Short Pin-fin Arrays Research Articles

Effects of heat loads on flow and heat transfer in the entrance region of a cooling duct with a staggered array of pin fins

Computational analysis based on SAS, a hybrid RANS-LES method, were performed to study the effects of heat load on the unsteady flow and heat transfer in the entrance region of a cooling duct with a staggered array of short pin fins under conditions relevant to gas turbines. Results obtained with heat loads ranging from 3.2 to 582 kW/m2 show density and velocity variations in the boundary layers created by heat transfer to affect where separation about each pin fin takes place, the row where unsteady vortex shedding starts, the frequency of shedding, and the production of turbulent kinetic energy. It also affected the vortical structures next to the endwall in how horseshoe vortices at the base of each pin interact with the shed vortices. These heat-transfer induced changes in the flow field were found to significantly reduce the rate at which row-averaged Nusselt number, Nu¯, increases along the duct in the entrance region and to enable Nu¯ to be nearly constant in the post-entrance region even though the Reynolds number is decreasing along the duct. Results obtained also show steady RANS to under predict Nu¯ throughout the duct because it could not account for the effects of vortex shedding, and unsteady RANS to under and then over predict Nu¯ along the duct because it could not predict which row vortex shedding first starts and then over predicts its effects on heat transfer. Correlations for Nu¯ were developed that account for heat load in the entrance and in the post-entrance regions.

Multiobjective Optimization of Staggered Elliptical Pin-Fin Arrays

This work presents a numerical procedure for multiple objectives to optimize staggered elliptic-shaped short pin-fin arrays. The multiobjective problem is to achieve an acceptable compromise between augmentation of turbulent heat transfer and reduction in friction loss. Four nondimensional variables, pin-fin height-to-channel height ratio, major axis length-to-channel height ratio, minor-axis length-to-channel height ratio, and pin-fin pitch-to-channel height ratio are chosen as design variables. The D-optimal method is used to determine the training points. The response surface method is used to approximate the Pareto optimal front with Reynolds-averaged Navier-Stokes analysis of the flow and heat transfer using the shear stress transport (SST) turbulence model. The Pareto-optimal solutions are obtained using a combination of an evolutionary algorithm and a local search.

Experimental Study of Pressure Drop and Heat Transfer in a Single-Phase Micropin-Fin Heat Sink

Abstract The pressure drop and heat transfer characteristics of a single-phase micropin-fin heat sink were investigated experimentally. Fabricated from 110 copper, the heat sink contained an array of 1950 staggered square micropin fins with 200×200μm2 cross section by 670μm height. The ratios of longitudinal pitch and transverse pitch to pin-fin equivalent diameter are equal to 2. De-ionized water was employed as the cooling liquid. A coolant inlet temperature of 25°C, and two heat flux levels, qeff″=50W∕cm2 and qeff″=100W∕cm2, defined relative to the platform area of the heat sink, were tested. The inlet Reynolds number ranged from 93 to 634 for qeff″=50W∕cm2, and from 127 to 634 for qeff″=100W∕cm2. The measured pressure drop and temperature distribution were used to evaluate average friction factor and local averaged heat transfer coefficient/Nusselt number. Predictions of the previous friction factor and heat transfer correlations that were developed for low Reynolds number (Re<1000) single-phase flow in short pin-fin arrays were compared to the present micropin-fin data. Moores and Joshi’s friction factor correlation (2003, “Effect of Tip Clearance on the Thermal and Hydrodynamic Performance of a Shrouded Pin Fin Array,” ASME J. Heat Transfer, 125, pp. 999–1006) was the only one that provided acceptable predictions. Predictions from the other friction factor and heat transfer correlations were significantly different from the experimental data collected in this study. These findings point to the need for further fundamental study of single-phase thermal/fluid transport process in micropin-fin arrays for electronic cooling applications.

