Post-CHF heat transfer during two-phase upflow boiling of R-407C in a vertical pipe

This paper reports a study of heat transfer in the post-critical heat flux (post-CHF) regime under forced convective upflow conditions in a uniformly heated vertical tube of 12.7 mm internal diameter and 3 m length. Experiments were conducted with non-azeotropic ternary refrigerant mixture R-407C for reduced pressures ranging from 0.37 to 0.75, mass flux values from 1200 to 2000 kg/m² s and heat flux from 50 to 80 kW/m². Data shows a considerable effect of system pressure on the post-CHF heat transfer coefficient for specified mass and heat fluxes. The post-CHF heat transfer coefficients for R-407C are compared with three existing correlations which are found to over predict the current data. A modified correlation to represent the experimental data for R-407C is presented.     

Experimental investigation of single-phase turbulent flow of R-134a in a multiport microchannel heat sink

This experimental study aims to investigate the heat transfer characteristics of single-phase turbulent flow of R-134a refrigerant in a rectangular multi-micro channel heat sink having 27 channels where each channel has a hydraulic diameter of 421 μm. Experimental results were obtained for inlet temperatures ranging from 24 to 33 °C, mass fluxes from 1485 to 2784 kg m^− 2 s^− 1 and wall heat fluxes from 3 to 24 kW m^− 2. The results indicate that the heat transfer coefficients are found to be higher at lower inlet temperatures than those at higher ones. In addition, when equal amount of heat supplied to the heat sink, the heat transfer coefficients increase with increasing the mass flux of refrigerant. They were also compared with 12 well-known correlations and it was seen that 4 of 12 were in good agreement with each other with the average deviation < 10%. The findings demonstrate that well-known correlations in fundamental sources can be used to predict the heat transfer coefficient of R-134a during its single phase flow in a multiport microchannel heat sink under turbulent regime.     

Experimental study of evaporation heat transfer characteristics of refrigerants R-134a and R-407C in horizontal small tubes

An experiment is carried out here to investigate the characteristics of the evaporation heat transfer for refrigerants R-134a and R-407C flowing in horizontal small tubes having the same inside diameter of 0.83 or 2.0 mm. In the experiment for the 2.0-mm tubes, the refrigerant mass flux G is varied from 200 to 400 kg/m² s, imposed heat flux q from 5 to 15 kW/m², inlet vapor quality x_in from 0.2 to 0.8 and refrigerant saturation temperature T_sat from 5 to 15 °C. While for the 0.83-mm tubes, G is varied from 800 to 1500 kg/m² s with the other parameters varied in the same ranges as those for D_i = 2.0 mm. In the study the effects of the refrigerant vapor quality, mass flux, saturation temperature and imposed heat flux on the measured evaporation heat transfer coefficient h_r are examined in detail. The experimental data clearly show that both the R-134a and R-407C evaporation heat transfer coefficients increase almost linearly and significantly with the vapor quality of the refrigerant, except at low mass flux and high heat flux. Besides, the evaporation heat transfer coefficients also increase substantially with the rises in the imposed heat flux, refrigerant mass flux and saturation temperature. At low R-134a mass flux and high imposed heat flux the evaporation heat transfer coefficient in the smaller tubes (D_i = 0.83 mm) may decline at increasing vapor quality when the quality is high, due to the partial dryout of the refrigerant flow in the smaller tubes at these conditions. We also note that under the same x_in, T_sat, G, q and D_i, refrigerant R-407C has a higher h_r when compared with that for R-134a. Finally, an empirical correlation for the R-134a and R-407C evaporation heat transfer coefficients in the small tubes is proposed.     

Flow pattern and heat transfer characteristics of R-134a refrigerant during flow boiling in a horizontal circular mini-channel

Flow boiling heat transfer of R-134a refrigerant in a circular mini-channel, 600 mm long with a diameter of 1.75 mm, is investigated experimentally in this study. The test section is a stainless steel tube placed horizontally. Flow pattern and heat transfer coefficient data are obtained for a mass flux range of 200–1000 kg/m² s, a heat flux range of 1–83 kW/m² and saturation pressures of 8, 10, and 13 bar. Five different flow patterns including slug flow, throat-annular flow, churn flow, annular flow and annular-rivulet flow are observed and the heat transfer coefficient data for different flow patterns are presented. The heat transfer coefficient increases with increasing heat flux but is mostly independent of mass flux and vapour quality. In addition, it is indicated from the experiments that the higher the saturation pressure, the lower is the heat transfer coefficient. Comparisons of the present data with the existing correlations are also presented.     

