Interface heat transfer for hydronic heating: heating tests of concrete blocks and numerical simulations

Abstract

ABSTRACT A comprehensive understanding of the heat transfer mechanism of hydronically-heated structures is crucial to predicting their heating performance. However, few studies considered the heat transfer at the interface of heating pipe and host material. In this paper, heat transfer at two critical interfaces for hydronically-heated concrete structures, e.g. pipe-concrete interface and concrete-air interface, has been tested and analyzed. Two concrete blocks, one with an embedded cross-linked polyethylene (PEX) pipe and one with an attached PEX pipe, were fabricated to represent heated bridge decks for the heat transfer study. The two blocks were tested inside a freezer box in a series of heating response tests at varied room and fluid temperatures. Measured temperatures at different depths were used to find the convection heat transfer coefficient and the thermal contact conductance between the concrete and PEX pipe through equations developed via finite difference method (FDM). Finite element models (FEM) of the two blocks were developed to verify the thermal contact values derived from a 1-D thermal resistance model. A comprehensive analysis of the temperature response of the models was performed, especially at the two interfaces, and the FEM models were able to predict the lab measurements accurately. The obtained contact heat transfer coefficient and the FEM models can be used for all hydronically-heated structures.