Table of contents

The safe operation of fossil-fuel-fired boilers is dependent to a large extent on the stability of the combustion process. Flame instability can be both inefficient and pollutant forming. but in extreme cases flame extinction can occur. Unless immediate action is taken in such circumstances, large quantities of unburnt fuel can be admitted to the combustion chamber and subsequently re-ignite explosively with serious consequences to both personnel and equipment. To eliminate such potentially dangerous situations. a need has been identified for reliable monitors capable of rapidly detecting the loss of flame. This review shows how advances in combustion systems have necessitated the continuing development of improved methods of flame failure detection to maintain acceptable performance characteristics.

A new fretting wear test rig was designed. The fretting contact consists of a sphere-on-plate arrangement. The temperature in a test can be set up to 1200 degrees C (specimen temperature 800 degrees C) and the normal load adjusted to between 130 and 600 N. The frequency can be between 1 and 60 Hz. The slip amplitude is controlled from 5 to 50 mu m by an eccentric cam arrangement. It is, therefore, possible to obtain partial slip fretting wear scars as well as full slip fretting wear scars. During fretting wear tests, the coefficient of friction can also be measured. One of the unique functions of the test rig is that the lower stationary specimen can be subjected to an adjustable static tensile load during tests. Hence, it becomes possible to obtain the relationship between applied tensile loads and fretting wear parameters, and to study the initiation of fatigue cracks by fretting.

A method and apparatus are described to carry out microhardness measurements as a function of temperature on amorphous solids to investigate the glass transformation. The Vickers hardness number H_v of a typical well characterised inorganic glassy polymer a-Se has been monitored as a function of temperature with the heating rate kept constant during the measurements. In the glass transformation region, H_V(T) was observed to go through a sharp fall. An empirical glass transition temperature, T_G, was defined at the inflection point of H_V(T) against T behaviour which depended on the heating rate, r. It is shown that the T_G against r data can be used to study the structural relaxation processes which control the mechanical properties in the T_G region. The activation energy for the structural relaxation process in a-Se, for example, was determined to be 2.77 eV over the temperature range 36-50 degrees C. The apparatus consisted of modifying a commercially available microhardness test instrument using a microcomputer and a data acquisition system, with an appropriate software and algorithm, to control the rate of heating of the sample.

The photoacoustic technique is applied to the simultaneous determination of the thermal conductivity and heat capacity of liquid crystal samples in their different mesophases. The results compare favourably with those reported in the literature.

A simple empirical equation for the measurement of the moisture content in a soil sample by using a microwave bridge to measure attenuation and phase shift through the sample is proposed. A calibration chart has been obtained from this equation for the convenient determination of the moisture content in a soil sample of unknown density. The validity and applicability of the equation has been confirmed by measurements on several soil samples of varying densities and thicknesses.

The authors describe an apparatus for the treatment of high-T_c oxide superconductors in flowing oxygen at pressures up to 1 MPa. The heating and annealing procedures are controlled by in situ measurements of the electrical resistance of the sample. The resistive properties at the annealing temperature are correlated with the superconducting (SC) properties of the novel oxide superconductor CaBiSrCu₂O_x.

The effects of spring and magnetic field distortions on the output voltage of an electromagnetic moving-coil geophone were studied theoretically and experimentally. Spring distortion is caused by vibrations of large amplitudes. Magnetic distortion is due to the nonuniformity of the magnetic flux through the moving coil. The perturbation method was employed to solve the nonlinear vibration equation. The calculated vibration velocity was then combined with the magnetic flux analysis to study the combined effects of spring and magnetic distortions on the moving-coil geophone. It was found that the third harmonic in geophone output is dominated by spring distortion while the second harmonic is caused primarily by magnetic distortion. The behaviours of geophone output predicted by the mathematical model were verified by experiments in which the coil was wound unevenly and the magnetic flux was kept constant.

A new target field method for designing coils to produce any desired field configuration is presented. This method has the advantage of giving coils of the minimum inductance consistent with the field specification, thus enabling the most rapid switching possible of gradient or main fields.

Presents a quantitative way of measuring turbidity by means of the turbidimetric method. The reasons for the diversity of readings are discussed and ways to eliminate this problem are proposed. The measurement of turbidity is replaced by measuring the total attenuation coefficient as the physical property. The substitution of the uni m^-1 from the coherent system of units (SI) for the existing turbidity units is proposed. The main reasons for error in the measurement of the total attenuation coefficient are presented by means of a radiometric analysis of the flowable head.

The properties of electromagnetic waves reflected by wet road surfaces have been investigated by computer modelling in the range of 0.5-10 GHz. The properties of the reflected wave depend on the thickness of the film and the concentration of salt in the water film. Reflection behaviour between 2 and 10 GHz is influenced mostly by the thickness of the film, whereas the reflection at frequencies below 1 GHz depends strongly on the salt concentration. Thus a dual frequency (e.g. 2.6 GHz and 0.5 GHz) reflectometer can be employed to detect simultaneously the thickness of a water film up to approximately 2 mm and its salt concentration up to 100 g l^-1.

