Upstream Disk Research Articles

Numerical investigation on the effects of HC geometry on the flow and heat transfer in the disk cavity system at the turbine vane root

In this study, we numerically investigate the flow and heat transfer in the sealing system at the vane root of a heavy-duty turbine with the aim of determining the optimal geometry of honeycomb cells within a certain parameter range. First, we analyze the impact of honeycomb cell geometry on flow in the sealing gaps, including flow rotation, energy dissipation in the teeth chamber and honeycomb cell, and frictional losses. Next, we examine the heat transfer capacity between the leakage flow and the surfaces of the rotor or stator, and establish the relationship between the stream function and the average heat transfer coefficient. Third, we compare the predicted core swirl ratio, the most important aerodynamic parameter in disk cavities, with those from equations in the literature. The decrease in core swirl ratio in the upstream disk cavity indicates that the use of different honeycomb cell geometries may lead to excessive axial thrust. Finally, we study the local and average heat transfer capacities on the turbine disk and introduce a correction factor (0.12–0.21) into the correlation to make it applicable to analogous turbine disks or rotor–stator cavities in other turbo machines with low radii honeycomb labyrinth seals. Based on our findings, we provide recommendations for the optimal honeycomb cell geometries for the two sealings for different targets. These results serve as a reference for designers of honeycomb labyrinth seals for gas turbines and other turbo machines.

Large eddy simulation of turbulent flow and heat transfer in a turbine disc cavity with impellers

The turbine disc cavity is an important component of a gas turbine, supplying airflow to the turbine blades and the sealing labyrinth. Impellers are installed in the cavity to increase pressure, but their mechanism was unknown in previous studies. Large eddy simulations were used in this study to reveal the effect of the impeller on turbulent flow and heat transfer in a turbine disc cavity. In addition, a nonlinear integral model capable of predicting velocity, pressure, and temperature was developed, which takes into account the heat transfer on the disc walls. This was the first engineering application of nonlinear solution of Ekman layer equations. The large eddy simulation results demonstrated that the vortex development is suppressed by the impeller, and the vortex in the impeller channel flows rapidly to the upstream disc, causing the Nusselt number on the upstream and downstream discs to increase and decrease, respectively. The turbulent heat flux in the central region of the cavity was low due to the destruction of the wall jet. The nonlinear integral model was used to calculate a working condition in <1 s, indicating that it has great potential for engineering applications.

Measurement and Analysis of Buoyancy-Induced Heat Transfer in Aero-Engine Compressor Rotors

Abstract The flow inside cavities between corotating compressor disks of aero-engines is driven by buoyancy, with Grashof numbers exceeding 1013. This phenomenon creates a conjugate problem: the Nusselt numbers depend on the radial temperature distribution of the disks, and the disk temperatures depend on the Nusselt numbers. Furthermore, Coriolis forces in the rotating fluid generate cyclonic and anticyclonic circulations inside the cavity. Such flows are three-dimensional, unsteady, and unstable, and it is a challenge to compute and measure the heat transfer from the disks to the axial throughflow in the compressor. In this paper, Nusselt numbers are experimentally determined from measurements of steady-state temperatures on the surfaces of both disks in a rotating cavity of the Bath compressor-cavity rig. The data are collected over a range of engine-representative parameters and are the first results from a new experimental facility specifically designed to investigate buoyancy-induced flow. The radial distributions of disk temperature were collected under carefully controlled thermal boundary conditions appropriate for analysis using a Bayesian model combined with the equations for a circular fin. The Owen-Tang buoyancy model has been used to compare predicted radial distributions of disk temperatures and Nusselt numbers with some of the experimentally determined values, taking account of radiation between the interior surfaces of the cavity. The experiments show that the average Nusselt numbers on the disk increase as the buoyancy forces increase. At high rotational speeds, the temperature rise in the core, created by compressibility effects in the air, attenuates the heat transfer, and there is a critical rotational Reynolds number for which the Nusselt number is a maximum. In the cavity, there is an inner region dominated by forced convection and an outer region dominated by buoyancy-induced flow. The inner region is a mixing region, in which entrained cold throughflow encounters hot flow from the Ekman layers on the disks. Consequently, the Nusselt numbers on the downstream disk in the inner region tend to be higher than those on the upstream disk.

