Problem 1: (a) Give three examples of each of natural (free) convection and forced convection which occur in industrial processes.
(b) The Grashof number and the Reynolds number appear in most correlations of experimental data for convective heat transfer. Explain, in a maximum of 150 words, the mechanisms of natural and forced convections with particular reference to the above non-dimensional groups.
(c) An appropriate correlation for heat transfer by natural convection from a horizontal pipe to the atmosphere is
Nu = 0.53Gr0.25Pr0.25
Where: Gr = αρ2d3(Ts - Tf) g/µ2
And: Pr = cpµ/k
Show that the above correlation can be simplified to:
When air has the values listed below
α = 3.007 * 10-3K-1, ρ = 1.086 kg m-3, cp = 1.0063 KJ kg-1 K-1
k = 2.816 * 10-5 kW m-1 K-1, µ = 1.962 * 10-5 kg m-1 s-1
Problem 2: Liquid ammonia is heated as it flows at a mean velocity of 2 m s-1 through a circular pipe. The pipe, which has an internal diameter of 75mm, is at a uniform temperature of 27oC, and the ammonia at a section 1.2 m from the inlet to the pipe has a temperature of -23oC. Use the following information to estimate the local heat transfer flux at l = 1.2m. Note, the properties of ammonia liquid have been taken at -23oC, except where stated.
Liquid ammonia properties:
Density = 60 kg m-3
Specific heat capacity = 4.86 kJ kg-1 K-1
Dynamic viscosity (at 27oC) = 1.19 * 10-4 kg m-1 s-1
Dynamic viscosity = 2.05 * 10-4kg m-1 s-1
Heat transfer correlations:
