Heat Transfer Performance of Pulsating - PowerPoint PPT Presentation

Study on the Surfactant Influence on the 以直线发电机为负载的热声斯特林 Heat Transfer Performance of Pulsating 发动机输出特性研究 Heat Pipe Xuehui Wang 1,2 , Bo Li 1 , Yuying Yan 1 , Xiaohong Han 2 , Guangming Chen 2 1. Fluids and Thermal Engineering Research Group, University of Nottingham, UK, NG7 2RD 2. Institute of Refrigeration and Cryogenics, Zhejiang University, Hangzhou, China 310051 Second ThermaSMART Annual Workshop, Kyushu University, Japan

Contents Introduction & Background Simulation/Experiment Results & Conclusions Discussions

1. Introduction & Background  Working principle: Due to the influence of the surface tension, a train of vapor slugs and liquid plugs will be formed once charging a small channel by a fluid The input heat in EV section will cause the un-balance of the slugs The heat will be dissipated by the oscillation of the slugs The surface tension, unbalanced oscillation motions define the PHP

1. Introduction & Background  The heat transfer performance of the PHP is greatly influenced by many parameters, and they can be divided into three groups Geometric Physical Operational parameters Parameters Properties Inner diameter; Surface tension; Charge ratio; Cross-section shape; Thermal conductivity; Inclination angle; Channel configuration; Specific heat; Gravity field; Number of turns; Viscosity; Heat flux; Length of section; … … … … … ...  Few studies showed the influence of surface tension on the heat transfer performance of PHP

2. Simulation/Experiment  Theoretical analysis on heat transfer For the liquid slug with two adjacent vapor plugs  Mass conservation  Momentum conservation  Energy conservation  Mass balance

2. Simulation/Experiment For the vapor plug with two adjacent liquid slugs  Mass conservation 液塞1 Q dm dm 2   v l v  tf i j i dt dt h q  j 1 fg Q vl m 1  Energy conservation 气塞  Q dm h + Q d v v fg v w i i i  k T ( T ) Q vl m 2  m h '' l w lv fg  液塞2  T T      Q 2 ( R ) L wi vi v w tf v l  i tf  u   2/3 1.34 Ca  Ca   2/3 r 1 3.35 Ca Thin film area  p ( T ) ˆ 2 M p  0.5 v _equ lv  m '' ( ) ( v )   ˆ  0.5 0.5 2 2 R T T lv v

2. Simulation/Experiment  The heat transfer of the liquid plug and vapor plug are analyzed based on the model presented Fig. 1 For a vapor plug Fig. 2 For a liquid slug        T T 10, T T T , T 10; p p 100 Pa        T T 10, T T T , T 10; p p 100 Pa w l l lv lv v d c w l l lv lv v d c For a typical long vapor plug, the heat transfer in the liquid conduction contributes a lot. For a liquid slug, the convection heat transfer ratio is great. However, the contribution of heat transfer in meniscus also play an important role, which is greatly influenced by surface tension of the working fluid .

2. Simulation/Experiment  Theoretical analysis on the force The length of slug/plugs are randomly and small compared with the length of pipe. So it is assumed the length distribution functions of the slugs in every single channel are the same  Force/Pressure drop caused by U-turn  Force/pressure drop by the gravity  Force/pressure drop by the flow resistance  Force/pressure drop by the capillary force F pf F f F C

2. Simulation/Experiment  Theoretical analysis on the force F F    F ' F F F   2 =  g 1 =  g f F ' F ' 0.50 0.6 surface tension=70mN/m L=10d, gravity 0.45 surface tension=60mN/m L=10d, capillary resistance surface tension=50mN/m Ratio of capillary resistance 0.5 L=5d, gravity surface tension=40mN/m 0.40 L=5d, capillary resistance 0.4 0.35 Ratio 0.30 0.3 0.25 0.2 0.20 0.15 0.1 1 2 3 4 5 1 2 3 4 5 Velosity(m/s) Velosity(m/s) For typical liquid slug, the capillary resistance could be the same level of the gravity of the it. Meanwhile, with the decrease of the surface tension of the working fluid the ratio of the capillary tension sufficiently decrease.

