{"id":449502,"date":"2026-01-01T20:41:23","date_gmt":"2026-01-01T20:41:23","guid":{"rendered":"https:\/\/innovationspace.ansys.com\/forum\/forums\/topic\/question-on-pre-exponential-factor-units\/"},"modified":"2026-01-01T20:41:23","modified_gmt":"2026-01-01T20:41:23","slug":"question-on-pre-exponential-factor-units","status":"publish","type":"topic","link":"https:\/\/innovationspace.ansys.com\/forum\/forums\/topic\/question-on-pre-exponential-factor-units\/","title":{"rendered":"Question on Pre-Exponential Factor Units"},"content":{"rendered":"

<p>I need help understanding what units Fluent uses for the pre-exponential factor when using chemical reactions. I understand that the units for pre-exponential factor are implied depending on the rest of the terms in the reaction rate equation and that the values that appear in the reaction window for both pre-exponential factor and activation energy in Fluent are automatically adjusted depending on the units in the Chemkin mechanism import. For an example with a first-order volumetric reaction:<\/p><p>R=k*C_nh3<\/p><p>Where “R” and “C_nh3” are in units of [mol\/m^3*s] and [mol\/m^3] respectively, meaning “k” and the pre-exponential factor have to be in units of [s^-1] to satisfy the units for “R”. Another example, which is actually what I’m currently trying to figure out, is a first-order surface reaction:<\/p><p>R=k*C_nh3<\/p><p>Where “R” and “C_nh3” are in units of [mol\/m^2*s] and [mol\/m^3] respectively, meaning “k” and the pre-exponential factor have to be in units of [m\/s] to satisfy the units for “R”. In Fluent, the default units for concentration and reaction rate are [kmol\/m^3] and [kmol\/m^3*s] OR [kmol\/m^2*s], respectively. I’ve seen while skimming through some older Chemkin documentation that the prexponential factor is prescribed in a mechanism assuming its component units are [mol] and [cm] by default, although I am unsure if that is how it still works current day in Fluent and Chemkin. <\/p><p>I have am trying to simulate a first-order surface reaction (NH3 = 3\/2H2 + 1\/2N2, R=k*C_nh3) where the Arrhenius parameters are given as:<\/p><p>A = 3.10*10^3 [m\/s]   ,   Ea = 1.16*10^8 [J\/kmol]<\/p><p>When I am setting up a text file for a Chemkin mechanism using a research paper as the source for the parameters, should I convert the prexponential from:<\/p><p> A = 3.10*10^3 [m\/s]  >  A = 3.10*10^5 [cm\/s]<\/p><p>So that Fluent automatically converts it to [m\/s]? And does Fluent automatically convert a [mol] unit in a pre-exponential factor to [kmol] instead? Thanks in advance.<\/p><\/p>\n","protected":false},"template":"","class_list":["post-449502","topic","type-topic","status-publish","hentry"],"aioseo_notices":[],"acf":[],"custom_fields":[{"0":{"_bbp_forum_id":["27792"],"_bbp_topic_id":["449502"],"_bbp_subscription":["302346","23673","629050"],"_bbp_author_ip":["152.86.124.122"],"_bbp_last_reply_id":["449610"],"_bbp_last_active_id":["449610"],"_bbp_last_active_time":["2026-01-02 21:52:49"],"_bbp_reply_count":["6"],"_bbp_reply_count_hidden":["0"],"_bbp_voice_count":["3"],"_bbp_engagement":["302346","23673","629050"],"_btv_view_count":["293"],"_bbp_topic_status":["unanswered"]},"test":"abtharpe42tntech-edu"}],"_links":{"self":[{"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/topics\/449502","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\/449502\/revisions"}],"wp:attachment":[{"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/media?parent=449502"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}