Experimental study of R-134a evaporation heat transfer in a narrow annular duct

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_r are examined in detail. For the duct gap of 2.0 mm, the refrigerant mass flux G is varied from 300 to 500 kg/m² s, imposed heat flux q from 5 to 15 kW/m², vapor quality x_m from 0.05 to 0.95, and refrigerant saturation temperature T_sat from 5 to 15 °C. While for the gap of 1.0 mm, G is varied from 500 to 700 kg/m² s with the other parameters varied in the same ranges as that for δ = 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_sat, 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.     

Condensation heat transfer coefficients of the zeotropic refrigerant mixture R-22/R-142b in smooth horizonal tubes

Heat transfer coefficients during condensation of the zeotropic refrigerant mixture R-22 with R-142b are presented. Measurements were obtained at different mass fractions in a smooth horizontal tube. All measurements were conducted at a high condensing saturation pressure of 2.43 MPa, which corresponds to a condensation temperature of 60 °C for R-22. The measurements were taken in 8.11 mm inner diameter smooth tubes with lengths of 1 603 mm. The heat transfer coefficients were determined with the Log Mean Temperature Difference equations. It was found that at low mass fluxes, between 40 kg·m⁻²·s⁻¹ to 350 kg·m⁻²·s⁻¹, the refrigerant mass fraction influences the heat transfer coefficient by up to a factor of two. The heat transfer coefficients decrease as the fraction of R-142b is increased. At high mass fluxes, of 350 kg·m⁻²·s⁻¹ and more the heat transfer coefficients were not strongly influenced by the refrigerant mass fraction. The average heat transfer coefficient decreased by only 7% as the refrigerant mass fraction changed from 100% R-22 to 50%/50% R-22/R142b.     

Effect of tube diameter on boiling heat transfer of R-134a in horizontal small-diameter tubes

The boiling heat transfer of refrigerant R-134a flow in horizontal small-diameter tubes with inner diameter of 0.51, 1.12, and 3.1 mm was experimentally investigated. Local heat transfer coefficient and pressure drop were measured for a heat flux ranging from 5 to 39 kW/m², mass flux from 150 to 450 kg/m² s, evaporating temperature from 278.15 to 288.15 K, and inlet vapor quality from 0 to 0.2. Flow patterns were observed by using a high-speed video camera through a sight glass at the entrance of an evaporator. Results showed that with decreasing tube diameter, the local heat transfer coefficient starts decreasing at lower vapor quality. Although the effect of mass flux on the local heat transfer coefficient decreased with decreasing tube diameter, the effect of heat flux was strong in all three tubes. The measured pressure drop for the 3.1-mm-ID tube agreed well with that predicted by the Lockhart–Martinelli correlation, but when the inner tube diameter was 0.51 mm, the measured pressure drop agreed well with that predicted by the homogenous pressure drop model. With decreasing tube diameter, the flow inside a tube approached homogeneous flow. The contribution of forced convective evaporation to the boiling heat transfer decreases with decreasing the inner tube diameter.     

Investigation on heat transfer and pressure drop during swirl flow boiling of R-134a in a horizontal tube

An experimental investigation has been carried out to study the effect of twisted tape inserts on heat transfer enhancement and pressure drop in a horizontal tube during swirl flow boiling of R-134a. The test-evaporator was an electrically heated horizontal copper tube and twisted tapes with different twist ratios of 6, 9, 12 and 15 were inserted one by one. The data were acquired at the refrigerant mass velocities of 54, 86, 114 and 136 kg/s m². The twisted tape inserts increases the boiling heat transfer coefficients and the pressure drop across the test-evaporator. An empirical correlation has also been developed to predict the swirl flow pressure drop in the test-evaporator.     

Local heat transfer coefficient for pool boiling of R-134a and R-123 on smooth and enhanced tubes

The current paper presents experimental investigation of nucleate pool boiling of R-134a and R-123 on enhanced and smooth tubes. The enhanced tubes used were TBIIHP and TBIILP for R-134a and R-123, respectively. Pool boiling data were taken for smooth and enhanced tubes in a single tube test section. Data were taken at a saturation temperature of 4.44 °C. Each test tube had an outside diameter of 19.05 mm and a length of 1 m. The test section was water heated with an insert in the water passage. The insert allowed measurement of local water temperatures down the length of the test tube. Utilizing this instrumentation, local heat transfer coefficients were determined at five locations along the test tube. The heat flux range was 2.5–157.5 kW/m² for the TBIIHP tube and 3.1–73.2 kW/m² for the TBIILP tube. The resulting heat transfer coefficient range was 4146–23255 W/m². °C and 5331–25950 W/m². °C for both tubes, respectively. For smooth tube testing, the heat flux ranges were 7.3–130.7 kW/m² and 7.5–60.7 kW/m² for R-134a and R-123, respectively; with resulting heat transfer coefficient ranges of 1798.9–11,379 W/m². °C and 535.4–3181.8 W/m². °C. The study provided one of the widest heat flux ranges ever examined for these types of tubes and showed significant structure to the pool boiling curve that had not been traditionally observed. Additionally, this paper presented an investigation of enhanced tubes pool boiling models.     

