30th June 2011 1Enrico Da Riva , V. Rao

Parametric study using Empirical Results

June 30th 2011Bdg 298

Enrico Da Riva,Vinod Singh Rao

CFD-2011-03-GTKNA-62(Micro channel)

30th June 2011 2Enrico Da Riva , V. Rao

PDdevice = PDmanifold + PDmicrochannels+ PDpipe

where PD : Pressure Drop

Estimation of the pressure drop in the Microchannel device

30th June 2011 3Enrico Da Riva , V. Rao

The pressure drop in a fluid flow is proportional to the dynamic pressure as given by the relation:

The value of K(=3*2.1) is chosen according to the experimental data provided @15°C to obtain the pressure

drop @ -25°C

Estimation of pressure drop in manifold

30th June 2011 4Enrico Da Riva , V. Rao

The pressure drop across the microchannel and the inlet-outlet pipe is calculated using churchill correlations flow in tubes.

where

Re : Reynolds number f: friction factor

where L: Length w: Mass flow rate

D: hydraulic diameter

Estimation of pressure drop in the microchannel

30th June 2011 5Enrico Da Riva , V. Rao

Number of microchannels: 300Length of microchannel: 40mm Depth of microchannel: 100µm Width of channel: 100µm Mass flow rate:7 g/s

Density @ 15°C :1700.85 Kg/m3 Kinematic viscosity @ 15°C : .425 cSt Inlet velocity: 1.75 m/s Inlet Reynolds number: 410.99

Density @ -25°C :1805.25 Kg/m3

Kinematic viscosity @ -25°C : .8 cSt Inlet velocity:1.65 m/s Inlet Reynolds number:205.71

» Flow is laminar in the microchannels

Dimensions and flow regime in the microchannels

30th June 2011 6Enrico Da Riva , V. Rao

The pressure drop varies linearly with the mass flow rate in the laminar region.

The variation of the kinematic viscosity with temperature also has a huge effect on the pressure drop . The pressure drop at a lower temperature is quite high as compared to the higher temperature with same mass flow rate.

Effect of the mass flow rate on Pressure drop across the microchannels

30th June 2011 7Enrico Da Riva , V. Rao

Length of microchannel: 40mmDepth of microchannel: 100µmFin-width: 100µm Mass flow rate: 7 g/sec

Increasing the number of channels with fin-width constant increases the pressure drop exponentially as the hydraulic diameter decreases quite fast.

Effect of the microchannel dimension/Number of microchannels on Pressure drop across the

microchannels

30th June 2011 8Enrico Da Riva , V. Rao

Length of microchannel: 40mm Depth of mircochannel: 100µmMicrochannel-width:100µm Mass flow rate: 7 g/sec

Increasing the number of channels with channel width constant decreases the pressure drop as there is smaller mass flow rate in each channel.

Effect of the microchannel dimension/Number of microchannels on Pressure drop across the

microchannels

30th June 2011 9Enrico Da Riva , V. Rao

Inlet mass flow rate: 7g/sInlet diameter: 1.4 mmManifold cross section: 280µm X 1.7mm

Density @ 15°C :1700.85 Kg/m3 Kinematic viscosity @ 15°C : .425 cSt Inlet velocity: 2.67 m/s Inlet Reynolds number: 8806.94

Manifold velocity: 8.64 m/s Manifold Reynolds number: 9781.55

Density @ -25°C :1805.25 Kg/m3

Kinematic viscosity @ -25°C : .8 cSt Inlet velocity:2.51 m/s Inlet Reynolds number: 4408.11

Manifold velocity: 8.14 m/s Manifold Reynolds number: 4895.93

Flow is turbulent in the manifold

Effect of the mass flow rate on Pressure drop across the manifold

30th June 2011 10Enrico Da Riva , V. Rao

Dimensions:Inlet diameter of the pipe: 1.4 mmDepth of the manifold: 280 µmWidth of the manifold: 1.7mm

Temperature: 15°C

Effect of the mass flow rate on Pressure drop across the manifold

30th June 2011 11Enrico Da Riva , V. Rao

Effect of the mass flow rate on Pressure drop across the manifold.

30th June 2011 12Enrico Da Riva , V. Rao

With decrease in temperature , the pressure drop in microchannel become more prominent but for the current operating point, manifold pressure drop is quite important.

Effect of the mass flow rate on Pressure drop across the manifold.

30th June 2011 13Enrico Da Riva , V. Rao

Effect of the mass flow rate on Pressure drop across the inlet-outlet pipes.

30th June 2011 14Enrico Da Riva , V. Rao

The change in the steepness of the curve may be due to the transition from laminar region to turbulent flow.

Effect of the mass flow rate on Pressure drop across the Pipes.

30th June 2011 15Enrico Da Riva , V. Rao

1.) The pressure drop in the manifold would be dominant at the design temperature and design mass flow rate.

2.) The problem is due to the small cross section of the manifold.

3.) Before looking at the microchannels, we must work on the manifold.

4.) Temperature has a strong influence on the pressure drop in the microchannel, but this is not so important with the present design since the dominant pressure drop is in the manifold.

5.) Using multiple inlet-outlet system to reduce pressure drop in the manifold.

Conclusions & Suggestions

30th June 2011 16Enrico Da Riva , V. Rao

Working fluid perflourohexane @Temperature -25 °C

Mass flow: 7 g/s or 2*3.5 g/s

Manifold velocity:4.07 m/s Inlet velocity: 1.25 m/s Manifold Reynolds number: 2447Inlet Reynolds number:2204

Pressure drop with dual inlet-outlet

30th June 2011 17Enrico Da Riva , V. Rao

The required pressure drop for mass flow rate of 7 g/s is about 4 bar

Pressure drop with dual inlet-outlet

30th June 2011 18Enrico Da Riva , V. Rao

For laminar flow Nusselt number : 3.2Hydraulic diameter of channels : 100 µm

Thermal conductivity of Perflourohexane @ -25 °C: .06275 W/(m-k) Heat transfer coefficient (Perflourohexane):2008 W/(m2 k)

Heat transfer coefficient