Visual observation of two-phase flow pattern of R-22, R-134a, and R-407C in a 6.5-mm smooth tube

Two-phase flow pattern and friction characteristics for R-22, R-134a, and R-407C inside a 6.5 mm smooth tube are reported in this study. The range of mass flux is between 50 and 700 kg/(m² s). The experimental data show that the two-phase friction multipliers are strongly related to the flow pattern. For a stratified, wavy flow pattern a mass-flux dependence of the multipliers is seen. The flow pattern transition for the mixture refrigerant shows a considerable delay, compared with that of pure refrigerant.     

Condensation heat transfer characteristics of R-22, R-134a and R-410A in a single circular microtube

The condensation heat transfer coefficients of R-22, R-134a and R-410A in a single circular microtube were investigated experimentally. The experiments are conducted without oil in the refrigerant loop. The test section is a smooth, horizontal copper tube of 1.77 mm inner diameter. The experiments were conducted at mass flux of 450-1050 kg/m² s, saturation temperature of 40 °C. The test results showed that in case of single-phase flow, the single-phase Nusselt Number measured by experimental data was higher than that calculated by Gnielinski correlation. In case of two-phase flow, the condensation heat transfer coefficient of R-410A was higher than that of R-22 and R-134a at the given mass flux. The condensation heat transfer coefficient of R-22 showed almost a similar value to that of R-134a. Most of the existing correlations which were proposed in the large diameter tube failed to predict condensing heat transfer. And also, recently proposed correlation in the single circular microtube is considered not adequate for small diameter tube. Therefore, it is necessary to develop accurate and reliable correlation to predict heat transfer characteristics in the single circular microtube.     

Convective boiling of halocarbon refrigerants flowing in a horizontal copper tube – an experimental study

An experimental study of convective boiling of refrigerants R-22, R-134a and R-404A in a 12.7 mm internal diameter, 2 m long, horizontal copper tube has been performed. Experiments involved a relatively wide range of operational conditions. Experiments were performed at the evaporating temperatures of 8°C and 15°C. Quality, mass velocity and heat flux varied in the following ranges: 5% to saturated vapor, 50–500 kg/(s m²); and 5–20 kW/m². Effects of these physical parameters over the heat transfer coefficient have been investigated. High quality experiments were also performed up to the point of the tube surface dryout, a mechanism which was investigated from the qualitative point of view. Two heat transfer coefficient correlations from the literature have been evaluated through comparisons with experimental data. Deviations varied in the range from −25% to 42%.     

Condensation heat transfer characteristics of R-22, R-134a and R-410A in small diameter tubes

The condensation heat transfer of pure refrigerants, R-22, R-134a and a binary refrigerant R-410A flowing in small diameter tubes was investigated experimentally. The condenser is a countflow heat exchanger which refrigerant flows in the inner tube and cooling water flows in the annulus. The heat exchanger is smooth, horizontal copper tube of 1.77, 3.36 and 5.35 mm inner diameter, respectively. The length of heat exchanger is 1220, 2660 and 3620 mm, respectively. The experiments were conducted at mass flux of 200–400 kg/m² s and saturation temperature of 40°C. The main results were summarized as follows: in case of single-phase flow, the single-phase Nusselt Number measured by experimental data was higher than that calculated by Gnielinski and Wu and Little correlation. The new single-phase correlation based on the experimental data was proposed in this study. In case of two-phase flow, the condensation heat transfer coefficient of R-410A for three tubes was slightly higher than that of R-22 and R-134a at the given mass flux. The condensation heat transfer coefficient of R-22 showed almost a similar value to that of R-134a. The condensation heat transfer coefficient for R-22, R-134a and R-410A increased with increasing mass flux and decreasing tube diameter. Most of the existing correlations which were proposed in the large diameter tube failed to predict condensation heat transfer. Therefore, the new condensation heat transfer correlation based on the experimental data was proposed in the present study.     

