Subcooled boiling flow in liquid-cooled internal combustion engines

Doktorsavhandling, 2022

Technical components that generate heat need to be cooled by removing the excess heat in order to protect their structural integrity. Internal combustion engines are either air cooled or liquid cooled. Liquid cooling is a more efficient way of removing the excess heat from internal combustion engines with high specific power, i.e., high power per cylinder volume, owing to the higher thermal conductivity and specific heat capacity of liquids compared to air. Such engines are often cooled with a liquid coolant, which is a mixture of ethylene glycol and water.
The coolant flows through various parts of the engine structure through carefully designed coolant flow passages, which constitute the coolant jacket. The flow of liquid coolant removes excess heat from the engine metal structure by what is known as heat transfer by forced convection. Certain critical regions in the engine experience high temperatures, such that the temperature of the engine structure at the solid-coolant interface is above the saturation temperature of the coolant. This results in occurrence of nucleate boiling locally in that region, which is the vaporization of the coolant in the form of bubbles close to the heated surface. The heat transfer rate involved in nucleate boiling is exponentially higher compared to that in forced convection, due to the phase change process involved. Local nucleate boiling removes heat efficiently from the critical regions without over cooling the non-critical regions, resulting in precision cooling. However, excessive boiling results in coalescence and agglomeration of vapor bubbles, resulting in the formation of a thin vapor film next to the surface of the metal. This is known as film boiling, which drastically reduces the heat transfer rate and, thereby, prevents cooling. Encountering film boiling might be detrimental to the structural integrity of the engine. Therefore, it is important to understand this limit and extract the potential of controlled local nucleate boiling for efficient cooling of internal combustion engines. Extracting this potential is the primary focus of this thesis.
Nucleate boiling is a complicated phenomenon involving complex interactions between the solid metal, the liquid coolant and the coolant vapor bubbles. Nucleate boiling is influenced by several factors, such as operating pressure of the cooling system, coolant flow velocity, coolant bulk temperature, thermo-physical properties of the coolant, surface roughness of the solid etc. The complexities involved and the factors influencing nucleate boiling are thoroughly discussed in this thesis. Furthermore, a numerical model is developed and proposed to analyse boiling occurring in the coolant jacket of internal combustion engines. The model not only captures the onset of boiling and estimates the boiling heat flux with reasonably good accuracy, but also includes a parameter which indicates occurrence of excessive boiling. This parameter is a useful tool to avoid occurrence of film boiling with sufficient safety margin during engine design and development. The model results are first validated with data from simple academic experiments and then with data from tests conducted on a passenger car engine.