{"id":166837,"date":"2022-02-28T21:43:08","date_gmt":"2022-02-28T21:43:08","guid":{"rendered":"\/forum\/forums\/topic\/negative-absolute-pressure-in-heat-pipe\/"},"modified":"2022-03-01T08:43:54","modified_gmt":"2022-03-01T08:43:54","slug":"negative-absolute-pressure-in-heat-pipe","status":"closed","type":"topic","link":"https:\/\/innovationspace.ansys.com\/forum\/forums\/topic\/negative-absolute-pressure-in-heat-pipe\/","title":{"rendered":"Negative absolute pressure in heat pipe"},"content":{"rendered":"<div class=\"Item-Body\">\n<div class=\"Message userContent\">\n<p>Hello,<\/p>\n<p>I am trying to simulate a heat pipe. In a previous thread I mentioned the simulation diverging and I was able to identify the root cause being the absolute pressure becoming negative whenever I activate a UDF for saturation pressure. <\/p>\n<p>My question would be, can the UDF, which is just an implementation of the Antoine equation, cause the absolute pressure to be negative? Here is the UDF code:<\/p>\n<p>#include &quot;udf.h&quot;<\/p>\n<p>DEFINE_PROPERTY(saturation_temp,c,t)<\/p>\n<p>{<\/p>\n<p>real sat_t;<\/p>\n<p>real vapor_p;<\/p>\n<p>real static_p = C_P(c,t);<\/p>\n<p>real operating_p = RP_Get_Real (&quot;operating-pressure&quot;);<\/p>\n<p>vapor_p = static_p + operating_p;<\/p>\n<p>sat_t = (1723.6425\/(8.05573-((double)log10((double)vapor_p\/133.322)))-233.08)+273.15;<\/p>\n<p>return sat_t;<\/p>\n<p>}<\/p>\n<p><\/p>\n<\/p>\n<p>And this is a snapshot of my solution methods. I am using VOF multiphase flow with liquid and vapor water as the two phases. The controls tab is unchanged (relaxation factors, etc.)<\/p>\n<div class=\"embedExternal embedImage\">\n<div class=\"embedExternal-content\">\n<a class=\"embedImage-link\" href=\"\/forum\/wp-content\/uploads\/forum-uploads\/285\/UB03MB7VO2DR.png\" rel=\"nofollow noopener\" target=\"_blank\"><br \/>\n<img decoding=\"async\" class=\"embedImage-img\" src=\"\/forum\/wp-content\/uploads\/forum-uploads\/285\/UB03MB7VO2DR.png\" alt=\"sol-method.png\" \/><br \/>\n<\/a><\/p>\n","protected":false},"template":"","class_list":["post-166837","topic","type-topic","status-closed","hentry","topic-tag-absolute-pressure-1","topic-tag-multiphase","topic-tag-phase-change"],"aioseo_notices":[],"acf":[],"custom_fields":[{"0":{"_btv_view_count":["544"],"_bbp_likes_count":["0"],"_bbp_subscription":["249574"],"_bbp_topic_status":["unanswered"],"_bbp_status":["publish"],"_bbp_topic_id":["166837"],"_bbp_forum_id":["27792"],"_bbp_engagement":["22555","249574"],"_bbp_voice_count":["2"],"_bbp_reply_count":["1"],"_bbp_last_reply_id":["206967"],"_bbp_last_active_id":["206967"],"_bbp_last_active_time":["2022-03-01 08:43:54"]},"test":"fjulioumich-edu"}],"_links":{"self":[{"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/topics\/166837","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\/166837\/revisions"}],"wp:attachment":[{"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/media?parent=166837"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}