{"id":460614,"date":"2026-07-01T17:42:59","date_gmt":"2026-07-01T17:42:59","guid":{"rendered":"https:\/\/innovationspace.ansys.com\/forum\/forums\/topic\/laminar-flame-speed-correction-in-fluent\/"},"modified":"2026-07-01T17:42:59","modified_gmt":"2026-07-01T17:42:59","slug":"laminar-flame-speed-correction-in-fluent","status":"publish","type":"topic","link":"https:\/\/innovationspace.ansys.com\/forum\/forums\/topic\/laminar-flame-speed-correction-in-fluent\/","title":{"rendered":"Laminar Flame Speed Correction in Fluent"},"content":{"rendered":"

<p>Dear experts,<\/p><p>I am trying to better understand the proper way to specify or correct the laminar flame speed in ANSYS Fluent for gas-air premixed \/ partially premixed combustion simulations. I would appreciate any guidance or experience from experts who are familiar with Fluent combustion modeling.<\/p><p>According to Section 8.3.2.6. Laminar Flame Speed in the Fluent Theory Guide, the laminar flame speed can be specified or evaluated using several approaches: constant<\/strong>; user-defined function<\/strong>; Metghalchi-Keck<\/strong>; prepdf-polynomial<\/strong>; laminar-flame-speed-computed<\/strong>; laminar-flame-speed-library<\/strong>.<\/p><p>The reason for this question is that the laminar flame speed used by Fluent may not always be suitable for all gas-air combustion conditions. In addition, during flame propagation, the remaining unburned mixture is not necessarily still at the initial temperature and pressure. Due to heat transfer, compression, expansion, and local flow-field evolution, the temperature and pressure of the unburned gas may change significantly, under both adiabatic and non-adiabatic conditions. Therefore, it seems necessary to consider whether the laminar flame speed should depend on the local unburned mixture composition, temperature, and pressure, rather than only on the initial condition.<\/p><p>Based on the available Fluent options, my current understanding and questions are as follows.<\/p><p>1. constant<\/strong><\/p><p>For partially premixed combustion, especially when the unburned mixture composition is non-uniform, a constant laminar flame speed does not seem appropriate. The local equivalence ratio or mixture fraction may vary spatially, so a single constant value may not represent the actual flame propagation behavior.<\/p><p>2. Metghalchi-Keck<\/strong><\/p><p>The Metghalchi-Keck method appears to be limited to several predefined fuels or fuel groups. I am not sure whether it is suitable for general gas-air combustion systems, especially when the user wants to use a specific kinetic mechanism or customized fuel composition.<\/p><p>3. prepdf-polynomial<\/strong><\/p><p>From my understanding, the prepdf-polynomial<\/strong> method establishes laminar flame speed values at 20 mixture-fraction points between the specified lower and upper mixture-fraction limits. Fluent then uses these values to construct a piecewise-linear relation.<\/p><p>In the Fluent interface, these values seem to be editable in the Piecewise Linear Dialog Box<\/strong>. My question is: can these tabulated laminar flame speed values be manually replaced by user-generated flame speed data?<\/p><p>For example, if laminar flame speeds are calculated externally using Chemkin, Cantera, or another 1-D premixed flame solver, can the resulting data be entered into this panel to define a customized (S_L)-mixture-fraction relation?<\/p><p>However, even if this is possible, this method seems mainly to define laminar flame speed as a function of mixture fraction or composition. It is not clear whether it can account for the effects of local unburned gas temperature and pressure during the CFD calculation.<\/p><p>4. laminar-flame-speed-computed<\/strong><\/p><p>The Fluent Theory Guide states: “For the laminar-flame-speed-computed method, the laminar flame speed is computed by the Ansys Chemkin premix flame reactor model using 1-D premixed laminar flames in physical space.”<\/p><p>I have not used this option before, so I would like to ask how this method actually works.<\/p><p>Does laminar-flame-speed-computed<\/strong> calculate the laminar flame speed based on the imported Chemkin reaction mechanism and thermodynamic data? If so, does it consider the required mixture composition, temperature, and pressure conditions?<\/p><p>More specifically, is the laminar flame speed calculated dynamically during the CFD simulation according to the local unburned mixture state, or is it precomputed before the CFD calculation and then stored in some form of lookup table?<\/p><p>If it is computed dynamically, would this significantly increase the computational cost? If it is precomputed, which variables are included in the table, such as mixture fraction, equivalence ratio, temperature, pressure, enthalpy, or progress variable?<\/p><p>This method looks potentially suitable, because it may be able to connect laminar flame speed with detailed chemistry. However, the practical implementation details are still not clear to me.<\/p><p>5. laminar-flame-speed-library<\/strong><\/p><p>The laminar-flame-speed-library<\/strong> option seems to use Fluent’s internal flame-speed library. Since the speed data in the panel cannot be modified directly, this option may not be suitable when customized flame speed data are required.<\/p><p>6. user-defined function<\/strong><\/p><p>A common approach seems to be using a UDF to introduce a temperature- and pressure-dependent laminar flame speed correlation, for example (S_L = S_{L,0}(T_u\/T_0)^a(P\/P_0)^b), where (T_u) is the unburned gas temperature, (P) is the local pressure, and (a), (b) are empirical exponents.<\/p><p>For the premixed \/ partially premixed combustion model, I understand that the relevant official macro is the turbulent premixed source term macro:<\/p><p>DEFINE_TURB_PREMIX_SOURCE(name, c, t, turb_flame_speed, source)<\/code><\/p><p>My question is whether this UDF approach is the recommended way to introduce a customized (S_L(T_u,P,\\phi)) relation in Fluent, or whether it only modifies the turbulent flame-speed source term rather than directly prescribing the laminar flame speed.<\/p><p>In other words, if the objective is to define a physically corrected laminar flame speed as a function of local unburned gas temperature, pressure, and composition, is DEFINE_TURB_PREMIX_SOURCE<\/code> the most appropriate method, or is there a more direct Fluent-supported approach?<\/p><p>7. New input parameter<\/strong><\/p><p>In some Fluent panels, there is also a New Input Parameter<\/strong> option. It looks as if it might allow the user to define a parameter or function, but I have not found clear official documentation explaining whether it can be used for laminar flame speed specification.<\/p><p>Is this option only intended for Workbench parameterization, or can it be used to define a physical function such as laminar flame speed?<\/p><p>8. Possible double-counting of temperature and pressure effects<\/strong><\/p><p>A further concern is whether temperature and pressure effects may be counted twice.<\/p><p>If Fluent’s internal laminar flame speed model already includes some dependence on temperature and pressure, and a user additionally applies a correction such as (S_L = S_{L,0}(T_u\/T_0)^a(P\/P_0)^b), would this amplify the temperature and pressure effects unrealistically?<\/p><p>For example, if (S_{L,0}) is not a true reference value at (T_0) and (P_0), but is already affected by Fluent’s internal correction, then applying another correction may lead to an overestimated correction may lead to an overestimated flame speed.<\/p><p>Therefore, I would like to understand how Fluent treats temperature and pressure effects in each laminar-flame-speed option, and how to avoid double-counting when using customized correlations or UDFs.<\/p><p>In summary, I would be grateful for advice on the following points:<\/p><\/p>\n

