Equivalent-homogeneous-layers-set method for time-dependent heat transfer through hollow-block walls

Abstract

An equivalent-homogeneous-layer-set (EHLS) method to account for the time-dependent heat transfer through a wall composed by hollow-blocks is proposed. This method considers the two-dimensional heat transfer due to conduction through the solid frame, the heat transfer by natural convection inside the air-cavities, and the heat transfer by radiation between air-cavity surfaces. The EHLS method gives an equivalent wall, composed by a set of homogeneous layers, that has a similar thermal behavior than the original hollow-block wall. Convection and radiation inside the air-cavities are considered by using temperature dependent heat coefficients and a radiative correction factor. This radiative correction factor is calculated for different values of the in-cavity fractional area and different aspect ratios of the air-cavities by comparing numerical results of the EHLS method with results of an experimentally validated two-dimensional model, in steady-state. This experimentally validated two-dimensional model, for time-dependent periodic conditions, is used to validate the EHLS method. Differences are up to 3.4% for the energy transferred through the wall, and 7% for both the sol–air decrement factor and the lag-time when the hollow-block wall is in a non-air conditioned room and up to 6% for the total thermal load when it is in an air-conditioned room. The EHLS method can be implemented into whole-building simulations programs and can be extended for other constructive systems for walls or roofs that have air-cavities inside, using the appropriate convective and radiative coefficients and radiative correction factor.