Heat Exchanger Design
To increase the rate of heat transfer, what can be done?
Q = U A Tln
U = (hi, kwall and ho)
1
o
o
ln
o
w
ti
i
o
ii
o
o h
1R
d
d
kR
d
d
h
1
d
d
U
1
h
f
d
kNuh
f
h
k
dhNu
Nanofluids (Method – 3)
How?, Then answer may be micro-channels (Method – 4)
Fins (Method – 1)
Flow rate (velocity) (Method – 2)
Microfluidics – channel sizes
2
Microchannel Technology
3
44
Flow channel classification
• Channel classification based on hydraulic diameter is intended to
serve as a simple guide for conveying the dimensional range
under consideration.
• Channel size reduction has different effects on different
processes.
• Deriving specific criteria based on the process parameters may
seem to be an attractive option,
but considering the number of processes and parameters that
govern transitions from regular to microscale phenomena (if
present),
a simple dimensional classification is generally adopted in
literature.
55
Fig. 1.1. Ranges of channel diameters employed in various
applications, Kandlikar and Steinke (2003).
66
Table 1.1 Channel dimensions for different types of flow for
gases at one atmospheric pressure.
• The classification proposed by Mehendale et al. (2000)
divided the range:
from 1 to 100 μm as microchannels,
100 μm to 1mm as meso-channels,
1 to 6 mm as compact passages, and
greater than 6 mm as conventional passages.
77
Table 1.2: Channel dimensions for different types of flow for
gases at one atmospheric pressure.
Channel dimensions (μm)
Gas Continuum
flow
Slip flow Transition
flow
Free molecular
flow
Air > 67 0.67–67 0.0067–0.67 < 0.0067
Helium > 194 1.94–194 0.0194–1.94 < 0.0194
Hydrogen > 123 1.23–123 0.0123–1.23 < 0.0123
88
Table 1.3: Channel classification scheme.
Channel Description Value
Conventional channels > 3mm
Minichannels 3 mm ≥ D > 200 μm
Microchannels 200 μm ≥ D >10 μm
Transitional Microchannels 10 μm ≥ D > 1 μm
Transitional Nanochannels 1 μm ≥ D > 0.1 μm
Nanochannels 0.1 μm ≥ D
D: smallest channel dimension
99
Non-circular channels
• In the case of non-circular channels,
it is recommended that the minimum channel dimension;
for example, the short side of a rectangular cross-section
should be used in place of the diameter „D‟.
• We will use the above classification scheme for defining
minichannels and microchannels.
• This classification scheme is essentially employed for ease in
terminology;
the applicability of continuum theory or slip flow conditions
for gas flow needs to be checked for the actual operating
conditions in any channel.
10
Table: Fanning friction factor and Nusselt number for fully developed
laminar flow in ducts, derived from Kakac et al. (1987).
10
1111
Basic heat transfer and pressure drop
considerations• The effect of hydraulic diameter on heat transfer and pressure drop
is illustrated in Figs. 1.2 and 1.3 for water and air flowing in a square channel under constant heat flux and fully developed laminar flow conditions.
• The heat transfer coefficient „h‟ is unaffected by the flow Reynolds number (Re) in the fully developed laminar region, since the “Nu” is constant in laminar flow regime.
• It is given by: Eq. 1
• where k is the thermal conductivity of the fluid and Dh is the hydraulic diameter of the channel.
• The Nusselt number (Nu) for fully developed laminar flow in a square channel under constant heat flux conditions is 3.61.
hD
kNuh
1212
• Figure 1.2 shows the variation of h for flow of water and air
with channel hydraulic diameter under these conditions.
• The dramatic enhancement in h with a reduction in channel size
is clearly demonstrated.
Fig. 1.2. Variation of the heat transfer coefficient with channel
size for fully developed laminar flow of air and water.
1313
• On the other hand, the friction factor f varies inversely with Re,
since the product „f · Re‟ remains constant during fully
developed laminar flow.
