Engineering topics

Hess, C. F.

Publications and source records attributed to Hess, C. F..

12 records

NASA NTRS1990

Nonlinear spacial instability of a fluid sheet

The mechanism of nonlinear distortion of a fluid sheet leading to atomization is investigated numerically with the use of vortex dynamics and experimentally by means of holography. The configuration investigated consists of a planar fluid sheet emerging from a rectangular slit with and without coflowing air. The numerical model is two-dimensional, inviscid, and includes surface tension effects. The experimental results indicate the existence of well-defined three-dimensional structures. These are formed mainly by the nonlinear interaction of transverse and streamwise disturbances. The transverse disturbances are associated with the Kelvin-Helmholtz instability while the streamwise disturbances appear related to streamwise vortices possibly originating inside the nozzle.

Rangel, R. H.↗

NASA NTRS1989

Holographic diagnostics of breakup phenomena of impinging jets for liquid rocket motor applications

The use of pulsed laser holography in analyzing the atomization of impinging jets is evaluated using a holographic camera system capable of recording a 3D image of the atomization process in 3 x 10 exp -8 seconds. The holocamera and the experimental configuration and procedure are described in detail. Two nozzles were fabricated to produce 1-mm diameter liquid water jets that impinged at 60 and 90 deg. Holograms were obtained for each nozzle at liquid velocities of 20 and 40 m/s. Analysis of the holograms revealed the evolution of the jets from the plane of emergence through the region of droplet formation. Features of interest included the growth of disturbances prior to the breakup region and the effect of these disturbances on the formation of ligaments and liquid globules. Preliminary analyses show that waves of atomized liquid formed after impingement are closely correlated to waves measured on the jets, leading to the possibility of controlling the atomization process at the nozzle.

Hess, C. F.↗

NASA NTRS1986

Optical technique to characterize heavy rain

A technique to measure the size and velocity of raindrops is presented and experimental and numerical results discussed. The technique bases the droplet size on the absolute intensity of the light scattered when the droplets cross a laser-based probe volume. The velocity is obtained from the Doppler frequency. A size range of 0.1 mm to 9 mm was covered with overlapping ranges. Experiments were conducted with simulated rain sprays except that the local LWC was much larger than the values encountered in natural rain. The accuracy of the size measurement was not strongly affected by spray interference, but it showed a clear dependence with the droplet shape. A simple numerical model based on the light reflected from ellipsoids attempts to explain the relationship between droplet shape and absolute scattered light.

Hess, C. F.↗

NASA NTRS1985

A technique to measure the size of particles in laser Doppler velocimetry applications

A method to measure the size of particles in Laser Doppler Velocimeter (LDV) applications is discussed. Since in LDV the velocity of the flow is assocated with the velocity of particles to establish how well they follow the flow, in the present method the interferometric probe volume is surrounded by a larger beam of different polarization or wavelength. The particle size is then measured from the absolute intensity scattered from the large beam by particles crossing the fringes. Experiments using polystrene particles between 1.1 and 3.3 microns and larger glass beads are reported. It is shown that the method has an excellent size resolution and its accuracy is better than 10% for the particle size studied.

Hess, C. F.↗

NASA NTRS1985

Optical technique to study the impact of heavy rain on aircraft performance

A laser based technique was investigated and shown to have the potential to obtain measurements of the size and velocity of water droplets used in a wind tunnel to simulate rain. A theoretical model was developed which included some simple effects due to droplet nonsphericity. Parametric studies included the variation of collection distance (up to 4 m), angle of collection, effect of beam interference by the spray, and droplet shape. Accurate measurements were obtained under extremely high liquid water content and spray interference. The technique finds applications in the characterization of two phase flows where the size and velocity of particles are needed.

Hess, C. F.↗

NASA NTRS1985

Research study of droplet sizing technology leading to the development of an advanced droplet sizing system

An instrument to measure the size and velocity of droplets was developed. The instrument uses one of two techniques, as appropriate. In the first technique two small laser beams of one color identify the center of a larger laser beam of a different color. This defines a region of almost uniform intensity where the light scattered by the individual droplets can be related to their size. The first technique uses the visibility of a Doppler burst and validates it against the peak intensity of the signal's pedestal. Results are presented for monodisperse, bimodal, trimodal, and polydisperse sprays produced by the Berglund-Liu droplet generator and a pressure nozzle. Size distributions of a given spray obtained using three different size ranges show excellent self-consistency in the overlapping region. Measurements of sprays of known characteristics exhibit errors in the order of 10%. The principles of operation and design criteria of the instrument are discussed in great detail.

