Comparative study of heat transfer and pressure drop during flow boiling and flow condensation in minichannels

This paper was presented at the 4th Micro and Nano Flows Conference (MNF2014), which was held at University College, London, UK. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute, ASME Press, LCN London Centre for Nanotechnology, UCL University College London, UCL Engineering, the International NanoScience Community, www.nanopaprika.eu.In the paper a method developed earlier by authors is applied to calculations of pressure drop and heat transfer coefficient for flow boiling and also flow condensation for some recent data collected from literature for such fluids as R245fa, R600a, R134a, R1234yf and other. The modification of interface shear stresses between flow boiling and flow condensation in annular flow structure is considered through incorporation of the so called blowing parameter. The shear stress between vapor phase and liquid phase is generally a function of non-isothermal effects. The mechanism of modification of shear stresses at the vapor-liquid interface has been presented in detail. In case of annular flow it contributes to thickening and thinning of the liquid film, which corresponds to condensation and boiling respectively. There is also a different influence of heat flux on the modification of shear stress in the bubbly flow structure, where it affects the bubble nucleation. In that case the effect of applied heat flux is considered. As a result a modified form of the two-phase flow multiplier is obtained, in which the non-adiabatic effect is clearly pronounced

A numerical study of bubble growing during saturated and sub-cooled flow boiling in micro channels

This paper was presented at the 4th Micro and Nano Flows Conference (MNF2014), which was held at University College, London, UK. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute, ASME Press, LCN London Centre for Nanotechnology, UCL University College London, UCL Engineering, the International NanoScience Community, www.nanopaprika.eu.A CFD study of bubbles growing in a mini-channel with a diameter of 0.64 mm has been done. Coupled level set and volume of fluid (CLSVOF) method is applied to capture the two phase interface. Geo-reconstruct method is used to re-construct the two-phase interface. A constant velocity inlet boundary with mass flux 335 /2 and a heated boundary wall with constant heat flux (10/2 ) is applied. Both saturated and sub-cooled inlet condition are studied. The growth of bubbles and the transition of flow regime differs each other under these two conditions. Sub-cooling significantly lowers the bubble growth rate. However, it does not affect the heat transfer coefficient at the same level due to its complicated heat transfer mechanism

Parametric effects on dryout of propane in a vertical circular mini-channel

This paper was presented at the 3rd Micro and Nano Flows Conference (MNF2011), which was held at the Makedonia Palace Hotel, Thessaloniki in Greece. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, Aristotle University of Thessaloniki, University of Thessaly, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute.This article presents dryout results of propane in a vertical circular mini channel made of stainless steel with an internal diameter of 1.70 mm and a heated length of 245 mm. The experiments are performed at three saturation temperatures of 23 degrees C, 33 degrees C and 43 degrees C. Mass flux is varied from 100 kg/m2s to 500 kg/m2s.The heat flux is increased in steps up to occurrence of dryout. The effect of different parameters such as mass flux, vapour quality and saturation temperature on the dryout heat flux is investigated. The results show that the dryout heat flux increases with the increase in mass flux and with the decrease of vapour quality. Almost no effect of saturation temperature on the dryout heat flux is observed. The generalised CHF correlations developed for macro and micro scale from the literature are compared with experimental results. Correlations developed by Callizo et al. (2008), Bowring (1972) and Katto and Ohno (1984) gave reasonably good predictions

Flow boiling pressure drop of R134a in micro diameter tubes: Experimental results and assessment of correlations

