The relationship between the volumetric heat release rate and radiation of non-premixed hydrogen-oxygen flames at atmospheric and elevated pressure is investigated. Both the radiation of the excited hydroxyl radical (OH*) and the continuous blue radiation are considered. To physically interpret radiation and heat release, the phenomena are first analyzed within laminar flames following a hybrid approach: a pressurized jet flame experiment is set up to correctly measure the OH* and blue radiation. The heat release rate is obtained from a complementary CFD simulation. Radiation and heat release are clearly uncorrelated for changes in pressure. Spatially, radiation and heat release occur at separate locations. To further scrutinize the laminar flame structure, non-premixed counterflow flame simulations are performed. By considering statistical ensembles of flamelets, these findings are transferred onto turbulent flames. As before, no general direct proportionality between radiation and heat release rate is observed because of flame straining. A technique for correcting these effects is applied, and its potential is evaluated. The impact of self-absorption of OH* radiation at elevated pressures on its interpretation is discussed.
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