| The present research is a two-part experimental study of nucleate boiling heat transfer from microporous enhanced, flat and finned surfaces in FC-72. In the first part, the effects of pressure and subcooling on the pool boiling heat transfer of microporous enhanced flat and finned surfaces are studied. The test surfaces, solid copper blocks with 1-cm2 bases and 5 x 5 square pin-fin arrays of 0 (flat), 2, 4 and 8 mm fin lengths, were tested at an absolute pressure range of 30–150 kPa and a subcooling range of 0–50 K in FC-72. The differences between pure and gassy subcooling as well as finned surface orientation were also investigated. Results showed that, in general, the effects of pressure and subcooling on both nucleate boiling and CHF were consistent with the prevailing trends in the literature, however, subcooling was found to significantly affect the high heat flux region of the microporous finned surface nucleate boiling curve. The enhancement of CHF from increased liquid subcooling was greater for the microporous surfaces than the plain surfaces. Correlations for both nucleate boiling and CHF are presented.; In the second part, the effects of velocity and subcooling on the flow boiling heat transfer from microporous enhanced finned surfaces are studied. The test surfaces were identical to those in the first part of this study but only the flat (plain only), 4 and 8 mm finned surfaces were used. The test surfaces were flush-mounted into a 1.46 cm wide x 1.27 cm high horizontal flow channel and tested at fluid velocities ranging from 0.5–4 m/s (Reynolds numbers from 18,700 to 174,500) and pure subcoolings from 4–20 K at an inlet pressure of 100 kPa. The effect of surface microstructure on flow boiling heat transfer from flat surfaces was also studied. Results showed that both the plain and microporous coated finned surfaces' boiling curves were affected by velocity and subcooling and was attributed to a significant portion of the finned surfaces' area remaining under single-phase convection conditions. The effect of fin length on the flow boiling heat transfer was similar to previously tested pool boiling results. |
