The objective of this experiment is to investigate the unsteady heat transfer of R-407C subcooled flow boiling (SCFB) due to the oscillating refrigerant flow rate within a narrow tube oriented horizontally for application in energy efficiency air conditioning system. The refrigerant flow rate is input in the cycle of triangular oscillations. Influences of oscillation amplitude (ΔG/G‾) and oscillation period (tp) of the periodic flow rate on SCFB heat transfer are studied. The present results disclose that either ΔG/G‾ or tp has minor impact on time-average SCFB heat transfer performance. However, obvious changes in the temporal variations of the wall temperature on the pipe (Tw) and convective heat transfer coefficient (hr) are found, leading to three heat transfer mechanisms of single phase convection, intermittent nucleate boiling, and persistent subcooled boiling for various input heat fluxes. The Tw of persistent subcooled boiling presents the reversed tendency compared to that of single phase convection due to the variation of the triangular R-407C oscillation. In the case of intermediate heat flux, the unusual boiling phenomenon, called intermittent nucleate boiling, can be obtained depending on the R-407C mass flux. The visualized bubble images are also provided to further describe the intermittent nucleate boiling.

An experiment was performed in this study to investigate the effect of duct spanwise inclinations on the lon-gitudinal vortex pattern in an air mixed convective flow. Key parameters, including Reynolds (Re) numbers from 20 to 30 and Grashof (Gr) numbers from 4071 to 10,855, are investigated. The spatial and temporal attributes of longitudinal vortex rolls (LVRs) are examined via flow field visualization and transient temperature measure-ment in details. Compared with the corresponding non-sloping case, the duct inclination significantly influences the LVR evolution. Due to the non-zero spanwise buoyancy, the vortex roll tends to be asymmetrical across the span and causes the delayed onset of lower LVRs. The temperature fluctuates significantly with time adjacent to the upper sidewall at the larger tilt angle. In an inclined duct, lower LVRs will stretch spanwise to squeeze upper vortex rolls with a decrease in Re number. Increasing the Gr number is beneficial to LVR formation for a smaller spanwise-inclined angle. However, the increase in either Gr number or tilt angle makes the vortex flow relatively unstable. We also provide an empirical expression to determine the vortex onset as a function of Re, Pr, Gr numbers, and the tilted angle.

The present work pertains to experimental studies on the unsteady phenomena in the saturated flow boiling of R407C stemming from the oscillatory refrigerant flow rate with the triangle shape in the horizontal slender annular channel. Attention is mainly paid to examine the effects of different parameters, including mean mass flux, amplitude and period of the oscillating mass flux, saturation temperature of R407C, input thermal flux, and gap size on the boiling diagrams and heat transfer coefficients. Firstly, the steady-state saturated flow boiling is studied as baseline cases. It was concluded that beyond the onset of nucleate boiling region, the mass flux of refrigerant has little influences on the boiling diagrams, which indicates that the fully developed nucleate boiling is the major mechanism. The saturated flow boiling heat transfer coefficient is enhanced as the gap size is reduced. Then, the experiment focuses on the temporal changes in the wall temperature Tw and the heat transfer coefficient hr stemming from oscillatory flow conditions. When the mass flux is oscillated periodically with the triangle shape, the Tw and hr are also oscillated temporally and enhanced with increasing in either the amplitude or frequency of oscillating mass flux. At higher input thermal flux, Tw decreases with the decreasing mass flux, which has opposite tendency compared to the single-phase convection with lower thermal flux. For intermediate exerted thermal flux, there is a negligible oscillation in the Tw and hr for most cases; the amplitudes of oscillations are lower than 0.2 °C. Besides, the significant oscillations in the wall temperature and heat transfer coefficient are noted the case with large period of the oscillating mass flux.

