Cooling Flow Research Articles

An Investigation of Coolant Within Serpentine Passages of a High-Pressure Axial Gas Turbine Blade

Cooling flow behavior is investigated within the multiple serpentine passages with turbulators on the leading and trailing walls of an axial gas turbine blade operating at design-corrected conditions with accurate external flow conditions. Pressure and temperature measurements at midspan within the passages are obtained using miniature butt-welded thermocouples and miniature Kulite pressure transducers. These measurements, as well as airfoil surface pressure field data from a full computational fluid dynamics (CFD) simulation, are used as boundary conditions for a model that provides quantitative values of film-cooling blowing ratio for each film-cooling hole on the blade. The model accounts for the continuously changing cross-sectional area and shape of the channels, frictional pressure loss, convective heat transfer from the solid portion of the blade, massflow reduction as coolant bleeds out through film-cooling or impingement holes, compressibility effects, and the effects of blade rotation. The results of the model provide detailed coolant ejection information for a film-cooled rotating turbine airfoil operating at design-corrected conditions and also account for the highly variable freestream conditions on the airfoil. While these values are commonly known for simpler experimental geometries, they have previously either been unknown or estimated crudely for full-stage experiments of this nature. The better-quantified cooling parameters provide a bridge for better comparison with the wealth of film-cooling work already reported for simplified geometries. The calculation also shows the significant range in blowing ratio that can arise even among a single row of cooling holes associated with one of the turbulated passages, due to significant changes in both coolant and local freestream massfluxes.

Force Based Measurement Method for Cooling Flow Quantification

AbstractQuantification of heat exchanger performance in its operative environment is in many engineering applications an essential task, and the air flow rate through the heat exchanger core is an important optimizing parameter. This paper explores an alternative method for quantifying the air flow rate through compact heat exchangers positioned in the underhood of a passenger car. Unlike conventional methods, typically relying on measurements of direct flow characteristics at discrete probe locations, the proposed method is based on the use of load-cells for direct measurement of the total force acting on the heat exchanger. The air flow rate is then calculated from the force measurement. A direct comparison with a conventional pressure based method is presented as both methods are applied on a passenger car’s radiator tested in a full scale wind tunnel using six different grill configurations. The measured air flow rates are presented and discussed over a wide range of test velocities. The advantages and draw backs of both approaches are compared and discussed in detail. The proposed method is non-intrusive, leaving the heat exchanger core intact, with no need for integration of measurement points over the core region. Due to the measuring principle, the load-cell method will inherently over-predict the air-flow rate. This error is quantified and an empirical correction function is investigated. This paper shows that the corrected force based method determines the air flow rate through a heat exchanger with an accuracy similar to that of traditional pressure/velocity methods while offering a considerable number of advantages.

Abstract TP8: A Novel and Thermally Efficient System for Focal Cerebral Hypothermia

Background: Focal cerebral hypothermia via intracarotid cold saline infusion (CSI) is an emerging therapy for acute stroke. Systems of cold saline delivery tested in humans have been thermally inefficient and failed to achieve target cortical temperatures. We applied a set of thermodynamic enhancements to improve thermal efficiency of CSI systems. Methods: We replicated the previously described CSI system composed of a cold saline bag (4°C), connected to a 5-Fr diagnostic angiographic catheter immersed in a model of aortic counterflow at 37°C and infusing at a rate of 33 cc/min. Three adjustments were introduced to the system: catheter insulation via a shuttle catheter, higher catheter flow rate (50 cc/min) and external cooling of the system. Those were collectively labeled Cold Preservation via External Cooling, Insulation and Supplemented flow (Cold-PRECISE). We compared the standard system with Cold-PRECISE and with each adjustment separately. Primary outcome was temperatures collected at tubing outlet, catheter tip and the estimated perfusate temperature. We used ANCOVA to compare primary outcomes adjusting for potential confounders. Results: Individual system adjustments improved thermal efficiency at all primary endpoints (Table 1). Compared to standard CSI, insulation and higher flow rate reduced perfusate temperatures by 0.33 and 1.96°C respectively (p <0.001). External cooling reduced tubing outlet by 8.95°C (p 0.03) but had no statistically significant impact on perfusate temperature. The collective implementation of system adjustments had a synergistic effect with catheter tip and perfusate temperatures of 19.57 ± 0.34 and 29.87 ± 0.13 vs 27.13 ± 0.13 and 34.43 ± 0.05 respectively (p <0.001). Conclusion: Cold-PRECISE system preserved coldness of locally infused saline, achieving perfusate temperatures that are estimated to meet target cortical temperatures. The clinical efficacy of this system should be evaluated in future clinical trials.

