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Heat Transfer > Convection > Flashcards

Convection Flashcards

1
Q

What Is convection?

A

The removal of heat from a surface by a fluid.

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2
Q

What is forced convection?

A

The removal of heat from a surface by a fluid whose motion is generated by an external force.

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3
Q

What is natural convection?

A

The removal of heat from a surface by a fluid whose motion is not generated by any external source but only by density differences in the fluid occurring due to temperature gradients.

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4
Q

What is Newton’s Law of Cooling?

A

Is the basic equation used to calculate the steady state convection heat transfer in both heating and cooling.

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5
Q

What is Newton’s Law of Cooling dependent on?

A

Is dependent on the convective heat transfer coefficient, surface area, the temperature of the environment and of the cooling body’s surface.

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6
Q

What is the Nusselt number?

A

Is the ratio of convective to conductive heat transfer across or normal to the boundary.

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7
Q

What is the Prandtl number?

A

It represents the ratio of momentum diffusion to the thermal diffusion. The values used are for the fluid, not for the surface material

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8
Q

What is the Rayleigh number?

A

It is used as a indicator of the primary heat transfer mechanism.

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9
Q

What is the critical value?

A

The critical value is a number that is specific to the fluid and the subscript written with the symbol designates the characteristic dimension with which the critical values have been correlated to.

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10
Q

What is the correlation between the Rayleigh number and critical value?

A

If the Rayleigh number is less than that critical value, heat transfer is primarily by conduction. When the Rayleigh number is greater the, heat transfer is primarily by convection.

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11
Q

What is the Reynolds number?

A

The Reynolds number is used to determine which of the three flow regimes is applicable.

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12
Q

What are the three flow regimes?

A

The three flow regimes are laminar flow, transient flow, and turbulent flow. Which cover a range of values for the Reynolds number depending on the surface and flow of the fluid.

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13
Q

External flow vs internal flow for Reynolds number?

A

For internal flow the characteristic dimension becomes the inner diameter instead of the outer diameter and the average fluid velocity is used instead of the infinite velocity.

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14
Q

How does turbulent flow effect the average velocity in internal flow?

A

In fully turbulent flow the average velocity is equal to the bulk velocity.

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15
Q

How does laminar flow effect the average velocity in internal flow?

A

In fully developed laminar flow the average velocity is equal to 50 % of the maximum velocity.

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16
Q

When is the Nusselt equation used?

A

It is used to find the film coefficient of a flat plate or a large diameter cylinder in a stationary fluid.

17
Q

What is the Rayleigh number used for in the Nusselt equation?

A

The Rayleigh number is used to find the empirical constants.

18
Q

What does condensation do to surfaces thermally?

A

When a vapor condenses on a cooler surface, it creates a film that insulates the surface and creates a thermal resistance or falls/flows from the surface and removes thermal energy.

19
Q

What is film wise condensation?

A

A continuous film of condensate that covers the entire surface and flows smoothly down under the action of gravity and falls off.

20
Q

What is drop wise condensation?

A

A discontinuous film (caused by impurities or irregularities) of condensate that covers the surface and flows smoothly down under the action of gravity and falls off.

21
Q

How is the Nusselt equation related to condensation?

A

It can be used to find the film coefficient for the film wise condensation of a pure saturated vapor.

22
Q

How is the nusselt equation effected for flow through noncircular ducts?

A

The diameter is replaced with a hydraulic diameter.

23
Q

Why are spheres used for fluid heat transfer?

A

Spheres have the smallest surface area to volume ratio, so spherical tanks are used where heat transfer is to be minimized.

24
Q

When is the logarithmic mean temperature used?

A

When calculating the heat transfer for a tube whose temperature difference changes along its length, in a heat exchanger.

25
Q

When is the NTU method used?

A

To find the heat exchanger efficiency or effectiveness.

26
Q

What is the heat exchanger effectiveness?

A

It is defined as the ratio of the actual heat transfer to the maximum possible heat transfer.

27
Q

What is first step the NTU method?

A

The first step is to calculate the thermal capacity rates for the two fluids.

28
Q

What is second step the NTU method?

A

The second step is to find the effectiveness using the minimum thermal capacity rate. This can either be the hot or cold fluid.

29
Q

What is the last step in the NTU method?

A

The last step is to use the correct NTU equation for the heat exchangers operating conditions.

Heat Transfer (3 decks)

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