Experiments on the characteristics of evaporation of R410A in brazed plate heat exchangers with different geometric configurations

Experiments on the evaporative heat transfer and pressure drop in the brazed plate heat exchangers were performed with refrigerants R410A and R22. The plate heat exchangers with different 45°, 35°, and 20° chevron angles are used. Varying the mass flux of refrigerant (13–34 kg/m² s), the evaporating temperature (5, 10 and 15 °C), the vapor quality (0.9–0.15) and heat flux (2.5, 5.5 and 8.5 kW/m²), the evaporation heat transfer coefficients and pressure drops were measured. The heat transfer coefficient increases with increasing vapor quality and decreasing evaporating temperature at a given mass flux in all plate heat exchangers. The pressure drop increases with increasing mass flux and quality and with decreasing evaporating temperature and chevron angle. It is found that the heat transfer coefficients of R410A are larger than those of R22 and the pressure drops of R410A are less than those of R22. The empirical correlations of Nusselt number and friction factor are suggested for the tested PHEs. The deviations between correlations and experimental data are within ±25% for Nusselt number and ±15% for friction factor.     

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.     

Condensation from superheated vapor flow of R744 and R410A at subcritical pressures in a horizontal smooth tube

This paper presents experimentally determined heat transfer coefficients for condensation from a superheated vapor of CO₂ and R410A. The superheated vapor was flowed through a smooth horizontal tube with 6.1 mm ID under almost uniform temperature cooling at reduced pressures from 0.55 to 0.95, heat fluxes from 3 to 20 kW m^?2, and superheats from 0 to 40 K. When the tube wall temperature reaches the saturation point, the measured results show that the heat transfer coefficient gradually starts deviating from the values predicted by a correlation valid for single-phase gas cooling. This point identifies the start of condensation from the superheated vapor. The condensation starts earlier at higher heat fluxes because the tube wall temperature reaches the saturation point earlier. The heat transfer coefficient reaches a value predicted by correlations for condensation at a thermodynamic vapor quality of 1. The measured heat transfer coefficient of CO₂ is roughly 20–70% higher than that of R410A at the same reduced pressures. This is mainly because the larger latent heat and liquid thermal conductivity of CO₂, compared to that of R410A, increase the heat transfer coefficient.     

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.     

Comparative investigations of the condensation of R134a and R404A refrigerants in pipe minichannels

The present paper describes the results of experimental investigations of heat transfer and pressure drop during the condensation of the R134a and R404A refrigerants in pipe minichannels with internal diameters d = 0.31–3.30 mm. The results concern investigations of the local heat transfer coefficient and a pressure drop in single mini-channels. The results were compared with calculations according to the correlations proposed by other authors. Within the range of the examined parameters of the condensation process in mini-channels produced from stainless steel, it was established that the values of the heat transfer coefficient may be described with Akers et al. and Shah correlations within a limited range of the mass flux density of the refrigerant and the mini-channel diameter. A pressure drop during the condensation of these refrigerants is described in a satisfactory manner with Friedel and Garimella correlations. On the basis of the experimental investigations, the authors proposed their own correlation for the calculation of local heat transfer coefficient α_x.     

A complete evaluation method for the experimental data of flow boiling in smooth tubes

A complete solution for boiling phenomena in smooth tubes has been giving as a procedure regarding with the calculation of convective heat transfer coefficient and pressure drop using accurate experimental data validated by flow regime maps and sight glasses on the experimental facility. The experimental study is conducted in order to investigate the effect of operating parameters on flow boiling convective heat transfer coefficient and pressure drop of R134a. The smooth tube having 8.62 mm inner diameter and 1100 mm length is used in the experiments. The effect of mass flux, saturation temperature and heat flux is researched in the range of 290–381 kg/m² s, 15–22 °C and 10–15 kW/m², respectively. The experiments revealed that the heat transfer coefficient and pressure drop are significantly affected by mass flux for all tested conditions. Moreover, the experimental results are compared with well-known heat transfer coefficient and frictional pressure drop correlations given in the literature. In addition, 122 number of heat transfer and pressure drop raw experimental data is given for researchers to validate their theoretical models.     

