D. Ewing

Heat transfer to impinging round jets with triangular tabs

Abstract Experiments were performed to characterize the heat transfer enhancement produced by adding arrays of triangular tabs to the exit of turbulent round impinging jets issuing from a long pipe. For small nozzle-to-plate distances the local heat transfer was increased more than 25% in a series of distinct regions surrounding the impingement region. The largest increase in the average Nusselt number occurred for a nozzle-to-plate distance of approximately 4 diameter. In this case, the average Nusselt number was increased by 20% for the impingement region but only approximately 10% for the region with a radius of 3 jet diameters. Measurements of the velocity field were performed in free jets with tab arrays to investigate how the tabs modify the development of the flow.

Characterization of the Soot Deposition Profiles in Diesel Engine Exhaust Gas Recirculation (EGR) Cooling Devices Using a Digital Neutron Radiography Imaging Technique

A non-destructive neutron radiography technique was used to measure the thickness of diesel soot deposited in the tubes of exhaust gas recirculation (EGR) cooling devices. Measurements were performed to characterize the fouling in single-tube and three-tube devices for laminar and turbulent flows. Measurements were also performed to characterize the effect that the design of the inlet header had on the deposition characteristics in the device. The analysis of the neutron images showed that the soot deposition in the single-tube device occurred at a faster rate for a turbulent flow than for a laminar flow. The deposition thickness decreased along the tubes for both flow regimes. More soot deposited in the center tube of the three-tube bundle for the expansion angle 45° inlet header suggesting there was an uneven distribution of the exhaust gas flow in the tube bundle. For the device with the expansion angle 60° inlet header the soot was approximately more evenly distributed along the tubes.

Three-Dimensional Turbulent Wall Jets Issuing from Moderate-Aspect-Ratio Rectangular Channels

The development of three-dimensional turbulent wall jets emanating from long channels with outlet cross-sectional aspect ratios from 1 to 8 was investigated by measuring the mean and turbulent flowfields using hot-wire anemometry. The turbulent velocity profiles indicate that the core of the jet behaves like a two-dimensional wall jet before the interaction of the lateral shear layers. Contours of the full flowfield indicate that the turbulent mechanism that causes the lateral growth of the three-dimensional wall jets is located in the lateral shear layers near the wall. Increasing the outlet aspect ratio separates the lateral shear layers, causing a wider core region of two-dimensional wall-jet development that, in turn, delays the onset of far-field three-dimensional wall-jet development. The development of the different aspect-ratio wall jets collapsed onto a single curve when the streamwise coordinate was normalized by the square root of the channel cross-sectional area and the vertical and lateral jet half-widths were normalized by the height and width of the channel, respectively.

Heat Transfer Enhancement in Axial Taylor-Couette Flow

An experimental investigation was performed to determine the effect that surface roughness has on the heat transfer in an axial Taylor-Couette flow. The experiments were performed using an inner rotating cylinder in a stationary water jacket for Taylor numbers of 106 to 5×107 and axial Reynolds numbers of 900 to 2100. Experiments were performed for a smooth inner cylinder, a cylinder with two-dimensional rib roughness and a cylinder with three-dimensional cubic protrusions. The heat transfer results for the smooth cylinder were in good agreement with existing experimental data. The change in the Nusselt number was relatively independent of the axial Reynolds number for the cylinder with rib roughness. This result was similar to the smooth wall case but the heat transfer was enhanced by 5% to 40% over the Taylor number range. The Nusselt number for the cylinder with cubic protrusions exhibited an axial Reynolds number dependence. For a low axial Reynolds number of 980, the Nusselt number increased with the Taylor number in a similar way to the other test cylinders. At higher axial Reynolds numbers, the heat transfer was initially independent of the Taylor number before increasing with Taylor number similar to the lower Reynolds number case. In this higher axial Reynolds number case the heat transfer was enhanced by up to 100% at the lowest Taylor number of 1×106 and by approximately 35% at the highest Taylor number of 5×107 .Copyright

On the Design of an Aero-Engine Nose Cone Anti-Icing System Using a Rotating Heat Pipe

The feasibility of using a rotating heat pipe to anti-ice the nose cones of small turbofan aero-engines is investigated. A stationary jacket evaporator design was used to transport heat into the rotating heat pipe located along the central fan shaft of the engine. The rotating heat pipe condenser was made an integral part of the nose cone using a high conductivity, lightweight material and the tip of the nose cone. The use of heating channels along the nose cone and passive heat transfer enhancement in the evaporator were also investigated. The computational model used to predict the heat transfer performance is outlined. The overall heat transfer to the nose cone was 0.8–1.2 kW using water in the heat pipe and 0.4–0.75kW using ethanol. The heating channels were not effective due to the small contact area with the nose cone. The heat transfer enhancement in the evaporator increased the total heat transfer modestly and the temperature of the nose cone increased over the contact area made with the high conductivity material. The results show that rotating heat pipes are a feasible nose cone anti-icing technology.

