Transient internal forced convection under dynamic thermal loads: in clean-tech and automotive applications

This research aims to address the thermal behavior of emerging engineering applications with dynamic thermal characteristics. Such applications include: i) clean-tech systems, e.g., powertrain and propulsion systems of Hybrid/Electric/Fuel Cell Vehicles (HE/E/FCV); ii) sustainable/renewable power generation systems (wind, solar, tidal); and iii) information technology (IT) systems (e.g., data centers, e-houses, and telecommunication facilities). In this research, transient internal forced-convection was used to model the thermal characteristics of the cooling systems in the above-mentioned applications. In addition, sinusoidal heat flux was considered, since arbitrary loads can be modeled by a superposition of sinusoidal waves using a Fourier transformation series. Additionally, benchmark driving cycles were used to investigate the thermal characteristics of a cooling system in the context of the real-world application of HE/E/FCV. Firstly, the energy equation was solved analytically for a steady tube flow under an arbitrary time-dependent thermal load. Then sinusoidal heat flux was taken into account, and closed-form relationships were obtained to predict the temperature distribution inside the fluid and the Nusselt number. Finally, the presented results were validated using a commercially available software program: ANSYS Fluent. In the next step, the energy equation was solved analytically for tube flow with an arbitrary flow rate and a given time-dependent heat flux. Sinusoidal heat flux and flow rate were then taken into account; closed-form series solutions for the temperature distribution and Nusselt number of the tube flow were presented. An independent numerical simulation was also performed to validate the models.Additionally, new testbeds were designed and built and a comprehensive experimental study was performed to analyze the thermal behavior of a tube flow under arbitrary time-dependent heat flux. It was shown that there was an excellent agreement between the experimental data and the predictions of the developed models.As a result of the above work, a new model was developed that predicts the minimum instantaneous flow rate to maintain the temperature at a given level under an arbitrary time-dependent heat flux. Compared to conventional steady-state designs, the developed model can result in up to a 50% energy savings while maintaining the temperature of the system below the targeted value.