Experimental study of horizontal flattened tubes performance on condensation of R600a vapor

An empirical setup has been established to study heat transfer and pressure drop characteristics during condensation of R600a, a hydrocarbon refrigerant, in a horizontal plain tube and different flattened channels. Round copper tubes of 8.7 mm I.D. were deformed into flattened channels with different interior heights of 6.7 mm, 5.2 mm and 3.1 mm as test sections. The test conditions include heat flux of 17 kw/m², mass velocity in the range of 154.8–265.4 kg/m²s and vapor quality variation from approximately 10% to 80%. Results indicate that flattening the tubes causes significant enhancement of heat transfer coefficient which is also accompanied by simultaneous augmentation in flow pressure drop. Therefore, the overall performance of the flattened tubes with respect to heat transfer enhancement considering the pressure drop penalty is analyzed. It is concluded that the flattened tube with 5.2 mm inner height tube has the best overall performance. Due to the failure of pre-existing correlations for round tube condensation heat transfer, a new correlation is proposed which predicts 90% of the entire data within ± 17% error.     

Convective boiling of pure and mixed refrigerants: An experimental study of the major parameters affecting heat transfer

An experimental study is carried out to investigate the characteristics of the evaporation heat transfer for different fluids. Namely, pure refrigerants fluids (R22 and R134a), azeotropic and quasi-azeotropic mixtures (R404A, R410A, R507) and zeotropic mixtures (R407C and R417A).The test section is a smooth, horizontal, stainless steel tube (6 mm ID, 6 m length) uniformly heated by the Joule effect. The flow boiling characteristics of the refrigerant fluids are evaluated in 250 different operating conditions. Thus, a data-base of more than 2000 data points is produced.The experimental tests are carried out varying: (i) the refrigerant mass fluxes within the range 200–1100 kg/m² s; (ii) the heat fluxes within the range 3.50–47.0 kW/m²; (iii) the evaporating pressures within the range 3.00–12.0 bar.In this study, the effect on measured heat transfer coefficient of vapour quality, mass flux, saturation temperature, imposed heat flux, thermo-physical properties are examined in detail.     

Investigation of heat transfer and pressure drop of CO2 two-phase flow in a horizontal minichannel

An innovative cooling system based on evaporative CO₂ two-phase flow is under investigation for the tracker detectors upgrade at CERN (European Organization for Nuclear Research). The radiation hardness and the excellent thermodynamic properties emphasize carbon dioxide as a cooling agent in the foreseen minichannels. A circular stainless steel tube in horizontal orientation with an inner diameter of 1.42 mm and a length of 0.3 m has been used as a test section to perform the step-wise scanning of the vapor quality in the entire two-phase region. To characterize the heat transfer and the pressure drop depending on the vapor quality in the tube, measurements have been performed by varying the mass flux from 300 to 600 kg/m² s, the heat flux from 7.5 to 29.8 kW/m² and the saturation temperature from ?40 to 0 °C (reduced pressures from 0.136 to 0.472). Heat transfer coefficients between 4 kW/m² K and 28 kW/m² K and pressure gradients up to 75 kPa/m were registered. The measured data was analyzed corresponding to the dependencies on heat flux, mass flux and saturation temperature. A database has been established containing about 2000 measurement points. The experimental data was compared with common models recently developed by Cheng et al. [1], [2] to cross check their applicability. The overall trends and experimental data were reproduced as predicted by the models before the dryout onset, and deviations have been analyzed. A modified friction factor for the pressure drop model [1] in mist flow has been proposed based on the experimental data.     

Prediction of frictional pressure drop of R134a during condensation inside smooth and corrugated tubes

In the present paper, in order to understand the accuracy of 38 different correlations derived by various researchers in this field, the correlations were executed for condensation frictional pressure drop. To accomplish this goal, experimental data provided from authors' previous publications encompassing 412 points for two smooth tubes, and 334 points for five corrugated tubes, have been utilized so as to compare the determined results. The experimental setup is composed of a 2.5 m double tube for horizontal configuration and smooth and corrugated tubes at the inner diameters of 0.0081 m, while the applied mass flux range spans between 709 and 1974 kg m^− 2 s^− 1. The average quality of vapor and saturation pressure ranges lie within 0.09 to 0.97, and 10 to 13 bar, respectively. Determining the frictional pressure drop in two-phase flows does not involve corrugated tube geometry in the calculation of friction factor, to make this available, a slight alteration that requires the replacement of a correlation with another one in the literature was suggested with regard to friction factor approach. As a result of this, it was noticed that performances of some correlations were optimized to predict the frictional pressure drop in corrugated tubes. Additionally, the most effective correlations have been selected for the horizontal double pipe heat exchanger having smooth and corrugated tubes. Finally, alteration of the condensation pressure drop with Reynolds number are presented using both experimental data and best predictive correlations.     

