Chapter 13b

Question 1

Radiation heat exchange depends on

Answer 1

orientation of the surfaces relative to each other accounted for by view factor

Question 2

View factor is

Answer 2

purely geometric quantity, independent of surface properties and temp

Question 3

view factor also called

Answer 3

shape factor configuration factor and angle factor

Question 4

diffuse view factor based on

Answer 4

assumption that the surfaces are diffuse emitters and diffuse reflectors

Question 5

specular view factor

Answer 5

assumption that the surfaces are diffuse emitters and specular reflectors

Question 6

Fij

Answer 6

the fraction of the radiation leaving surface i that strikes surface j directly

Question 7

Radiosity is

Answer 7

total rate at which radiation leaves differential surfaces via emission and reflection in all direction

Question 8

radiosity =

Answer 8

J = pi()*I

Question 9

Heat transfer from surface Q. =

Answer 9

JA = pi()I*A

Question 10

Reciporcity relation for view factors

Answer 10

A1F12 = A2F21

Question 11

F11=

Answer 11

fraction of radiation leaving surface 1 that strikes itself directly
non zero for concave surfaces

Question 12

underlying assumption of view factor

Answer 12

radiation a surface receives from a source is directly proportional to the angle the surface subtends when viewed from the source
case when surfaces are isothermal and diffuse emitters and reflectors and separated by nonparticipating medium such as air

Question 13

issue with assumption of view factor

Answer 13

only the case if the radiation coming off the source is uniform in all directions throughout its surface and the medium between the surfaces doe not absorb emit or scatter radiation

Question 14

What is the reciprocity relation

Answer 14

F12 = F21 if A1 = A2
A1F12 = A2F21

Question 15

radiatio analysis on an enclosure consisting of N surfaces requires the evaluation of

Answer 15

N^2 view factors

Question 16

The summation rule

Answer 16

sum of the view factors from a surface i of an enclsure to all surfaces of the enclosure including to itself must equal unity
sum of Fij from j=1 to N =1

Question 17

total number of viewfactors that need to be evaluated directly for an N surface enclosure =

Answer 17

1/2 N(N-1)

Question 18

The superposition rule

Answer 18

view factor from a surface i to a surface j is equal to the sum of the view factors surface i to the parts of surface j
F1(2,3) = F12 + F13

Question 19

The superposition rule fraction of surfaces

Answer 19

F(2,3)1 = (A2F21 + A3F31)/A2 + A3

Question 20

The symmetry rule

Answer 20

two or more surfaces the possess symmetry about a third surface will have identical view factors from that surface
if surface j and k are symmetric about surface i
Fij=Fik and Fji = Fki

Question 21

View factors between infinitely long surfaces

Answer 21

the crossed strings method

Fij = (sum of crossed strings - sum of uncrossed strings)/(2 * string on surface i)

Question 22

When can crossed strings method be applied

Answer 22

changes and ducts vlong in one direction relative to other directions, therefore consider 2D
modeled as inf long

Question 23

net rate of radiation when approximated as blackbodies

Answer 23

Q.12 = Radiation leaving the entire surface 1 that strikes surface 2 - Radiation leaving the entire surface 2 that strikes surface 1
=A1Eb1F12 - AEb2F21
=A1F12sigma(T1^4 - T2^4)

Question 24

Net heat transfer from any surface i of an N surface enclosure =

Answer 24

Q.i = sum from j=1 to N AiFijsigma(Ti^4-Tj^4)

Question 25

issue with non black surfaces heat transfer

Answer 25

reflections make very complicated

Question 26

what are assumptions to get around reflection

Answer 26

opaque diffuse gray | isothermal surfaces and incoming and outgoing radiation is uniform

Question 27

What is radiosity J

Answer 27

the total radiation energy leaving a surface per unit time per unit area reflected radiation + emitted radiation

Question 28

for surface that is gray and opaque with is radiosity

Answer 28

epsilon = alhpa alhpa + rho = 1 J = radiation emitted by surface i + radiation reflected by surface i = EPSi*Ebi + RHOiGi = EPSi*Ebi + (1 - EPSi)Gi (W/m^2)

Question 29

Net radiation heat transfer to or from a surface

Answer 29

Q.i = (radiation leaving a surface i) - (radiation incident on entire surface i) = Ai (Ji-Gi)

Question 30

Net radiation heat transfer to or from a surface Q.i =

Answer 30

Ai*EPSi/(1-EPSi) * (Ebi - Ji) = (Ebi - Ji)/Ri

Question 31

surface resistance to radiation

Answer 31

Ri = 1 - EPSi / Ai*EPSi

Question 32

What is a reradiating surface

Answer 32

surface modeled as adiabatic since their back sides are well insulated and the net heat transfer through them is zeo Ji = Ebi = sigma * T^4

Question 33

Space resistance to radiation

Answer 33

Rij = 1/Ai*Fij

Question 34

Draw a network representation of net radiation heat transfer from surface i to the remaining N surfaces of an enclosure

Answer 34

see book

Question 35

the net radiation flow from a surface through its surface resistance =

Answer 35

the sum of the radiation flows from that surface to all other surfaces through corresponding space resistance

Question 36

Direct method If net heat transfer rate is given for radiation analysis of an enclosure

Answer 36

Q.i = Ai sum from j=1 to N of Fij (Ji-Jj)

Question 37

Direct If surfaces with specified temperature Ti is given for radiation analysis of an enclosure

Answer 37

sigma*T^4 = Ji + (1-EPSi)/EPSi * sum from j=1 to N of Fij (Ji-Jj)

Question 38

When is the network method no longer practical

Answer 38

for enclosures with more than three of four surfaces

Question 39

Draw the schematic of a two surface enclosure

Answer 39

see book

Question 40

Draw schematic of a three surface enclosure

Answer 40

see book