Effect of Height Difference on The Performance of Two phase - - PowerPoint PPT Presentation

Effect of Height Difference on The Performance of Two phase Thermosyphon Loop Used in Air-conditioning System

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Department of Building Science Tsinghua University Speaker: Zhang Penglei, a PhD candidate

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Contents

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Click to add Title 1 Click to add Title 2 Click to add Title 3 Click to add Title 4

Background

Experimental method Results and discussion Conclusions

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Background

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Condenser Evaporator Downcomer Riser

qe

Two-phase thermosyphon loop (TPTL)

 wickless gravity assisted heat pipe, two-phase

natural circulation loop, separated heat pipe

 Consists of evaporator, condenser, riser, and

downcomer

 Condenser must be higher than the evaporator  Gas/ gas-liquid rises in the riser, and liquid

flows back in the downcomer due to the gravity

High heat transfer coefficient Excellent isothermality Flexibility Relatively long distance Natural circulation without pump, compressor

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Background-traditional TPTLs

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Condenser Evaporator Downcomer Riser

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Cooling of electronic components/ light water reactor

 Fixed heat flux  Large heat flux (>20 kW/m)  Large temperature difference (>40C)  Large refrigerant charge to avoid

dryout

 Larger condenser than evaporator

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Background-TPTLs in air-conditioning system

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Exhausted air Fresh air

Heat recovery for free

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IT device

IDC

Free cooling for spaces with large rejected heat (computer rooms, Internet data centers)

 Small heat flux (<10 kW/m)  Small temperature difference (<30 oC)  Dryout is allowable, charge should be moderate

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Background-effect of height difference

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Height difference ∆H

Larger height difference Larger liquid head Larger driving force Better performance

? ? ? Q/ M ΔH

 Height difference is a main influencing factor, especially in air-conditioning system,

where it vary a lot, due to the restricted mounting locations in the building

 Driving force resulting from the liquid head keep balance with the pressure drop  In the common sense: larger height difference, larger liquid head, larger driving

force, then better performance, is that always true???

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Contents

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Click to add Title 1 Click to add Title 2 Click to add Title 3 Click to add Title 4

Background

Experimental method

Results and discussion Conclusions

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Experimental setups

8 Schematic diagram of the experimental setups

F

P F F ∆P

Condenser Evaporator Riser Downcomer Coriolis Mass Flowmeter High temperature water tank Low temperature water tank Pump Water flowmeter Water flowmeter Pump Valve Differential pressure gauge Pressure gauge T T T T T T T T T T T T T T T T T T

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Experimental setups

9 Evaporator

T6 T5 T4 T3 T2 T1 T7 T8 T9

12 16 Refrigerant outlet Refrigerant inlet Water

• utlet

Water inlet Liquid level detector 450

T15 T10 T11 T12 T13 T14 T16 T17 T18

500 12 16 Refrigerant inlet Refrigerant outlet Water

• utlet

Water inlet

Condenser

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Experimental setups

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F

P F F ∆P

Riser Downcomer Ball valve

 Riser and downcomer: Transparent

flexible pipe (Φ12)，PU pipe

 The condenser is fixed on a lifter,

height difference 0-1.5 m

 A ball valve is set at the outlet of

evaporator to change the circulation flow resistance Lifter

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Experimental setups

11 Coriolis mass flowmeter

0.0% 0.5% 1.0% 1.5% 2.0% 200 400 600 800 1000 10 20 30 40 50

• Max. error (% of flowrate)

Pressure drop (Pa) Flowrate (kg h-1) Pressure drop

• Max. error

F

P F ∆P

e

( )

P in

• u t

Q c V t t   

Tout Tin V Pressure drop and maximum error of the Coriolis flowmeter

 A Coriolis mass flowmeter with high

precision and low pressure drop to measure the flow rate of refrigerant

 Heat transfer rate is calculated by the water

side of evaporator

 Liquid level and flow pattern is observed

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Experimental setups

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Items Specifications Evaporator 3 parallel double pipes; Pipe length: 0.45 m Diameter of inner pipe: 12 mm; Diameter of outer pipe: 16 mm; Condenser 3 parallel double pipes; Pipe length: 0.45 m Diameter of inner pipe: 12 mm; Diameter of outer pipe: 16 mm; Riser Length: 2.0 m; Inner diameter: 12 mm Downcomer Length: 2.0 m; Inner diameter: 12 mm Height difference 0-1.5 m Refrigerant R134a

Specifications of the TPTL heat exchanger

Items Range Uncertainty Type Thermocouple

• 10-50 oC

<0.1 oC T type Pressure transducer 0-2.5 MPa <0.2% UNIK 5000 Differential pressure transducer 0-50 kPa <0.2% UNIK 5000 Coriolis mass flowmeter (for refrigerant) 0-1000 kg h-1 <0.05% MASS 6000 Flowmeter (for water) 0-10 m3 h-1 <0.1 m3 h-1 LXSR-E

