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Abstract: This paper computes the thermal delay in multilayer systems. The heat transfer by conduction across multilayer systems under unsteady boundary conditions is simulated using analytical solutions. The solutions, obtained as Green''s functions, are established by imposing temperature and heat flux continuity at the media interfaces of the various layers. The unsteady state conditions are dealt with by first computing the solution in the frequency domain and then applying (fast) inverse Fourier transforms into space-time. After validating the analytical formulation by comparing these results with those obtained experimentally, the thermal delay is computed for different multilayer systems. It is computed as the difference between the time the thermal variation is recorded in one of the system''s surfaces and the time it appears at the opposite surface. The paper calculates the computed thermal delay for construction walls made of cork, mineral wood and extruded polystyrene panels. The thermal properties of these materials have been previously defined experimentally. In the simulations, the systems are subjected to temperature changes that vary according a sine function, with the aim of defining the environmental exterior temperature variation over a period of several days. [Copyright &y& Elsevier]

Abstract: This paper focuses on discussing four myths and concerned conventional practices of film cooling research guided by a series of computational simulations. The issues that have been discussed include: (a) the film cooling effectiveness (φ) is given a constant (0.6) to calculate the heat flux ratio between film-cooled and no-film cases; (b) the adiabatic wall temperature is the driving temperature of film cooling; (c) the adiabatic film heat transfer coefficient can be obtained from an isoenergetic film experiment (T j = T g); (d) using a heated surface can provide a simplified approach to simulate the film-cooling condition. The result shows that the adiabatic wall temperature is not always the driving temperature (i.e. T aw is not always larger than T w), but T aw does act well as the reference temperature in correctly predicting the heat flux direction and giving an always positive adiabatic film heat transfer coefficient. Using a constant value for φ is questionable and may lead to un-realistic or false HFR. The conventional equation of HFR was not theoretically exact and can lead to an error of up to 20%. A revised HFR equation is provided in this paper. The dominant energy passage in turbine airfoil film cooling is always from hot combustion gas flows into the airfoil; therefore, employing a heated wall as a boundary condition with hotter main stream flow and cold film injection to simulate the actual film cooling condition is found to be fundamentally questionable. A conjugate simulation that includes wall heat transfer and internal flow cooling shows that reversed heat transfer from blade to gas that gives a negative HFR is possible, due to the heat conduction from the downstream hotter region into the near film hole cooler region. [Copyright &y& Elsevier]