Mass Flux Range Research Articles

Analysis of Two-Phase Pressure Drop Fluctuation Characteristics in a Single Mini-Channel

Two-phase pressure drop fluctuations during flow boiling in a single mini-channel were experimentally investigated. Degassed water was tested in circular cross section mini-channels with the hydraulic diameter of 1.0 mm at liquid mass fluxes range of 21.19-84.77 kg m-2 s-1 and heat fluxes of 0~155.75 kW m-2. Effects of heat flux and mass flux on pressure drop fluctuations were discussed based on the time and frequency domain analysis of the measured pressure drop. Two types of fluctuations were identified, which are the incipient boiling fluctuation (IBF) and the explosive boiling fluctuation (EBF) respectively. The IBF is a low frequency low amplitude fluctuation, which relates to the bubble dynamics when incipient boiling occurs. It is sensitive to the thermal and flow conditions. With the increase of heat flux and mass flux, the IBF is suppressed. The EBF is a low frequency high amplitude fluctuation, which occurs near the critical heat flux.

Effect of gravity vector on flow boiling heat transfer, flow pattern map, and pressure drop of R245fa refrigerant in mini tubes

Organic Rankine cycles have been proposed to address the on-going issue of global warming by recovering waste heat. R245fa refrigerant has high performance as a working fluid in such cycles. This study looks at the flow boiling and heat transfer mechanism of this refrigerant in a 3-mm stainless-steel tube. Horizontal flow and vertical flow are both considered. The flow pattern in mini tubes is visualized, and flow pattern maps are presented. Annular flow, slug flow, and bubbly flow are present in the horizontal flow. The vertical flow shows annular flow, churn flow, slug flow, and bubbly flow. Heat transfer coefficient data are also given for a mass flux range of 200–700kg/m2s, heat flux of 10–40kW/m2, and inlet quality of 0.1–0.7. The pressure drop and heat transfer mechanism are discussed. The horizontal flow data were compared with the vertical upward flow data. The experimental data for heat transfer coefficient and pressure drop was compared to the well-known correlations from the literature. The correlation by Liu & Winterton for heat transfer coefficient and the homogenous equation of Cicchitti et al. for pressure drop was found to wok the best for this range of experimental conditions. Also the flow patterns were well-predicted by using Ong and Thome correlation. The flow pattern base model of Cioncolini and Thome was able to capture most of the heat transfer coefficient and pressure drop data in annular flow. The results can be used for designing evaporators to consider the differences between vertical and horizontal flow regarding phase-change characteristics in heat exchangers orientation.

An experimental study of flow boiling frictional pressure drop of R134a and evaluation of existing correlations

A series of experiments on flow boiling frictional pressure drop of R134a in three horizontal circular smooth copper tubes with inner diameters of 1.002, 2.168, and 4.065mm were conducted. A total of 397 experimental data points are obtained for a mass flux range of 185–935kg/m2s, a heat flux range of 18.0–35.5kW/m2, a saturation pressure range of 0.578–0.82MPa, and a vapor quality range of 0.03–1.0. The effects of mass flux, heat flux, vapor quality, saturation pressure, and diameter on the frictional pressure drop are analyzed and the results indicate that the frictional pressure drop increases with increasing mass flux, increases with vapor quality until a peak and then drops, decreases with increasing saturation pressure and hydraulic diameter, and almost keep constant with heat flux. Thirty-two existing correlations of two-phase frictional pressure drop are compared with the experimental data. The best correlation has the mean absolute deviation of 15.0%. Besides, an experimental database of flow boiling frictional pressure drop is complied, which contains 3244 data points, including 397 from the current experiments and 2847 from 27 published papers. The 32 existing correlations are evaluated with the database. The best correlation has the mean absolute deviation of 24.5%. The work provides a guide for selecting a suitable correlation for specific applications of flow boiling frictional pressure drop.

Experimental investigation on heat transfer characteristics of supercritical CO2 cooled in horizontal helically coiled tube

An experimental investigation on the heat transfer characteristics of supercritical CO2 during gas cooling process in a helically coiled tube is conducted. The experimental data are obtained over a mass flux range of 79.6–238.7 kg m−2 s−1, an inlet pressure range of 7.5–9.0 MPa and a mean bulk temperature of 23.0–53.0 °C. The effects of mass flux, bulk temperature and pressure on the heat transfer coefficient for helically coiled tubes are investigated. A comparative analysis of the gravitational buoyancy and the heat transfer coefficient is carried out between helically coiled tubes and straight tubes. A new heat transfer correlation of the supercritical CO2 in the horizontal helically coiled tube is proposed based on the experimental data. The maximum error between the predicted results of the new correlation and the experimental data is 20%.

