Observations of boiling behaviors and measurements of critical heat flux (CHF) were carried out for saturated water boiling on a horizontal, upward-facing plate at pressures from atmospheric to 7MPa. The primary bubbles diminish in size almost in inverse proportion to pressure and commence to coalesce in the very low heat flux region. The diameter of detached coalesced bubbles increases with increases in the heat flux and reaches about 10 mm even at a pressure of 5MPa. Detachment frequency of the coalesced bubbles was unaffected by the heat flux and pressure. The CHF predicted based on the macrolayer dryout model agrees well with the measured data.
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