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??