material measurement laboratory cryogenic engineering conference 2015 06-29-2015 11:45 am...
TRANSCRIPT
Material Measurement Laboratory
Cryogenic Engineering Conference 201506-29-2015
11:45 AM
Single-phase ambient and cryogenic temperature heat transfer coefficients in
microchannel
Seungwhan Baek and Peter E BradleyNIST
Boulder, CO 80305 USA
Material Measurement Laboratory 2
Contents
• Introduction
• Experiments
• Results
• Discussion
• Summary
Material Measurement Laboratory 3
Introduction• Microscale J-T cryocooler development
– Requires microchannel heat exchanger – Requires microchannel heat transfer characteristics
CCompressor
Microchannel Heat exchanger
J-T valve
Evaporator
Microchannel Heat exchanger
Microchannel Heat exchanger
Early prototype
Revised MCC Design
Material Measurement Laboratory 4
Thermal design of MCC heat exchanger
• Operating condition of MCC – Fluid flow in microchannel ( Dh < 100 μm )– Extremely low flowrate ( Re < 100 )
– Low pressure ratio (Pr<4:1)
• Cooling Performance of MCC depends on– Heat exchanger performance– Need heat exchanger/heat transfer characteristics
for MCC operating condition – No previous research at these condition)
• Two different heat exchanger in MCC– Recuperative HX: Single phase (gas or liquid)
• Phase I research
– Isothermal HX: two-phase (gas+liquid)• Phase 2 research
Recuperative HX
Isothermal HX
Dh: hydraulic diameter, Re: Reynolds number
Material Measurement Laboratory 5
Thermal design of heat exchanger
• Heat transfer coefficient (h) required to determine geometry of heat exchanger (Aht), heat transfer amount ()
– Geometry: Heat transfer area (Aht=L x W), thickness of the wall (th)
• Nusselt number– Dimensionless number to define heat transfer performance – Nusselt number (heat transfer characteristic) is dependent on fluid property (Pr) &
operating condition (Re)
Wall
Flow channelL
W
H
th
Flow direction
h= �̇�𝐴h𝑡(𝑇 𝑓 −𝑇𝑤)
Aht: heat transfer area (m2): heat amount (W)Tf: fluid temperature (K)Tw: wall temperature (K)
𝑁𝑢=h𝐷h
𝑘 𝑓𝑁𝑢= 𝑓 (𝑅𝑒 ,𝑃𝑟)
h: heat transfer coefficient (W/m2K)kf: thermal conductivity of fluid (W/mK)Dh: hydraulic diameter (m)Re: Reynolds number Pr: Prandtl number
Material Measurement Laboratory 6
10 100 1000 100001E-3
0.01
0.1
1
10
100
Yang et al (2012) Choi et al. (1991) Wu & Little (1983) Morini et al. (2012)N
usse
lt nu
mbe
r (N
u)
Reynolds number (Re)
Nusselt number: Previous research
Review of heat transfer and pressure drop characteristics of single and two-phase microchannels, Asadi, 2014, (Review paper)
• Theoretical Nusselt number: Nu=4.36 const. (circular channel, Re < 2000)
• From previous research: single-phase Nu # correlation for Re < 2000• No experiment at low temperature (T < 200 K), low flow rate (Re = 10 - 50)• Experiments do not follow theory, Differ from each other, No recent research
• Need to verify heat transfer characteristic for better design of MCC
From previous research, Nu=f(Re, Pr) for Re < 2000
Laminar TurbulentMCC Operating
regime
Researcher Diameter Correlation
Kays (1970) Dh > 1mm
Sieder & Tate (1996) Dh > 1mm
Shah and London (1978) Dh > 1mm
Shah and London (1978) Dh > 1mm
