Particle based method to include interstitial gas contribution to conductive heat transfer in dense granular systems

Abstract

Roads are a key element of infrastructure connecting people, goods, and services. Increasing the sustainability of road construction helps balance the ecological impact of economic growth. Some road materials like hot mix asphalt (HMA) can be recycled and reused in new road construction. Increasing the amount of reclaimed asphalt product (RAP) in new road construction reduces waste and cost in raw materials but requires additional energy to reheat the RAP. Therefore, improving the thermal efficiency of HMA production is a necessary step for increasing RAP usage. Simulation offers a tool to improve the thermal efficiency of mixing equipment, but only if the numerical models can capture the relevant physics at the industrial scale.

While conductive heat transfer is the dominant mode in some methods of aggregate and RAP heating because these materials have low thermal conductivity, much of the heat transfer will occur through interstitial gases. Multi-phase methods are often computationally expensive, placing many industrial scale applications beyond reach. The objective of this study is to create a tool to make the simulation of heat transfer in dense granular systems where the gas contribution is important tractable at the industrial scale. To that end, a simple particle-based model is proposed that includes the contribution of the interstitial gases to conduction in dense granular materials.

The model was developed using a multi-scale approach to correlate the effect of interstitial gases on conduction heat transfer in randomly packed samples to the local packing structure. The model was then validated against an experiment of a static, heat column of glass beads and found to better approximate the temperature distribution than a more complex and computationally expensive method. The model was then tested in a dense, dynamic system by comparing the results to experimental data for glass beads in an indirectly heated rotating drum. The model was shown to capture the bulk average temperature rise in the glass beads. Further virtual tests were performed to validate the model against an experiment of mixing hot and cold aggregate particles in a flighted, inclined drum. Results from the simulation show a good agreement with the experiment for the temperature rise in the cold particles demonstrating the model’s ability to capture the gas contribution in dynamic systems of irregular shaped particles. Finally, the model was used to improve heat exchange to recycled asphalt in a full-scale mixer, demonstrating the model’s value to improve equipment design.