This paper was presented at the 3rd Micro and Nano Flows Conference (MNF2011), which was held at the Makedonia Palace Hotel, Thessaloniki in Greece. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, Aristotle University of Thessaloniki, University of Thessaly, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute.This article has been made available through the Brunel Open Access Publishing Fund.The experimental results of two phase flow boiling pressure drop of R134a in vertical micro diameter stainless steel tubes are presented in this paper. The tests were conducted using four tubes; one tube with an inner diameter of 0.52 mm and 100 mm heated length and three tubes with an inner diameter of 1.1 mm and different heated lengths (150, 300 and 450 mm). Other experimental conditions include: mass flux range of 200 – 500 kg/m2 s, system pressure range of 6 – 10 bar, inlet sub-cooling of about 5K and heat flux range of 1 – 140 kW/m2. The results indicated that the total measured two phase pressure drop increases with increasing mass flux, heat flux (exit quality) and decreasing system pressure and tube inner diameter. The test section heated length was found to have a significant effect on the measured pressure drop per metre length. The total measured two phase pressure drop results were also compared with eighteen macro and micro scale models and correlations

Experimental investigation of two-phase pressure drop in a microchannel

This paper was presented at the 2nd Micro and Nano Flows Conference (MNF2009), which was held at Brunel University, West London, UK. The conference was organised by Brunel University and supported by the Institution of Mechanical Engineers, IPEM, the Italian Union of Thermofluid dynamics, the Process Intensification Network, HEXAG - the Heat Exchange Action Group and the Institute of Mathematics and its Applications.An experimental study of two-phase pressure drop in a horizontal circular microchannel is presented in this paper. Test tube is made of fused silica having an internal diameter of 781μm with a total length of 261mm and a heated length of 191mm.The external surface of the test tube is coated with an electrically conductive thin layer of ITO (Indium Tin Oxide) for direct heating of test section. Refrigerant R134a was used as the working fluid and mass flux during the experiments was varied from 100 to 650 kg/m2sec. Experiments were performed at a system pressure of 7.70 bar (corresponding to saturation temperature of 30oC). Two-phase frictional pressure drop characteristics with different mass flux, vapour fraction and heat flux were explored in detail. Finally, the prediction capability of well known available correlations in the literature, developed for macrochannels and others especially developed for microchannels was also checked. The homogeneous model predicts the data fairly well with a mean absolute deviation (MAD) of 19% and 69% of data within ±20% error band. The Müller-Steinhagen and Heck (1986) correlation developed for macrochannels predicts the data with a MAD of 19% and 61% of data within ±20% error band. The Mishima and Hibiki (1996) correlation, developed for microchannels, also shows fairly good approximation of the data with MAD of 19% and 57% of data within ±20% error band

Pressure drop across micro-pin heat sinks under boiling conditions

Two-phase pressure drop was studied in four different micro pin fin heat sinks. Micro pin fin heat sinks used in the current studies were operated under boiling conditions using water and R-123 as working fluids. It was observed that once boiling was initiated severe temperature fluctuations and flow oscillations were recorded for three of the micro pin fin heat sinks, which was characterized as unstable boiling. Pressure drop signals were presented just before and after the unstable boiling conditions. Flow images and FFT (fast Fourier Transform) profiles of pressure signals were used to explain experimental results and unstable nature in flow boiling observed in the three of the devices. Stable boiling conditions where the temperature and pressure drop had a steady and stable profile could be only obtained from one micro pin fin heat sink at high mass velocities. The two-phase pressure drop in this hydrofoil-based micro pin fin heat sink has been investigated using R-123 as the working fluid. Two-phase frictional multipliers have been obtained over mass fluxes from 976 to 2349 kg/m2. It has been found that the two-phase frictional multiplier is strongly dependent on flow pattern. The theoretical prediction using Martinelli parameter based on the laminar fluid and laminar gas flow represented the experimental data fairly well for the spray-annular flow. For the bubbly and wavy-intermittent flow, however, large deviations from the experimental data were recorded. The Martinelli parameter was used successfully to determine the flow patterns, which were bubbly, wavy-intermittent, and spray-annular flow in the current study

