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 Department of Building Science Tsinghua University Speaker: Zhang Penglei , a PhD candidate 1

Contents Click to add Title Background 1 Click to add Title 2 Experimental method Click to add Title 3 Results and discussion Click to add Title 4 Conclusions 2 Tsinghua University

Background  wickless gravity assisted heat pipe, two-phase natural circulation loop, separated heat pipe Condenser  Consists of evaporator, condenser, riser, and Riser 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 Downcomer High heat transfer coefficient Excellent isothermality q e Evaporator Flexibility Relatively long distance Natural circulation without pump, compressor Two-phase thermosyphon loop (TPTL) 3 Tsinghua University

Background-traditional TPTLs Condenser Riser Downcomer q e Evaporator  Fixed heat flux  Large heat flux (>20 kW/m)  Large temperature difference (>40C)  Large refrigerant charge to avoid Cooling of electronic components/ light dryout water reactor 4  Larger condenser than evaporator Tsinghua University

Background-TPTLs in air-conditioning system Fresh air 6 Exhausted air IDC IT device Free cooling for spaces with large Heat recovery for free rejected heat (computer rooms, Internet data centers)  Small heat flux (<10 kW/m)  Small temperature difference (<30 o C)  Dryout is allowable, charge should be moderate 5 Tsinghua University

Background-effect of height difference Larger height difference Q / M ? Larger liquid head ? Height Larger driving difference force ∆ H ? Δ H Better performance  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??? 6 Tsinghua University

Contents Click to add Title Background 1 Click to add Title Experimental method 2 Click to add Title Results and discussion 3 Click to add Title Conclusions 4 7 Tsinghua University

Experimental setups T T Differential ∆ P Condenser pressure gauge T T T T Riser T Water flowmeter Valve T F P T Pump T Pressure gauge T Evaporator T Low temperature T T T water tank T Water Downcomer flowmeter T F Pump T F High temperature Coriolis Mass Flowmeter water tank Schematic diagram of the experimental setups 8 Tsinghua University

Experimental setups Refrigerant inlet Refrigerant outlet T18 T9 Water Water T10 T5 outlet outlet 12 12 T11 T4 16 16 Liquid T15 T12 500 T16 T6 T3 T7 level detector 450 T13 T2 T17 T8 T14 T1 Water Water inlet inlet Refrigerant outlet Refrigerant inlet Evaporator Condenser 9 Tsinghua University

Experimental setups ∆ P Riser Ball valve F P Lifter  Riser and downcomer: Transparent flexible pipe ( Φ 12) ， PU pipe Downcomer  The condenser is fixed on a lifter, F height difference 0-1.5 m F  A ball valve is set at the outlet of evaporator to change the circulation flow resistance 10 Tsinghua University

Experimental setups 1000 2.0% Pressure drop 800 Max. error Max. error (% of flowrate) ∆ P 1.5% Pressure drop (Pa) 600 1.0% 400 0.5% T out P 200 0 0.0% 0 10 20 30 40 50 Flowrate (kg h -1 ) T in Pressure drop and maximum error of the Coriolis flowmeter F V F  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    Q c V t t ( ) P in o u t e Coriolis mass flowmeter  Liquid level and flow pattern is observed 11 Tsinghua University

Experimental setups Specifications of the TPTL heat exchanger Items Specifications 3 parallel double pipes; Pipe length: 0.45 m Evaporator Diameter of inner pipe: 12 mm; Diameter of outer pipe: 16 mm; 3 parallel double pipes; Pipe length: 0.45 m Condenser 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 transducers Items Range Uncertainty Type -10-50 o C <0.1 o C Thermocouple T type Pressure transducer 0-2.5 MPa <0.2% UNIK 5000 Differential pressure transducer 0-50 kPa <0.2% UNIK 5000 0-1000 kg h -1 Coriolis mass flowmeter (for refrigerant) <0.05% MASS 6000 0-10 m 3 h -1 <0.1 m 3 h -1 Flowmeter (for water) LXSR-E Test conditions Variables Measured points Temperature difference ( o C) 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 12 Tsinghua University Circulation flow resistance/ valve opening 25%, 50%, 100%

Contents Click to add Title Background 1 Click to add Title Experimental method 2 Click to add Title Results and discussion 3 Click to add Title Conclusions 4 13 Tsinghua University

Results and discussion 900 800 Condenser 700 Heat transfer rate (W) 600 500 Height 400 difference 300 Valve 200 Exp: Valve opening 100% 100 Exp: Valve opening 50% Evaporator Exp: Valve opening 25% 0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 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 14 Tsinghua University

Results and discussion saturated Condenser Condenser Liquid Downcomer Downcomer film Valve Valve Liquid column Liquid column Evaporator Evaporator Partially liquid filled phenomenon in experiment Traditional idea  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 15 Tsinghua University  It is just saturated at the outlet of condenser in case of partial liquid filled

Results and discussion Condenser Liquid Downcomer film Valve Liquid column Evaporator Liquid column height in the downcomer  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 16 Tsinghua University in the downcomer, and more likely to be fully liquid filled

Results and discussion 14 Exp: Valve opening 100% Exp: Valve opening 50% Exp: Valve opening 25% 12 Condenser -1 ) 10 Mass flow rate (kg h 8 Liquid Downcomer film 6 Valve 4 Not measured be calculated 2 Liquid column Evaporator 0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 Height difference (m) Refrigerant circulation flow rate  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 17 Tsinghua University

Results and discussion 0.5 14 Exp: Valve opening 100% Exp: Valve opening 100% Exp: Valve opening 50% Exp: Valve opening 50% Exp: Valve opening 25% Exp: Valve opening 25% 12 0.4 Liquid level in the evaporator (m) -1 ) 10 Mass flow rate (kg h 0.3 8 6 Liquid 0.2 level 4 Not measured be calculated 0.1 2 0 0.0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 Height difference (m) 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 Height difference (m) Liquid level in the evaporator Circulation flow rate  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 18 Tsinghua University

Results and discussion Larger height difference Condenser Condenser Larger Smaller Circulation flow Larger liquid Constant resistance Liquid head liquid head Downcomer Downcomer film Larger driving Constant Valve Valve force driving force Liquid column Liquid column Evaporator Evaporator Better Constant performance performance Fully liquid filled Partially liquid filled  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 19 Tsinghua University height difference does not always lead to larger liquid head