() The following equation arises in heat transfer from an object to a surroundin

Question

() The following equation arises in heat transfer from an object to a surrounding fluid: where T and Trare the temperatures of the object and fluid (both in oC), t is time in seconds, and and are constants. Constant is defined as: hA MC where A is the object surface area in m2, M is the object mass in kg and C is the object heat capacity in kJ/kg C. The quantity h is a convective heat transfer coefficient. (a) What are the units of ? (b) What are the units of a? (c) What are the units of h? (8) You find it necessary to calculate the Reynolds number for one of the process streams in plant you are supervising. The Reynolds number Re is dimensionless and is defined by: Re-AD where D is the inner pipe diameter, v is the fluid velocity, is the fluid density and is the fluid viscosity. A flow meter indicates the velocity is 5.2 m/s and the inner pipe diameter is 1.38 inches. Handbooks indicate that the fluid density is 7.1 Ibm/gal and that the viscosity is 0.055 poise. Use this information to calculate the Reynolds number

Explanation / Answer

7. From the equation

T - Tf = e-t

Units of = Units of temperature = Celsius

Since the term t is dimensionless, units of = 1/Units of time = second-1

and units of h will be watt/m2-C

8) 7.1 lbm/gal = 7.1 x 119.83 kg/m3 = 850.793 kg/m3

viscosity = 0.055 poise = 0.0055 Pa.s

diameter d= 1.38 inch = 0.03505 metres

Re= (850.793) x (5.2) x (0.03505) / (0.0055) = 28195

Since this is a pipe and Re>3000, flow is turbulent

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