Flow boiling heat transfer and pressure drop of R-134a in a mini tube: an experimental investigation

This paper presents the results of an experimental study carried out with R-134a during flow boiling in a horizontal tube of 2.6 mm ID. The experimental tests included (i) heat fluxes in the range from 10 to 100 kW/m², (ii) the refrigerant mass velocities set to the discrete values in the range of 240-930 kg/(m² s) and (iii) saturation temperature of 12 and 22 °C. The study analyzed the heat transfer, through the local heat transfer coefficient along of flow, and pressure drop, under the variation of these different parameters. It was possible to observe the significant influence of heat flux in the heat transfer coefficient and mass velocity in the pressure drop, besides the effects of saturation temperature. In the low quality region, it was possible to observe a significant influence of heat flux on the heat transfer coefficient. In the high vapor quality region, for high mass velocities, this influence tended to vanish, and the coefficient decreased. The influence of mass velocity in the heat transfer coefficient was detected in most tests for a threshold value of vapor quality, which was higher as the heat flux increased. For higher heat flux the heat transfer coefficient was nearly independent of mass velocity. The frictional pressure drop increased with the increase in vapor quality and mass velocity. Predictive models for heat transfer coefficient in mini channels were evaluated and the calculated coefficient agreed well with measured data within a range 35% for saturation temperature of 22 °C. These results extend the ranges of heat fluxes and mass velocities beyond values available in literature, and add a substantial contribution to the comprehension of boiling heat transfer phenomena inside mini channels.     

Heat transfer correlation for boiling flows in small rectangular horizontal channels with low aspect ratios

In the present experimental study, a correlation is proposed to represent the heat transfer coefficients of the boiling flows through horizontal rectangular channels with low aspect ratios. The gap between the upper and the lower plates of each channel ranges from 0.4 to 2 mm while the channel width being fixed to 20 mm. Refrigerant 113 was used as the test fluid. The mass flux ranges from 50 to 200 kg/m² s and the channel walls were uniformly heated up to 15 kW/m². The quality range covers from 0.15 to 0.75 and the flow pattern appeared to be annular. The modified Lockhart–Martinelli correlation for the frictional pressure drop was confirmed to be within an accuracy of ±20%. The heat transfer coefficients increase with the mass flux and the local quality; however the effect of the heat flux appears to be minor. At the low mass flux condition, which is more likely to be with the smaller gap size, the heat transfer rate is primarily controlled by the liquid film thickness. A modified form of the enhancement factor F for the heat transfer coefficient in the range of Re_LF200 well correlates the experimental data within the deviation of ±20%. The Kandlikar's flow boiling correlation covers the higher mass flux range (Re_LF>200) with 10.7% mean deviation.     

Condensation heat transfer and pressure drop characteristics of R-290, R-600a, R-134a and R-22 in horizontal tubes

This paper presents the experimental results of condensation heat transfer coefficients of hydrocarbon (HC) refrigerants R-290 and R-600a, hydrochlorofluorocarbon (HCFC) refrigerant R-22, and hydrofluorocarbon (HFC) refrigerant R-134a in a horizontal double-pipe heat exchanger having pipe inner diameters of 10.07, 7.73, 6.54, and 5.80 mm. The condensation process experiments were conducted at mass flux of 35.5–210.4 kg/m² s and condensation temperature of 40°C. The main results were summarized as follows: The average condensation heat transfer coefficients of R-290 and R-600a were higher than those of R-22 and R-134a. The pressure drops of the four refrigerants were in the order of R-600a > R-290 > R-134a > R-22. The pressure drops of R-600a, R-290, R-134a, and R-22 were approximately 6–15, 9.8–12.5, 4.3–6.7, and 2.1–4.6% higher, respectively, in the 10.7 mm diameter tubes compared to the 5.80 mm diameter tubes. Comparing the condensation heat transfer coefficients of our experimental results with those of other correlations, our experimental data in all the test tubes coincided best with that of Haraguchi et al.     

Subcooled flow boiling of R-134a in vertical channels of small diameter

Subcooled flow boiling heat transfer for refrigerant R-134a in vertical cylindrical tubes with 0.83, 1.22 and 1.70 mm internal diameter was experimentally investigated. The effects of the heat flux, q″ = 1–26 kW/m², mass flux, G = 300–700 kg/m² s, inlet subcooling, ΔT_sub,i = 5–15 °C, system pressure, P = 7.70–10.17 bar, and channel diameter, D, on the subcooled boiling heat transfer were explored in detail. The results are presented in the form of boiling curves and heat transfer coefficients. The boiling curves evidenced the existence of hysteresis when increasing the heat flux until the onset of nucleate boiling, ONB. The wall superheat at ONB was found to be essentially higher than that predicted with correlations for larger tubes. An increase of the mass flux leads, for early subcooled boiling, to an increase in the heat transfer coefficient. However, for fully developed subcooled boiling, increases of the mass flux only result in a slight improvement of the heat transfer. Higher inlet subcooling, higher system pressure and smaller channel diameter lead to better boiling heat transfer. Experimental heat transfer coefficients are compared to predictions from classical correlations available in the literature. None of them predicts the experimental data for all tested conditions.     

