An instantaneous condensing gas-fired water heater: Modeling and performance
Abstract
A mathematical model of an instantaneous gas-fired water heater was developed and verified experimentally. The water heater employed a ceramic fiber matrix burner, which emitted significant infrared radiation when heated by combustion. A copper finned tube multi-pass heat exchanger, located within the refractory-lined combustion chamber, was used to transmit thermal energy from the products of combustion to the water. The coil was designed to condense the water vapor present in the flue gases, thereby extracting the latent heat. Due to the high heat exchanger surface area to thermal input ratio, the water heater performed with a high efficiency (typically 90% or higher). The small surface area of the combustion chamber ensured low jacket losses. Convective transfer to the coil was described by means of experimentally determined coefficients obtained in a wind tunnel. The heat exchanger was operated at moderately low gas-side flow rates, under various degrees of surface wetting. Coils of several geometries were examined, and the influence of flow circuiting and external baffles was also studied. Correlations of Colburn j-factors for heat and mass transfer versus Reynolds number are presented. The model employed the experimental j-factors to describe the convective transfer occurring at the outer coil surface. The rectangular combustion chamber was modeled as a six surface enclosure bounding a uniform mixture of combustion product gases. All possible thermal radiation inter-reflections were accounted for by means of energy balances performed at each surface. Gas radiation was calculated by the exponential wide band model. The combustion gas temperature was obtained by performing an overall energy balance on the system. Flue losses were obtained by means of an empirical flue loss equation which accounts for vapor condensation. Two separate heat exchangers were installed in a prototype of the water heater, and baseline performance data were obtained. Model predictions were compared to these data, and in many cases close agreement was found. A model sensivity analysis indicated that improved predictions of heater performance could be obtained provided more accurate surface emissivity data were employed.
Degree
Ph.D.
Advisors
Goldschmidt, Purdue University.
Subject Area
Mechanical engineering
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