Recent Work on Boiling and Condensation in Microchannels

2009 ◽  
Vol 131 (4) ◽  
Author(s):  
Ping Cheng ◽  
Guodong Wang ◽  
Xiaojun Quan
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Pressure Drop ◽  
Cooling Rate ◽  
Recent Work ◽  
Mass Flux ◽  
Flow Boiling ◽  
Injection Flow ◽  
Stable Flow ◽  
Boiling Mode

Recent work on boiling of water and condensation of steam in single and parallel microchannels is reviewed in this paper. It is found that the amplitude and frequency of fluctuations of temperature and pressure during the unstable flow-boiling mode depend greatly on the inlet/outlet configurations and the exit vapor quality. By fabricating an inlet restriction on each microchannel or the installation of a throttling valve upstream of the test section, reversed flow of vapor bubbles can be suppressed resulting in a stable flow-boiling mode. Boiling heat transfer coefficient and pressure drop in microchannels under stable flow-boiling conditions are obtained. These data at high vapor qualities are found to be substantially different from the correlations obtained for flow-boiling in macrochannels. Microbubble emission boiling phenomena, which can defer the arrival of critical heat flux, exist in a partially heated Pyrex glass microchannel at sufficiently high heat flux and high inlet subcooling conditions. For condensation in a microchannel, transition from annular flow to slug/bubbly flow is investigated. The occurrence of the injection flow is owing to the instability of the liquid/vapor interface. The location, at which the injection flow occurs, depends on the mass flux and the cooling rate of steam. Increase in steam mass flux, decrease in cooling rate, and microchannel diameter tend to enhance the instability of the condensate film on the wall, resulting in the occurrence of injection flow further downstream at increasingly high frequency. The pressure drop in the condensing flow increases with the increase in mass flux and quality or with decreasing microchannel diameter. The existing correlations for pressure drop and heat transfer of condensing flow in macrochannels overestimate the experimental data in microchannels.

Boiling and Condensation in Microchannels

2008 ◽  
Author(s):  
Ping Cheng ◽  
Guodong Wang ◽  
Xiaojun Quan
Keyword(s):  
Pressure Drop ◽  
Cooling Rate ◽  
Mass Flux ◽  
Flow Boiling ◽  
Bubbly Flow ◽  
Reversed Flow ◽  
Injection Flow ◽  
Boiling Heat Transfer Coefficient ◽  
Stable Flow ◽  
Boiling Mode

The characteristics of boiling of water and condensation of steam in a microchannel under heating or cooling conditions are discussed in this paper. Stable and unstable boiling modes occur in a microchannel, depending on the nucleated bubble size in the microchannel. Stable boiling mode with constant temperature variations exists when the size of the nucleated bubble is less than that of the microchannel diameter, while unstable boiling mode exists when the size of the bubble is greater than the microchannel diameter. The latter is owing to the fact that when a bubble grows to the size of the microchannel, it will expand in both upstream and downstream directions. Subsequently, the reversed flow of vapor bubble is swept downstream by the incoming subcooled liquid, leading to large cyclic fluctuations of temperature and pressure. The amplitude and frequency of these fluctuations depend greatly on the inlet/outlet configurations and the exit vapor quality. By fabricating an inlet restriction on each microchannel, the reversed flow of vapor can be suppressed, resulting in a stable flow boiling mode. Boiling heat transfer coefficient and pressure drop in a microchannel under stable flow boiling conditions are obtained. These data at high vapor qualities are found to be substantially different from the correlations obtained for flow boiling in macrochannels. For condensation in a microchannel, mist flow, annular flow, injection flow, slug/bubbly flow exist depending on mass flux, condensation heat flux, and the location in the microchannel. The occurrence of the injection flow is owing to the instability of the liquid/vapor interface because the surface tension effect is predominant in microchannels. The location, at which the injection flow occurs, depends on the mass flux and the cooling rate of steam. Increase in steam mass flux, decrease in cooling rate and the microchannel diameter tend to enhance instability of the condensate film on the wall, resulting in occurrence of the injection flow further toward the outlet with an increase in occurrence frequency. At low mass fluxes, the pressure drop obtained for condensation in microchannels is substantially different from the correlation equations for macrochannels because of different flow patterns.

Effect of Inlet Restriction on Flow Boiling Heat Transfer in a Horizontal Microtube

2013 ◽  
Vol 135 (2) ◽  
Author(s):  
YanFeng Fan ◽  
Ibrahim Hassan
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Pressure Drop ◽  
Critical Heat Flux ◽  
Mass Flux ◽  
Boiling Heat Transfer ◽  
Flow Boiling ◽  
Mass Fluxes ◽  
Flow Boiling Heat Transfer ◽  
Low Mass

Flow boiling heat transfer in a horizontal microtube with inlet restriction (orifice) under uniform heating condition is experimentally investigated using FC-72 as working fluid. A stainless steel microtube with an inner diameter of 889 μm is selected as main microtube. Two microtubes with smaller diameters are assembled at the inlet of main microtube to achieve the restriction ratios of 50% and 20%. The experimental measurement is carried out at mass fluxes ranging from 160 to 870 kg/m2·s, heat fluxes varying from 6 to 170 kW/m2, inlet temperatures of 23 and 35 °C, and saturation pressures of 10 and 45 kPa. The effects of the orifices on two-phase pressure drop, critical heat flux (CHF), and flow boiling heat transfer coefficient are studied. The results show that the pressure drop caused by the orifice takes a considerable portion in the total pressure drop at low mass fluxes. This ratio decreases as the vapor quality or mass flux increases. The difference of normal critical heat flux in the microtubes with different orifice sizes is negligible. In the aspect of flow boiling heat transfer, the orifice is able to enhance the heat transfer at low mass flux and high saturation pressure, which indicates the contribution of orifice in the nucleate boiling dominated regime. However, the effect of orifice on flow boiling heat transfer is negligible in the forced convective boiling dominated regime.

scholarly journals Experimental Investigation on Flow Boiling Heat Transfer and Pressure Drop of a Low-GWP Refrigerant R1234ze(E) in a Horizontal Minichannel

Energies ◽  
2021 ◽  
Vol 14 (18) ◽  
pp. 5972
Author(s):  
Yu Xu ◽  
Zihao Yan ◽  
Ling Li
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Experimental Investigation ◽  
Pressure Drop ◽  
Mass Flux ◽  
Flow Boiling ◽  
Saturation Pressure ◽  
Nucleate Boiling ◽  
Vapor Quality ◽  
Frictional Pressure Drop

To protect the environment, a new low-GWP refrigerant R1234ze(E) was created to substitute R134a. However, its flow boiling performances have not received sufficient attention so far, which hinders its popularization to some extent. In view of this, an experimental investigation was carried out in a 1.88 mm horizontal circular minichannel. The saturation pressures were maintained at 0.6 and 0.7 MPa, accompanied by mass flux within 540–870 kg/m2 s and heat flux within 25–65 kW/m2. For nucleate boiling, a larger heat flux brings about a larger heat transfer coefficient (HTC), while for convective boiling, the mass flux and vapor quality appear to take the lead role. The threshold vapor quality of different heat transfer mechanisms is around 0.4. Additionally, larger saturation pressure results in large HTC. As for the frictional pressure drop (FPD), it is positively influenced by mass flux and vapor quality, while negatively affected by saturation pressure, and the influence of heat flux is negligible. Furthermore, with the measured data, several existing correlations are compared. The results indicate that the correlations of Saitoh et al. (2007) and Müller-Steinhagen and Heck (1986) perform best on flow boiling HTC and FPD with mean absolute deviations of 5.4% and 10.9%.

Flow Boiling of R134a and R245fa in a 1.1 mm Diameter Tube

2013 ◽  
Author(s):  
Emily A. Pike-Wilson ◽  
Mohamed M. Mahmoud ◽  
Tassos G. Karayiannis
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Pressure Drop ◽  
Mass Flux ◽  
Boiling Heat Transfer ◽  
Flow Boiling ◽  
Experimental Conditions ◽  
Local Flow ◽  
Flow Boiling Heat Transfer ◽  
Vapour Quality

New refrigerants are required for cooling systems due to the fact that refrigerants like R134a are about to be phased out. This paper presents a comparison between the flow boiling heat transfer and pressure drop results of refrigerants R245fa and R134a. The experiments with R245fa were conducted in a vertical cold drawn stainless steel tube with an inner diameter of 1.1 mm and heated length of 150 mm. Experimental conditions include: mass flux range 100–400 kg/m2s, heat flux range 10–60 kW/m2, pressures of 8 and 10 bar and 1.9 and 2.5 bar for R134a and R245fa corresponding to saturated temperatures 31 °C and 39 °C and exit vapour quality range 0–0.95. The data for R134a were obtained earlier using the same experimental facility at the same experimental conditions and with the same test tube. The results demonstrated that refrigerant properties have a significant effect on heat transfer and pressure drop. The pressure drop of R245fa is higher by up to 300% compared to that of R134a at similar conditions. In addition, the effect of mass flux and heat flux on the local flow boiling heat transfer coefficient was different. Heat transfer coefficients of R245fa showed a greater dependence on vapour quality. The agreement with past heat transfer correlations is better with R134a than with R245fa.

Heat Transfer and Pressure Drop of CO2 Flow Boiling in Microchannels

2000 ◽  
Author(s):  
Yuan Zhao ◽  
Majid Molki ◽  
Michael M. Ohadi
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Heat Transfer Coefficient ◽  
Pressure Drop ◽  
Transfer Coefficient ◽  
Mass Flux ◽  
Flow Boiling ◽  
Dominant Factor ◽  
Vapor Quality ◽  
Co2 Flow

Abstract An experimental investigation was performed to study the flow boiling heat transfer of CO2 in microchannels. Tests were conducted in a horizontal triangular microchannel with the hydraulic diameter of 0.86 mm. Heat to the test section was provided by direct electrical heating. Experiments were conducted with CO2 at saturation temperatures of 273 to 293 K, mass fluxes of 100 to 820 kg/m2s, heat fluxes of 3 to 23 kW/m2, and qualities of 20% to 85%. It was demonstrated that heat flux had an enhancing effect on the heat transfer coefficient, while mass flux had a negligible effect. Nucleate boiling mechanism is found to be the dominant factor for CO2 flow boiling in microchannels. Heat transfer coefficient degraded quickly at high vapor quality region (0.6–0.7), which is possibly due to flow mal-distribution. Pressure drop increases slightly with vapor quality and/or heat flux. Mass flux has a strong increasing effect on pressure drop.

A Study on Flow Boiling in Microchannel With Piranha Pin Fin

2015 ◽  
Author(s):  
X. Yu ◽  
C. Woodcock ◽  
Y. Wang ◽  
J. Plawsky ◽  
Y. Peles
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Fluid Flow ◽  
Pressure Drop ◽  
Heat Transfer Performance ◽  
Flow Boiling ◽  
Transfer Performance ◽  
Flow Conditions ◽  
Pin Fin ◽  
Flow And Heat Transfer

In this paper we reported an advanced structure, the Piranha Pin Fin (PPF), for microchannel flow boiling. Fluid flow and heat transfer performance were evaluated in detail with HFE7000 as working fluid. Surface temperature, pressure drop, heat transfer coefficient and critical heat flux (CHF) were experimentally obtained and discussed. Furthermore, microchannels with different PPF geometrical configurations were investigated. At the same time, tests for different flow conditions were conducted and analyzed. It turned out that microchannel with PPF can realize high-heat flux dissipation with reasonable pressure drop. Both flow conditions and PPF configuration played important roles for both fluid flow and heat transfer performance. This study provided useful reference for further PPF design in microchannel for flow boiling.

Experimental Study of Heat Transfer to Supercritical Pressure Water Flowing in a 2×2 Rod Bundle

2014 ◽  
Author(s):  
Han Wang ◽  
Qincheng Bi ◽  
Linchuan Wang ◽  
Haicai Lv ◽  
Laurence K. H. Leung
Keyword(s):  
Heat Transfer ◽  
Experimental Data ◽  
Heat Flux ◽  
Pressure Drop ◽  
Wall Temperature ◽  
Mass Flux ◽  
Heat Fluxes ◽  
Outer Diameter ◽  
Square Channel ◽  
Rod Bundle

An experiment has recently been performed at Xi’an Jiaotong University to study the wall temperature and pressure drop at supercritical pressures with upward flow of water inside a 2×2 rod bundle. A fuel-assembly simulator with four heated rods was installed inside a square channel with rounded corner. The outer diameter of each heated rod is 8 mm with an effective heated length of 600 mm. Experimental parameters covered the pressure of 23–28 MPa, mass flux of 350–1000 kg/m2s and heat flux on the rod surface of 200–1000 kW/m2. According to the experimental data, it was found that the circumferential wall temperature distribution of a heated rod is not uniform. The temperature difference between the maximum and the minimum varies with heat flux and/or mass flux. Heat transfer characteristics of supercritical water in bundle were discussed with respect to various heat fluxes. The effect of heat flux on heat transfer in rod bundles is similar with that in tubes or annuli. In addition, flow resistance reflected in the form of pressure loss has also been studied. Experimental results showed that the total pressure drop increases with bulk enthalpy and mass flux. Four heat transfer correlations developed for supercritical pressures water were compared with the present test data. Predictions of Jackson correlation agrees closely with the experimental data.

scholarly journals Effect of Metal Foam Insert Configurations on Flow Boiling Heat Transfer and Pressure Drop in a Rectangular Channel

Materials ◽  
2021 ◽  
Vol 14 (16) ◽  
pp. 4617
Author(s):  
Sanghyun Nam ◽  
Dae Yeon Kim ◽  
Youngwoo Kim ◽  
Kyung Chun Kim
Keyword(s):  
Heat Transfer ◽  
Pressure Drop ◽  
Mass Flux ◽  
Test Section ◽  
Boiling Heat Transfer ◽  
Metal Foam ◽  
Flow Boiling ◽  
Rectangular Channel ◽  
Saturation Pressure ◽  
Two Phase

Heat transfer under flow boiling is better in a rectangular channel filled with open-cell metal foam than in an empty channel, but the high pressure drop is a drawback of the empty channel method. In this study, various types of metal foam insert configurations were tested to reduce the pressure drop while maintaining high heat transfer. Specifically, we measured the boiling heat transfer and pressure drop of a two-phase vertical upward flow of R245fa inside a channel. To measure the pressure and temperature differences of the metal foam, differential pressure transducers and T-type thermocouples were used at both ends of the test section. While the saturation pressure was kept constant at 5.9 bar, the steam quality at the inlet of the test section was changed from 0.05 to 0.99. The channel height, moreover, was 3 mm, and the mass flux ranged from 133 to 300 kg/m2s. The two-phase flow characteristics were observed through a high-speed visualization experiment. Heat transfer tended to increase with the mean vapor quality, and, as expected, the fully filled metal foam channel offered the highest thermal performance. The streamwise insert pattern model had the lowest heat transfer at a low mass flux. However, at a higher mass flux, the three different insert models presented almost the same heat transfer coefficients. We found that the streamwise pattern model had a very low pressure drop compared to that of the spanwise pattern models. The goodness factors of the flow area and the core volume of the streamwise patterned model were higher than those of the full-filled metal foam channel.

Flow Boiling Heat Transfer, Pressure Drop, and Flow Pattern for CO2 in a 3.5mm Horizontal Smooth Tube

2009 ◽  
Vol 131 (9) ◽  
Author(s):  
Chang Yong Park ◽  
Pega Hrnjak
Keyword(s):  
Heat Transfer ◽  
Pressure Drop ◽  
Flow Pattern ◽  
Mass Flux ◽  
Boiling Heat Transfer ◽  
Flow Boiling ◽  
Flow Patterns ◽  
Transfer Coefficients ◽  
Heat Transfer Coefficients ◽  
Flow Boiling Heat Transfer

Abstract C O 2 flow boiling heat transfer coefficients and pressure drop in a 3.5mm horizontal smooth tube are presented. Also, flow patterns were visualized and studied at adiabatic conditions in a 3mm glass tube located immediately after a heat transfer section. Heat was applied by a secondary fluid through two brass half cylinders to the test section tubes. This research was performed at evaporation temperatures of −15°C and −30°C, mass fluxes of 200kg∕m2s and 400kg∕m2s, and heat flux from 5kW∕m2 to 15kW∕m2 for vapor qualities ranging from 0.1 to 0.8. The CO2 heat transfer coefficients indicated the nucleate boiling dominant heat transfer characteristics such as the strong dependence on heat fluxes at a mass flux of 200kg∕m2s. However, enhanced convective boiling contribution was observed at 400kg∕m2s. Surface conditions for two different tubes were investigated with a profilometer, atomic force microscope, and scanning electron microscope images, and their possible effects on heat transfer are discussed. Pressure drop, measured at adiabatic conditions, increased with the increase of mass flux and quality, and with the decrease of evaporation temperature. The measured heat transfer coefficients and pressure drop were compared with general correlations. Some of these correlations showed relatively good agreements with measured values. Visualized flow patterns were compared with two flow pattern maps and the comparison showed that the flow pattern maps need improvement in the transition regions from intermittent to annular flow.

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