1. Material Measurement Laboratory Cryogenic Engineering Conference 2015 06-29-2015 11:45 AM Single-phase ambient and cryogenic temperature heat transfer coefficients in microchannel Seungwhan Baek and Peter E Bradley NIST Boulder, CO 80305 USA

2. Material Measurement Laboratory Contents Introduction Experiments Results Discussion Summary 2

3. Material Measurement Laboratory Introduction Microscale J-T cryocooler development – Requires microchannel heat exchanger – Requires microchannel heat transfer characteristics Compressor Microchannel Heat exchanger J-T valve Evaporator Microchannel Heat exchanger Microchannel Heat exchanger Early prototype Revised MCC Design 3

4. Material Measurement Laboratory Thermal design of MCC heat exchanger Operating condition of MCC – Fluid flow in microchannel ( D h < 100 μm ) – Extremely low flowrate ( Re < 100 ) – Low pressure ratio (P r <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 D h : hydraulic diameter, Re: Reynolds number 4

5. Material Measurement Laboratory Thermal design of heat exchanger Wall Flow channel L W H th Flow direction h: heat transfer coefficient (W/m 2 K) k f : thermal conductivity of fluid (W/mK) D h : hydraulic diameter (m) Re: Reynolds number Pr: Prandtl number 5

6. Material Measurement Laboratory 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 Turbulent MCC Operating regime ResearcherDiameterCorrelation Kays (1970) D h > 1mm Sieder & Tate (1996) D h > 1mm Shah and London (1978) D h > 1mm Shah and London (1978)D h > 1mm Wu & Little (1983) D h = 150 μm Choi et al. (1991) D h = 81 μm Grigull and Tratz (1965) D h > 1mm Nu=4.36 Previous Nu# for single phase flow (gas) 6

7. Material Measurement Laboratory Experiments 7

8. Material Measurement Laboratory Experiments 180 μm110 μm65 μm Friction factor300 K gas N 2 Heat transfer Coefficient 70 K Liquid N 2 300 K Gas N 2 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) 8

9. Material Measurement Laboratory Experimental setup (schematic) 1 st stage 2 nd stage GM cryocooler Microchannel T=64 K Vacuum Chamber T inlet T wall_1 T wall_2 T wall_3 T outlet 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 2 nd stage Microchannel assembly GM-cryocooler 1 st stage 9

10. Material Measurement Laboratory Microchannel test section T out T in Thermal grease Heating element T w3 T w2 T w1 Epoxy Solder microchannel thermocouple L L/2 L/4 Flow ‘Not to scale’ 180 μm 380 μm Unit: mm 380 μm 36 AWG D o =130 μm E-type thermocouples Heating wire D o =160 μm Setup for D h =180 μm Heating length=3cm 10

11. Material Measurement Laboratory 139 μm SEM pictures of microchannels D h =180 μmD h =110 μmD h =65 μm D in (μm)18011065 D out (μm)380310160 D in /D out 0.470.350.4 Identical magnification Thickest wallThinnest wall 100 μm Dr. Baek’s hair cross section Not typical human 11

12. Material Measurement Laboratory Scale comparison 310 μm 160 μm 106 μm Dr. Baek hair 65 μm110 μm Stainless steel tube D h = 110 μm Stainless steel tube D h = 65 μm Dr. Baek Hair OD~100 μm OD=160 μm OD=310 μm Thermocouple tip 300 μm Not able to measure fluid temperature and wall temperature separately. 36AWG 12

13. Material Measurement Laboratory ‘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 T inlet T wall,x T outlet Constant heat flux heater x L 13

14. Material Measurement Laboratory Result & Discussion 14

15. Material Measurement Laboratory Friction factor measurements (Gas, 300 K) Hydraulic characteristic of fluid in microchannels – Friction factor follows conventional theory Laminar Turbulent Experimental friction factor Laminar theory friction factor Turbulent theory friction factor (Blasius equation) 15

16. Material Measurement Laboratory Nu # measurement Nu # degrades from Re < 1000 Nu #: 180 μm > 65 μm > 110 μm Similar trend with previous research (Morini, Choi, Little) Nu=4.36 Laminar Turbulent Nu=4.36 Laminar Turbulent Nu # degrades from Re > 200 Nu # : 180 μm ~= 65 μm ~= 110 μm No other research to compare Gas N 2 (300 K) Liquid N 2 (70 K ) 16

17. Material Measurement Laboratory Scaling effect Non-D numberEffectIgnored when KnKnudsengas rarefactionKn < 0.001 MaMachflow compressibilityMa < 0.3 BrBrinkmanviscous heatingBr < 0.005 λLambdaaxial conduction of wallλ < 0.01 PePecletaxial conduction of fluidPe > 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 Microchannel Phase Kn (<0.001) Ma (<0.3) Br (<0.005) λ (<0.01) Pe (>50) ReRe=1Re=3000Re=1Re=3000Re=1Re=3000 D h =180 μm gas0.000070.00060.102.180.005102340 liquid0.000010.00030.040.100.00023200 D h =110 μm gas0.000120.00180.271.630.006102400 liquid0.000020.00070.110.800.00025200 D h =65 μm gas0.000210.00310.461.100.00282430 liquid0.000050.00100.20 0.000452700 17

18. Material Measurement Laboratory 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 Baek et al., 2014, Cryogenics 60 49-61 Lin T-Y and Kandlikar S G, 2012 Journal of Heat Transfer 134 (2) 020902 (1) 18

19. Material Measurement Laboratory Comparison of Experiment & Nu app Comparison shows identical trend with experiment & equation (1). Comparison implies actual Nu=4.36 holds in low Re # flow. 19

20. Material Measurement Laboratory Summary 20

21. Material Measurement Laboratory Summary Design of heat exchangers influence development of MCC – High uncertainty in operation  Due to very small D h, 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 Nu app imply validation of Nu = 4.36 In laminar flow for single-phase fluid. 21

22. Material Measurement Laboratory Experiment vs Previous work 22

23. Material Measurement Laboratory Thank you! Test Specimen is available! Ask Peter & Seungwhan for observation! 23