be operated under widely varying heat fluxes in practical systems. While a general analysis of all factors influencing heat pipe operation is rather difficult, it is possible to obtain the necessary relations for the operating range of interest using the results presented above. The dependence of the convective heat transfer coefficient on wick porosity is shown in Fig. 5 for different combinations of working fluid and design materials under conditions of film evaporation. Heat transfer decreases as porosity increases, with this decrease being greater as the ratio of wick material to working fluid conductivity increases. The onset of nucleate boiling results in a considerable increase in the rate of heat transfer. It is necessary to understand how heat pipe design parameters affect the transition from film to nucleate boiling in order to control the radiator heat transfer behaviour appropriately. Equation (6) shows that the heat flux at which nucleate boiling begins depends both on the size of the heat pipe and the details of the wick structure. Equation (4) shows that, while the heat flux at which the transition from film evaporation to nucleate boiling takes place is monotonically dependent on size, there is a distinct maximum with respect to porosity. Wick porosities less than 60% or greater than 90% result in the onset of nucleate boiling under relatively low flux conditions. Boiling intensity depends on the structural and geometrical parameters of the wick, though to a different extent for the different liquids. Equations (3)-(5) show that as wick thickness increases, the boiling intensity of the liquids thermodynamically similar to water falls more rapidly than does that of ammonia. Thus the use of ammonia as the heat pipe working fluid makes the heat pipe less sensitive to deviations from the ideal
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