Gary W. Rankin

Numerical Simulation of the Cold Gas Dynamic Spray Process

A computational fluid dynamic (CFD) model of the cold gas dynamic spray process is presented. The gas dynamic flow field and particle trajectories within an oval-shaped supersonic nozzle as well as in the immediate surroundings of the nozzle exit, before and after the impact with the target plane, are simulated. Predicted nozzle wall pressure values compare well with experimental data. In addition, predicted particle velocity results at the nozzle exit are in qualitative agreement with those obtained using a side-scatter laser Doppler anemometer (LDA). Details of the pattern of the particle release into the surroundings are visualized in a convenient manner.

An experimental investigation of laminar axisymmetric submerged jets

Detailed measurements of the velocity profiles in a laminar axisymmetric submerged jet of water were taken using a laser-Doppler anemometer. A non-intrusive measurement technique is particularly advantageous in this application owing to the unstable nature of the laminar jet and the destabilizing effect which objects submerged in the jet have. Flow visualization was employed to ensure that all of the measuring points were located within the laminar region of the jet. The variation of centreline velocity, jet half-radius and velocity-profile shape are investigated for various Reynolds numbers and axial distances. Emphasis is placed on the jet-development region; however, data from the fully developed region are also presented. Particular attention is given to determine the proper non-dimensional groups which are required to collapse the data. The predictions of a simple boundary-layer analysis are used as a guide in this regard and found to give an accurate representation of the flow field. Velocity-profile data were taken at sufficiently small radial increments to allow a determination of the jet kinematic momentum using the basic integral definition. Although approximately constant, a slight variation with axial distance is indicated. The momentum initially decreases, and then increases gradually to a value greater than that at the tube exit. An attempt to explain the trend of the variation is made using certain hypotheses regarding the velocity and pressure conditions at the tube exit.

Evaluation of the basic systems of equations for turbulence measurements using the Monte Carlo technique

A numerical experiment has been carried out to evaluate two of the methods available for finding the time-averaged mean velocity and the Reynolds stresses of a turbulent flow field using hot wires. The conventional method is based on the series expansion of the response equation, subsequent truncation of the series and time averaging. The improved method is based on squaring and time averaging without neglecting any terms. The method adopted to evaluate these two methods is based on the Monte Carlo simulation of a pseudo turbulent flow field using random-number generators and the corresponding hot-wire response, for a prescribed set of conditions, by assuming an appropriate model for the hot-wire response. The simulated hot-wire response and the calibration constants are then perturbed about their mean values to study the effects of errors in these quantities. The perturbed response is used to compute the time-averaged flow field by the two methods. The deviation of these values from the generated pseudo values, averaged over large number of trials, is used as the criterion to evaluate the methods. This procedure is also used to estimate the errors due to truncation in the conventional method, to study the effect of turbulence-intensity levels and to study the effects of measurement errors. The results indicate that the choice of the method for determining the time-averaged quantities should be based on the turbulence-intensity level and the measurement errors likely to be encountered. The conventional method yields reliable mean-velocity results for turbulence intensities as high as 50% with second-order turbulence correction. If measurement errors are within reasonable limits and the turbulence level is below 20%, the conventional method yields reliable results for Reynolds stresses. The improved method should be used to determine the time-averaged flow field for turbulence intensity above 40–50%. The error in the yaw sensitivity parameter k has an insignificant effect on the mean velocity and Reynolds stresses computed by both methods. By accurately determining the sensitivity s of the hot wire, the accuracy of the measured mean velocity and Reynolds stresses can be improved significantly. An improved method of carrying out the uncertainty analysis for measurements, based on the Monte Carlo technique, has also been outlined.

Performance Characteristics of a Microscale Ranque–Hilsch Vortex Tube

The results of an experimental investigation of the energy separation performance of a microscale Ranque-Hilsch vortex tube are presented. The supply channel Reynolds number of a microscale Ranque-Hilsch vortex tube is varied over a considerable range, which extends into the laminar flow regime in order to determine the minimum conditions for cooling. Experiments are conducted for a fixed geometry and control valve setting. At low Reynolds numbers based on the inlet tube hydraulic diameter and average velocity, the results exhibit an increase in dimensionless temperature in both the hot and cold outlets as the Reynolds number is increased from zero, reaching maximum values below 500 and 1000, respectively. The hot outlet dimensionless temperature decreases after reaching its maximum and achieves a minimum value at a Reynolds number below 1500. It then increases steadily with further increases in Reynolds number. The cold outlet dimensionless temperature decreases steadily after the maximum to become negative at a Reynolds number of approximately 1800. This implies that the cooling effect occurs at Reynolds numbers consistent with turbulent flow. The performance characteristics of the microscale vortex tube operating at higher inlet pressures of 200 kPa, 300 kPa, and 400 kPa with an average inlet temperature of 293.6 K are also presented for cold air mass ratio values over the range of 0.05-0.95. An increase in the inlet pressure causes the values of the dimensionless cold temperature difference to increase over the whole range of the cold air mass fraction. An unstable operation is observed at a length to diameter ratio of approximately 10, causing radial mixing between the cold and hot flow streams and a dramatic change in the cold mass flow fraction plot.