Characterization of Two-Phase Flow in Microchannels

Aluminum multi-port microchannel tubes are currently utilized in automotive air conditioners for refrigerant condensation. Recent research activities are directed toward developing other air conditioning and refrigeration systems with microchannel condensers and evaporators. Three parameters are necessary to analyze a heat exchanger performance: heat transfer, pressure drop, and void fraction. The purpose of this investigation is the experimental investigation of void fraction and frictional pressure drop in microchannels. A flow visualization analysis is another important goal for two-phase flow behavior understanding and experimental analysis. Experiments were performed with a 6-port and a 14-port microchannel with hydraulic diameters of 1.54 mm and 1.02 mm, respectively. Mass fluxes from 50 to 300 kg/s.m2 (range of most typical automotive applications) are operated, with quality ranging from 0% to 100% for two-phase flow experiments. R410A, R134a, and air-water mixtures are used as primary fluids. The results from the flow visualization studies indicate that several flow configurations may exist in multi-port microchannel tubes at the same time while constant mass flux and quality flow conditions are maintained. Flow mapping of the fluid regimes is accomplished by developing functions that describe the fraction of time or the probability that the fluid exists in an observed flow configuration. Experimental analysis and flow observations suggest that pressure drop and void fraction in microchannel is dependent on the most probable flow regime at which the two-phase mixture is flowing. In general, correlations for void fraction and pressure drop predictions are based in a separated flow model and do not predict the experimental results in the range of conditions investigated. A flow regime based model is developed for pressure drop and void fraction predictions in microchannels.Air Conditioning and Refrigeration Project 10

Supersonic wind tunnel nozzles: A selected, annotated bibliography to aid in the development of quiet wind tunnel technology

This bibliography, with abstracts, consists of 298 citations arranged in chronological order. The citations were selected to be helpful to persons engaged in the design and development of quiet (low disturbance) nozzles for modern supersonic wind tunnels. Author, subject, and corporate source indexes are included to assist with the location of specific information

One-dimensional mechanistic model for flow boiling pressure drop in small- to micro- passages

This paper was presented at the 2nd Micro and Nano Flows Conference (MNF2009), which was held at Brunel University, West London, UK. The conference was organised by Brunel University and supported by the Institution of Mechanical Engineers, IPEM, the Italian Union of Thermofluid dynamics, the Process Intensification Network, HEXAG - the Heat Exchange Action Group and the Institute of Mathematics and its Applications.Accurate predictions of two-phase pressure drop in small to micro diameter passages are necessary for the design of compact and ultra-compact heat exchangers which find wide application in process and refrigeration industries and in cooling of electronics. A semi-mechanistic model of boiling two-phase pressure drop in the confined bubble regime is formulated, following the three-zone approach of Thome et al. (2004) for heat transfer. The total pressure drop is calculated by time-averaging the respective pressure drop values of single-phase liquid, elongated bubble with a thin liquid film and single-phase vapour. The model results were compared with experimental data collected for a wide range of diameter tubes (4.26, 2.88, 2.02, 1.1 and 0.52 mm) for R134a at 6 – 12 bar

Microfluidic systems for in situ formation of nylon 6,6 membranes.

A microfluidics based, localised formation of nylon 6,6 membranes has been undertaken. The study demonstrates the feasibility of maintaining stable aqueous/organic interfaces for xylene within simple linear flow channels. Glass fabricated structures were used with adipoyl chloride and hexamethylenediamine in the organic and aqueous phases, respectively, in order to achieve nylon 6,6 interfacial polymerisation. Localised membrane formation was investigated in flow channels of different geometries over a wide range of flow rates (500–4000 μl/min), with Reynolds numbers ranging from 8.4 to 67.2. The results demonstrate that interfacial polymerisation occurs consistently over a wide range of flow rates and of flow entry angles for dual aqueous/organic solvent input. However, creation of uniform planar film structures required careful optimisation, and these were best achieved at 2000 μl/min with a flow entry angle of 45°. The resulting membranes had thicknesses in the range between 100 and 300 μm. Computational modelling of the aqueous/organic flow was performed in order to characterise flow stability and wall shear-stress patterns. The flow arrangement establishes a principle for the fabrication of micromembrane structures designed for low sample volume separation, where the forming reaction is a facile and rapid interfacial process