S. Ashforth-Frost

Velocity and turbulence characteristics of a semiconfined orthogonally impinging slot jet

Abstract An experimental investigation to determine velocity and turbulence characteristics of a semiconfined (where the nozzle is integral to a confinement plate) impinging slot jet has been undertaken. Jet nozzle to impingement plate spacings of 4 and 9.2 jet widths, at a Reynolds number of 20,000, were considered in detail. Measurements parallel to the impingement plate were made by using hot-wire anemometry, up to 9 jet widths from the axis and across the jet plane, to document the jet exit conditions and jet development. Measurements show that the potential core of the jet is longer for the semiconfined configuration when compared with the unconfined, owing to limited entrainment and spreading of the jet. In the impinging jet, velocity and turbulence data were related directly to heat transfer. Low levels of turbulence observed in the stagnation region and its subsequent development in the wall jet suggest that transition to a turbulent wall jet occurs when the impingement plate is placed within the potential core of the jet. This coincides with the development of secondary maxima in heat transfer. When the impingement plate is placed in the developing jet, heat transfer decreases monotonically along the plate, and the data show that the jet is effectively turbulent at the stagnation point.

An improved cross correlation technique for particle image velocimetry

The standard cross correlation technique frequently used in particle image velocimetry to extract velocity vectors necessitates the assumption that the velocity gradients inside the interrogation area are negligible. However, the procedure is generally video-based, so such an assumption may no longer be valid. This is particularly so in re-circulation zones, in which the distortion between images can be dramatic. A new iterative procedure for re-building the second image, based on velocity gradients of particles due to displacement, rotation and shear, has been proposed. This improved cross correlation algorithm has been shown to be considerably more accurate for simulated uniform, re-circulating and bi-axial shearing flows, and has been applied to the case of natural convection due to a heated horizontal cylinder.

Evaluating convective heat transfer coefficients using neural networks

Liquid crystal thermography combined with transient conduction analysis is often used to deduce local values of convective heat transfer coefficients. Neural networks based on the backpropagation algorithm have been successfully applied to predict heat transfer coefficients from a given set of experimentally obtained conditions. Performance characteristics studied on numerous network configurations relevant to this application indicate that a 3-6-3-1 arrangement yields the least errors with convergence improving directly with both the global learning rates and those of individual layers.

Heat transfer characteristics of a slot jet impinging on a semi-circular convex surface

Abstract Surface heat transfer characteristics of a heated slot jet impinging on a semi-circular convex surface have been investigated by using the transient heating liquid crystal technique. Free jet velocity, turbulence and temperature characteristics have been determined by using a combination of an X-wire and a cold wire anemometry. The parametric effects of jet Reynolds number (ReW) ranging from 5600 to 13,200 and the dimensionless slot nozzle-to-impingement surface distance (Y/W) ranges from 2 to 10 on the local circumferential heat transfer have been studied. Local circumferential Nusselt number (NuS) decreases with increasing the dimensionless circumferential distance (S/W) from its maximum value at the stagnation point up to S/W=3.1. The transition in the wall jet from laminar to turbulent flow was completed by about 3.3⩽S/W⩽4.2 which coincided with a secondary peak in heat transfer. Correlations of local and average Nusselt numbers with ReW, Y/W and S/W have been established for the stagnation point and the circumferential distribution. The rate of decay of average circumferential Nusselt numbers around the semi-circular convex surface is much faster than that which occurs laterally along the flat surface. As Y/W increases, the effect of surface curvature becomes apparent and the difference between the flat surface correlation and the convex surface becomes more pronounced.

Effect of nozzle geometry and semi-confinement on the potential core of a turbulent axisymmetric free jet

The effects of nozzle geometry and confinement on the potential core and subsequent axial development of a turbulent axisymmetric air jet at a Reynolds number of 22 500 have been studied. Four jet exit conditions, namely, flat and fully developed velocity profiles for unconfined and semi-confined cases were investigated. Mean velocity and turbulence profiles were measured using laser-Doppler anemometry. Liquid crystal thermography used in steady state enabled optimal nozzle to plate spacing to be established for maximum heat transfer. Preliminary results presented here indicate that the length of the potential core is greater for the fully developed jet exit profile and is further extended by semi-confinement. The semi-confinement reduces the stagnation point heat transfer by up to ten per cent.