Boiling heat transfer in a hydrofoil-based micro pin fin heat sink

Flow boiling of R-123 in a hydrofoil-based micro pin fin heat sink was investigated. Average two-phase heat transfer coefficients were obtained over effective heat fluxes ranging from 19 to 312 W/cm² and mass fluxes from 976 to 2349 kg/m² s. The paper presents a flow map, which divides the data into three flow pattern regions: bubbly, wavy intermittent and spray-annular flows. Heat transfer coefficient trends and flow morphologies were used to infer boiling heat transfer mechanisms. Existing conventional scale correlations for circular tubes resulted in large scatter and were not able to predict the heat transfer coefficients accurately.     

Evaporation heat transfer and pressure drop characteristics of R-290 (propane), R-600 (butane), and a mixture of R-290/R-600 in the three-lines serpentine small-tube bank

This paper reports an experimental investigation of heat-transfer and pressure-drop behavior of R-290, R-600, and R-290/R-600 in the three-lines serpentine small-diameter (2.46 mm) tube bank. Heat transfer coefficients and pressure drop characteristics are measured for a range of heat flux (5–21 kW/m²), mass flux (250–500 kg/m² s), equilibrium mass quality (0–0.86), and the fixed mixture composition ((R-290/R-600 (55 wt.%/45 wt.%)). The results show that the flow boiling heat transfer coefficients for R-290, R-600, and R-290/R-600 are 1.66–1.96-fold, 1.28–1.38-fold and 1.57–1.88-fold greater as compared with those for R-134a under equal heat and mass fluxes. Also, the two-phase flow frictional pressure drop for R-600, R-290/R-600 and R-290 are 1.41–1.60-fold, 1.32–1.50-fold and 1.22–1.40-fold smaller as compared with that for R-134a. A new heat transfer correlation was presented by using a superposition model to predict the experimental data for both pure refrigerants and refrigerant mixtures. Experimental results were compared with several correlations which predict the evaporative heat transfer, which are in good agreement with experimental data.     

Hydrodynamic cavitation and boiling in refrigerant (R-123) flow inside microchannels

This research article investigates the effect that hydrodynamic cavitation has on heat transfer. The fluid medium is refrigerant R-123 flowing through 227 μm hydraulic diameter microchannels. The cavitation is instigated by the inlet orifice. Adiabatic tests were conducted to study the two-phase cavitating flow morphologies and hydrodynamic characteristics of the flow. Diabatic experiments were performed resulting in surface temperatures under heat fluxes up to 213 W/cm² and mass velocities from 622 kg/m² s to 1368 kg/m² s. Results were compared to non-cavitating flows at the same mass velocities. It was found that the cavitating flows can significantly enhance the heat transfer. The heat transfer coefficient of the cavitating flows was larger than the non-cavitating flows by as much as 84%. Single-phase and two-phase heat transfer coefficients have been elucidated and employed to deduce the heat transfer mechanism prevailing under boiling conditions with and without the presence of cavitation.     

Boiling heat transfer of refrigerant R-600a/R-290-oil mixtures in the serpentine small-diameter U-tubes

Flow boiling experiments for refrigerant R-600a, and R-290 mixed with the lubricating oil (EMKARATE) in the serpentine small-diameter (2.46 mm) U-tubes are reported. The tests were conducted at the nominal inlet pressure of 186.2 kPa, vapor qualities (0–0.76), mass flux of 100–320 (kg/m²s) and inlet oil concentrations from 0 to 5 mass% oil. It was noted that a significant degradation of heat transfer coefficients presented at high qualities and high oil concentrations. The present study investigated that whether the average heat transfer coefficient increased as mass fluxes and the numbers of the U-bend increased. In addition, the ratios (the enhanced heat transfer factor, EF) for both R-600a and R-290 refrigerants increased as the oil concentration increased up to 1% then decreased with each increase in oil concentration. Moreover, pressure dropped during evaporation increased with the addition of a lubricant, mass fluxes and the numbers of the U-turn. The values of the pressure drop penalty factor PF were generally larger than l and increased rapidly as the oil concentration and the numbers of the U-turn increased.     

