{"id":456663,"date":"2026-04-19T05:58:57","date_gmt":"2026-04-19T05:58:57","guid":{"rendered":"https:\/\/innovationspace.ansys.com\/forum\/forums\/topic\/question-about-using-expression-for-co-mixing-at-supply-inlet-in-fluent\/"},"modified":"2026-04-19T05:58:57","modified_gmt":"2026-04-19T05:58:57","slug":"question-about-using-expression-for-co-mixing-at-supply-inlet-in-fluent","status":"publish","type":"topic","link":"https:\/\/innovationspace.ansys.com\/forum\/forums\/topic\/question-about-using-expression-for-co-mixing-at-supply-inlet-in-fluent\/","title":{"rendered":"Question about using expression for CO\u2082 mixing at supply inlet in Fluent"},"content":{"rendered":"

<p class=”isSelectedEnd”>Hello everyone,<\/p><p class=”isSelectedEnd”>I am currently performing a CFD simulation using ANSYS Fluent to study CO\u2082 distribution in a classroom with multiple occupants. The model includes supply air (SA), return air (RA), and CO\u2082 sources from human breathing.<\/p><p class=”isSelectedEnd”>I initially attempted to simulate HVAC recirculation by defining the CO\u2082 concentration at the supply inlet using an expression:<\/p><p class=”isSelectedEnd”>C_SA = 0.3 × C_OA + 0.7 × Average(CO\u2082 at RA)<\/p><p class=”isSelectedEnd”>where:<\/p><ul data-spread=”false”><\/p>\n

  • C_OA is the outdoor air CO\u2082 concentration (500 ppm),<\/li>\n
  • C_RA is the area-weighted average CO\u2082 at the return outlet.<\/li>\n<\/ul>\n

    <p class=”isSelectedEnd”>However, I encountered several issues:<\/p><\/p>\n

      \n
    1. The indoor CO\u2082 concentration becomes unrealistically high (around 1800–2000 ppm).<\/li>\n
    2. The spatial distribution becomes overly uniform, losing expected airflow-driven gradients.<\/li>\n
    3. I am not sure whether Fluent updates the “Average()” expression at every iteration, or if it remains fixed during the solution process.<\/li>\n
    4. I suspect this setup creates a feedback loop where the boundary condition depends on the solution itself.<\/li>\n<\/ol>\n

      <p class=”isSelectedEnd”>My questions are:<\/p><ul data-spread=”false”><\/p>\n

    5. Is it appropriate to directly use expressions referencing surface averages (e.g., Average(CO\u2082 at RA)) as boundary conditions in Fluent?<\/li>\n
    6. Does Fluent update such expressions every iteration, or only at initialization?<\/li>\n
    7. Would it be more correct to implement this type of recirculation using an iterative (manual) coupling approach instead?<\/li>\n
    8. Are there recommended practices for modeling HVAC recirculation (mixing of return and outdoor air) in Fluent without using UDF?<\/li>\n<\/ul>\n

      <p class=”isSelectedEnd”>Any insights or references would be greatly appreciated.<\/p><p>Thank you!<br><br>\"\u5df2\u4e0a\u50b3\u7684\u5716\u50cf\"`\"\"<\/p><\/p>\n","protected":false},"template":"","class_list":["post-456663","topic","type-topic","status-publish","hentry"],"aioseo_notices":[],"acf":[],"custom_fields":[{"0":{"_bbp_forum_id":["27792"],"_bbp_topic_id":["456663"],"_bbp_subscription":["670665"],"_bbp_author_ip":["140.116.37.39"],"_bbp_last_reply_id":["0"],"_bbp_last_active_id":["456663"],"_bbp_last_active_time":["2026-04-19 05:58:57"],"_bbp_reply_count":["0"],"_bbp_reply_count_hidden":["0"],"_bbp_voice_count":["1"],"_bbp_engagement":["670665"],"_btv_view_count":["14"],"_bbp_topic_status":["unanswered"],"_bbp_notification_enabled":["670665"]},"test":"didi06071120gmail-com"}],"_links":{"self":[{"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/topics\/456663","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\/456663\/revisions"}],"wp:attachment":[{"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/media?parent=456663"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}