| This dissertation presents an experimental study of the instantaneous, local heat flux, from the exhaust gases to a straight pipe extension of the exhaust port of a four cylinder spark-ignition engine. Both the steady-state and transient heat flux characteristics were studied, over a wide range of engine operating conditions, from 1000 rpm, light load, through 4000 rpm, full load Transient heat flux measurements were made at two locations on the inner surface of the test section. Additionally, the test section included two gas-temperature thermocouples, a steady-state heat-flux sensor, a series of external wall temperatures and a fast-response exhaust-gas-pressure sensor. The transient heat flux sensors consisted of a fast-response surface thermocouple, and a thermocouple recessed several millimeters from the surface.;During the exhaust valve-open period, the surface heat flux history was characterized by a double peak. The largest peak resulted from the action of high-velocity, blowdown gases exiting the combustion chamber at the beginning of the exhaust process. The significantly lower second peak was due to the slower piston-driven gas motion that occurred during the displacement phase. During the closed-valve period, the local surface heat flux was found to be relatively low (10 to 45 kW/m2) in comparison to the peak heat flux (80 to 288 kW/m2), and to exhibit minimal decay.;The Convective Augmentation Factor (CAF), which is defined as the ratio of the measured heat flux to the corresponding heat flux for fully-developed turbulent pipe flow, was found to decrease with increasing Reynolds number and increasing axial distance from the entrance of the test section. The local steady-state heat flux was well-correlated by a Nusselt-Reynolds number relationship that included entrance-effects. These effects were found to be the major contributor to the local heat transfer augmentation.;A quasi-steady-state analysis was conducted, by employing the steady-state correlation developed in this study and the instantaneous exhaust gas temperatures and velocities exhaust predicted by an engine simulation. The predicted heat flux histories from this analysis were in good agreement with the corresponding measured histories during blowdown, however, predictions were relatively poor during the displacement phase, both in magnitude and behavior. |