Experimental study of evaporation heat transfer of R-134a inside a corrugated tube with different tube inclinations

Experimental heat transfer studies during evaporation of R-134a inside a corrugated tube have been carried out. The corrugated tube has been provided with different tube inclination angles of the direction of fluid flow from horizontal, α. The experiments were performed for seven different tube inclinations, α, in a range of − 90° to + 90° and four mass velocities of 46, 81, 110 and 136 kg m^− 2 s^− 1 for each tube inclination angle during evaporation of R-134a. Data analysis demonstrate that the tube inclination angle, α, affects the boiling heat transfer coefficient in a significant manner. The effect of tube inclination angle, α, on heat transfer coefficient, h, is more prominent at low vapor quality and mass velocity. In the low vapor quality region, the heat transfer coefficient, h, for the + 90° inclined tube is about 62% more than that of the − 90° inclined tube. The results also showed that at all mass velocities, the highest average heat transfer coefficient were achieved for α = + 90°. An empirical correlation has also been developed to predict the heat transfer coefficient during flow boiling inside a corrugated tube with different tube inclinations.     

Flow-boiling heat transfer of R-134a-based nanofluids in a horizontal tube

The influence of nanoparticles on the flow-boiling of R-134a and R-134a/polyolester mixtures is quantified for flows of low vapor quality (x < 20%) over a range of mass fluxes (100 < G < 400 kg/m² s). With direct dispersion of SiO₂ nanoparticles in R-134a, the heat transfer coefficient decreases (as much as 55%) in comparison to pure R-134a. This degradation is, in part, due to difficulties in obtaining a stable dispersion. However, excellent dispersion is achieved for a mixture of R-134a and polyolester oil with CuO nanoparticles, and the heat transfer coefficient increases more than 100% over baseline R-134a/polyolester results. In the range of these experiments, nanoparticles have an insignificant effect on the flow pressure drop with the R-134a/POE/CuO nanofluid.     

Condensation heat transfer and pressure drop of HFC-134a in a helically coiled concentric tube-in-tube heat exchanger

The two-phase heat transfer coefficient and pressure drop of pure HFC-134a condensing inside a smooth helically coiled concentric tube-in-tube heat exchanger are experimentally investigated. The test section is a 5.786 m long helically coiled double tube with refrigerant flowing in the inner tube and cooling water flowing in the annulus. The inner tube is made from smooth copper tubing of 9.52 mm outer diameter and 8.3 mm inner diameter. The outer tube is made from smooth copper tubing of 23.2 mm outer diameter and 21.2 mm inner diameter. The heat exchanger is fabricated by bending a straight copper double-concentric tube into a helical coil of six turns. The diameter of coil is 305 mm. The pitch of coil is 35 mm. The test runs are done at average saturation condensing temperatures ranging between 40 and 50 °C. The mass fluxes are between 400 and 800 kg m⁻² s⁻¹ and the heat fluxes are between 5 and 10 kW m⁻². The pressure drop across the test section is directly measured by a differential pressure transducer. The quality of the refrigerant in the test section is calculated using the temperature and pressure obtained from the experiment. The average heat transfer coefficient of the refrigerant is determined by applying an energy balance based on the energy rejected from the test section. The effects of heat flux, mass flux and, condensation temperature on the heat transfer coefficients and pressure drop are also discussed. It is found that the percentage increase of the average heat transfer coefficient and the pressure drop of the helically coiled concentric tube-in-tube heat exchanger, compared with that of the straight tube-in-tube heat exchanger, are in the range of 33–53% and 29–46%, respectively. New correlations for the condensation heat transfer coefficient and pressure drop are proposed for practical applications.     

Experimental investigation on two-phase flow boiling heat transfer of five refrigerants in horizontal small tubes of 0.5, 1.5 and 3.0 mm inner diameters

An experimental investigation on two-phase flow boiling heat transfer with refrigerants of R-22, R-134a, R-410A, C₃H₈ and CO₂ in horizontal circular small tubes is presented. The experimental data were obtained over a heat flux range of 5–40 kW m^?2, mass flux range of 50–600 kg m^?2 s^?1, saturation temperature range of 0–15 °C, and quality up to 1.0. The test section was made of stainless steel tubes with inner diameters of 0.5, 1.5 and 3.0 mm, and lengths of 330, 1000, 1500, 2000 and 3000 mm. The experimental data were mapped on Wang et al. (1997) [5] and Wojtan et al. (2005) [6] flow pattern maps. The effects of mass flux, heat flux, saturation temperature and inner tube diameter on the heat transfer coefficient are reported. The experimental heat transfer coefficients were compared with some existing correlations. A new boiling heat transfer coefficient correlation that is based on a superposition model for refrigerants in small tubes is presented with 15.28% mean deviation and ?0.48% average deviation.     

