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  • 1.
    Ali, Rashid
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Phase Change Phenomena During Fluid Flow in Microchannels2010Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Phase change phenomena of a fluid flowing in a micro channel may be exploited to make the heat exchangers more compact and energy efficient. Compact heat exchangers offer several advantages such as light weight, low cost, energy efficiency, capability of removing high heat fluxes and charge reduction are a few to mention. Phase change phenomena in macro or conventional channels have been investigated since long but in case of micro channels, fewer studies of phase change have been conducted and underlying phenomena during two-phase flow in micro channels are not yet fully understood. It is clear from the literature that the two-phase flow models developed for conventional channels do not perform well when extrapolated to micro scale.

    In the current thesis, the experimental flow boiling results for micro channels are reported. Experiments were conducted in circular, stainless steel and quartz tubes in both horizontal and vertical orientations. The internal diameters of steel tubes tested were 1.70 mm, 1.224 mm and the diameter of quartz tube tested was 0.781 mm. The quartz tube was coated with a thin, electrically conductive, transparent layer of Indium-Tin-Oxide (ITO) making simultaneous heating and visualization possible. Test tubes were heated electrically using DC power supply. Two refrigerants R134a and R245fa were used as working fluids during the tests. Experiments were conducted at a wide variety of operating conditions.

    Flow visualization results obtained with quartz tube clearly showed the presence of confinement effects and consequently an early transition to annular flow for micro channels. Several flow pattern images were captured during flow boiling of R134a in quartz tube. Flow patterns recorded during the experiments were presented in the form of Reynolds number versus vapour quality and superficial liquid velocity versus superficial gas velocity plots. Experimental flow pattern maps so obtained were also compared with the other flow pattern maps available in the literature showing a poor agreement. Flow boiling heat transfer results for quartz and steel tubes indicate that the heat transfer coefficient increases with heat flux and system pressure but is independent on mass flux and vapour quality. Experimental flow boiling heat transfer coefficient results were compared with those obtained using different correlations from the literature. Heat transfer experiments with steel tubes were continued up to dryout condition and it was observed that dryout conditions always started close to the exit of the tube. The dryout heat flux increased with mass flux and decreased with exit vapour quality. The dryout data were compared with some well known CHF correlations available in the literature. Two-phase frictional pressure drop for the quartz tube was also obtained under different operating conditions. As expected, two-phase frictional pressure drop increased with mass flux and exit vapour quality.

  • 2.
    Ali, Rashid
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Palm, Björn E.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Dryout Characteristics During Flow Boiling of R134a in Vertical Circular Minichannels2011In: International Journal of Heat and Mass Transfer, ISSN 0017-9310, E-ISSN 1879-2189, Vol. 54, no 11-12, p. 2434-2445Article in journal (Refereed)
    Abstract [en]

    In this paper, the experimental results of dryout during flow boiling in minichannels are reported and analysed. Experiments were carried out in vertical circular minichannels with internal diameters of 1.22 mm and 1.70 mm and a fixed heated length of 220 mm. R134a was used as working fluid. Mass flux was varied from 50 kg/m(2) s to 600 kg/m(2) s and experiments were performed at two different system pressures corresponding to saturation temperatures of 27 degrees C and 32 degrees C. Experimental results show that the dryout heat flux increases with mass flux and decreases with tube diameter while system pressure has no clear effect for the range of experimental conditions covered. Finally, the prediction capabilities of the well known critical heat flux (CHF) correlations are also tested.

