A numerical study of pulse-combustor jet impingement heat transfer

A pulsating jet generated by a pulse combustor has been experimentally demonstrated as a technique for impingement heat transfer enhancement relative to a steady jet. The enhancement factor was as high as 2.5. Despite such potential, further studies of this technique have been limited, let alone industrial applications. The ultimate goal of the Pulsed Air Drying project at the Institute of Paper Science and Technology is to develop this technique to commercialization for industrial applications such as paper drying. The main objective of the research in this dissertation is to provide a fundamental basis for the development of the technology. Using CFD simulations, the research studied the characteristics of pulsating single-slot-nozzle jet impingement flows and heat transfer on stationary and moving surfaces. In addition, in order to understand basic flow characteristics of pulse-combustor jets, a simplified model of Helmholtz pulse combustors was developed. The model was used to recommend a strategy to generate a pulsating jet having large amplitude of velocity oscillation. And based on this model, pulsating jets in the simulations were characterized as those at the tailpipe exit of a pulse combustor. The impingement conditions were similar to those in conventional impingement hoods for paper drying. Parameter studies included the effects of jet velocity oscillation amplitude, pulsation frequency, mean jet velocity, tailpipe width, and impingement surface velocity. Simulation results showed that the amplitude of jet velocity oscillation was the most important parameter for heat transfer enhancement, in which two mechanisms were identified: high impinging jet velocity during the positive cycle and strong re-circulating flows in the impingement zone during the negative cycle of jet velocity oscillation. As for the improvement by the pulsating jets relative to steady jets, the maximum heat transfer enhancement and energy saving factors were 1.8 and 3.0, respectively, which were very encouraging for further development of the technology.Ph.D.Committee Co-Chair: Ahrens, Fred; Committee Co-Chair: Patterson, Tim; Committee Member: Aidun, Cyrus; Committee Member: Empie, Jeff; Committee Member: Frederick, Ji

Characteristics of Pulsating Flows in a Pulse Combustor

Pulsating flows in a Helmholtz pulse combustor tailpipe were numerically simulated by a commercial CFD software package, FLUENT. The effects of ambient temperature on the characteristics of the pulsating tailpipe flows were studied. Two study cases, with high and low levels of ambient temperature, were simulated with compressible flow equations. An additional case, with high ambient temperature, was simulated with incompressible (temperature-dependent density) flow equations. Results showed that the effect of ambient temperature on the mean temperature profile in the tailpipe was limited to the distance where the ambient fluid traveled into the tailpipe during the period of flow reversal. In this region, the amplitude of mass flow rate oscillation significantly increased, due to higher density associated with low ambient temperature. The overall effects of cooler ambient temperature included an increase in mean pressure at the entrance of the tailpipe and a decrease in the magnitude of velocity amplitude profile along the tailpipe. Interestingly, the mean velocities along the tailpipe, even at the tailpipe exit, were not affected by the cooler ambient air. The mean velocity at the exit corresponded to the higher temperature of fresh fluid from upstream, which was not affected by the ambient temperature, driven out of the tailpipe in each oscillation cycle. The linear acoustic theory with appropriate assumptions could be used to calculate the magnitude of the profiles of velocity amplitude along the tailpipe as a fair approximation, at least for the study cases in this thesis.M.S.Committee Chair: Dr. Timothy Patterson; Committee Member: Dr. Cyrus Aidun; Committee Member: Dr. Frederick Ahren