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Rotating machinery like gas turbine engines consist of rotating components like gas turbine disks that are exposed to high thermal loads and stresses due to centrifugal force acting on them, and thus effective cooling is essential. The disks are additionally exposed to hot turbine gas ingress as an adverse pressure gradient is created in the stator-rotor cavity due to outward pumping of near-disk flow. Impingement of a coolant jet on such a surface can effectively provide cooling and also counter this adverse gradient. There also exists a circular relative velocity between the disk and wall-jet formed by impinging air as well as the air in the disk vicinity. Thus, the study of heat transfer under rotating conditions becomes important. With the advent of advanced manufacturing techniques, it is possible to produce disks with novel heat transfer enhancement features such as pin-fins which have not been previously explored. They create unique flow features in addition to increasing the surface area. The present study numerically investigates the heat transfer over rotating surfaces under jet impingement with square, triangular, cylindrical and dome-shaped pin-fins. This will inform the optimization of pin geometry for better heat transfer effectiveness and uniformity. Three different jet Reynolds numbers between 5000 and 18000 based on the jet diameter have been studied for three different Rotational Reynolds numbers based on the disk diameter. Transient non-conjugate heat transfer analysis was conducted, and the technique was compared with experimental results on square pin-fins and a smooth circular disk. The heat transfer enhancement greatly depends on the flow separation and pin area normal to the flow. A heat transfer enhancement of close to 2.2 was observed for triangular fins for the lowest rotation speed, for the highest rotation speed, the highest enhancement of close to 1.7 was observed in the cylindrical pins. [ABSTRACT FROM AUTHOR]

The primary focus of this paper is to study the film cooling performance for a row of cylindrical holes each supplemented with two symmetrical antivortex holes, which branch out from the main holes. The antivortex design was originally developed at NASA-Glenn Research Center by James Heidmann, coauthor of this paper. This 'antivortex' design is unique in that it requires only easily machinable round holes, unlike shaped film cooling holes and other advanced concepts. The hole design is intended to counteract the detrimental vorticity associated with standard circular cross-section film cooling holes. The geometry and orientation of the antivortex holes greatly affect the cooling performance downstream, which is thoroughly investigated. By performing experiments at a single mainstream Reynolds number of 9683 based on the freestream velocity and film hole diameter at four different coolant-to-mainstream blowing ratios of 0.5, 1, 1.5, and 2 and using the transient IR thermography technique, detailed film cooling effectiveness and heat transfer coefficients are obtained simultaneously from a single test. When the antivortex holes are nearer the primary film cooling holes and are developing from the base of the primary holes, better film cooling is accomplished as compared to other antivortex hole orientations. When the antivortex holes are laid back in the upstream region, film cooling diminishes considerably. Although an enhancement in heat transfer coefficient is seen in cases with high film cooling effectiveness, the overall heat flux ratio as compared to standard cylindrical holes is much lower. Thus cases with antivortex holes placed near the main holes certainly show promising results. [DOI: 10.1115/1.2950059]

Metal-based microchannel heat exchangers (MHEs) are of current interest due to the combination of high heat transfer performance and improved mechanical integrity. Efficient methods for fabrication and assembly of functional metal-based MHEs are essential to ensure the economic viability of such devices. In this paper, the results on fabrication, assembly, and heat transfer testing of Cu- and Al-based MHE prototypes are reported. Efficient fabrication of Cu- and Al-based high-aspect-ratio microscale structures (HARMSs) has been achieved through molding replication using surface-engineered metallic mold inserts. Replicated metallic HARMSs were assembled through eutectic bonding to form entirely Cu- and Al-based MHE prototypes, on which heat transfer tests were conducted to determine the average rate of heat transfer from electrically heated Cu blocks placed outside the MHEs to water flowing within the molding replicated microchannel arrays. Experimentally observed heat transfer rates are higher as compared to those from previous studies on microchannel devices with similar geometries. The potential influence of microchannel surface profile on heat transfer rates is discussed. The present results illustrate the potential of metal-based MHEs in wide-ranging applications. Index Terms--Heat transfer characteristics, metal-based microchannel heat exchangers (MHEs), molding replication.