Labile trace metal contribution of the runoff collector to a semi-urban river.

In this study, the distribution of labile trace metals (LTMs; Cd, Co, Cr, Cu, Ni, Pb, and Zn) in a semi-urban runoff collector was examined to assess its influence to a natural aqueous system (Jalle River, Bordeaux, France). This river is of high importance as it is part of a natural reserve dedicated to conserving aquatic flora and fauna. Two sampling campaigns with a differing precipitation condition (period 1, spring season; and period 2, summer season associated with storms) were considered. Precipitation and water flow were monitored. The collector is active as it is receptive to precipitation changes. It influences the river through discharging water, contributing LTMs, and channeling the mass fluxes. During period 2 where precipitation rate is higher, 25% of the total water volume of the river was supplied by the collector. LTMs were detected at the collector. Measurements were done by using diffusive gradient in thin films (DGT) probes deployed during 1, 7, and 14days in each period. The results showed that in an instantaneous period (day 1 or D1), most of these trace metals are above the environmental quality standards (Cd, Co, Cr, and Zn). The coefficient of determination (r (2) > 0.50) employed confirmed that the LTM concentrations in the downstream can be explained by the collector. While Co and Cr are from the upstream and the collector, Cd, Cu, and Zn are mostly provided by the collector. Ni, however, is mostly delivered by the upstream. Using the concentrations observed, the river can be affected by the collector in varying ways: (1) adding effect, resulting from the mix of the upstream and the collector (if upstream ˂ downstream); (2) diluted (if upstream ˃ downstream); and (3) conservative or unaffected (upstream ~ downstream). The range of LTM mass fluxes that the collector holds are as follows: (1) limited range or ˂10g/day, Cd (0.04-1.75g/day), Co (0.08-05.42g/day), Ni (0.06-1.45g/day), and Pb (0.08-9.89g/day); (2) moderate range or 11-50g/day, Cr (0.23-33.26g/day) and Cu (0.77-37.88g/day); and (3) wide range or ˃50g/day, Zn (26.33-676.61g/day). Hence, the collector is a major source of concern in terms of contamination. This is as the water with considerable LTMs is channeled openly to the river without any treatment.

Enhancement of Critical Heat Flux in Tight Rod Bundle with Wire Spacer

The experiment of CHF (critical heat flux) was conducted for water boiling two-phase flow in three-pin tight rod bundle. The effects of with and without wire spacers and the pitch to diameter ratio p/d on CHF were investigated under the conditions of mass flux range 250-430 kg/(m 2 ·s), inlet temperature from 70 °C to 92 °C and the pressure of 0.1 MPa. The CHF was enhanced by wire spacers in comparison with the results of CHF without wire spacers. The CHF was enhanced by reducing the p/d from 1.18 to 1.10 under the same flow rate condition, although it did not change appreciably with the change of p/d under the same mass flux condition.

Nonintrusive Optical Validation of Two-Phase Flow Regimes in a Small-Diameter Tube

While the majority of studies examining the relationship between flow regime and thermofluid performance rely on subjective flow regime determination, objective techniques are needed to more firmly establish the nature and repeatability of these phenomena. This study describes a nonintrusive optical flow regime characterization methodology used to study horizontal, adiabatic, two-phase flow of water–water vapor and water–nitrogen gas in small (8.84 mm) diameter tubes. The method relies on shining a fiber-optic light source through the top of a borosilicate glass tube at the outlet of a smooth copper tube, using a CMOS camera to capture light rings resulting from total internal reflection at the liquid–vapor interface, and extracting a film thickness profile from successive images. Using these unique temporally varying film profiles, quantitative identification measures were developed for the primary flow regimes, including the ability to explain and quantify the more subtle transitions that exist between dominant regimes as the two-phase flows progress from saturated liquid to qualities of 0.32. Application of this methodology has shown the Taitel–Dukler, Ullmann–Brauner, and Wojtan et al. phenomenological flow regime maps to capture the salient features and transition boundaries, with varying accuracy, for small-diameter two-phase flow in a mass flux range of 15 to 230 kg/m2-s.