Plasma sources based on small, commercially available, Nd:YAG and KrF excimer lasers are described, with particular emphasis on the potential use of such sources in x-ray lithography. The results obtained from the characterisation of prototype systems are extrapolated to allow the specification of a source that could be used in a production system, and the predicted properties of this source are compared with other possibilities for x-ray lithography.

The output frequencies of some commercially available index-guided laser diodes have been stabilised to the linear absorption maxima of the rubidium D lines (780 nm and 795 nm). The linewidth of these lasers was determined to be approximately=30 MHz (FWHM) from interferometric fringe visibility measurements, equivalent to a coherence length of a few metres. Allan variance measurements were made on the beat between two Rb-stabilised lasers, and the stability found to be 4*10^-10 of the optical frequency at tau =1 s. The reproducibility is approximately 3 parts in 10⁸ of the optical frequency, using a peak-to-peak modulation depth of 100 MHz. The frequencies of these stabilised lasers have been measured by interferometry to +or-1*10^-7 (3 sigma uncertainty) of the optical frequency. Doppler-free saturated rubidium spectra have been observed in a third derivative form.

A novel device is described which provides the possibility for accurate sensing of laser beam or interference fringe positions, utilising a pair of photoconductive elements that differentiates the light intensity with respect to the lateral coordinate. The device consists of a microstripline T structure with an InP:Fe chip incorporated in the middle. The chip has a metallic layer divided into three parts by two narrow photoconductive gaps, separated by a distance of 350 mu m. The central part is connected to a microstripline that constitutes the output branch, while the two side parts are connected to two similar microstriplines that are biased with DC voltages of equal magnitudes but opposite polarities. The differential mode of operation gives improved resolution as compared with ordinary photodetectors. In the case of gaussian beam position measurements a lateral resolution of 10 mu m could be demonstrated experimentally, and in measurements of interference fringe positions the resolution 2 pi /60 rad was achieved. In both cases these limits were set by the mechanical translation stage used. The temporal resolution was set by the carrier lifetime of the material, 200 ps.

A technique for continuous phase shifting of a linearly polarised light wave is described, which is based on a simple polarisation rotating scheme. Using two rotatable and one fixed fibre cranks as phase plates arranged in cascade, an endless all-fibre linear polarisation phase shifter can be constructed. This device also operates as a linear polarisation rotator.

The capability of flowmeters to function at small Reynolds numbers (<200) is vital in many applications where low flows or high viscosities are encountered, such as the metering of oil, respiratory flows, domestic gas and point-of-sale metering. The principle of operation of one such fluidic flowmeter, the target meter, is explained by means of flow visualisations and dynamic pressure distribution analyses. The performance characteristics of the meter are reported in detail. Minimum Reynolds numbers as low as 54 are attained. Also it is shown that the difficult calibration zones encountered in conventional fluidic flowmeters during jet transition to turbulence can be eliminated.

An innovative technique to measure the thermal lag, or response time, of a cold-wire temperature sensor is presented and an electronic compensation scheme is developed for measuring temperature fluctuations to about 1000 Hz using commercially available sensors. In order to measure the response accurately, a new methodology, utilising pulse heating of the sensor with a chopped laser beam, is illustrated. The square-wave energy pulse produced by the optically-chopped laser beam is sensed by the wire and its output is fed to a spectrum analyser for determining the high-frequency roll-off of the sensor. From these data, the time constant of the given sensor may be determined at various mean air velocities. The measured 3 dB frequency point is compared with the theoretical response equations derived for each sensor. An electronic compensation circuit is described which corrects for the high-frequency roll-off of the temperature sensor as a function of mean air velocity. The scheme provides a real-time response correction to the output of the temperature sensor in a fluctuating medium over a velocity range of 0-10 m s^-1.

A new thermal mass-flow meter for the measurement of fuel consumption in automobiles and light aircraft and of liquid flows in industrial processes has been designed. A maximum flow rate of 10 ml s^-1 can be measured without having to resort to a bypass or to a very long measurement pipe. The nonlinearity is less than 1% for gasoline or kerosine. The measurement accuracy is relatively insensitive to viscosity and internal contamination of the meter. The response time is 10 s. The dimensions of the actual measurement device are 200*20*20 mm³. The tube connection has an inner diameter of 13 mm.

Major improvements in the resolution of the photochromic dye method for measuring the instantaneous velocity profile for pulsatile flow are described. Specifically, improvements in the optical design have reduced the trace width from approximately 1.2 to 0.2 mm. Detailed studies were conducted using a 2.9 Hz sinusoidal flow superimposed on a steady flow through a straight rigid tube of 4.85 mm inside diameter. Velocity profiles were measured throughout the flow cycle and these, together with the wall shear stresses, were compared with those predicted from the Womersley rigid-tube flow equations. The excellent agreement between the experimental and predicted values illustrates the accuracy of the experimental technique. The authors have also developed a split-beam measurement method whereby the velocity profiles at three different positions along a flow vessel can be measured simultaneously. This technique provides an instantaneous description of the spatial relationship of the flow field, and is illustrated for pulsatile flow through a constricted tube.

Utilising the results of the theory of double-resonator dye lasers a novel TEA N₂ laser-pumped quenched dye laser arrangement is presented. By the use of two quenched lasers in cascade 12-15 ps pulses are obtained. The main advantage of this system is its simplicity and reliability.