2. Experimental rig  The rig for the experimental rig is shown below Data collecting Cooling module module PHP module Heating module  Criteria for the performance (superheat) (thermal resistance)

3. Results &Discussions  The influence of the surfactant on the startup characteristics of the PHP 80 DI water DI water 80 10 ppm concentration 10 ppm concentration 60 20 ppm concentration 40 ppm concentration 20 ppmconcentration Superheat(K) 40 40 ppmconcentration 60 startup 20 Startup q=637 q=955 q=1274 q=1592 q=1911 Superheat(K) 0 0 2000 4000 6000 8000 40 Time(s) DI water 80 10 ppm concentration startup 20 ppm concentration 60 20 40 ppm concentration Superheat(K) 40 q=1592 q=1274 q=1911 q=955 q=637 20 0 q=637 q=955 q=1274 q=1592 q=1911 0 0 2000 4000 6000 8000 10000 0 2000 4000 6000 8000 Time(s) Time(s) The surfactant solution decreases the start up power of the pulsating heat pipe

3. Experimental results  The influence of the surfactant on the heat transfer performance of the PHP 1.6 1.0 DI water DI water 1.4 Thermal resistance(K/W) 10 ppm concnetration 0.8 10 ppm concentration 20 ppm concnetration 40 ppm concnetration 20 ppm concentration 0.6 1.2 Thermal resistance(K/W) 40 ppm concentration 0.4 1.0 0.2 0 4000 8000 12000 16000 20000 2 ) heat flux ( W/m 0.8 1.4 DI water 10 ppm concentration 1.2 0.6 20 ppm concentration Thermal resistance(K/W) 40 ppm concentration 1.0 0.8 0.4 0.6 0.4 0.2 0.2 0 4000 8000 12000 16000 20000 0 4000 8000 12000 16000 20000 2 ) Heat flux ( W/m 2 ) Heat flux ( W/m The existence of the surfactant greatly enhances the heat transfer of the PHP

2. Simulation/Experiment  The influence of the surfactant on the dry-out characteristics of the PHP  The influence of the surfactant on the dry-out characteristics of the PHP 180 180 DI water DI water 10 ppm concentration 20 ppm concentration dryout 10 ppm concentration 40 ppm concentration q=19427 150 Temperature( ℃ ) 20 ppm concentration 160 q=11465 q=13057 q=14650 q=16242 q=17384 dryout 40 ppm concentration 120 Temperature( ℃ ) 140 90 0 2000 4000 6000 8000 10000 Time(s) DI water 120 180 10 ppm concentration 20 ppm concentration q=17834 q=19427 q=21019 40 ppm concentration Temperature(℃) 160 dryout q=13057 q=14650 q=16242 140 100 120 q=16242 q=9873 q=11465 q=13057 q=14650 100 80 0 2000 4000 6000 8000 10000 0 2000 4000 6000 8000 Time(s) Time(s) The existing of surfactant increases the dry-out heat flux of the pulsating heat pipe

3. Conclusions For the PHP with surfactant solutions, they start at a lower heat flux. Furthermore, the temperature fluctuates at a lower level.  Decrease the superheat of the bubble The heat transfer performance of the PHP  Better wetting of the wall significantly improves by using surfactant solutions as the working fluid  Decrease the capillary resistance The experimental results indicated that the dry- out heat fluxes are higher when the working fluids are surfactant solutions

4. Discussions 1. Is there a ‘best condition’ or ‘dead condition’ for the PHP? Best: making best use of the heat; Dead: the unbalanced flow is balanced Input heat Charging a pipe Heat dissipated Plugs and slugs Oscillation motions positive EV negative

4. Discussions 2. Whether the initial distribution of the vapor plug and liquid slug affect the performance of the PHP? If so, how to build the analytical model? length