Abstract
KEYWORDS - heat transfer, waste heat recovery, internal combustion engines, pulsating flow.
ABSTRACT
Research and design in the field of internal combustion engines (ICE) seek to achieve high performance while conserving fuel economy and low pollutant emissions. Waste heat recovery represents a promising way to improve ICE efficiency. In the past few years the potential of several technologies which transform the engine waste heat into a more useful form of energy, either mechanical or electrical, have been examined. The objective of this study is to investigate the potential of various designs of heat energy storage system (HESS) to temporally recover thermal energy from hot gases and to restore heat to a cold gas for a conventional ICE application. Seven different geometrical designs of HESS are investigated. The basic geometric shape is a hollow cylinder with crossbars. Hot and cold gas flows are alternately forced to pass through the HESS in opposite directions. In order to evaluate the influence of thickness, distance between bars, number of rows of bars in the axial direction, on pressure losses and on thermal behaviour of the HESS, a parametric study is performed. The thermodynamic conditions similar to the exhaust engine gases are reproduced with a conventional ICE and the HESS is placed into an external manifold connected to the exhaust side of the ICE. The engine is operated at 2000 rpm and 1 bar of intake pressure. The maximum hot gas temperature is here limited at 180°C, and the maximum mass flow rate is 32 kg/h.
Efficiency of each HESS to recover and to restore energy is evaluated separately for each of the two phases flow and compared with the results of a reference case, corresponding to an empty pipe without any HESS. In the optimal cases, a recovery efficiency about two times higher than in the reference case and a capacity to restore energy 1.6 times higher than in the reference case, are obtained. When combining the two heat transfer phases in a global recovery/restoration coefficient, an efficiency more than two times higher than in the reference case is reached. The results also point out that in the optimum case, thanks to the low thermal inertia of the HESS, about 40 seconds less than in the reference case are needed to restore the same quantity of energy into a cold air.