Thermal Behavior of Radial Foil Bearings Supporting an Oil-Free Gas Turbine: Design of the Cooling Flow Passage and Modeling of the Thermal System

Gas foil bearings (GFBs) have many noticeable advantages over the conventional rigid gas bearings, such as frictional damping of the compliance structure and tolerance to the rotor misalignment, so they have been successfully adopted as the key element that makes possible oil-free turbomachinery. As the adoption of the GFB increases, one of the critical elements for its successful implementation is thermal management. Even though heat generation inside the GFB is small due to the low viscosity of the lubricant, many researchers have reported that the system might fail without an appropriate cooling mechanism. The objective of the current research is to demonstrate the reliability of GFBs installed in the hot section of a micro-gas turbine (MGT). For the cooling of the GFBs, we designed a secondary flow passage and thermohydrodynamic (THD) analysis has been done for temperature prediction. In the analysis, the 3D THD model for the radial GFB extended to include the surrounding structure, such as the plenum, chamber, and the rotor in the solution domain by solving global mass and energy balance equations. In the MGT, the pressurized air discharged from the compressor wheel was used as the cooling air source, and it was injected into the plenum between two radial GFBs. We monitored the pressure and temperature of the cooling air along the secondary flow passage during the MGT operation. No thermal instability occurred up to the maximum operation speed of 43,000 rpm. The test results also showed that the pressure drop between the main reservoir and the plenum increases with an increasing operation speed, which indicated an increased cooling air flow into the plenum. The plenum and bearing sleeve temperature was maintained close to the cooling air source temperature for the entire speed due to a sufficient cooling air flow into the bearing. In addition, the direct injection of the cooling air from the main stream lowered the bearing sleeve temperature by 5–20 °C over the injection through the reservoirs. The predicted plenum and bearing sleeve temperatures with the developed THD model show good agreement with the test data.

CFD Modeling of Film Cooling Flow with Inclined Jets

Film cooling has been widely used to control temperature of high temperature and high pressure blades. In a film cooled blade the air taken from last compressor stages is ejected through discrete holes drilled on blade surface to provide a cold layer between hot mainstream and turbine components. A comprehensive understanding of phenomena concerning the complex interaction of hot gasses with coolant flows in a vane passage plays a major role in the definition of a well performing film cooling scheme. In this study turbulent film cooling flow has been studied numerically. The computational simulation is conducted by employing the Reynolds Averaged Navier-­Stokes (RANS) approach. The standard k-e model with enhanced wall treatment has been implemented for modeling the turbulent flow. Effects of different cooling holes temperature have been studied on the surface of the blade and as results show the temperature of the surface reduces significantly as the temperatures of the cooling holes decreases.

Effect of Local Cooling and Blood Flow Restriction on Muscle Weakness and Atrophy Caused by Detraining

Effect of Local Cooling and Blood Flow Restriction on Muscle Weakness and Atrophy Caused by Detraining

Upstream and Downstream Step Curvature Effects on Film Cooling Effectiveness and Flow Structures

In this study, computational simulations were made using ANSYS CFX to predict the influence of upstream and downstream step curvature radii on the film cooling effectiveness. Five upstream steps and four downstream steps with different curvature radii were studied. The curvature radii of these steps of R = 1.64D, 2D, 3D, 4D, and 5D were used. The film cooling effectiveness (η) of these steps was investigated and compared with experiment. The near-field flow phenomena such as the pressure coefficient and the jet trajectories were discussed. Results indicate that the step position in the upstream of a film hole has a higher effect on the film cooling effectiveness compared with the downstream position. The film cooling effectiveness increases in the case of the upstream curved steps at lower radii due to entrapping more coolant in the region upstream of a hole. The results show that the pressure difference has an important effect to draw the coolant in the upstream region of a film hole compared with the case without step.