• The frictional pressure drop per unit length for the flow of an
incompressible fluid is given by:
Eq (2)
where „ pf /L’ is the frictional pressure gradient, „f’ is the
Fanning friction factor, G is the mass flux, and ρ is the fluid
density.
• For fully developed laminar flow, we can write:
f · Re = C Eq (3)
where Re is the Reynolds number, Re=GDh/μ, and C is a
constant, C =14.23 for a square channel.
D
Gf2
L
p 2
f
1414
• Figure 1.3 shows the variation of pressure gradient with the channel size for a square channel with G =200 kg/(m2 s), and for air and water assuming incompressible flow conditions.
• These plots are for illustrative purposes only, as the above assumptions may not be valid for the flow of air, especially in smaller diameter channels.
• It is seen from Fig. 1.3 that the pressure gradient increases dramatically with a reduction in the channel size.
• The balance between the heat transfer rate and pressure drop becomes an important issue
in designing the coolant flow passages for the high-flux heat removal encountered in microprocessor chip cooling.
1515
Fig. 1.3. Variation of pressure gradient with channel size for
fully developed laminar flow of air and water.
Delphi Micro Channel Evaporator
16
Aquaforce Aircooled chiller microchannel coil
Microchannel Reactor Concept
17
Close integration of the exothermic synthesis and steam generation
18
Micro-scale heat exchangers - Introduction
• Micro-scale heat exchangers
or micro structured heat exchangers are heat exchangers
in which a fluid flows in a lateral direction in a confined area
such as a tube or small cavity that dimensions are below the size of 1mm.
• Typically the fluid flows through a cavity which is called a mirochannel.
• This technology exploits enhanced heat transfer resulting
from structurally constraining streams to flow in microchannels,
which reduces thermal resistance to transferring heat.
19
• Fluid flowing through the channels on a plate evaporates or
condenses, and heat is transferred.
• Micro heat exchangers have been demonstrated with
high convective heat transfer coefficients ranging form
10,000 to 35,000 W/(m2-°C), or
about one order of magnitude higher than typically seen
in conventional heat exchangers
with very low pressure drops, typically 1 or 2 psi.
• The basic operating principle of these devices goes back to the
convective heat transfer within the flows of the microchannels.
20
• The convective heat transfer equation is
(1)
• In this equation
„h‟ is the heat transfer coefficient of the microscale heat exchanger,
„Nu‟ is the Nusselt number which is about 3.66 (for circular channels),
„kf‟ is the thermal conductivity of the working fluid, and
„de‟ is the equivalent diameter of the microchannel which the fluid flows
through.
• From this equation one can tell see how
the size of the channel directly affects the heat transfer coefficient of the
heat exchanger,
as the diameter is decreased, the heat transfer coefficient increases.
e
f
f
e
f
e
D
kNuh,or
k
Dh
k
DhNu
21
Different Types of Microscale Heat Exchangers
• The different types of microscale heat exchangers are the same as the different classifications of conventional heat exchangers.
• They have either one or two passages for the fluid to flow through.
• One fluid:
When there is only one fluid and one passage in the heat exchanger the fluid is used to transfer the heat to another location.
Application of this kind of heat exchangers is usually found in electronics to transfer heat into the fluid and out of the electronic device.
• Two Fluids:
When there are two fluids and two passages they are usually classified by the direction in which the fluids flow by each other.
Microscale heat exchangers can either be cross flow or counter flow heat exchangers.
22
• Counter Flow
Counter flow micro scale heat exchangers work the same way as
macro-scale counter flow heat exchangers.
In a counter flow heat exchanger the two fluids flow in opposite
directions of each other.
The fluids enter the heat exchanger at opposite ends.
The cooler fluids exits the counter flow microscale heat exchanger
at the end where the hot fluid enters therefore the cooler fluid will
approach the inlet temperature of the hot fluid.
Counter flow microscale heat exchangers are more efficient than
cross flow microscale heat exchangers.
