Effsys expand May 18, 2016

Numerical study of the transitions of flow boiling in micro-channels Qingming Liu, Björn Palm Department of energy technology KTH

Contents 1 Introduction 2 Numerical methods 3 Boudary conditions 4 Results 5 Conclusions

Micro electronic chips microchannels MEMS(micro electronic and mechanic systems) Micro electronic chips Fuel cells Micro heat exchangers

Contents transitions

Flow boiling in 1,24 mm tube q= 15 kw/m2 Bubbly flow , confined bubbly flow , elongated bubbly flow, annular flow

Numerical methods The Coupled Level Set and VOF method: higher accuracy on the gas-liquid interface curvature simulation than conventional VOF method A Non-equilibrium phase change model: higher accuracy on the interface temperature calculation than the equilibrium model

Validation of the interface curvature model (CLSVOF) Single Taylor bubbles move in a channel

Validation of the interface temperature model A gas bubble grows in super heated liquids 1-D Stefan problem

Single bubble Simulation on the flow and heat transfer characteristics of confined bubbles in micro-channels. ASME 10th International Conference on Nanochannels, Microchannels, and Minichannels, 2012

Effect of surface tension 𝜎𝑐𝑜𝑠𝜃= 𝜎 𝑔𝑠 − 𝜎 𝑙𝑠 (𝜎 =0.0039, 0.0078, 0.0023 𝑛 𝑚 , Inlet Re = 383, D=0.64 mm)

The motion of the bubble can enhancing micro convection by creating eddies between the bubble and wall. Re= 1698 q= 30kw/m2

Results and Discussion Heat transfer Wall temperature fluid temperature Re= 1698 q= 30kw/m2

Validation of the coalescence

Transition 1 A numerical study of bubble coalescence during flow boiling in micro-channels. APPLIED THERMAL ENGINEERING, Apr 2016 Effect of sub cooling on bubble coalescence. Micro and Nano flow conference London, Sept 2014

Three pairs of bubbles Saturated Sub-cooled Comparison with experiment Owaihaib 2009

𝐹𝑖𝑔. 9.𝑇𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒 𝑐𝑜𝑛𝑡𝑜𝑢𝑟 𝑆𝑎𝑡𝑢𝑟𝑎𝑡𝑒𝑑 S𝑢𝑏𝑐𝑜𝑜𝑙𝑒𝑑

Current work Validation The causes of transition (Taylor bubble coalescence)

Adiabatic validation Revellin, R. (2005). "Experimental two-phase fluid flow in microchannels."

A validation case Fig.6. Bubble shapes comparison with an experiment[32], the time span between two consecutive pictures is 0.2 ms . q=180 kw/m2, G=400 kg/m2,D=0.4 mm,Tsat= 304.15 K

Future plan The consequence of coalescence: 1 effect on heat transfer, 2 bubble terminal velocity 3 film thickness.