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A numerical study has been carried out to investigate the boiling flow instability of a reactor thermosyphon system. The numerical model solves the conservation equations of mass, momentum and energy applicable to a two-fluid and three-field steam–water system using a finite difference technique. The computer code MONA was used for this purpose. The code was applied to the thermosyphon system of an EO (ethylene oxide) chemical reactor in which the heat released by a catalytic reaction is carried by boiling water under natural circulation conditions. The steady-state characteristics of the reactor thermosyphon system were predicted using the MONA code and conventional two-phase flow models in order to understand the model applicability for this type of thermosyphon system. The two-fluid model was found to predict the flow closest to the measured value of the plant. The stability behaviour of the thermosyphon system was investigated for a wide range of operating conditions. The effects of power, subcooling, riser length and riser diameter on the boiling flow instability were determined. The system was found to be unstable at higher power conditions which is typical for a Type II instability. However, with an increase in riser diameter, oscillations at low power were observed as well. These are classified as Type I instabilities. Stability maps were predicted for both Type I and Type II instabilities. Methods of improving the stability of the system are discussed.

In electrically driven guinea pig left atria, positive inotropic responses to (−)-isoprenaline and the selective β 1 -adrenoceptor agonist RO363 were obtained in the absence and in the presence of the functional antagonists adenosine, carbachol, gallopamil, nifedipine, and Ro 03-7894. Each of the functional antagonists reduced the maximum response to both agonists and produced nonparallel right-ward shifts in the cumulative concentration effect curves. For both agonists, dissociation constants (k a ) were calculated using the equation described by Furchgott (1966) for irreversible antagonism. For RO363, which is a partial agonist with high agonist activity, the equations outlined for functional interaction by Mackay (1981) were also employed to calculate k a values. The k a values obtained by each method were compared with the dissociation constants (K D ) for the two agonists determined from their ability to displace the radioligand (−)-[ 125 l]iodocyano-pindolol from β 1 -adrenoceptors in guinea pig left atrial membrane preparations. The estimates of k A varied substantially from K D values. The K D values were taken as more accurate estimates of the true values for the dissociation constants because a high degree of correlation exists between pK D and pD 2 values for a number of other β-adrenoceptor agonists that behave as partial agonists and between pK D and pK B values for a number of β-adrenoceptor antagonists. Thus it appears that there are serious limitations in the current theory for using functional antagonism as a means of obtaining agonist dissociation constants.