Quarter Quiz 2

Time-dependent temperature change; capacitors

...in non-steady state heat transfer problems can be approximated by adding... to the thermal circuit model for the system.

ratio of the surface area to volume of the object

Application of the lumped capacitance method to approximate the time dependent temperature change of an object is most appropriate when the ratio of the surface area to volume of the object is large.

False

Assuming one could trip the boundary layer on a flat plate into turbulence at any location x along the plate that one wanted, to absolutely maximize heat or mass transfer one should trip the boundary layer into turbulence at the very front edge of the plate (x=0).

most of the temperature or concentration change being compressed into a small viscous sublayer near the surface

At the transition from a laminar to turbulent boundary layer in flow past a flat plate, the sudden increase in heat and mass transfer is the direct result of turbulent mixing in the viscous sublayer .

is attributed a reversal of flow direction near the surface

Boundary layer separation for a cylinder in cross-flow

an increasing pressure field acting on the fluid as it moves around the cylinder

Boundary layer separation for a cylinder in cross-flow is attributed to

1.67

Consider a flat plate of length L with TSB9000 super special oil flowing over the top. The parameters describing the plate and oil are below.

3.54

Consider a flat plate with TSB9000 super special oil flowing over the top. The parameters describing the flow and oil are below. V= 6.5 m/s ν= 23 x 10-6 m2/s Rex,c= 5 x 105 If the plate should have a boundary layer that is one half laminar, followed by one half turbulent flow, how long should the plate (in meters to 1 decimal place) be? Warning! Make sure to adjust your answer to reflect the entire length of the plate!

be reduced by one-half

Consider a object of fixed volume with Biot number characteristic length LC. If the surface area is suddenly doubled, how would the calculated Biot number change? It would...

2

Consider a system in which the heat transfer behavior is governed by the following relationships:

1.8

Consider a system in which the heat transfer behavior is governed by the following relationships: When the Reynolds number is 389, the ratio of the Schmidt (Sc) to Prandtl (Pr) numbers is found to be: 6 When the Reynolds number is increased to 1,905, the ratio of the average Sherwood to average Nusselt numbers, to the nearest tenth (1 decimal place) will be:

-kA

Consider a temperature profile in a material of thermal conductivity k, where A and B are constants: T(x)=ABx4+Ax−AB The heat flux at x = 0 is:

less than one to avoid overestimating the value of the heat transfer coefficient

Consider the Zukauskas relationship for a tube bank: A typical value for C2, the correction factor for banks of tubes with less than 20 rows, is

The absorptivity and emissivity of the surface are equal.

Consider the following expression for radiation heat transfer: The derivation and use of this equation is based on which of the following assumptions?

positive

Consider the temperature profile in the above drawing superimposed on the coordinate system as drawn with the (+) positive x-direction pointing left. When Fourier's Law is applied to the above system, the sign of heat flux is:

False

For a cylinder in cross-flow, keeping the flow laminar with a Reynolds numbers less than 2x10^5 is likely to avoid boundary layer separation.

conduction

Fourier's Law is used to describe heat transfer associated with _______

All of the above

In HW problem 1.37, if one wanted to consider that some heat might be conducted from the chamber through the legs, what additional information would be needed?

thermal energy being conducted within the glass plate.

In HW problem 1.59, the plate glass would shatter if it experienced a difference in temperature greater than 15 ˚C/mm. This information was important for calculating the heat transfer associated with the:

flow past each fin could be treated as flow over a flat plate

In Problem 7.22, the heat transfer coefficient was approximated by assuming flow past each fin could be treated as flow over a flat plate .

-Free electron migration -Phonons (vibrational waves) -Transfer of translational energy through molecular collisions -Transfer of rotational and vibrational energy through molecular collisions

Match each material group with the physical phenomenon contributing the most to its thermal conductivity.

0.71

The Nusselt relation for heat transfer involving a cylinder of diameter D in cross flow is: C= 0.14 m= 0.5 If the cylinder diameter is doubled, with all other conditions kept the same, by what factor (to 2 decimal places) will the heat transfer coefficient (h) change?

c

The average Nusselt number for a surface of characteristic length L is formed from which collection of variables:

d

The average Sherwood number for a sur

d

The average Sherwood number for a surface of characteristic length L is formed from which collection of variables:

enhance thermal conductivity.

The gap above is 0.2 mm. In HW problem 1.53, what was the purpose of adding paste to the gap?

false

The heat and mass transfer analogy says that if the Nusselt and Sherwood numbers for two geometricaly analogous systems are equivalent, the heat transfer and mass transfer coefficients for those systems will be identical.

laminar

The largest local heat or mass transfer coefficient that can be achieved on a flat plate would occur in the... boundary layer region.

is governed by convective heat transfer to an adjacent fluid

The temperature distribution within one-dimensional plane wall at an initial time (t = 0), at steady state (t ⟶∞), and at several intermediate times are as shown. The temperature profiles suggest the boundary condition at x = L is is governed by convective heat transfer to an adjacent fluid .

as the fluid passes into/through the tube bank, its velocity increases relative to its value outside the bank

The use of the maximum Reynolds number accounts for the fact that

Prandtl, Nusselt

To convert a Sherwood correlation into a relationship good for predicting heat transfer characteristics, one can simply replace the Schmidt number with the ______number and the Sherwood number with the _________number.

Prandtl; Nusselt

To convert a Sherwood correlation into a relationship good for predicting heat transfer characteristics, one can simply replace the Schmidt number with the... number and the Sherwood number with the...

False

True or False: Assuming one could trip the boundary layer on a flat plate into turbulence at any location x along the plate that one wanted, to absolutely maximize heat or mass transfer one should trip the boundary layer into turbulence at the very front edge of the plate (x=0).

False

True or False: The heat and mass transfer analogy says that if the Nusselt and Sherwood numbers for two geometrically analogous systems are equivalent, the heat transfer and mass transfer coefficients for those systems will be identical.

False

True or false: The lumped capacitance method is useful for approximating the spatial temperature distribution inside of an object as a function of time.

False

True or false: The lumped capacitance method works best when the resistance to convection heat transfer from an object is low, and the resistance to conduction inside the object is high.

rows of tubes at the same spacing

When considering flow over an aligned bank of tubes with an initial Reynolds number maximum of 105, doubling the tubes in a row at the same spacing will produce an increase in pressure drop associated with flow through the system.

tubes in a row at the same spacing

When considering flow over an aligned bank of tubes with an initial Reynolds number maximum of 105, doubling... will be least likely to increase the pressure drop associated with flow through the system.

The difference in thermal energy between the entering silicon powder and the exiting silicon sheet.

Which of the following phenomena were a source of heat loss from the cylindrical chamber that needed to be considered in HW problem 1.37?

false

for a cylinder in cross-flow, keeping the flow laminar with a Reynolds number less than 2x10^5 is likely to avoid boundary layer separation

false

the heat and mass transfer analogy says that if the Nusselt and Sherwood numbers for two geometrically analogous systems are equivalent, the heat and mass transfer coefficients for those systems will be identical

true

the reynolds number used to predict the type of flow in the boundary layer region over a flat plate depends on the distance from the plate's front edge

false

when calculating heat transfer associated with a bank of tubes, the use of the log mean temperature accounts for the changing temperature of the tube surfaces