Flow and Heat Transfer Mechanisms in a Rotating Compressor Cavity Under Centrifugal Buoyancy-Driven Convection

This paper presents a systematic study of flow and heat transfer mechanisms in a compressor disc cavity with an axial throughflow under centrifugal buoyancy-driven convection, comparing with previously published experimental data. Wall-modelled large-eddy simulations are conducted for six operating conditions, covering a range of rotational Reynolds number (3.2x10^5 - 2.2x10^6), buoyancy parameter (0.11 - 0.26) and Rossby number (0.4 - 0.8). Numerical accuracy and computational efficiency of the simulations are considered. Wall heat transfer predictions are compared with measured data with a good level of agreement. A constant rothalpy core occurs at high Eckert number, appearing to reduce the driving buoyancy force. The flow in the cavity is turbulent with unsteady laminar Ekman layers observed on both discs except in the bore flow affected region on the downstream disc cob. The shroud heat transfer Nusselt number-Rayleigh number scaling agrees with that of natural convection under gravity for high Rayleigh numbers. Disc heat transfer is dominated by conduction across unsteady Ekman layers, except on the downstream disc cob. The disc bore heat transfer is close to a pipe flow forced convection correlation. The unsteady flow structure is investigated showing strong unsteadiness in the cavity that extends into the axial throughflow.

Flow and Heat Transfer Mechanisms in a Rotating Compressor Cavity Under Centrifugal Buoyancy-Driven Convection

This paper presents a systematic study of flow and heat transfer mechanisms in a compressor disc cavity with an axial throughflow under centrifugal buoyancy-driven convection, comparing with previously published experimental data. Wall-modelled large-eddy simulations are conducted for six operating conditions, covering a range of rotational Reynolds number (3.2 × 105 – 2.2 × 106), buoyancy parameter (0.11 – 0.26) and Rossby number (0.4 – 0.8). Numerical accuracy and computational efficiency of the simulations are considered. Wall heat transfer predictions are compared with measured data with a good level of agreement. A constant rothalpy core occurs at high Eckert number, appearing to reduce the driving buoyancy force. The flow in the cavity is turbulent with unsteady laminar Ekman layers observed on both discs except in the bore flow affected region on the downstream disc cob. The shroud heat transfer Nusselt number-Rayleigh number scaling agrees with that of natural convection under gravity for high Rayleigh numbers. Disc heat transfer is dominated by conduction across unsteady Ekman layers, except on the downstream disc cob. The disc bore heat transfer is close to a pipe flow forced convection correlation. The unsteady flow structure is investigated showing strong unsteadiness in the cavity that extends into the axial throughflow.

A Spatiotemporal Water Vapor–Deep Convection Correlation Metric Derived from the Amazon Dense GNSS Meteorological Network

Deep atmospheric convection, which covers a large range of spatial scales during its evolution, continues to be a challenge for models to replicate, particularly over land in the tropics. Specifically, the shallow-to-deep convective transition and organization on the mesoscale are often not properly represented in coarse-resolution models. High-resolution models offer insights on physical mechanisms responsible for the shallow-to-deep transition. Model verification, however, at both coarse and high resolution requires validation and, hence, observational metrics, which are lacking in the tropics. Here a straightforward metric derived from the Amazon Dense GNSS Meteorological Network (~100 km × 100 km) is presented based on a spatial correlation decay time scale during convective evolution on the mesoscale. For the shallow-to-deep transition, the correlation decay time scale is shown to be around 3.5 h. This novel result provides a much needed metric from the deep tropics for numerical models to replicate.

Twin Tube Shock Absorber Thermo-Mechanical Coupling Simulation

During the working process, temperature of the oil increases and its viscosity decreases due to the damping force, resulting in changes in operating characteristics of the shock absorber. This paper firstly analyzes the mechanism of themogenesis and heat transfer means of shock absorber in details, and then establishes the thermodynamic model of shock absorber with the energy conservation law and the first law of thermodynamics on this basis. After a comparison between the simulation results and the experimental results, causes of error will be analyzed. The study shows that the more influential correlations are the internal flow convection correlation used between the oil of the different chambers and the tubes of the shock absorber and the external flow convection correlation between the outside tube of the shock absorber and the ambient air.

