Critical heat ~ ux in a long \ rectangular channel subjected to one ! sided heating *

Flow boiling in a straight\ rectangular channel was investigated to explore the conditions leading to critical heat ~ux "CHF#[ The ~ow channel had a 4[9×1[4 mm cross!section and 090[5 mm heated length^ heat was applied to the shorter dimension along only one wall[ Tests were performed with FC!61 liquid over a velocity range of 9[14 to 09 m s and at outlet subcoolings of 2\ 05 and 18>C[ Video images captured at CHF at various ~ow conditions revealed that vapor coalesces into a series of patches resembling a wavy vapor layer which propagates along the heated wall allowing liquid to contact the wall only at discrete locations[ Measurements indicate that vapor patch length\ vapor patch height and liquid contact length increase along the ~ow direction but decrease with increasing subcooling and velocity[ The measurements also reveal that the ratio of liquid!to!vapor length remains approximately constant along the heated wall for a particular subcooling[ These observations and measurements represent key contributions to critical heat ~ux modeling e}orts presented in Part II of this study[ Þ 0887 Elsevier Science Ltd[ All rights reserved[