ENH: Surface film - heat transfer only active where film is present

src/regionModels/surfaceFilmModels/thermoSingleLayer/thermoSingleLayer.C

@@ -252,10 +252,22 @@ tmp<fvScalarMatrix> thermoSingleLayer::q(volScalarField& hs) const
 {
     dimensionedScalar Tstd("Tstd", dimTemperature, 298.15);
 
+    volScalarField htcst(htcs_->h());
+    volScalarField htcwt(htcw_->h());
+
+    forAll(alpha_, i)
+    {
+        htcst[i] *= max(alpha_[i], ROOTVSMALL);
+        htcwt[i] *= max(alpha_[i], ROOTVSMALL);
+    }
+
+    htcst.correctBoundaryConditions();
+    htcwt.correctBoundaryConditions();
+
     return
     (
-      - fvm::Sp(htcs_->h()/Cp_, hs) - htcs_->h()*(Tstd - TPrimary_)
-      - fvm::Sp(htcw_->h()/Cp_, hs) - htcw_->h()*(Tstd - Tw_)
+      - fvm::Sp(htcst/Cp_, hs) - htcst*(Tstd - TPrimary_)
+      - fvm::Sp(htcwt/Cp_, hs) - htcwt*(Tstd - Tw_)
     );
 }