Heat and Mass Transfer 2(2+0)

Lesson-24 Problems on Radiation

Problem 7.7    A furnace inside temperature of 2250 K has a glass circular viewing of 6 cm diameter. If the transmissivity of glass is 0.08, make calculations for the heat loss from the glass window due to radiation.

Solution:    

Problem 7.4    A thin metal plate of 4 cm diameter is suspended in atmospheric air whose temperature is 290 K. the plate attains a temperature of 295 K when one of its face receives radiant energy from a heat source at the rate of 2 W. If heat transfer coefficient on both surfaces of the plate is stated to be 87.5 W/m²-deg, workout the reflectivity of the plates.

Solution:         

Problem 7.9    A black body of total area 0.045 m² is completely enclosed in a space bounded by 5 cm thick walls. The walls have a surface area 0.5 m² and thermal conductivity 1.07 W/m-deg. If the inner surface of the enveloping wall is to be maintained at 215°C and the outer wall surface is at 30°C, calculate the temperature of the black body. Neglect the difference between inner and outer surfaces areas of enveloping material.

Solution:      

Problem 7.12  A furnace radiation at 2000K. Treating it as a black body radiation, calculate the

(i)                 Monochromatic radiant flux density at 1 μm wavelength.

(ii)               Wavelength at which emission is maximum and the corresponding radiant flux density

(iii)             Total emissive power, and

(iv)             Wavelength λ such that emission from 0 to λ is equal to the emission from λ to ∞.

Solution:    

Problem 7.16  A polished metal pipe 5 cm outside diameter and 370 K temperature at the outer surface is exposed to ambient conditions at 295 K temperature. The emissivity of the surface is 0.2 and the convection coefficient of heat transfer is 11.35 W/m²-deg. Calculate the heat transfer by radiation and natural convection per metre length of the pipe. Take thermal radiation constant  

What would be the overall coefficient of heat transfer by the combined mode of convection and radiation?

Solution:        

Problem 7.18  A gray surface has an emissivity  at a temperature of 550 K source. If the surface is opaque, calculate its reflectivity for a black body radiation coming from a 550 K source.

(b)         A small 25 mm square hole is made in the thin-walled door of a furnace whose inside walls are at 920 K. if the emissivity of the walls is 0.72, calculate the rate at which radiant energy escapes from the furnace through the hole to the room.

Solution:        

Problems on Radiation

Problem 1: The temperature of the flame in a furnace is 1900 K, find (a) monochromatic energy emission at 1μ per m² (b) λ_max (c)  monochromatic energy emission at λ_max and at 1900 K. (d) Ratio of E_bλ / (E_bλ)_max (e) Total energy emitted/m².

Solution:          

 

Problem 2: Liquid oxygen (boiling temperature = - 182°C) is to be stored in a spherical container of 30 cm diameter. The system is insulated by an evacuated space between inner space and surrounding 45 cm ID concentric sphere. For both sphere ε = 0.03 and temperature of the outer sphere is 30°C. Estimate the rate of heat flow by radiation to the Oxygen in the container. Determine the rate at which liquid oxygen would evaporate at -182 °C. Latent Heat of oxygen = 215 KJ/Kg. Solution:  The heat flow between the two concentric sphere by radiation is given by

 

Therefore  rate of evaporation of oxygen  =  = 0.0455 Kg/hr.

Problem 3:Determine in W radiation heat loss from each meter of 20 cm diameter heating pipe when it is placed centrally in the brick duct of square section 30 cm side.

Temperature of pipe surface = 200°C

Brick duct temperature = 20°C

Emissivity of the pipe surface = 0.8

 Brick duct emissivity = 0.9

Assume only radiation heat transfer between pipe and brick duct.

If the system is in steady state condition then find the surface heat transfer coefficient of the brick duct assuming the temperature of the surroundings of the duct is 10°C.