Effects of the rotation number on flow and heat transfer in contra-rotating disk cavity with superposed flow

The turbulent fluid flow and convective heat transfer in counter-rotating disk cavity with central axial air inflow and radial air outflow are numerically studied based on the finite volume method. Efforts are focused upon the influence of the rotation number Rt on the flow structure, cooling performance, sealing effect, and surface tangential friction characteristics in the cavity. The stagnation point where the radial outward flow along the upstream disk driven by the rotation force meets the radial outward flow along downstream disk driven by the combination of rotation force and inflow inertial force moves from upstream disk wall to the shroud with increasing Rt. At the Rt far smaller than 1, the fluids in the core region between two disks rotate with the upstream disk like a rigid body, and the tangential velocity of the rotating core decreases with the increase of the disk cavity radius, which is different from the Batchelor-type flow. At the Rt larger than 1, the fluids on the upstream disk side rotate like the Batchelor-type flow, while the sandwich rotation disappears in the fluid on the downstream disk side. The temperature on the upstream disk wall increases and then decreases with increasing values of Rt, and the critical value of Rt for the change of temperature variation is assessed to be at about Rt = 0.69. The temperature and radial temperature gradient of the downstream disk wall decrease with increasing Rt. With increasing Rt by increasing the disk rotation rate, the pressures near the downstream disk decrease, while the frictional moments on rotating disks increase. Due to the effect of flow structure, the frictional moment on the upstream disk is smaller than that on the downstream disk.

Influence of rotation speed on the steady cooling effects in the turbine disc cavity

The flow and heat transfer characteristics in the counter-rotating turbine disc cavity is numerically simulated using the rng k-ε turbulence model, and the influence of rotation speed on the steady flow and the cooling effects is further investigated. The results indicate that the influence of rotation speed in low speed range on the upstream disc surface heat transfer depends on the relative driving action of rotating radial force and intake inertial force on the flow near the upstream disc. With respect to the downstream disc, the convective heat transfer coefficient on the disc surface in the lower radius region decreases while the one in the higher radius region increases with the increase of rotation speed.

Numerical study on the flow over two circular disks in tandem arrangement

The flows over the two circular disks in tandem arrangement at low Reynolds numbers are numerically studied using large-eddy simulations. Both disks have the same aspect ratio (χ = d/w = 5, where d and w are, respectively, the diameter and the thickness of the disk), while the upstream disk has a coaxial hole in D diameter. First, the effect of the Reynolds number on the near-wake evolution of the tandem disks with spacing of l/d = 0.1 is investigated. Compared with a single circular disk, it is found that the upstream apertured disk (D/d = 0.2) delays the wake transition. Four bifurcations, i.e., “Steady state,” “Zig-zig,” “Zig-Zag,” and “Weakly chaotic” modes, are captured at critical Reynolds numbers of 178, 207, 212, and 275, respectively. Second, the effect of disk spacing on near-wake evolution is studied for Re = 200 and D/d = 0.2. The wake is steady and planar-symmetric at l/d = 1, in which a totally new wake mode characterized by a three-thread wake is observed. The wake becomes unsteady but planar-symmetric at l/d = 1.5. At l/d = 2, however, the planar-symmetric wake structures are all broken. The upstream disk wake retains planar-symmetry, while the downstream disk wake is similar to the “Reflectional-symmetry-broken” mode in a single disk wake. The planar symmetry is recovered for both disks at l/d = 6. It is found that for l/d &amp;gt; 3, the interaction between the two disks becomes weaker as the spacing increases. Finally, the effect of disk spacing on the drag and lift coefficients for each disk is examined. When disk spacing is constant, the lift coefficient of the upstream disk is always lower than that of the downstream disk. The drag coefficient of each disk in tandem is smaller than that of a single disk. With the increase in disk spacing, the drag coefficient of the upstream disk changes little while the drag coefficient of the downstream disk increases rapidly.

Electrochemical Generator-Collector Measurements to Decipher Inter-Electrode Communication Mechanisms in Advanced Batteries

