Methods
Experimental Setup
Particle Image Velocimetry was performed on the 1-D scale-up Solar Oven at three discrete water temperatures (30o C, 50o C, and 70o C) in order to characterize how the role of convection currents changes over the heating of the oven. One way to predict and describe velocity distribution in convection cells is the dimensionless Grashof number:
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The oven was heated with 15 halogen lamps located 24" from the top of the oven in an array of:
Figure 3 - Lighting arrangement of halogen lamps above PIV setup.
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Figure 6 - Band Pass Filter schematic and generic Matlab code used to process the data.
Results
PIV was run at three temperatures and vector maps were created displaying mean velocity at each subwindow. Additionally, video was captured from both the scientific camera and a handheld camera showing convection behavior dynamically. The handheld video is accompanied on the CD of this report. We assumed in doing PIV that convection movement was relatively two-dimensional, so convection cell strength can be characterized by looking at the maximum U and V mean velocities (Table 1). Averaging the two values, we found that convection cell strength increases by 12% from 30oC to 50oC and by 27% from 50oC to 70oC.
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Figure 7 - Vector map of mean displacements at 30°C.
Figure 8 - Vector map of mean displacements at 50°C.
Figure 9 - Vector map of mean displacements at 70°C.
Critical Theory Issues and Challenges
The investigation of the heating mechanisms behind food cooking is an interesting project for the solar oven team to delve into. It adds knowledge that will potentially affect the way in which future ovens are constructed and/or used. The effectiveness of these changes is largely unknown without testing, so care must be taken to give advice without first knowing all of the performance implications. We believe that showing the handheld video of the PIV experiment being run would help users of the oven visualize and understand how heating takes place in the oven.
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