Micro-heat exchangers are heat exchangers where fluid flows in confined spaces less than 1 mm in dimensions. They have compact sizes, high heat transfer coefficients between 10,000-35,000 W/m2K, and high surface area to volume ratios of 20,000-30,000 m2/m3. Micro-heat exchangers are lighter weight and have lower costs than conventional heat exchangers. They can be modeled using computational fluid dynamics or tested experimentally to determine effectiveness, pressure loss, and other performance characteristics. Common fabrication methods include chemical etching, LIGA, and diffusion bonding.

1. MICRO-HEAT
EXCHANGERS
Zubair Ahmed
GNITC
μ

2. • An Heat exchanger in which
fluid flows in a confined space
(tubes or cavity) whose
dimensions are less than
micrometer ( < 1 mm ) .
DEFINITION

3. • Compact size
• High heat transfer coefficients
(10,000 – 35,000 W/m2 k )
h = Nu * k / D
• High surface are to volume
ratio
( 20,000 – 30,000 m2/m3 )
• Lower costs due to less
material being used in
fabrication
• Light Weight
WHY??

4. Same classification as
conventional macro-scale heat
exchangers
• Parallel Flow
• Counter Flow
• Cross Flow
TYPES

5. • Conventional Equation validFACT THAT
Studied by
• CFD ( Computational Fluid Dynamics ) Approach
• Experimental Approach

6. CFD APPROACH
Energy equation
Momentum equation

7. Continuity equation
Energy equation for wall
Applying Boundary Equations

8. WE DETERMINE
Effectiveness
Pumping Power
ΔPt = ΔPh +ΔPc = (Phi –Pho) + (Pci – Pco )Pressure Loss

9. • Obtain inlet and outlet temperatures of hot and cold
fluid
• Obtain log mean temperature difference
• Calculate overall heat transfer coefficient
EXPERIMENTAL

10. • Reynolds number
• Pressure drop
• Then the performance index is calculated

11. ANALYSIS
What goes in making a successful μHEX

12. • Nanofluids ( water + alumina ) heat capacity, heat
transfer coeff.
• Heat transfer coefficient is varying along the lenght
• impact of gravity on the fluid through the microchannel
heat exchanger was found to be small
• The heat transfer in microstructure heat exchangers can be
by the hydraulic diameter of microchannels
• Enhancement can also be made by changing the design from
microchannels to more complex geometries.
• Relatively high pressure losses, high pump work
• Variable cross-section reduces pressure losses

13. • Chemical Etching
• LIGA ( lithography , moulding ,
electroplatin)
• Diffusion Bonding
HOW TO MAKE

14. • Chemical etching is a process by which
a strong acid or base is used to remove
material to shape microchannels.
• directional material removal rate
dependency are required
• Potassium Hydroxide or Hydrogen
Fluoride
CHEMICAL
ETCHING

15. • Diffusion bonding is a welding process
by which two surfaces are bonded
together under high temperature and
mechanical pressure in a vacuum or
non-oxygen environment
• 0.5 - 0.8 times the melting
temperature for the material
DIFFUSI0N
BONDING

16. • high pressure loss
• wall roughness
• Inadequate heat absorption
• High manufacturing cost
• Slower production
OBSTACLES

17. THANK YOU
Questions??