Heat Transfer Contributions of Pins and Endwall in Pin-Fin Arrays: Effects of Thermal Boundary Condition Modeling

Short pin-fin arrays are often used for cooling turbine airfoils, particularly near the trailing edge. An accurate heat transfer estimation from a pin-fin array should account for the total heat transfer over the entire wetted surface, which includes the pin surfaces and uncovered endwalls. One design question frequently raised is the actual magnitudes of heat transfer coefficients on both pins and endwalls. Results from earlier studies have led to different and often contradicting conclusions. This variation, in part, is caused by imperfect or unrealistic thermal boundary conditions prescribed in the individual test models. Either pins or endwalls, but generally not both, were heated in those previous studies. Using a mass transfer analogy based on the naphthalene sublimation technique, the present experiment is capable of revealing the individual heat transfer contributions from pins and endwalls with the entire wetted surface thermally active. The particular pin-fin geometry investigated, S/D = X/D = 2.5 and H/D = 1.0, is considered to be one of the optimal array arrangement for turbine airfoil cooling. Both inline and staggered arrays with the identical geometric parameters are studied for 5000 ≤ Re ≤ 25,000. The present results reveal that the general trends of the row-resolved heat transfer coefficients on either pins or endwalls are somewhat insensitive to the nature of thermal boundary conditions prescribed on the test surface. However, the actual magnitudes of heat transfer coefficients can be substantially different, due to variations in the flow bulk temperature. The present study also concludes that the pins have consistently 10 to 20 percent higher heat transfer coefficient than the endwalls. However, such a difference in heat transfer coefficient imposes very insignificant influence on the overall array-averaged heat transfer, since the wetted area of the uncovered endwalls is nearly four times greater than that of the pins.

Turbulent heat transfer and pressure drop in an internally finned equilateral triangular duct

Experiments were carried out to determine average heat transfer coefficients and friction factors for turbulent flow through equilateral triangular ducts with pin fins. The measurements were made by using a triangular duct heat exchanger. The fluids in the heat exchanger were air and water, and the average heat transfer coefficients were determined by measuring the overall heat transfer coefficients of the heat exchanger. To attain fully developed conditions at the entrance and exit, the heat exchanger was built with additional lengths before and after the test section (30 hydraulic diameters). The triangular ducts of the heat exchanger consisted of two metal walls (brass) and a wall of a less conductive material. The metal walls of the air side were finned with an array of short pin fins. Due to their high thermal conductivity and the small temperature variation of the water, these two walls were practically isothermal. The third wall of the duct was well insulated and can be considered adiabatic. The results are presented in dimensionless form, in terms of Nusselt numbers and friction factors as functions of the Reynolds number. The pin fin efficiency, which depends on the heat transfer coefficient, is also determined as a function of dimensionless parameters. It was found that the fins increase both the heat transfer and the pressure drop compared with an unfinned (smooth) duct. A better comparison between finned and smooth ducts was made for the same pumping power and heat transfer area. The results show that the Nusselt number for the finned duct, in the present work, is always greater than the one for the smooth duct.

Convective mass transfer and pressure loss characteristics of staggered short pin-fin arrays

The effect of fin shape on the mass transfer and pressure loss of a ten-row staggered short pin-fin array in fully developed approaching flow is investigated experimentally using three different fin shapes. The Reynolds number, based on approach-velocity and fin diameter, ranges from 3000 to 18000. The fin shape affects the row-by-row variation of Sherwood number. The results indicate that the stepped-diameter circular fin arrays have not only a larger mass transfer coefficient, but also a smaller pressure loss compared to a uniform-diameter circular fin array. In stepped-diameter circular fin arrays, the effect of step-length-to-total-length ratio on Sherwood number is small, but the total mass transfer rate and pressure loss change significantly with this ratio, owing to differences in mass transfer area and free flow area, respectively.

Effect of Trailing-Edge Ejection on Local Heat (Mass) Transfer in Pin Fin Cooling Channels in Turbine Blades

Experiments are conducted to study the local heat transfer distribution and pressure drop in a pin fin channel that models the cooling passages in modern gas turbine blades. The detailed heat/mass transfer distribution is determined via the naphthalene sublimation technique for flow through a channel with a 16-row, staggered 3 × 2 array of short pin fins (with a height-to-diameter ratio of 1.0, and streamwise and spanwise spacing-to-diameter ratios of 2.5) and with flow ejection through holes in one of the side walls and at the straight flow exit (to simulate ejection through holes along the trailing edges and through tip bleed holes of turbine blades). The pin fin heat/mass transfer and the channel wall heat/mass transfer are obtained for the straight-flow-only and the ejection-flow cases. The results show that the regional pin heat/mass transfer coefficients are generally higher than the corresponding regional wall heat/mass transfer coefficients in both cases. When there is side wall flow ejection, a portion of the flow turns to exit through the ejection holes and the rate of heat/mass transfer decreases in the straight flow direction as a result of the reducing mass flow rate along the channel. The rate of cooling air flow through a pin fin channel in a gas turbine blade must be increased to compensate for the “loss” of the cooling air through trailing edge ejection holes, so that the blade tip is cooled sufficiently.

