Due to its comparatively large thermal conductivity, water is a preferred fluid for convection cooling. However, in applications involving electronic devices, watermust notcome into contact with the devices, whichwould therefore have to be hermetically sealed. To circumvent related design and operational complexities andto ensure that the devices are not rendered inoperable bycontact with the coolant, a dielectric fluid is commonlyused in lieu of water. Many gases have excellent dielectric characteristics, and despite its poor heal transferproperties, air is the common choice for electronic cooling. However, there is an alternative, which involves aclass of perfluorinated liquids that are excellentdielectrics and have heat transfer properties superior tothose of gases.Consider the microchannel chip cooling application of Problem 8.109 but now for a perfluorinatedliquid with properties of c p = 1050 J/kg ⋅ K , k = 0.065 W/m ⋅ K , μ = 0.0012 N ⋅ s/m 2 , and Pr = 15 . (a) For channel dimensions of H = 200 μ m, W = 50 μ m, and S = 20 μ m. a chip thermal conductivity of k c h = 140 W/m ⋅ K and width L = 10mm. a channel base temperature ( x = 0) of T s = 350 K , achannel inlet temperature of T m , i = 290 K , and aflow rate of m 1 = 10 − 4 kg/s per channel, determinethe outlet temperature and the chip power dissipation for the dielectric liquid. (b)Consider the foregoing conditions, but with air at a flow rate of m 1 = 10 − 6 kg/s used as the coolant. Using properties of c p = 1007 J/kg ⋅ K,k = 0.0263 W/m ⋅ K , and μ = 185 × 10 − 7 N ⋅ s/m 2 , determine the air outlet temperature and the chip power dissipation.
Due to its comparatively large thermal conductivity, water is a preferred fluid for convection cooling. However, in applications involving electronic devices, watermust notcome into contact with the devices, whichwould therefore have to be hermetically sealed. To circumvent related design and operational complexities andto ensure that the devices are not rendered inoperable bycontact with the coolant, a dielectric fluid is commonlyused in lieu of water. Many gases have excellent dielectric characteristics, and despite its poor heal transferproperties, air is the common choice for electronic cooling. However, there is an alternative, which involves aclass of perfluorinated liquids that are excellentdielectrics and have heat transfer properties superior tothose of gases.Consider the microchannel chip cooling application of Problem 8.109 but now for a perfluorinatedliquid with properties of c p = 1050 J/kg ⋅ K , k = 0.065 W/m ⋅ K , μ = 0.0012 N ⋅ s/m 2 , and Pr = 15 . (a) For channel dimensions of H = 200 μ m, W = 50 μ m, and S = 20 μ m. a chip thermal conductivity of k c h = 140 W/m ⋅ K and width L = 10mm. a channel base temperature ( x = 0) of T s = 350 K , achannel inlet temperature of T m , i = 290 K , and aflow rate of m 1 = 10 − 4 kg/s per channel, determinethe outlet temperature and the chip power dissipation for the dielectric liquid. (b)Consider the foregoing conditions, but with air at a flow rate of m 1 = 10 − 6 kg/s used as the coolant. Using properties of c p = 1007 J/kg ⋅ K,k = 0.0263 W/m ⋅ K , and μ = 185 × 10 − 7 N ⋅ s/m 2 , determine the air outlet temperature and the chip power dissipation.
Solution Summary: The author explains the water outlet temperature and chip power dissipation for dielectric liquid.
Due to its comparatively large thermal conductivity, water is a preferred fluid for convection cooling. However, in applications involving electronic devices, watermust notcome into contact with the devices, whichwould therefore have to be hermetically sealed. To circumvent related design and operational complexities andto ensure that the devices are not rendered inoperable bycontact with the coolant, a dielectric fluid is commonlyused in lieu of water. Many gases have excellent dielectric characteristics, and despite its poor heal transferproperties, air is the common choice for electronic cooling. However, there is an alternative, which involves aclass of perfluorinated liquids that are excellentdielectrics and have heat transfer properties superior tothose of gases.Consider the microchannel chip cooling application of Problem 8.109 but now for a perfluorinatedliquid with properties of
c
p
=
1050
J/kg
⋅
K
,
k
=
0.065
W/m
⋅
K
,
μ
=
0.0012
N
⋅
s/m
2
,
and
Pr
=
15
.
(a) For channel dimensions of
H
=
200
μ
m,
W
=
50
μ
m, and
S
=
20
μ
m. a chip thermal conductivity of
k
c
h
=
140
W/m
⋅
K
and width L = 10mm. a channel base temperature (x = 0) of
T
s
=
350
K
, achannel inlet temperature of
T
m
,
i
=
290
K
, and aflow rate of
m
1
=
10
−
4
kg/s per channel, determinethe outlet temperature and the chip power dissipation for the dielectric liquid. (b)Consider the foregoing conditions, but with air at a flow rate of
m
1
=
10
−
6
kg/s used as the coolant. Using properties of
c
p
=
1007
J/kg
⋅
K,k
=
0.0263
W/m
⋅
K
,
and
μ
=
185
×
10
−
7
N
⋅
s/m
2
, determine the air outlet temperature and the chip power dissipation.
2-D: A fin may be manufactured as an integral part of a surface by using a casting or extrusion
process, or it may be separately brazed or adhered to the surface. From thermal considerations,
which option is preferred?
2-E: What is the difference between steady-state and transient heat transfer processes? Give an
example for each of them.
2-F: What is the physical interpretation of the Biot number?
2-G: For flow over a flat plate, sketch variation of local convective heat transfer coefficient, h(x),
versus the distance along the plate x for laminar, transition, and turbulent flow regimes.
Design a hydrocooling unit that can cool fruits andvegetables from 30 to 5°C at a rate of 20,000 kg/h under thefollowing conditions:The unit will be of flood type, which will cool the productsas they are conveyed into the channel filled with water. Theproducts will be dropped into the channel filled with water atone end and be picked up at the other end. The channel canbe as wide as 3 m and as high as 90 cm. The water is to becirculated and cooled by the evaporator section of a refrigerationsystem. The refrigerant temperature inside the coils is tobe 22°C, and the water temperature is not to drop below 1°Cand not to exceed 6°C.Assuming reasonable values for the average product density,specific heat, and porosity (the fraction of air volume ina box), recommend reasonable values for (a) the water velocitythrough the channel and (b) the refrigeration capacity ofthe refrigeration system.
Q3/Condensing steam at 1 atm is used to maintain a vertical plate 20 cm high and 3.0 m wide at a
constant temperature of 100°C. The plate is exposed to room air at 20°C. What flow rate of air will
result from this heating process? What is the total heating supplied to the room air?
Ans. 0.0034 kg/s, 278W
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Principles of Heat Transfer (Activate Learning wi...Mechanical EngineeringISBN:9781305387102Author:Kreith, Frank; Manglik, Raj M.Publisher:Cengage Learning