Numerical analyses of a laboratory test of a geothermal bridge deck externally heated under controlled temperature

Abstract

Hydronically heated bridges, whose shallow geothermal energy is extracted from underground loops, are often used for bridge deicing. However, the existing heated deck designs employ embedded pipe loops, which are only applicable to new bridges. A new externally heated hydronic bridge deck has been developed for existing bridges. Heating tests of the hydronic deck were performed in an environmental chamber under conditions of controlled water and ambient temperature. In order to model the newly tested hydronic deck and understand the heat transfer processes, a 3-dimensional multi-physics model of the heated deck was developed in COMSOL. Transient analysis of the model was first performed and fully calibrated with the laboratory tests, and a thermal contact model was used to model a poor contact at the bottom of the bridge deck. The calibrated finite element model was further verified by the steady-state results of 15 environmental chamber tests. The heating responses of the heated deck at freezing ambient temperatures were also simulated and showed the same trend as those simulated at above-freezing conditions. Heat flux and heat energy balance were performed on the heated deck to determine the heat energy flow and heating efficiency. The numerical analysis indicates that approximately 76% of total supplied heat can be transferred to the deck top surface independent of the variation of ambient temperature. Close to the bridge deck surface, one-dimensional upward heat transfer can be assumed for heat flux analysis within the heated bridge deck. The developed model is capable of accurately modeling the heat transfer processes in the externally heated deck, and can be used for design analyses of such a system.