In this study, the time-periodical flow rate oscillation for R-410A in horizontal narrow annuli is experimentally investigated in details for its heat transfer feature. The influence of average level of refrigerant mass flux G over bar, saturation temperature Tsat, applied heat flux q, and amplitude Delta G/G over bar and period tp of oscillating flow rate on the evaporative heat transfer of R-410A in a horizontal narrow annulus is presented. For the experimentation, the gap of the employed annular pipe is 2 mm, whereas the ranges of G over bar, T-sat, and q are between G over bar = 300 to 400 kg/(m(2)s), T-sat = 5 degrees C, 10 degrees C and 15 degrees C, and q = 5 to 15 kW/m(2), respectively. Particularly, the focus is on the mean level of flow rate oscillations G over bar, and the impact of Delta G/G over bar and tp on time periodic evaporation. The experimental results disclose that the time-averaged hr has minor reliance on the Delta/G over bar and t(p), but the instantaneous T-w and h(r) periodically oscillate with time in the oscillation frequency as the oscillating of the mass flux since the bubble formation and growth are highly dependent on the mass flow rate. Different from the single-phase heat transfer, the T-w decreases with the reducing refrigerant flow rate, indicating the better the evaporative heat transfer on the heating surface in the periodic evaporation. This trend is reversed for the single-phase convection. These unusual variations in the Tw and hr with the oscillating mass flux are ascribed to the change of the steam quality and liquid film thickness. Besides, the amplitude of the T-w oscillation changes non-monotonously in association with the vapor quality for a particular G over bar, T-sat, q , and Delta G/G over bar and t(p). Finally, the correlations are provided to explore the dependency of hr on the oscillating mass flux and vapor quality of refrigerant R-410A under the mass flux oscillation. (c) 2021 Elsevier Ltd. All rights reserved.

In this investigation, the evaporative heat transfer characteristics of R410A flow in a slender annular duct under an oscillatory heat flux are studied experimentally with emphasis on the influences of mean mass flux, heat flux, and oscillating amplitude and period of heat flux. The applied heat flux is varied in ranges of q = 5,0 0 0 to 15,0 0 0 W m(-2) and the mean mass fluxes of 300 and 400 kg.(m -2s -1) are considered, respectively. The results reveal that increasing either oscillating amplitude or period of heat flux enhances the oscillation of surface temperature and evaporation heat transfer coefficient (HTC). The prevailing influence of heat flux oscillations on the evaporation heat transfer causes notable variations in the vapour quality and liquid film thickness. At a high level of vapour quality, the thickness of refrigerant liquid film on the hot surface is decreased, leading to the increase of the evaporation HTC. Furthermore, the oscillation in the wall temperature is slightly stronger as the period of heat flux oscillation boosts in the range of 120 s to 600 s. Based on the present measured data, some correlations are provided for the evaporation HTC of R410A as the function of the heat flux and vapour quality. (C)& nbsp;2022 Elsevier Ltd. All rights reserved.

The present research is concerned with the experimental work on R-410A subcooled flow boiling in horizontal annuli. Firstly, boiling curves and boiling heat transfer coefficients are used to explore effects of input heat flux (q), liquid mass flux (G), inlet refrigerant subcooling (ΔTsub), saturation temperature (Tsat) and annular gap (δ). The results indicate the obvious temperature drop occurs during onset of nucleate boiling (ONB) and increasing G and ΔTsub enlarges the temperature undershoot. With the increasing q, the heat transfer coefficient is raised apparently. Higher ΔTsub and larger δ both lead to lower boiling heat transfer coefficients, but varying G and Tsat has small influence on the boiling heat transfer. Secondly, the visualization of bubble behavior discloses increasing q improves bubble size, formation, and departure. With the increase in G, ΔTsub, Tsat and δ, the bubble growth is suppressed, leading to the reduction in bubble detachment diameter (dp). Moreover, the bubble detachment frequency (fb) and mean density of active nucleation site (Nac) are reduced with either the increase in ΔTsub and δ or the decrease in Tsat. Finally, empirical correlations are proposed for dp, fb, Nac, and total heat flux, which yields satisfactory agreement with our measured data.

Subcooled flow boiling (SFB) is a complicated phenomenon regarding the significant heat transfer and vapor bubble formation, which is extremely critical either in daily life or industrial application. The present work pertains to the experimental study on heat transfer characteristics and bubble behaviors in narrow horizontal annuli with emphasis on diverse refrigerants of R-134a, R–407C and R-410A. From experimental results, the significant temperature undershoot during ONB is observed and the level of temperature drop is R-134a > R–407C > R-410A. Enhancing the input heat flux increases boiling heat transfer coefficient (hr) for all three refrigerants. The hr of R-410A always remains higher than R-134a and R–407C stemming from the physical property of lower surface tension. The visualization of bubble behavior addresses that enhancing the input heat flux improves buoyancy force, resulting in the increases of bubble departure size and departure frequency. To achieve a quantitative analysis of bubble characteristics, three indexes of bubble detachment diameter (dp), bubble departure frequency (fb) and the active nucleation site density (Nac) are introduced to explore bubble dynamic behaviors in SFB. The results show that the refrigerant of R-410A exhibits the smallest dp but the highest fb and Nac among these three refrigerants.