  • 3.
    Ali, Rashid
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Palm, Björn E.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Claudi, Martin-Callizo
    Maqbool, Mohammad H.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Flow Patterns and Flow Pattern Maps for Microchannels2010In: 2010 3rd International Conference on Thermal Issues in Emerging Technologies, Theory and Applications - Proceedings, ThETA3 2010, 2010, p. 33-42Conference paper (Refereed)
    Abstract [en]

    Dense packaging of electronic components generates very high heat fluxes and therefore results in challenges for proper thermal management of such components. Microchannel based evaporators with phase changing liquids are regarded as a promising solution for such high heat flux cooling applications. Due to confinement of flow and differences in the relative importance of governing phenomena, the two-phase flow and heat transfer characteristics of microchannels have been shown to be different from those of conventional sized channels. The fact that microchannel is an attractive cooling option but at the same time there is a clear lack of understanding of related hydrodynamic and thermal transport phenomena which provides an impetus for microchannel research. This paper presents the flow patterns and flow pattern maps obtained for an experimental study of R134a during flow boiling in a horizontal microchannel. The microchannel was a fused silica tube, the outer surface of which was coated with thin, transparent and electrically conductive layer of Indium-Tin-Oxide (ITO). The microchannel was 781 m in internal diameter and 191 mm in heated length. Operating parameters during the experiments were: mass flux 100-400 kg/m2 s, heat flux 5-45 kW/m2, saturation temperature 25 and 30 °C. A High speed camera was used with a close up lens to capture the flow patterns evolved along the channel. Flow pattern maps are presented in terms of superficial gas and liquid velocity and in terms of Reynolds number and vapor quality plots. The results are compared with some flow pattern maps for conventional and micro scale channels available in literature.

  • 4.
    Ali, Rashid
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Palm, Björn E.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Maqbool, Mohammad H.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    A Visualization Study During Flow Boiling of R134a In A Horizontal Microchannel2010In: ASME 2010 8th International Conference on Nanochannels, Microchannels, and Minichannels Collocated with 3rd Joint US-European Fluids Engineering Summer Meeting, ICNMM2010, 2010, p. 85-94Conference paper (Refereed)
    Abstract [en]

    In this paper, the experimental flow boiling visualization results of a microchannel are presented and discussed. A series of visualization experiments have been conducted in a horizontal, circular, uniformly heated microchannel, to record the two-phase flow patterns evolved during the boiling process and to study the ebullition process. A high speed camera (REDLAKE HG50LE) with a maximum of 100000 fps together with tungsten lights was used to capture the images along the test section. Microchannel was made of circular fused silica tube having an internal diameter of 0.781 mm and a uniformly heated length of 191 mm. Outside of the test tube was coated with a thin, electrically conductive layer of Indium Tin Oxide (ITO) for direct heating of the test section. Refrigerant R134a was used as working fluid and experiments were performed at two different system pressures corresponding to saturation temperatures of 25 degrees C and 30 degrees C. Mass flux was varied from 100 kg/m(2)s to 400 kg/m(2)s and heat flux ranged from 5 kW/m(2) to 45 kW/m(2). Visualization results show that the bubble growth is restricted by the tube diameter which results in very short existence of isolated bubbly flow regime except essentially restricted to a very short length of test tube. Flow patterns observed along the length were: Isolated bubble, elongated bubble, slug flow, semi annular and annular flow. Rigorous boiling and increased coalescence rates were observed with increase in heat flux. Bubble frequency was observed to increase with both heat and mass flux. A comparison with our previous flow boiling visualization studies, carried out for a test tube of 1.33 mm internal diameter, shows that the number of active nucleation sites is less while the bubble frequency is higher for the current study. Mean bubble length and bubble velocity during elongated bubble flow pattern have also been calculated from the images obtained during the tests.

  • 5.
    Ali, Rashid
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Palm, Björn E.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Maqbool, Mohammad H.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Experimental Investigation of Two-phase Pressure Drop in a Microchannel2011In: Heat Transfer Engineering, ISSN 0145-7632, E-ISSN 1521-0537, Vol. 32, no 13/14, p. 1126-1138Article in journal (Refereed)
    Abstract [en]

