During the 1980s, solar thermal electric technology was commercialized with the installation of 350 MW of electrical power capacity in the California desert. The technology involved heating a heat transfer oil in receiver tubes placed at the focus of line-focus, parabolic trough solar concentrators. The heat transfer oil was then used to generate steam which, in turn, powered a steam turbine/electrical generator. Since the transfer of heat from the oil to the steam creates a temperature drop and a resulting loss in thermal efficiency, alternatives have been considered. In one alternative, steam would be generated directly inside the receiver tubes. Consider an example in which a heat flux of 50,000 W/m² is absorbed on the outside surface of a 12.7 mm i.d., stainless steel 316 tube with a wall thickness of 1.245 mm. Inside the tube, saturated liquid water at 300°C is flowing at a rate of 100 kg/hr. Determine the maximum tube wall temperature if the steam quality is to be increased to 0.5. Assume μ_v = 2.0 x 10^–5 kg/(ms). Neglect any heat losses from the outside of the receiver tube.

GIVEN

• 12.7 mm i.d. tube with flowing, boiling water inside
• 50,000 W/m² heat flux at tube o.d.
• 100 kg/hr water enters the tube at saturated liquid conditions, 300°C
• Absolute viscosity of the steam is μ_v = 2 x 10^–5 kg/(ms)
• Tube heat losses are negligible

ASSUMPTIONS

• The method of Chen is applicable