Specifications of the transducers

Variables Measured points Temperature difference (oC) 30 (35/5) Refrigerant charge/liquid level (m) 0.9 m (optimal value) Height difference (m) 0, 0.3, 0.6, 0.9, 1.2, 1.5 Circulation flow resistance/ valve opening 25%, 50%, 100%

Test conditions

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Contents

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Click to add Title 1 Click to add Title 2 Click to add Title 3 Click to add Title 4

Background Experimental method

Results and discussion

Conclusions

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Results and discussion

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0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 100 200 300 400 500 600 700 800 900

Exp: Valve opening 100% Exp: Valve opening 50% Exp: Valve opening 25%

Heat transfer rate (W) Height difference (m)

Heat transfer rate

 For the loop with larger flow resistance (valve opening 25%), increases continuously .For the

loop with lower flow resistance (valve opening 100% and 75% ) , first increases sharply, then slowly, finally remains constant,

 TPTL performs worse with larger circulation resistance

Condenser Evaporator Valve Height difference

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Results and discussion

15 Traditional idea

Condenser Evaporator Valve Downcomer Liquid film Liquid column

saturated  The downcomer is not always fully liquid filled (traditional idea)  downcomer can be partially liquid filled in some cases, the lower part of the downcomer is full of

liquid while the upper part is only surrounded by a hollow liquid film

 It is just saturated at the outlet of condenser in case of partial liquid filled

Condenser Evaporator Valve Downcomer Liquid column

Partially liquid filled phenomenon in experiment

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Results and discussion

16 Liquid column height in the downcomer

Condenser Evaporator Valve Downcomer Liquid film Liquid column

 First, liquid column keep consistent with height difference, then can not keep up with the

increases of height difference, fully liquid filled turn to partially liquid filled

 the loop with larger circulation resistance (small opening of the valve) has higher liquid column

in the downcomer, and more likely to be fully liquid filled

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Results and discussion

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0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 2 4 6 8 10 12 14

Exp: Valve opening 100% Exp: Valve opening 50% Exp: Valve opening 25%

Mass flow rate (kg h

• 1)

Height difference (m) Not measured be calculated

Refrigerant circulation flow rate

Condenser Evaporator Valve Downcomer Liquid film Liquid column

 When the downcomer is fully liquid filled, with the increase of height difference, the refrigerant

mass flow rate and the heat transfer rate increase sharply, since liquid head increase shsrply

 when the downcomer is partially liquid filled, the refrigerant mass flow rate and the heat

transfer rate rises slowly even remains constant, since the liquid head risers slowly

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Results and discussion

18 Liquid level in the evaporator

 when the height difference is small, especially for the loop with larger circulation resistance, the

liquid level in the evaporator is very low and much refrigerant stays in the condenser

 the liquid head is not sufficient to drive the refrigerant to the evaporator, therefore the TPTL

performs badly

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 0.0 0.1 0.2 0.3 0.4 0.5

Liquid level in the evaporator (m) Height difference (m)

Exp: Valve opening 100% Exp: Valve opening 50% Exp: Valve opening 25%

Liquid level 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 2 4 6 8 10 12 14

Exp: Valve opening 100% Exp: Valve opening 50% Exp: Valve opening 25%

Mass flow rate (kg h

• 1)

Height difference (m) Not measured be calculated

Circulation flow rate

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Results and discussion

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Larger height difference Larger liquid head Larger driving force Better performance Constant liquid head Constant driving force Constant performance

Condenser Evaporator Valve Downcomer Liquid film Liquid column Condenser Evaporator Valve Downcomer Liquid column

Circulation flow resistance

Fully liquid filled Partially liquid filled Larger Smaller

 When the circulation flow resistance is large, and the downcomer is fully liquid filled, larger

height difference, means larger liquid head and driving force, thus better performance

 When the circulation flow resistance is small, and the downcomer is partially liquid filled, larger

height difference does not always lead to larger liquid head

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Conclusions

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 TPTL begins to be widely used in air-conditioning system

 Different features from traditional field (cooling of electronics): small

temperature, small heat flux, small refrigerant charge

 Height difference has great effect on the performance of TPTL

 A visual experimental setup is developed to investigate the effect

• f height difference on the performance of TPTL

 The common sense “ fully liquid filled downcomer” and “ larger height

difference lead to better performance” is not always true in the air- conditioning system

 The downcomer can be “partially liquid filled”, and liquid head remains

constant with the increase of height difference, when the circulation flow resistance is small

 Overall, increase of height difference will enhance the performance, and it is

more obvious when the circulation flow resistance is large

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0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 200 400 600 800 1000

Heat transfer rate /W Static liquid level /m

T=6

• C

T=12

• C

T=18

• C

T=24

• C

T=30

• C

T=36

• C

Height difference: 1.5m Valve opening: 100%

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5 10 15 20 25 30 35 40 0.0 0.5 1.0 1.5

Liquid column height /m Temperature difference (

• C)

1.9m 1.7m 1.5m 1.3m 1.1m 0.9m 0.7m 0.5m Height difference: 1.5 m Valve opening: 100%