Pool boiling performance of finned surfaces in R-113

Performance of horizontal copper heaters with a transverse fin structure was investigated for pool boiling heat transfer and critical heat flux limits. Data were obtained for 5.1 and 7.6 cm diameter structured cooper and brass heaters in saturated R-113 boiling at pressures ranging between 0.037 and 1 atm. The fin structure consisted of 0.16 cm×0.16 cm×0.32 cm high square fins with an interfin spacing of 0.16 cm. Following a similar methodology to Haley and Westwater¹, a numerical analysis of the heat transfer phenomenon was performed by solving the one-dimensional fin conduction equation with a non-linear heat transfer boundary condition obtained from the previously reported data for R-113 boiling on plain surfaces. The predictions agreed with the data at the 1 atm pressure levels but showed deviations at the low pressure levels. The results showed that, compared with plain surface heaters of the same diameters the finned structured surfaces investigated: (a) decreased the wall temperature differences for a given heat flux and saturated pool boiling conditions, thus improving the nucleate boiling heat transfer coefficients, and (b) increased the critical heat flux limits, calculated as the power input divided by the heater projected area, by a factor of 2–2.5.     

An experimental investigation of flow boiling in an asymmetrically heated rectangular microchannel

By using unique experimental techniques and carefully constructed experimental apparatus, the characteristics of flow boiling of water in microscale were investigated using a single horizontal rectangular microchannel. A polydimethylsiloxane rectangular microchannel (D_h = 103.5 and 133 μm) was fabricated by using the replica molding technique, a kind of soft lithography. A piecewise serpentine platinum microheater array on a Pyrex substrate was fabricated with the surface micromachining MEMS technique. Real time flow visualization of the phase change phenomena inside the microchannel was performed using a high speed CCD camera with microscope. The experimental local boiling heat transfer coefficients were studied, and single bubble inception, growth, and departure, as well as elongated bubble behavior were analyzed to elucidate the microscale heat transfer mechanisms. Tests were performed for mass fluxes of 77.5, 154.9, and 309.8 kg/m² s and heat fluxes of 180–500 kW/m². The effects of mass flux, heat flux, and vapor qualities on flow boiling heat transfer in a microchannel were studied.     

Heat transfer and pressure drop of R-134a condensation in a coiled,double tube

An experimental study was carried out to investigate condensation heat transfer and pressure drop characteristics of R-134a in a coiled double tube oriented with its helix axis in the vertical direction. Measurements were obtained at inlet pressure of 815 kPa for refrigerant mass flux ranging from 95 to 710 kg/m²s and cooling water Reynolds number varying from 1000 to 14000. Presented results illustrate the effects of refrigerant mass flux and average condensation temperature difference on the condensation heat transfer coefficient and pressure drop. Comparison with relevant data from other sources indicates a reasonable agreement. An empirical correlation was obtained for predicting condensation heat transfer coefficient. The present study may be considered of a practical and theoretical interest for the design of the helical double-tube condensers using R-134a as the working fluid. M. El-Sayed Mosaad is on leave from Mechanical Engineering Department, Mansoura University, Egypt.     

Two-phase frictional pressure drop multipliers for SUVA R-134a flowing in a rectangular duct

The adiabatic two-phase frictional multipliers for SUVA, R-134a flowing in a rectangular duct (with D_H = 4.8 mm) have been measured for three nominal system pressures (0.9 MPa, T_sat = 35.5 °C; 1.38 MPa, T_sat = 51.8 °C; and 2.41 MPa, T_sat = 75.9 °C) and three nominal mass fluxes (510, 1020 and 2040 kg/m²/s). The data is compared with several classical correlations to assess their predictive capabilities. The Lockhart–Martinelli model gives reasonable results at the lowest pressure and mass flux, near the operating range of most refrigeration systems, but gives increasingly poor comparisons as the pressure and mass flux are increased. The Chisholm B-coefficient model is found to best predict the data over the entire range of test conditions; however, there is significant disagreement at the highest pressure tested (with the model over predicting the data upwards of 100% for some cases). The data shows an increased tendency toward homogeneous flow as the pressure and flow rate are increased, and in fact the homogeneous model best predicts the bulk of the data at the highest pressure tested.     

