Experiments　were　conducted　in　a　vented　cylindrical　vessel　to　investigate　the　effect　of　hydrogen　ratio　(χ)　on　the　vented　deflagration　of　hydrogen/methane/air　mixtures,　with　ignition　source　located　in　the　center　of　the　chamber.　The　combustible　gas　mixtures　with　different　hydrogen　ratios　were　prepared　according　to　Dalton’s　law　of　partial　pressures.　The　pressure-time　histories　in　the　vessel　were　obtained　by　means　of　pressure　transducers,　while　high-speed　camera　and　schlieren　optics　were　used　to　record　the　flame　propagation　at　a　frequency　of　5000　Hz.　Results　indicate　that　χ　significantly　affects　the　pressure-time　histories.　In　some　cases,　three　pressure　peaks　of　p1,　p2,　and　p3can　be　observed　in　the　pressure　curves　in　the　vessel,　with　p1　caused　by　the　higher　volumetric　outflow　rate　than　that　of　the　production　by　combustion,　p2　produced　by　acoustically　enhanced　combustion,　and　p3　resulting　from　the　maximum　combustion　area.　p1　remained　almost　constant,　and　its　amplitude　is　slightly　greater　than　the　static　burst　pressure　of　the　aluminum　foil.　p2　occurred　at　χ　ranging　from　0.45　to　0.9,　increased　with　an　increase　in　χ.　p3　only　appeared　in　the　case　of　χ　=　1.　When　χ　was　between　0.45　and　0.9,　p2　was　the　dominant　pressure　peak.　Moreover,　the　size　and　period　of　fine　cells,　formed　on　the　flame　surface,　are　significantly　affected　by　χ,　and　their　period　is　close　to　that　of　the　acoustic　oscillations.　When　χ＜0.3,　there　is　only　one　pressure　peak　p1　inside　the　vessel;　For　χ　＞　0.3,　the　maximum　internal　overpressure　increased　as　χ　was　increased.　When　the　flame　rushed　outside　the　vent,　it　ignited　the　combustible　cloud　and　resulted　in　an　external　explosion.　For　χ　＜　0.15,　the　external　explosion　could　not　be　monitored　because　the　external　explosion　was　very　weak;　the　external　explosion　(pext)　occurred　as　χ　＞　0.15　and　its　intensity　increased　with　χ.　©　2025　Science　Press.　All　rights　reserved.