In one dimension in the x-direction the rate of heat transfer or heat flux is expressed according to Fourier’s law as outlined in Sect. 1.1.q . ′′ x =−k⋅dTdx q.x′′=−k⋅dTdxwhere k is the thermal conductivity. For simplicity the mathematical presentation of the heat transfer phenomena is here in general made for one-dimensional cases only. Corresponding presentations in two and three dimensions can be found in several textbooks such as [1, 2].