Example Calculation, Large Bi Case Cooling of a Perspex Plate in Water

2 Example Calculation, Large Bi Case Cooling of a Perspex Plate in Water 

A vertical Perspex plate, 3-cm thick, 10-cm high and initially at 100°C, is cooled by immersion in a large water bath at 20°C. Estimate the time taken for the centre of the plate to reach 25°C. 

2 Estimate hand, hence, Bi. As AT changes with time from 80 to 5°C, an average AT of 25°C is taken to estimate Gr. 

From Table 2, for natural convection from a vertical plate with GrPr = 1.533 x 109, 

Bi is large, and so the thermal resistance of the water film may be neglected. 

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