A New Approach to Calculating Fuel Temperature in TRIGA Mark I Research Reactors

Often a forced convection heat transfer coefficient is used to calculate the peak fuel temperature for a rectangular lattice TRIGA core even though the core is cooled by natural convection. The arguments for applying a forced convection empirical relationship are examined and another relationship is suggested. The peak fuel temperature was calculated using two different correlations, Dittus-Boelter and natural convection, for pool temperatures of 30°C and 60°C. The Dittus-Boelter correlation predicted a fuel temperature rise of 1.85°C for this difference in pool temperature, contrary to the predicted rise of 25.64°C from natural convection relationships. Experimental data shows that the relationship of fuel temperature rise with increasing pool temperature is more accurately represented by the natural convection correlation than with Dittus-Boelter. Using a derived natural convection correlation, the calculated peak fuel temperatures then closely match measured data. A procedure was developed to access convective heat transfer coefficient changes in the cladding gap as a function of reactor power for the hot channel which are similar to those presented in literature.

Turbulent Forced Convection Correlation for the In-Tube Flow of Binary Gas Mixtures With 0.1<Pr<0.7

Turbulent forced convection correlations are available in the literature for gases (Pr ∼ 0.7), but the test data leave a gap in the range of Prandtl (Pr) number between 0.1 and 0.7 occupied by binary gas mixtures. In this paper we develop a turbulent forced convection correlation for the Nusselt (Nu) number of in-tube binary gas mixtures for the ranges of Reynolds (Re) number between 104 and 106 and Prandtl (Pr) number between 0.1 and 0.7. A fully connected back-propagation Artificial Neural Network (ANN) is used to learn the pattern of Nu as a function of Re and Pr. Available test data in the range of 0.001 &lt; Pr &lt; 0.1 and 0.7 &lt; Pr &lt; 1000 are provided to the ANN. The test data are separated in two sets to train and test the neural network. A training set with 80% of the data is used to predict a testing set with the remaining 20% of the data. After the network is trained, we make use of the excellent nonlinear interpolation capabilities of ANNs, to predict values of Nu for the sought range 0.1 &lt; Pr &lt; 0.7. These predictions are later used to generate a correlation that aptly covers the complete range of Prandtl numbers.

Enhanced Boiling on Microconfigured Surfaces

New results are presented for pool boiling from vertical, smooth and regularly microconfigured etched silicon surfaces, in saturated water at 1 atm. All specimens were 1.27 cm square and approximately 300 μm thick. The etched microstructures were hexagonal dimples and rectangular trenches. The dimples were 4.1 μm deep and 11.5 μm across, on 22 μm centers. The trenches were 51 μm deep, 12.6 μm wide and 101 μm long, with repeat distances of 22 and 110 μm, in the two directions. The surface densities of the microstructures were 2 × 105 per cm2 for the dimples and 0.4 × 105 per cm2 for the trenches. Electrical heating was accomplished by applying an electrical potential across the phosphorous doped dry side of the silicon specimen substrate. The hexagonally dimpled specimen in the nominal nucleate pool boiling region had heat transfer increased by a factor of 4.2 over that of the smooth specimens. The heat transfer enhancement was a factor of 3.1 over the smooth specimen data, for the trenched specimen data. In the nominally convective-vaporization regime, both the smooth and microconfigured specimens had as much as 5 times the heat transfer compared to a uniform flux natural convection correlation. Comparable heat transfer measurements in subcooled water verified the experimental procedure and also indicated that only a small fraction of this large enhancement may be explained by edge effects, on these small heaters.

Heat Transfer by Free Convection From a Horizontal Wire to Carbon Dioxide in the Critical Region

The free-convection heat transfer from a 0.015-in-dia horizontal platinum wire to carbon dioxide in its critical region is experimentally investigated. The bulk fluid temperature and pressure are varied from 48 deg F to 136 deg F and 1000 psia to 1300 psia, respectively. Wire temperatures up to 1600 deg F are used. The results do not show the sharp rise in the heat flux curves that has been reported in a recent investigation. From the present measurements it appears that the usual free-convection correlation can be used even near the critical state, providing the properties are suitably evaluated.