<\/p>\n\n\n\n
In COMSOL<\/strong>: h_top = 14.4, h_bot = 7.2 W\/m\u00b2K are fixed constants<\/strong> from t=0. Full convective cooling is applied from the very first second.<\/p>\n\n\n\n In TS<\/strong>: h is recomputed every timestep from the Churchill & Chu correlation at At t=0, So early in the transient:<\/p>\n\n\n\n TS has far less cooling during the transient \u2192 rises faster and higher. At SS both converge to the same h, so SS temperatures should match closely. This matches exactly what the chart shows.<\/p>\n\n\n\n Simple fix<\/strong>: evaluate h at a representative \u0394T from the start (e.g., use the current local temperature rather than T_mean, or initialise h at the SS value and let it update). The most direct fix to match the COMSOL setup would be to use fixed h values:<\/p>\n\n\n\n Or keep temperature-dependent h but evaluate at a better representative temperature than T_mean.no, I want fix that in comsol, not in TS, teach me how to defin ht and hb in comsol as fucntion of position and temperature.<\/p>\n\n\n\n In COMSOL, you define h as an expression in T directly in the Heat Flux<\/strong> boundary condition. Here’s how:<\/p>\n\n\n\n Top surface<\/strong> \u2014 Heat Flux BC:<\/p>\n\n\n\n Bottom surface<\/strong> \u2014 Heat Flux BC:<\/p>\n\n\n\n Alternatively, use the built-in “Convective heat flux”<\/strong> type and enter In COMSOL: h_top = 14.4, h_bot = 7.2 W\/m\u00b2K are fixed constants from t=0. Full convective cooling is applied from the very first second. In TS: h is recomputed every timestep from the Churchill & Chu correlation at T_mean: At t=0, T_mean = T_amb, so \u0394T = 0, Ra = 0, h = 0. TS […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":[],"categories":[1],"tags":[],"_links":{"self":[{"href":"https:\/\/laserphotonics.uk\/index.php?rest_route=\/wp\/v2\/posts\/2667"}],"collection":[{"href":"https:\/\/laserphotonics.uk\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/laserphotonics.uk\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/laserphotonics.uk\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/laserphotonics.uk\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=2667"}],"version-history":[{"count":1,"href":"https:\/\/laserphotonics.uk\/index.php?rest_route=\/wp\/v2\/posts\/2667\/revisions"}],"predecessor-version":[{"id":2668,"href":"https:\/\/laserphotonics.uk\/index.php?rest_route=\/wp\/v2\/posts\/2667\/revisions\/2668"}],"wp:attachment":[{"href":"https:\/\/laserphotonics.uk\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=2667"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/laserphotonics.uk\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=2667"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/laserphotonics.uk\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=2667"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}T_mean<\/code>:<\/p>\n\n\n\nT_mean = float(np.mean(T)) # starts at T_amb = 20\u00b0C\nht = calc_h_top(T_mean) # \u2192 h \u2248 0 at t=0 !\n<\/code><\/pre>\n\n\n\nT_mean = T_amb<\/code>, so \u0394T = 0<\/code>, Ra = 0<\/code>, h = 0<\/code>. TS starts with essentially zero convective cooling<\/strong> and only builds up h as the board heats. The full h=14.4 isn’t reached until T_mean \u2248 55\u00b0C (\u0394T \u2248 35K above ambient).<\/p>\n\n\n\nTime<\/th> COMSOL h_top<\/th> TS h_top (approx)<\/th><\/tr><\/thead> t = 0 s<\/td> 14.4 W\/m\u00b2K<\/td> ~0<\/td><\/tr> t = 60 s<\/td> 14.4 W\/m\u00b2K<\/td> ~5\u20138<\/td><\/tr> t = 300 s<\/td> 14.4 W\/m\u00b2K<\/td> ~11\u201313<\/td><\/tr> SS<\/td> 14.4 W\/m\u00b2K<\/td> 14.4<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n # Option: fix h to SS calibrated values (matches COMSOL setup)\nht = calc_h_top(T_AMB + 35.0) # evaluate at design \u0394T=35K, constant\nhb = calc_h_bottom(T_AMB + 35.0)\n<\/code><\/pre>\n\n\n\n
\n\n\n\nStep 1 \u2014 Define parameters (Global Definitions > Parameters)<\/h3>\n\n\n\n
Name<\/th> Expression<\/th> Description<\/th><\/tr><\/thead> T_amb<\/code><\/td>293.15[K]<\/code><\/td>Ambient temperature<\/td><\/tr> k_air<\/code><\/td>0.0271[W\/(m*K)]<\/code><\/td>Air conductivity<\/td><\/tr> nu_air<\/code><\/td>1.655e-5[m^2\/s]<\/code><\/td>Kinematic viscosity<\/td><\/tr> alpha_air<\/code><\/td>2.34e-5[m^2\/s]<\/code><\/td>Thermal diffusivity<\/td><\/tr> L_c<\/code><\/td>0.0292[m]<\/code><\/td>Characteristic length (A\/P)<\/td><\/tr> H_SCALE<\/code><\/td>1.76<\/code><\/td>Nu calibration multiplier<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n
\n\n\n\nStep 2 \u2014 Define variables (Global Definitions > Variables)<\/h3>\n\n\n\n
dT_surf = max(T - T_amb, 0.001[K])\nT_film = (T + T_amb) \/ 2\nbeta_air = 1 \/ T_film\nRa = 9.81[m\/s^2] * beta_air * dT_surf * L_c^3 \/ (nu_air * alpha_air)\nNu_top = if(Ra < 1e7, 0.54 * Ra^0.25, 0.15 * Ra^(1\/3))\nNu_bot = 0.27 * Ra^0.25\nh_top_fn = Nu_top * k_air \/ L_c * H_SCALE\nh_bot_fn = Nu_bot * k_air \/ L_c * H_SCALE\n<\/code><\/pre>\n\n\n\nmax(..., 0.001)<\/code> avoids Ra=0 at t=0 when T=T_amb.<\/p><\/blockquote>\n\n\n\n
\n\n\n\nStep 3 \u2014 Apply to boundary conditions<\/h3>\n\n\n\n
General inward heat flux<\/code><\/li>h_top_fn * (T_amb - T)<\/code><\/li><\/ul>\n\n\n\nh_bot_fn * (T_amb - T)<\/code><\/li><\/ul>\n\n\n\nh_top_fn<\/code> in the h field \u2014 COMSOL will evaluate it locally at each surface node’s current T automatically.<\/p>\n\n\n\n
\n\n\n\nWhat this achieves<\/h3>\n\n\n\n