The present study pertains to the experimental study on the R-134a subcooled flow boiling stemming from oscillatory heat flux in narrow annuli oriented horizontally with focus on heat transfer characteristics. The test section used in the experiment is composed of a Pyrex glass tube, which is completely designed to measure the flow boiling heat transfer coefficient (FBHTC). In this experiment, the annular gap of the horizontal duct is in the range of 1-5 mm with governing parameters used for practical applications in air conditioning and refrigerating devices. Particularly, the focus is on the effects of heat flux oscillation mean value (q), amplitude (Delta q) and period (tp) on the heat transfer features of subcooled flow boiling. The experimental results show that the obvious temperature undershoot takes place during onset of nucleate boiling (ONB) for R-134a oscillating subcooled flow boiling. Increasing either Delta q or tp has relatively little effect on the boiling curve and FBHTC at a given heat flux, while the annular gap plays an important role. Moreover, increasing either the amplitude or period of incident heat flux enhances the wall temperature and FBHTC; while an increase in mean heat flux tends to enhance the wall temperature, but has negligible effect on FBHTC for the flow boiling. Then, a flow pattern map is provided to describe the boundary that distinguishes the different boiling modes of the oscillating heat flux to the subcooled R-134a flow. Finally, a parametric analysis is carried out to further understand the effects of Delta q and tp on the heating wall temperature.

In this paper, a comprehensive experimental investigation is performed to study the bubble characteristics during the alternating subcooled flow boiling with the oscillated refrigerant flow rates. The R-407C refrigerant is selected as the refrigerant. Different parameters related to the bubble characteristics are investigated during the temporal alternating R-407C flow boiling. In addition, a number of relationships are proposed for these parameters as the function of R-470C mass flux. The flow regime map for this type of boiling is also investigated. The results indicated that the intermittent boiling can be observed for the wider ranges of the Boiling number. For the long oscillation period of the mass flux, the oscillation vibration of the subcooled flow boiling is quite strong. The bubble departure frequency boosts with increasing the mean mass flux. Eventually, the active nucleation site density is enhanced with increasing the mean saturation temperature of refrigerant.

In this work, an experimental investigation is accomplished on the bubble dynamics of alternating saturated flow boiling of refrigerant R-407. The refrigerant flow rate is oscillated with time approximating the form of the inverted triangular wave to study bubble behaviors of saturated flow boiling. The effects of different parameters, including average mass flux G¯, period of oscillating mass flux tp, amplitude of oscillating mass flux ΔG, and mean saturation temperature T¯sat of refrigerant R-407, on bubble structures, mean bubble departing size and frequency, and mean density of active nucleation site are investigated. Measured results show that the times in which the bubble nucleation phenomenon is started or terminated are associated with the oscillatory flow conditions. The bubble departure size is raised considerably with the decreasing mass flux in the first half of the alternating cycle. However, an opposite trend is observed in the second half of the temporal alternating cycle where the mass flux is raised. The period of oscillation in the mass flux has negligible influence on the bubble dynamics. Finally, more changes in the bubble departing frequency and the densities of the active bubble nucleation sites are observed as the amplitude of oscillation in the mass flux increases.

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.

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.

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.

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.

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.

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.

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 R-134a mass flux G¯ varies from 200 to 500 kg/m2 s, imposed heat flux ranges from 0 to 45 kW/m2, and the amplitude of the mass flux oscillation changes from 0 to 30% of G¯ with the period of the mass flux oscillation varied from 20 to 120 s for the inlet liquid subcooling ranging from 0 to 6 °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 Tw 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 dp, f and nac 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.

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 400kg/m2s, mean imposed heat flux q¯ ranges from 0 to 10W/cm2 and the amplitude of the heat flux oscillation Δq is fixed at 10–50% of q¯ with the period of the heat flux oscillation varied from 10 to 30s. 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¯, a larger Δq/q¯, and a longer tp they exhibit stronger oscillations. Besides, a slight time lag in Tw 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¯−Δq. The opposite processes take place for the heat flux raised to the high level of q¯+Δq. 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.

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.