John M. Kuhlman

Variation of entrainment in annular jets

Mesure des distributions de vitesse moyenne pour des jets axisymetriques circulaires et annulaires entrant dans un environnement stagnant. On en deduit des donnees sur la decroissance de la vitesse de ligne des centres et sur la croissance de la largeur du jet

Three-component velocity measurements in a turbulent recirculating bubble-driven liquid flow

Abstract Three-dimensional mean and RMS bubble and liquid velocity measurements have been obtained in a turbulent, recirculating, bubble-driven liquid flow. Measurements have been made using a three-component laser Doppler anemometer. Bubble column turbulent liquid circulation for the present experiments is due to injection of a single vertical jet of air along the column centerline. Measured distributions of mean and RMS bubble and liquid velocities are consistent with the existence of a toroidal recirculating liquid flow, as is observed in flow visualization. Measured mean circumferential (swirl) velocity is essentially zero. RMS velocities in all three directions are of the same order of magnitude as the mean velocity, with radial RMS velocity being slightly larger than the other two components. Relative velocities between the bubbles and liquid are significant only near the air jet and near the free surface.

Performance of high-power gas-flow spark gaps

Results of an experimental investigation of the performance of repetitively pulsed gas-flow spark gaps are presented. Interelectrode gas-flow velocity and turbulence levels have been documented using pitot and hot-wire probes while transient arc debris behavior has been studied using schlieren optical techniques. Typical electrical recovery curves are presented at various gas velocities. Analysis of the recovery, flow, and optical diagnostic data has led to the development of a simple model for the recovery of a spark gap with gas flow.

ACCURACY STUDIES OF A TWO-COMPONENT DOPPLER GLOBAL VELOCIMETER (DGV)

A two-component Doppler Global Velocimeter (DGV) system is described, and preliminary velocity measurements obtained from the DGV system to quantify its accuracy are summarized. This DGV system uses molecular iodine vapor cells as frequency discriminating filters to determine the Doppler shift of laser light which is scattered off of seed particles in a flow, from which the flow velocity is determined. Results are presented for velocity distributions over the surface of a rotating wheel. A two-component Point Doppler Velocimeter (PDV) system has already been developed, and its accuracy demonstrated to be on the order of better than ±1 m/sec. Presently, the accuracy of the two component DGV system is being investigated. The two-component DGV system uses four 8 bit Hitachi CCD cameras, and a Matrox Genesis frame grabber for image acquisition. Image acquisition software has been developed, along with the required image processing software, including the required image warping and pixel registration routines, calibration, averaging, and so forth. For the rotating wheel results, RMS noise levels are observed which are on the order of ±1 m/sec, while total velocity range errors are observed to be between ±1-2 m/sec. However, there is also a zero velocity offset which is observed to be on the order of -20 m/sec; the cause of this offset is presently unknown. In the future, data from the two-component DGV system will be compared with similar data which has already been obtained with the twocomponent PDV system. This will allow a careful comparison of the accuracies of the two systems, and

Results for a Two-Component Doppler Global Velocimeter

A two-component Doppler global velocimeter (DGV) system is described, and velocity measurements obtained to quantify its accuracy are presented. Molecular iodine vapor cells are used as frequency discriminating e lters to determine the Doppler shift of laser light that is scattered off of seed particles in a e ow, from which velocity is determined. Results are presented for velocity distributions over the surface of a rotating wheel, a fully developed pipe e ow, and a freejet. For the rotating wheel DGV results, rms noise levels display standard deviations of § 1 m/s, whereas total velocity range errors are between § 1 and 2 m/s out of 59 m/s. The rms error is dominated by residual errors in the e at-e eld correction, and the 8-bit camera resolution, whereas velocity range errors are largely ine uenced by iodine cell calibration accuracy. Measurements for fully developed turbulent pipe e ow and axisymmetric jet e ow show good agreement with pitot-static probe data. A random velocity offset error due to iodine cell side arm temperature variations of about 4 ‐5 m/s has been found to be the dominant error source in the present DGV system. A reference tab has been used to record the zero velocity signals in the pipe and jet e ows.

Influence of the Coulomb Force on Spray Cooling

Effects of the Coulomb electrical body force on heat transfer performance of an instrumented spray cooling experiment are reported. Heat transfer performance is documented for a range of spray volume flow rates and heater power levels using the dielectric liquids, FC‐72 and HFE‐7000, sprayed onto a Thick Film Resistor (TFR) heater; along with flow visualization results using a transparent Indium‐Tin Oxide (ITO) heater. Two Coulomb force electrode geometries show modest but consistent improvements in heat transfer (order of 5–15%), but only at heat fluxes where boiling of the liquid film occurs. Flow visualization shows a highly contorted liquid film forming on the heater surface. These flow visualization results are used to aid in the estimation of characteristic time scales governing the effects of surface tension, gravity, heating of the liquid film, and vaporization of the film. For the present dense liquid sprays, it is concluded that none of these time scales are as short as the average time between ...