Heat Transfer and Pressure Drop for Short Pin-Fin Arrays With Pin-Endwall Fillet

The effects of array configuration and pin-endwall fillet on the heat transfer and pressure drop of short pin-fin arrays are investigated experimentally. The pin-fin element with endwall fillet, typical in actual turbine cooling applications, is modeled by a spool-like cylinder. The arrays studied include an in-line and a staggered array, each having seven rows of five pins. These arrays have the same geometric parameters, i.e., H/D = 1, S/D = X/D = 2.5, and the Reynolds number ranging from 5 × 103 to 3 × 104. One of the present results shows that the staggered array always has a higher array-averaged heat transfer coefficient than its in-line counterpart. However, the pressure drop for the staggered array is higher compared to the in-line configuration. These trends are unaffected by the existence of the pin-endwall fillet. Another significant finding is that an array with pin-endwall fillet generally produces lower heat transfer coefficient and higher pressure drop than that without endwall fillet. This leads to the conclusion that pin-endwall fillet is undesirable for heat transfer augmentation. In addition, nai¨ve use of the heat transfer results obtained with perfectly circular cylinders tends to overestimate the pin-fin cooling capability in the actual turbine. The effects of endwall fillet on the array heat transfer and pressure drop are much more pronounced for the staggered array than for the inline array; however, they diminish as the Reynolds number increases.

Length to Diameter Ratio and Row Number Effects in Short Pin Fin Heat Transfer

Recently, several experiments concerning heat transfer from short pin fins have been conducted with the results indicating lower heat transfer from short pin fins than from longer pin fins found in tube banks and other similar configurations. Assessments of the effect of the number of pin rows and row geometry have also been made. It was felt that there was a need to determine the relative contribution of pin length to diameter ratio and pin row geometry on the heat transfer. Array-averaged heat transfer coefficients on pin and endwall surfaces were measured for two configurations of staggered arrays of short pin fins (length to diameter ratio of 4). One configuration contained eight streamwise rows of pins, while the other contained only four rows. Results showed that both the eight-row and the four-row configurations for an Lp/D of 4 exhibit higher heat transfer than in similar tests on shorter pin fins (Lp/D of 1/2 and 2). It was also found that for this Lp/D ratio the array-averaged heat transfer was slightly higher with eight rows of staggered pins than with only four rows.

Developing Heat Transfer in Rectangular Ducts With Staggered Arrays of Short Pin Fins

Streamwise heat transfer variation, overall array heat transfer, and overall flow friction behavior are presented for large aspect ratio ducts containing uniformly spaced staggered arrays of circular pin fins which span the entire duct height. The array geometries investigated have short pin heights (length-to-diameter ratio of 1) and moderate spanwise (transverse) and streamwise (longitudinal) pin spacings (pitch-to-diameter ratios of 1.5 and 2.5). Such staggered array geometries are typical of applications found in internally cooled gas turbine engine airfoils. The uncovered duct walls comprise a substantial fraction of the total heat transfer area. Ten pin rows in the streamwise direction are utilized in all the experiments, with a segmented construction allowing determination of spanwise-averaged heat transfer coefficients resolved to a single row spacing in the streamwise direction. Comparisons are made with results of prior studies which are mainly restricted to flow normal to banks of circular cylinders with long length-to-diameter ratios.

Heat-Transfer Coefficients for Staggered Arrays of Short Pin Fins

Short pin fins are often used to increase the heat transfer to the coolant in the trailing edge of a turbine blade. Due primarily to limits of casting technology, it is not possible to manufacture pins of optimum length for heat-transfer purposes in the trailing edge region. In many cases the pins are so short that they actually decrease the total heat-transfer surface area compared to a plain wall. A heat-transfer data base for these short pins is not available in the literature. Heat-transfer coefficients on pin and endwall surfaces were measured for several staggered arrays of short pin fins. The measured Nusselt numbers when plotted versus Reynolds numbers were found to fall on a single curve for all surfaces tested. The heat-transfer coefficients for the short pin fins (length to diameter ratios of 1/2 and 2) were found to be about a factor of two lower than data from the literature for longer pin arrays (length to diameter ratios of about 8).