Microheat Exchangers
- 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