MATHEMATICAL MODELING OF AN INSTANTANEOUS GAS FIRED WATER HEATER

GREGORY MICHAEL MAXWELL, Purdue University

Abstract

A mathematical model has been developed to simulate the thermal performance of a instantaneous gas fired water heater. The model allows for variation in heater geometry (including several types of finned tube/baffle heat exchanger configurations), ambient conditions, and operating conditions (such as excess air, input firing rate, water flow rate and inlet temperature). Experimental temperature data were obtained for a range of operating conditions and heat exchanger configurations. The data were compared to model predictions and were found to agree well for most heater components modeled except for the heater floor where a major portion of the thermal radiation from the reradiating refractory tiles and the emitting products of combustion is blocked by the burner assembly. A sensitivity analysis for some of the heater input parameters was performed to examine their effect on heater thermal efficiency. The results showed that improvements in heater performance could be achieved if the combustion chamber height was reduced, the emissivity of the refractory surfaces was decreased, the heat exchanger emissivity was increased, the amount of excess air used for combustion was decreased, and the ratio of heat transfer surface area to thermal input was increased. From these results a new heater design was proposed and modeled. The new design showed an improvement in thermal efficiency of more than 5% over the current design. A numerical method was developed to calculate the radiation view factor between a planar surface and complex surfaces such as a finned tube heat exchanger or a series of burner tubes. The numerical method is based on the experimental method of Eckert and uses a ray firing technique and a computer graphics terminal. View factors calculated for simple geometries were compared to view factors calculated using the numerical method. Excellent agreement (<0.1% error) was found between the calculations. An experimental apparatus (based on the work of Eckert) was used to calculate the view factor between a differential surface element and several finned tube geometries. These view factors were compared to view factors calculated using the numerical method and were found to be in good agreement. The numerical method was then used to calculate view factors for the various geometries found in the water heater. When the view factors were incorporated into the heat transfer models, the calculated heater floor temperatures and overall thermal performance of the heater more closely matched the experimental values.

Degree

Ph.D.

Subject Area

Mechanical engineering

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