Precision cooling for C02 reduction

Licentiate thesis, 2019

Global climate is adversely affected by increasing emissions of carbon dioxide and other green-house gases. The road transport sector contributes significantly to these emissions. Irrespective of the source of energy– Internal Combustion Engines (ICEs) or electric motors, cooling is an important factor that affects the operating performance and more importantly, the efficiency. Efficient cooling positively impacts operating efficiency by reducing fuel consumption and thereby leads to reduced CO2 emissions. The high power density of modern day ICE causes excessive thermal loads to be exerted on the engine components. This reiterates the demand for efficient cooling. While the convectional cooling system grows in size, it also extracts more power from the engine output. It is, therefore, desired to provide efficient cooling of the hot spots, while avoiding over-cooling of the regions with low or moderate temperature. This method, known as precision cooling, improves local cooling and minimizes overall cooling. This in turn reduces the heat losses and thereby improves the thermodynamic efficiency of the ICE.

Local boiling can be an efficient way to implement precision cooling. The heat transfer involving nucleating and collapsing vapor bubbles near the surface of a hot metal is known as nucleate boiling. This phenomenon can positively impact cooling, as a significant amount of heat is extracted from the hot metal for the evaporation of the coolant to its vapor phase. Thus, heat is efficiently transferred locally near the hot spot through the vapor bubbles. However, excessive boiling could be counter productive and can lead to formation of a thin vapor film with low thermal conductivity on the metal surface. This film reduces heat transfer, prevents cooling and can eventually lead to material breakdown. Hence, it is extremely important to use nucleate boiling without the risk of having film boiling. Therefore, accurate estimation of boiling heat flux is the first step towards utilizing the potential of nucleate boiling. The main focus of this work is on numerical models to estimate the wall boiling heat flux. Such numerical models can be used in conjunction with Computational Fluid Dynamics (CFD) to analyse the heat transfer inside an ICE coolant jacket. A semi-mechanistic boiling model, based on established existing models in literature, has been proposed. Experiments performed on simplified geometries, representative of the areas in the ICE where boiling can be encountered, are used for validating the new model. The results from the validation study show that boiling is affected by properties of the flow, fluid and the solid. In addition to an improvement in accuracy of predicting the boiling heat flux, the model also provides a conservative measure to limit boiling and ensure the adverse effects of excessive boiling are not encountered. Finally, the limitations in the current model are discussed along with a possible solution for improvement.