Effect of Pin-fins On Jet Impingement Heat Transfer Over a Rotating Disk

Many engineering applications consist of rotating components which experience high heat load. For instance, applications like the gas turbine engine consist of rotating disks and the study of heat transfer over such rotating surfaces is of particular interest. In the case of gas turbines, the disk also needs to be protected from the ingress of hot turbine gases caused by the low pressure region created due to the radially outward pumping of fluid close to the rotating surface. Present experimental study investigates the effects of introducing pin-fins on heat transfer over surface of a rotating gas turbine disk. Experiments were conducted at rotational Reynolds numbers (ReR) of 5487 - 12803 and jet Reynolds numbers (Re) of 5000 - 18000, nozzle to target spacing (z/d = 2 - 6), impingement eccentricities (e = 0 -0.67), angles of impingement (0°-20°), and the pin fin height (Hf = 3.05mm - 19.05mm). Steady state temperature measurements were taken using thermocouples embedded in the disk, and area average Nusselt number (Nu) was calculated. The results have been compared with those for a smooth aluminum disk. Nu was significantly enhanced by the presence of pin-fins. The enhancement was higher for lower Re and the maximum enhancement was found to be 3.9 times that of a smooth disk for Re = 5000. Qualitative visualization of flow field has been performed for smooth and the pin-fin case using the commercial simulation package Ansys Fluent to further understand the flow features that result in the enhancement.

Eddy resolving simulation of mixed convection in a rotating annular cavity with one heated disk and axial throughflow: the effect of the surface macro-relief

We present the results of hybrid RANS/LES computations of non-isothermal buoyant flow in a rapidly revolving enclosure with paraxial transit stream of the cooling air. Foil heat flux meters mounted on the disk surface in the base experiment are mimicked by means of the grid resolved macro-relief. The results obtained using the relief and smooth disk models are collated with available measurements. According to the simulation, the addition of the relief has resulted in switching from two to three pairs of cyclonic/anti-cyclonic global circulations, and the overall heat transfer rate has increased by 20%. It has been found also that the sensor readings can be up to 25% higher than the heat flux averaged over the circumference at the same radius. Despite this distinct effect of the surface relief, the local heat transfer rate is still underestimated considerably as compared to measurements.

Capacity of roller mill for cement grinding

Engineering for the cement industry is part of the heavy industry. The cement industry is the main supplier of raw materials for the production of concrete and reinforced concrete. For grinding cement, two types of mills are used - ball and roller. Recent decades have proven the great effectiveness of a vertical roller mill for grinding raw materials. Its effectiveness, combined with the implementation of drying, grinding and separation in one unit, gives it an undeniable advantage over a ball mill. This explains the significant increase in the share of roller mills in the cement mill market. The grinding process in such mills occurs due to abrasion, respectively, in the process of work wear of the rubbing parts of the mill occurs. The work evaluated the performance of a mill with smooth disk rolls. During the study, the cause of the destruction of the sectors of the mill produced by FLSmidth, operating in the Russian Federation, was identified. The study revealed the causes of the destruction of the details of the roller mill: with the simultaneous impact of the workload and the displacement of the sectors resulting from intensive wear, the total equivalent stresses exceed the value of the endurance limit under cyclic loading. Therefore, the accumulation of fatigue damage to the material, the formation and growth of cracks, which adversely affects the performance of the mill. A number of measures have been proposed to increase the operability of mills of this design.