Wu & Little (1983) Dh = 150 μm
Choi et al. (1991) Dh = 81 μm
Grigull and Tratz (1965) Dh > 1mm
Nu=4.36
Previous Nu# for single phase flow (gas)
Material Measurement Laboratory 7
Experiments
Material Measurement Laboratory 8
Experiments
180 μm 110 μm 65 μm
Friction factor 300 K gas N2
Heat transfer Coefficient
70 K Liquid N2
300 K Gas N2
• Measure the single phase heat transfer coefficients• Nitrogen • microchannels (180 μm, 110 μm, 65 μm)
• Operating condition• cryogenic liquid flow (~70 K) • ambient gas flow(~300 K)
Material Measurement Laboratory 9
Experimental setup (schematic)
1st stage
2nd stage
GM cryocooler
Microchannel
T=64 K
Vacuum Chamber
Tinlet Twall_1 Twall_2 Twall_3Toutlet
Pressure sensor Pressure sensor
Compressor
Recuperator
Radiation shield
Flow conditioner
Constant heat flux
‘Not to scale’
Mass flow meter
Vacuum chamber
Feedthrough collar +
Test section
Compressor
GM-cryocooler 2nd stage
Microchannel assembly
GM-cryocooler 1st stage
Material Measurement Laboratory 10
Microchannel test section
Tout
Tin
Thermal grease
Heating element
Tw3Tw2Tw1
Epoxy
Solder
microchannel
thermocouple
LL/2L/4
Flow ‘Not to scale’
180 μm
380 μm
Unit: mm
380 μm36 AWG Do=130 μm E-type thermocouples Heating wire
Do=160 μm
Setup forDh=180 μm
Heating length=3cm
Material Measurement Laboratory 11
139 μm
139 μm
SEM pictures of microchannels
Dh=180 μm Dh=110 μm Dh=65 μm
Din (μm) 180 110 65Dout (μm) 380 310 160Din/Dout 0.47 0.35 0.4
Identical magnification
Thickest wallThinnest wall
100 μm
Dr. Baek’s hair cross sectionNot typical human
Material Measurement Laboratory 12
Scale comparison
310 μm 160 μm
106 μm
Dr. Baek hair
65 μm110 μm
Stainless steel tube
Dh= 110 μm
Stainless steeltubeDh= 65 μm
Dr. Baek Hair
OD~100 μmOD=160 μm
OD=310 μm
Thermocouple tip300 μm
Not able to measure fluid temperature and wall temperature separately.
36AWG
Material Measurement Laboratory 13
‘Classic’ Nu# estimation method
1. Find energy input to fluid
2. Estimate fluid temperature inside microchannel (based on linear temperature profile)
3. Measure the wall temperature
4. Determine the heat transfer coefficient
5. Calculate the Nusselt number
Microchannel
Tinlet Twall,x Toutlet
Constant heat fluxheater
𝑄 𝑓𝑙𝑜𝑤=�̇�𝑐𝑝 (𝑇 𝑜𝑢𝑡−𝑇 𝑖𝑛)=�̇� (𝑖𝑜𝑢𝑡−𝑖𝑖𝑛)
x
L
𝑄 𝑓𝑙𝑜𝑤𝑥𝐿
=�̇�𝑐𝑝 (𝑇 𝑓 , 𝑥−𝑇 𝑖𝑛 ) 𝑇 𝑓 , 𝑥=𝑄𝑓𝑙𝑜𝑤
�̇�𝑐𝑝𝐿𝑥+𝑇 𝑖𝑛
𝑇𝑤𝑎𝑙𝑙 ,𝑥
h=𝑄 𝑓𝑙𝑜𝑤
𝐴𝐻𝑇 (𝑇𝑤−𝑇 𝑓 ,𝑥)=
�̇� (𝑖𝑜𝑢𝑡−𝑖𝑖𝑛 )𝐴𝐻𝑇 (𝑇𝑤−𝑇 𝑓 , 𝑥)
𝑁𝑢=h𝐷h
𝑘 𝑓
Material Measurement Laboratory 14
Result & Discussion
Material Measurement Laboratory 15
Friction factor measurements (Gas, 300 K)
• Hydraulic characteristic of fluid in microchannels– Friction factor follows conventional theory
10 100 1000 100001E-3
0.01
0.1
1
10 180 m Exp. 110 m Exp. 65 m Exp.
Laminar Theory: 16/Re Turbulent: Blasius
f - fr
ictio
n fa
ctor
Reynolds number
Laminar Turbulent
Experimental friction factor
Laminar theory friction factor
Turbulent theory friction factor (Blasius equation)
Material Measurement Laboratory 16
Nu # measurement
• Nu # degrades from Re < 1000• Nu #: 180 μm > 65 μm > 110 μm• Similar trend with previous research
(Morini, Choi, Little)
10 100 1000 100001E-3
0.01
0.1
1
10
100 65 m Gas N
2 Exp.
110 m Gas N2 Exp.
