relationship (Eqs. 9, 10) (m 2 /s) bCoefficient in Eq. (24b) which should be defined trough the initial condition δ (t = 0) = 0 C p Specific heat capacity (J/kg) E L (n, β, t) Squared-error function in accordance with the Langford criterion (Eq. 34) E LT (n, β, t) Squared-error function in accordance with the Langford criterion (Eq. 35) E Mq (p, β, t) Squared-error function in accordance with the Langford criterion and fixed flux BC problem (Eq. 76) e LT (n, β, t) Squared-error sub-function in accordance with the Langford criterion (Eq. 35) e Lq (p, β) Squared-error sub-function in accordance with the Langford criterion and the fixed flux BC problem k Thermal conductivity (W/mK) k (T) Temperature-dependent thermal conductivity (W/mK) k 0 Thermal conductivity of the linear problem (β = 0) (W/mK) m Dimensionless parameter of the nonlinearity (power-law diffusivity) n Dimensionless exponent of the parabolic profile p Dimensionless exponent of the parabolic profile of the assumed profile used to solve Eq. (48) (m) Abstract Closed form approximate solutions to nonlinear transient heat conduction with linearly temperaturedependent thermal diffusivity have been developed by the integral-balance integral method under transient conditions. The solutions uses improved direct approaches of the integral method and avoid the commonly used linearization by the Kirchhoff transformation. The main steps in the new solutions are improvements in the integration technique of the double-integration technique and the optimization of the exponent of the approximate parabolic profile with unspecified exponent. Solutions to Dirichlet and Neumann boundary condition problems have been developed as examples by the classical Heat-balance integral method (HBIM) and the Double-integration method (DIM). Additional examples with HBIM and DIM solutions to cases when the Kirchhoff transform is initially applied have been developed.
List of symbolsThermal diffusivity (m 2 /s) a 0 Thermal diffusivity of the linear problem (β = 0) (m 2 /s) a p Thermal diffusivity coefficient in the case of power-law non-linear