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In COMSOL: h_top = 14.4, h_bot = 7.2 W/m²K 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:
T_mean = float(np.mean(T)) # starts at T_amb = 20°C
ht = calc_h_top(T_mean) # → h ≈ 0 at t=0 !
At t=0, T_mean = T_amb, so ΔT = 0, Ra = 0, h = 0. TS starts with essentially zero convective cooling and only builds up h as the board heats. The full h=14.4 isn’t reached until T_mean ≈ 55°C (ΔT ≈ 35K above ambient).
So early in the transient:
Time
COMSOL h_top
TS h_top (approx)
t = 0 s
14.4 W/m²K
~0
t = 60 s
14.4 W/m²K
~5–8
t = 300 s
14.4 W/m²K
~11–13
SS
14.4 W/m²K
14.4
TS has far less cooling during the transient → 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.
Simple fix: evaluate h at a representative ΔT 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:
# Option: fix h to SS calibrated values (matches COMSOL setup)
ht = calc_h_top(T_AMB + 35.0) # evaluate at design ΔT=35K, constant
hb = calc_h_bottom(T_AMB + 35.0)
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.
In COMSOL, you define h as an expression in T directly in the Heat Flux boundary condition. Here’s how:
Alternatively, use the built-in “Convective heat flux” type and enter h_top_fn in the h field — COMSOL will evaluate it locally at each surface node’s current T automatically.
What this achieves
At t=0: T=T_amb, dT≈0, Ra≈0, h≈0 (matches TS behaviour)
At SS: Ra≈71,000, h_top≈14.4, h_bot≈7.2 W/m²K (matches calibrated values)
COMSOL evaluates the expression locally per element at each timestep — better than TS which uses T_mean globally
The transient h buildup will match TS more closely