Heat Transfer During Near-Critical-Pressure Condensation of Refrigerant Blends

Heat transfer during condensation of refrigerant blends R404A and R410A flowing through horizontal tubes with 0.76 ≤ D ≤ 9.4 mm at nominal Pr = 0.8–0.9 was investigated. Local heat transfer coefficients were measured for the mass flux range 200 < G < 800 kg m−2 s−1 in small quality increments over the entire vapor–liquid region. Heat transfer coefficients increased with quality and mass flux, while the effect of reduced pressure was not very significant within this range of pressures. The heat transfer coefficients increased with a decrease in diameter. Correlations from the literature were not able to predict the condensation heat transfer coefficient for these fluids at these near-critical pressures over the wide range of tube diameters under consideration. A new flow-regime based model for heat transfer in the wavy, annular, and annular/wavy transition regimes, which predicts 91% of the data within ±25%, is proposed.

Transient Analysis of Microchannel Heat Sink during Single Phase and Flow Boiling Conditions

In the present work, an experimental investigation has been made to analyze the performance of microchannel heat sink under transient operating conditions. The transient analysis has been made by estimating the response time for different input heat flux and coolant mass flow rate. Analysis has been made for rectangular cross-section microchannels fabricated on a copper block of size 25.7 × 12 × 10 mm3. Twelve (12) numbers of microchannels are fabricated in the copper block. The width and depth of individual channels are 400 μm and 750 μm respectively. Performance analysis has been made for both single phase and flow boiling conditions of the coolant flow using deionized water as coolant. Experiments have been performed for coolant mass flux (G) range of 90 - 250 kg/m2s and input heat flux (q) range of 20 - 300 kW/m2 respectively. It has been observed that at constant input heat flux, response time decreases with the increase in coolant mass flux during single phase cooling. However this trend is not strongly followed during the two-phase or flow boiling cooling condition.

An experimental study on characteristics of post-CHF heat transfer in the high subcritical pressure region near to the critical pressure

Supercritical pressurized water cooled reactor (SCWR), which has a once-through water cooled reactor for supplying supercritical pressure steam at high temperature to a turbine system, is one of the next generation reactors for the purpose of improving economic efficiency and safety. In the SCWR system, water pressure passes through the critical pressure during startup, shutdown and in case of loss of coolant accident (LOCA). Similarly, in fossil-fired power plants, once-through boilers of sliding pressure type operate at supercritical pressure during nominal load, but they work near the critical pressure during partial load. In the high subcritical pressure region slightly below the critical pressure, critical heat flux (CHF) phenomenon readily occurs at relatively low heat flux and in a high subcooled region, and then it causes serious damages to fuel rods or boiler tubes due to the abrupt temperature rise in the rod or tube. In this study, experiments on post-CHF heat transfer in such the high subcritical pressure region near the critical pressure (reduced pressure range of 0.924 up to 0.992) in the range of mass flux 400-1000 kg/(m(2).s) with a circular tube of 4.4 mm ID in vertical upward flow. HCFC22 and HFC134a were used as the test fluids. Based on the obtained data, characteristics of the post-CHF heat transfer were examined and found to be classified into two distinctive types mainly according to flow rate conditions. The influences of pressure, flow rate and heat flux on the characteristics were also clarified.

Pressure drop characteristics of R134a during flow boiling in a single rectangular micro-channel

The pressure-drop characteristics during flow boiling in a single rectangular micro-channel with hydraulic diameter of 0.68mm are presented. In the present study, pressure drop was studied at heat flux range of 7.63–49.46kW/m2, mass flux range of 600–1400kg/m2s, and saturation temperature of 23, 27 and 31°C. Experimental results indicated that the total pressure was dominated by frictional pressure drop. The increase of mass flux also increased the frictional pressure gradient, whereas the increase of saturation temperature reduced the frictional pressure gradient. In addition, heat flux also had an insignificant effect on the frictional the pressure gradient. A new correlation was also proposed for effective design of micro-channel heat exchanger.

Boiling Heat Transfer and Pressure Drop of a Refrigerant R32 Flowing in a Small Horizontal Tube

In this study, experiments were performed to examine characteristics of flow boiling heat transfer and pressure drop of a low global warming potential refrigerant R32 flowing in a horizontal copper circular tube with 1.0 mm inside diameter for the development of a high-performance heat exchanger using small-diameter tubes or minichannels for air conditioning systems. Axially local heat transfer coefficients were measured in the range of mass fluxes from 30 to 400 kg/(m2·s), qualities from 0.05 to 1.0, and heat fluxes from 2 to 24 kW/m2 at the saturation temperature of 10°C. Pressure drops were also measured in the rage of mass fluxes from 30 to 400 kg/(m2·s) and qualities from 0.05 to 0.9 at the saturation temperature of 10°C under adiabatic condition. In addition, two-phase flow patterns were observed through a sight glass fixed at the tube exit with a digital camera. The characteristics of boiling heat transfer and pressure drop were clarified based on the measurements and the comparison with data of R410A obtained previously. Also, measured heat transfer coefficients were compared with two existing correlations.