Both established lithium-ion and emerging sodium- and magnesium-ion battery chemistries require improved cycle and calendar life in order for electric vehicles to gain widespread market penetration. One of the most serious lifetime problems for these nonaqueous batteries is electrolyte degradation from unwanted side reactions. Electrolyte reduction into insoluble products causes formation and growth of the solid electrolyte interphase (SEI) and has been well-studied. However, both oxidation and reduction can also result in soluble gaseous or liquid products, which then can migrate to the opposite electrode and contribute to electrochemical activity. Such inter-electrode ‘communication’ reactions may be detrimental to battery lifetime; for example, dissolution of transition metals from the cathode and their deposition at the anode is known to accelerate capacity fade [1]. However, communication can also be beneficial when harmful products generated from the cathode are consumed by reduction at the anode[2]. Controlling chemical communication between electrodes for improved battery lifetime requires better understanding of the chemical reactions that generate soluble products. Electrochemical methods are particularly advantageous because they can directly assess the redox activity of reaction products. In particular, four-electrode generator-collector measurements such as the rotation ring-disk electrode (RRDE) are commonly used to quantify reaction selectivity in fuel cell catalysis. Despite their utility, these methods are seldom applied to nonaqueous battery research. In this work, we apply generator-collector measurements and other electroanalytical techniques in order to understand how chemical communication between electrodes affects cell lifetime. In one application of our approach, the RRDE is used to probe mechanisms of transition metal incorporation into the SEI. Fig. 1(a) shows SEI formation on RRDE. The platinum ring was cycled through SEI formation potentials of 3.0-0.1 V while the gold disk electrode was held at a potential of 4.8 V and the entire assembly rotated at 400 rpm. In contrast to a standard battery, the potentials of the upstream `cathode' (disk) and downstream `anode' (ring) are controlled separately, and current does not necessarily balance between the two. For clarity, only the first of three formation cycles are shown. When the upstream disk electrode was coated with particles of LiNi0.5Mn1.5O4 (red curve), SEI formation showed additional peaks corresponding to reduction of metal cations, or formation of metal-catalyzed reduction products. After SEI formation, 0.01M ferrocene was added to the electrolyte and the disk was held at 3.5V to oxidize ferrocene while the ring was cycled to low potentials for ferrocenium reduction [3]. The ring current was normalized by the disk oxidation current and is shown in Fig. 1(b). After forming SEI on the ring, the through-film mediator reduction is subject to kinetic and transport limitations (blue), in contrast to the clean Pt ring (grey). These limits are greatly decreased when the SEI is formed in the presence of cathode particles (red), demonstrating that the film is less electronically passivating and ionically resistive. Further experiments show that the through-film diffusivity increases by a factor of eight, while the through-film exchange current density increases by several orders of magnitude. These observations suggest that Mn induces both higher porosity due to mechanical cracking or gas evolution as well as electron conduction through metallic Mn centers. Future work will discuss the mechanism of passivation in greater detail and our development of improved methods for studying these and related reactions. [1] J. A. Gilbert, I. A. Shkrob, and D. P. Abraham, “Transition Metal Dissolution, Ion Migration, Electrocatalytic Reduction and Capacity Loss in Lithium-Ion Full Cells,” J. Electrochem. Soc., vol. 164, no. 2, pp. A389–A399, 2017. [2] D. J. Xiong, R. Petibon, M. Nie, L. Ma, J. Xia, and J. R. Dahn, “Interactions between Positive and Negative Electrodes in Li-Ion Cells Operated at High Temperature and High Voltage,” J. Electrochem. Soc., vol. 163, no. 3, pp. 546–551, 2016. [3] M. H. Tang and J. Newman, “Electrochemical Characterization of SEI-Type Passivating Films Using Redox Shuttles,” J. Electrochem. Soc., vol. 158, no. 5, pp. A530–A536, 2011. Figure 1

Gas-Cooling of Multiple Short Inline Disks in Flow Along Their Axis

Abstract To learn about cooling of gas quenched batches, this paper reports on numerical predictions of local and average convective heat transfer coefficients, and overall pressure drops, on batches of five axially aligned, constant temperature, short cylindrical disks (25 cm diameter, 5 cm thickness) with and without a concentric hole, with interdisk spacings of 5–20 cm, in axial turbulent flows of 20 bar nitrogen gas at inlet speeds from 10 m/s to 100 m/s, corresponding to Reynolds numbers (Re) between 3.27×106 and 32.7×106. The heat transfer coefficients along the disk surfaces vary strongly up to a worse case of two orders of magnitude for the upstream disk. This nonuniformity is much lower for the disks downstream, especially after spacing is increased beyond 0.1 m. As expected, the upstream disk exhibited rather different heat transfer coefficients than the ones downstream, the magnitude of the heat transfer coefficient and its uniformity increased with the interdisk spacing, and varied by a factor of about 4–5 along the surfaces. The average heat transfer coefficient (Nusselt number, Nu) on the disks increased approximately with Reynolds number as Re0.85. Re did not have much influence on the nonuniformity of Nu on the disk surfaces. The overall pressure drop along the flow increases with the interdisk spacing, rising by about 60 % as the spacing is increased from 0.05 m to 0.20 m. The presence of a hole increases the heat transfer coefficient in all cases. Some suggestions for reducing the heat transfer coefficient nonuniformity are made.

