Masters Theses

Devdutt Dash

Date of Award

5-1994

Degree Type

Thesis

Degree Name

Master of Science

Major

Mechanical Engineering

Major Professor

Roy J. Schulz

Committee Members

John P. Foote, K.C. Reddy

Abstract

Modern energy conversion and power systems require compact and highly effective heat exchanger systems that are economical and achieve process requirements. Indirectly fired combustion systems for power plants must use heat exchangers operating with hot products of combustion at atmospheric pressure on one side, and clean pressurized air on the other. The air is indirectly heated before being expanded through a power turbine. Studies at UTSI and elsewhere show that the heat transfer coefficient on the hot gas side limits the efficiency of the heat exchanger and determines its size. Therefore to make the heat exchanger compact and efficient, convective heat transfer augmentation is required.

Convective heat transfer augmentation incurs an increase in the pressure drop associated with the gas flow. This increases the pumping power required by the system. It thus becomes essential to balance the trade off between an increase in the amount of heat transferred and the increase in pump work required to move the fluids through the heat exchanger. This research is an experimental study of convective heat transfer and its enhancements in a bayonet tube heat exchanger. The bayonet style heat exchanger holds promise in high temperature (1650 °K and greater) heat exchangers, which will probably be made, at least partially, of ceramics. The enhancement technique investigated in the present study is based on swirl flow. Swirl flow has been found to be effective in increasing heat transfer in tubes. However, its effect in annulus flows and bayonet tubes has not been extensively studied. In addition, the complex nature of the flow phenomenon in bayonet tubes makes it difficult to directly apply standard heat transfer and pressure drop correlations.

Inlet swirl was generated in a standard double-pipe bayonet heat exchanger using a four vane swirler with a 70° vane angle. The swirler was positioned in the entrance of the annular section of the exchanger. The heat transfer characteristics of the bayonet tube with a non uniform temperature boundary condition was studied for annular flow in the Reynolds number regime of 8900 to 17870. The bayonet tube was fully instrumented with thermocouples for wall and gas temperature measurements. The resulting temperature distributions were analyzed to determine heat flux and surface heat transfer coefficient distributions in the system. The analysis accounted for both radiation heat transfer, and non-developed flow conditions, for various Reynolds numbers, in computing the heat transfer coefficients. Experimentation and subsequent analysis has shown that swirl flow indeed enhances the heat transfer performance of bayonet tubes by approximately two to three times over the (actual) heat transfer coefficient for non-swirl, straight-flow conditions, for the Reynolds numbers investigated.

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