Evaluation of the performance of the empirical correlations used to predict R134a's boiling frictional pressure drop inside smooth and corrugated tubes

Owing to the generalization problem, there aren't sufficient empirical correlations for two-phase flows. So as to investigate the thermal features of the two-phase flow in smooth and enhanced tubes, a suitable procedure of the models and correlations related with the heat transfer coefficients, friction factors and two-phase multipliers are needed because a significant variation in thermal properties happens during phase-change. Comparison of frictional pressure drop of R134a during flow boiling phenomena occurred in a smooth and 5 enhanced tubes with well-known empirical correlations were performed in this study. The apparatus has 0.85 m long double tube for vertical configuration as a test section that includes smooth and corrugated copper tubing having inner diameters of 0.0087 m, and the range of mass fluxes are between 200 and 400 kg m^− 2 s^− 1. The average vapor qualities vary from 0.14 to 0.86, and saturation pressure interval is between 4.5 and 5.7 bar. The mean boiling heat transfer coefficient of R134a is determined via energy balance in the test section. The estimation performance of 36 empirical correlations in literature proposed for convective boiling flows in smooth and corrugated tubes are evaluated by means of authors' database (350 data points for vertical tubes). Boiling trend lines have been plotted for the change of vapor quality, liquid phase Reynolds numbers with gas phase ones. In addition, the most successful correlations are confirmed their predictabilities for the vertical adjusted evaporator having smooth and corrugated tubes using the database of authors' earlier publications in open sources.     

Boiling heat transfer and pressure drop of ammonia-lithium nitrate solution in a plate generator

This paper presents the thermal and pressure-drop experimental evaluation of a fusion plate heat exchanger (PHE) during boiling conditions of a solution of lithium nitrate in ammonia. The data are representative of the working conditions of generators in single-effect absorption chillers. The solution flow rate and outlet temperature were modified in the ranges of 0.041–0.083 kg/s and 78–95 °C, respectively. Correlations for single-phase-flow heat transfer are used to characterize the boiling heat transfer. The influences of the heat flux, mass flux and exit-vapour quality are analyzed. Boiling heat-transfer coefficients and correlations for the Nusselt number are obtained. Results are compared with Cooper’s and Ayub’s correlations for boiling heat transfer. Pressure drop in the solution side was also measured and one correlation was obtained to characterize the frictional pressure drop under boiling conditions.     

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 of R407C/oil mixtures inside corrugated and micro-fin tubes

The paper reviews the evaporation of R407C data currently available in the literature. The experimental rig and procedure are discussed. Experimental data about the evaporation for the pure R407C and R407C/oil mixtures in two smooth tubes and two enhanced tubes are also presented. The performance benefits of the micro-fin tube and corrugated tube are quantified and discussed. During tests inlet vapour quality was set 0 and outlet quality 0.7. Mass flux density varied from about 250 to 500 kg/m² s. The experiments have been conducted for average saturation temperature 0 °C.     

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.     

Flow pattern maps for convective boiling of CO2 and R410A in a horizontal smooth tube: Experiments and new correlations analyzing the effect of the reduced pressure

In this work flow visualizations and measurements are made and analyzed to identify flow regime transitions, slug-to-intermittent, intermittent-to-annular and the dry-out inception, during the flow boiling of CO₂ in a horizontal smooth tube of 6.00 mm of internal diameter, varying the reduced pressure between 0.57 and 0.64, the mass velocity between 150 and 500 kg/m² s and the heat flux between 5 and 20 kW/m². Additional experiments for R410A show the effect of the reduced pressure over a wider range, from 0.19 to 0.52, varying the other operating parameters in the same ranges. All together, the new experimental dataset of 1420 observations and heat transfer measurements were utilized to determine the location of the flow pattern transitions, which showed a strong dependency of the vapor quality on the mass velocity for each transition line, for fixed reduced pressure. The dry-out inception line was influenced by the heat flux, as expected. The influence of the reduced pressure was also identified as an important parameter with a remarkable impact. This flow pattern database was then statistically compared with well established methods (Wojtan et al. (2005) [1] for R410A, Cheng et al. (2008) [2] for CO₂) and recent methods [3], [4], showing poor agreement in the determination of the intermittent-to-annular flow regime transition for all the methods and in the prediction of the dry-out inception for all the methods, except for Wojtan et al. (2005) [1]. Finally, new easy-to-use correlations are proposed to provide better agreement with the experimental dataset and to explicitly illustrate the effect of the reduced pressure in an effort to generalize this diabatic flow pattern map for broader application.     