23
Figure 2: Schematic of Counter Flow Heat Exchanger
24
• Cross Flow
Cross Flow microscale heat exchangers work the same way
as cross flow macro-scale heat exchangers.
In a cross flow heat exchanger one fluid flows perpendicular
to the second fluid.
One fluid flows through tubes or channels and the second
fluid passes around the tubes or channels at a 90° angle.
Cross flow micro heat exchangers are usually found in
applications where one of the fluids changes state therefore
having a two-phase flow.
25
Figure 4: Schematic of a Cross Flow Heat Exchanger
26
• To concurrently achieve the goals of high mass flow rate, low pressure drop, and high heat transfer rates,
the microscale cross flow heat exchanger comprises numerous parallel, but relatively short microchannels.
• The performance of these microscale heat exchangers is superior to the performance of previously available macro-scale heat exchangers.
• Typical channel heights are from a few hundred micrometers to about 2000 micrometers, and
typical channel widths are from around 50 micrometers to a few hundred micrometers.
• The use of microchannels in a cross flow microscale heat exchanger decreases the thermal diffusion lengths, allowing substantially greater heat transfer per unit volume or per unit mass than has been achieved with other heat exchangers.
• The cross flow microscale heat exchangers have performance characteristics that are superior to state of the art macro-scale heat exchanger designs.
27
28
Advantages over Macro-Scale Heat Exchangers
• Substantially better performance
Improves heat transfer coefficient with large number of
smaller channels
• Size
Smaller size allows for an increase in mobility and uses
• Light Weight
Lower weight reduces the structural and support
requirements
• Cost
Lower costs due to less material being used in fabrication
29
Disadvantages of Microscale Heat Exchangers
One of the main disadvantages of microchannel heat exchangers is the high
pressure loss that is associated with a small hydraulic diameter.
This prevents the uniform flow of the cooling material along the channel.
Microchannels are sometimes fairly long and absorb most of the heat along
the first section of the channel.
This makes them less able to absorb heat along later sections.
In order to get the maximum performance out of a microchannel heat
exchanger, there needs to be a balance between the desirable high heat
transfer coefficient and the undesiarable pressure loss.
Due to the small scale of microchannel passages, wall roughness can be very
important in determining how high the heat transfer coefficient is.
30
Applications of Microscale Heat Exchangers
• Microscale heat exchangers are being used to help along the
development of fuel cells.
• The compact microchannel fuel vaporizer (CMFV), which is a
microscale heat exchanger, is a main component of a microchannel
fuel processor that will hopefully enable fuel cell powered vehicles.
• Conventional heat exchangers are too large to be used in this
application, nor can they deliver the kind of performance needed in
this application.
• The microscale heat exchanger is also making possible a portable fuel
cell power supply.
• This power supply could make batteries obsolete.
31
• It will have a longer run time than a battery of comparable
weight.
• It could also be used in place of portable generators that operate
with an internal combustion engine.
• These fuel cells would operate more quietly and with a greater
efficiency than an engine driven generator.
• Problems with refueling a generator in a remote location could
also be solved be this new portable fuel cell.
32
Currently used in these Industries
• Automotive vehicles
• Commercial and Residential Heating/Cooling systems
• Aircrafts
• Manufacturing industries
• Cooling Electronic devices
Fundamentals of
Liquid Cooling
Thermal Management of Electronics
San José State University
Mechanical Engineering Department
Air as a Coolant
PROS:
• Simplicity
• Low Cost
• Easy to Maintain
• Reliable
CONS:
• Inefficient at heat removal
(low k and Pr)
• Low thermal capacitance
(low ρ and Cp)
• Large thermal resistance
Using Alternate Coolants
• As electronic components get smaller and heat transfer
requirements increase air becomes a less efficient coolant
• Liquid cooling provides a means in which thermal resistance can
be reduced dramatically
Types of Liquid Cooling
• Indirect –
The coolant does not come into contact with the electronics.
• Direct (Immersion) –
The coolant is in direct contact with the electronics.
Fluid Selection
• Is the fluid in direct contact with the electronics?
No.