Improved calibration of hot-wire anemometers

A new method of treating the data obtained for static calibration of hot wires has been developed. The method employs a Taylor series expansion of the nonlinear hot-wire equation about initial estimates of the required calibration constants. An iterative procedure using the Newton-Raphson technique then yields the new calibration constants. The new method is shown to be systematic and more accurate than the conventional method. Setting the new calibration constants on a lineariser makes the proces of linearisation more accurate.

An investigation of a micro-scale Ranque-Hilsch vortex tube.

The results of an experimental investigation of the energy separation performance of a micro-scale Ranque-Hilsch vortex tube are presented in this paper. The micro-scale vortex tube is 2 mm in diameter and constructed using a layered technique from multiple pieces of Plexiglas and aluminum. Four inlet slots, symmetrically located around the tube, form the vortex. The hydraulic diameter of each inlet slot and the orifice diameter for the cold exit are 229 and 800 microns respectively. The working fluid is low pressure, non-dehumidified compressed air at room temperature. The rate of the hot gas flow is varied by means of a control valve to achieve different values of cold mass fraction. The mass flow rates, temperatures and pressures of the supply and outlet flows are measured and the performance of the device presented. The supply channel Reynolds number is varied over a considerable range which extends into the laminar regime in order to determine the operating conditions for cooling. An increase in dimensionless temperature is found in both the cold and hot outlets as supply nozzle Reynolds number increases from zero. Maximum values occur at a Reynolds number of approximately 500 and the cold flow dimensionless temperature becomes negative at about 2500. Although the optimum cold mass ratio is higher than the conventional tubes, the effect on performance of tube length and cold exit diameter is similar to the conventional devices.Copyright

Fully developed flow of power law fluids in curved ducts with heat transfer

The fully developed flow of non-Newtonian fluids through curved square ducts with heat transfer has been numerically investigated. The momentum and energy equations are solved using a finite-difference scheme. The solutions are obtained for a range of the following nondimensional variables: Dean number Dn, curvature ratio δ, power law index n, and Prandtl number Pr. The results demonstrate the existence of dual solutions for a certain range of Dean number and power law index. The frictionat losses and heat transfer rate are found to increase with increases in Dn and n. The temperature field in the cross-sectional plane depends strongly on the secondary flow field and the Prandtl number. The effect of curvature ratio on the flow and heal transfer is also investigated.

3D Structure of the Rotor Wake of a Three Different Sweep Axial Fan

This experimental study, applied to a three different sweep axial fan (backward, radial and forward), aims at determining the 3D structure of the rotor wake from unsteady velocity measurements. The hot-film anemometry is used to measure the 3D unsteady velocity components in nearfield, downstream the fan. The data analysis leading to averaged and turbulent velocities, the components of the Reynolds’ stress tensor and the turbulent kinetic energy is presented, in order to illustrate the influence of the sweep. A spectral analysis is also performed.Copyright

Some studies on hot-wire calibration using Monte Carlo technique

A numerical experiment has been used to determine the sample size required to accurately determine the calibration constants of hot wires. The method is based on a Monte Carlo simulation of the hot-wire response, for a prescribed set of conditions, by assuming a heat transfer model for the hot wire. The simulated response is then analysed by curve fitting over a large number of trials to obtain the average values of calibration constants and the standard error of the estimate. The effects of the magnitude of error in the velocity, the range of velocity, the spacing of the data within the range and the methods of curve fitting were investigated. The use of subrange calibrations to cover a large velocity range is also investigated. The results indicate that the sample size required is in the range 20-30 for practical purposes. The nonlinear method of calibration consistently yields more accurate results than the linear method. The sample size necessary to obtain a given accuracy increases as the range and the magnitude of error in the velocity increases. The spacing of the sample within the range has negligible effects for higher velocity ranges. The subrange velocity calibration yields accurate results.

An Efficient Numerical Model to Determine Synthetic Jet Ejector Performance

An efficient computational fluid dynamic (CFD) model of a synthetic jet ejector (SJE) has been developed. The model allows a comprehensive investigation to be made of the effect of various operating parameters to achieve optimum performance of the device. The results of grid convergence, grid structure and time step are investigated in this regard. The model utilizes grid structure similarity in order to minimize the effect of the grid structure on the solution accuracy. A commercial finite volume solver is used for all simulations. The k‐e turbulence model is used along with the second‐order upwind scheme for spatial discretization and a second‐order implicit scheme for time advancement. Selected results are presented and discussed.