No description available.

No description available.

No description available.

No description available.

No description available.

No description available.

Boiling heat transfer on the outside of small diameter tubes in the range of 1.8-3.0mm has been investigated. Pool boiling was investigated at nominal atmospheric pressure for each of the tubes iIi isolation. The experiment was varied by investigating the effect of bubbles from a second tube mounted below by varying the heat flux on the upper tube. The upper tube diameter was changed from 3.00 to 2.32 and 1.83mm and in each case the lower tube was 3.00mm. Experimental results showed that the upper tube heat transfer coefficient was enhanced due to the combined mechanism of translating bubbles and turbulent convection at low to moderate heat fluxes. A compact tube bundle made up of 30 stainless steel tubes of outer diameter 3nun, pitch diameter ratio 1.5 and heating length of 50nun was designed to. permit the measurement of flow boiling heat transfer coefficient from tubes within the bundle. The heat flux tested was in the range of 4-21 kW/m2 and mass flux of 5.6-32.8 kg/m2s using distilled water, R-l13 and Flutec PPI at nominal atmospheric pressure as the working fluids. Results obtained showed that the heat transfer coefficient was predominantly dependent on the heat flux as opposed to mass flux. Macro scale models were compared with the experimental results and none of these models predicted the experimental results well. The Confinement number (Co) developed for flow boiling inside micro channels was applied to compact tube bundles and it was shown that confinement is expected ~o be significant for Co>0.63. Photographic studies. also showed that the diameter of the bubbles that were generated within the,bundle were greater than the tube diameter. As such, the sliding bubbles mechanism played less significant role in contributing to the heat transfer coefficient. The recent three-state correlation developed by Thome et al for flow boiling .he.at transfer in micro .channels was modified to predict the experimental results obtained using a compact tube bundle and it has been shown that the thin film evaporation was the dominant mechanism compared to the nucleate boiling. The results from the twin tube and compact bundle arrangement showed two regions coexist at any point in time; that part of the tube covered with liquid subject to nucleate boiling and the other part completely enveloped with vapour. This latter part is designated by the introduction of a factor p and this has been demonstrated experimentally and theoretically corroborated by a model based on a liquid part (i-p) and vapour part p.

No description available.

A theoretical analysis is made for the condensation of steam in a laminar film on the outer surface of an axisymmetrical body. The body rotates about a vertical axis and the film is assumed to drain under the combined influence of gravitational and centrifugal accelerations. A differential equation which governs the film thickness is derived and is numerically integrated for two types of axisymmetrical body to calculate the theoretical film to surface heat transfer coefficient. The first type of body is the cone. A decrease in the apex angle on stationary cones leads to an increase in the component of gravitational acceleration along the surface and to an increase in the theoretical heat transfer coefficient. However, an increase in the apex angle of a cone rotating at high speed increases the component of the centrifugal acceleration along the surface and increases the heat transfer coefficient. A comparison is made between the heat transfer coefficient for discs and for cones. The second type of body has a concave surface described by the rotation of a circular arc and represents part of a turbine rotor at the transition from shaft to blade disc. Theoretical film thicknesses and heat transfer coefficients are presented and discussed for bodies with an arc radius- of 0.2 m and shaft diameters between 0.002 m and 0.6 m. An apparatus for making measurements of heat transfer coefficient from steam to rotating axisymmetrical bodies with diameters up to 0.6 m, is described. The heat transfer results for steam condensing on cooled rotating cones with apex angles 10 , 20 and 60 are presented and discussed. The condensate film supports various patterns of waves as it drains along the surface of the 10 , 20 and 60 cones. At high speeds, drainage on the 10 and 20 cones is assisted by the formation and detachment of drops. The pattern of waves arid the mode of drainage are shown to be dependent on the apex angle, the speed of rotation, the cone diameter and the distance from the starting point of condensation. With either mode of drainage, the experimental heat transfer coefficients are generally larger than the theoretical laminar values.

An experimental investigation has been made into the evaporation of large freely falling, oscillating water drops in an atmosphere composed entirely of its own vapour at varying pressures. Experiments were carried out at pressures of 2, 4, 5 and 6 x 10(_5) N/m(^2). A photographic record was obtained of the drop at six observation points down the pressure vessel. From the drop diameter and the velocity of fall measured at the observation point the mass, Nusselt number, Reynolds number and coefficient of drag were calculated. With an increase in pressure the Nusselt numbers were found to increase and were much higher in value than those obtained by other investigators who had, however, used anchored drops in a moving atmosphere. Values obtained for small, freely falling drops were not comparable with the large drops investigated in this study due to the difference in dynamics of the larger drop falling within a pressurised atmosphere composed entirely of its own superheated vapour. From the experimental results it has been shown how the obtained heat-transfer coefficient decreased with the increase in excess temperature above the boiling point.