GG Tauri A: gas properties and dynamics from the cavity to the outer disk

Context. GG Tauri A is the prototype of a young triple T Tauri star that is surrounded by a massive and extended Keplerian outer disk. The central cavity is not devoid of gas and dust and at least GG Tauri Aa exhibits its own disk of gas and dust emitting at millimeter wavelengths. Its observed properties make this source an ideal laboratory for investigating planet formation in young multiple solar-type stars. Aims. We used new ALMA 13CO and C18O(3–2) observations obtained at high angular resolution (~0.2″) together with previous CO(3–2) and (6–5) ALMA data and continuum maps at 1.3 and 0.8 mm in order to determine the gas properties (temperature, density, and kinematics) in the cavity and to a lesser extent in the outer disk. Methods. By deprojecting, we studied the radial and azimuthal gas distribution and its kinematics. We also applied a new method to improve the deconvolution of the CO data and in particular better quantify the emission from gas inside the cavity. We perform local and nonlocal thermodynamic equilibrium studies in order to determine the excitation conditions and relevant physical parameters inside the ring and in the central cavity. Results. Residual emission after removing a smooth-disk model indicates unresolved structures at our angular resolution, probably in the form of irregular rings or spirals. The outer disk is cold, with a temperature <20 K beyond 250 au that drops quickly (∝r−1). The kinematics of the gas inside the cavity reveals infall motions at about 10% of the Keplerian speed. We derive the amount of gas in the cavity, and find that the brightest clumps, which contain about 10% of this mass, have kinetic temperatures 40−80 K, CO column densities of a few 1017 cm−2, and H2 densities around 107 cm−3. Conclusions. Although the gas in the cavity is only a small fraction of the disk mass, the mass accretion rate throughout the cavity is comparable to or higher than the stellar accretion rate. It is accordingly sufficient to sustain the circumstellar disks on a long timescale.

Study on drag reduction mechanism of rotating disk with micro-grooves

This paper presents the drag reduction mechanism of a rotating disk with micro-grooves. The flow characteristics of the micro-grooved disk at various rotating Reynolds numbers are investigated using experiments and large-eddy simulations. The results show that fluid in the gap between the disks undergoes circumferential movement, but fluid within the micro-grooves undergoes radial movement because of the dual function of wall rejection and boundary layer blockage. As a result, fluid within the micro-grooves moves very slowly and quietly. Hence, quiet and slow-moving fluid within the micro-grooves increases the thickness of the viscous sublayer and recedes mixed layer and suppresses the unstable motion. The mean relative velocity gradient of the immersed surface on the grooved disk becomes much lower than that of a smooth disk, and the contact area between the walls and the high-speed fluid is diminished. An interaction phenomenon between the micro-grooves and the gap could be discovered due to the micro-groove unenclosed structure. The interaction phenomenon makes the quiet fluid within the micro-grooves also suppresses the outside flow. Accordingly, a micro-grooved rotating disk has an obvious drag reduction effect compared with a rotating smooth disk.

A Bladeless Turbocompressor Concept: Shear Driven Gas Compression With Deformable Structures: Part 2 — Operating Principles and Theory

The theory underlying a novel method of gas compression driven by shear flow for next generation turbo-machinery is presented. The concept is based on the conversion of shaft power into hydrodynamic pressure and fluid flow that occur in the shear flow between a smooth rotating disk and a compliant surface counterface. This also holds for the inverse process, where gas expansion through the gap between the compliant surface and a shaft-mounted disk converts gas pressure into rotating power and torque. This is a logical evolutionary step that leverages the proven functionality of self-actuated fluid film compliant foil bearings and seals which operate in the hydrodynamic regime. Thus, as in these devices, the process of compression induced by shear flow is dominated by the balance between pressure and viscous forces which are in turn enhanced and controlled by tribological effects arising between the fluid film and the geometry of the counterface compliant surface. A model based on the compressible Reynolds equation coupled to the thin-plate theory formulation for compliant foil deflection is presented and parametrically solved to predict pressure, flow rate, and shear losses. The smooth disk and four-pad (sectored) compliant counterface effective size (7.6 mm < r < 14.1 mm), disk operating speed (50,000 to 360,000 rpm), nominal initial gap (0.03 mm < h0 < 0.635 mm), and overall operating conditions chosen for the parametric study correspond to those envisioned for eventual practical integration of miniaturized external combustion bladeless gas turbine engines and turbocompressors. Theoretical performance curves reporting flow versus pressure as well as compression power requirements versus speed were obtained. The predictions of the analysis are compared to results obtained experimentally on a proof of concept engine and presented in a companion paper. The simplicity of the bladeless geometry makes it amenable to deployment in multistage configurations, so that in conjunction with its foil bearing predecessors, this novel technology will result in low cost, ultra-high speed, high specific power and power density, high efficiency, oil-free and maintenance-free engines — attractive for many practical applications, ranging from military micro-UAV propulsion and portable power systems, to domestic combined heat and power turboalternators, and even micro-compressors for portable medical devices. As a point of reference, it is anticipated that a 10-stage bladeless compressor based on a compression stage as described herein would have a size comparable to that of a 355 mL soda can delivering a flow of 1 kg/min of compressed air.

