This paper reports the results of an experimental study in which 158-mm-dia disk and doughnut type 1-2 oil coolers were tested for various combinations of flow rates and clearances between the doughnut baffle and the shell. Oil coolers of 45 L per minute capacity were tested at the diametral clearances of 0.6 mm, 0.87 mm, and 1.25 mm between the doughnut baffle and shell for various flow rates of oil and water. It is found that the decrease in overall heat transfer coefficient with increasing clearance is not significant for most practical purposes. In another series of tests, oil coolers of 90 L per minute capacity with shell surface roughnesses of 4 and 19 microns and diametral clearances of 0.07 mm, 0.33 mm, and 0.59 mm were tested. The experimental results indicate that in this range of low clearances, the overall heat transfer coefficient increases with increasing clearance and surface roughness.

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