Experimental study of low pressure heat transfer on tubes and wall

Abstract

Experimental data are reported for water boiling at 850, 450 and 50 mbar pressures on the shell-side of a model industrial boiler slice. The boiler test section was 1 m high, 0.75 m wide and contained 36 electrically heated tubes. The tubes were 28.5 mm in diameter and 98 mm long. The design of the boiler ensured that the tubes were submerged in a liquid pool. The height of the liquid pool could be varied. The pool height was set to approximately 0.8 m for the tests carried out at a pressure of 850 mbar, submerging the top of the tube bundle by about 200 mm. Two pool heights were used for the tests carried out at a pressure of 50 mbar, one at approximately 0.8 m and another at approximately 2 m. The later submerged the top of the tube bundle by about 1.6 m. The tube heat flux was varied from 10‐65 kW/m2 for the tests at pressures 50 mbar and was varied within the range 10-70 kW/m2 for the test of 450 and 850 mbar. A near-symmetrical half of the tube bundle contained wall thermocouples. An additional 29 thermocouples were located throughout the liquid pool. The liquid pool temperature was found to be reasonably uniform and controlled by the pressure at the free surface. This led to a small amount of sub-cooling at a pressure of 850 mbar, up to 3 K, and a significant amount of sub-cooling at a pressure of 50 mbar, up to 16 K for the smaller pool height and up to 31 K at the larger pool height. The reasonably uniform pool temperature suggests that the liquid re-circulates within it. Boiling is found to occur at all heat fluxes at a pressure of 450 and 850 mbar, with the measured heat-transfer coefficients shown to be in broad agreement with nucleate boiling correlations available in the open literature. However, it is also consistent with a flow boiling process involving natural convection and nucleation, where the convection is driven by variations in liquid temperature on the walls of the tubes. This natural convection relies on an interaction between the tubes that produces mass fluxes in the range 46-87 kg/m2s, based on the approach area to the tube bundle. Boiling occurs only at the higher heat fluxes during the low level tests at a pressure of 50 mbar, with interactive natural convection being the dominant heat-transfer mechanism. The mass fluxes produced are in the range 28-70 kg/m2s. Boiling also occurs only at the higher heat fluxes during the high level tests at a pressure of 50 mbar. However, the convective heat transfer was more compatible with little interaction between the tubes, although some evidence suggests that the evaporator oscillates between interaction and isolated tube behaviour. Solids can come out of solution when some process fluids are evaporated. These solids can form beds of particles on the heated base of the evaporator vessel. The effect on base temperature of increasing the bed depth is experimentally investigated for water boiling at a pressure of 50 mbar absolute. The bed depth is varied from 0-32 mm using glass particles 500-600 μm in diameter. The evaporator used was a model industrial boiler slice. The tube heat flux was maintained at 65 kW/m² and the base heat flux varied within the range 0-45 kW/m². Out with the solid bed, the liquid temperature in the liquid pool is shown to be reasonably constant and close to the free surface saturation temperature. This indicates that fluid recirculation is taking place, with fluid flashing to the saturation temperature at the free surface before returning to the depths of the pool. The liquid temperature within to the solid bed is shown to be greater than that in the pool and to decrease with increasing base heat flux. The temperature of the base is shown to be subcooled in the absence of a solids bed. The presence of the bed induces boiling at most conditions, indicating that a strong convection current normally cools the base and that the base is insulated from this cooling by the bed. The bubbles formed within the bed increase in size with increasing bed depth and heat flux. Beneath the bed, the base temperatures decrease with increasing base heat flux and the base superheat increases with increasing bed depth until 16 mm, decreases at 24 mm and increase again at 32 mm.