Buoyancy-Affected Flow and Heat Transfer in Asymmetrically Heated Rotating Cavities

Finite-volume predictions are presented for the convective heat transfer rates in a rotating cavity, formed by two corotating plane disks and a peripheral shroud, and subjected to a radial outflow of cooling air. The heating of the disks is asymmetric, the air entering the cavity through a central hole in the cooler (upstream) disk. The predicted Nusselt number distributions for each disk are compared with unpublished data from the University of Sussex for dimensionless mass-flow rates in the range 2800 ⩽ Cw ⩽ 14,000 and rotational Reynolds numbers, Reθ, up to 5.2 × 106. A single-grid elliptic procedure was used with turbulent transport represented via a low-Reynolds-number k–ε model and the turbulence Prandtl number concept. In comparing the predicted and measured convective heat fluxes, it is important to consider the radiative heat exchange between the disks. This is estimated using a conventional view-factor approach based on black-body emission. Under conditions of asymmetric heating, rotationally induced buoyancy forces can exert significant effect on the flow structure, the induced motion tending to oppose that imposed by the radial outflow. Indeed, flow visualization studies have revealed that, as the rotational Reynolds number is increased (for a fixed value of Cw), the flow in the source region initially becomes oscillatory in nature, leading eventually to the onset of chaotic flow in which the usual Ekman layer structure does not persist in all angular planes. The extent to which the effects of such flow behavior can be captured by the steady, axisymmetric calculation approach used here is questionable, but it is found that the turbulence model (used previously for the prediction of heat transfer in symmetrically heated cavities) still leads to good (± 10 percent) predictive accuracy for the heated (downstream) disk. However, the predicted Nusselt numbers for the cooler (upstream) disk generally show little accord with experimental data, often signifying heat flow into the disk instead of vice versa. It is concluded that the modeling of the turbulent heat transport across the core region of the flow is erroneous, especially at high rotational Reynolds numbers: This is attributed to overestimated turbulence energy production in that region due to the action of the radial-circumferential component of shear stress (νw). Adoption of an algebraic-stress model for this shear stress is partly successful in removing the discrepancies between prediction and experiment.

Computation of Heat Transfer in Rotating Cavities Using a Two-Equation Model of Turbulence

The paper presents finite-difference predictions for the convective heat transfer in symmetrically heated rotating cavities subjected to a radial outflow of cooling air. An elliptic calculation procedure has been used, with the turbulent fluxes estimated by means of a low Reynolds number k–ε model and the familiar “turbulence Prandtl number” concept. The predictions extend to rotational Reynolds numbers of 3.7 × 106 and encompass cases where the disk temperatures may be increasing, constant, or decreasing in the radial direction. It is found that the turbulence model leads to predictions of the local and average Nusselt numbers for both disks that are generally within ± 10 percent of the values from published experimental data, although there appear to be larger systematic errors for the upstream disk than for the downstream disk. It is concluded that the calculations are of sufficient accuracy for engineering design purposes, but that improvements could be brought about by further optimization of the turbulence model.

The Performance of Two Butterfly Valves Mounted in Series

The results of an experimental investigation concerning the operating characteristics of two similar butterfly valves mounted in series are reported. Emphasis is given to the influence of the upstream valve disk angle, the downstream valve disk angle, the relative valve orientation, and the spacing between the valves. The dimensionless pressure drop, the mass flowrate coefficient, and the aerodynamic torque coefficient of each valve are used to characterize the system performance. The results show that the operating characteristics are strongly tied to the combined effect of the two valve disk angles. With noted exceptions, the valve disk orientation and spacing are secondary influences.

Computation of Forced Laminar Convection in Rotating Cavities

Finite difference solutions are presented for forced laminar convection in a rotating cylindrical cavity with radial outflow. This forms a simple model of the cooling flow between two compressor disks in a gas turbine engine. If the fluid enters the cavity from a uniform radial source, it is shown that the local Nusselt number changes from that of a “free disk” near the center of the cavity to that for Ekman layer flow at larger radii. With an axial inlet, the flow, and consequently, the heat transfer, is more complex. If vortex breakdown occurs, then the results are very similar to those for the radial inlet case, but otherwise a wall jet forms on the downstream disk, and the heat transfer from this disk may be several times that for the upstream disk. Variation of mean Nusselt number with rotational speed is qualitatively similar to previously published experimental measurements in turbulent flow. The effect of Prandtl number on heat transfer has also been demonstrated.