a Rectangular Two-Pass Channel (AR = 1:2) by Pin-Fin Arrays

To improve gas turbine performance, the operating temperature has been increased continuously. However, the heat transferred to the turbine blade is substantially increased as the turbine inlet temperature is increased. Cooling methods are therefore much needed for the turbine blades to ensure a long durability and safe operation. The blade tip region is exposed to the hot gas flows and is difficult to cool. A common way to cool the tip is to use serpentine passages with a 180° turn under the blade tip cap taking advantage of the three-dimensional turning effect and impingement. Increasing internal convective cooling is however required to increase the blade tip life. In this article, enhanced heat transfer of a blade tip has been investigated numerically. The computational models consist of a two-pass channel with a 180° turn and arrays of pin-fins mounted on the tip-cap, and a smooth two-pass channel. Inlet Reynolds numbers range from 100,000 to 600,000. The computations are 3-D, steady, and incompressible. The detailed 3-D fluid flow and heat transfer over the tip surfaces are presented. The overall performance of the two models is evaluated. It is found that due to the combination of turning, impingement, and pin-fin crossflow the heat transfer coefficient of the pin-finned tip might be a factor of 1.84 higher than that of a smooth tip. This augmentation is achieved at the expense of a penalty of pressure drop around 35%. It is suggested that the pin-fins could be used to enhance blade tip heat transfer and cooling.

HYBRID NUMERICAL METHOD FOR TRANSIENT ANALYSIS OF TWO-DIMENSIONAL PIN FINS WITH VARIABLE HEAT TRANSFER COEFFICIENTS

A hybrid numerical technique is used to investigate a two-dimensional cylindrical pin fin with arbitrary variable Biot numbers on the pin fin lateral and tip surfaces. By taking the Laplace transform with respect to time, the governing equation and boundary conditions are discretized by central finite difference then general solutions for dimensionless transient responses are obtained in the transform domain. A Laplace inverse technique is taken to achieve the inversion to the real domain. The transient distributions of temperature in the real domain are presented numerically. It is found that the variable Biot number can lead to an obvious temperature lagging effect base on the function of Biot numbers when pin fin immerses in variety physical environments. Moreover, the present method also can be applied to cases with different types of boundary conditions, such as time-dependent changes in boundary temperatures.