Investigation of nucleation and heat transfer during subcooled flow boiling on augmented surfaces

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Title: Investigation of nucleation and heat transfer during subcooled flow boiling on augmented surfaces
Author: Howell, Matthew
Abstract: Heat transfer characteristics of flat augmented surfaces in subcooled flow boiling with water at atmospheric pressure were obtained as functions of the surface orientation and Reynolds number. Data was collected with the heater surface temperature between 90C and 1 15C. The maximum temperature recorded by the thermocouple closest to the heater was between 200C and 220C. Five augmented surfaces and a base line plain polished aluminum surface were prepared and their performance was experimentally obtained in a 3 mm x 40 mm rectangular channel. The five augmented surfaces consisted of a sintered plain surface, microfin #1 (fin height- .132 mm, pitch - .635 mm, fin shape - round), sintered microfin #1, microfin #2 (fin height - .197 mm, pitch - .726 mm, fin shape - trapezoidal), and microfin #3 (fin height - .240 mm, pitch - .870 mm, fin shape - trapezoidal). Two microfin orientations of 0 and 20 were tested by rotating the heater surface. The heat transfer performance of the heater surface in the non-boiling (single-phase) and boiling regions were obtained over a Reynolds number range of 1565 to 7254 and compared with the plain surface performance. Microfin #3 and microfin #1 exhibited better heat transfer performance compared to the polished aluminum surface while microfin #2 transferred less heat compared to the polished aluminum surface. For microfin #1 nucleation started as soon as positive wall superheat was attained, while for microfin #2, microfin #3, and a polished aluminum surface a wall superheat of 6 to 10 C was required for inception of nucleation. A number of parameters affect the heat transfer from the augmented surfaces - subcooling, flow rate, swirl angle, surface finish, and surface geometry. The effect of these parameters was investigated for the six surfaces studied. Bubble activity was studied with the aid of video images obtained using a regular 30 frames per second video camera and a high speed video camera up to 1 000 frames per second under a magnification of up to 43 OX. As the degree of superheat increased beyond 10C, bubble activity became faster and could not be tracked by the high speed camera which only showed streaks of bubbles. It is thought that this increased bubble behavior continues with faster and smaller bubbles as the surface temperature continues to increase. Using experimental data taken by Mizo (1995), for flow boiling of water on a flat polished aluminum surface, existing pool boiling bubble growth models were investigate for possible extension into flow boiling. Extending existing pool boiling models to flow boiling proved very complex. Mikic and Rohsenow's [17] pool boiling bubble growth rate model is well accepted in literature. Limitations in applying this work to flow boiling were identified.
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Date: 1996-06

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