Study of a boiling dielectric fluid in a vertical mini-channel : application to the thermal management of battery packs

Summary :

This thesis is part of the industrial context of rapid development of electric vehicles, which includes the growing demand for more powerful and compact battery packs. These packs are generally made with lithium-ion battery cells and require a Battery Thermal Management System (BTMS) for an efficient cooling and to limit the thermal runaway hazard. Thermal runaway occurs when a cell is damaged, defective or when its temperature reaches a threshold value.In this case, the cell heats up spontaneously and its temperature rises rapidly, which can cause the expulsion of flammable gases, a chain reaction throughout the pack and ultimately a fire or explosion.The thermal management solution chosen in this thesis is the direct cooling of the immersed pack by a dielectric liquid circulating between the cells. The HFE-7100 has been selected as a working fluid for its eco-friendly coolant properties, its non-flammability, its attractive thermophysical properties, and especially for its boiling temperature at atmospheric pressure, which is very close to the estimated skin temperature at the beginning of a thermal runaway. The liquid can change phase to maintain the temperature in order to avoid thermal runaway, and to evacuate the heat in accidental situations.One scientific bottleneck for such a pack design, given the small spacing between cells or between cells and the pack casing, is the flow boiling in a vertical mini-channel. Confined boiling flow is a very efficient mode of heat transfer to dissipate high fluxes. The question to be addressed is whether such a flow can help in the hypothetical case of a thermal runaway.In this thesis, an experimental and a numerical approach are jointly conducted to study the cooling solution and the flow boiling.In a first step, the state of the art of Battery Thermal Management System and flow boiling in a mini-channel is presented. A general description of flow boiling and numerical simulation approaches of two-phase flows are proposed.In a second step, a test campaign is conducted with an experimental device to study the flow boiling of HFE-7100 in a vertical mini channel. The local heat transfer coefficient is obtained by using an inverse method. Heat transfer, critical heat flux (CHF), flow regimes and pressure drops are the main parameters studied. The influence of different parameters such as mass flow rate, subcooling, pressure and surface structuring is analyzed.In a third step, a numerical modeling approach is proposed. The simulation of the two-phase flow of HFE-7100 in the mini-channel of the test section is proposed with an Eulerian approach. The numerical models are validated from the experimental data. Finally, a battery module immersed by HFE-7100 is modeled, and a thermal runaway scenario of a cell is suggested and simulated. Calculations show that the cooling system is an effective and promising solution to dissipate heat and prevent thermal runaway propagation.