Flow Boiling in Microchannels,Abhiishek Velichala Anand Vijaykumar Eniola Eniket Nellie Rajarova,Courtesy- M K Moharan (IITK),INTRODUCTION,What is a Microchannel ?Need to dissipate high heat fluxes in MEMS and electronic cooling devices.Pioneering work on Microchannels: Investigated the upper limit of burnout conditions on small diameter tubes1964. Tuckerman and Pease demonstrated the importance of microchannels in cooling of integrated circuitry1981. Classification :,Flow Boiling in a Microchannel,Flow boiling in a channel is nothing but boiling occurring due the flow of a liquid with certain velocity whose walls are subjected to heatMicrochannel flows are driven by pumps with mean flow velocities in the range of few millimeters per second to many centimeters per secondFlow in microchannels will be mainly laminar,Microchannel Vs Conventional Channel,The 2 phase heat transfer correlations for microchannels are different from conventional channelsSurface tension forces are more dominant and gravity forces are negligible in microchannelsSeparate studies need to be conducted for microchannels,Why microchannel? Features and motivation,Higher Surface Density Higher Heat Transfer Coefficients(h=Nu.k/D, Nu = constant for laminar flow) Low Thermal Resistance Volumetric heat transfer rate depends inversely on square of the channel diameter,Non-dimensional groups,Some of the important non-dimensional numbers in flow boiling through microchannel are given below:Capillary number , Ca = V / Weber number, We = Dh G2/K1 = (q/Ghfg )2 * (l / g)K2 = (q/hfg )2 * (D/ g )why do we need dimensionless groups ?They are useful at arriving at key basic relationships among system variables that are valid for different fluids under different operating conditions.,FLOW BOILING REGIMES Ref - Effects of channel dimension, heat flux, and mass flux on flow boiling regimes in microchannels -Tannaz Harirchian, Suresh V. Garimella,Heat Transfer Mechanism,Flow boiling heat transfer in microchannel is often assumed to be the result of two different mechanisms, nucleate boiling and convective boiling,Rapid Evaporation (Nucleate Boiling),Liquid Motion (convective Boiling),Courtesy- Kandlikar,Continued,The local heat transfer coefficient (h) is calculated as a sum of the two contributions i.e., nucleate and convectiveCorrelations have been developed to find out the h value for different flow conditionsFor low heat fluxes, convective heat transfer mechanism is dominant whereas nucleate boiling was the dominant mode of heat transfer for high heat fluxesFor Re 300, flow boiling mechanism is nucleate boiling dominant . Since most of the flows through microchannels have low Re, heat transfer during flow boiling in microchannels is seen to be nucleate boiling dominant,Effect of Parameters,Effect of Contact Angle Effect of gravity Effect of surface tension Effect of channel dimensions Effect of quality Effect of Heat Flux on convective heat transfer coefficient Effect of mass flux , Effect of molecular Mass(M) Effect of Buoyancy Effect of Tsat , Tsuper, Reynolds Number, Tsub,Difference in channel dimension macro and micro,Flow Instability,Instability occurs due to rapid expansion of nucleating bubble. The bubble expansion in the reverse direction to the overall flow direction, and introduction of vapor into the inlet manifold. Instabilities can be characterized by flow visualizations and pressure drop fluctuations.,Methods to reduce Instabilities,Introduction of artificial nucleation cavities of the right size. Introduction of pressure drop elements at the entrance to each channel is expected to reduce the reverse flow condition. The PDEs introduce a significant increase in the flow resistance in the reversed flow direction.,CHF in microchannels,CHF is one of the most important thermal-hydraulic transition phenomena in flow/pool boiling and is of significant engineering importance.It sets the upper limit of heat flux for many engineering systems and marks the transition from a very effective heat transfer mode to a very ineffective one. The occurrence of CHF must be regarded as an undesirable condition, as it will cause overheating of an individual channel or even the entire substrate containing the microchannels. The transition corresponds to dryout of the liquid film on the tube wall.,The sharp reduction in the local heat transfer coefficient follows CHF conditions. Parameters influencing ChF: Tube Diameter, Channel length Inlet Subcooling Satuaration Temperature Mass Flux Design with the following combination of characteristicsshort channel length, (ii) a low saturation temperature, (iii) a large mass flux, (iv) a large subcooling, and (v) a large microchannel diameter for the chosen fluid. Instability and experimental uncertainties are responsible for the low values of CHF reported in literature.,Variation of heat transfer coefficient and wall temperature with wall heat flux in the 400 m x 400 m microchannels, G = 630 kg/m2s (Harirchian and Garimella 2008a).,