    \n
  1. <p>For gas-air premixed \/ partially premixed combustion, which Fluent laminar flame speed option is most appropriate when (S_L) should depend on local unburned mixture composition, temperature, and pressure?<\/p><\/li>\n
  2. <p>Can the prepdf-polynomial<\/strong> values in the Piecewise Linear Dialog Box<\/strong> be manually replaced by externally calculated laminar flame speed data?<\/p><\/li>\n
  3. <p>How exactly does laminar-flame-speed-computed<\/strong> work: real-time Chemkin calculation during CFD, or precomputed lookup table before the CFD calculation?<\/p><\/li>\n
  4. <p>Is DEFINE_TURB_PREMIX_SOURCE(name, c, t, turb_flame_speed, source)<\/code> the correct UDF route for introducing a customized (S_L(T_u,P,\\phi)) relation, or does it only affect the turbulent flame speed source term?<\/p><\/li>\n
  5. <p>How can one avoid double-counting temperature and pressure effects when combining Fluent’s built-in laminar flame speed model with a user-defined correction?<\/p><\/li>\n<\/ol>\n

    <p>Any clarification, practical experience, or reference to relevant Fluent documentation would be highly appreciated.<\/p><p>Thank you very much.<\/p><\/p>\n","protected":false},"template":"","class_list":["post-460614","topic","type-topic","status-publish","hentry","topic-tag-fluids"],"aioseo_notices":[],"acf":[],"custom_fields":[{"0":{"_bbp_forum_id":["27792"],"_bbp_topic_id":["460614"],"_bbp_subscription":["704212"],"_bbp_author_ip":["104.164.48.209"],"_bbp_last_reply_id":["0"],"_bbp_last_active_id":["460614"],"_bbp_last_active_time":["2026-07-01 17:42:59"],"_bbp_reply_count":["0"],"_bbp_reply_count_hidden":["0"],"_bbp_voice_count":["1"],"_bbp_engagement":["704212"],"_btv_view_count":["13"],"_bbp_topic_status":["unanswered"],"_bbp_notification_enabled":["704212"],"_bbp_likes_count":["1"]},"test":"abousialj0gmail-com"}],"_links":{"self":[{"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/topics\/460614","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/topics"}],"about":[{"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/types\/topic"}],"version-history":[{"count":0,"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/topics\/460614\/revisions"}],"wp:attachment":[{"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/media?parent=460614"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}