Calibrating for viewing angle effect during heat transfer measurements on a curved surface

Abstract Liquid crystal thermography (LCT) has been widely used for the determination of surface heat transfer distribution. However, this technique is sensitive to illumination and viewing angle and therefore limited to surfaces with only slight curvature. A liquid crystal calibration technique using true-colour image processing system has now been developed to alleviate the effect of viewing angle on oblique/curved surfaces. Application of the calibration and transient liquid crystal thermographic techniques and uncertainty analysis to a heated air slot jet impinging on a semi-cylindrical convex surface has been demonstrated. It is shown that the local heat transfer coefficient may be overestimated by up to 39.1% at a viewing angle of 69° from the normal under test conditions. However, the overall uncertainty in heat transfer coefficient can be significantly reduced from the maximum value of 36.3% to within 11.1% by using the implemented viewing calibration technique.

NUMERICAL PREDICTION OF SEMI-CONFINED JET IMPINGEMENT AND COMPARISON WITH EXPERIMENTAL DATA

SUMMARY The standard k-c eddy viscosity model of turbulence in conjunction with the logarithmic law of the wall has been applied to the prediction of a fblly developed turbulent axisymmetric jet impinging within a semi-conbed space. A single geometry with a Reynolds number of 20,000 and a nozzle-to-plate spacing of two diameters has been considered with inlet boundary conditions based on measured profiles of velocity and turbulence. Velocity, turbulence and heat transfer data have been obtained using laser-Doppler anemometry and liquid crystal thermography respectively. In the developing wall jet, numerical results of heat transfer compare to within 20% of experiment where isotropy prevails and the trends in turbulent kinetic energy are predicted. However, stagnation point heat transfer is overpredicted by about 300%, which is attributed directly to the turbulence model and inapplicability of the wall function. Jet impingment flows are frequently used in industrial practice for their high heat and mass transfer rates. Their employment is common but also diverse and typical applications include many heating, cooling and drying processes such as the manufacture of printed wiring boards, printing processes, production of foodstuffs, de-icing of aircraft wings and cooling of turbine aerofoils. The high heat transfer rates are especially needed to achieve short processing times for product quality or owing to temporal limitations of the process and/or for energy efficiency. The fluid dynamic structure of such processes is extremely complex and as such it is often reduced to that of understanding a single impinging jet, which will be turbulent except at very low Reynolds numbers. Even when the practical application is simplified, the necessary experimental rigs can be cumbersome and expensive, not to mention the time-consuming data acquisition, validation and analysis. Numerical simulations are an alternative to the experimental approach and can provide a fast and economic solution which will describe the flow or at least identify trends in the flow or heat transfer distribution. In this case the designer needs to be aware of the reliability and limitations of the numerical solutions for that particular geometry under investigation. An assessment of the model can only be obtained by comparison with experiment. Since numerical solutions are problem-dependent, there is a need for reliable experimental data specific to the jet impingement geometry to facilitate a direct assessment.

Heat transfer from a flat plate to a turbulent axisymmetric impinging jet

Abstract The heat transfer due to a single axisymmetric impinging jet at a Reynolds number of 20 000 and nozzle-to-plate spacings of 1—8 has been determined using a combined constant heat flux/constant temperature electrocaloric test plate. Both unconfined and semi-confined geometries have been considered for a uniform mean exit velocity. The exit conditions and axial development of the jet have been quantified using hot-wire anemometry. Careful design of the composite heater plate has reduced experimental uncertainties in the local Nusselt number to 5 per cent.

The role of neural networks in fluid mechanics and heat transfer

The recent applications of Artificial Neural Networks (ANNs) to fluid mechanics and heat transfer are presented. ANNs have proved beneficial by their capability in modelling complex nonlinear problems as well as providing a fast, automatic method in some applications. In heat transfer the backpropagation model has been predominant and it has also been widely used in fluid mechanics. However, flow visualization has witnessed a substantial application of unsupervised learning algorithms. Finally, a novel technique that uses two ART2 (Adaptive Resonance Theory) networks to determine fluid flow velocities has been developed by the authors resulting in accuracies of up to 96.4%

Using ART2 networks to deduce flow velocities

A novel algorithm for obtaining flow velocity vectors using ART2 networks (based on adaptive resonance theory) is presented. The method involves tracking the movement of groups of seeding particles in a fluid space through the analysis of two successive images. Simulated flows, created artificially by shifting the particles through known distances or rotating through known angles, were used to establish the accuracy of the technique in predicting displacements. Accuracies were quantified by comparison with known displacements and were found to improve with increasing displacement, angle of rotation and size of the sampling window. In addition, the technique has been extended to derive qualitative and quantitative information for a practical case of natural convective flow.