Water will normally be used due to the fact that it is cheap and has superior thermal properties.
Yes.
A dielectric must be used.
Consideration must be given to the thermal properties of different dielectric fluids.
Microchannels
• Microchannels are most commonly used for indirect liquid
cooling of IC‟s and may be:
Machined into the chip itself.
Machined into a substrate or a heat sink and then attached
to a chip or array of chips.
Microchannels
• Example: Thermal
Conduction Module used
on IBM 3080X/3090
series
• Heat is transmitted
through an intermediate
structure to a cold plate
through which a coolant
is pumped
Incropera, pg. 3
Microchannels
• Rth,h – Conduction Resistance through the chip
• Rth,c – Contact Resistance at the Chip/Substrate Interface
• Rth,sub – 3-D Conduction Resistance in the substrate (spreading resistance)
• Rth,cnv – Convection Resistance from the substrate to the coolant
Incropera, pg. 155
Note that this network ends with the mean fluid temperature.
If we use the inlet fluid temperature, we also need to include Rcaloric
Contact Resistance
• Contact resistance is proportional to roughness, and inversely proportional to
pressure.
• Contact resistance depends on several factors
Surface roughness
Pressure holding the two surfaces together
Type of the fluid in the void space between two surfaces
The interface temperature
• Keeping the viscous liquid like “glycerin” in the interface, reduces the contact
resistance between two aluminum surfaces by a “factor of 10” at a given pressure.
• As a thumb rule, the “typical contact resistance” is approximately equal to “5 mm
of additional thickness”.
• Contact resistance may be reduced by use of special bonding greases or insertion
of a soft metal foil between the two surfaces.41
essurePr
RoughnesscetanresisContact
Motivating Example
Laminar flow through a rectangular channel:
Kandlikar and Grande, pg. 7 Kandlikar and Grande, pg. 8
Pressure Drop in Microchannels
• The pressure drop due to forcing a fluid through a small channel may produce design limitations.
V is the mean flow velocity
L is the flow length
ρ is the fluid density
f is the friction factor, depends on the
aspect ratio.
Limitations may include:
1) Pumping Power
2) Mechanical Stress Limitation of
the Chip Materialh
2
D
VLf2p
Pressure Drop Example
• If chip power increases
mass flow rate must
increase
• If mass flow rate
increases pressure drop
increases Kandlikar and Grande, pg. 9
)TT(c
Qm
i,wo,wp
.
Optimization of Microchannels
• How should the channels in the silicon substrate be designed for optimal
heat transfer?
• Should the channel be deep or shallow?
• Make sure to give a valid reason.
• The channels should be deep so that the hydraulic diameter is small but the channel surface area is large.
• Caution: Making the channels too small may result in unreasonable pressure drop.
Kandlikar and Grande, pg. 9
Microchannel - Issues
• Liquids + Electronics
Self-explanatory
• Fouling Leading to Clogging
Clogging prevents flow of liquids through a channel
Local areas where heat is not pulled away from components at a high enough rate are developed
Microchannel - Issues
• Mini-Pumps
Able to move liquid through the channel at a required rate
Able to produce large pressure heads to overcome the large pressure drop associated with the small channels
Tradition rotary pumps can not be used due to their large size and power consumption
For information on some current solutions refer to
http://www.electronics-cooling.com/html/2006_may_a3.html
Current Research for Single Phase Convection in
Microchannels
• Surface Area
Adding protrusions to the channels to increase surface area.
Adding and arranging fins in a manner that is similar to a
compact heat exchanger.
Microstructures
Silicon
Substrate
Examples of different
geometries:
• Staggered Fins
•Posts
•T-Shaped Fins
Kandlikar and Grande, pg. 10
Current Research for Single Phase Convection
in Microchannels
• Manufacturing Technology
Reducing cost of manufacturing
Producing enhanced geometries
For further information refer to article by Kandlikar and Grange
• Justifying deviation from classical theory for friction and heat transfer coefficients when microchannel diameters become small
Lack of a good analytical model
Surface Roughness
Accurate measurements of system parameters
Ect.