High resolution infrared measurements of single-phase flow of R245fa and R236fa within a compact plate heat exchanger, Part 1: Experimental setup and pressure drop results

This two-part paper presents experimental work to characterize thermal and hydraulic performances of a compact plate heat exchanger. Upward single-phase heat transfer and pressure drop of low pressure, liquid refrigerants R245fa and R236fa was investigated within a plate prototype fabricated with a 1 mm pressing depth and a chevron angle of 65°. Specifically in Part 1, experiments were carried out for Reynolds numbers ranging from 34 to 1615, corresponding to a range of mass flux from 8 to 384 kg m−2 s−1 and Prandtl numbers ranging from 4.9 to 6.5. The Fanning friction factor was found to be strongly dependent on the Reynolds number and the experimental pressure drop database was validated against several prediction methods available in the open literature. Fine resolution infrared measurements were performed to obtain local (pixel-by-pixel) heat transfer coefficient, fully described in Part 2, while the technique developed to reduce the experimental data is detailed in Part 1. Finally, a frictional pressure drop prediction method, which captured the entire experimental databank within a bandwidth of ± 20% and mean absolute error of 8.2%, was also provided.

Modeling and validation of interfacial area transport equation in subcooled boiling flow

The first comprehensive validation of the interfacial area transport equation in subcooled boiling is presented and shown to perform exceptionally when compared with experimental data. The formulation and closure of the bubble layer averaged interfacial area transport equation is reviewed along with the treatment of the two-fluid model in subcooled boiling. Interfacial area concentration source and sink terms in subcooled boiling are presented including the bubble interaction mechanisms (random collision and turbulent impact), as well as phase change terms (wall nucleation and condensation). Additionally, the volume source terms from phase change are described and discussed in terms of their significance to the interfacial area transport equation. The validation of the interfacial area transport equation with a recently proposed wall nucleation source term is shown to have excellent prediction at low and elevated pressure, as well as a wide range of mass flux. With new confidence in the wall nucleation source term, the interfacial area concentration in subcooled boiling can be accurately predicted. Due to its strong dependence in the modeling of active nucleation site density, bubble departure frequency, and departure diameter, the calculation is shown to be very sensitive to wall temperature.

An experimental study of flow boiling heat transfer of R134a and evaluation of existing correlations

A series of experiments on flow boiling heat transfer coefficient of R134a in three horizontal circular smooth copper tubes with inner diameters of 1.002, 2.168, and 4.065mm were conducted. A total of 397 experimental data points are obtained for a mass flux range of 185–935kg/m2s, a heat flux range of 18.0–35.5kW/m2, a saturation pressure range of 0.578–0.82MPa, and a vapor quality range of 0.03–1.0. The effects of heat flux, saturation pressure, vapor quality, mass flux, and diameter on the heat transfer coefficient are analyzed and the results indicate that the heat transfer coefficient is higher for smaller tubes and for higher saturation pressure while the effects of heat flux, mass flux, and vapor quality are different for different tubes. There are 28 existing correlations of flow boiling heat transfer compared with the experimental data, and the comparison results indicate that the best correlation has a mean absolute deviation of 12.1% and has strong ability to trace the tendency of the heat transfer coefficient with vapor quality. Besides, an experimental database of R134a flow boiling heat transfer is complied, which contains 3000 data points, including 397 from the current experiments and 2603 from 21 published papers. The 28 existing correlations are evaluated with the database, providing a guide for selecting a suitable correlation for specific applications of R134a flow boiling heat transfer.

A Review of Recent Empirical Correlations for the Calculation of Determination of R134a’s Convective Heat Transfer Coefficient in Vertical Condensers

Condensation heat transfer of R134a which flowed in the inner tube and cooling water which flowed in the annulus were experimentally investigated inside a smooth tube-in-tube heat exchanger in author’s previous studies. In this current work, the investigational setup consists of a 0.5m long double tube and the inner tube is made up of smooth copper tubing of 9.52mm outer diameter and 8.1mm inner diameter. The experiments are carried out for the mass flux range of 260-515kgm-2s-1 and the heat flux range of 11.3-55.3kWm-2 and also at a mean saturation temperature interval of 40-50°C. The mean heat transfer coefficient of R134a is found through executing an energy equivalence predicated on the energy flow from the test section. The predictability of actual correlations in open sources proposed for two-phase annular flow are tested by using authors’ experimental database. Alterations of vapor quality and saturation temperature difference with calculated condensation heat transfer coefficients are also shown according to different condensing temperatures and mass fluxes. Moreover, the independency of annular flow heat transfer empirical equations from tube orientation and the overall practicality for a vertical tube is validated.

