[en] An experimental study of CHFG (Critical Heat Flux in Gap) has been performed to investigate the inherent cooling mechanism in a hemispherical narrow gap. The objectives of the CHFG test are to measure critical power using distilled water and Freon R-113 with experimental parameters such as a system pressure from 0.1 to 1.0 MPa and a gap thickness of 0.5, 1.0, 2.0, 5.0, and 10.0 mm. The experimental results on critical power are compared with the existing correlation, developed in flat plate, annuli, and spherical gaps. The distilled water data on critical power are also compared with the Freon R-113 data. The CHFG test results have shown that the measured values of critical power are much lower than the predictions made by empirical CHF correlations applicable to fiat plate gaps and annuli. A CCFL brings about local dryout and finally, global dryout in a hemispherical narrow gap thickness of 0.5, 1.0, and 2.0 mm. An increase in gap thickness of 10.0 mm leads to the pool boiling condition. The measured critical powers using Freon R-113 in hemispherical narrow gaps of 1-mm and 2-mm thickness are 51.5% and 44. 5% of that using water, respectively, which is different from the pool boiling condition with approximately 17%. For this reason, the boiling mechanism in the hemispherical gaps of small size is different from that in pool conditions. The measured critical power using R-113 in a 5-mm gap thickness is 21.6% of that using water, which is similar to the pool boiling condition. (author)