This study is aimed at elucidating the relationship between the hydrodynamic and thermal phenomena in horizontal boiler tubes. It focuses on the empirical evaluation of the wall temperature distribution in a uniformly heated, 6-m-long, water-carrying pipe, operated at near-atmospheric pressure, over a range of imposed heat fluxes and water flow rates. The results reveal the existence of a complex temperature field on the pipe wall, incorporating circumferentially isothermal and anisothermal regions and a significant axial variation along the top of the pipe. For moderate flow rates these variations were found to give rise to two distinct anisothermal zones: a low-quality zone near the inlet and a moderate-quality zone approximately midway along the pipe. Physical models and semi-analytic relations for the liquid and vapor velocities, constituting the boundaries between isothermal and anisothermal operation, are presented.

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