Microchannel size effects on local flow boiling heat transfer to a dielectric fluid

Heat transfer with liquid–vapor phase change in microchannels can support very high heat fluxes for use in applications such as the thermal management of high-performance electronics. However, the effects of channel cross-sectional dimensions on the two-phase heat transfer coefficient and pressure drop have not been investigated extensively. In the present work, experiments are conducted to investigate the local flow boiling heat transfer of a dielectric fluid, Fluorinert FC-77, in microchannel heat sinks. Experiments are performed for mass fluxes ranging from 250 to 1600 kg/m² s. Seven different test pieces made from silicon and consisting of parallel microchannels with nominal widths ranging from 100 to 5850 μm, all with a nominal depth of 400 μm, are considered. An array of temperature sensors on the substrate allows for resolution of local temperatures and heat transfer coefficients. The results of this study show that for microchannels of width 400 μm and greater, the heat transfer coefficients corresponding to a fixed wall heat flux as well as the boiling curves are independent of channel size. Also, heat transfer coefficients and boiling curves are independent of mass flux in the nucleate boiling region for a fixed channel size, but are affected by mass flux as convective boiling dominates. A strong dependence of pressure drop on both channel size and mass flux is observed. The experimental results are compared to predictions from a number of existing correlations for both pool boiling and flow boiling heat transfer.     

Flow boiling heat transfer of HFO1234yf and R32 refrigerant mixtures in a smooth horizontal tube: Part I. Experimental investigation

HFO1234yf has been proposed for mobile air-conditioners due to its low global warming potential (GWP) and performance comparable to that of R134a. However, its performance is inferior to that of R410A. This makes it difficult to be applied to residential air-conditioners. In order to apply the low-GWP refrigerant to residential air-conditioners, refrigerant mixtures of HFO1234yf and R32 are proposed, and their flow boiling heat transfer performances were investigated at two mass fractions (80/20 and 50/50 by mass%) in a smooth horizontal tube with an inner diameter of 2 mm. The experiments were conducted under heat fluxes ranging from 6 to 24 kW/m² and mass fluxes ranging from 100 to 400 kg/m² s at the evaporation temperature of 15 °C. The measured heat transfer coefficients were compared with those of pure HFO1234yf and R32. The results showed that the heat transfer coefficients of the mixture with an R32 mass fraction of 20% were 10–30% less than those of pure HFO1234yf for various mass and heat fluxes. When the mass fraction of R32 increased to 50%, the heat transfer coefficients of the mixture were 10–20% greater than those of pure HFO1234yf under conditions of large mass and heat fluxes. Moreover, the heat transfer coefficients of the mixtures were about 20–50% less than that of pure R32. The performances of the mixtures were examined at different boiling numbers. For refrigerant mixture HFO1234yf and R32 (80/20 by mass%), the nucleate boiling heat transfer was noticeably suppressed at low vapor quality for small boiling numbers, whereas the forced convective heat transfer was significantly suppressed at high vapor quality for large boiling numbers. This indicates that the heat transfer is greatly influenced by the mass diffusion resistance and temperature glide of the mixture.     

Performance of naphthalene thermosyphons with non-condensable gases – Theoretical study and comparison with data

Naphthalene thermosyphons are efficient heat transfer devices that operate within 250 and 400 °C. There is a lack of literature about naphthalene thermosyphons, especially with the presence of non-condensable gases (NCG). Thermal circuit resistance models, considering or not NCG, are developed. NCG–vapor flat front hypothesis is adopted. Condensation and evaporation heat transfer coefficients are obtained from literature correlations. Thermal resistance data provided from naphthalene thermosyphon charged with argon, is obtained using especial experimental setup. Two combinations of correlations provided good comparison with data, for thermosyphons with and without NCG. These models are successfully applied for heat exchanger design.     

Experimental study on condensation heat transfer enhancement and pressure drop penalty factors in four microfin tubes

Heat transfer and pressure drop characteristics of four microfin tubes were experimentally investigated for condensation of refrigerants R134a, R22, and R410A in four different test sections. The microfin tubes examined during this study consisted of 8.92, 6.46, 5.1, and 4 mm maximum inside diameter. The effect of mass flux, vapor quality, and refrigerants on condensation was investigated in terms of the heat transfer enhancement factor and the pressure drop penalty factor. The pressure drop penalty factor and the heat transfer enhancement factor showed a similar tendency for each tube at given vapor quality and mass flux. Based on the experimental data and the heat-momentum analogy, correlations for the condensation heat transfer coefficients in an annular flow regime and the frictional pressure drops are proposed.