    Experimental results of two-phase pressure drop in a horizontal circular microchannel are reported in this paper. A test tube was made of fused silica having an internal diameter of 781 mu m with a total length of 261 mm and a heated length of 191 mm. The outer surface of the test tube was coated with an electrically conductive thin layer of ITO (indium tin oxide) for direct heating of the test section. Refrigerants R134a and R245fa were used as the working fluids, and mass flux during the experiments was varied between 100 and 650 kg/m(2)-s. Experiments were performed at two different system pressures corresponding to saturation temperatures of 25 degrees C and 30 degrees C for R134a and at three different system pressures corresponding to saturation temperatures of 30 degrees C, 35 degrees C, and 40 degrees C for R245fa. Two-phase frictional pressure drop characteristics with variation of mass flux, vapor fraction, saturation temperature, and heat flux were explored in detail. Finally, the prediction capability of some well-known correlations available in the literature, some developed for macrochannels and others especially developed for microchannels, was assessed.

  • 6.
    Ali, Rashid
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Palm, Björn E.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Maqbool, Mohammad H.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Flow Boiling Heat Transfer Characteristics of a Minichannel up to Dryout Condition2010In: MNHMT2009, VOL 2, New York: AMER SOC MECHANICAL ENGINEERS , 2010, p. 25-34Conference paper (Refereed)
    Abstract [en]

    In this paper the experimental flow boiling heat transfer results of a minichannel are presented. A series of experiments was conducted to measure the heat transfer coefficients in a minichannel made of stainless steel (AISI 316) having an internal diameter of 1.7mm and a uniformly heated length of 220mm. R134a was used as working fluid and experiments were performed at two different system pressures corresponding to saturation temperatures of 27 degrees C and 32 degrees C. Mass flux was varied from 50 kg/m(2) s to 600 kg/m(2) s and heat flux ranged from 2kW/m(2) to 156 kW/m(2). The test section was heated directly using a DC power supply. The direct heating of the channel ensured uniform heating and heating was continued until dry out was reached. The experimental results show that the heat transfer coefficient increases with imposed wall heat flux while mass flux and vapour quality have no considerable effect. Increasing the system pressure slightly enhances the heat transfer coefficient. The heat transfer coefficient is reduced as dryout is reached. It is observed that dryout phenomenon is accompanied with fluctuations and a larger standard deviation in outer wall temperatures.

  • 7.
    Ali, Rashid
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Palm, Björn E.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Maqbool, Mohammad H.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Flow Boiling Heat Transfer Characteristics of a Minichannel up to Dryout Condition2011In: Journal of heat transfer, ISSN 0022-1481, E-ISSN 1528-8943, Vol. 133, no 8, p. 081501-Article in journal (Refereed)
    Abstract [en]

    In this paper, the experimental flow boiling heat transfer results of a minichannel are presented. A series of experiments was conducted to measure the heat transfer coefficients in a minichannel made of stainless steel (AISI 316) having an internal diameter of 1.70 mm and a uniformly heated length of 220 mm. R134a was used as a working fluid, and experiments were performed at two different system pressures corresponding to saturation temperatures of 27 degrees C and 32 degrees C. Mass flux was varied from 50 kg/m(2) s to 600 kg/m(2) s, and heat flux ranged from 2 kW/m(2) to 156 kW/m(2). The test section was heated directly using a dc power supply. The direct heating of the channel ensured uniform heating, which was continued until dryout was reached. The experimental results show that the heat transfer coefficient increases with imposed wall heat flux, while mass flux and vapor quality have no considerable effect. Increasing the system pressure slightly enhances the heat transfer coefficient. The heat transfer coefficient is reduced as dryout is reached. It is observed that the dryout phenomenon is accompanied with fluctuations and a larger standard deviation in outer wall temperatures.