180 m Gas N2 Exp.
Nu
ssel
t n
um
ber
Reynolds number
Nu=4.36
Laminar Turbulent
10 100 1000 100001E-3
0.01
0.1
1
10
100
65 m Liquid N2 Exp.
110 m Liquid N2 Exp.
180 m Liquid N2 Exp.
Nu
ssel
t n
um
ber
Reynolds number
Nu=4.36
Laminar Turbulent
• Nu # degrades from Re > 200 • Nu # : 180 μm ~= 65 μm ~= 110 μm• No other research to compare
Gas N2 (300 K) Liquid N2 (70 K )
Material Measurement Laboratory 17
Scaling effectNon-D number Effect Ignored when
Kn Knudsen gas rarefaction Kn < 0.001
Ma Mach flow compressibility Ma < 0.3
Br Brinkman viscous heating Br < 0.005
λ Lambda axial conduction of wall λ < 0.01
Pe Peclet axial conduction of fluid Pe > 50
• Scaling effect can influence the thermal behavior of fluid flow in microchannels*
*Guo Z-Y and Li Z-X, 2003 International Journal of Heat and Mass Transfer 46 (1) 149-159
MicrochannelPhase Kn
(<0.001)
Ma (<0.3) Br
(<0.005)
λ(<0.01)
Pe (>50)
Re Re=1 Re=3000 Re=1 Re=3000 Re=1 Re=3000
Dh=180 μmgas 0.00007 0.0006 0.10
1.52.18 0.005 10 2340
liquid 0.00001 0.0003 0.04 0.10 0.0002 3 200
Dh=110 μmgas 0.00012 0.0018 0.27 1.63 0.006 10 2400
liquid 0.00002 0.0007 0.11 0.80 0.0002 5 200
Dh=65 μmgas 0.00021 0.0031 0.46
8.71.10 0.002 8 2430
liquid 0.00005 0.0010 0.20 0.20 0.0004 5 2700
Material Measurement Laboratory 18
Nu # degradation• Nu # degradation is related to axial conduction effect through the wall.• Axial conduction changes temperature profile to ‘non-linear’. (Baek et al, 2014)• Non-linear temperature profile violates the assumption in classic Nu # measurement.• Classic Nu # measurement including axial conduction effect leads to estimate ‘apparent
Nu #’.
• Apparent Nu # is neither actual nor theoretical Nu #.
• Apparent Nu # (Lin & Kandlikar, 2012) – Nu # degrades due to axial conduction effect with classic Nu# measurement method
2
1
1 4(RePr)
app app
theoryw wtheory x
f f
Nu h
Nuk ANu h
k A
Baek et al., 2014, Cryogenics 60 49-61Lin T-Y and Kandlikar S G, 2012 Journal of Heat Transfer 134 (2) 020902
(1)
Material Measurement Laboratory 19
Comparison of Experiment & Nuapp
• Comparison shows identical trend with experiment & equation (1).
• Comparison implies actual Nu=4.36 holds in low Re # flow.
10 100 1000 100001E-3
0.01
0.1
1
10
100 Simulations for Liquid N
2, eqn. (6)
Simulations for Gas N2, eqn. (6)
110 m
180 m
65 m
110 m
65 m Liquid N2 Exp.
110 m Liquid N2 Exp.
180 m Liquid N2 Exp.
65 m Gas N2 Exp.
110 m Gas N2 Exp.
180 m Gas N2 Exp.
App
aren
t Nus
selt
num
ber
Reynolds number
180 m
65 m
Material Measurement Laboratory 20
Summary
Material Measurement Laboratory 21
Summary
• Design of heat exchangers influence development of MCC– High uncertainty in operation Due to very small Dh, Low Re# , Low temperature
• The hydraulic and thermal characteristics of fluid in the microchannel are investigated by the experiments.
– Friction factors : comparable to macro-scale tubes– Nu # : decreased value @ low Re# , which are affected by axial conduction
• Axial conduction effect influence the fluid & wall temperature profile to become non-linear.
• Comparison of experimental result and theoretically derived Nuapp imply validation of Nu = 4.36 In laminar flow for single-phase fluid.
Material Measurement Laboratory
Experiment vs Previous work
22
Material Measurement Laboratory 23
Thank you!
Test Specimen is available!Ask Peter & Seungwhan for
observation!