Gas Turbine Cooling Flows and Their Influence in Output

This article presents the importance of understanding cooling flow monitoring especially when applied to land-based gas turbines. Cooling flows are necessary for the engine to function; however, too much cooling has a negative impact on the performance and output. Strategically placed instrumentation in the cooling flow delivery system can monitor the health and hence the output of the gas turbine generator utilized in a simple or combined cycle operation. In order to monitor cooling flows, a good approach is to look at disc cavity temperatures as well as bypass valve positions. It is best to trend both bypass valve position and disc cavity temperatures over a range of temperatures and engine load operation to get a better idea if the orifice plates in the main lines are sized properly. A quick way to determine whether there are cooling issues in an engine or not is to trend disc cavity temperature and bypass valve positions {AQ: Edits have been made in this sentence “A quick…valve positions.” for better readability. Please check and correct if necessary.}

Coupling of Cooling, Solidification and Gravity-driven Flow

The gravity-driven spread of fluids with a free surface can be strongly influenced by solidification of the fluid when it experiences heat loss from its surface and is cooled below its solidification temperature. Examples of interplay between heat loss, solidifica- tion and flow are found in lava flows, which take on many morphologies depending on the fluid properties and flow conditions. Laboratory experiments with viscous polyethylene glycol wax and yield-strength slurry flows under cold water have been useful in rationalising observed morphological types in terms of dynamical regimes depending on cooling rate and extrusion rate. Scaling analyses have also achieved some success in predicting the spreading rates of solidifying flows on both a horizontal floor and a sloping plane.

Investigating the star formation histories of the brightest cluster galaxies

This article is devoted to the study of the central stellar populations of the brightest cluster galaxies (BCGs). High signal-to-noise ratio, long-slit spectra for a sample of 39 galaxies were fitted against two stellar population models, Pegase.HR (P.HR) and Vazdekis/MILES, to determine the star formation histories of the galaxies using full spectrum fitting, to investigate, in particular, whether a single stellar population (SSP) or composite stellar population model provides the better description. Monte Carlo simulations and χ2 maps were used to check the reliability of the solutions. The ages and [Fe/H] were (i) compared with those derived with the Lick indices and (ii) tested against the internal galaxy properties (the velocity dispersions and absolute K-band magnitudes) and the properties of the host cluster environment (X-ray temperatures, luminosities, offsets and the presence of cooling flows (CFs)), to determine whether any statistically significant correlations existed. The results indicate that 79 per cent of the BCG sample could be represented by an SSP fit, while the remaining 21 per cent likely experienced more than one star formation epoch. The correlations showed that the BCGs hosted in CF clusters are generally found closer to the centre of the clusters, while the BCGs in non-CF clusters are generally found further away. The main results suggested that at least some of the galaxies in the BCG sample had a more complex star formation history than first assumed and that the presence of CFs in the clusters could account for some, but not all, of the star formation activity of BCGs.

