An Experimental Investigation of In-Tube Evaporation of Pure Ammonia in a Smooth and a Microfin Tube, Part I—Heat Transfer (RP-866)

Abstract

Average and sectional-average heat transfer coefficients were measured for the evaporation of pure ammonia inside horizontal smooth and microfin tubes. The test sections were tube-in-tube, counterflow heat exchangers with ammonia flowing in the inner tube and liquid R-134a in the annulus. Both the smooth and the microfin test sections were constructed with a 3.048 m (10.0 ft) long aluminum tube placed inside a 19.0 mm (0.75 in.) outside diameter (OD) outer tube. Data were collected at mass fluxes of 9, 27, 47 and 61 kg/(m2·s) (6600, 20,000, 35,000 and 45,000 lb/h·ft2), saturation temperatures of 5, −10 and −20°C (41, 14 and −4°F) and heat fluxes of 820, 2710, and 5430 W/m2 (260, 860, and 1720 Btu/h·ft2). Average heat transfer coefficients were measured for a 15 to 95% quality range at each saturation temperature for all mass fluxes. Sectional-average heat transfer coefficients were measured for each saturation temperature and heat flux at nominal test-section vapor qualities of 10, 25, 50, 75, and 95%. The improved surface-wetting characteristics of the microfin tube were found to increase the heat transfer coefficient relative to that measured in the smooth tube. This effect was greatest at low mass fluxes when the fluid flow was laminar and stratified. A comparison of the experimental data with current correlations available for evaporation showed that no correlation accurately predicted the heat transfer coefficients over the entire range of conditions.