The results were compared with the results of previous heat transfer and pressure-drop studies and were generally found to be in good agreement. An empirical correlation was developed for the heat transfer results that accounts for the effects of twist ratio, wall-to-gas temperature ratio, and tube length and includes most of the previous single-phase results.As the operating temperatures of nuclear reactors and rocket nozzles increase and their heat fluxes go higher, the problem of getting the most heat out of the system with the least amount of pressure drop becomes increasingly important. The enhancement of heat transfer from tubes, rods, plates, etc., by means of various promoters has been studied for a number of years at Oak Ridge National Laboratory in connection with its various reactor development programs. The study discussed here is related to the gas-cooled reactor program; its purpose was to investigate the use of full-length, full-diameter twisted tapes inside tubes as a means of increasing heat transfer in gaseous systems.Although twisted tapes have been used for over forty years, the nature of their effect on heat transfer and pressure drop is still not completely understood. They have been used in both single and two-phase systems and for heating and cooling. The increase in heat transfer they produce apparently is due to a combination of effects, the most significant of which are an increased path length and an increase in the amount of mixing due to centrifugal forces and/or turbulence level.In order to establish the limits for this investigation, it was assumed that future high-performance gas-cooled reactors would be likely to have heat fluxes of the order of 106 B.t.u./hr:sq.ft.
Heat transfer and pressure-drop experiments have been performed for gas flow inside nine, 1/2-in-OD, 0.035-in. wall thickness, A-nickel, spirally corrugated tubes. The corrugations, which varied from 0.003–0.028 in. deep, were formed by pulling the tubes through a rotating head containing four embossing tools; corrugation-spacing-to-corrugation-depth ratios (P/e) ran from 16–41. The data, for heat transfer to nitrogen, at approximately 200 psig, were correlated by an expression of the form NNu,B (NPr,B)−0.4 × (Tw/TB)0.5 = A(NRe,B)m, where all the physical properties were evaluated at bulk gas conditions. The exponent, m, on the Reynolds number was observed to be consistently greater (0.854–0.900) than the value of 0.8 found for smooth tubes; the constant, A, varied from 0.0095–0.0195 with no apparent correlation with P/e. Friction factors, measured with adiabatic airflow, were found to be up to 1.7 times that for smooth tubes. Tubes of this geometry were found to be very effective in enhancing heat transfer. On an equal pumping power basis, for example, a tube with P/e = 22 had a heat transfer coefficient 22 percent greater than a smooth tube.
(FEVy) were recorded before the study and before, during, and after each working shift, and bronchial responsiveness to methacholine was measured at the beginning and end of the study. Twenty three subjects received nedocromil and.22 received a placebo in a double blind design; there was no difference in baseline characteristics between the two groups. At the end of the study, no differences were found between the nedocromil and placebo groups in the prevalence of symptoms or development of new symptoms during the study. The drug had no effect on changes in methacholine PD,, or FEVI. As in previous studies, new symptoms developing during the season were more common in atopic subjects and were associated with a fall in methacholine PD20. It is concluded that nedocromil has no effect on the development of new symptoms in grain workers. The mechanisms underlying these symptoms require further study.Grain dust has both acute and long term effects on the airways.`5 A few affected subjects develop allergic asthma (due to durum wheat,6 storage mites,7 or unknown allergens8) but in general the mechanisms responsible for these effects have not been established. Studies in seasonal workers in the Western Australia grain industry have shown that symptoms developing in subjects newly exposed to grain are associated with a fall in forced expiratory volume in one second (FEVI) and increased bronchial responsiveness. These changes appear soon after exposure at work and do not result from the presence of asthma, which is rare.9 lo Bronchial responsiveness is also increased in grain workers without allergic asthma." These observations suggest that inflammation of airways, perhaps implicating mast cells, may contribute to the development of obstruction of airways in grain workers.
The equations describing the flow produced by the interaction of a potential vortex with a stationary surface are solved by means of similarity transformations. In previous attempts at solutions, use was made of the boundary-layer approximations (in which case no solution can be obtained) or a particular form was assumed for one of the velocity components. In this analysis, where the only restriction is that the flow be similar, solutions are obtained for tangential Reynolds numbers up to 4.75. The numerical method employed is based on a recently proposed technique for solving ordinary differential equations with asymptotic boundary conditions.
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