***If you are interested in this take a look at:
Palm, B. “Heat Transfer in Microchannels”. Microscale Thermophysical Engineering 5:155-175, 2001. Taylor Francis, 2001.
Current Research for Single Phase Convection in
Microchannels
Jet Impingement
• Benefits of using a jet in
thermal management of a
surface:
A thin hydrodynamic
boundary layer is formed
A thin thermal boundary
layer is formed
Incropera, pg. 56
Classifying Impinging Jets
• Jets can be:
Free-Surface – discharged
into an ambient gas
Submerged – discharged
into a liquid of the same
type
• Cross Sections:
Circular
Rectangular
• Confinement:
Confined – Flow is
confined to a region after
impingement
Unconfined – Flow is
unconfined after
impingement
Classify the Following Jets
• Liquid jet released into
ambient gas
• Liquid release into
liquid of the same type
Incropera, pg. 56
Incropera, pg. 65
Classify the Following Jets
Unconfined, circular, free-
surface jet
Unconfined, circular,
submerged jet
Incropera, pg. 56
Incropera, pg. 65
Nozzle Design
• Nozzles are designed to create different jet
characteristics
Example: Sufficiently long nozzles will produce both fully
developed laminar or turbulent jets (Shown in b)
Incropera, pg. 58
Flow Regions
• Stagnation Region –
Jet flow is decelerated normal to the impingement surface and accelerated parallel to it.
Hydrodynamic and thermal boundary layers are uniform.
• Wall Jet Region –
Boundary layers begin to grow
Incropera, pg. 62
Degradation of Heat Transfer During Jet
Impingement
• Splattering –
Droplets are eject from the wall jet region due to
the distance the nozzle is from the heat source, and
the surface tension of the jet fluid
• Hydraulic Jump –
An abrupt increase in film thickness and reduction in film
velocity occurring in the wall jet region
Confining Fluid Flow
• Adding a confining wall:
Adds low and high pressure regions
Sometimes adds secondary stagnation regions
Degrades convection heat transfer
Decreases space needed to use jet impingement
Incropera, pg. 69
Two-Phase Boiling in Microchannels
• Fluid entering microchannels is heated to the point where it
boils
• Flow in microchannels is highly unpredictable and can
produce large voids and multiple flow regimes inside of tubes
• No accurate analytical models currently exist; many analytical
models have errors ranging from 10% to well over 100%
Flow Regimes in Two-Phase Applications
Garimella, pg. 107
Immersion (Direct) Cooling
• In direct cooling electronics are immersed into a dielectric liquid
• Closed loop systems (Transformer cooling with oils) are
normally used due to
both the cost of the liquids used and
the environmental issues associated with the liquids escaping
into the atmosphere
Typical Liquids Used in Immersion
Cengel, pg. 920
Boiling Used in Immersion(Reuse of the cooling liquids)
• Electronics expel heat into the liquid
• Vapor bubbles are formed in the liquid
• The vapor is collected at the top of the enclosure where it
comes in contact with some sort of heat exchanger
• The vapor condenses and returns to the liquid portion of the
reservoir
Cengel, pg. 918
Boiling Used in Immersion
• Electronics dissipate heat through the liquid
• Vapor bubbles are generated
• As vapor bubbles rise they come in contact with the cooler liquid produced by an immersed heat exchange and they implode
*The prior example is more efficient due to the heat transfer coefficient associated with condensation
Cengel, pg. 919
Cray-2 Supercomputer
• Cold fluid enters between the
circuit modules
• Convection occurs, pulling
heat from the electronics to the
liquid
• The heated fluid is pumped to
a heat exchanger
• Heat is transfer from the
immersion liquid to chilled
water in the heat exchanger Incropera, pg. 6
Concerns with Immersion
• Introduction of incompressible gasses into a vapor space
This will limit the amount of condensation that is allowed
to occur and degrade heat transfer
• Leakage
Environmental Concerns
Reliability