On the Failure of a Gas Foil Bearing: High Temperature Operation Without Cooling Flow

Implementing gas foil bearings (GFBs) in micro gas turbine engines is a proven approach to improve system efficiency and reliability. Adequate thermal management for operation at high temperatures, such as in a gas turbine or a turbocharger, is important to control thermal growth of components and to remove efficiently mechanical energy from the rotor mainly. The paper presents a test rotor supported on GFBs operating with a heated shaft and reports components temperatures and shaft motions at an operating speed of 37 krpm. An electric cartridge heater loosely inserted in the hollow rotor warms the test system. Thermocouples and noncontact infrared thermometers record temperatures on the bearing sleeve and rotor outer diameter (OD), respectively. No forced cooling air flow streams were supplied to the bearings and rotor, in spite of the high temperature induced by the heater on the shaft outer surface. With the rotor spinning, the tests consisted of heating the rotor to a set temperature, recording the system component temperatures until reaching thermal equilibrium in ∼ 60 min, and stepping the heater set temperature by 200 °C. The experiments proceeded without incident until the heater set temperature equaled 600 °C. Ten minutes into the test, noise became apparent and the rotor stopped abruptly. The unusual operating condition, without cooling flow and a too large increment in rotor temperature, reaching 250 °C, led to the incident which destroyed one of the foil bearings. Post-test inspection evidenced seizure of the hottest bearing (closest to the heater) with melting of the top foil at the locations where it rests on the underspring crests (bumps). Analysis reveals a notable reduction in bearing clearance as the rotor temperature increases until seizure occurs. Upon contact between the rotor and top foil, dry-friction quickly generated vast amounts of energy that melted the protective coating and metal top foil. Rather than a reliability issue with the foil bearings, the experimental results show poor operating procedure and ignorance on the system behavior (predictions). A cautionary tale and a lesson in humility follow.

Thermomechanical Stress Distributions in a Gas Turbine Blade Under the Effect of Cooling Flow Variations

Thermomechanical stresses in gas turbine blades are investigated. Attention is focused on effects caused by varying the cooling airflow that runs through the blade interior, keeping constant a mainstream condition around the blade surface. Stress concentration was predicted numerically under engine real operating conditions. Temperature distributions in the metal blade surface produced by convective boundary conditions were linked with heat conduction within the blade using a conjugate solution. Results of stress concentration in the blade material for reduced cooling flow rate, blocked cooling ducts, and rotation rate were obtained. It is shown that temperature and stress distributions are a strong function of position in blade interior material and surface. Thermomechanical stress concentration was observed in the leading edge, with the endwall region affected by large stress concentration. Stress magnitude increments were found for combined cyclic thermal heating and sustained mechanical loads on specific planes of airfoil span for reduced cooling flow. Also, large stress gradients between leading and trailing regions of the blade were observed. The study reveals that blocking channels increase stresses in the central region of blade transversal cross section.

The jet feedback mechanism (JFM): From supernovae to clusters of galaxies

AbstractWe study the similarities of jet‐medium interactions in several quite different astro‐physical systems using 2D and 3D hydrodynamical numerical simulations, and find many similarities. The systems include cooling flow (CF) clusters of galaxies, core collapse supernovae (CCSNe), planetary nebulae (PNe), and common envelope (CE) evolution. The similarities include hot bubbles inflated by jets in a bipolar structure, vortices on the sides of the jets, vortices inside the inflated bubbles, fragmentation of bubbles to two and more bubbles, and buoyancy of bubbles. The activity in many cases is regulated by a negative feedback mechanism. In CF clusters we find that heating of the intra‐cluster medium (ICM) is done by mixing hot shocked jet gas with the ICM, and not by shocks. Our results strengthen the jet feedback mechanism (JFM) as a common process in many astro‐physical objects. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Experimental and Numerical Study of Gap Size and Cooling Flow Rate Effects on Tip Flow of Gas Turbine Blade

In this paper the flow near blade tip of gas turbines was investigated for blowing ratios M = 0, 0.9, 1.7 and 3, and gap 0.3, 1 and 1.5 percent of blade span. The structure of the flow near blade tip was identified using PIV. Velocity profiles from vertical PIV planes around the blade show the effects of blowing ratio on flow field distribution. Vortex flows were observed in the pressure side of leading edge for M = 0 or larger. Instead, the trailing edge region has high velocity magnitude but attached to blade wall. Numerical simulations were conducted in order to investigate the effects of varying the tip gap. Results show that leakage flow in suction side of trailing edge does not perturb too much the flow structure as in leading edge. It is observed that part of cooling flow redistributes as leakage from the tip cavity towards the suction side as expected. However, leakage on pressure side with high momentum rates was found as well, mainly on leading edge. It is confirmed that leakage mass flow distributes unevenly through blade chord as a function of the tip-gap size, cooling flow and passage conditions of the mainstream with maximum value on trailing edge. The paper shows that increments of blowing ratio reduce the contribution of coolant to leakage flow.

