Convection Flashcards
In laminar flow momentum and heat transfer is mainly at the _____ level.
Molecular
At constant pressure the value of coefficient of cubical expansion for gases could be evaluated as _____.
1/T(K)
In flow over a flat plate the temperature and velocity gradients above the boundary layer _____.
is zero
In convection, temperature and velocity gradients vary only in the _____.
Boundary layer
In fully developed flow in a pipe under constant heat flux condition the temperature difference between the wall and the fluid will _____.
Be constant
In flow through pipes the type of flow is decided by the conditions at _____.
Entry
In turbulent flow the velocity at a point varies randomly about a _____ velocity.
mean
In viscous oil flow thermal boundary layer will be _____ than hydrodynamic boundary layer.
thinner
In turbulent flow momentum and heat transfer is due to _____ between layers.
macroscopic mixing
The value of transition Reynolds number in the case of flow over flat plate is _____.
5 × 105
In pipe flow, convection coefficient at entrance region will be _____ compared to the fully developed region.
higher
Reyleigh number is the product of _____ numbers.
Grashof and Prandtl
In turbulent flow in rough pipes the Stanton number is related to _____.
friction factor
The thickness of the hydrodynamic boundary layer is defined as the distance from the surface at which velocity _____ is nearly zero.
gradient
Flow transition is generally judged by _____ number.
Reynolds
As compared to forced convection the additional force encountered in free convection analysis is _____.
buoyancy
The thickness of hydrodynamic boundary layer in flow over plate in turbulent flow is proportional to _____.
Rex^(–0.2)
If thermal diffusivity equals momentum diffusivity, then the ratio of thermal and velocity boundary layer thickness will be _____ .
equal to
In pipe flow the analogy method of convection analysis relates _____ number to friction factor.
Stanton
In turbulent flow over flat plate the convection coefficient will be proportional to the distance raised to the power _____.
-0.2
Along the thickness in the boundary layer velocity and temperature gradients _____.
decrease
In pipe flow under constant wall heat flux conditions the convection coefficient will be _____ as compared to constant wall temperature condition.
higher
n some cases where both free convection and forced convection may contribute to heat transfer, the criterion for the determination of predominance of either is the value of _____.
Gr/Re2
In laminar flow the velocity at a location with respect to time _____.
is constant
Graetz number is defined as the product of _____.
Reynolds, Prandtl and D/L
The velocity in the case of free convection boundary layer is zero at _____.
wall and boundary layer thickness
The ratio of momentum diffusivity to thermal diffusivity is called _____ number.
The ratio of momentum diffusivity to thermal diffusivity is called _____ number.
Prandtl
In free convection the average value of h will be _____ for short vertical plate as compared to a longer vertical plate.
higher
In the case of flow over flat plate the Reynolds number along the length will _____ continuously.
increase
In flow through non-circular sections _____ replaces diameter.
Hydraulic diameter
Peclet number is the product of _____ numbers.
Reynolds and Prandtl
In fully developed laminar flow through pipes the convection coefficient _____.
is constant
In laminar flow, the average convection coefficient along the length will _____.
decrease
The dimensionless number which replaces the Reynolds number in the case of free convection is _____.
Grashof number
In flow over spheres, and cylinders the characteristic length used in the calculation of dimensionless number related to hydrodinamic aspects is _____.
diameter
Hydraulic mean diameter is defined as _____.
4A/P
In liquid METAL FLOW over a flat plate, thermal boundary layer will be _____ than hydrodynamic boundary layer.
thicker
F/T: Reynolds number is the ratio between viscous force and buoyant force.
False
F/T: With convection on the surface any amount of additional insulation cannot reduce the heat flow through a hollow spherical insulation of the same material to half the original flow rate.
True
F/T: Momentum and displacement thickness will be more compared to boundary layer thickness.
False
F/T: Grashof number is the ratio between buoyant force and viscous force.
True
F/T: In turbulent flow in pipes Nusselt is proportional to Re0.8.
True
F/T: If Prandtl number is greater than one, the thermal boundary layer will be thicker compared to hydrodynamic boundary layer.
False
F/T: In free convection, Rayleigh number is similar to Peclet number in forced convection.
True
F/T: Inertia force rather than buoyant force plays a more important role in free convection.
False
F/T: Lower values of kinematic viscosity will lead to higher value of h both in free and forced convection.
True
F/T: In cases where both modes of convection may contribute the ratio Gr/Re2 is a measure of the importance of either mode.
True
F/T: In the case of small hollow cylinders or spheres, with outside convection the thermal resistance may decrease by the addition of insulation.
True
F/T: In turbulent flow the velocity at point varies about an average value.
True
F/T: In fully developed flow through a pipe, under laminar flow conditions, average Nusselt number is constant.
True
F/T: In pipe flow for similar velocity conditions water will have a higher convection coefficient compared to liquid metal.
False
F/T: Other conditions remaining the same as viscosity increases the boundary layer thickness will decrease.
False
F/T: The local value of convection coefficient in laminar flow over a flat place will decrease along the length.
True
F/T: In flow-through tube banks of tubes closer pitch will lead to higher values of h.
True
F/T: In laminar flow Nusselt is a function of Re0.8.
False
F/T: In flow over a flat plate over length L, the average convection coefficient will be equal to (4/3) hL.
False
F/T: The value of convection coefficient for the same flow velocity will be lower in the case of water as compared to air.
False
F/T: The hydraulic mean diameter for an annulus is Do2 – Di2.
False
