HEAT TRANSFER IN A PULSE COMBUSTION WATER HEATER
Pulse combustion devices offer several potential advantages over conventional burners, including enhanced efficiency, reduced size, automatic rejection of exhaust gases and consistency of operation over long time periods. This project was conducted in order to acquire an understanding of the heat transfer characteristics and pressure oscillations occurring in pulse combustion devices. A gas-fired pulse combustion water heater was utilized. This apparatus was available prior to the current research, but in order to accomplish the goal of the present investigation it was necessary to modify the system in several ways. Experimental investigations were then conducted in order to study the dependence of the unit's pressure oscillations, heat transfer characteristics and thermal performance on various system parameters. Relevant measurements of temperatures, pressures and flow rates were obtained during each test. Conclusions were drawn from comparisons of heat transfer results and pressure results. Flapper valves used to increase the pressure amplitude in the combustion chamber were found to enhance the thermal efficiency by various amounts, depending on the conditions, up to a maximum of 23%. The highest thermal efficiency obtained experimentally was 84.8%. A refined thermal analysis was applied to the experimental data obtained from one particular test for which the thermal efficiency was calculated to be 67.8%. Detailed calculations were performed in order to determine the heat transfer occurring in the various components of the system. Based on these calculations, 31% of the input energy was lost through the stack and only 1.2% was lost through other paths. The remaining 67.8% of the energy input was transferred to the water. It was found that approximately 64% of the heat transfer to the water took place in the flue tube section, with 26% occurring directly from the combustion chamber through the water cooling jacket and 10% occurring through the heat exchanger end plates from the combustion chamber and exhaust chamber. It was also estimated that the contribution of the gas radiation to the total heat transfer in the flue tube section was approximately 22%.
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