A Bladeless Turbocompressor Concept: Shear Driven Gas Compression With Deformable Structures: Part 1 — Experimental Proof of Concept

A novel concept for shear flow driven gas compression that could enable next generation turbomachinery has been designed and experimentally demonstrated. In order to achieve this, a prototype proof-of-concept compliant foil-based bladeless turbo-compressor device was developed and used to conduct a gas compression parametric study. The principle underpinning the operation of this device is the conversion of shaft power into hydrodynamically generated pressure that occurs in the shear flow between a smooth rotating disk and a compliant surface. The present compliant foil bladeless turbocompressor (CFBT) is an evolutionary derivative of self-acting compliant foil bearings and seals, which operate in the hydrodynamic regime. Thus, as in these devices, the process of compression induced by shear flow is dominated by the balance between pressure and viscous forces, which are in turn enhanced and controlled by tribological effects arising between the shear layer and the deformable geometry of the compliant surface. The single shaft foil bearing based proof-of-concept CFBT presented is powered by a permanent magnet motor capable of reaching speeds up 360,000 rpm, and consists of two independent compression stages mounted on opposite ends of the shaft. Each compression stage consists of a smooth disk with the effective corresponding counterface of radii 7.6 mm < r < 14.1 mm, with one of each disk’s surfaces facing a four-pad compliant foil surface mounted on the housing. The nominal initial gap separating each of the disks from their corresponding compliant foils is nominally h0 = 0.025 mm and 0.4 mm, respectively. In this configuration, air is entrained from opposite directions through axial intakes and turned 90° as it undergoes shear between the rotating disk and the compliant foil pads of each of the stages, inducing a net radially-oriented outward flow, which is then collected in the quasi-volute of the respective stage. The system is heavily instrumented, with each of the quasi-volutes fitted with thermocouples, pressure probes and a flow meter. An experimental parametric study was performed compressing standard temperature and pressure air for varying speeds up to 360,000 rpm. Performance curves reporting flow vs. pressure as well as compression power requirements vs. speed were obtained for the individual compression stages. The experimental results on the proof of concept turbocompressor are analyzed in the context of the theoretical foundations presented in a companion paper (Heshmat and Cordova, 2017), showing excellent correlation. It is anticipated that due to its simple bladeless geometry, application of this novel technology in conjunction with foil bearings will result in low cost, ultra-high speed, high efficiency, high specific power, miniaturized turbocompressors and high power density oil-free and maintenance-free machines, such as compressors, meso-scale gas turbines, or turbogenerators. Attractive applications for this technology range from military micro-UAV propulsion and portable power systems, to domestic combined heat and power (CHP) turboalternators and medical devices such as portable oxygen concentrators and CPAP (Continuous Positive Air Pressure) machines.

Boundary Layer Measurements on a Rotating Disk

Dynamic stall occurs on helicopter rotor blades, wind turbine blades, and even insect and bird wings. Although most studies on dynamic stall are conducted assuming two-dimensional behavior, it has been shown that this phenomenon is highly three-dimensional. Recent studies of dynamic stall on a rotating blade of helicopter in forward flight and wind turbines in yaw have shown that the nature of the radial flow near the surface has first-order significance in the stalled flow field. Past literature suggests that the boundary layer over a rotating disk has been correlated to that over a swept wing. Drawing a similar parallel it is hypothesized that fundamental insights into dynamic stall may be derived from studying the boundary layer over a rotating blade in dynamic stall is similar to that over a rotating disk. Particle image velocimetry (PIV) is used to investigate the boundary layer development on a rotating disk. The first set of measurements was conducted using PIV with a micro lens attachment to the camera. Next, a microscope was used to conduct μ-PIV measurements. The measured radial velocity profiles show substantially higher radial jet peak velocities than the analytical solution for a mirror-smooth disk. This difference is narrowed down to the effect of surface roughness of the painted disk, representative of reality on rotor blades used in PIV. This results in a much higher effective viscosity in the near-surface layer, contributing to additional radial flow, as seen from the centrifugal pump literature. However, the non dimensional radial velocity profile exhibits the expected self similar behavior at various radial locations.