Hess, C. F.↗

NASA NTRS1984

Nonintrusive optical single-particle counter for measuring the size and velocity of droplets in a spray

A technique for measuring nonintrusively, and in real time, the size and velocity of droplets in a spray is presented. A small beam identifies the center of a larger beam, thus defining a region of almost uniform intensity, and only droplets crossing through such a center are measured. The size is obtained from the absolute scattered light and the velocity from the modulated signal produced by the interferometric pattern. A self-calibrating algorithm is also discussed. Results are presented for a spray of predictable chracteristics.

Hess, C. F.↗

NASA NTRS1984

Spray characterization with a nonintrusive technique using absolute scattered light

A technique to measure the size and velocity of particles is discussed, and results are presented. In this technique two small laser beams of one color identify the center of a laser beam of a different color. This defines a region of almost uniform intensity where the light scattered by the individual particles can be related to their sizes. A variation of this technique that uses two polarizations of the same color of laser beam is also presented. Results are presented for monodisperse, bimodal, trimodal, and polydisperse sprays produced by the Berglund-Liu droplet generator and a pressure nozzle. Size distributions obtained at three different ranges for the same spray show excellent self-consistency in the overlapping regions. Measurements of a spray of known characteristics exhibit errors in the order of 10 percent.

Hess, C. F.↗

NASA NTRS1984

Development and implementation of advanced diagnostic techniques

Two techniques were identified which offer great potential in the measurement of sprays. The first is referred to as IMAX, and it consists of a nonintrusive pulse height analyzer. The second is referred to as Visibility/Intensity (V/I) and it performs a size measurement by examining the visibility and the pedestal intensity that the IMAX technique provides a larger dynamic range and higher accuracy than V/I. It also shows that the two-color IMAX concept provides a higher S/N primarily because of the high efficiency in spectrally separating the two signals. The size distribution of two kinds of sprays are reported. The first spray was produced by a Berglund-Liu droplet generator with dispersion air. The second spray was produced by a pressure nozzle.

Hess, C. F.↗

NASA NTRS1984

A technique combining the visibility of a Doppler signal with the peak intensity of the pedestal to measure the size and velocity of droplets in a spray

A technique combining the visibility of a Doppler signal and the intensity of the scattered light to measure the size and velocity of particles is presented. It is shown that using only the visibility technique can lead to large errors under many conditions such as dense sprays. It is also shown that this error is considerably reduced and very high resolution is obtained by combining the visibility with the intensity of the scattered light. An instrument was developed using this new concept and measurements were performed in sprays of known characteristics. The results of monodispersed, bimodal and trimodal sprays are reported.

Hess, C. F.↗

NASA NTRS1982

Holographic studies of the vapor explosion of vaporizing water-in-fuel emulsion droplets

Holographic studies were performed which examined the fragmentation process during vapor explosion of a water-in-fuel (hexadecane/water) emulsion droplet. Holograms were taken at 700 to 1000 microseconds after the vapor explosion. Photographs of the reconstructed holograms reveal a wide range of fragment droplet sizes created during the explosion process. Fragment droplet diameters range from below 10 microns to over 100 microns. It is estimated that between ten thousand and a million fragment droplets can result from this extremely violent vapor explosion process. This enhanced atomization is thus expected to have a pronounced effect on vaporization processes which are present during combustion of emulsified fuels.

Sheffield, S. A.↗

NASA NTRS1982

The application of nonintrusive optical methods for physical measurements in combustion

Laser optical diagnostic techniques have proven to be effective for making physical measurements in hostile combustion environments. Two major and complementary approaches have been applied; namely, single event imaging and multi-signal detection methods. Single event imaging methods (e.g., pulsed laser holography) can provide the essentially instantaneous observation of physical events, e.g., fuel atomization, droplet breakup, particle and surface combustion, condensed phase formation, and flow field visualization. Multi-signal detection methods, which involve the processing of signals produced by the light scattered from the interaction of particles and/or droplets with an incident coherent ray, can provide information on flow velocity, turbulence, fluid shear stresses, and particle/droplet size and velocity. Recent results using both approaches are presented.

Wuerer, J. E.↗