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MICRO-HEAT EXCHANGERS
Zubair AhmedGNITC
μ
• An Heat exchanger in which fluid flows in a confined space (tubes or cavity) whose dimensions are less than micrometer ( < 1 mm ) .
DEFINITION
• 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??
Same classification as conventional macro-scale heat exchangers• Parallel Flow• Counter Flow• Cross Flow
TYPES
• Conventional Equation valid
FACT THAT
Studied by
• CFD ( Computational Fluid Dynamics ) Approach• Experimental Approach
CFD APPROACH
Energy equation
Momentum equation
Continuity equation
Energy equation for wall
Applying Boundary Equations
WE DETERMINE
Effectiveness
Pumping Power
ΔPt = ΔPh +ΔPc = (Phi –Pho) + (Pci – Pco ) Pressure Loss
• Obtain inlet and outlet temperatures of hot and cold fluid
• Obtain log mean temperature difference
• Calculate overall heat transfer coefficient
EXPERIMENTAL
• Reynolds number
• Pressure drop
• Then the performance index is calculated
ANALYSIS
What goes in making a successful μHEX
• 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
• Chemical Etching• LIGA ( lithography , moulding ,
electroplatin)• Diffusion Bonding
HOW TO MAKE
• 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
CHEMICALETCHING
• 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
DIFFUSI0NBONDING
• high pressure loss • wall roughness • Inadequate heat absorption• High manufacturing cost• Slower production
OBSTACLES
THANK YOU
Questions??