  • 8.
    Ali, Rashid
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Palm, Björn E.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Maqbool, Mohammad H.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Flow Boiling Heat Transfer of Refrigerants R134a and R245fa in a Horizontal Microchannel2010In: Proceedings of 2nd European Conference on Microfluidics, 2010Conference paper (Refereed)
  • 9.
    Ali, Rashid
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Palm, Björn E.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Maqbool, Mohammad H.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Flow Boiling Heat Transfer Of Refrigerants R134a And R245fa In A Horizontal Micro-Channel2012In: Experimental heat transfer, ISSN 0891-6152, E-ISSN 1521-0480, Vol. 25, no 3, p. 181-196Article in journal (Refereed)
    Abstract [en]

    Micro-channel-based evaporators are a promising option for high heat flux cooling applications. Micro-channels offer several advantages, including a smaller coolant inventory, superior heat transfer performance, compactness, lightness of weigh. Despite being attractive, the governing phenomena in micro-channels, especially during phase change, are less understood. This article reports the experimental flow boiling heat transfer results of refrigerants R134a and R245fa in a horizontal micro-channel. A series of experiments was conducted to measure the heat transfer coefficients in a circular micro-channel made of fused silica having an internal diameter of 781 mu m and a uniformly heated length of 191 mm. The outer surface of the test tube was coated with a thin, electrically conductive layer of indium-tin-oxide. The surface coating with the electrically conductive layer of indium-tin-oxide made it possible to visualize the flow boiling process simultaneously with uniform heating of the test section. R134a and R245fa were used as working fluids and experiments were performed at a system pressure of 7.7 bar for R134a and at 1.8 bar for R245fa, corresponding to saturation temperature of 30 degrees C. Mass flux was varied from 175 kg/m(2)s to 500 kg/m(2)s, and heat flux ranged from 5 kW/m(2) to 60 kW/m(2). A high-speed camera was used to capture the images in the case of flow boiling of R134a. The experimental results indicated that the heat transfer coefficient increased with heat flux while the mass flux proved to have a negligible effect on heat transfer coefficient.

  • 10.
    Ali, Rashid
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Palm, Björn
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Maqbool, Mohammad H.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Experimental investigation of two phase pressure drop in a microchannel2009In: Proceedings of 2nd Micro & Nano flows Conference, Academic Conferences Publishing, 2009Conference paper (Refereed)
  • 11.
    Ali, Rashid
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology.
    Palm, Björn
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Martin-Callizo, C.
    Maqbool, Muhammad Hamayun
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Study of flow boiling characteristics of a microchannel using high speed visualization2013In: Journal of heat transfer, ISSN 0022-1481, E-ISSN 1528-8943, Vol. 135, no 8, p. 081501-Article in journal (Refereed)
    Abstract [en]

    This paper presents the visualization results obtained for an experimental study of R134a during flow boiling in a horizontal microchannel. The microchannel used was a fused silica tube having an internal diameter of 781 lm, a heated length of 191mm, and was coated with a thin, transparent, and electrically conductive layer of indium-tin-oxide (ITO) on the outer surface. The operating parameters during the experiments were: mass flux 100-400 kg/m2 s, heat flux 5-45 kW/m2, saturation temperatures 25 and 30 °C, corresponding to saturation pressures of 6.65 bar and 7.70 bar and reduced pressures of 0.163 and 0.189, respectively. A high speed camera with a close up lens was used to capture the flow patterns that evolved along the channel. Flow pattern maps are presented in terms of the superficial gas and liquid velocity and in terms of the Reynolds number and vapor quality plots. The results are compared with some flow pattern maps for conventional and micro scale channels available in the literature. Rigorous boiling and increased coalescence rates were observed with an increase in the heat flux.

  • 12.
    Maqbool, Mohammad H.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Palm, Björn E.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Khodabandeh, Rahmatollah
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Ali, Rashid
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Two phase heat transfer of ammonia in a mini/micro channel2010In: Proceedings of the 8th International Conference on Nanochannels, Microchannels and Minichannels, 2010, ASME Press, 2010, p. 1639-1647Conference paper (Refereed)
    Abstract [en]

    Experiments have been performed to investigate heat transfer in a circular vertical mini channel made of stainless steel (AISI 316) with internal diameter of 1.70 mm and a uniformly heated length of 245 mm using ammonia as working fluid. The experiments are conducted for a heat flux range of 15 to 350 kW/m(2) and mass flux range of 100 to 500 kg/m(2)s. The effects of heat flux, mass flux and vapour quality on the heat transfer coefficient are explored in detail. The experimental results show that the heat transfer coefficient increases with imposed wall heat flux while mass flux and vapour quality have no considerable effect. Experimental results are compared to predictive methods available in the literature for boiling heat transfer. The correlations of Cooper et al. [1] and Shah [3] are in good agreement with our experimental data.

