On the heat transfer characteristics of liquid-gas taylor flows in minichannels

Heat transfer enhancement is of significant importance to many thermal management applications. Multi phase flows are an effective way to meet increasing thermal demands. This work investigates heat transfer enhancement potential associated with slug or Taylor flows within minichannels. The primary focus is upon understanding the mechanisms leading to enhanced heat transfer and the effects of using different liquid phases and varying flow parameters. High speed images were used to obtain mean bubble velocity, liquid film thickness and void fraction measurements within a 1/16" diameter channel. Pressure drop data was obtained using a differential pressure transducer while infrared thermography was used to capture high resolution experimental wall temperatures subjected to a constant heat flux boundary condition. Experiments were conducted for a range of liquid slug lengths and void fractions, ensuring results spanned both the thermal entrance and thermally developed flow regions. Flow dynamics, pressure drop and heat transfer characteristics were investigated using water and oils as the liquid phase while air was used as the segmenting phase throughout. An added novel aspect to this work is incorporating microencapsulated phase change material (MPCM) suspensions into conventional liquid-gas flows. Interfacial tension between the phases and hence, Capillary number was observed to have a significant influence on the both the flow structure and associated pressure drop. Phase change particles are seen to result in significantly increased interfacial pressure drop contributions. Nusselt number enhancements were observed throughout when data was reduced to account for void fraction but the gaseous void was also noted to increase as the bubble moved along the channel, due to increased pressure drop and compressibility effects. As a result, mean length void fraction values across the heated test section were required to collapse experimental data in place of the dynamic gas quality. Some scatter is observed in thermally developed Nu values and is believed to be a result of Capillary number effects. Nu oscillations that are seen for water-air slug flows are observed to dampen for flows with increased film thickness magnitudes. Heat transfer characteristics of MPCM suspension flows were investigated with mass particle concentrations ranging from 5.03-30.2%. Deviation from the Graetz solution was observed during the phase change region. This was caused by wall temperatures reaching the onset melt temperature before the bulk mean of the fluid and this is due to temperature gradients in the radial direction. A novel model was developed and validated to predict local Nu values which utilised an asymptotic limit blending approach. MPCM slug flows were observed to result in higher heat transfer rates in the thermal entrance region. It is postulated that this results from surfactants inducing flow within the liquid film. Thermally developed Nu values are a combination of individual enhancements due to fluid recirculation within the liquid slugs and absorption of latent heat. Enhancement due to latent heat is also observed to be a function of void fraction within the flow. The ratio of hydraulic pumping power to the heat removal rate was used in an optimisation procedure to determine most favourable flow parameters. By comparing the slug flow regime and single phase Hagen-Poiseuille flow under the same conditions, an optimum was identified. It was determined that void fraction should be kept at a minimum and optimum slug length was dependent on inverse Graetz number and the ratio of Capillary to Reynolds number. Maximum enhancement for slug flows over Poiseuille flow was observed for increasing values of both x* and Ca/Re. Design maps were put forward which identify the level of enhancement attainable for specific flow configurations with up to 50% reduction in the ratio of (Q 􀀆 pump/Q 􀀆 heat)tp/sp being attainable. Slug flows demonstrate significant enhancements over conventional single phase flows, hence, demonstrating potential for the use in thermal management applications.