Experimental Study on Film Cooling Flow Characteristics of Turbine Stator Blade at Different Setting Angles

The technology of film cooling is one of the most effective means of protecting the turbine blades. In this paper, flow structures of the turbine stator blade with six hole-rows at different blowing ratio（M=0.5, 1.0 and 1.5）and setting angles（β=40°, 50°, 60°, 70°, 80° and 90°） was measured by PIV in piston flow type of low-speed wind tunnel laboratory. Velocity was analyzed. Results show that: velocity gradient of suction side was much higher than pressure side and increased with setting angle reduction; Adherence of film is influenced by setting angle and blowing ratio, when M=1.0 and β=70° anchorage dependent is best and suction side is greater than pressure side.

Use of an Immersed Mesh for High Resolution Modeling of Film Cooling Flows

The development of a high pressure turbine requires the accurate prediction of flow within and around film cooling holes. However, the length scales inherent to film cooling flows produce a large disparity against those of the mainstream flow; hence they cannot be resolved by a mesh generated for an aerodynamics analysis. Furthermore, the process of meshing cooling holes is not only time consuming but cumbersome; thus making the parametric study of film cooling effectiveness for a given blade geometry, using hole geometry and distribution, very difficult in a design environment. In this paper an immersed mesh block (IMB) approach is proposed which allows the refined mesh of a cooling hole to be immersed into the coarser mesh of a nozzle guide vane (NGV) and solved simultaneously while maintaining mass conservation. By employing two-way coupling, the flow physics in and around cooling holes is able to interact with the mainstream; hence the length scales of both types of flow are appropriately resolved. A generic cooling hole design can then be mapped to a given aerofoil geometry multiple times to achieve an appropriate distribution of cooling holes. The results show that for a realistic transonic blade, a configuration consisting of up to 200 cooling holes can be efficiently and accurately calculated—while retaining the original aerodynamic mesh but with a much enhanced resolution for the film cooling.

Effect of Cooling Flow on the Operation of a HotRotor-Gas Foil Bearing System

Gas foil bearings (GFBs) operating at high temperature rely on thermal management procedures that supply needed cooling flow streams to keep the bearing and rotor from overheating. Poor thermal management not only makes systems inefficient and costly to operate but could also cause bearing seizure and premature system destruction. This paper presents comprehensive measurements of bearing temperatures and shaft dynamics conducted on a hollow rotor supported on two first generation GFBs. The hollow rotor (1.36 kg, 36.51 mm OD and 17.9 mm ID) is heated from inside to reach an outer surface temperature of 120 °C. Experiments are conducted with rotor speeds to 30 krpm and with forced streams of air cooling the bearings and rotor. Air pressurization in an enclosure at the rotor mid span forces cooling air through the test GFBs. The cooling effect of the forced external flows is most distinct when the rotor is hottest and operating at the highest speed. The temperature drop per unit cooling flow rate significantly decreases as the cooling flow rate increases. Further measurements at thermal steady state conditions and at constant rotor speeds show that the cooling flows do not affect the amplitude and frequency contents of the rotor motions. Other tests while the rotor decelerates from 30 krpm to rest show that the test system (rigid-mode) critical speeds and modal damping ratio remain nearly invariant for operation with increasing rotor temperatures and with increasing cooling flow rates. Computational model predictions reproduce the test data with accuracy. The work adds to the body of knowledge on GFB performance and operation and provides empirically derived guidance for successful rotor-GFB system integration.