  • 13.
    Maqbool, Mohammad H.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Palm, Björn
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Khodabandeh, Rahmatollah
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Ali, Rashid
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Two-phase pressure drop of ammonia in a mini/micro-channel2010In: Proceedings of the  8th International Conference on Nanochannels, Microchannels and Minichannels, 2010, ASME Press, 2010, p. 1731-1739Conference paper (Refereed)
    Abstract [en]

    Experiments have been performed to investigate two-phase pressure drop in a circular vertical mini-channel made of stainless steel (AISI 316) with internal diameter of 1.70 mm and a uniformly heated length of 245 mm using ammonia as working fluid. The experiments are conducted for heat flux range of 15 to 350 kW/m(2) and mass flux range of 100 to 500 kg/m(2)s. A uniform heat flux is applied to the test section by DC power supply. Two phase frictional pressure drop variation with mass flux, vapour quality and heat flux was determined. The experimental results are compared to predictive methods available in literature for frictional pressure drop. The Homogeneous model and the correlation of Muller Steinhagen et al. [14] are in good agreement with our experimental data with MAD of 27% and 26% respectively.

  • 14.
    Maqbool, Muhammad Hamayun
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Palm, Björn
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Khodabandeh, Rahmatollah
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Ali, Rashid
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Experimental Investigation of Two Phase Pressure Drop in a vertical mini-channel at three saturation pressures2010In: Proceedings of the 2nd European Conference on Microfluidics - Microfluidics 2010 - Toulouse, December 8-10, 2010, SHF , 2010Conference paper (Refereed)
    Abstract [en]

    Experiments are performed to study the behavior of  the two phase flow pressure drop of ammonia , in a vertical mini channel made of  stainless steel having an internal diameter of 1.224 mm and a heating length of 245 mm. The test conditions are: mass fluxes from 200 to 500 kg/m2s, heat fluxes from 20 to 340 kW/m2 and saturation temperatures of 23oC,33oC and 43oC. The experimental results are compared to well known correlations for frictional pressure drop in macro and micro scale channels.

  • 15.
    Martin Callizo, Claudi
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Ali, Rashid
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Palm, Björn
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    New experimental results on flow boiling of R-134a in a vertical microchannel2007In: Proc. 10th UK Heat Transfer, Academic Conferences Publishing, 2007Conference paper (Refereed)
    Abstract [en]

    With the rapid development of micro electronic devices and micro manufacturing technology, heat transfer in microchannels has become increasingly important. However, the governing phenomena are not yet well understood.This paper presents new experimental results on flow boiling of R-134a in a vertical microchannel. Local heat transfer coefficients are measured along a single metal tube with internal diameter of 640 μm and a uniformly heated length of 213 mm, for three different mass fluxes, 250, 400 and 600 kg/m2s, two system pressures, 7.70 and 8.87 bar at the inlet (corresponding to saturation temperatures of 30 and 35 ºC), heat fluxes ranging from 5 to 70 kW/m2, and vapour qualities up to 0.89.From the experimental results it is clear that the heat transfer coefficient increases with heat flux and system pressure and does not change with vapour quality or mass flux when the quality is less than 0.45–0.50. For higher vapour qualities, the heat transfer coefficient decreases with vapour quality and the effect of the heat flux on the heat transfer coefficient seems to diminish. This deterioration of the heat transfer coefficient is believed to be caused by the occurrence of intermittent dry out in this vapour quality range.The experimental heat transfer coefficients are compared against predictions from three generalized models, two developed specifically for microscale geometries and one for conventional channels, showing unsatisfactory agreement, especially for the high vapour quality range.

