Heat Transfer (Set 3) Flashcards
As one of the transport phenomena, the concepts in heat transfer are crucial for design, operation, and maintenance of heat transfer equipment such as heat exchangers and insulation.
The Grashoff number is defined as the ratio of the
buoyancy to viscous forces.
According to Reynolds analogy, Stanton number is equal to (where, f = Fanning friction factor)
f/2
The absorptivity of a body is equal to its emissivity
under thermal equilibrium.
Absorptivity and refletivity of a perfect black body are respectively
1 and 0
In thermal radiation for a black body (where, ε is emissivity and α is absorptivity)
α = 1; ε = 1
Pick out the wrong statement.
S.I. unit of fouling factor is Watt/m2.°K.
When warm and cold liquids are mixed, the heat transfer is mainly by
convection
A cube, sphere & a thin circular plate (all having same mass and made of same material) are all heated to 300°C and allowed to cool in natural air. Which one will cool the slowest?
Sphere
Vent pipes are provided in a condenser to
remove non-condensable gases.
Heat transfer by natural convection is enhanced in system with
high co-efficient of thermal expansion.
In case of ____ boiling, the bubbles formed on a submerged hot surface get absorbed in the mass of the liquid.
pool
A body cools down from 75°C to 70°C in 10 minutes. It will cool down from 70° C to 65° C in ____ minutes.
> 10
In forced convection, the Nusselt number is a function of
Re and Pr
At what value of Prandtl number, the hydrodynamic and thermal boundary layers are identical?
1
A dilute aqueous solution is to be concentrated in an evaporator system. High pressure steam is available. Multiple effect evaporator system is employed, because
total amount of vapor produced per Kg of feed steam in a multiple effect system is much higher than in a single effect.
Heat transfer efficiency leading of energy conservation in a heat exchanger can be achieved by
keeping the heat transfer surface clean.
Trap is used to remove ____ from steam pipe lines.
condensate
A composite flat wall of a furnace is made of two materials ‘A’ and ‘B’. The thermal conductivity of ‘A’ is twice of that of material ‘B’, while the thickness of layer of ‘A’ is half that of B. If the temperature at the two sides of the wall are 400 and 1200°K, then the temperature drop (in °K) across the layer of material ‘A’ is
160
Tube pitch is the ____ of tube diameters and the clearances.
sum
With increase in the distance between the heat source and the object receiving the heat, the radiation heat transfer
decreases
The equivalent diameter for the annulus of a double pipe heat exchanger, whose inner pipe has fins on the outside is ____ compared to the same size pipes without fins.
less
Natural convection is characterised by
Grashoff number
Nusselt number for full developed, laminar, constant property flow in a pipe at uniform heat flux is
4.364
Thickness of thermal boundary layer is more compared to that of hydrodynamic boundary layer, when the value of Prandtl number is
< 1