Fabrication of

Atomic layer deposition (ALD) provides a promising approach for deposition of ultrathin low-defect-density tunnel barriers, and it has been implemented in a high-vacuum magnetron sputtering system for in situ deposition of ALD-Al₂O₃ tunnel barriers in superconductor-insulator-superconductor Josephson junctions. A smooth ALD-Al₂O₃ barrier layer was grown on an Al-wetted Nb bottom electrode and was followed with a top Nb electrode growth using sputtering. Preliminary low temperature measurements of current-voltage characteristics of the Josephson junctions made from these trilayers confirmed the integrity of the ALD-Al₂O₃ barrier layer. However, the <i>I</i>_c<i>R</i>_N product of the junctions is much smaller than the value expected from the Ambegaokar-Baratoff formula suggesting a significant pair-breaking mechanism at the interfaces.

\hbox{Nb/Al}_{2}\hbox{O}_{3}/\hbox{Nb}

A one-dimensional steady state model was developed to predict the heat transfer performance of a shell (liquid)-and-tube (gas) heat exchanger used as a cooling device for exhaust gas recirculation (EGR) application where there is a significant temperature drop across the device. The predictions of the model results were compared with experimental measurements and the trends were found to be in good agreement for most of the transitional and turbulent regimes. The results showed that the exit gas temperature increases with increasing gas mass flow rate at fixed gas inlet temperature and coolant flow rate. It was also found that the exit gas temperature was essentially independent of the coolant flow rate for the typical operating range but did depend on the coolant inlet temperature. It was observed that the pressure drop across the cooling device was not a strong function of the gas inlet temperature. The heat-transfer effectiveness of the cooling device was found to slightly depend on the gas mass flow rate and inlet gas temperature. A preliminary analysis showed that fouling in the EGR cooling device can have a significant effect on both the thermal and hydraulic performance of the cooling device.Copyright

Josephson Junctions Using In Situ Magnetron Sputtering and Atomic Layer Deposition

The development of three-dimensional turbulent wall jets were formed using rectangular channels of moderate aspect ratios from Ar = 1 to 8 have been investigated using profiles and contours of the turbulent velocities in the region x/h = 3 to 60. The decay of the streamwise velocity and the lateral and vertical growth of the jet half widths could be collapsed by scaling by the square root of area and normalizing by the initial width and height of the jet. Although this scaling collapses the mean flow contours, the Reynolds stresses and the streamwise vorticity differ indicating that the physics of the flow is different.

The Heat Transfer Characteristics of Exhaust Gas Recirculation (EGR) Cooling Devices

The evolution of the large-scale structures in the impinging round jet were studied by measuring the fluctuating pressure on the impingement surface for nozzle-to-plate distances of 2.0, 3.0 and 4.0 nozzle diameters. It is found that the large-scale vortex ring structures played a much more dominant role when the nozzle-to-plate spacing was 2.0 diameters than for either 3.0 or 4.0 diameters. The results for a nozzle-to-plate spacing of 3.0 nozzle diameters more closely resembles the spacing of 4.0 diameters. The convection velocity of the different azimuthal modes were deduced from radial cross-spectra measurements. It was found that the convection velocity of all the azimuthal modes were similar and the convection speed for the structures measured with the fluctuating pressure were independent of nozzle-to-plate distance.Copyright

The Rectangular Wall Jet. Part 1: The Effect of Varying Aspect Ratio

The local mass transfer in a 203mm diameter back to back bend arranged in a S-configuration was measured at a Reynolds number of 300,000. A dissolving wall method using gypsum dissolution to water at 40°C was used, with a Schmidt number of 660. The experiment was performed in a flow loop by flowing water through the test section. The topography of the unworn and the worn inner surface was quantified using nondestructive X-ray Computed Tomography (CT) scans. The two scanned surfaces were aligned to a common coordinate system using commercial software and in-house routines. The local mass transfer rate was obtained from the local change in radius over the flow time. Two regions of high mass transfer were present: (i) along the intrados of the first bend near the inlet and (ii) at the exit of the extrados of the first bend that extends to the intrados of the second bend. The latter was the region of highest mass transfer in the S-bend.Copyright