  • 16.
    Martin-Callizo, Claudi
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Ali, Rashid
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Palm, Björn E.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Dryout incipience and critical heat flux in saturated flow boiling of refrigerants in a vertical uniformly heated microchannel2008In: PROCEEDINGS OF THE 6TH INTERNATIONAL CONFERENCE ON NANOCHANNELS, MICROCHANNELS, AND MINICHANNELS, PTS A AND B, NEW YORK: AMER SOC MECHANICAL ENGINEERS , 2008, p. 705-712Conference paper (Refereed)
    Abstract [en]

    The prediction of dryout occurrence is crucial in the design and safety of compact heat exchangers. For this purpose, a series of experimental tests have been performed to investigate dryout incipience and critical heat flux (CHF) in a circular vertical microchannel with internal diameter of 640 mu m and a uniformly heated length of 213 mm using refrigerants R-134a, R-22 and R-245fa as working fluids. The effects of mass flux, system pressure, and refrigerant on the dryout heat flux and vapour quality are explored in detail. Finally, the experimental CHF results are compared to existing correlations both specially developed for microchannels and for macroscale geometries. The comparison shows good agreement with the classical Katto and Ohno (1984) correlation, developed for conventional large tubes. On the other hand, none of the microchannel correlations predicts in agreement the experimental data.

  • 17.
    Martin-Callizo, Claudi
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Palm, Björn E.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Owhaib, Wahib
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Ali, Rashid
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Flow Boiling Visualization of R-134a in a Vertical Channel of Small Diameter2010In: Journal of heat transfer, ISSN 0022-1481, E-ISSN 1528-8943, Vol. 132, no 3, p. 031001-Article in journal (Refereed)
    Abstract [en]

    The present work reports on flow boiling visualization of refrigerant R-134a in a vertical circular channel with an internal diameter of 1.33 mm and 235 mm in heated length. A quartz tube with a homogeneous Indium Tin Oxide coating is used to allow heating and simultaneous visualization. Flow patterns have been observed along the heated length with the aid of high-speed complementary metal oxide semiconductor (CMOS) digital camera. From the flow boiling visualization, seven distinct two-phase flow patterns have been observed: isolated bubbly flow, confined bubbly flow, slug flow, churn flow, slug-annular flow, annular flow, and mist flow. Two-phase flow pattern observations are presented in the form of flow pattern maps. The effects of the saturation temperature and the inlet subcooling degree on the two-phase flow pattern transitions are elucidated. Finally, the experimental flow pattern map is compared with models developed for conventional sizes as well as to a microscale map for air-water mixtures available in literature, showing a large discrepancy.

  • 18.
    Martin-Callizo, Claudi
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Palm, Björn
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Owhaib, Wahib
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Ali, Rashid
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Flow boiling visualization of R-134a in a vertical channel of small diameter2007In: 2007 Proceedings of the ASME/JSME Thermal Engineering Summer Heat Transfer Conference - HT 2007 Volume 1, 2007, p. 135-143Conference paper (Refereed)
    Abstract [en]

    The present work reports on flow boiling visualization of refrigerant R-134a in a vertical circular channel with internal diameter of 1.33 mm and 23 5 mm in heated length. Quartz tube with a homogeneous ITO-coating is used allowing heating and simultaneous visualization. Flow patterns have been observed along the heated length with the aid of a digital camera with close-up lenses. From the flow boiling visualization, seven distinct two-phase flow patterns have been observed: Isolated bubbly flow, confined bubbly flow, slug flow, churn flow, slug-annular flow, annular flow, and mist flow. Two-phase flow pattern observations are presented in the form of flow pattern maps. Finally, the experimental flow pattern map is compared to models developed for conventional sizes as well as to a microscale map for air-water mixtures available in the literature, showing a large discrepancy.

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