Investigation of Port Level Refrigerant Flow Mal-distribution in Microchannel Heat Exchanger

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Investigation of Port Level Refrigerant Flow Mal-distribution in Microchannel Heat Exchanger Zhenning Li, Jiazhen Ling, Vikrant Aute, Reinhard Radermacher 12

1 Investigation of Port Level Refrigerant Flow Mal-distribution in Microchannel Heat Exchanger Zhenning Li, Jiazhen Ling, Vikrant Aute, Reinhard Radermacher 12 th IEA Heat Pump Conference, 2017 Center for Environmental Energy Engineering Department of Mechanical Engineering University of Maryland, College Park, MD 20742

Micro-channel HX Benefits Improved heat transfer against tube-fin HX 65% lower refrigerant pressure drop Up to 10% lower refrigerant charge (due to

2 Micro-channel HX Benefits Improved heat transfer against tube-fin HX 65% lower refrigerant pressure drop Up to 10% lower refrigerant charge (due to smaller internal volume) Compact design, i.e., about 2-3 times higher surface area-to-volume ratio Opportunities for weight reduction Hence, cost reduction 2

3 MCHX Mal-distribution Literature Review Authors Kim and Sin (2006) Marchitto et al. (2007) Ahmad et al.(2008) Kim et al (2012) Header geometry Header orientation Parameters Investigated Tube Number Tube protrusion depths Ref. mass flux Ref. quality Ref. property Maldistribution Level Header Port Method Kim et al (2013) Zou and Hrnjak (2013a) Hwang et al (2007) Dario et al (2015) Liu and Hrnjak (2016) Zou and Hrnjak (2016) Anbumeenaksh et al (2016) + Sim. Sim. 3

4 Objective Explore the impact of port level refrigerant mal-distribution on MCHX condenser and evaporator performance Quantify port level refrigerant flow maldistribution induced by air propagation Explore the effect of operating parameters on port mal-distribution (e.g. ref. mass flux, frontal air velocity) Explore the influence of geometry parameters on MCHX performance (e.g. number of ports) 4

5 Approach: Port-by-Port Flow Simulation Known: Equalized outlet pressure at merging point Unknown: Refrigerant mass flow rate distribution Res TubePair P1 sttube, out - P2 ndtube, out = DP Tube, scale Res PortPair P1 stport, out - P2 ndport, out = DP Port, scale Res HeaderPair P1 stheader - P2ndHeader = DP Tube, scale

Dimension of Simulated MCHX Tube Length (m) 0.

0254 Tube thickness (m) 0.0018 Port diameter (m) 0.

arrangement 13-13-6-4 Fin density (FPI) 20 Louver length (m) 0.

6 Dimension of Simulated MCHX Tube Length (m) Total number of tubes 36 Tube depth (m) Tube thickness (m) Port diameter (m) Number of ports per tube 18 Number of passes 4 Pass arrangement Fin density (FPI) 20 Louver length (m) Louver angle (deg.) 30 Louver pitch (m) Average Fin height (m) Air Air Air Port level finite control volume

7 Correlations and Test Conditions Heat transfer correlation Pressure drop correlation Air side (Louver fin) Chang and Wang R-410A side Vapor Two-Phase Liquid Dittus- Shah for Shah for Dittus- Boelter condenser evaporator Boelter Chang et al. Blasius Chen et al. Blasius Operation Mode Air dry bulb ( C) Air wet bulb ( C) Average air velocity (m/s) R410A inlet Tsat ( C) R410A inlet DeltaT/x R410A mass flux (kg/m2s) Condenser K superheat Evaporator x=

8 Case Study: Condenser 1 st port seeing fresh air has largest mass flow rate Mass flow decreases in each port along air flow direction 2.79% capacity degradation Pass #1 and #2 are mostly in two-phase region Pass #3 and #4 are mostly in sub-cooling region Top tube has end effect due to larger fin area, and has more fresh air on top Pass #1 Pass #2 Pass #3 Pass #4

Case Study: Evaporator 1 st port seeing fresh air has smallest mass flow rate, opposite to condenser Mass flow increases in each port along air flow direction 3.

9 Case Study: Evaporator 1 st port seeing fresh air has smallest mass flow rate, opposite to condenser Mass flow increases in each port along air flow direction 3.52% capacity degradation Mal-distribution effect fades along downstream passes Pass #1 and #2 are mostly in two-phase Pass #3 and #4 are mostly in superheat Top tube still has end effect Pass #1 Pass #2 Pass #3 Pass #4

10 Normalized mass flow rate Maldistribution in Sample Tubes Mass flow decreases in condenser, increases in evaporator Mal-distribution effect pass #1 > pass #2 > pass #3 & #4 Tubes in two phase region have more pronounced maldistribution than vapor/liquid phase Tubes From Condenser Tube #2, Pass #1, ΔTin = 8.4 K Tube #14, Pass #2, xin = 0.41 Tube #36, Pass #4, ΔTin = -5K Normalized mass flow rate Tubes From Evaporator Tube #36, Pass #4,ΔTin =0.2K Tube #14, Pass #2,xin = 0.83 Tube #2, Pass #1, xin = Microchannel port number Microchannel port number

11 Effect of Frontal Air Velocity Larger frontal air velocity induces more flow mal-distribution Standard deviation(std) of port mal-distribution for sample tubes are below 3%. Normalized mass flow rate Tube #2 From Condenser Vair = 1 m/s Vair = 2 m/s Vair = 3 m/s Vair = 4 m/s 3.00% 2.00% 1.00% STD of Port Mass Flow Rate 2.79% 2.31% 1.49% 1.02% Microchannel port number 0.00% 1 m/s 2 m/s 3 m/s 4 m/s

12 Actual Capacity [Ton] Mal-distribution Impact on HX Performance Select a 50 ton MCHX condenser (nominal capacity for a roof-top unit ) Parametric study by varying number of ports Large number of ports induce significant capacity degradation (18.9% when N ports/tube = 36) Number of Ports Per Tube (Q uniform -Q maldistribution )/Q uniform 20% 16% 12% 8% 4% 0% Number of Ports Port Tube

13 Conclusion For condenser, port mass flow decreases along air flow direction, while evaporator has opposite trend Mal-distribution in upstream passes are more pronounced than in downstream passes Two phase tubes have more significant maldistribution than single phase tubes Large air velocity can induce more mal-distribution Large number of ports is detrimental to MCHX performance (18.9% capacity degradation observed) Future work Choose number of ports with caution Design variable port size MCHX for port flow uniformity 13

14 14 Thank You

Design Optimization of Variable Geometry Microchannel Heat Exchangers

Purdue University Purdue e-pubs International Refrigeration and Air Conditioning Conference School of Mechanical Engineering 2014 Design Optimization of Variable Geometry Microchannel Heat Exchangers Long