无法进行微分方程模拟

sol = NDSolve[{Laplacian[u[t, x, y], {x, y}] == D[u[t, x, y], t],
DirichletCondition[u[t, x, y] == 0, x == 0 || x == 10 || y == 0],
DirichletCondition[u[t, x, y] == 100, y == 10], u[0, x, y] == 0},
u, {t, 20, 100}, {x, 0, 10}, {y, 0, 10}];
Manipulate[
DensityPlot[u[t, x, y] /. sol, {x, 0, 10}, {y, 0, 10},
PlotLegends -> Automatic, ColorFunction -> "TemperatureMap",
PlotRange -> All, PlotPoints -> 50], {t, 20, 100},
ControlPlacement -> Top]
热传导模拟，为什么在这个范围内密度图始终不变