The design of an apparatus to determine the film coefficient of water at high heat fluxes

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Virginia Polytechnic Institute

The thesis is concerned with the design of an apparatus to determine the film coefficients for the conditions of forced convention, local boiling of sub-cooled water and the two phase flow of water-steam mixtures. Inasmuch as the agitating action of the bubbles during boiling of liquid permits high rate of heat transfer, information concerning convective heat transfer under conditions of local boiling and the two phase flow is of importance to the development of high heat flux heat exchangers which incorporate a phase change of coolant.

The variation of the film coefficient with water temperature, flow rate, heat input rate, and fluid pressure may be investigated with this apparatus. The range of variables is

Heat flux: 2.5 x 10⁵ to 5 x 10⁵ B/hr ft² Inlet temperature: 100 to 300°F Mass velocity: 0.52 x 10⁶ to 1.7 x 10⁶ 1b/hr ft² Pressure: Up to 200 psia

The basic scheme of this apparatus is to pump deionized water through an electrically heard test section which is a stainless steel tub, 0.465 in O. D. x 0.020 in. thickness x 5ft length, where the fluid temperature, surface temperature and fluid pressure are measured by thermocouples and pressure gages. The electrical power to the test section is supplied by a welding transformed. Pressurization of the system is accomplished by throttling the flow at the discharge end of the test section through the exhaust valves. The fluid leaving the test section is condensed and sub-cooled by a vertical vapor-in-tube type heat exchanger so that the fluid can be recirculated again. An auxiliary ion-exchanger system is provided for deionization of the test water so that the deposition of scale on the heating surface can be prevented.

With the above apparatus, local clause of fluid temperature, surface temperature and heat flux may be obtained and consequently a “point” determination of the film coefficient is possible.