An Investigation into the Accuracy of Orifice Based Flow Estimates for Pulsating Compressible Flows 2014-01-1154
Orifices, flow nozzles and arbitrarily shaped flow obstructing flow measurement devices are widely used to estimate EGR flow rates in engines, and also used to model flow restricting components like valves in engine analysis tools such as GT-Power. The standard assumptions about the flow discharge coefficient and its variation with Reynolds number are based on investigations of orifices across steady non-pulsating flows, widely reported in literature. In this work, the discharge coefficient for steady state pulsating flow as well as accelerating pulsating flow, commonly encountered during steady state and dynamic engine operation respectively, were investigated by installing an orifice on the exhaust side of a naturally aspirated diesel engine, while making reference flow measurements with a Laminar Flow Element on the intake side. ‘Snap Throttle’ tests were performed to accelerate the flow on the exhaust side with a sudden increase in exhaust gas temperature and accompanying decrease in density. Contrary to reported literature based on steady non-pulsating flow, a linear and directly proportional relationship between coefficient of discharge and Reynolds number was determined for steady state operation at Reynolds numbers exceeding 45,000. The coefficient of discharge changed by about 11% as the Reynolds number increased from approximately 45,000 to 103,000, corresponding to part load and full load steady state conditions at three engine speeds. Compressibility effects were observed to be important. During dynamic operation, the primary source of inaccuracy for the orifice based estimate was determined to be the slow temperature measurement rather than dynamic pressure waves corresponding to the acceleration of flow during the snap throttle event. Another objective of this work was to understand the role of factors other than the fuel-Oxygen ratio in the production of smoke spikes during dynamic operation. Such factors include injector dribbling/overflow caused by pulsations in the fuel injection system, wall impingement, lack of mixture preparation while the full load in-cylinder flow field is established, and the relatively low cylinder wall temperature during the initial period of full load operation. Engine opacity during the snap throttle event was observed to track the fuel-Oxygen ratio, suggesting that these factors were of secondary importance.