Test: Modes of Heat Transfer - 1 - Mechanical Engineering MCQ

Steady two-dimensional heat conduction takes place in the body shown in the figure below. The normal temperature gradients over surfaces P and Q can be considered to be uniform. The temperature gradient  at surface Q is equal to 10 k/m. Surfaces P and Q are maintained at constant temperatures as shown in the figure, while the remaining part
of the boundary is insulated. The body has a constant thermal conductivity of 0.1 W/m.K. The values of and at surface P are:

A plane wall is 25 cm thick with an area of 1 m², and has a thermal conductivity of 0.5 W/mK. If a temperature difference of 60°C is imposed across it, what is the heat flow?  

Which one of the following expresses the thermal diffusivity of a substance in terms of thermal conductivity (k), mass density (ρ) andspecific heat (c)?   

Match List-I with List-II and select the correct answer using the codes given below the lists:       

 

Temperature profiles for four cases are shown in the following figures and are labelled A, B, C and D.

Match the above figures with  
1. High conductivity fluid
2. Low conductivity fluid
3. Insulating body
4. Guard heater

Select the correct answer using the codes given below:

Assertion (A): Thermal diffusivity is a dimension less quantity.

Reason (R): In M-L-T-Q system the dimensions of thermal diffusivityare [L²T^-1]          

Assertion (A): Thermal diffusivity is a dimensionless quantity.

Reason (R): In M-L-T-Q system the dimensions of thermal diffusivityare [L²T^-1]        

Assertion (A): Hydrogen cooling is used for high capacity electrical generators.
Reason (R): Hydrogen is light and has high thermal conductivity ascompared to air 

Assertion (A): Cork is a good insulator.

Reason (R): Good insulators are highly porous. 

An oil cooler in a high performance engine has an outside surface area 0.12 m² and a surface temperature of 65 degree Celsius. At any intermediate time air moves over the surface of the cooler at a temperature of 30 degree Celsius and gives rise to a surface coefficient equal to 45.4 W/ m ² K. Find out the heat transfer rate?

A wall of thickness 0.6 m has width has a normal area 1.5 m² and is made up of material of thermal conductivity 0.4 W/mK. The temperatures on the two sides are 800°C. What is the thermal resistance of the wall?  

A composite wall of a furnace has 2 layers of equal thickness having thermal conductivities in the ratio of 3 : 2. What is the ratio of the temperature drop across the two layers? 

One dimensional unsteady state heat transfer equation for a sphere with heat generation at the rate of 'q' can be written as

For the three-dimensional object shown in thefigure below, five faces are insulated. Thesixth face (PQRS), which is not insulated,interacts thermally with the ambient, with aconvective heat transfer coefficient of 10 W/m².K. The ambient temperature is 30°C. Heatis uniformly generated inside the object at therate of 100 W/m³. Assuming the face PQRS tobe at uniform temperature, its steady statetemperature is:

In a composite slab, the temperatureat the interface (Tinter) between twomaterials is equal to the average ofthe temperatures at the two ends.Assuming steady one-dimensionalheat conduction, which of thefollowing statements is true aboutthe respective thermalconductivities?

Heat flows through acomposite slab, as shownbelow. The depth of the slabis 1 m. The k values are inW/mK. the overall thermalresistance in K/W is:

A stainless steel tube (k_s = 19 W/mK) of 2 cm ID and 5 cm OD is insulated with 3 cm thick asbestos (k_a = 0.2 W/mK). If the temperature difference between the innermost and outermost surfaces is 600°C, the heat transfer rate per unit length is: