Browsing by Author "Chen, C. A."

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Bubble characteristics in time periodic evaporation flow of R-134a in a narrow annular pipe due to heat flux oscillation
(2016-12)
Chen, C. A.
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Lin, T. F.
;
Yan, Wei-Mon
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機械工程學系
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Department of Mechanical Engineering
In this work, bubble characteristics of periodic evaporation flow with refrigerant R-134a in a horizontal narrow annular pipe were examined experimentally in details. Attention is focused on the time periodic evaporation flow characteristics affected by the mean levels, amplitudes, and periods of the heat flux oscillation. The photos of the R-134a time periodic evaporating flow taken from the duct side are presented to show the change of the dominant two-phase flow pattern in the duct with the experimental parameters. The results show that at the low vapor quality, the bubbles get smaller with time and become less crowded in the duct in the first half of the cycle in which the R-134a heat flux decreases. The changes of the bubble characteristics with the instantaneous heat flux become more pronounced for an increase in the amplitude of the heat flux oscillation. At the very high mean vapor quality the bubble nucleation can be barely seen in the entire periodic cycle since the liquid film covering the heating surface is very thin. In addition, the duct flow is dominated by the annular two-phase flow at all time. (C) 2016 Elsevier Ltd. All rights reserved.
Bubble characteristics in time periodic saturated flow boiling of R-134a in a narrow annular pipe due to heat flux oscillation
(2016-11)
Chen, C. A.
;
Lin, T. F.
;
Yan, Wei-Mon
;
機械工程學系
;
Department of Mechanical Engineering
In this work, experiments have been conducted to investigate how the imposed time periodic heat flux oscillation affects the bubble characteristics of saturated flow boiling with refrigerant R-134a in a horizontal narrow annular pipe. The test section for the horizontal annular duct consists of an outer pipe made of Pyrex glass and an inner heated copper pipe, intending to facilitate the visualization of boiling processes. A cartridge heater is installed inside the inner pipe to provide the required heat flux to the refrigerant flow in the narrow annular duct. In particular, attention is focused on the time periodic saturated flow boiling characteristics affected by the mean levels, amplitudes, and periods of the heat flux oscillation. The results show that the bubble departure diameter, bubble frequency and active nucleation site density are found to oscillate periodically in time as well and at the same frequency as the imposed heat flux oscillation. Furthermore, in the boiling the resulting oscillation amplitudes of the bubble parameters, such as the bubble departure diameter, bubble frequency and active nucleation site density, get larger for a longer period and a larger amplitude of the imposed heat flux oscillation and for a higher mean imposed heat flux. (C) 2016 Elsevier Ltd. All rights reserved.
Bubble dynamics in evaporation flow of R-134a in narrow annular ducts due to flow rate oscillation
(2019-01-01)
Chen, C. A.
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Lin, T. F.
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Ali, Hafiz Muhammad
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Amani, Pouria
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Yan, Wei-Mon
This article aims to investigate the influence of imposed flow rate oscillation on the bubble behaviors of evaporative flow of R-134a in horizontal narrow annular ducts. Particularly, the effects of amplitudes and periods of the oscillating flow rates along with the vapor quality of the R-134a on the bubble characteristics are examined in details. Moreover, the R-134a flow photos captured at the middle axial location are presented to illustrate the shift between the annular two-phase flow pattern and the bubble nucleation at different operating conditions and time instances. The observations reveal that the imposed mass flow rate oscillation results in the fluctuation of bubble departure diameter and frequency along with the density of active nucleation sites with the same frequencies. The larger amplitudes and longer periods of the mass flow rate oscillation lead to the stronger fluctuations in bubble dynamics. Furthermore, it is found that the bubble nucleation on the heating surface is the dominant mechanism at the low vapor quality of 0.05. However, the annular two-phase flow prevails over a relatively large portion of the periodic cycle at the dryness fraction of 0.5 and over the entire cycle at the dryness fraction of 0.95. Finally, the evaporative heat transfer of R-134a subject to the flow rate oscillation is correlated as a function of the mass flow rate and the vapor quality.
Experimental study of R-134a evaporation heat transfer in a narrow annular duct
(2010-04-01)
Chen, C. A.
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Lee, C. Y.
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Lin, T. F.
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機械工程學系
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Department of Mechanical Engineering
An experiment is carried out here to investigate the evaporation heat transfer and associated evaporating flow pattern for refrigerant R-134a flowing in a horizontal narrow annular duct The gap of the duct is fixed at 1.0 and 2 0 mm. In the experiment, the effects of the duct gap, refrigerant vapor quality, mass flux and saturation temperature and imposed heat flux on the measured evaporation heat transfer coefficient h, are examined in detail. For the duct gap of 2.0 mm, the refrigerant mass flux G is varied from 300 to 500 kg/m(2) s, imposed heat flux q from 5 to 15 kW/m(2), vapor quality x(m) from 0.05 to 0.95, and refrigerant saturation temperature T(sat) from 5 to 15 degrees C. While for the gap of 1.0 mm, G is varied from 500 to 700 kg/m(2)s with the other parameters varied in the same ranges as that for delta = 2 0 mm The experimental data clearly show that the evaporation heat transfer coefficient increases almost linearly with the vapor quality of the refrigerant and the increase is more significant at a higher G. Besides, the evaporation heat transfer coefficient also rises substantially at increasing q. Moreover, a significant increase in the evaporation heat transfer coefficient results for a rise in T,,,. but the effects are less pronounced in the narrower duct at a low imposed heat flux and a high refrigerant mass flux. Furthermore, the evaporation heat transfer coefficient increases substantially with the refrigerant mass flux except at low vapor quality. We also note that reducing the duct gap causes a significant increase in h(r). In addition to the heat transfer data, photos of R-134a evaporating flow taken from the duct side show the change of the dominant two-phase flow pattern in the duct with the experimental parameters. Finally, an empirical correlation for the present measured heat transfer coefficient for the R-134a evaporation in the narrow annular ducts is proposed. (C) 2009 Elsevier Ltd. All rights reserved
Experimental study on time periodic evaporation heat transfer of R-134a in annular ducts due to wall heat flux oscillation
(2017-03)
Chen, C. A.
;
Lin, T. F.
;
Yan, Wei-Mon
;
機械工程學系
;
Department of Mechanical Engineering
An experimental study was carried out to examine the time periodic evaporation heat transfer for refrigerant R-134a flowing in horizontal narrow annular ducts subjected to wall heat flux oscillation in the form of triangular waves. The experiment was performed for the gap of the annular duct delta = 1.0, 2.0 and 5.0 mm. Besides, the amplitude of the imposed heat flux oscillation Delta q was set at 0, 10, 30, 50, 80 and 100% of mean imposed heat flux (q) over bar. Moreover, the period of the heat flux oscillation t(p) was fixed at 2, 20, 30, 60,120 and 600 s. Attention was mainly paid to examining the effects of the amplitude and period of the wall heat flux oscillation on the time periodic evaporation heat transfer in the narrow annular pipe. The measured evaporation heat transfer data are expressed in terms of the evaporation heat transfer coefficient. The thermal characteristics of the oscillatory evaporation heat transfer were illustrated by showing the time variations of the instantaneous heated pipe wall temperature T-w, and evaporation heat transfer coefficient h(r). Measured results showed that the time-average heat transfer coefficients for the oscillatory evaporation heat transfer of R-134a were not affected to a noticeable degree by the amplitude and period of the imposed heat flux oscillation. Besides, the stronger wall temperature oscillation was noticed for a longer period and a larger amplitude of the imposed heat flux oscillation and for a higher mean imposed heat flux. (C) 2016 Elsevier Ltd. All rights reserved.
Oscillatory subcooled flow boiling heat transfer of R-134a and associated bubble characteristics in a narrow annular duct due to flow rate oscillation
(2013-08-01)
Wang, S. L.
;
Chen, C. A.
;
Lin, T. F.
;
機械工程學系
;
Department of Mechanical Engineering
An experiment is conducted here to investigate how an imposed time periodic flow rate oscillation in the form of a triangular wave affects the long time subcooled flow boiling heat transfer and associated bubble characteristics of refrigerant R-134a in a horizontal narrow annular duct. In the experiment the mean R134a mass flux (G) over bar varies from 200 to 500 kg/m(2) s, imposed heat flux ranges from 0 to 45 kW/m(2), and the amplitude of the mass flux oscillation changes from 0 to 30% of (G) over bar with the period of the mass flux oscillation varied from 20 to 120 s for the inlet liquid subcooling ranging from 0 to 6 degrees C. The duct gap is fixed at 2.0 mm. The results indicate that the inlet liquid subcooling significantly affects the oscillatory flow boiling heat transfer characteristics. Besides, when the imposed heat flux is close to that for the onset of stable flow boiling, intermittent flow boiling appears. The intermittent boiling prevails in a very different range of the Boiling number for a change in the inlet subcooling. Moreover, in the subcooled boiling the heated wall temperature, bubble departure diameter and frequency, and active nucleation site density also oscillate periodically in time. Furthermore, in the persistent boiling at high imposed heat flux the resulting T-w oscillation is stronger for a higher inlet liquid subcooling and for a longer period and a larger amplitude of the mass flux oscillation. And for a larger amplitude of the mass flux oscillation, stronger temporal oscillations in d(p), f and n(ac) are noted. Finally, a flow regime map is provided to delineate the boundaries separating different boiling regimes for the oscillatory R-134a subcooled flow boiling in the annular duct. (C) 2013 Elsevier Ltd. All rights reserved.
Saturated flow boiling heat transfer of R-407C and associated bubble characteristics in a narrow annular duct
(2008-07-15)
Hsieh, F. C.
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Li, K. W.
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Lie, Y. M.
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Chen, C. A.
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Lin, T. F.
Experiments are conducted here to investigate how the channel size affects the R-407C saturated flow boiling heat transfer and associated bubble characteristics in a horizontal narrow annular duct. The gap of the duct is fixed at 1.0 and 2.0 mm in this study. The measured data indicate that the saturated flow boiling heat transfer coefficient increases with a decrease in the duct gap. Besides, raising the imposed heat flux can cause a significant increase in the boiling heat transfer coefficients. However, the effects of the refrigerant mass flux and saturated temperature on the boiling heat transfer coefficient are milder. The results from the flow visualization show that the mean diameter of the bubbles departing from the heating surface decreases noticeably at increasing R-407C mass flux. Moreover, the bubble departure frequency increases at reducing duct size and at a high imposed heat flux many bubbles generated from the cavities in the heating surface tend to merge together to form big bubbles. Meanwhile comparison of the present heat transfer data for R-407C with R-134a in the same duct and with some existing correlations is conducted. Furthermore, correlation for the present R-407C saturated flow boiling heat transfer data is proposed. Additionally, the present data for some quantitative bubble characteristics such as the mean bubble departure diameter and frequency and the active nucleation site density are also correlated. (C) 2008 Elsevier Ltd. All rights reserved.
Saturated flow boiling heat transfer of R-410A and associated bubble characteristics in a narrow annular duct
(2011-11-01)
Chen, C. A.
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Li, K. W.
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Lie, Y. M.
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Lin, T. F.
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機械工程學系
Experiments are conducted here to investigate how the channel size affects the R-410A saturated flow boiling heat transfer and associated bubble characteristics in a horizontal narrow annular duct. The gap of the duct is fixed at 1.0 and 2.0 mm in this study. The measured data indicate that the saturated flow boiling heat transfer coefficient increases with increasing refrigerant mass flux and saturated temperature and with a decrease in the gap size. Besides, raising the imposed heat flux can cause a significant increase in the boiling heat transfer coefficient. The results from the flow visualization show that the mean diameter of the bubbles departing from the heating surface decreases slightly with increasing refrigerant mass flux and saturated temperature. Moreover, the bubble departure frequency increases at reducing duct size and increasing mass flux. And at a high imposed heat flux many bubbles generated from the cavities in the heating surface tend to merge together to form big bubbles. Meanwhile, comparisons of the present heat transfer data for R-410A with R-407C and R-134a in the same duct and with some existing correlations are conducted. Furthermore, an empirical correlation for the present R-410A saturated flow boiling heat transfer data is proposed. (C) 2011 Elsevier Ltd. All rights reserved.
Subcooled flow boiling heat transfer and associated bubble characteristics of FC-72 on a heated micro-pin-finned silicon chip
(2010-11-01)
Chang, W. R.
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Chen, C. A.
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Ke, J. H.
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Lin, T. F.
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機械工程學系
Experiments are conducted here to investigate subcooled flow boiling heat transfer and associated bubble characteristics of FC-72 on a heated micro-pin-finned silicon chip flush-mounted on the bottom of a horizontal rectangular channel. In the experiments the mass flux is varied from 287 to 431 kg/m(2) s, coolant inlet subcooling from 2.3 to 4.3 degrees C, and imposed heat flux from 1 to 10 W/cm(2). Besides, the silicon chips contain three different geometries of micro-structures, namely, the smooth, pin-finned 200 and pin-finned 100 surfaces. The pin-finned 200 and 100 surfaces, respectively, contain micro-pin-fins of size 200 mu m x 200 mu m x 70 mu m (width x length x height) and 100 mu m x 100 mu m x 70 mu m. The measured data show that the subcooled flow boiling heat transfer coefficient is reduced at increasing inlet liquid subcooling but is little affected by the coolant mass flux. Besides, adding the micro-pin-fin structures to the chip surface can effectively raise the single-phase convection and flow boiling heat transfer coefficients. Moreover, the mean bubble departure diameter and active nucleation site density are reduced for rises in the FC-72 mass flux and inlet liquid subcooling. Increasing coolant mass flux or reducing inlet liquid subcooling results in a higher mean bubble departure frequency. Furthermore, larger bubble departure diameter, higher bubble departure frequency, and higher active nucleation site density are observed as the imposed heat flux is increased. Finally, empirical correlations for the present data for the heat transfer and bubble characteristics in the FC-72 subcooled flow boiling are proposed. (C) 2010 Published by Elsevier Ltd.
Subcooled flow boiling heat transfer of R-407C and associated bubble characteristics in a narrow annular duct
(2009-06-01)
Chen, C. A.
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Chang, W. R.
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Li, K. W.
;
Lie, Y. M.
;
Lin, T. F.
An experiment is conducted here to investigate how the channel size affects the subcooled flow boiling heat transfer and the associated bubble characteristics of refrigerant R-407C in a horizontal narrow annular duct with the gap of the duct fixed at 1.0 and 2.0 mm. The measured boiling curves indicate that the temperature overshoot at ONB is relatively significant for the subcooled flow boiling of R-407C in the duct. Besides, the subcooled flow boiling heat transfer coefficient increases with a reduction in the duct gap, but decreases with an increase in the inlet liquid subcooling. Moreover, raising the heat flux imposed on the duct can cause a significant increase in the boiling heat transfer coefficients. However, the effects of the refrigerant mass flux and saturated temperature on the boiling heat transfer coefficient are slighter. Visualization of the subcooled flow boiling processes in the duct reveals that the bubbles are suppressed to become smaller and less dense by raising the refrigerant mass flux and inlet subcooling. Raising the imposed heat flux, however, produces positive effects on the bubble population, coalescence and departure frequency. Meanwhile, the present heat transfer data for R-407C are compared with the R-134a data measured in the same duct and with some existing correlations. We also propose empirical correlations for the present data for the R-407C subcooled flow boiling heat transfer and some quantitative bubble characteristics such as the mean bubble departure diameter and frequency and the active nucleation site density. (C) 2009 Elsevier Ltd. All rights reserved.
Time periodic evaporation heat transfer of R-134a in a narrow annular duct due to mass flow rate oscillation
(2018-03-01)
Chen, C. A.
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Lin, T. F.
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Yan, Wei-Mon
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Amani, Mohammad
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機械工程學系
An investigation into the effect of mass flow rate oscillation on the R-134a evaporation heat transportation in a horizontal annular duct was experimentally conducted. The experiments were performed at different amplitudes (10, 20, and 30%) and periods (20, 30, 60, and 120 s) of the mass flux oscillation in a duct with different gap sizes (1.0, 2.0 and 5.0 mm). In this regard, the time variations of evaporation heat transportation coefficient and the heated wall temperature were also analyzed for the thermal characteristics of the oscillatory evaporation heat transfer. Measured results showed that the amplitude and period of the mass flux oscillation insignificantly affected the time-average heat transportation coefficient for the R-134a oscillatory evaporation heat transfer. However, the larger amplitudes and longer periods of the mass flux oscillation led to stronger wall temperature oscillations. (C) 2017 Elsevier Ltd. All rights reserved.
Time periodic flow boiling heat transfer of R-134a and associated bubble characteristics in a narrow annular duct due to flow rate oscillation
(2010-09-01)
Chen, C. A.
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Chang, W. R.
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Lin, T. F.
;
機械工程學系
;
Department of Mechanical Engineering
An experiment is conducted here to investigate the effects of the imposed time periodic refrigerant flow rate oscillation in the form of nearly a triangular wave on refrigeriant R-134a flow boiling heat transfer and associated bubble characteristics in a horizontal narrow annular duct with the duct gap fixed at 2.0 mm. The results indicate that when the imposed heat flux is close to that for the onset of stable flow boiling, intermittent flow boiling appears in which nucleate boiling on the heated surface does not exist in an entire periodic cycle. At somewhat higher heat flux persistent boiling prevails. Besides, the refrigerant flow rate oscillation only slightly affects the time-average boiling curves and heat transfer coefficients. Moreover, the heated wall temperature, bubble departure diameter and frequency, and active nucleation site density are found to oscillate periodically in time as well and at the same frequency as the imposed mass flux oscillation. Furthermore, in the persistent boiling the resulting heated wall temperature oscillation is stronger for a longer period and a larger amplitude of the mass flux oscillation. And for a larger amplitude of the mass flux oscillation, stronger temporal oscillations in the bubble characteristics are noted. The effects of the mass flux oscillation on the size of the departing bubble and active nucleation site density dominate over the bubble departure frequency, causing the heated wall temperature to decrease and heat transfer coefficient to increase at reducing mass flux in the flow boiling, opposing to that in the single-phase flow. But they are only mildly affected by the period of the mass flux oscillation. However, a short time lag in the wall temperature oscillation is also noted. Finally, a flow regime map is provided to delineate the boundaries separating different boiling regimes for the R-134a flow boiling in the annular duct. (C) 2010 Published by Elsevier Ltd.
Time periodic saturated flow boiling heat transfer of R-134a in a narrow annular duct due to heat flux oscillation
(2017-03)
Chen, C. A.
;
Lin, T. F.
;
Yan, Wei-Mon
;
機械工程學系
;
Department of Mechanical Engineering
The time periodic saturated boiling heat transfer in a horizontal annulus was investigated experimentally where the walls are under an oscillating heat flux. The fluid enters the duct with zero vapor quality (saturated liquid state). The amplitude of the imposed heat flux oscillation Delta q varies from 0% to 50% of mean imposed heat flux (q) over bar and four different periods of heat flux oscillation t(p) including 20, 30, 60 and 120 s are applied to the system. The measured data display that when the applied heat flux is close to that for the onset of stable flow boiling, intermittent flow boiling appears in which nucleate boiling on the heated surface only exists in a partial interval of each periodic cycle and the heat flux oscillation does not noticeably affect the time-average boiling curves and heat transfer coefficients. Besides, the heated wall temperature and evaporating flow pattern are found to oscillate periodically in time as well and at the same frequency as the imposed heat flux oscillation. Furthermore, in the persistent boiling the resulting oscillation amplitudes of the heated surface temperature, heat transfer coefficient gets larger for a longer period and larger amplitude of the imposed heat flux oscillation and for a higher mean imposed heat flux. The substantial time lag in the heated surface temperature oscillation is observed. In the first half of the periodic cycle in which the heat flux reduces with time, after the time lag the heated wall temperature decreases with time. The inverse processes occur in the second half of the cycle in which imposed heat flux increases with time. Finally, flow regime maps are provided to explain the boundaries separating different boiling regimes for the R-134a saturated boiling in the duct. (C) 2016 Elsevier Ltd. All rights reserved.
Transient oscillatory saturated flow boiling heat transfer and associated bubble characteristics of FC-72 over a small heated plate due to heat flux oscillation
(2012-01-31)
Wang, S. L.
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Chen, C. A.
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Lin, Y. L.
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Lin, T. F.
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機械工程學系
An experiment is carried out here to investigate the transient oscillatory boiling heat transfer and associated bubble characteristics of FC-72 flow over a small circular plate subject to a time varying heat flux with the plate flush mounted on the bottom of a horizontal rectangular channel. At the inlet the flow is maintained at saturated liquid state with zero vapor quality. The imposed heat flux oscillates periodically with time in the form of rectangular waves. In the experiment the FC-72 mass flux G varies from 300 to 400 kg/m(2)s, mean imposed heat flux (q) over bar ranges from 0 to 10 W/cm(2) and the amplitude of the heat flux oscillation Delta q is fixed at 10-50% of (q) over bar with the period of the heat flux oscillation varied from 10 to 30 s. The experimental results show that the time-averaging FC-72 oscillatory boiling heat transfer characteristics resemble that for stable flow boiling. However, the imposed heat flux oscillation causes significant temporal oscillations in the heated plate temperature, boiling heat transfer coefficient, bubble departure diameter and frequency, and active nucleation site density. These physical quantities oscillate at the same frequency as the heat flux oscillation and at a higher (q) over bar, a larger Delta q/(q) over bar, and a longer t(p) they exhibit stronger oscillations. Besides, a slight time lag in T-w oscillation is seen. Moreover, the size of departing bubbles, active nucleation site density and bubble departure frequency decrease as the heat flux is reduced to the low level of (q) over bar - Delta q. The opposite processes take place for the heat flux raised to the high level of (q) over bar + Delta q. Furthermore, at the mean imposed heat flux close to that for the ONB in the stable boiling we observe intermittent boiling in the flow. A regime map is provided to delineate the boundaries among single-phase liquid flow, intermittent boiling and persistent boiling. (C) 2011 Elsevier Ltd. All rights reserved.