Journal of Mechanical Engineering Science

Where a rotor runs within a clearance space, the clearance being comparable with rotor mass unbalance, the synchronous whirling behaviour of the rotor may be considerably affected by intermittent interaction with the stator at the clearance position. Discontinuity and jump phenomena may occur: in general, behaviour will be different with increasing speed from that with decreasing speed, and in either case zones may exist in which rotor-stator interaction is possible but not certain. In the analysis here presented, rotor and stator are regarded as linear multi-degree-of-freedom systems including damping; dry friction at the clearance space is taken into account. Discussion is limited to cases with radial symmetry, and interaction is assumed limited to the position of the clearance space. Polar receptances are used to establish equilibrium conditions with interaction, and speed zones are defined within which interaction may occur. Some hypothetical cases are fully explored, demonstrating that rotor-stator interactions may occur in a variety of forms and circumstances.

Interactions with dry friction counterwhirling are also considered. Some experimental results on counterwhirl within a ball bearing are given and qualitatively compared with theory.

The combustion processes in a dual-fuel engine are examined through the computation and analysis of effective rates of heat release. Methane was adopted as the primary fuel and the effect of deliberately introducing carbon dioxide along with the methane was also examined to simulate operation on methane sources containing carbon dioxide as a major contaminant. An examination of the various heat release records suggests that dual-fuel combustion generally appears to undergo two distinct phases. The first phase is associated mainly with the consumption of the pilot and some associated gaseous fuel, while the second phase is dependent on the concentrations and the quality of the gaseous fuel employed. Under light audible knock, the first phase gets prolonged and persists over a much longer period than under knock-free combustion. The rates of burning are excessively high and project well over the second phase. This is indirectly indicative of the auto-ignition nature of knock and the simultaneous rapid burning of the pilot together with a very substantial fraction of the gaseous charge.

Tension and torsion creep recovery tests on commercially pure copper at 150, 200 and 250°C are reported. Recoverable strain can range from a negligible percentage to approaching 100 per cent of the total creep strain developed, depending on stress, creep period preceding unloading, and current recovery time. An empirical equation relating creep recovery to these characteristics of the initial creep test is given.

Nearly all creep data are obtained under closely controlled constant conditions of load and temperature. Most high-temperature plant, however, is subject to cyclic variations of both, due to periodic off load or reduced load conditions.

Test data for a C-Mn steel are presented for various cycles of stress and/or temperature. The effects of cycling of stress or temperature, and of in-phase changes of both, are predictable within reasonable limits. It is shown, however, that with out-of-phase changes in stress and temperature, i.e. high stress/low temperature to low stress/high temperature conditions, a very rapid acceleration of strain can occur.

Results are given of direct-stress random-loading and block-loading fatigue tests on sharply notched mild steel specimens subjected to a tensile mean stress. It is shown that the random-loading tests are more damaging than the multi-level block tests simulating the same conditions.

Methods of endurance prediction in variable-amplitude conditions are discussed. In random-loading tests the hypothetical S-N curve method gives the better prediction provided that a suitable value of the slope parameter, d, is chosen; this value is not predictable from two-level block tests. The Palmgren-Miner method gave optimistic predictions.

There was little difference between the two prediction methods when applied to the results of the multi-level block tests and the value of d suggested by two-level tests was satisfactory.

A previous paper demonstrated that high pressure face seals have an inherent stiffness due to elastic distortions. At that time a simplified treatment of the elastic and thermal problems was presented and agreement between this theory and experimental results was found to be reasonable. Because of the predominant effect of the elastic distortions and their importance in determining the stability of a seal, the authors of this paper have determined a more accurate method of solution of the elastic problem.

The results of this theory give closer agreement with the published results obtained experimentally, and demonstrate that the simplified treatment is valid only if the seal has a large face to thickness ratio. Solutions for square-section seals are presented in non-dimensional form, leakage and parting force due to fluid pressure being expressed as ratios of the rigid values. Although only one sealing arrangement is treated the elastic solution can be applied to any body symmetrical about an axis.

The theory of evaporating flow given in this paper is believed to be an advance on what has hitherto been available in that it makes due allowance for the effect of vapour condensation which occurs simultaneously with vapour expansion. The basic theory is applicable without modification to any fluid undergoing evaporating flow in the annular mode, given suitable correlations for wall and interphase friction factors as defined in the paper. Preliminary correlations, which can only be very rough, are provided for water. A lot of experimental work on a variety of fluids would be required to achieve a position from which general correlations could be developed. The theory incidentally permits prediction of critical outlet conditions and some interesting comparisons with available experimental data have been made.

The response of a mechanical system to the displacement excitation at a support is determined in terms of the natural frequencies and modes of the rigidly supported system. Receptances in closed and series forms are obtained for torsional and flexural systems and their properties are discussed. Support receptances are used to determine the modal and frequency changes caused by the introduction of flexibility at a beam support. The case of a beam on two flexible supports is then analysed using the cross-receptances between supports.

An experimental study of the head of water behind the nappe of a rectangular weir suggests that it is linearly related to the discharge head over the weir.

Mechanical Engineering Science Monographs are separate non-periodic publications intended to provide a forum for the more lengthy specialized contributions of permanent value for which space is not available in The Journal of Mechanical Engineering Science.