{"id":352414,"date":"2024-02-19T22:33:34","date_gmt":"2024-02-19T22:33:34","guid":{"rendered":"\/forum\/forums\/topic\/flamelet-model-chi_st-calculation\/"},"modified":"2024-02-19T22:33:34","modified_gmt":"2024-02-19T22:33:34","slug":"flamelet-model-chi_st-calculation","status":"closed","type":"topic","link":"https:\/\/innovationspace.ansys.com\/forum\/forums\/topic\/flamelet-model-chi_st-calculation\/","title":{"rendered":"Flamelet Model $\\chi_{st}$ calculation"},"content":{"rendered":"

Dear Fluent Combustion community,<\/p>\n

In the Fluent’s Theory Manual Section 8.1.4.2.3 Embedding Diffusion Flamelets in Turbulent Flames<\/strong>, I have noticed that the Favre mean scalar dissipation rate  ($\\chi_{st}$)  is modelled as $\\tilde{\\chi_{st}}=2 \\frac{\\widetilde{f^{‘2}} \\tau}$ where $\\tau$ is the time scale and and the Favre mean of the mean mixture fraction variance is \\widetilde{f^{‘2}}.. According to literature, (example Theoretical and Numercial Combustion by Poinsot and Veynante) this is not correct and is equal to the mean of the scalar dissipation rate ($\\tilde{\\chi}$).<\/p>\n

$\\tilde{\\chi_{st}}$ field need then be obtained from $tilde{\\chi} field using the following relationship<\/p>\n

<\/p>\n

Has anyone else noticed this? Should this be corrected?<\/p>\n

Also according to the manual, the modelling method that is used to model the $\\chi$ across the flamelet is attributed to the paper by Kim and Williams – “Extinction of diffusion flames with nonunity Lewis numbers”, 1997. (DOI: https:\/\/doi.org\/10.1023\/A:1004282110474). After reading this paper, I couldnt find anything about scalar dissipation rates or diffusion timescales. But their previous paper “Structures of Flow and Mixture Fraction Fields for Counterflow Diffusion Flames with Small Stoichiometric Mixture Fractions”, 1993,<\/a> presents a formula for $T_{s}$  (eq 54 below) which is the diffusion time at the location of the flamelet where the mixture fraction is equal to the mixture fraction at stoichiometry. Inverse of this would give the scalar dissipation rate at stoichiometry.<\/p>\n

   <\/p>\n

 <\/p>\n

I am confused as to how the Fluent theory manual arrived at Eq. 8-43 (section 8.1.4.3 Flamelet Generation).<\/p>\n

I appreciate any light that the combustion community can shed on this matter! ????<\/p>\n

Thanks! Mana <\/p>\n","protected":false},"template":"","class_list":["post-352414","topic","type-topic","status-closed","hentry","topic-tag-flamelet-1","topic-tag-non-premixed-combustion","topic-tag-reduced-order-model"],"aioseo_notices":[],"acf":[],"custom_fields":[{"0":{"_bbp_subscription":["88420","101181"],"_bbp_author_ip":["23.206.193.154"]," _bbp_last_reply_id":["0"]," _bbp_likes_count":["0"],"_btv_view_count":["672"],"_bbp_topic_status":["unanswered"],"_bbp_topic_id":["352414"],"_bbp_forum_id":["27792"],"_bbp_engagement":["88420","101181"],"_bbp_voice_count":["2"],"_bbp_reply_count":["6"],"_bbp_last_reply_id":["361437"],"_bbp_last_active_id":["361437"],"_bbp_last_active_time":["2024-04-10 06:02:57"]},"test":"hmanatungadeakin-edu-au"}],"_links":{"self":[{"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/topics\/352414","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\/352414\/revisions"}],"wp:attachment":[{"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/media?parent=352414"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}