The present study investigates the effects of coolant injection on adiabatic film effectiveness and heat transfer coefficients from a plane and recessed tip of a high pressure turbine first stage rotor blade. Three cases where coolant is injected from (a) five orthogonal holes located along the camber line, (b) seven angled holes located near the blade tip along the pressure side, and (c) combination cases when coolant is injected from both tip and pressure side holes were studied. The pressure ratio (inlet total pressure to exit static pressure for the cascade) across the blade row was 1.2, and the experiments were run in a blow-down test rig with a four-blade linear cascade. The Reynolds number based on cascade exit velocity and axial chord length was 8.61 x [10.sup.5] and the inlet and exit Mach numbers were 0.16 and 0.55, respectively. A transient infrared technique was used to measure adiabatic film effectiveness and heat transfer coefficient simultaneously for three blowing ratios of 1.0, 2.0, and 3.0. For all the cases, gap-to-blade span ratio of 1% was used. The depth-to-blade span ratio of 0.0416 was used for the recessed tip. Pressure measurements on the shroud were also taken to characterize the leakage flow and understand the heat transfer distributions. For tip injection, when blowing ratio increases from 1.0 to 2.0, film effectiveness increases for both plane and recessed tip and heat transfer coefficient decreases for both plane and recessed tip. At blowing ratio 3.0, lift-off is observed for both cases. In case of pressure side coolant injection and for plane tip, lift-off is observed at blowing ratio 2.0 and reattachments of jets are observed at blowing ratio 3.0. But, almost no effectiveness is observed for squealer tip at all blowing ratios with pressure side injection with reduced heat transfer coefficient along the pressure side. For combination case, very high effectiveness is observed at blowing ratio 3.0 for both plane and recessed blade tip. It appears that for this high blowing ratio, coolant jets from the tip hit the shroud first and then reattach back onto the blade tip with very high heat transfer coefficients for both plane and recessed blade tip. [DOI: 10.1115/1.2366540]

The effect of jet pulsation and duty cycle on ,film effectiveness and heat transfer was investigated on a film hole located on the circular leading edge of a blunt body. A transient infrared technique was used to measure both heat transfer coefficients and film effectiveness from a single test. Detailed Frossling number and film effectiveness distributions were obtained for all flow conditions. Jet pulsing frequencies of 5 Hz, 10 Hz, and 20 Hz have been studied. The effect of duty cycle created by the valve opening and closing times was also set at different levels of 10%, 25%, 50%, and 75% of designated 100% fully open condition for different blowing ratios from 0.25 to 2.0. The combination of pulse frequency and duty cycle was investigated for different blowing ratios on a single leading edge hole located at 22 deg from geometric leading edge. Results indicate that higher effectiveness and lower heat transfer coefficients are obtained at the reduced blowing ratios, which result from reduced duty cycles. The effect of varying the pulsing frequency from 5 Hz to 20 Hz is not discernable beyond the level of experimental uncertainty. Effective blowing ratio due to lowering of the duty cycle at a given blowing ratio seems to play a more important role in combination with pulsing, which provides improved cooling effectiveness at lower heat transfer coefficients.

In film cooling situations, there is a need to determine both local adiabatic wall temperature and heat transfer coefficient to fully assess the local heat flux into the surface. Typical film cooling situations are termed three temperature problems where the complex interaction between the jets and mainstream dictates the surface temperature. The coolant temperature is much cooler than the mainstream resulting in a mixed temperature in the film region downstream of injection. An infrared thermography technique using a transient surface temperature acquisition is described which determines both the heat transfer coefficient and film effectiveness (nondimensional adiabatic wall temperature) from a single test. Hot mainstream and cooler air injected through discrete holes are imposed suddenly on an ambient temperature surface and the wall temperature response is captured using infrared thermography. The wall temperature and the known mainstream and coolant temperatures are used to determine the two unknowns (the heat transfer coefficient and film effectiveness) at every point on the test surface. The advantage of this technique over existing techniques is the ability to obtain the information using a single transient test. Transient liquid crystal techniques have been one of the standard techniques for determining h and [eta] for turbine film cooling for several years. Liquid crystal techniques do not account for nonuniform initial model temperatures while the transient IR technique measures the entire initial model distribution. The transient liquid crystal technique is very sensitive to the angle of illumination and view while the IR technique is not. The IR technique is more robust in being able to take measurements over a wider temperature range which improves the accuracy of h and [eta]. The IR requires less intensive calibration than liquid crystal techniques. Results are presented for film cooling downstream of a single hole on a turbine blade leading edge model.

The effect of squealer geometries on the blade tip leakage flow and associated heat transfer is analyzed for a scaled up high pressure turbine blade in a low-speed wind tunnel facility. Results reveal that the presence of the squealer alters the tip gap flow field significantly and produces lower overall heat transfer coefficients.