Accuracy Study of a 2-Component Point Doppler Velocimeter (PDV)

A two-component Point Doppler Velocimeter (PDV) which has recently been developed is described, and a series of velocity measurements which have been obtained to quantify the accuracy of the PDV system are summarized. This PDV system uses molecular iodine vapor cells as frequency discriminating filters to determine the Doppler shift of laser light which is scattered off of seed particles in a flow. The majority of results which have been obtained to date are for the mean velocity of a rotating wheel, although preliminary data are described for fully-developed turbulent pipe flow. Accuracy of the present wheel velocity data is approximately ± 1 % of full scale, while linearity of a single channel is on the order of ± 0.5 % (ie, ± 0.6 m/sec and ± 0.3 in/sec, out of 57 m/sec, respectively). The observed linearity of these results is on the order of the accuracy to which the speed of the rotating wheel has been set for individual data readings. The absolute accuracy of the rotating wheel data is shown to be consistent with the level of repeatability of the cell calibrations. The preliminary turbulent pipe flow data show consistent turbulence intensity values, and mean axial velocity profiles generally agree with pitot probe data. However, there is at present an offset error in the radial velocity which is on the order of 5-10 % of the mean axial velocity.

Potential flow past axisymmetric bodies at angle of attack

In the present work, Karamchetis3 suggestion to use higher order line singularity techniques has been developed to allow accurate calculation of incompressible flow past an axisymmetric body at angle of attack. Either piecewiseconstant or linearly-varying line source or line doublet singularities have been utilized for the axial flow, while either constant or linear doublets, whose axes are aligned with the crossflow, have been utilized for the crossflow solution. For flows at angle of attack, the two velocity distributions are superimposed to obtain the resulting three dimensional surface velocity and pressure distributions. Details of the derivation of the method have been given in Refs. 7 and 8. This Note presents results of a convergence study using this axial singularity method, where solution accuracy has been investigated for ellipsoids of slenderness ratios between two and ten, both for axial and inclined flow. Effects of singularity type, element number, element size distribution, and singularity line inset distance have been investigated. Further, a paneling scheme has been developed which yields accurate results for the class of axisymmetric bodies having continuous body slopes, but having discontinuous jumps in curvature, such as occurs at the juncture between an ellipsoidal or ogival nose and cylindrical body.

Three component velocity measurements in an axisymmetric jet using LDA

A three-component laser Doppler velocimeter (LDV) has been used to acquire three-dimensional mean and fluctuating velocity measurements in a low-speed air jet entering a stagnant ambient, over the first 16 jet exit diameters along the jet centerline trajectory. These data are consistent with previous measurements in axisymmetric, turbulent jets. A comparison between radial traverses through the jet centerline in two orthogonal directions is also self-consistent. Mean velocity and Reynolds stress data approach a self-preserving behavior by x/D = 16. However, the RMS turbulence fluctuations were not yet self-preserving at this axial location, as expected based upon previous experimental studies. These data confirm the ability to obtain reliable three-dimensional velocity data using the present three-dimensional LDV system.

Spray Cooling Droplet Impingement Model

Spray Cooling is one of the leading techniques proposed for rejection of high heat flux levels necessary in future aircraft and spacecraft electronics and weapons packages. Experimental research and computational CFD require significant amounts of time to setup and execute to completion. Therefore, a faster Monte Carlo based simulation has been proposed, in order to aid in the development of differing spray cooling conditions and applications. MATLAB has been used to model the surface impingement behavior of several hundred thousand spray droplets exiting a Spraying Systems FullJet 1/8-G spray nozzle. Previously measured Phase Doppler Anemometry measurements were used to match the droplet diameter and radial flux distributions of the simulation to actual spray conditions. Typical spray characteristics for the Spraying Systems nozzle are: a flow rate of 1.05x10 -s m 3 /s, a normal droplet velocity of 12 m/s, a droplet Sauter mean diameter of 48 m, and heat flux values ranging from 50-100 W/cm 2 . These spray conditions result in the following nondimensional parameters: We, 300-1350, Re, 750-3500, and Oh, 0.01-0.025. The simulation produces a surface map depicting different heat transfer regions present on the heater surface; these include: undisturbed thin liquid film, fresh liquid craters, boiling craters and dried out craters. A combination of flow boiling and liquid convection was used to calculate heat transfer values based on the distribution of liquid on the heater surface. For one assumed heater power and spray flow rate, the simulation produced a Nusselt number of 22.3, while experimental efforts produced a Nusselt number of 55.5 for the same heater power level and one spray flow rate.