Comparison of electrical and laser spark emission spectroscopy for fuel concentration measurements

Emission spectra from electrical and laser sparks in flowing methane–air mixtures of various compositions have been collected and analysed. The differences and similarities between the electrical and laser sparks in the context of their emission are discussed. The emission spectra from the laser spark were characterized by a weak continuum, onto which several strong atomic lines and some molecular bands were superimposed, in contrast to the spectra of electrical spark where a strong continuum, few atomic lines and several strong molecular bands were evident, making thus the laser spark spectroscopy a more accurate technique to measure hydrocarbon concentration. For both types of spark, the total intensity of the CN chemiluminescence around 388 nm was found to correlate almost linearly with fuel concentration in methane–air mixtures.     

A new rosette design for more reliable hole-drilling residual stress measurements

A new six-element strain gage rosette is presented that can greatly improve residual stress measurement accuracy when using the hole-drilling method. The proposed rosette consists of three pairs of sector-shaped radial and circumferential gages connected as half-bridges. This rosette design increases effective strain sensitivity by a factor of 2.3 compared with a standard ASTM rectangular rosette, and can identify stresses at one-third greater depths from the measurement surface. Experimental measurements confirm theoretical strain response calculations within 3–4 percent. Apart from a small increase in time to complete the electrical connections, the practical use of the proposed rosette is identical to that of a conventional three-element rosette.     

Probe and optical diagnostics for scalar measurements in premixed flames

 Procedures which allow the measurement of scalars in premixed recirculating flames using digitally-compensated thermocouples or, alternatively, laser-Rayleigh scattering, are described together with the error sources associated with the use of these techniques. The procedure for the numeric compensation of the thermocouple signals included experimental data on the characteristics of the effect of velocity and temperature on the time constant of the thermocouple and was optimised to guarantee the temporal resolution of the system. The Rayleigh system included its calibration in a propane/air flame and was optimised to avoid contamination of Mie scattering from small particles. The effect of the photomultiplier shot noise on the Rayleigh signal is quantified and used to compensate the results. The two techniques are shown to give rise to results in close agreement and to be suitable to characterise the turbulence nature of strongly recirculating premixed flames. Received: 15 November 1996/Accepted: 2 September 1997     

光纤陀螺用光纤光源的新型自动温度控制

光源良好的平均波长稳定性是保证光纤陀螺标度因子稳定性的重要条件。而光纤光源平均波长的变化主要源于环境温度的变化。为了使光源获得更好的输出特性,需要对光源泵浦温度进行精密控制。文中阐述了一种基于FPGA和MAX1968芯片设计的光纤陀螺用光纤光源泵浦温度自动控制(ATC)技术。控制过程中提出了一种新的控制算法--递进式PID。与传统PID算法相比,递进式PID算法的最大特点是其各个参数可以随外界环境而变化。经试验测定,泵浦的温度稳定性能够稳定在±0.03℃以内,因而泵浦具有很好的平均波长稳定性。     

Monofiber optical probes for gas detection and gas velocity measurements: optimised sensing tips

Optical probes are a very common tool for the investigation of gas–liquid flow dynamics. A single sensitive tip gives access to the phase indicator function, while using two tips some distance apart provides estimates of the gas velocity. Recently, it has been shown that the gas velocity could also be inferred from a monofiber probe provided that its latency length is known. To improve the capabilities of this new technique, an optimisation of the probe geometry, based on optical simulations and controlled piercing experiments, has been undertaken. In this first paper, conical probes (1C) produced using a new manufacturing technique are considered. Although they are effective for simultaneous gas velocity measurements, their actual response is sensitive to small geometrical defects occurring at their tips. Therefore, calibrations on well controlled interfaces appear necessary both to check the presence of pre-signals responsible for incorrect phase detections, and to establish a significant rise-time/velocity correlation.     

Fabrication of rigid and flexible refractive-index-matched flow phantoms for flow visualisation and optical flow measurements

A method for the construction of both rigid and compliant (flexible) transparent flow phantoms of biological flow structures, suitable for PIV and other optical flow methods with refractive-index-matched working fluid is described in detail. Methods for matching the in vivo compliance and elastic wave propagation wavelength are presented. The manipulation of MRI and CT scan data through an investment casting mould is described. A method for the casting of bubble-free phantoms in silicone elastomer is given. The method is applied to fabricate flexible phantoms of the carotid artery (with and without stenosis), the carotid artery bifurcation (idealised and patient-specific) and the human upper airway (nasal cavity). The fidelity of the phantoms to the original scan data is measured, and it is shown that the cross-sectional error is less than 5% for phantoms of simple shape but up to 16% for complex cross-sectional shapes such as the nasal cavity. This error is mainly due to the application of a PVA coating to the inner mould and can be reduced by shrinking the digital model. Sixteen per cent variation in area is less than the natural patient to patient variation of the physiological geometries. The compliance of the phantom walls is controlled within physiologically realistic ranges, by choice of the wall thickness, transmural pressure and Young’s modulus of the elastomer. Data for the dependence of Young’s modulus on curing temperature are given for Sylgard 184. Data for the temperature dependence of density, viscosity and refractive index of the refractive-index-matched working liquid (i.e. water–glycerol mixtures) are also presented.     

Two-point laser Doppler velocimetry measurements in a Mach 1.2 cold supersonic jet for statistical aeroacoustic source model

Single and multi-point laser Doppler velocimetry measurements performed in a cold Mach 1.2 jet flow are used to assess those properties of the aerodynamic field most relevant in the generation of turbulence mixing noise. Single point measurements yield mean velocity profiles, turbulence intensity profiles and power spectral densities of both the velocity and Reynolds stress fields at seven axial stations between the jet exit and the end of the potential core. The longitudinal components of the second-order and fourth-order two-point velocity correlation tensor are obtained from a series of multi-point LDV measurements, whence a cartography of integral space and time scales, convection velocities and acoustic compactness is effected. These results are used to examine differences between subsonic and supersonic jet aerodynamics in terms of their sound generating potential. Finally analytical expressions are proposed for the spatial and temporal parts of the longitudinal correlation coefficient function. These are scaled using the integral space and time scales of the velocity and Reynolds stress fields, and excellent agreement is found with experimentally determined functions.Nomenclature  c_o  Sound speed - D  Exit nozzle diameter - f, f  Spatial correlation function - g, g  Temporal correlation function - f_St  Frequency based Strouhal number - _i  2nd-order integral length scales in i-th direction -  4th-order integral length scales in i-th direction - M_c  Convective Mach number - M_j Jet exit Mach number - q Quantity q evaluated at location y into the flow -  Time average of the quantity q - r Radial distance from the jet exit - r_0.5  Radial location of the shear layer axis - r* Normalised radial coordinate - r_ij Second-order velocity correlation - r_ijkl Fourth-order velocity correlation - St Jet Strouhal number - U_c Convective velocity - U_e Subsonic coflow velocity - U_j Jet exit velocity - U_i Mean part of u_i - u_i Local velocity in i-th direction - u_ti Fluctuating part of u_i - x Distance from the exit nozzle - y Location test point - _i Variance of the velocity component u_i - _ij Variance of the velocity product u_iu_j -  Constant value - _ij Kronecker delta - _c Shear layer thickness - t_i Interarrival time between two ldv samples -  Separation distance in the moving pattern (components _i ) -  Polynomial function -  Separation distance in the fixed pattern (components _i ) -  Time delay -  2nd-order time scale in the fixed pattern -  2nd-order time scale in the moving pattern -  4th-order time scale in the fixed pattern -  4th-order time scale in the moving pattern -  Typical radian frequency where

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