Thermophysical properties and heat transfer performance of Al2O3/R-134a nanorefrigerants

The past decade has seen the rapid development of nanofluids science in many aspects. In recent years, refrigerant-based nanofluids have been introduced as nanorefrigerants due to their significant effects over heat transfer performance. This study investigates the thermophysical properties, pressure drop and heat transfer performance of Al₂O₃ nanoparticles suspended in 1, 1, 1, 2-tetrafluoroethane (R-134a). Suitable models from existing studies have been used to determine the thermal conductivity and viscosity of the nanorefrigerants for the nanoparticle concentrations of 1 to 5 vol.%. The pressure drop, pumping power and heat transfer coefficients of nanorefrigerant in a horizontal smooth tube have also been investigated with the same particle concentration at constant velocity of 5 m/s and uniform mass flux of 100 kg/m² s. In this study, the thermal conductivity of Al₂O₃/R-134a nanorefrigerant increased with the augmentation of particle concentration and temperature however, decreased with particle size intensification. In addition, the results of viscosity, pressure drop, and heat transfer coefficients of the nanorefrigerant show a significant increment with the increase of volume fractions. Therefore, optimal particle volume fraction is important to be considered in producing nanorefrigerants that can enhance the performance of refrigeration systems.     

Heat transfer and pressure drop during HFC refrigerant saturated vapour condensation inside a brazed plate heat exchanger

This paper presents the heat transfer coefficients and the pressure drop measured during HFC refrigerants 236fa, 134a and 410A saturated vapour condensation inside a brazed plate heat exchanger: the effects of saturation temperature (pressure), refrigerant mass flux and fluid properties are investigated. The heat transfer coefficients show weak sensitivity to saturation temperature (pressure) and great sensitivity to refrigerant mass flux and fluid properties. A transition point between gravity controlled and forced convection condensation has been found for a refrigerant mass flux around 20 kg/m²s that corresponds to an equivalent Reynolds number around 1600–1700. At low refrigerant mass flux (G_r < 20 kg/m²s) the heat transfer coefficients are not dependent on mass flux and are well predicted by the Nusselt [20] analysis for vertical surface: the condensation process is gravity controlled. For higher refrigerant mass flux (G_r > 20 kg/m²s) the heat transfer coefficients depend on mass flux and are well predicted by Akers et al. [21] equation: forced convection condensation occurs. In the forced convection condensation region the heat transfer coefficients show a 25–30% increase for a doubling of the refrigerant mass flux.The frictional pressure drop shows a linear dependence on the kinetic energy per unit volume of the refrigerant flow and therefore a quadratic dependence on mass flux.HFC-410A shows heat transfer coefficients similar to HFC-134a and 10% higher than HFC-236fa together with frictional pressure drops 40-50% lower than HFC-134a and 50–60% lower than HFC-236fa.     

Evaporation heat transfer and pressure drop of HFC-134a in a helically coiled concentric tube-in-tube heat exchanger

The two-phase heat transfer coefficient and pressure drop of HFC-134a during evaporation inside a smooth helically coiled concentric tube-in-tube heat exchanger are experimentally investigated. The test section is a 5.786-m long helically coiled tube with refrigerant flowing in the inner tube and heating water flowing in the annulus. The inner tube is made from copper tubing of 9.52 mm outer diameter and 7.2 mm inner diameter. The heat exchanger is fabricated by bending a straight copper tube into a spiral coil. The diameter of coil is 305 mm. The test run are done at average saturated evaporating temperatures ranging between 10 and 20 °C. The mass fluxes are between 400 and 800 kg m⁻² s⁻¹ and the heat fluxes are between 5 and 10 kW m⁻². The inlet quality of the refrigerant in the test section is calculated using the temperature and pressure obtained from the experiment. The pressure drop across the test section is directly measured by a differential pressure transducer. The effects of heat flux, mass flux and, evaporation temperature on the heat transfer coefficients and pressure drop are also discussed. The results from the present experiment are compared with those obtained from the straight tube reported in the literature. New correlations for the convection heat transfer coefficient and pressure drop are proposed for practical applications.