Flow pattern transition models and correlations for flow boiling in mini-tubes

An evaluation of models and correlations predicting flow patterns in mini-tubes is described in this paper and final recommendations are made for a way forward. Flow boiling patterns of R245fa in a 1.1mm diameter copper tube were used in this evaluation. The experiments covered an experimental range of mass flux 100–400kg/m2s, heat flux 3–25kW/m2, inlet pressure of 1.85 and inlet subcooling of 5K. Hysteresis was evident in these experiments across the whole range, with obvious changes in the flow patterns between increasing and decreasing heat flux. The four main flow patterns were bubbly, slug, churn and annular flow. Confined flow was also evident. For increasing heat flux, only annular flow was evident but all the flow patterns were evident with decreasing heat flux. Therefore, the evaluation of flow pattern maps carried out in this study was based on the decreasing heat flux data, as this covered the full range of flow patterns. The evaluation of more than ten models and correlations demonstrated that there is no general model that can predict accurately all flow pattern transition boundaries. Only one model succeeded in predicting all transition boundaries very well, except the bubbly to slug transition. Thus, a new modification on this boundary is proposed in this paper that could predict the experimental data used in this study very well.

Experimental investigation on heat transfer and pressure drop of supercritical water flows in an inclined rifled tube

This paper presents an investigation of heat transfer behavior and pressure drop characteristics of subcritical and supercritical water flows inside an inclined rifled tube for near-supercritical and supercritical flows. The operating pressure range was from 15 to 28MPa, the mass flux range was from 600 to 1000kg/m2s and wall heat flux was from 300 to 500kW/m2. The maximum inner diameter of pipe was 19.5mm and the inclination angle of the tube was 20° with respect to the horizontal plane. The heat transfer to water and pressure drop gradients at various operating pressures for various heat fluxes and mass fluxes were studied. The results show that the effect of mass flux variations on heat transfer at supercritical pressures is more considerable than this effect at subcritical pressures. The same behavior was found for the pressure drop gradient. It was found that as the ratio of the mass flux to the heat flux exceeded 2.46kg/kWs, heat transfer enhancement occurred near the pseudo critical point for supercritical pressures while maximum heat transfer at subcritical pressures occurred at ratio of the mass flux to the heat flux value of 1.65kg/kWs for inclined rifled tube.

Evaporation heat transfer and pressure drop of R-410A in three 7.0 mm O.D. microfin tubes having different inside geometries

R-410A evaporation heat transfer and pressure drop data are provided for three 7.0 mm O.D. microfin tubes. The microfin tubes had different helix angle, fin height and fin apex angle. Tests were conducted for a range of quality (0.2 ~ 0.8), mass flux (216 ~ 390 kg/m2s), heat flux (9 ~ 17 kW/m2) and saturation temperature (8 ~ 12°C). It was found that three microfin tubes yielded approximately the same heat transfer coefficients. Microfin tube with larger inter-fin spacing or smaller helix angle yielded lager pressure drop. Both heat transfer coefficient and pressure drop increased as mass flux or quality increased. However, they decreased as saturation temperature increased. The range of heat transfer enhancement factor (1.37 ~ 1.97) was comparable with that of pressure drop penalty factor (1.22 ~ 1.77). Data are compared with available heat transfer and pressure drop correlations.

Pressure drop during near-critical-pressure condensation of refrigerant blends

This paper presents the results of an experimental study on pressure drop during condensation of refrigerant blends R404A and R410A at near-critical pressures inside tubes of 0.76 ≤ D ≤ 9.4 mm. Local frictional pressure gradients were measured at reduced pressures (Pr) of 0.8 and 0.9 for the mass flux range 200 ≤ G ≤ 800 kg m−2 s−1, in small quality increments. The data were compared with adiabatic and diabatic two-phase pressure drop correlations from the literature. These correlations were not able to adequately predict the pressure drop over the range of tube sizes and mass fluxes under consideration here at high reduced pressures. A new correlation for pressure drop is introduced, which accounts for property variations and microchannel effects. The correlation predicts 85% of the data within ±25%, with an average deviation between model predictions and data of ±14%.