Prediction of Axial and Circumferential Flow Conditions in a High Temperature Foil Bearing With Axial Cooling Flow

A successful implementation of gas foil bearings (GFBs) into high temperature turbomachinery requires adequate thermal management to maintain system reliability and stability. The most common approach for thermal management in a GFB-rotor system is to supply pressurized air at one end of the bearing to remove hot spots in the bearings and control thermal growth of components. This technical brief presents test data for a laboratory rotor-GFB system operating hot to identify the flow characteristics of axial cooling streams flowing through the thin film region and underneath the top foil. A bulk flow model is used for description of the fluid motion and includes the Hirs’ friction factor formulation for smooth surfaces. Laminar flow prevails through the thin film gas region; while for the cooling flow between the top foil and bearing housing, a transition from laminar flow to turbulent flow occurs as the cooling flow rate increases. Large cooling flow rate and the ensuing turbulent flow conditions render limited effectiveness in controlling temperatures in a test rotor-GFB system.

ACHANDRA-VLA INVESTIGATION OF THE X-RAY CAVITY SYSTEM AND RADIO MINI-HALO IN THE GALAXY CLUSTER RBS 797

We present a study of the cavity system in the galaxy cluster RBS 797 based on Chandra and VLA data. RBS 797 (z = 0.35), is one of the most distant galaxy clusters in which two pronounced X-ray cavities have been discovered. The Chandra data confirm the presence of a cool core and indicate an higher metallicity along the cavity directions. This is likely due to the AGN outburst, which lifts cool metal-rich gas from the center along the cavities, as seen in other systems. We find indications that the cavities are hotter than the surrounding gas. Moreover, the new Chandra images show bright rims contrasting with the deep, X-ray deficient cavities. The likely cause is that the expanding 1.4 GHz radio lobes have displaced the gas, compressing it into a shell that appears as bright cool arms. Finally we show that the large-scale radio emission detected with our VLA observations may be classified as a radio mini-halo, powered by the cooling flow (CF), as it nicely follows the trend P_radio vs. P_CF predicted by the re-acceleration model.

Superiority of Gold versus Platinum Irrigated Tip Catheter Ablation of the Pulmonary Veins and the Cavotricuspid Isthmus: A Randomized Study Comparing Tip Temperatures and Cooling Flow Requirements

In order to optimize power delivery into the myocardium during radiofrequency ablation (RFA) without overheating the electrode tip, active cooling of the tip electrode as well as electrode tips made of gold have evolved. Recently, an externally irrigated gold tip electrode ablation catheter has been developed to combine the advantages of these 2 technologies. We sought to investigate the procedural parameters tip temperature, delivered power and cooling flow requirements of the irrigated gold tip catheter in comparison to the conventional irrigated platinum iridium (Pt) tip catheter in pulmonary vein isolation (PVI) and cavotricuspid isthmus (CTI) ablation. Sixty patients referred for first PVI were randomized into ablation with irrigated gold tip catheter versus irrigated Pt tip catheter. Forty-nine patients received ablation of CTI following PVI. Mean and standard deviation from all measurements were calculated for each patient. During RFA of pulmonary veins, mean catheter tip temperature was significantly lower in the gold group (35.4 ± 0.9 °C vs 38.2 ± 0.8 °C, P < 0.001), and total amount of delivered energy was higher (1303.1 ± 81.1 W vs 1223.7 ± 115.6 W, P = 0.004). During CTI ablation, necessary saline flow was almost 2.5-fold lower in the gold group (22.5 ± 5.9 mL/min vs 52.5 ± 9.7 mL/min, P < 0.001), accompanied by significantly lower tip temperature (39.1 ± 0.6 °C vs 40.5 ± 1.4 °C, P < 0.001). The irrigated gold tip electrode allows to deliver significantly more energy at a lower electrode tip temperature in RFA of PV and CTI in comparison to the irrigated Pt tip electrode. The required saline flow during CTI ablation is much lower than in Pt.