Fluid flow and heat transfer around circular cylinder – flat and curved plates combinations

Experimental studies were carried out to investigate the fluid flow and heat transfer around a heated circular cylinder which was placed at various distances of a wall boundary with different geometries (flat or curved plate) with subcritical Reynolds number ranging from 3.5×10³ to 10⁴. The effects of plate geometry (aspect ratio: W|H=1.0,1.5 and 2.0, and rim angle, φ=0°,60°,90°, and 120°) and gap ratio, (G|D=0.0,0.86,2.0,7.0,10.0) on the fluid flow and heat transfer characteristics (static pressure around cylinder surface, wake width, base pressure, pressure drag coefficients, velocity distribution, and both local and mean Nusselt numbers) were presented. Also flow visualization was carried out to illustrate the flow patterns around the cylinder at various gap ratios (G|D). It was found that the heat transfer and fluid flow characteristics are dependent on the plate geometry at all tested gap ratios, except for G|D=7.0 and 10.0, they are independent of the plate geometry.     

Evaporation heat transfer and flow characteristics of R-134a flowing through internally grooved tubes

This article describes experimental investigations of the heat transfer coefficient and pressure drop of R-134a flowing inside internally grooved tubes. The test tubes are one smooth tube and four grooved tubes. All test tubes are made from type 304 stainless steel, have an inner diameter of 7.1 mm, are 2,000 mm long and are installed horizontally. The test section is uniformly heated by a DC power supply to create evaporation conditions. The groove depth of all grooved tubes is fixed at 0.2 mm. The experimental conditions are conducted at saturation temperatures of 20, 25 and 30°C, heat fluxes of 5, 10 and 15 kW/m², and mass fluxes of 300, 500 and 700 kg/m² s. The effects of groove pitch, mass flux, heat flux, and saturation temperature on heat transfer coefficient and frictional pressure drop are discussed. The results illustrate that the grooved tubes have a significant effect on the heat transfer coefficient and frictional pressure drop augmentations.     

Heat transfer in bubble layers at elevated pressures

Heat transfer with steam condensation under moderate pressure on the surface of a horizontal tube immersed in a bubbling layer was experimentally investigated. A copper test section 16 mm in outer diameter and 400 mm in length was placed in a bubbling column 455 mm in diameter. Experiments were made under pressures of 0.14–0.8 MPa, with void fraction 0.04–0.23, vapor superficial velocities 0.05–0.42 m/s, liquid-wall temperature differences 47–105 K, and heat flux densities 0.12–0.8 MW/m². The heat transfer process in the bubbling layer is shown to be of a high intensity: with moderate values of steam content, heat transfer coefficients reach 12–14 kW/(m² · K). Data obtained showed that the known correlations do not consider the influence of pressure on heat transfer. For the first time, data on radial steam content distribution under pressures higher than atmospheric were obtained by an electroprobe method. A table of experimental data is presented.     

Condensation pressure drop of HFC-134a and R-404A in a smooth and micro-fin U-tube

The frictional pressure drop during condensation of HFC-134a and R-404A in a smooth (8.56 mm ID) and micro-fin U-tubes (8.96 mm ID) are experimentally investigated. Different from previous studies, the present experiments are performed for various condensing temperatures. The test runs are done at average saturated condensing temperatures ranging from 35 °C to 60 °C. The mass fluxes are between 90 and 800 kg/m²s. The experimental results indicate that the average frictional pressure drop increases with mass flux but decreases with increasing condensing temperature for both smooth and micro-fin-tubes. The average frictional pressure drops of HFC-134a and R-404A for the micro-fin-tubes were 1-1.7 and 1-2.1 times larger than that in smooth tube respectively. New correlations based on the data gathered during the experimentation for predicting frictional pressure drop are proposed for wide range of operating conditions.     

