{"id":247445,"date":"2023-01-25T15:08:15","date_gmt":"2023-01-25T15:08:15","guid":{"rendered":"\/forum\/forums\/topic\/critical-heat-flux-boiling-model-always-diverged\/"},"modified":"2023-01-25T15:08:15","modified_gmt":"2023-01-25T15:08:15","slug":"critical-heat-flux-boiling-model-always-diverged","status":"closed","type":"topic","link":"https:\/\/innovationspace.ansys.com\/forum\/forums\/topic\/critical-heat-flux-boiling-model-always-diverged\/","title":{"rendered":"Critical Heat Flux boiling model always diverged"},"content":{"rendered":"

Hello everyone,<\/p>\n

I am currently simulating water boiling in a micro tube with low pressure and microgravity.  <\/span>The inlet radius is 0.5 mm, the near inlet part was set to isotherm wall, followed by > 6mm heating walls. The axial length is about 7 mm, the outlet radius is 0.25 mm.  <\/span>I ran the case at 10 kPa pressure, -1e-20 m\/s2 gravity in X-direction (I’d like to set it to zero, but the boiling model requires it to calculate some parameters like bubble departure diameter), using the 2D axisymmetric condition. The mesh has a structured grid of about 25400 cells and a focused inflation layer in the wall surfaces, I estimated the y plus is about 50. The heat flux of outer and inner walls was set to 75 000 W\/m2 and 60 000 W\/m2 respectively, it’s supposed to be enough to evaporate all the low mass-flow liquid, so I chose the Critical Heat Flux wall boing model.<\/p>\n

<\/p>\n

<\/p>\n

For the FLUENT setups, I am using :<\/p>\n

 <\/p>\n

Gravity On in the -x direction<\/p>\n

Multiphase -> Eulerian -> Boiling Model -> Critical Heat Flux<\/p>\n

Energy On<\/p>\n

SST k-ω model with Low-Re Correction<\/p>\n

Phase Interactions:<\/p>\n

Forces (I tried just keep Drag (Universal) and Dispersion Force (Burns) because of some ANSYS staff’s answers from this Forum, but it didn’t work)<\/p>\n

Drag model -> Ishii<\/p>\n

Lift model -> Moraga or tomiyama<\/p>\n

Turbulent dispersion force -> lopez-de-bertodano<\/p>\n

Surface tension -> Constant -> 0.071 N\/m or 0.032 N\/m<\/p>\n

Heat transfer -> ranz-marshall<\/p>\n

Mass transfer -> Boiling from Liquid to Vapor at 318.96 K saturated temperature (10 KPa)<\/p>\n

Bubble departure diameter -> Tolubinski-Kostanchuk<\/p>\n

Frequency of bubble departure -> Cole<\/p>\n

Nucleation site density -> Lemmert-Chawla<\/p>\n

Area influence coefficient -> Delvalle-Kenning<\/p>\n

Interfacial area -> ia-particle<\/p>\n

<\/p>\n

<\/p>\n

<\/p>\n

I defined the fluid properties referring to the water properties corresponding to 10 KPa. I kept the reference temperature by default here, I’m not sure of the exact value I should input.<\/p>\n

<\/p>\n

 <\/p>\n

Since I set up this case based on replacing mesh into CHF boiling tutorial VMFL067_FLUENT-1 and modifying boundary conditions, some inlet and outlet profiles like inlet k and ω, outlet k and ω, the outlet backflow temperatures were set by default or in line with the tutorial case. In addition, the inlet mass flow rate is 0.00005 kg\/s and outlet gauge pressure is 5400 Pa.<\/p>\n

<\/p>\n

<\/p>\n

 <\/p>\n

For a solver, I tried Coupled with and without Pseudo time, but both eventually got crushed. I also tried a transient solver.<\/p>\n

<\/p>\n

I increased the heating wall heat flux from 10 000 W\/m2 gradually, there was no vapor generation with relatively low heat flux. After the heat flux increased to 75 000 W\/m2, it seemed OK before the fluid reached the saturation temperature; With further iterations, the mode would diverge, resulting in the discontinuous vapor phase and invalid pressure contours. However, I’ve seen in previous work that the model achieves convergence but I am not getting the same type of convergence when this high heat flux has been applied.<\/p>\n

<\/p>\n

After liquid water reached saturation temperature<\/p>\n

<\/p>\n

Does anybody know how to achieve convergence for this CHF modeling?<\/p>\n

By the way, it would be good if anyone can explain the reference temperature when defining the fluid properties and inlet\/outlet turbulence profiles further.<\/p>\n

Any help that can be provided is very much appreciated!<\/p>\n","protected":false},"template":"","class_list":["post-247445","topic","type-topic","status-closed","hentry","topic-tag-boiling-flow-in-fluent"],"aioseo_notices":[],"acf":[],"custom_fields":[{"0":{"_bbp_subscription":["273395","852","287123"],"_bbp_author_ip":["23.220.96.181"]," _bbp_last_reply_id":["0"]," _bbp_likes_count":["0"],"_btv_view_count":["1570"],"_bbp_topic_status":["unanswered"],"_bbp_status":["publish"],"_bbp_topic_id":["247445"],"_bbp_forum_id":["27792"],"_bbp_engagement":["852","273395","287123"],"_bbp_voice_count":["3"],"_bbp_reply_count":["9"],"_bbp_last_reply_id":["315012"],"_bbp_last_active_id":["315012"],"_bbp_last_active_time":["2023-11-06 11:36:01"],"_bbp_likes_count":["0"]},"test":"shuai-ren-001student-uni-lu"}],"_links":{"self":[{"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/topics\/247445","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\/247445\/revisions"}],"wp:attachment":[{"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/media?parent=247445"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}