Forced convection heat transfer to a single and two-phase steam/water mixture in a helical coil with radiant heating
Vafaie, Foad N
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The purpose of this study has been to perform an analytical and experimental investigation into the heat transfer characteristics for a once-through steam generator with a helical flow geometry. The application is the receiver for a fixed hemispherical mirror solar concentrator on the Crosbyton Solar Power Project. In this design, the working fluid, water, goes successively through the single-phase liquid, two-phase and super-heated vapor regimes in a once-through, helically wrapped tube bundle subjected to nonuniform, asymmetrical radiant heating. Individual segments of the radiation profile for the receiver were simulated using high intensity, line source quartz lamps providing concentrations of up to 240 suns. A segment of the helical coil was instrumented and mounted in the radiation field. Test section instrumentation included inlet fluid flow rate, inlet and exit fluid pressure and temperature, and 42 type K thermocouples welded to the surface of the coil. The incident radiation field was mapped using a Gordon type heat flux transducer calibrated for the quartz lamp spectrum. An upstream hot oil heat exchanger provided inlet fluid conditions to the test section consistent with the segment of the receiver being simulated e.g., single-phase or two-phase. The range of test conditions included an incident radiation flux of 6,000 to 27,000 Btu/hr-ft , a flow rate of 100 to 500 Ibm/hr, an inlet pressure of 100 to 500 psia, and an inlet quality of 0 to 70 percent. A numerical analysis was developed to predict the local internal heat transfer coefficients and fluid state based on the measurements obtained in the test procedure discussed above. The analysis included consideration of the externally applied radiation field, convective and radiative heat losses, two-dimensional conduction through the tube wall, and.convection to the internal fluid. The results show that there is a significant change in the angtilar variation of the internal heat transfer coefficient at low quality when compared with values for high quality. At low quality, the highest internal heat transfer coefficients are obtained on the tube surface farthest from the axis of the coil. At high qualities, this changes to yield the highest coefficients on the tube surface closest to the coil axis. Factors contributing to this phenomena include radial acceleration, induced secondary flow, flow acceleration along the flow axis, and the onset of film boiling. The integrated average values of the heat transfer coefficient for the subcooled liquid and two-phase were correlated against dimensionless parameters of the flow such as the Reynolds number, Prandtl number, boiling number and tube-to-coil diameter ratio.