Flow boiling performance in horizontal microfinned copper tubes with the same geometric characteristics

This article reports an experimental investigation on flow boiling heat transfer and pressure drop of refrigerant R-134a in a smooth horizontal and two microfinned tubes from different manufacturers with the same geometric characteristics. Experiments have been carried out in an experimental facility developed for change of phase studies with a test section made with 9.52 mm external diameter, 1.5 m long copper tubes, electrically heated by tape resistors wrapped on the external surface. Tests have been performed under the following conditions: inlet saturation temperature of 5 °C, vapor qualities from 5% to 90%, mass velocity from 100 to 500 kg/s m², and a heat flux of 5 kW/m². Experimental results indicated that the heat transfer performance was basically the same for both microfin tubes. The pressure drop is higher in the microfinned tubes in comparison to the smooth tube over the whole range of mass velocities and vapor qualities. The enhancement factor, used to evaluate the combination of heat transfer and pressure drop, is higher than one for both tubes for mass velocities lower than 300 kg/s m². Values lower than one have been obtained for both tubes in the mass velocity upper range as a result of a significant pressure drop increment not followed by a correspondent increment in the heat transfer coefficient. Some images, illustrating the flow patterns, were obtained from the visualization section, located in the exit of the test section with the same internal diameter of the tested tube.     

Effect of EHD on heat transfer enhancement during two-phase condensation of R-134a at high mass flux in a horizontal smooth tube

In this study, effect of electrohydrodynamic (EHD) on the condensation heat transfer enhancement and pressure drop of pure R-134a are experimentally investigated. The test section is a 2.5 m long counterflow double tube heat exchanger with refrigerant flowing in the inner tube and cooling water flowing in the annulus. The inner tube is made from smooth horizontal copper tubing of 9.52 mm outer diameter. The electrode is made from stainless steel wire of 1.47 mm diameter. The test runs are performed at average saturated temperatures ranging between 40 and 60°C, mass flux ranging between 200 and 600 kg/m² s, heat flux ranging between 10 and 20 kW/m² and applied voltage at 2.5 kV. For the presence of the electrode, the experimental results indicate that the maximum heat transfer enhancement ratio is around 30% while the maximum increase in pressure drop is about 25%.     

Flow boiling visualization in a vertical circular minichannel at high vapor quality

This paper reports on an experimental study of saturated flow boiling of R134a inside a circular vertical quartz tube coated with a transparent heater. The inner diameter of the tube was 1.33 mm and the heated length 235.5 mm. The flow pattern at high vapor qualities and the dryout of the liquid film were studied using a high speed CCD camera at the mass fluxes 47.4 and 124.4 kg/m² s in up flow at 6.425 bar. The heat fluxes ranged from 5 to 13.6 kW/m² for the lower mass flux and from 20 to 32.4 kW/m² for the higher mass flux.

The behavior of the flow close to dryout was found to be different at low and high mass flux. At low mass flux the location of the liquid front fluctuated with waves passing high up in the tube. In between the waves, a thin film was formed, slowly evaporating without breaking up.

At high mass flux the location of the liquid front was more stable. In this case the liquid film was seen to break up into liquid streams and dry zones on the tube wall.     

Flow boiling heat transfer study of R-134a/R-290/R-600a mixture in 9.52 and 12.7 mm smooth horizontal tubes: Experimental investigation

A detailed experimental investigation is carried out to study the flow boiling heat transfer behavior of R-134a/R-290/R-600a (91%/4.068%/4.932% by mass) refrigerant mixture in smooth horizontal tubes of diameter 9.52 and 12.7 mm. The heat transfer coefficients of the mixture are experimentally measured under varied heat flux conditions for stratified flow patterns using a coaxial counter-current heat exchanger test section. The tests are conducted for refrigerant inlet temperatures between ?9 and 5 °C and mass flow rates ranging from 3 to 5 g s^?1. Kattan–Thome–Favrat maps are used to confirm the flow patterns for the tested conditions. The magnitude of the heat transfer coefficient with respect to flow patterns and different mechanisms of boiling are discussed. The heat transfer coefficient of the refrigerant mixture is also compared with that of R-134a for selected working conditions. The significance of nucleate boiling in the overall heat transfer process under these testing conditions is highlighted.